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Uranium and Depleted Uranium

(Updated December 2009)

http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Uranium-Resources/Uranium-and-Depleted-Uranium/#.UhXJX3-N6So

• The basic fuel for a nuclear power reactor is uranium – a heavy metal able to release abundant concentrated energy.
• Uranium occurs naturally in the Earth's crust and is mildly radioactive.
• Depleted uranium is a by-product from uranium enrichment.
• The health hazards associated with any uranium are much the same as those for lead.

The Earth's uranium (chemical symbol U) was apparently formed in supernovae up to about 6.6 billion years ago (see information page on The Cosmic Origins of Uranium). Its radioactive decay provides the main source of heat inside the Earth, causing convection and continental drift. As decay proceeds, the final product, lead, increases in relative abundance.

Uranium was discovered by Martin Klaproth, a German chemist, in 1789 in the mineral pitchblende, and was named after the planet Uranus. It occurs in most rocks in concentrations of 2 to 4 parts per million and is as common in the Earth's crust as tin, tungsten and molybdenum and about 40 times as common as silver. It is also found in the oceans, at an average concentration of 1.3 parts per billion. There are a number of locations in different parts of the world where it occurs in economically-recoverable concentrations. When mined, it yields a mixed uranium oxide product, U3O8. Uraninite, or pitchblende, is the most common uranium mineral.

In the past, uranium was also used to colour glass (from as early as 79 AD) and deposits were once mined in order to obtain its decay product, radium. This element was used in luminous paint, particularly on the dials of watches and aircraft instruments up to the 1950s, and in medicine for the treatment of disease.

For many years from the 1940s, virtually all of the uranium that was mined was used in the production of nuclear weapons, but this ceased to be the case in the 1970s. Today the only substantial use for uranium is as fuel in nuclear reactors, mostly for electricity generation. Uranium-235 is the only naturally-occurring material which can sustain a fission chain reaction, releasing large amounts of energy.

While nuclear power is the predominant use of uranium, heat from nuclear fission can be used for industrial processes. It is also used for marine propulsion (mostly naval). And small nuclear reactors are important for making radioisotopes.

The uranium atom

Uranium is one of the heaviest of all the naturally-occurring elements and has a specific gravity of 18.7. Its melting point is 1132ºC.^a

Like other elements, uranium occurs in slightly differing forms known as isotopes. These isotopes differ from each other in the number of neutron particles in the nucleus. Natural uranium as found in the Earth's crust is a mixture of three isotopes: uranium-238 (U-238), accounting for 99.275%; U-235 – 0.720%; and traces of U-234 – 0.005%.

The isotope U-235 is important because under certain conditions it can readily be split, yielding a lot of energy. It is therefore said to be 'fissile'. Meanwhile, like all radioactive isotopes, it decays. U-238 decays very slowly, its half-life^b being the same as the age of the Earth. This means that it is barely radioactive, less so than many other isotopes in rocks and sand. Uranium-238 has a specific radioactivity of 12.4 kBq/g, and U-235 80 kBq/g, but the smaller amount of U-234 is very active (231 MBq/g) so the specific radioactivity of natural uranium (25 kBq/g) is about double that of U-238 despite it consisting of over 99% U-238.^c In decay it generates 0.1 watts/tonne and this is enough to warm the Earth's mantle.

Uranium fission

The nucleus of the U-235 isotope comprises 92 protons and 143 neutrons (92 + 143 = 235). When the nucleus of a U-235 atom is split in two by a neutron^d, some energy is released in the form of heat, and two or three additional neutrons are thrown off. If enough of these expelled neutrons split the nuclei of other U-235 atoms, releasing further neutrons, a chain reaction can be achieved. When this happens over and over again, many millions of times, a very large amount of heat is produced from a relatively small amount of uranium.

It is this process, in effect 'burning' uranium, which occurs in a nuclear reactor. In a nuclear reactor the uranium fuel is assembled in such a way that a controlled fission chain reaction can be achieved. The heat created by splitting the U-235 atoms is then used to make steam which spins a turbine to drive a generator, producing electricity.

Whereas the U-235 atom is 'fissile', the U-238 atom is said to be 'fertile'. This means that it can capture a neutron and become (indirectly) plutonium-239, which is fissile. Pu-239 is very much like U-235, in that it can fission following neutron capture, also yielding a lot of energy^e. Because there is so much U-238 in a reactor core (most of the fuel), these reactions occur frequently, and in fact about one-third of the energy yield typically comes from burning Pu-239.

Both uranium and plutonium were used to make bombs before they became important for making electricity and radioisotopes. But the type of uranium and plutonium for bombs is different from that in a nuclear power plant. Bomb-grade uranium is highly enriched (>90% U-235, instead of about 3.5-5.0% in a power plant); bomb-grade plutonium is fairly pure (>90%) Pu-239 and is made in special reactors.

Uranium as a fuel for nuclear power

About 14% of the world's electricity is generated from uranium in nuclear reactors¹. This amounts to over 2700 billion kWh, as much as from all sources worldwide in 1988.² It comes from about 435 nuclear reactors with a total output capacity of about 370,000 MWe operating in 30 countries. Over 50 more reactors are under construction and another 430 are on the drawing board³. A typical 1000 megawatt (MWe) reactor can provide enough electricity for a modern city of close to one million people, about 7 billion kWh per year.

Belgium, Bulgaria, Czech Republic, Finland, France, Hungary, South Korea, Lithuania, Slovakia, Slovenia, Sweden, Switzerland and Ukraine all get 30% or more of their electricity from nuclear reactors. Germany and Japan derive more than a quarter of their electricity from uranium. The USA has over 100 reactors operating, supplying 20% of its electricity⁴.

Nuclear power stations and fossil-fuelled power stations of similar capacity have many features in common. Both require heat to produce steam to drive turbines and generators. In a nuclear power station, however, the fissioning of uranium atoms replaces the burning of coal or gas. The chain reaction that takes place in the core of a nuclear reactor is controlled by rods which absorb neutrons. They are inserted or withdrawn to set the reactor at the required power level. The fuel elements are surrounded by a substance called a moderator to slow the speed of the emitted neutrons and thus enable the chain reaction to continue^f. Water, graphite and heavy water are used as moderators in different types of reactors.

Sources of uranium

Uranium is widespread in many rocks, and even in seawater. However, like other metals, it is seldom sufficiently concentrated to be economically recoverable. Where it is, we speak of an orebody. In defining what is ore, assumptions are made about the cost of mining and the market price of the metal. Uranium resources are therefore calculated as tonnes recoverable up to a certain cost.

Australia's uranium resources are about 25% of the world's total, but Canada has been the world's leading producer to 2008, then being overtaken by Kazakhstan^g. Other countries with known resources include Russian Federation, USA, South Africa, Namibia, Niger, Brazil and Ukraine. Many more countries have smaller deposits which could be mined. (See information page on Supply of Uranium).

Uranium is sold only to countries which are signatories of the Nuclear Non-Proliferation Treaty, and which allow international inspection to verify that it is used only for peaceful purposes.

From uranium ore to reactor fuel

Uranium ore can be mined by underground or open-cut methods, depending on its depth. After mining, the ore is crushed and ground up. Then it is treated with acid to dissolve the uranium, which is then recovered from solution. Uranium may also be mined by in situ leaching (ISL), where it is dissolved from the orebody in situ and pumped to the surface.

The end product of the mining and milling stages, or ISL, is uranium oxide concentrate (U3O8). Before it can be used in a reactor for electricity generation, however, it must undergo a series of processes to produce a useable fuel.

For most of the world's reactors, the next step in making a useable fuel is to convert the uranium oxide into a gas, uranium hexafluoride (UF6), which enables it to be enriched^h. Enrichment increases the proportion of the U-235 isotope from its natural level of 0.7% to 3-5% (see information page on Uranium Enrichment). This enables greater technical efficiency in reactor design and operation, particularly in larger reactors, and allows the use of ordinary water as a moderator. A by-product (sometimes considered a waste product) of enrichment is depleted uranium (about 86% of the original feed).

After enrichment, the UF6 gas is converted to uranium dioxide (UO2) which is formed into fuel pellets. These fuel pellets are placed inside thin metal tubes which are assembled in bundles to become the fuel elements for the core of the reactor. UO2 has a very high melting point – 2865°C (compared with uranium metal – 1132°C).

Used reactor fuel is removed from the reactor and stored, either to be reprocessed or disposed of underground.

The uranium orebody contains both U-235 and (mostly) U-238. About 95% of the radioactivity in the ore is from the U-238 decay series. This has 14 radioactive isotopes in secular equilibrium, thus each represents 7% of the total. (In the case of Ranger ore - with 0.3% U308 - it has about 450 kBq/kg, so irrespective of the mass proportion, 32 kBq/kg per nuclide in that decay series.) When the ore is processed, the U-238 and the very much smaller masses of U-234 (and the U-235) are removed. The balance becomes tailings, and at this point has about 86% of its original intrinsic radioactivity. However, with the removal of most U-238, the following two short-lived decay products (Th-234 & Pa-234) soon disappear, leaving the tailings with a little over 70% of the radio-activity of the original ore after several months. The controlling long-lived isotope then becomes Th-230 which decays with a half life of 77,000 years to radium-226 followed by radon-222.

Recycled (reprocessed) uranium

Uranium comprises about 96% of used fuel. When used fuel is reprocessed, both plutonium and uranium are recovered separately.

Uranium recovered from reprocessing used nuclear fuel is mostly U-238 with about 1% U-235, so it needs to be converted and re-enriched. This is complicated by the presence of impuritiesⁱ and two isotopes in particular, U-232 and U-236, which are formed by or following neutron capture in the reactor, and increase with higher burn-up levels^j. U-232 is largely a decay product of Pu-236, and increases with storage time in used fuel, peaking at about ten years. Both U-232 and U-236 decay much more rapidly than U-235 and U-238, and one of the daughter products of U-232 emits very strong gamma radiation, which means that shielding is necessary in any plant handling material with more than very small traces of it. U-236, comprising about 0.5% of recovered uranium, is a neutron absorber which impedes the chain reaction, and means that a higher level of U-235 enrichment is required in the product to compensate.

Because they are lighter than U-238, both U-232 and U-236 tend to concentrate in the enriched (rather than depleted) output, so reprocessed uranium (RepU) that is re-enriched for fuel must be segregated from enriched fresh uranium. Enriched RepU has an activity of over 250 kBq/g, which compares with 82kBq/g (most of this being from U-234) for enriched fresh uranium. The presence of U-236 in particular means that most reprocessed uranium can normally be recycled only once. In the future, laser enrichment techniques may be able to remove these difficult isotopes.

Uranium from thorium

Thorium, as well as uranium, can be used as a nuclear fuel. Although not fissile itself, Th-232 will absorb slow neutrons to produce uranium-233 (U-233)^k, which is fissile (and long-lived). The irradiated fuel can then be unloaded from the reactor, the U-233 separated from the thorium, and fed back into another reactor as part of a closed fuel cycle.

U-233 has higher neutron yield per neutron absorbed than U-235 or Pu-239. Given a start with some other fissile material (U-233, U-235 or Pu-239) as a driver, a breeding cycle similar to but more efficient than that with U-238 and plutonium (in conventional thermal neutron reactors) can be set up. The driver fuels provide all the neutrons initially, but are progressively supplemented by U-233 as it forms from the thorium. However, the intermediate product protactinium-233 (Pa-233) is a neutron absorber which diminishes U-233 yield. (See information page on Thorium).

Other uses of uranium-fuelled reactors

There are also other uses for uranium-fuelled nuclear reactors. Over 200 small nuclear reactors power some 150 ships, mostly submarines, but ranging from icebreakers to aircraft carriers. These can stay at sea for very long periods without having to make refuelling stops. In most such vessels the steam drives a turbine directly geared to propulsion.

The heat produced by nuclear reactors can also be used directly rather than for generating electricity. In Russia, for example, it is used to heat buildings and elsewhere it provides heat for a variety of industrial processes such as water desalination. In the future, high-temperature reactors could be used for industrial processes such as thermochemical production of hydrogen.

Radioisotope production in uranium fuelled reactors

Radioactive materials (radioisotopes) play a key role in the technologies that provide us with food, water and good health and have become a vital part of modern life. They are produced by bombarding small amounts of particular elements with neutrons. Using relatively small special purpose nuclear reactors (usually called research reactors), a wide range of radioisotopes can be made at low cost. The use of radioisotopes has become widespread since the early 1950s, and there are now some 280 research reactors in 56 countries producing them.

In medicine, radioisotopes are widely used for diagnosis, and also for treatment and research. Radioactive chemical tracers emit gamma radiation which provides diagnostic information about a person's anatomy and the functioning of specific organs. Radiotherapy also employs radioisotopes in the treatment of some illnesses, such as cancer. More powerful gamma sources are used to sterilise syringes, bandages and other medical equipment. About one in two people in Western countries is likely to experience the benefits of nuclear medicine in their lifetime, and gamma sterilisation of equipment is almost universal. (See information page on Radioisotopes in Medicine.)

In the preservation of food, radioisotopes are used to inhibit the sprouting of root crops after harvesting, to kill parasites and pests, and to control the ripening of stored fruit and vegetables. Irradiated foodstuffs are accepted by world and national health authorities for human consumption in an increasing number of countries. They include potatoes, onions, dried and fresh fruits, grain and grain products, poultry and some fish. Some prepacked foods can also be irradiated.

Agriculturally, in the growing crops and breeding livestock, radioisotopes also play an important role. They are used to produce high-yielding, disease- and weather-resistant varieties of crops, to study how fertilisers and insecticides work, and to improve the productivity and health of domestic animals. Industrially, and in mining, they are used to examine welds, to detect leaks, to study the rate of wear of metals, and for on-stream analysis of a wide range of minerals and fuels. (See information page on Radioisotopes in Industry.)

Environmentally, radioisotopes are used to trace and analyse pollutants, to study the movement of surface water, and to measure water runoffs from rain and snow, as well as the flow rates of streams and rivers.

Most household smoke detectors use a radioisotope (americium-241) derived from the plutonium formed in nuclear reactors. These alarms save many lives. (See information page on Smoke Detectors and Americium.)

Depleted uranium

Every tonne of natural uranium produced and enriched for use in a nuclear reactor gives about 130 kg of enriched fuel (3.5% or more U-235). The balance is depleted uranium tails (U-238, typically with 0.25-0.30% U-235). This major portion has been depleted in its fissile U-235 isotope (and, incidentally, U-234) by the enrichment process. It is commonly known as DU if the focus is on the actual material, or tails if the focus is on its place in the fuel cycle and its U-235 assay.

DU tails are either stored as UF6 or (especially in France) de-converted back to U3O8, which is more benign chemically and thus more suited for long-term storage. It is also less chemically toxic. Every year over 50,000 tonnes of depleted uranium joins already substantial stockpiles in the USA, Europe and Russia. World stock is about 1.5 million tonnes.

Some DU is drawn from these stockpiles to dilute high-enriched (>90%) uranium released from weapons programs, particularly in Russia, and destined for use in civil reactors (see information page on Military Warheads as a Source of Nuclear Fuel). This weapons-grade material is diluted about 25:1 with depleted uranium, or 29:1 with depleted uranium that has been enriched slightly (to 1.5% U-235) to minimise levels of (natural) U-234 in the product.

Some, assaying 0.25-0.40% U-235, is sent to Russia for re-enrichment, using surplus plant capacity there to produce either natural uranium equivalent or low-enriched uranium (4-5% U-235).

Some DU is used for mixed oxide (MOX) fuel, by mixing with plutonium (see information page on Mixed Oxide (MOX) Fuel).

Other uses depend on the metal's very high density (1.7 times that of lead). Hence, where maximum mass must fit in minimum space, such as aircraft control surface and helicopter counterweights, yacht keels, etc, it is often well suited. Until the mid 1970s it was used in dental porcelains. In addition it is used for radiation shielding, being some five times more effective than lead in this role.

Also because of its density, it is used as solid slugs or penetrators in armour-piercing projectiles, alloyed with abut 0.75% titanium. DU is pyrophoric, so that upon impact about 30% of the projectile atomises and burns to uranium oxide dust. It was widely used in the 1990/91 Gulf War (300 tonnes) and less so in the 1998/99 Kosovo War (11 tonnes).

Health aspects of DU

Depleted uranium is not classified as a dangerous substance radiologically, though it is a potential hazard in large quantities, beyond what could conceivably be breathed. Its emissions are very low, since the half-life of U-238 is the same as the age of the Earth (4.5 billion years). There are no reputable reports of cancer or other negative health effects from radiation exposure to ingested or inhaled natural or depleted uranium, despite much study.

However, uranium does have a chemical toxicity about the same as that of lead, so inhaled fume or ingested oxide is considered a health hazard. Most uranium actually absorbed into the body is excreted within days, the balance being laid down in bone and kidneys. Its biological effect is principally kidney damage. The World Health Organization (WHO) has set a tolerable daily intake level for uranium of 0.6 microgram/kg body weight, orally. (This is about eight times our normal background intake from natural sources.) Standards for drinking water and concentrations in air are set accordingly.

Like most radionuclides, it is not known as a carcinogen, or to cause birth defects (from effects in utero) or to cause genetic mutations. Radiation from DU munitions depends on how long since the uranium has been separated from the lighter isotopes so that its decay products start to build up. Decay of U-238 gives rise to Th-234, Pa-234 (beta emitters) and U-234 (an alpha emitter)^m. On this basis, in a few months, DU is weakly radioactive with an activity of around 40 kBq/g quoted. (If it is fresh from the enrichment plant and hence fairly pure, the activity is 15 kBq/g, compared with 25 kBq/g for pure natural uranium. Fresh DU from enriching reprocessed uranium has U-236 in it and more U-234 so is about 23 kBq/g.)

In 2001, the UN Environment Programme (UNEP) examined the effects of nine tonnes of DU munitions having been used in Kosovo, checking the sites targeted by it⁵. UNEP found no widespread contamination, no sign of contamination in water of the food chain and no correlation with reported ill-health in NATO peacekeepers. A two-year study⁶ by Sandia National Laboratories in USA reported in 2005 that consistent with earlier studiesⁿ, reports of serious health risks from DU exposure during the 1991 Gulf War are not supported by medical statistics or by analysis.

An editorial in the Radiological Protection Bulletin of the UK's National Radiation Protection Board stated: "DU is radioactive and doses from inhalation of dust or from handling bare spent rounds need to be assessed properly. However, the scientific consensus at present is that the risks are likely to be small and easily avoidable, especially compared with the other risks the armed forces have to take in war."⁸

Thus DU is clearly dangerous for military targets, but for anyone else – even in a war zone – there is little hazard. Ingestion or inhalation of uranium oxide dust resulting from the impact of DU munitions on their targets is the main possible exposure route.

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Further Information

Notes

a. See also Webelements' uranium webpage [Back]

b. The half-life is the time it takes for a radionuclide to lose half of its own radioactivity. [Back]

c. The becquerel (Bq) is a unit or measure of actual radioactivity in material (as distinct from the radiation it emits), with reference to the number of nuclear disintegrations per second (1 Bq = 1 disintegration/sec). For further information on units of radioactivity see the Units of radiation and radioactivity section in the information page on Radiation and Nuclear Energy [Back]

d. U-235 can fission following capture of a low-energy (or 'thermal') neutron to form a new compound nucleus, which then splits into two daughter fragments and two or three neutrons (average around 2.5), releasing energy in the process. For further information on nuclear fission, see WNA's Some Physics of Uranium education paper [Back]

e. Sometimes Pu-239 simply captures a neutron without splitting, and it becomes Pu-240. Because the Pu-239 is either progressively burned or becomes Pu-240, the longer the fuel stays in the reactor the more Pu-240 accumulates in it. The significance of this is that when the used fuel is removed after about three years, the plutonium in it is not suitable for making weapons – because Pu-240 has a relatively high rate of spontaneous fission – but can be recycled as fuel (see also information page on Plutonium). [Back]

f. Neutrons released in fission are initially fast (velocity about 10⁹ cm/sec, or energy above 1 MeV), but fission in U-235 is most readily caused by slow (thermal) neutrons (velocity about 10⁵ cm/sec, or energy about 0.02 eV). A moderator material comprising light atoms thus surrounds the fuel rods in a reactor to slow down the neutrons in elastic collisions. For further information, see WNA's education paper on Some Physics of Uranium [Back]

g. Kazakhstan is now reported to be the world's leading uranium producer – see Kazakhstan takes top spot in 2009, World Nuclear News (5 January 2010) [Back]

h. For reactors which use natural uranium as their fuel (and which require graphite or heavy water as a moderator) the U3O8 concentrate simply needs to be refined and converted directly to uranium dioxide. [Back]

i. Recovered uranium (especially from earlier military reprocessing) may be contaminated with traces of fission products. Over 2002-06 USEC cleaned up 7400 tonnes of technetium-contaminated uranium from the US Department of Energy. [Back]

j. Recovered uranium also contains a higher proportion of U-234 than fresh reactor fuel. As well as having a greater specific activity than both U-235 and U-238, the presence of U-234 alters the reactivity as it absorbs neutrons. [Back]

k. Neutron absorption by Th-232 produces Th-233, which has a half-life of about 22 minutes. This undergoes beta decay to form Pa-233 (half-life 27 days), most of which forms U-233 by further beta decay. Around 11% of the U-233 is converted by further neutron absorption to U-235, which is the fissile isotope of uranium used in conventional nuclear reactors. A small amount of the Pa-233 and U-233 forms U-232 in the reactor. Separated U-233 is therefore always contaminated with traces of U-232, which has a 69-year half-life but whose daughter products^l, particularly thallium-208, are strong gamma emitters with very short half-lives. This creates significant problems in handling the bred U-233 and makes it easy to detect, hence conferring proliferation resistance. [Back]

l. The decay chain of U-232 has six short-lived decay products before Tl-208, which precedes stable Pb-208. [Back]

m. U-238 (half-life 4.5 billion years) decays to thorium-234 (half-life 24 days), which beta decays to protactinium-234 (half-life one minute), which beta decays to U-234 (alpha emitter, half-life 246,000 years). [Back]

n. For example, a 2001 paper by the Australasian Radiation Protection Society (ARPS)⁷, which quotes several studies, concludes that health risks associated with the levels of DU exposure experienced during the Gulf War are essentially zero. A summary of the ARPS statement reads as follows:

Some military personnel involved in the 1991 Gulf War complained of continuing stress-like symptoms for which no obvious cause was found. These symptoms were at times attributed to the use of depleted uranium in shells and other missiles, which are said to have caused toxic effects. Similar complaints arose from later fighting in the Balkans (Kosovo). Because of the latency period for the induction of cancer by radiation, it is not credible that any cases of radiation-induced cancer could in the short term be attributed to the Kosovo conflict. Furthermore, extensive studies have concluded that no radiological health hazard should be expected from exposure to depleted uranium. The risk from external exposure is essentially zero, even when pure metal is handled. No detectable increases of cancer, leukaemia, birth defects or other negative health effects have ever been observed from radiation exposure to inhaled or ingested natural uranium concentrates, at levels far exceeding those likely in areas where DU munitions have been used. This is mainly because the low radioactivity per unit mass of uranium means that the mass needed for significant internal exposure would be virtually impossible to accumulate in the body – and DU is less than half as radioactive as natural uranium. [Back]

References

1. Key World Energy Statistics 2009, OECD International Energy Agency, 9 rue de la Fédération, 75739 Paris Cedex 15, France (2009) [Back]

2. Table 8.2a of Annual Energy Review 2008, U.S. Energy Information Administration, Report No. DOE/EIA-0384(2008) [Back]

3. World Nuclear Association table of World Nuclear Power Reactors & Uranium Requirements [Back]

4. World Nuclear Association table of Nuclear share figures [Back]

5. Depleted Uranium in Kosovo: Post-Conflict Environmental Assessment United Nations Environment Programme (2001) [Back]

6. Albert C. Marshall, An Analysis of Uranium Dispersal and Health Effects Using a Gulf War Case Study, Sandia National Laboratories, USA, SAND2005-4331 (July 2005) [Back]

7. Potential Health Effects of Depleted Uranium in Munitions, Australasian Radiation Protection Society media release (8 February 2001) [Back]

8. Michael Clark, Editorial, Radiological Protection Bulletin No. 229, P3 (March 2001), National Radiological Protection Board [Back]

General sources

World Health Organization fact sheet on depleted uranium

US Department of Energy Depleted UF6 Management Information Network website (http://web.ead.anl.gov/uranium/)

Radiation Protection Bulletin No. 167, P.13-16 (July 1995), National Radiological Protection Board

Balkans Task Force Final Report, The Kosovo Conflict – Consequences for the Environment & Human Settlements, United Nations Environment Programme and United Nations Centre for Human Settlements (Habitat), ISBN 9280718011 (1999)

Management of Depleted Uranium, OECD Nuclear Energy Agency, OECD Publishing, ISBN 9789264195257 (Aug 2001)

Bulletin of the Atomic Scientists, Volume 55, Number 6, (November-December 1999)

The Cosmic Origins of Uranium

(November 2006)

• Uranium drives 16% of our electricity worldwide, yet this fact pales into insignificance when we consider the role uranium has played in the evolution of the Earth.
• The Earth's uranium was produced in one or more supernovae over 6 billion years ago.
• Uranium later became enriched in the continental crust.

Geologists and geochemists have been studying the abundance, distribution and chronometric potential of the isotopes of uranium for more than a century. Their work stems from Klaproth's discovery in 1789 of the heaviest naturally occurring element, Becquerel's demonstration in 1896 that uranium salts are radioactive, Boltwood's conclusion in 1905 that lead as well as helium is a decay product of uranium, and Rutherford's suggestion in 1906 of the geological time-keeping potential of radioactivity. From a geochemical point of view, some of the major questions are:

1. Where did the uranium now in the Earth come from?
2. What effects has the comparatively trivial uranium content of the Earth had on the evolution of the planet and, conversely, are there feedbacks controlling the geochemical cycle of uranium that vary secularly (i.e. over long, indefinite periods of time)?
3. Can we track through time the way uranium has been recycled through the exosphere, crust and mantle of the Earth?
4.  

Cosmic abundance of elements

For many years, since the 1930s, a large number of scientists have been occupied with determining the abundances of the elements and their isotopes in the objects comprising the solar system, and with accounting for the abundance patterns observed. In fact, spectroscopic measurements show that the abundances of elements in stars vary and that there is no single applicable 'cosmic abundance' pattern.

Closer to home, there are major differences in abundances of the elements in the various planets that orbit our hydrogen-helium dominated Sun. The terrestrial planets, including the Earth, are relatively depleted in the potentially gaseous or volatile elements (hydrogen, helium, carbon and neon) and are dominated by elements of low and even atomic number (oxygen, magnesium, silicon and iron). On this scale, uranium - the abundance of which in the Sun is only 10 ^-12 that of hydrogen - is an exceedingly rare element. Furthermore, measurements of oxygen isotopes in meteorites show that the solar system as a whole is not homogeneous in terms of isotopic ratios. All these variations point to a conclusion that multiple sources were involved in the production of proto-solar system material.

Where did uranium come from?

Cosmochemists have been concerned not only with patterns and secular trends of abundance of the elements in galaxies but also with the origins of abundance anomalies in particular stars and with theories on the synthesis of different nuclei to account for these observations. According to the theories developed, the Earth's uranium was produced in one or more supernovae ("An explosive brightening of a star in which the energy radiated by it increases by a factor of ten billion ... A supernova explosion occurs when a star has burned up all its available nuclear fuel and the core collapses catastrophically." - Oxford Dictionary of Physics). The main process concerned was the rapid capture of neutrons on seed nuclei at rates greater than disintegration through radioactivity. The neutron fluxes required are believed to occur during the catastrophically explosive stellar events called supernovae. Gravitational compression of iron (the island of nuclear stability, incapable of further exothermic fusion reactions) and sudden collapse in the centre of a massive star triggers the explosive ejection of much of the star into space, together with a flood of neutrons. Remnants of hundreds of supernovae have been found, and we "witnessed" one in the Magellanic Clouds in 1987.

So, we know that the Earth's uranium was produced through this process in one or more supernovae, and that this material was inherited by the solar system of which the Earth is a part.

We might further ask how long ago this synthesis of uranium occurred. Given

• the present day abundances of U-235 and U-238 in the various 'shells' forming our planet,
• a knowledge of the half-lives of these isotopes, and
• the age of the Earth (c 4.55 billion years) - known from various radiometric 'clocks', including those of the uranium-to-lead decay chains.

We can calculate the abundances of U-235 and U-238 at the time the Earth was formed. Knowing further that the production ratio of U-235 to U-238 in a supernova is about 1.65, we can calculate that if all of the uranium now in the solar system were made in a single supernova, this event must have occurred some 6.5 billion years ago.

This 'single stage' is, however, an oversimplification. In fact, multiple supernovae from over 6 billion to about 200 million years ago were involved. Additionally, studies of the isotopic abundances of elements, such as silicon and carbon in meteorites, have shown that more than ten separate stellar sources were involved in the genesis of solar system material. Thus the relative abundance of U-235 and U-238 at the time of formation of the solar system:

• cannot be inverted to a 'single stage' model age,
• is essentially an accidental and unique value, and
• reflects the input of the explosive debris of many progenitor stars.

Enrichment in Earth's crust

Many analyses have been made of the uranium in the rocks forming the continental and oceanic crusts, and in samples of the Earth's mantle exposed as uplifted slices in mountain belts or as 'xenoliths' in basalts and kimberlites (hosts of diamonds).

We can have some confidence that these measurements are robust for the crust and upper mantle of the Earth, but less confidence that we know the abundance of uranium in the lower mantle and the outer and inner cores. While on average the abundance of uranium in meteorites is about 0.008 parts per million (gram/tonne), the abundance of uranium in the Earth's 'primitive mantle' - prior to the extraction of the continental crust - is 0.021 ppm. Allowing for the extraction of a core-forming iron-nickel alloy with no uranium (because of the characteristic of uranium which makes it combine more readily with minerals in crustal rocks rather than iron-rich ones), this still represents a roughly twofold enrichment in the materials forming the proto-Earth compared with average meteoritic materials.

The present-day abundance of uranium in the 'depleted' mantle exposed on the ocean floor is about 0.004 ppm. The continental crust, on the other hand, is relatively enriched in uranium at some 1.4 ppm. This represents a 70-fold enrichment compared with the primitive mantle. In fact, the uranium lost from the 'depleted' oceanic mantle is mostly sequestered in the continental crust.

It is likely that the process or processes which transferred uranium from the mantle to the continental crust are complex and multi-step. However, for at least the past 2 billion years they have involved:

1. formation of oceanic crust and lithosphere through melting of the mantle at mid-ocean ridges,
2. migration of this oceanic lithosphere laterally to a site of plate consumption (this is marked at the surface by a deep-sea trench),
3. production of fluids and magmas from the downgoing (subducted) lithospheric plate and overriding mantle 'wedge' in these subduction zones,
4. transfer of these fluids/melts to the surface in zones of 'island arcs' (such as the Pacific's Ring of Fire),
5. production of continental crust from these island arc protoliths, through remelting, granite formation and intra-crustal recycling.

All through this crust-forming cycle, the lithophile character of uranium is manifest in the constancy of the potassium to uranium ratio at about 10,000 in the rock range from peridotite to granite. Because we would like to keep track of how uranium is distributed in the Earth, the abundance and isotopic characteristics of lead - the radiogenic daughter of U-235 and U-238 - are useful parameters. Table 1 below highlights the relatively low abundance of lead in the Earth's mantle and the consequent high uranium to lead ratio, compared with meteorites. The difference in abundance can most likely be explained by lead's volatile nature and tendency to combine with iron, with lead being lost during terrestrial accretion and core separation. One of the consequences, of course, of these high ratios is the comparatively high radiogenic/non-radiogenic content of Pb-207/Pb-204 and Pb-206/Pb-204 in the Earth's crust and mantle compared with meteorites or the earth's core. (Pb-207 is the final stable decay product of U-235, and Pb-206 is that of U-238. Pb-204 is non-radiogenic.

Table 1

	U abundance (ppm)	Pb abundance (ppm)	U/Pb ratio
Meteorites	0.008	2.470	0.003
Primitive mantle	0.021	0.185	0.113
Continental crust	1.4	12.6	0.111

The figure given for the continental crust is an average of the entire crust. Of course, local concentration of uranium can far exceed these values, ranging up to 50 ppm disseminated in some granites, to much higher values in ore deposits. In fact, in the geological past, local concentrations of uranium have occasionally achieved natural criticality, for example the Oklo reactors in Gabon.

Energy source

Convection in the outer core and the mantle, whereby heat is transferred by movement of heated matter, governs many of the Earth's endogenous processes.

The convection in the core may be driven by the heat released during progressive solidification of the core (latent heat of crystallisation) and leads to the self-sustaining terrestrial dynamo which is the source of the Earth's magnetic field. Heat transfer from the core at the core/mantle boundary is also believed to trigger upwellings of relatively hot, and hence low density, plumes of material. These plumes then ascend, essentially without gaining or losing heat, and undergo decompression melting close to the Earth's surface at 'hot spots' like Hawaii, Reunion and Samoa.

However, the primary source of energy driving the convection in the mantle is the radioactive decay of uranium, thorium and potassium. In the present Earth, most of the energy generated is from the decay of U-238 (c 10^-4 watt/kg). At the time of the Earth's formation, however, decay of both U-235 and K-40 would have been subequal in importance and both would have exceeded the heat production of U-238.

A simple way of viewing the process of plate tectonics - the formation and disposal of oceanic lithosphere - is that this is the mechanism by which the mantle sheds heat. Conversely, 'mantle plumes/hot spots' are the way the core sheds heat. In terms of total heat loss from the Earth at present, plate activity constitutes about 74%, hot spots account for approximately 9% and radiogenic heat lost directly from the continental crust is some 17%. The Earth is well insulated thermally and the heat loss from the surface now can reflect heat generation a considerable time in the past.

Measurements of heat have led to estimates that the Earth is generating between 30 and 44 terawatts of heat, much of it from radioactive decay. Measurements of antineutrinos have provisionally suggested that about 24 TW arises from radioactive decay. Professor Bob White provides the more recent figure of 17 TW from radioactive decay in the mantle. This compares with 42-44 TW heat loss at the Earth's surface from the deep Earth. The balance comes from changes in the core. (There is very much greater heat loss arising from incident solar radiation, which is quite distinct.)

Atmosphere and greenhouse effect, the role of plants

Apart from the fundamental physical and chemical differentiation of the Earth driven by plate tectonics, lithosphere formation and destruction are also critical for many processes in the outer layer of the atmosphere. We know, for example, that during periods of enhanced oceanic lithosphere formation, such as occurred during the Cretaceous period some 100 million years ago, the mid-ocean ridges stood higher, triggering flooding of the low-lying portions of the continents. In fact the Laurasian part of the former Pangea supercontinent was drowned to a greater extent than the Gondwana part, maybe reflecting some deep-seated thermal/compositional contrast. The effects were manifold and include:

1. enhanced carbon dioxide release causing increased carbon dioxide content of the ocean and the atmosphere,
2. diminished continental surface area leading to a reduction in the titration through weathering of atmospheric carbon dioxide,
3. sustained high atmospheric carbon dioxide levels leading to an enhanced greenhouse effect and warmer climate.

Secular changes have taken place in several atmospheric processes, including a change in composition, from relatively reducing to astonishingly oxidising. The odd-looking "equation" for atmosphere production is:

CO₂ + H₂ = N₂ + O₂

where the primary, volcanically degassed inputs to the atmosphere are on the left, and the cumulative abundant components are on the right hand side of the equation. Nitrogen is a trace volcanic emission, not utilised to any great extent in surface processes - including the trivial effect of organic life - and merely accumulates in the atmosphere. The Earth's distance from the Sun, together with the greenhouse feedback, allows surface temperatures to be generally sustained within the condensation range for water. Carbon dioxide dissolves in water and is also sequestered in calcite by inorganic and organic precipitation as limestone.

The remarkable feature of our atmosphere is the presence of molecular oxygen released through photosynthesis, the process by which green plants manufacture their carbohydrates from atmospheric carbon dioxide and water:

6CO₂ + 6H₂O = C₆H₁₂O₆ + 6O₂

Photosynthesis can be traced back in time to about 3.8 billion years. For a while, the oxygen released was consumed through oxidation of reduced ferrous compounds at the Earth's surface. Ultimately, the oxygen started to accumulate in the atmosphere as free oxygen some 2.5 billion years ago.

In addition to many other effects, the change in the redox character of the exosphere led to a fundamental change in the way uranium was transported in the weathering-erosion-deposition cycle. Whereas under reduced conditions uranium is relatively insoluble and stable as uraninite (UO₂), under oxidising conditions it becomes soluble (U⁶⁺) and readily transported. Since 2.5 billion years ago ore deposits of uranium have been formed primarily where reduction of uranium-bearing fluids was achieved, for example by bacteria or through contact with graphitic shales.

Uranium distribution through time

The oxidising atmosphere also led to an increased concentration of uranium in the oceans and thence via transport in recirculating hydrothermal fluids to relative enrichment in the oceanic crust. The enhanced uranium transport from the exosphere to the Earth's interior - via subduction of oceanic lithosphere and the subsequent rehomogenisation of this lithosphere into the Earth's mantle - has had a significant effect on the present distribution of uranium in the Earth, and may account for some curious inconsistencies in the isotopic characteristics of the mantle. For example, whereas the time-integrated Pb-208 (stable final decay product of Th-232)/Pb-206 values of mid-ocean ridge basalts indicate mantle source values of Th/U of about 4, the measured values of Th/U and systematics of short-lived Th-U disequilibria indicate a ratio of about 2. It is likely that since about 2.5 billion years ago injections of uranium into the mantle have been effective in the reduction of the thorium to uranium ratio on an (upper) mantle-wide scale.

An additional net effect is the selective reinjection of uranium as opposed to lead - which is mostly stripped out in subduction zones and returned immediately to the crust - into the mantle. We know that overall, most basalts are being produced from a mantle with an enhanced uranium/lead ratio and with apparent 'future' ages, compared with the lead isotopic ratios characteristic of a closed system, single stage evolution of uranium/lead in the Earth. This feature is sometimes referred to by geochemists as the 'lead paradox', and may in part relate to the feedback influence of an oxidising, life-triggered exosphere on the interior of the Earth.

Natural nuclear reactors in the Earth's crust

At Oklo in Gabon, West Africa, about 2 billion years ago, at least 17 natural nuclear reactors commenced operation in a rich deposit of uranium ore. Each operated at about 20 kW thermal. At that time the concentration of U-235 in all natural uranium was 3.7 percent instead of 0.7 percent as at present (U-235 decays much faster than U-238, whose half-life is about the same as the age of this planet.).

These natural chain reactions, started spontaneously by the presence of water acting as a moderator, continued for about two million years before finally dying away. During this long reaction period about 5.4 tonnes of fission products as well as 1.5 tonnes of plutonium together with other transuranic elements were generated in the orebody. The initial radioactive products have long since decayed into stable elements but study of the amount and location of these has shown that there was little movement of radioactive wastes during and after the nuclear reactions. Plutonium and the other transuranics remained immobile.

Georeactor theory

A quite different view of the role of uranium in the Earth is the theory that much of the uranium in the primordial planet sunk to the core and has formed a core there, some 8 km across, which has been fissioning ever since. The depletion of U-235 over geological time has not terminated the reaction because this core is a fast reactor (not requiring any moderator) which breeds plutonium-239 from the U-238. The georeactor theory has relatively little supporting evidence, and is not widely supported.

Sources:
This paper, apart from the last two sections, and an addition quantifying internal energy generation, is by Prof. Richard Arculus, Australian National University, and is used with the author's permission. It is based on a paper presented by Professor Arculus at the Uranium Institute Mid-Term Meeting in Adelaide on 17 April 1996.
New Scientist 7/8/04.

Supply of Uranium

(updated August 2012)
http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Uranium-Resources/Supply-of-Uranium/#.UhXLY3-N6So

• Uranium is a relatively common metal, found in rocks and seawater. Economic concentrations of it are not uncommon.
• Its availability to supply world energy needs is great both geologically and because of the technology for its use.
• Quantities of mineral resources are greater than commonly perceived.
• The world's known uranium resources increased 15% in two years to 2007 due to increased mineral exploration.

Uranium is a relatively common element in the crust of the Earth (very much more than in the mantle). It is a metal approximately as common as tin or zinc, and it is a constituent of most rocks and even of the sea. Some typical concentrations are: (ppm = parts per million). 

Very high-grade ore (Canada) - 20% U	200,000 ppm U
High-grade ore - 2% U,	20,000 ppm U
Low-grade ore - 0.1% U,	1,000 ppm U
Very low-grade ore* (Namibia) - 0.01% U	100 ppm U
Granite	3-5 ppm U
Sedimentary rock	2-3 ppm U
Earth's continental crust (av)	2.8 ppm U
Seawater	0.003 ppm U

* Where uranium is at low levels in rock or sands (certainly less than 1000 ppm) it needs to be in a form which is easily separated for those concentrations to be called "ore" - that is, implying that the uranium can be recovered economically.  This means that it need to be in a mineral form that can easily be dissolved by sulfuric acid or sodium carbonate leaching.

An orebody is, by definition, an occurrence of mineralisation from which the metal is economically recoverable. It is therefore relative to both costs of extraction and market prices. At present neither the oceans nor any granites are orebodies, but conceivably either could become so if prices were to rise sufficiently.

Measured resources of uranium, the amount known to be economically recoverable from orebodies, are thus also relative to costs and prices. They are also dependent on the intensity of past exploration effort, and are basically a statement about what is known rather than what is there in the Earth's crust - epistemology rather than geology. See Appendix 2 for mineral resource and reserve categories.

Changes in costs or prices, or further exploration, may alter measured resource figures markedly. At ten times the current price, seawater might become a potential source of vast amounts of uranium. Thus, any predictions of the future availability of any mineral, including uranium, which are based on current cost and price data and current geological knowledge are likely to be extremely conservative.

From time to time concerns are raised that the known resources might be insufficient when judged as a multiple of present rate of use. But this is the Limits to Growth fallacy, a major intellectual blunder recycled from the 1970s, which takes no account of the very limited nature of the knowledge we have at any time of what is actually in the Earth's crust. Our knowledge of geology is such that we can be confident that identified resources of metal minerals are a small fraction of what is there. Factors affecting the supply of resources are discussed further and illustrated in the Appendix.

Uranium availability

With those major qualifications the following Table gives some idea of our present knowledge of uranium resources. The total and several country figures are lower than two years earlier due to economic factors, notably inflation of production costs. It can be seen that Australia has a substantial part (about 31 percent) of the world's uranium, Kazakhstan 12 percent, and Canada and Russia 9 percent each.   

Known Recoverable Resources of Uranium 2011

	tonnes U	percentage of world
Australia	1,661,000	31%
Kazakhstan	629,000	12%
Russia	487,200	9%
Canada	468,700	9%
Niger	421,000	8%
South Africa	279,100	5%
Brazil	276,700	5%
Namibia	261,000	5%
USA	207,400	4%
China	166,100	3%
Ukraine	119,600	2%
Uzbekistan	96,200	2%
Mongolia	55,700	1%
Jordan	33,800	1%
other	164,000	3%
World total	5,327,200

Reasonably Assured Resources plus Inferred Resources, to US$ 130/kg U, 1/1/11, from OECD NEA & IAEA, Uranium 2011: Resources, Production and Demand ("Red Book").  The total to US$ 260/kg U is 7,096,600 tonnes U, and Namibia moves up ahead of Niger.

 

Current usage is about 68,000 tU/yr.  Thus the world's present measured resources of uranium (5.3 Mt) in the cost category around present spot prices and used only in conventional reactors, are enough to last for about 80 years.  This represents a higher level of assured resources than is normal for most minerals.  Further exploration and higher prices will certainly, on the basis of present geological knowledge, yield further resources as present ones are used up. 

An initial uranium exploration cycle was military-driven, over 1945 to 1958. The second cycle was about 1974 to 1983, driven by civil nuclear power and in the context of a perception that uranium might be scarce. There was relatively little uranium exploration between 1985 and 2003, so the significant increase in exploration effort since then could conceivably double the known economic resources despite adjustments due to increasing costs. In the two years 2005-06 the world’s known uranium resources tabulated above and graphed below increased by 15% (17% in the cost category to $80/kgU). World uranium exploration expenditure is increasing, as the the accompanying graph makes clear. In the third uranium exploration cycle from 2003 to the end of 2011 about US$ 10 billion was spent on uranium exploration and deposit delineation on over 600 projects. In this period over 400 new junior companies were formed or changed their orientation to raise over US$ 2 billion for uranium exploration. About 60% of this was spent on previously-known deposits. All this was in response to increased uranium price in the market and the prospect of firm future prices.

The price of a mineral commodity also directly determines the amount of known resources which are economically extractable. On the basis of analogies with other metal minerals, a doubling of price from present levels could be expected to create about a tenfold increase in measured economic resources, over time, due both to increased exploration and the reclassification of resources regarding what is economically recoverable.

This is in fact suggested in the IAEA-NEA figures if those covering estimates of all conventional resources (U as main product or major by-product) are considered - another 7.6 million tonnes (beyond the 5.3 Mt known economic resources), which takes us to 190 years' supply at today's rate of consumption. This still ignores the technological factor mentioned below. It also omits unconventional resources (U recoverable as minor by-product) such as phosphate/ phosphorite deposits (up to 22 Mt U), black shales (schists) and lignite (0.7 Mt U), and even seawater (up to 4000 Mt), which would be uneconomic to extract in the foreseeable future, although Japanese trials using a polymer braid have suggested costs a bit over $250/kgU. 

Research proceeds.

 

It is clear from this Figure that known uranium resources have increased almost threefold since 1975, in line with expenditure on uranium exploration. (The decrease in the decade 1983-93 is due to some countries tightening their criteria for reporting.  If this were carried back two decades, the lines would fit even more closely.  The change from 2007 to 2009 is due to reclassifying resources into higher-cost categories.)  Increased exploration expenditure in the future is likely to result in a corresponding increase in known resources, even as inflation increases costs of recovery and hence tends to decrease the figures in each cost category.

About 20% of US uranium came from central Florida's phosphate deposits to the mid 1990s, as a by-product, but it then became uneconomic.  With higher uranium prices today the resource is being examined again, as is another lower-grade one in Morocco.  Plans for Florida extend only to 400 tU/yr at this stage. See also companion paper on Uranium from Phosphate Deposits.

Coal ash is another easily-accessible though minor uranium resource in many parts of the world.  In central Yunnan province in China the coal uranium content varies up to 315 ppm and averages about 65 ppm.  The ash averages about 210 ppm U (0.021%U) - above the cut-off level for some uranium mines.  The Xiaolongtang power station ash heap contains over 1000 tU, with annual arisings of 190 tU.  Recovery of this by acid leaching is about 70% in trials. This project has yet to announce any commercial production, however.

Widespread use of the fast breeder reactor could increase the utilisation of uranium 50-fold or more. This type of reactor can be started up on plutonium derived from conventional reactors and operated in closed circuit with its reprocessing plant. Such a reactor, supplied with natural or depleted uranium for its "fertile blanket", can be operated so that each tonne of ore yields 60 times more energy than in a conventional reactor.

see also WNA position paper.

Reactor Fuel Requirements

The world’s power reactors, with combined capacity of some 375 GWe, require about 68,000 tonnes of uranium from mines or elsewhere each year.  While this capacity is being run more productively, with higher capacity factors and reactor power levels, the uranium fuel requirement is increasing, but not necessarily at the same rate.  The factors increasing fuel demand are offset by a trend for higher burn-up of fuel and other efficiencies, so demand is steady.  (Over the years 1980 to 2008 the electricity generated by nuclear power increased 3.6-fold while uranium used increased by a factor of only 2.5.)   

Reducing the tails assay in enrichment reduces the amount of natural uranium required for a given amount of fuel.  Reprocessing of used fuel from conventional light water reactors also utilises present resources more efficiently, by a factor of about 1.3 overall.

Today's reactor fuel requirements are met from primary supply (direct mine output - 78% in 2009) and secondary sources: commercial stockpiles, nuclear weapons stockpiles, recycled plutonium and uranium from reprocessing used fuel, and some from re-enrichment of depleted uranium tails (left over from original enrichment).  These various secondary sources make uranium unique among energy minerals.

Nuclear Weapons as a source of fuel

An important source of nuclear fuel is the world's nuclear weapons stockpiles. Since 1987 the United States and countries of the former USSR have signed a series of disarmament treaties to reduce the nuclear arsenals of the signatory countries by approximately 80 percent.

The weapons contained a great deal of uranium enriched to over 90 percent U-235 (ie up to 25 times the proportion in reactor fuel). Some weapons have plutonium-239, which can be used in mixed-oxide (MOX) fuel for civil reactors.  From 2000 the dilution of 30 tonnes of military high-enriched uranium has been displacing about 10,600 tonnes of uranium oxide per year from mines, which represents about 15% of the world's reactor requirements.

Details of the utilisation of military stockpiles are in the paper Military warheads as a source of nuclear fuel.

Other secondary sources of uranium

The most obvious source is civil stockpiles held by utilities and governments.  The amount held here is difficult to quantify, due to commercial confidentiality.  As at January 2009 some 129,000 tU total inventory was estimated for utilities, 10,000 tU for producers and 15,000 tU for fuel cycle participants, making a total of 154,000 tU (WNA Market Report).   These reserves are expected not to be drawn down, but to increase steadily to provide energy security for utilities and governments.

Recycled uranium and plutonium is another source, and currently saves 1500-2000 tU per year of primary supply, depending on whether just the plutonium or also the uranium is considered.  In fact, plutonium is quickly recycled as MOX fuel, whereas the reprocessed uranium (RepU) is mostly stockpiled.  See also  Processing of Used Nuclear Fuel for Recycle  paper.

Re-enrichment of depleted uranium (DU, enrichment tails) is another secondary source.  There is about 1.5 million tonnes of depleted uranium available, from both military and civil enrichment activity since the 1940s, most at tails assay of 0.25 - 0.35% U-235.  Non-nuclear uses of DU are very minor relative to annual arisings of over 35,000 tU per year.  This leaves most DU available for mixing with recycled plutonium on MOX fuel or as a future fuel resource for fast neutron reactors.  However, some that has relatively high assay can be fed through under-utilised enrichment plants to produce natural uranium equivalent, or even enriched uranium ready for fuel fabrication.  Russian enrichment plants have treated 10-15,000 tonnes per year of DU assaying over 0.3% U-235, stripping it down to 0.1% and producing a few thousand tonnes per year of natural uranium equivalent.  This Russian program treating Western tails has now finished, but a new US one is expected to start when surplus capacity is available, treating about 140,000 tonnes of old DU assaying 0.4% U-235.

International fuel reserves

There have been three major initiatives to set up international reserves of enriched fuel, two of them multilateral ones, with fuel to be available under International Atomic Energy Agency (IAEA) auspices despite any political interruptions which might affect countries needing them. The third is under US auspices, and also to meet needs arising from supply disruptions.

In November 2009 the IAEA Board approved a Russian proposal to create an international "fuel bank" or guaranteed reserve of low-enriched uranium under IAEA control at the International Uranium Enrichment Centre (IUEC) at Angarsk. This Russian LEU reserve was established a year later and comprises 120 tonnes of low-enriched uranium as UF6, enriched 2.0 - 4.95% U-235 (with 40t of latter), available to any IAEA member state in good standing which is unable to procure fuel for political reasons. It is fully funded by Russia, held under safeguards, and the fuel will be made available to IAEA at market rates, using a formula based on spot prices. Following an IAEA decision to allocate some of it, Rosatom will transport material to St Petersburg and transfer title to IAEA, which will then transfer ownership to the recipient.

This initiative complements the proposed IAEA fuel bank by making more material available to the IAEA for assurance of fuel supply to countries without their own fuel cycle facilities. The 120 tonnes uranium as UF6 is equivalent to two full fuel loads for a typical 1000 MWe reactor, and is (in 2011) worth some US$ 250 million.

In December 2010 the IAEA board resolved to establish a similar guaranteed reserve of low-enriched uranium, the IAEA LEU bank, with the support of $50 million from the US-based Nuclear Threat Initiative (NTI) organization and US billionaire Warren Buffett, plus a matching $107 million from the US government ($50 million), the EU ($32 million), UAE ($10 million), Kuwait ($10 million) and Norway ($5 million). The IAEA is drawing up a framework that defines the "fuel bank's" structure, access and location. It will comprise a physical stock of UF6 at enrichment levels ranging up to 4.95% U-235 and owned by the IAEA, which shall "be responsible for storing and protecting" it. A comprehensive Host State Agreement will need to provide for the IAEA facility to be extraterritorial. Kazakhstan has offered to host it, and the IAEA has examined two proffered sites.  See IAEA Factsheet.

In 2005 the US government announced plans for the establishment of a mechanism to ensure fuel supply for use in commercial reactors in foreign countries where there has been supply disruption. The fuel would come from downblending 17.4 tonnes of high-enriched uranium (HEU). In August 2011 US Department of Energy announced an expanded scope for the program so it would also serve US utility needs, and now be called the American Assured Fuel Supply (AFS). At that point most of the downblending of the HEU had been completed, and the scheme was ready to operate. The AFS will comprise about 230 tonnes of low-enriched uranium (with another 60t from downblending being sold on the market to pay for the work). The AFS program is administered by the US National Nuclear Safety Administration, foreign access must be through a US entity, and the fuel will be sold at current market prices. The 230 t amount is equivalent to about six reloads for a 1000 MWe reactor. 

Thorium as a nuclear fuel

Today uranium is the only fuel supplied for nuclear reactors. However, thorium can also be utilised as a fuel for CANDU reactors or in reactors specially designed for this purpose. Neutron efficient reactors, such as CANDU, are capable of operating on a thorium fuel cycle, once they are started using a fissile material such as U-235 or Pu-239. Then the thorium (Th-232) atom captures a neutron in the reactor to become fissile uranium (U-233), which continues the reaction. Some advanced reactor designs are likely to be able to make use of thorium on a substantial scale.

The thorium fuel cycle has some attractive features, though it is not yet in commercial use. Thorium is reported to be about three times as abundant in  the earth's crust as uranium. The 2009 IAEA-NEA "Red Book" lists 3.6 million tonnes of known and estimated resources as reported, but points out that this excludes data from much of the world, and estimates about 6 million tonnes overall. See also companion paper on Thorium.

Main references
OECD NEA & IAEA, 2010, Uranium 2009: Resources, Production and Demand
WNA 2009 Market Report
UN Institute for Disarmament Research, Yury Yudin (ed) 2011, Multilateralization of the Nuclear Fuel Cycle - The First Practical Steps.

Appendix 1. ---- (Sept 2005)

Substantially derived from 2003 WNA Symposium paper by Colin MacDonald, Uranium: Sustainable Resource or Limit to Growth? - supplemented by his 2005 WNA Symposium paper and including a model "Economic adjustments in the supply of a 'non-renewable' resource" from Ian Hore-Lacy.

The Sustainability of Mineral Resources
with reference to uranium

It is commonly asserted that because "the resources of the earth are finite", therefore we must face some day of reckoning, and will need to plan for "negative growth". All this, it is pointed out, is because these resources are being consumed at an increasing rate to support our western lifestyle and to cater for the increasing demands of developing nations. The assertion that we are likely to run out of resources is a re-run of the "Limits to Growth" argument (Club of Rome 1972 popularised by Meadows et al in Limits of Growth at that time. (A useful counter to it is W Berckerman, In Defence of Economic Growth, also Singer, M, Passage to a Human World, Hudson Inst. 1987). In the decade following its publication world bauxite reserves increased 35%, copper 25%, nickel 25%, uranium and coal doubled, gas increased 70% and even oil increased 6%.) fashionable in the early 1970s, which was substantially disowned by its originators, the Club of Rome, and shown up as nonsense with the passing of time. It also echoes similar concerns raised by economists in the 1930s, and by Malthus at the end of the 18th Century.

In recent years there has been persistent misunderstanding and misrepresentation of the abundance of mineral resources, with the assertion that the world is in danger of actually running out of many mineral resources. While congenial to common sense if the scale of the Earth's crust is ignored, it lacks empirical support in the trend of practically all mineral commodity prices and published resource figures over the long term. In recent years some have promoted the view that limited supplies of natural uranium are the Achilles heel of nuclear power as the sector contemplates a larger contribution to future clean energy, notwithstanding the small amount of it required to provide very large amounts of energy.

Uranium supply news is usually framed within a short-term perspective. It concerns who is producing with what resources, who might produce or sell, and how does this balance with demand? However, long-term supply analysis enters the realm of resource economics. This discipline has as a central concern the understanding of not just supply/demand/price dynamics for known resources, but also the mechanisms for replacing resources with new ones presently unknown. Such a focus on sustainability of supply is unique to the long view. Normally-functioning metals markets and technology change provide the drivers to ensure that supply at costs affordable to consumers is continuously replenished, both through the discovery of new resources and the re-definition (in economic terms) of known ones.

Of course the resources of the earth are indeed finite, but three observations need to be made: first, the limits of the supply of resources are so far away that the truism has no practical meaning. Second, many of the resources concerned are either renewable or recyclable (energy minerals and zinc are the main exceptions, though the recycling potential of many materials is limited in practice by the energy and other costs involved). Third, available reserves of 'non-renewable' resources are constantly being renewed, mostly faster than they are used.

There are three principal areas where resource predictions have faltered:

• predictions have not accounted for gains in geological knowledge and understanding of mineral deposits;
• they have not accounted for technologies utilised to discover, process and use them;
• economic principles have not been taken into account, which means that resources are thought of only in present terms, not in terms of what will be economic through time, nor with concepts of substitution in mind.

What then does sustainability in relation to mineral resources mean? The answer lies in the interaction of these three things which enable usable resources (Some licence is taken in the use of this word in the following, strictly it is reserves of minerals which are created) effectively to be created. They are brought together in the diagram below.

Numerous economists have studied resource trends to determine which measures should best reflect resource scarcity (Tilton, J. On Borrowed Time? Assessing the threat of mineral depletion, Resources for the Future, Washington DC 2002). Their consensus view is that costs and prices, properly adjusted for inflation, provide a better early warning system for long-run resource scarcity than do physical measures such as resource quantities.

Historic data show that the most commonly used metals have declined in both their costs and real commodity prices over the past century. Such price trends are the most telling evidence of lack of scarcity. Uranium has been a case in point, relative to its late 1970s price of US$ 40/lb U₃O₈.

An anecdote underlines this basic truth: In 1980 two eminent professors, fierce critics of one another, made a bet regarding the real market price of five metal commodities over the next decade. Paul Ehrlich, a world-famous ecologist, bet that because the world was exceeding its carrying capacity, food and commodities would start to run out in the 1980s and prices in real terms would therefore rise. Julian Simon, an economist, said that resources were effectively so abundant, and becoming effectively more so, that prices would fall in real terms. He invited Ehrlich to nominate which commodities would be used to test the matter, and they settled on these (chrome, copper, nickel, tin and tungsten). In 1990 Ehrlich paid up - all the prices had fallen.

However, quantities of known resources tell a similar and consistent story. To cite one example, world copper reserves in the 1970s represented only 30 years of then-current production (6.4 Mt/yr). Many analysts questioned whether this resource base could satisfy the large expected requirements of the telecommunications industry by 2000. But by 1994, world production of copper had doubled (12 Mt/yr) and the available reserves were still enough for another 30 years. The reserve multiple of current production remained the same.

   

Metal Prices

Another way to understand resource sustainability is in terms of economics and capital conservation. Under this perspective, mineral resources are not so much rare or scarce as they are simply too expensive to discover if you cannot realise the profits from your discovery fairly soon. Simple economic considerations therefore discourage companies from discovering much more than society needs through messages of reduced commodity prices during times of oversupply. Economically rational players will only invest in finding these new reserves when they are most confident of gaining a return from them, which usually requires positive price messages caused by undersupply trends. If the economic system is working correctly and maximizing capital efficiency, there should never be more than a few decades of any resource commodity in reserves at any point in time.

Resource levels

The fact that many commodities have more resources available than efficient economic theory might suggest may be partly explained by two characteristics of mineral exploration cycles. First, the exploration sector tends to over-respond to the positive price signals through rapid increases in worldwide expenditures (which increases the rate of discoveries), in particular through the important role of more speculatively-funded junior exploration companies. Exploration also tends to make discoveries in clusters that have more to do with new geological knowledge than with efficient capital allocation theory. As an example, once diamonds were known to exist in northern Canada, the small exploration boom that accompanied this resulted in several large discoveries - more than the market may have demanded at this time. These patterns are part of the dynamics that lead to commodity price cycles. New resource discoveries are very difficult to precisely match with far-off future demand, and the historic evidence suggests that the exploration process over-compensates for every small hint of scarcity that the markets provide.

Another important element in resource economics is the possibility of substitution of commodities. Many commodity uses are not exclusive - should they become too expensive they can be substituted with other materials. Even if they become cheaper they may be replaced, as technology gains have the potential to change the style and cost of material usage. For example, copper, despite being less expensive in real terms than 30 years ago, is still being replaced by fibre optics in many communication applications. These changes to materials usage and commodity demand provide yet another dimension to the simple notion of depleting resources and higher prices.

In summary, historic metals price trends, when examined in the light of social and economic change through time, demonstrate that resource scarcity is a double-edged sword. The same societal trends that have increased metals consumption, tending to increase prices, have also increased the available wealth to invest in price-reducing knowledge and technology. These insights provide the basis for the economic sustainability of metals, including uranium.

Geological Knowledge

Whatever minerals are in the earth, they cannot be considered usable resources unless they are known. There must be a constant input of time, money and effort to find out what is there. This mineral exploration endeavour is not merely fossicking or doing aerial magnetic surveys, but must eventually extend to comprehensive investigation of orebodies so that they can reliably be defined in terms of location, quantity and grade. Finally, they must be technically and economically quantified as mineral reserves. That is the first aspect of creating a resource. See Appendix 2 for mineral resource and reserve categories.

For reasons outlined above, measured resources of many minerals are increasing much faster than they are being used, due to exploration expenditure by mining companies and their investment in research. Simply on geological grounds, there is no reason to suppose that this trend will not continue. Today, proven mineral resources worldwide are more than we inherited in the 1970s, and this is especially so for uranium.

Simply put, metals which are more abundant in the Earth's crust are more likely to occur as the economic concentrations we call mineral deposits. They also need to be reasonably extractable from their host minerals. By these measures, uranium compares very well with base and precious metals. Its average crustal abundance of 2.7 ppm is comparable with that of many other metals such as tin, tungsten, and molybdenum. Many common rocks such as granite and shales contain even higher uranium concentrations of 5 to 25 ppm. Also, uranium is predominantly bound in minerals which are not difficult to break down in processing.

As with crustal abundance, metals which occur in many different kinds of deposits are easier to replenish economically, since exploration discoveries are not constrained to only a few geological settings. Currently, at least 14 different types of uranium deposits are known, occurring in rocks of wide range of geological age and geographic distribution. There are several fundamental geological reasons why uranium deposits are not rare, but the principal reason is that uranium is relatively easy both to place into solution over geological time, and to precipitate out of solution in chemically reducing conditions. This chemical characteristic alone allows many geological settings to provide the required hosting conditions for uranium resources. Related to this diversity of settings is another supply advantage ?the wide range in the geological ages of host rocks ensures that many geopolitical regions are likely to host uranium resources of some quality.

Unlike the metals which have been in demand for centuries, society has barely begun to utilise uranium. As serious non-military demand did not materialise until significant nuclear generation was built by the late 1970s, there has been only one cycle of exploration-discovery-production, driven in large part by late 1970s price peaks (MacDonald, C, Rocks to reactors: Uranium exploration and the market. Proceedings of WNA Symposium 2001). This initial cycle has provided more than enough uranium for the last three decades and several more to come. Clearly, it is premature to speak about long-term uranium scarcity when the entire nuclear industry is so young that only one cycle of resource replenishment has been required. It is instead a reassurance that this first cycle of exploration was capable of meeting the needs of more than half a century of nuclear energy demand.

Related to the youthfulness of nuclear energy demand is the early stage that global exploration had reached before declining uranium prices stifled exploration in the mid 1980s. The significant investment in uranium exploration during the 1970-82 exploration cycle would have been fairly efficient in discovering exposed uranium deposits, due to the ease of detecting radioactivity. Still, very few prospective regions in the world have seen the kind of intensive knowledge and technology-driven exploration that the Athabasca Basin of Canada has seen since 1975. This fact has huge positive implications for future uranium discoveries, because the Athabasca Basin history suggests that the largest proportion of future resources will be as deposits discovered in the more advanced phases of exploration. Specifically, only 25% of the 635,000 tonnes of U₃O₈ discovered so far in the Athabasca Basin could be discovered during the first phase of surface-based exploration. A sustained second phase, based on advances in deep penetrating geophysics and geological models, was required to discover the remaining 75%.

Another dimension to the immaturity of uranium exploration is that it is by no means certain that all possible deposit types have even been identified. Any estimate of world uranium potential made only 30 years ago would have missed the entire deposit class of unconformity deposits that have driven production since then, simply because geologists did not know this class existed.

Technology

It is meaningless to speak of a resource until someone has thought of a way to use any particular material. In this sense, human ingenuity quite literally creates new resources, historically, currently and prospectively. That is the most fundamental level at which technology creates resources, by making particular minerals usable in new ways. Often these then substitute to some degree for others which are becoming scarcer, as indicated by rising prices. Uranium was not a resource in any meaningful sense before 1940.

More particularly, if a known mineral deposit cannot be mined, processed and marketed economically, it does not constitute a resource in any practical sense. Many factors determine whether a particular mineral deposit can be considered a usable resource - the scale of mining and processing, the technological expertise involved, its location in relation to markets, and so on. The application of human ingenuity, through technology, alters the significance of all these factors and is thus a second means of "creating" resources. In effect, portions of the earth's crust are reclassified as resources. A further aspect of this is at the manufacturing and consumer level, where technology can make a given amount of resources go further through more efficient use.(aluminium can mass was reduced by 21% 1972-88, and motor cars each use about 30% less steel than 30 years ago)

An excellent example of this application of technology to create resources is in the Pilbara region of Western Australia. Until the 1960s the vast iron ore deposits there were simply geological curiosities, despite their very high grade. Australia had been perceived as short of iron ore. With modern large-scale mining technology and the advent of heavy duty railways and bulk shipping which could economically get the iron ore from the mine (well inland) through the ports of Dampier and Port Hedland to Japan, these became one of the nation's main mineral resources. For the last 45 years Hamersley Iron (Rio Tinto), Mount Newman (BHP-Billiton) and others have been at the forefront of Australia's mineral exporters, drawing upon these 'new' orebodies.

Just over a hundred years ago aluminium was a precious metal, not because it was scarce, but because it was almost impossible to reduce the oxide to the metal, which was therefore fantastically expensive. With the discovery of the Hall-Heroult process in 1886, the cost of producing aluminium plummeted to about one twentieth of what it had been and that metal has steadily become more commonplace. It now competes with iron in many applications, and copper in others, as well as having its own widespread uses in every aspect of our lives. Not only was a virtually new material provided for people's use by this technological breakthrough, but enormous quantities of bauxite world-wide progressively became a valuable resource. Without the technological breakthrough, they would have remained a geological curiosity.

Incremental improvements in processing technology at all plants are less obvious but nevertheless very significant also. Over many years they are probably as important as the historic technological breakthroughs.

To achieve sustainability, the combined effects of mineral exploration and the development of technology need to be creating resources at least as fast as they are being used. There is no question that in respect to the minerals industry this is generally so, and with uranium it is also demonstrable. Recycling also helps, though generally its effect is not great.

Economics

Whether a particular mineral deposit is sensibly available as a resource will depend on the market price of the mineral concerned. If it costs more to get it out of the ground than its value warrants, it can hardly be classified as a resource (unless there is some major market distortion due to government subsidies of some kind). Therefore, the resources available will depend on the market price, which in turn depends on world demand for the particular mineral and the costs of supplying that demand. The dynamic equilibrium between supply and demand also gives rise to substitution of other materials when scarcity looms (or the price is artificially elevated). This then is the third aspect of creating resources.

The best known example of the interaction of markets with resource availability is in the oil industry. When in 1972 OPEC suddenly increased the price of oil fourfold, several things happened at both producer and consumer levels.

The producers dramatically increased their exploration effort, and applied ways to boost oil recovery from previously 'exhausted' or uneconomic wells. At the consumer end, increased prices meant massive substitution of other fuels and greatly increased capital expenditure in more efficient plant. As a result of the former activities, oil resources increased dramatically. As a result of the latter, oil use fell slightly to 1975 and in the longer perspective did not increase globally from 1973 to 1986. Forecasts in 1972, which had generally predicted a doubling of oil consumption in ten years, proved quite wrong.

Oil will certainly become scarce one day, probably before most other mineral resources, which will continue to drive its price up. As in the 1970s, this will in turn cause increased substitution for oil and bring about greater efficiencies in its use as equilibrium between supply and demand is maintained by the market mechanism. Certainly oil will never run out in any absolute sense - it will simply become too expensive to use as liberally as we now do.

Another example is provided by aluminium. During World War II, Germany and Japan recovered aluminium from kaolinite, a common clay, at slightly greater cost than it could be obtained from bauxite.

Due to the operation of these three factors the world's economically demonstrated resources of most minerals have risen faster than the increased rate of usage over the last 50 years, so that more are available now, notwithstanding liberal usage. This is largely due to the effects of mineral exploration and the fact that new discoveries have exceeded consumption.

Replacement of uranium

A characteristic of metals resource replacement is that the mineral discovery process itself adds a small cost relative to the value of the discovered metals. As an example, the huge uranium reserves of Canada's Athabasca Basin were discovered for about US$1.00/kgU (2003 dollars, including unsuccessful exploration). Similar estimates for world uranium resources, based on published IAEA exploration expenditure data and assuming that these expenditures yielded only the past uranium produced plus the present known economic resources categories at up to US$80/kg (Uranium 2003: Resources, Production and demand. Nuclear Energy Agency and IAEA, OECD Publications 2004) yields slightly higher costs of about US$1.50/kgU. This may reflect the higher component of State-driven exploration globally, some of which had national self-sufficiency objectives that may not have aligned with industry economic standards.

From an economic perspective, these exploration costs are essentially equivalent to capital investment costs, albeit spread over a longer time period. It is, however, this time lag between the exploration expense and the start of production that confounds attempts to analyse exploration economics using strict discounted cash flow methods. The positive cash flows from production occur at least 10-15 years into the future, so that their present values are obviously greatly reduced, especially if one treats the present as the start of exploration. This creates a paradox, since large resource companies must place a real value on simply surviving and being profitable for many decades into the future; and, without exploration discoveries, all mining companies must expire with their reserves. Recent advances in the use of real options and similar methods are providing new ways to understand this apparent paradox. A key insight is that time, rather than destroying value through discounting, actually adds to the option value, as does the potential of price volatility. Under this perspective, resource companies create value by obtaining future resources which can be exploited optimally under a range of possible economic conditions. Techniques such as these are beginning to add analytical support to what have always been intuitive understandings by resource company leaders - that successful exploration creates profitable mines and adds value to company shares.

Since uranium is part of the energy sector, another way to look at exploration costs is on the basis of energy value. This allows comparisons with the energy investment cost for other energy fuels, especially fossil fuels which will have analogous costs related to the discovery of the resources. From numerous published sources, the finding costs of crude oil have averaged around US$ 6/bbl over at least the past three decades. When finding costs of the two fuels are expressed in terms of their contained energy value, oil, at US$ 1050/MJ of energy, is about 300 times more expensive to find than uranium, at US$ 3.4/MJ. Similarly, the proportion of current market prices that finding costs comprise are lower for uranium. Its finding costs make up only 2% of the recent spot price of US$ 30/lb ($78/kgU), while the oil finding costs are 12% of a recent spot price of US$ 50/bbl.

By these measures, uranium is a very inexpensive energy source to replenish, as society has accepted far higher energy replacement costs to sustain oil resources. This low basic energy resource cost is one argument in favour of a nuclear-hydrogen solution to long-term replacement of oil as a transportation fuel.

Forecasting replenishment

Supply forecasters are often reluctant to consider the additive impacts of exploration on new supply, arguing that assuming discoveries is as risky and speculative as the exploration business itself. Trying to predict any single discovery certainly is speculative. However, as long as the goal is merely to account for the estimated total discovery rate at a global level, a proxy such as estimated exploration expenditures can be used. Since expenditures correlate with discovery rate, the historic (or adjusted) resources discovered per unit of expenditure will provide a reasonable estimate of resource gains to be expected. As long as the time lag between discovery and production is accounted for, this kind of dynamic forecasting is more likely to provide a basis for both price increases and decreases, which metals markets have historically demonstrated.

Without these estimates of uranium resource replenishment through exploration cycles, long-term supply-demand analyses will tend to have a built-in pessimistic bias (i.e. towards scarcity and higher prices), that will not reflect reality. Not only will these forecasts tend to overestimate the price required to meet long-term demand, but the opponents of nuclear power use them to bolster arguments that nuclear power is unsustainable even in the short term. In a similar fashion, these finite-resources analyses also lead observers of the industry to conclude that fast breeder reactor technology will soon be required. This may indeed make a gradual appearance, but if uranium follows the price trends we see in other metals, its development will be due to strategic policy decisions more than uranium becoming too expensive.

The resource economics perspective tells us that new exploration cycles should be expected to add uranium resources to the world inventory, and to the extent that some of these may be of higher quality and involve lower operating cost than resources previously identified, this will tend to mitigate price increases. This is precisely what has happened in uranium, as the low-cost discoveries in Canada's Athabasca Basin have displaced higher-cost production from many other regions, lowering the cost curve and contributing to lower prices. Secondary uranium supplies, to the extent that they can be considered as a very low-cost mine, have simply extended this price trend.

The first exploration and mining cycle for uranium occurred about 1970 to 1985. It provided enough uranium to meet world demand for some 80 years, if we view present known resources as arising from it. With the rise in uranium prices to September 2005 and the concomitant increase (boom?) in mineral exploration activity, it is clear that we have the start of a second such cycle, mid 2003 to ??. The price increase was brought about by diminution of secondary supplies coupled with a realization that primary supplies needed to increase substantially.

Several significant decisions on mine development and increased exploration by major producers will enable this expansion of supply, coupled with smaller producers coming on line. The plethora of junior exploration companies at the other end of the spectrum which are finding no difficulty whatever in raising capital are also a positive sign that a vigorous new exploration and mining cycle is cranking up. From lows of around US$ 55 million per year in 2000, world uranium exploration expenditure rose to about US$ 110 million in 2004 and is expected to be US$ 185 million in 2005, half of this being from the junior exploration sector. The new cycle is also showing considerable regional diversification. Measured from 1990, cycle 2 totals US$ 1.5 billion to 2005, compared with a total of about three times this figure (uncorrected) for the whole of the first cycle.

Depletion and sustainability

Conversely, the exhaustion of mineral resources during mining is real. Resource economists do not deny the fact of depletion, nor its long-term impact - that in the absence of other factors, depletion will tend to drive commodity prices up. But as we have seen, mineral commodities can become more available or less scarce over time if the cost-reducing effects of new technology and exploration are greater than the cost-increasing effects of depletion.

One development that would appear to argue against economic sustainability is the growing awareness of the global depletion of oil, and in some regions such as North America, natural gas. But oil is a fundamentally different material. This starts with geology, where key differences include the fact that oil and gas were formed by only one process: the breakdown of plant life on Earth. Compared with the immense volumes of rock-forming minerals in the Earth? crust, living organisms on top of it have always been a very tiny proportion. But a more important fact is that the world has consumed oil, and recently natural gas as well, in a trajectory of rapid growth virtually unmatched by any other commodity. Consumption growth rates of up to 10% annually over the past 50 years are much higher than we see for other commodities, and support the contention that oil is a special depletion case for several reasons: its geological occurrence is limited, it has been inexpensive to extract, its energy utility has been impossible to duplicate for the price, and its resulting depletion rates have been incredibly high.

This focus on rates of depletion suggests that one of the dimensions of economic sustainability of metals has to do with their relative rates of depletion. Specifically, it suggests that economic sustainability will hold indefinitely as long as the rate of depletion of mineral resources is slower than the rate at which it is offset. This offsetting force will be the sum of individual factors that work against depletion, and include cost-reducing technology and knowledge, lower cost resources through exploration advances, and demand shifting through substitution of materials.

An economic sustainability balance of this type also contemplates that, at some future point, the offsetting factors may not be sufficient to prevent irreversible depletion-induced price increases, and it is at this point that substituting materials and technologies must come into play to take away demand. In the case of rapid oil depletion, that substitute appears to be hydrogen as a transport fuel. Which raises the question of how the hydrogen is produced, and nuclear energy seems the most likely means of that, using high-temperature reactors.

From a detached viewpoint all this may look like mere technological optimism. But to anyone closely involved it is obvious and demonstrable. Furthermore, it is illustrated by the longer history of human use of the Earth's mineral resources. Abundance, scarcity, substitution, increasing efficiency of use, technological breakthroughs in discovery, recovery and use, sustained incremental improvements in mineral recovery and energy efficiency - all these comprise the history of minerals and humankind.

 Appendix 2.

 Mineral Resources and Reserves

The International Template for Reporting of Exploration Results, Mineral Resources and Mineral Reserves (July 2006) integrates the minimum standards being adopted in national reporting codes worldwide with recommendations and interpretive guidelines for the Public Reporting of Exploration Results, Mineral Resources and Mineral Reserves. The definitions (below) in this edition of the International Reporting Template are either identical to, or not materially different from those definitions used in the countries represented on the Committee for Mineral Reserves International Reporting Standards (CRIRSCO), notably Australia, whose JORC code was the basis of these international definitions, and Canada (NI 43-101 code).

A ‘Mineral Resource’ is a concentration or occurrence of material of intrinsic economic interest in or on the Earth’s crust in such form, quality and quantity that here are reasonable prospects for eventual economic extraction. The location, quantity, grade, geological characteristics and continuity of a Mineral Resource are known, estimated or interpreted from specific geological evidence, sampling and knowledge. Mineral Resources are sub-divided, in order of increasing geological confidence, into Inferred, Indicated and Measured categories.

An ‘Inferred Mineral Resource’ is that part of a Mineral Resource for which tonnage, grade and mineral content can be estimated with a low level of confidence. It is inferred from geological evidence and assumed but not verified geological and/or grade continuity. It is based on information gathered through appropriate techniques from locations such as outcrops, trenches, pits, workings and drill holes which is limited or of uncertain quality and reliability.

An ‘Indicated Mineral Resource’ is that part of a Mineral Resource for which tonnage, densities, shape, physical characteristics, grade and mineral content can be estimated with a reasonable level of confidence. It is based on exploration, sampling and testing information gathered through appropriate techniques from locations such as outcrops, trenches, pits, workings and drill holes. The locations are too widely or inappropriately spaced to confirm geological and/or grade continuity but are spaced closely enough for continuity to be assumed.

A ‘Measured Mineral Resource’ is that part of a Mineral Resource for which tonnage, densities, shape, physical characteristics, grade and mineral content can be estimated with a high level of confidence. It is based on detailed and reliable exploration, sampling and testing information gathered through appropriate techniques from locations such as outcrops, trenches, pits, workings and drill holes. The locations are spaced closely enough to confirm geological and grade continuity.

A ‘Mineral Reserve’ (or Ore Reserve) is the economically mineable part of a Measured and/or Indicated Mineral Resource. It includes diluting materials and allowances for losses, which may occur when the material is mined. Appropriate assessments and studies will have been carried out, and include consideration of and modification by realistically assumed mining, metallurgical, economic, marketing, legal, environmental, social and governmental factors. These assessments demonstrate at the time of reporting that extraction could reasonably be justified. Mineral or Ore Reserves are sub-divided in order of increasing confidence into Probable Mineral/Ore Reserves and Proved Mineral/Ore Reserves.

A ‘Probable Mineral Reserve’ (or Probable Ore Reserve) is the economically mineable part of an Indicated, and in some circumstances, a Measured Mineral Resource. It includes diluting materials and allowances for losses which may occur when the material is mined. Studies to at least Pre-Feasibility level will have been carried out, including consideration of and modification by realistically assumed mining, metallurgical, economic, marketing, legal, environmental, social and governmental factors. The results of the studies demonstrate at the time of reporting that extraction could reasonably be justified.

A ‘Proved Mineral Reserve’ (or proved Ore Reserve) is the economically mineable part of a Measured Mineral Resource. It includes diluting materials and allowances for losses which may occur when the material is mined. Studies to at least Pre-Feasibility level will have been carried out, including consideration of and modification by realistically assumed mining, metallurgical, economic, marketing, legal, environmental, social and governmental factors. These studies demonstrate at the time of reporting that extraction is justified.

Uranium from Phosphates

(Updated June 2013)

• Rock phosphate deposits contain many million tonnes of uranium, which may be extracted as a by-product of making fertilisers.
• Some 20,000 tonnes of uranium has already been obtained from these rock phosphate deposits, but the process became uneconomic in the 1990s.
• Higher uranium prices and process refinements have changed the economic situation.

In addition to the 5.4 million tonnes of uranium in known recoverable resources, there are substantial amounts comprising what is known as "unconventional resources". Chief among these is rock phosphate, or phosphorite. Estimates of the amount available range from 9 to 22 million tonnes of uranium.
About 20% of US uranium came from central Florida's phosphate deposits to the mid 1990s, as a by-product, but it then became uneconomic. From 1981 to 1992 US production averaged just over 1000 tU per year, then fell way sharply and finished in 1998. The IAEA "Red Book" also reports significant US production 1954-62. With higher uranium prices today the US resource is being examined again, as is a lower-grade one in Morocco. Plans for Florida extend only to 400 tU/yr at this stage.
Cameco and Uranium Equities Ltd are setting up a demonstration plant in the USA using a refined process – PhosEnergy – and estimate that some 7700 tU could be recovered annually as by-product from phosphate production, including 2300 tU/yr in the USA.

Phosphate rock production for fertilizer in 2010, million tonnes

Algeria	2
Australia	2.8
Brazil	5.5
China	65
Egypt	5
Israel	3
Jordan	6
Morocco & W. Sahara	26
Russia	10
South Africa	2.3
Syria	2.8
Tunisia	7.6
USA	26
Other countries	11.7
World total in 2010	176 Mt

Source: IAEA¹

Phosphate rock (phosphorite) is a marine sedimentary rock which contains 18-40% P₂O₅, as well as some uranium and all its decay products, often 70 to 200 ppmU, and sometimes up to 800 ppm. The main mineral in the phosphate rock is apatite, and most commonly, fluorapatite – Ca₅(PO₄)₃F or Ca₁₀(PO₄)₆(F,OH)₂. This is insoluble, so cannot directly be used as a fertiliser (unless in very acid soils) so must first be processed. This is normally in a wet process phosphoric acid (WPA) plant where it is first dissolved in sulphuric acid. About 2-4% fluorine is usually present. There are about 400 plants using this wet process worldwide, with capacity of some 50 million tonnes P₂O₅ per year.
Some phosphate deposits – about 4% of total known – are igneous, created by magmatic extrusion as an alkaline chimney. The main mineral is apatite, with some fluorapatite.
When phosphate rock is treated with sulphuric acid in sub stoichiometric quantity, normal superphosphate is formed. If more sulphuric acid is added, a mixture of phosphoric acid and gypsum (calcium sulphate) is obtained. After the gypsum is filtered out, the resultant phosphoric acid can be treated to recover uranium.

The basic reaction is:
Ca₃(PO₄)₂ + 3H₂SO₄ + 6H₂O ==> 2H₃PO₄ + 3CaSO₄.2H₂O  – exothermic
An improved higher-temperature process produces hemihydrate: CaSO₄.1/2H₂O
Fluorides need to be controlled as gases and in effluents (HF, fluorosilicic acid) and about half the fluorine reports with the gypsum. In the process a lot of crud is generated, and this was disposed of with gypsum tailings, despite its low-level radioactivity.
After the gypsum precipitation stage, triple superphosphate is obtained by reacting the phosphoric acid with further phosphate rock. Otherwise, various ammonium phosphate fertilizers can be produced by reacting the phosphoric acid with ammonia.
The uranium is normally recovered from the phosphoric acid (H₃PO₄ – bearing about 28% P₂O₅) by some form of solvent extraction (SX). Kamorphos is developing a simpler version of this.
PhosEnergy, an ion exchange (IX) process, represents a step-change refinement of the old processes. It was announced in 2009, offering uranium recovery costs of $25-30 /lb U₃O₈, compared with historical costs of double this.^a A demonstration plant was built in Adelaide, South Australia, and shipped to the USA to operate at a US fertilizer producer, where it was commissioned in May 2012. The test campaign with four trials on two feed sources was successful, showing recoveries of over 92% at $20-25/lb operating cost, and with Cameco very fully involved. Full evaluation of the project operation with an engineering study was reported in March 2013, and $18/lb operating cost with capital cost of $156 million for a base case 400 t/yr U₃O₈ plant were quoted. Cameco reaffirmed its commitment in subscribing a further $4 million.
The process involves taking 27% acid, redox reduction to ferrous iron giving oxidation of uranium, primary IX ,then recycle of acid with improved quality and a secondary IX uranium recovery being much the same as that in Cameco’s US ISL operations, so potential synergy. Bottom line of process is 95% U recovery, no radwaste, $18/lb cost, improved acid quality for main plant, $120 million bolt-on plant cost.
The next phase of PhosEnergy commercialisation is expected to be a continuous on-site demonstration scale operation at the site of an existing phosphate producer. This phase will underpin a Definitive Feasibility Study (DFS) and be the basis for a full-scale commercial facility, which might be constructed and commissioned within three years of the commencement of a DFS.
As well as the PhosEnergy project, Cameco is involved independently through Nukem with CF Industries in developing a facility to recover about 400 tU/yr from Florida phosphates.
In the USA eight plants for the recovery of uranium from phosphoric acid have been built and operated since the 1970s (six in Florida, two in Louisiana). Plants have also been built in Canada, Spain, Belgium (for Moroccan phosphate), Israel, and Taiwan. 
Brazil is planning a new plant with uranium as co-product with phosphate from 0.08%U ore in igneous rock, to operate from 2012 at 1000 tU/yr. Morocco has by far the largest known resources of uranium in phosphate rock. Egypt reports 42,000 tU in phosphate rocks, at 50-200 ppm U.
The potential amount of uranium able to be recovered from WPA phosphoric acid streams is over 11,000 tonnes U per year (global P2O5 production in 2010 was 33.6 Mt). The economic benefit will be both in the value of the uranium and in reduced regulatory demands on disposal of low-level radioactive wastes arising from the WPA process. Estimated uranium production costs will put the new process in the lowest quartile of new uranium production.

Santa Quiteria and Itataia mines, Brazil
This has reserves of 340 Mt of phosphate containing 140,000 tU at Santa Quiteria and 80,000 tU at Itataia, grading 0.054% U in P2O5, giving 1270 tU/yr from about 2015, 970 tU of this from Itataia.
USA
The country has reserves of 1400 Mt phosphates containing 170,000 tU. At 9.6 Mt/yr P2O5 production, this would yield 2300-2680 tU/yr by-product. Nukem and CF Industries were planning a 430 tU/yr uranium recovery operation at CF's Plant City.
Jordan
The country has reserves of 1500 Mt phosphates containing up to 140,000 tU. At 676,000 t/yr P2O5 production the uranium potential is 135 tU/yr. The government is putting out the Qatrana phosphorites for tender to develop, containing 52 Mt phosphate and 22,000 tU with vanadium.
Egypt
The country has reserves of 100 Mt phosphates containing 40,000 tU.
Tunisia
The country has reserves of 100 Mt phosphates containing 50,000 tU. At 1.6 Mt/yr P2O5 production, this would yield 265 tU/yr by-product.
Morocco
The country has reserves of 50 billion tonnes of phosphates containing 6.9 MtU. At 4.8 Mt/yr P2O5 production, this would yield 960 tU/yr by-product. There is some expectation of 1900 tU/yr production from 2017.

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Further Information

Notes

a. The PhosEnergy process is being developed by Uranium Equities Limited (UEQ) through a US registered company, Urtek LLC. Cameco secured rights to earn up to a 73% interest in the technology, and initially paid $12.5 of the $16.5 million required for this to UEQ and a further $4.5 million for the founder's 10%. UEQ subsequently agreed to pay Cameco a share of that 10%, to hold 27% of the rights for the process. Cameco paid a further $4 million in March 2013 to reach 73% share. On the basis of its earlier 70% interest, Cameco agreed to provide funding for a minimum of 50% of UEQ’s portion of capital expenditure for the construction of the first commercial plant, repayable out of earnings. Cameco and UEQ are seeking to enter commercial arrangements with phosphate producers where the process would provide a technical solution for the recovery of uranium from phosphates. The capital required to install the process would be in exchange for off-take from the facility. [Back]

References

1. International Atomic Energy Agency, Safety Reports series No. 78, Radiation Protection And Management of NORM Residues in the Phosphate Industry (2013) [Back]

General sources

Guzman, ETR et al., Uranium in Phosphate Rock and Derivatives (1995)
WISE, Uranium recovery from phosphates

Nuclear Power Reactors

(updated July 2013)

http://world-nuclear.org/info/Nuclear-Fuel-Cycle/Power-Reactors/Nuclear-Power-Reactors/#.UhXG5X-N6So

• Most nuclear electricity is generated using just two kinds of reactors which were developed in the 1950s and improved since.
• New designs are coming forward and some are in operation as the first generation reactors come to the end of their operating lives.
• Around 13% of the world's electricity is produced from nuclear energy, more than from all sources worldwide in 1960.

A nuclear reactor produces and controls the release of energy from splitting the atoms of certain elements. In a nuclear power reactor, the energy released is used as heat to make steam to generate electricity. (In a research reactor the main purpose is to utilise the actual neutrons produced in the core. In most naval reactors, steam drives a turbine directly for propulsion.)

The principles for using nuclear power to produce electricity are the same for most types of reactor. The energy released from continuous fission of the atoms of the fuel is harnessed as heat in either a gas or water, and is used to produce steam. The steam is used to drive the turbines which produce electricity (as in most fossil fuel plants).

The world's first nuclear reactors operated naturally in a uranium deposit about two billion years ago. These were in rich uranium orebodies and moderated by percolating rainwater. Those at Oklo in west Africa, each less than 100 kW thermal, together consumed about six tonnes of that uranium.

Today, reactors derived from designs originally developed for propelling submarines and large naval ships generate about 85% of the world's nuclear electricity. The main design is the pressurised water reactor (PWR) which has water at over 300°C under pressure in its primary cooling/heat transfer circuit, and generates steam in a secondary circuit. The less numerous boiling water reactor (BWR) makes steam in the primary circuit above the reactor core, at similar temperatures and pressure. Both types use water as both coolant and moderator, to slow neutrons. Since water normally boils at 100°C, they have robust steel pressure vessels or tubes to enable the higher operating temperature. (Another type uses heavy water, with deuterium atoms, as moderator. Hence the term ‘light water’ is used to differentiate.) 

Components of a nuclear reactor

There are several components common to most types of reactors:

Fuel. Uranium is the basic fuel. Usually pellets of uranium oxide (UO₂) are arranged in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core.*
* In a new reactor with new fuel a neutron source is needed to get the reaction going.  Usually this is beryllium mixed with polonium, radium or other alpha-emitter. Alpha particles from the decay cause a release of neutrons from the beryllium as it turns to carbon-12.  Restarting a reactor with some used fuel may not require this, as there may be enough neutrons to achieve criticality when control rods are removed.

Moderator. Material in the core which slows down the neutrons released from fission so that they cause more fission. It is usually water, but may be heavy water or graphite.

Control rods. These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or to halt it.*  In some PWR reactors, special control rods are used to enable the core to sustain a low level of power efficiently. (Secondary control systems involve other neutron absorbers, usually boron in the coolant – its concentration can be adjusted over time as the fuel burns up.)
* In fission, most of the neutrons are released promptly, but some are delayed. These are crucial in enabling a chain reacting system (or reactor) to be controllable and to be able to be held precisely critical.

Coolant. A fluid circulating through the core so as to transfer the heat from it.  In light water reactors the water moderator functions also as primary coolant. Except in BWRs, there is secondary coolant circuit where the water becomes steam. (See also later section on primary coolant characteristics)

Pressure vessel or pressure tubes. Usually a robust steel vessel containing the reactor core and moderator/coolant, but it may be a series of tubes holding the fuel and conveying the coolant through the surrounding moderator.

Steam generator.(not in BWR) Part of the cooling system where the high-pressure primary coolant bringing heat from the reactor is used to make steam for the turbine, in a secondary circuit. Essentially a heat exchanger like a motor car radiator*. Reactors may have up to four 'loops', each with a steam generator.

* These are large heat exchangers for transferring heat from one fluid to another – here from high-pressure primary circuit in PWR to secondary circuit where water turns to steam. Each structure weighs up to 800 tonnes and contains from 300 to 16,000 tubes about 2 cm diameter for the primary coolant, which is radioactive due to nitrogen-16 (N-16, formed by neutron bombardment of oxygen, with half-life of 7 seconds). The secondary water must flow through the support structures for the tubes. The whole thing needs to be designed so that the tubes don't vibrate and fret, operated so that deposits do not build up to impede the flow, and maintained chemically to avoid corrosion. Tubes which fail and leak are plugged, and surplus capacity is designed to allow for this. Leaks can be detected by monitoring N-16 levels in the steam as it leaves the steam generator. 

Containment. The structure around the reactor and associated steam generators which is designed to protect it from outside intrusion and to protect those outside from the effects of radiation in case of any serious malfunction inside. It is typically a metre-thick concrete and steel structure.

There are several different types of reactors as indicated in the following Table.

Nuclear power plants in commercial operation

Reactor type	Main Countries	Number	GWe	Fuel	Coolant	Moderator
Pressurised Water Reactor (PWR)	US, France, Japan, Russia, China	271	270.4	enriched UO2	water	water
Boiling Water Reactor (BWR)	US, Japan, Sweden	84	81.2	enriched UO2	water	water
Pressurised Heavy Water Reactor 'CANDU' (PHWR)	Canada	48	27.1	natural UO2	heavy water	heavy water
Gas-cooled Reactor (AGR & Magnox)	UK	17	9.6	natural U (metal), enriched UO2	CO2	graphite
Light Water Graphite Reactor (RBMK & EGP)	Russia	11 + 4	10.4	enriched UO2	water	graphite
Fast Neutron Reactor (FBR)	Russia	1	0.6	PuO2 and UO2	liquid sodium	none
	TOTAL	436	399.3

GWe = capacity in thousands of megawatts (gross)
Source: Nuclear Engineering International Handbook 2011, updated to 1/1/12
For reactors under construction: see paper  Plans for New Reactors Worldwide.

Fuelling a nuclear power reactor 

Most reactors need to be shut down for refuelling, so that the pressure vessel can be opened up. In this case refuelling is at intervals of 1-2 years, when a quarter to a third of the fuel assemblies are replaced with fresh ones. The CANDU and RBMK types have pressure tubes (rather than a pressure vessel enclosing the reactor core) and can be refuelled under load by disconnecting individual pressure tubes.

If graphite or heavy water is used as moderator, it is possible to run a power reactor on natural instead of enriched uranium. Natural uranium has the same elemental composition as when it was mined (0.7% U-235, over 99.2% U-238), enriched uranium has had the proportion of the fissile isotope (U-235) increased by a process called enrichment, commonly to 3.5 - 5.0%. In this case the moderator can be ordinary water, and such reactors are collectively called light water reactors. Because the light water absorbs neutrons as well as slowing them, it is less efficient as a moderator than heavy water or graphite.

During operation, some of the U-238 is changed to plutonium, and Pu-239 ends up providing about one third of the energy from the fuel.

In most reactors the fuel is ceramic uranium oxide (UO₂ with a melting point of 2800°C) and most is enriched. The fuel pellets (usually about 1 cm diameter and 1.5 cm long) are typically arranged in a long zirconium alloy (zircaloy) tube to form a fuel rod, the zirconium being hard, corrosion-resistant and permeable to neutrons.* Numerous rods form a fuel assembly, which is an open lattice and can be lifted into and out of the reactor core. In the most common reactors these are about 3.5 to 4 metres long.
*Zirconium is an important mineral for nuclear power, where it finds its main use. It is therefore subject to controls on trading. It is normally contaminated with hafnium, a neutron absorber, so very pure 'nuclear grade' Zr is used to make the zircaloy, which is about 98% Zr plus tin, iron, chromium and sometimes nickel to enhance its strength. 

Burnable poisons are often used (especially in BWR) in fuel or coolant to even out the performance of the reactor over time from fresh fuel being loaded to refuelling. These are neutron absorbers which decay under neutron exposure, compensating for the progressive build up of neutron absorbers in the fuel as it is burned. The best known is gadolinium, which is a vital ingredient of fuel in naval reactors where installing fresh fuel is very inconvenient, so reactors are designed to run more than a decade between refuellings.

The power rating of a nuclear power reactor

Nuclear power plant reactor power outputs are quoted in three ways:

• Thermal MWt, which depends on the design of the actual nuclear reactor itself, and relates to the quantity and quality of the steam it produces.
• Gross electrical MWe indicates the power produced by the attached steam turbine and generator, and also takes into account the ambient temperature for the condenser circuit (cooler means more electric power, warmer means less). Rated gross power assumes certain conditions with both.
• Net electrical MWe, which is the power available to be sent out from the plant to the grid, after deducting the electrical power needed to run the reactor (cooling and feed-water pumps, etc.) and the rest of the plant.*

* Net electrical MWe and gross MWe vary slightly from summer to winter, so normally the lower summer figure, or an average figure, is used. If the summer figure is quoted plants may show a capacity factor greater than 100% in cooler times. Some design options, such as powering the main large feed-water pumps with electric motors (as in EPR) rather than steam turbines (taking steam before it gets to the main turbine-generator), explains some gross to net differences between different reactor types. The EPR has a relatively large drop from gross to net MWe for this reason.

The relationship between these is expressed in two ways:

• Thermal efficiency %, the ratio of gross MWe to thermal MW. This relates to the difference in temperature between the steam from the reactor and the cooling water. It is often 33-37%.
• Net efficiency %, the ratio of net MWe achieved to thermal MW. This is a little lower, and allows for plant usage.

In WNA papers and figures and WNN items, generally net MWe is used for operating plants, and gross MWe for those under construction or planned/proposed. 

Pressurised Water Reactor (PWR)

This is the most common type, with over 230 in use for power generation and several hundred more employed for naval propulsion. The design of PWRs originated as a submarine power plant. PWRs use ordinary water as both coolant and moderator. The design is distinguished by having a primary cooling circuit which flows through the core of the reactor under very high pressure, and a secondary circuit in which steam is generated to drive the turbine.  In Russia these are known as VVER types - water-moderated and -cooled.

A PWR has fuel assemblies of 200-300 rods each, arranged vertically in the core, and a large reactor would have about 150-250 fuel assemblies with 80-100 tonnes of uranium.

Water in the reactor core reaches about 325°C, hence it must be kept under about 150 times atmospheric pressure to prevent it boiling. Pressure is maintained by steam in a pressuriser (see diagram). In the primary cooling circuit the water is also the moderator, and if any of it turned to steam the fission reaction would slow down. This negative feedback effect is one of the safety features of the type. The secondary shutdown system involves adding boron to the primary circuit.

The secondary circuit is under less pressure and the water here boils in the heat exchangers which are thus steam generators. The steam drives the turbine to produce electricity, and is then condensed and returned to the heat exchangers in contact with the primary circuit.

Boiling Water Reactor (BWR)

This design has many similarities to the PWR, except that there is only a single circuit in which the water is at lower pressure (about 75 times atmospheric pressure) so that it boils in the core at about 285°C. The reactor is designed to operate with 12-15% of the water in the top part of the core as steam, and hence with less moderating effect and thus efficiency there.  BWR units can operate in load-following mode more readily then PWRs.

The steam passes through drier plates (steam separators) above the core and then directly to the turbines, which are thus part of the reactor circuit. Since the water around the core of a reactor is always contaminated with traces of radionuclides, it means that the turbine must be shielded and radiological protection provided during maintenance. The cost of this tends to balance the savings due to the simpler design. Most of the radioactivity in the water is very short-lived*, so the turbine hall can be entered soon after the reactor is shut down.

* mostly N-16, with a 7 second half-life

A BWR fuel assembly comprises 90-100 fuel rods, and there are up to 750 assemblies in a reactor core, holding up to 140 tonnes of uranium. The secondary control system involves restricting water flow through the core so that more steam in the top part reduces moderation.

Pressurised Heavy Water Reactor (PHWR or CANDU)

The PHWR reactor design has been developed since the 1950s in Canada as the CANDU, and more recently also in India.  PHWRs generally use natural uranium (0.7% U-235) oxide as fuel, hence needs a more efficient moderator, in this case heavy water (D₂O).** The PHWR produces more energy per kg of mined uranium than other designs.

** with the CANDU system, the moderator is enriched (ie water) rather than the fuel, - a cost trade-off.

The moderator is in a large tank called a calandria, penetrated by several hundred horizontal pressure tubes which form channels for the fuel, cooled by a flow of heavy water under high pressure in the primary cooling circuit, reaching 290°C. As in the PWR, the primary coolant generates steam in a secondary circuit to drive the turbines. The pressure tube design means that the reactor can be refuelled progressively without shutting down, by isolating individual pressure tubes from the cooling circuit.

A CANDU fuel assembly consists of a bundle of 37 half metre long fuel rods (ceramic fuel pellets in zircaloy tubes) plus a support structure, with 12 bundles lying end to end in a fuel channel. Control rods penetrate the calandria vertically, and a secondary shutdown system involves adding gadolinium to the moderator. The heavy water moderator circulating through the body of the calandria vessel also yields some heat (though this circuit is not shown on the diagram above). 

Newer PHWR designs such as the Advanced Candu Reactor (ACR) have light water cooling and slightly-enriched fuel.

CANDU reactors can readily be run on recycled uranium from reprocessing LWR used fuel, or a blend of this and depleted uranium left over from enrichment plants. About 4000 MWe of PWR can then fuel 1000 MWe of CANDU capacity, with addition of depleted uranium. Thorium may also be used in fuel.

Advanced Gas-cooled Reactor (AGR)

These are the second generation of British gas-cooled reactors, using graphite moderator and carbon dioxide as primary coolant. The fuel is uranium oxide pellets, enriched to 2.5-3.5%, in stainless steel tubes. The carbon dioxide circulates through the core, reaching 650°C and then past steam generator tubes outside it, but still inside the concrete and steel pressure vessel (hence 'integral' design). Control rods penetrate the moderator and a secondary shutdown system involves injecting nitrogen to the coolant.

The AGR was developed from the Magnox reactor, also graphite moderated and CO₂ cooled, and one of these is still operating in UK to late 2014. They use natural uranium fuel in metal form. Secondary coolant is water.

Light water graphite-moderated reactor (RBMK)

This is a Soviet design, developed from plutonium production reactors. It employs long (7 metre) vertical pressure tubes running through graphite moderator, and is cooled by water, which is allowed to boil in the core at 290°C, much as in a BWR. Fuel is low-enriched uranium oxide made up into fuel assemblies 3.5 metres long. With moderation largely due to the fixed graphite, excess boiling simply reduces the cooling and neutron absorbtion without inhibiting the fission reaction, and a positive feedback problem can arise, which is why they have never been built outside the Soviet Union. See appendix on RBMK Reactors for more detail.

Advanced reactors

Several generations of reactors are commonly distinguished. Generation I reactors were developed in 1950-60s and only one is still running today. They mostly used natural uranium fuel and used graphite as moderator. Generation II reactors are typified by the present US fleet and most in operation elsewhere. They typically use enriched uranium fuel and are mostly cooled and moderated by water. Generation III are the Advanced Reactors evolved from these, the first few of which are in operation in Japan and others are under construction and ready to be ordered. They are developments of the second generation with enhanced safety. There is no clear distinction Gen II to Gen III.

Generation IV designs are still on the drawing board and will not be operational before 2020 at the earliest, probably later. They will tend to have closed fuel cycles and burn the long-lived actinides now forming part of spent fuel, so that fission products are the only high-level waste. Of seven designs under development, 4 or 5 will be fast neutron reactors. Four will use fluoride or liquid metal coolants, hence operate at low pressure. Two will be gas-cooled. Most will run at much higher temperatures than today’s water-cooled reactors. See Generation IV Reactors paper.

More than a dozen (Generation III) advanced reactor designs are in various stages of development. Some are evolutionary from the PWR, BWR and CANDU designs above, some are more radical departures. The former include the Advanced Boiling Water Reactor, a few of which are now operating with others under construction. The best-known radical new design has the fuel as large 'pebbles' and uses helium as coolant, at very high temperature, possibly to drive a turbine directly.

Considering the closed fuel cycle, Generation 1-3 reactors recycle plutonium (and possibly uranium), while Generation IV are expected to have full actinide recycle.

Fast neutron reactors (FNR)

Some reactors (only one in commercial service) do not have a moderator and utilise fast neutrons, generating power from plutonium while making more of it from the U-238 isotope in or around the fuel. While they get more than 60 times as much energy from the original uranium compared with the normal reactors, they are expensive to build. Further development of them is likely in the next decade, and the main designs expected to be built in two decades are FNRs. If they are configured to produce more fissile material (plutonium) than they consume they are called Fast Breeder Reactors (FBR). See also Fast Neutron Reactors and Small Reactors papers.

Floating nuclear power plants

Apart from over 200 nuclear reactors powering various kinds of ships, Rosatom in Russia has set up a subsidiary to supply floating nuclear power plants ranging in size from 70 to 600 MWe. These will be mounted in pairs on a large barge, which will be permanently moored where it is needed to supply power and possibly some desalination to a shore settlement or industrial complex. The first has two 40 MWe reactors based on those in icebreakers and will operate at a remote site in Siberia. Electricity cost is expected to be much lower than from present alternatives.

The Russian KLT-40S is a reactor well proven in icebreakers and now proposed for wider use in desalination and, on barges, for remote area power supply. Here a 150 MWt unit produces 35 MWe (gross) as well as up to 35 MW of heat for desalination or district heating. These are designed to run 3-4 years between refuelling and it is envisaged that they will be operated in pairs to allow for outages, with on-board refuelling capability and used fuel storage. At the end of a 12-year operating cycle the whole plant is taken to a central facility for 2-year overhaul and removal of used fuel, before being returned to service. Two units will be mounted on a 21,000 tonne barge. A larger Russian factory-built and barge-mounted reactor is the VBER-150, of 350 MW thermal, 110 MWe. The larger VBER-300 PWR is a 325 MWe unit, originally envisaged in pairs as a floating nuclear power plant, displacing 49,000 tonnes. As a cogeneration plant it is rated at 200 MWe and 1900 GJ/hr. See also Nuclear Power in Russia paper.

Lifetime of nuclear reactors

Most of today's nuclear plants which were originally designed for 30 or 40-year operating lives. However, with major investments in systems, structures and components lives can be extended, and in several countries there are active programs to extend operating lives. In the USA most of the more than one hundred reactors are expected to be granted licence extensions from 40 to 60 years. This justifies significant capital expenditure in upgrading systems and components, including building in extra performance margins.

Some components simply wear out, corrode or degrade to a low level of efficiency. These need to be replaced. Steam generators are the most prominent and expensive of these, and many have been replaced after about 30 years where the reactor otherwise has the prospect of running for 60 years. This is essentially an economic decision. Lesser components are more straightforward to replace as they age. In Candu reactors, pressure tube replacement has been undertaken on some plants after about 30 years operation.

A second issue is that of obsolescence. For instance, older reactors have analogue instrument and control systems. Thirdly, the properties of materials may degrade with age, particularly with heat and neutron irradiation. In respect to all these aspects, investment is needed to maintain reliability and safety. Also, periodic safety reviews are undertaken on older plants in line with international safety conventions and principles to ensure that safety margins are maintained.

Another important issue is knowledge management (KM) over the full lifecycle from design, through construction and operation to decommissioning for reactors and other facilities. This may span a century and involve several countries, and involve a succession of companies. The plant lifespan will cover several generations of engineers. Data needs to be transferable across several generations of software and IT hardware, as well as being shared with other operators of similar plants.* Significant modifications may be made to the design over the life of the plant, so original documentation is not sufficient, and loss of design base knowledge can have huge implications (eg Pickering A and Bruce A in Ontario). Knowledge management is often a shared responsibility and is essential for effective decision-making and the achievement of plant safety and economics.

* ISO15926 covers portability and interoperability for lifecycle open data standard. Also EPRI in 2013 published Advanced Nuclear Technology: New Nuclear Power Plant Information Handover Guide.  

See also section on Ageing, in Safety of Nuclear Power Reactors paper.

Load-following capacity

Nuclear power plants are essentially base-load generators, running continuously. This is because their power output cannot readily be ramped up and down on a daily and weekly basis, and in this respect they are similar to most coal-fired plants. (It is also uneconomic to run them at less than full capacity, since they are expensive to build but cheap to run.) However, in some situations it is necessary to vary the output according to daily and weekly load cycles on a regular basis, for instance in France, where there is a very high reliance on nuclear power.

While BWRs can be made to follow loads reasonably easily without burning the core unevenly, this is not as readily achieved in a PWR. The ability of a PWR to run at less than full power for much of the time depends on whether it is in the early part of its 18 to 24-month refueling cycle or late in it, and whether it is designed with special control rods which diminish power levels throughout the core without shutting it down. Thus, though the ability on any individual PWR reactor to run on a sustained basis at low power decreases markedly as it progresses through the refueling cycle, there is considerable scope for running a fleet of reactors in load-following mode. See further information in the Nuclear Power in France paper.

As fast neutron reactors become established in future years, their ability to load-follow will be a benefit. 

Primary coolants

The advent of some of the designs mentioned above provides opportunity to review the various primary coolants used in nuclear reactors. There is a wide variety – gas, water, light metal, heavy metal and salt: 

Water or heavy water must be maintained at very high pressure (1000-2200 psi, 7-15 MPa) to enable it to function above 100°C, as in present reactors. This has a major influence on reactor engineering. However, supercritical water around 25 MPa can give 45% thermal efficiency – as at some fossil-fuel power plants today with outlet temperatures of 600°C, and at ultra supercritical levels (30+ MPa) 50% may be attained.

Water cooling of steam condensers is fairly standard in all power plants, because it works very well, it is relatively inexpensive, and there is a huge experience base. Water is a lot more effective than air for removing heat.

Helium must be used at similar pressure (1000-2000 psi, 7-14 MPa) to maintain sufficient density for efficient operation. Again, there are engineering implications, but it can be used in the Brayton cycle to drive a turbine directly.

Carbon dioxide was used in early British reactors and their AGRs which operate at much higher temperatures than light water reactors. It is denser than helium and thus likely to give better thermal conversion efficiency. There is now interest in supercritical CO₂ for the Brayton cycle.

Sodium, as normally used in fast neutron reactors at around 550ºC, melts at 98°C and boils at 883°C at atmospheric pressure, so despite the need to keep it dry the engineering required to contain it is relatively modest. It has high thermal conductivity. However, normally water/steam is used in the secondary circuit to drive a turbine (Rankine cycle) at lower thermal efficiency than the Brayton cycle. In some designs sodium is in a secondary circuit to steam generators. Sodium does not corrode the metals used in the fuel cladding or primary circuit, nor the fuel itself if there is cladding damage.

Lead or lead-bismuth eutectic in fast neutron reactors are capable of higher temperature operation. They are transparent to neutrons, aiding efficiency, and since they do not react with water the heat exchanger interface is safer. They do not burn when exposed to air. However, they are corrosive of fuel cladding and steels, which originally limited temperatures to 550°C. With today's materials 650°C can be reached, and in future 700°C is in sight, using oxide dispersion-strengthened steels. A problem is that Pb-Bi yields toxic polonium (Po-210) activation products. Pb-Bi melts at a relatively low 125°C (hence eutectic) and boils at 1670°C, Pb melts at 327°C and boils at 1737°C but is very much more abundant and cheaper to produce than bismuth, hence is envisaged for large-scale use in the future, though freezing must be prevented. The development of nuclear power based on Pb-Bi cooled fast neutron reactors is likely to be limited to a total of 50-100 GWe, basically for small reactors in remote places. In 1998 Russia declassified a lot of research information derived from its experience with submarine reactors, and US interest in using Pb or Pb-Bi for small reactors has increased subsequently. The Gen4 Module (Hyperion) reactor will use lead-bismuth eutectic which is 45% Pb, 55% Bi. A secondary circuit generating steam is likely.

Molten fluoride salt boils at 1400°C at atmospheric pressure, so allows several options for use of the heat, including using helium in a secondary Brayton cycle circuit with thermal efficiencies of 48% at 750°C to 59% at 1000°C, or manufacture of hydrogen. Fluoride salts have very low vapour pressure even at red heat, have reasonably good heat transfer properties, are not damaged by radiation, do not react violently with air or water, and are inert to some common structural metals.

Low-pressure liquid coolants allow all their heat to be delivered at high temperatures, since the temperature drop in heat exchangers is less than with gas coolants. Also, with a good margin between operating and boiling temperatures, passive cooling for decay heat is readily achieved.

The removal of passive decay heat is a vital feature of primary cooling systems, beyond heat transfer to do work. When the fission process stops, fission product decay continues and a substantial amount of heat is added to the core. At the moment of shutdown, this is about 6.5% of the full power level, but after an hour it drops to about 1.5% as the short-lived fission products decay. After a day, the decay heat falls to 0.4%, and after a week it will be only 0.2%. This heat could melt the core of a light water reactor unless it is reliably dissipated, as shown in 2011 at Fukushima, where about 1.5% of the heat was being generated when the tsunami disabled the cooling. In passive systems, some kind of convection flow is relied upon. 

Top AHTR line is potential, lower one practical today. See also paper on Cooling Power Plants.

There is some radioactivity in the cooling water flowing through the core of a water-cooled reactor, due mainly to the activation product nitrogen-16, formed by neutron capture from oxygen. N-16 has a half-life on only 7 seconds but produces high-energy gamma radiation during decay. It is the reason that access to a BWR turbine hall is restricted during actual operation.

Nuclear reactors for process heat

Producing steam to drive a turbine and generator is relatively easy, and a light water reactor running at 350°C does this readily. As the above section and Figure show, other types of reactor are required for higher temperatures. A 2010 US Department of Energy document quotes 500°C for a liquid metal cooled reactor (FNR), 860°C for a molten salt reactor (MSR), and 950°C for a high temperature gas-cooled reactor (HTR). Lower-temperature reactors can be used with supplemental gas heating to reach higher temperatures, though employing an LWR would not be practical or economic. The DOE said that high reactor outlet temperatures in the range 750 to 950°C were required to satisfy all end user requirements evaluated to date for the Next Generation Nuclear Plant. 

Primitive reactors

The world's oldest known nuclear reactors operated at what is now Oklo in Gabon, West Africa. About 2 billion years ago, at least 17 natural nuclear reactors achieved criticality in a rich deposit of uranium ore. Each operated intermittently at about 20 kW thermal, the reaction ceasing whenever the water turned to steam so that it ceased to function as moderator. At that time the concentration of U-235 in all natural uranium was 3.7 percent instead of 0.7 percent as at present. (U-235 decays much faster than U-238, whose half-life is about the same as the age of the Earth.) These natural chain reactions, started spontaneously by the presence of water acting as a moderator, continued overall for about 2 million years before finally dying away.

During this long reaction period about 5.4 tonnes of fission products as well as 1.5 tonnes of plutonium together with other transuranic elements were generated in the orebody. The initial radioactive products have long since decayed into stable elements but close study of the amount and location of these has shown that there was little movement of radioactive wastes during and after the nuclear reactions. Plutonium and the other transuranics remained immobile.

Sources:
Wilson, P.D., 1996, The Nuclear Fuel Cycle, OUP.