Uranium and Depleted Uranium
(Updated December 2009)
- 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-lifeb 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.
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 neutrond, 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 energye. 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 continuef. 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 Kazakhstang. 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 enrichedh. 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 levelsj. 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.)
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.
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 productsl, 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]
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]
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)