Uranium and Depleted Uranium
(August 2007)
- The basic fuel for a nuclear power reactor is uranium - a very heavy metal containing abundant concentrated energy.
- It is mildly radioactive and occurs naturally in the Earth's crust.
- Depleted uranium is a by-product or waste product of uranium enrichment.
- The health hazards associated with any uranium are much the same as those for lead.
Uranium was apparently formed in super novae about 6.6 billion years ago. While it is not common in the solar system, today 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. See also paper on Cosmic Origins and Gelogical Role of Uranium.Uranium was discovered by Martin Klaproth, a German chemist, in 1789 in the mineral pitchblende, and was named after the planet Uranus.
Uranium (chemical symbol U) is slightly more abundant than tin and about 40 times as common as silver. 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. 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.
The melting point of uranium is 1132°C.
see also: Webelements information.
Uses
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.
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, and in medicine for the treatment of disease.
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 nuclear reactors are important for making radioisotopes.
The Uranium Atom
On a scale arranged according to the increasing mass of their nuclei, uranium is the heaviest of all the naturally-occurring elements (Hydrogen is the lightest). Uranium has a specific gravity of 18.7.
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' and we use the expression 'nuclear fission'.
Meanwhile, like all radioactive isotopes, it decays. U-238 decays very slowly, its half-life 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 natural uranium is 25 kBq/g. 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, 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.
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. Water, graphite and heavy water are used as moderators in different types of reactors.
Most nuclear reactors require natural uranium (having 0.7% U-235) to be enriched, so as to increase the proportion of the fissile isotope U-235 about five- or six-fold. (see below)
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.
Uranium and Plutonium
Whereas the U-235 atom is 'fissile', the U-238 atom is said to be 'fertile'. This means that it can capture one of the neutrons which are flying about in the core of the reactor and become (indirectly) plutonium-239, which is fissile. Pu-239 is very much like U-235, in that it fissions when hit by a slow neutron and this also yields a lot of energy.
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 comes from "burning" Pu-239.
But sometimes a Pu-239 atom 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 is in it. The significance of this is that when the spent fuel is removed after about three years, the plutonium in it is not suitable for making weapons but can be recycled as fuel. See also Plutonium Paper .
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. Enrichment increases the proportion of the U-235 isotope from its natural level of 0.7% to 3 - 4%. 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 (or waste product) of enrichment is depleted uranium (about 89% 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 - 2800°C.
Enriched uranium, with the lighter isotopes concentrated, has an activity of 82 kBq/g, most of this being from the U-234. (Enriched reprocessed uranium has an activity of over 250 kBq/g, largely due to U-234 and U-236.)
For reactors which use natural uranium as their fuel (and hence which require graphite or heavy water as a moderator) the U3O8 concentrate simply needs to be refined and converted directly to uranium dioxide.
Used reactor fuel is removed and stored, either to be reprocessed or disposed of underground
Reprocessed Uranium
When used nuclear fuel is reprocessed, both plutonium and uranium are recovered separately. Uranium comprises about 96% of that spent fuel.
The composition of reprocessed uranium depends on the time the fuel has been in the reactor, but it is mostly U-238. Typically it will have about 1% U-235 and small amounts of U-232 and U-236. The former is a gamma-emitter, making the material difficult to handle, even with trace amounts. The latter, comprising about 0.5% of the material, is a neutron absorber which means that if reprocessed uranium is used for fresh fuel it must be enriched slightly more than is required for natural uranium. In the future, laser enrichment techniques may be able to remove these isotopes.
Reprocessed uranium (especially from earlier military reprocessing) may also be contaminated with traces of fission products. Over 2002-06 USEC successfully cleaned up 7400 tonnes of technetium-contaminated uranium from the US Department of Energy.
Nuclear power
Over 16% of the world's electricity is generated from uranium in nuclear reactors. This amounts to over 2600 billion kWh, as much as from all sources worldwide in 1960.
It comes from about 440 nuclear reactors with a total output capacity of about 370 000 MWe operating in 31 countries. A further thirty reactors are under construction and another 240 are on the drawing board.
Belgium, Bulgaria, Finland, France, Germany, Hungary, Japan, South Korea, Lithuania, Slovakia, Slovenia, Spain, Sweden, Switzerland and Ukraine all get 30% or more of their electricity from nuclear reactors. The USA has over 100 reactors operating, supplying 20% of its electricity. The UK gets about a quarter of its electricity from uranium.
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 reserves are therefore calculated as tonnes recoverable up to a certain cost.
Australia's reserves are about 25% of the world's total, but Canada is the world's leading producer. Other countries with reserves include Canada, USA, South Africa, Namibia, Brazil and Kazakhstan. China may also have substantial deposits of uranium. Many more countries have smaller deposits which could be mined.
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.
Radioisotopes
Radioisotopes have become a vital part of modern life. Using relatively small special purpose nuclear reactors, a wide range of radioactive materials (radioisotopes) can be made at low cost. For this reason their use has become widespread since the early 1950s, and there are now some 280 "research" reactors in 56 countries producing them.
Radioisotopes play an important part in the technologies that provide us with food, water and good health. They are
produced by bombarding small amounts of particular elements with neutrons.
In medicine, radioisotopes are widely
used for diagnosis 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.
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.
Most household smoke detectors use a radioisotope (Americium-241) derived from the plutonium formed in nuclear reactors. These alarms save many lives.
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.
Other reactors
There are also other uses for 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. High-temperature reactors can also be used for industrial processes such as thermochemical production of hydrogen.
Nuclear weapons
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%); bomb-grade plutonium is fairly pure (>90%) Pu-239 and is made in special reactors.
Today a lotof military high-enriched uranium is becoming available for electricity production. It is diluted about 25:1 with depleted uranium before being used as reactor fuel.
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 (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.
DU is stored either as UF6 or it is de-converted back to U3O8, which is more benign chemically and thus more suited for long-term storage. It is also less toxic. Every year over 50,000 tonnes of depleted uranium joins already substantial stockpiles in USA, Europe and Russia. World stock is about 1.2 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. 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.
Other uses are more mundane, and 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 Kuwait war (300 tonnes) and less so in Kosovo (11 tonnes).
See also US Dept of Energy Depleted Uranium web site.
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. WHO has set a Tolerable Daily Intake level for U 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. If thorium-234 (half-life 24 days), protactinium-234 (half-life 1 minute) and U-234 have built up through decay of U-238, then Th-234 and Pa-234 will give rise to some beta emissions and U-234 is an alpha emitter. 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 Program examined the effects of nine tonnes of DU munitions having been used in Kosovo, checking the sites targeted by it. 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 study by Sandia National Laboratories in USA reported in 2005 that consistent with earlier studies, reports of serious health risks from DU exposure during the 1991 Gulf War are not supported by medical statistics or by analysis.
Thus DU is clearly dangerous for people in vehicles which are 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. See also Appendix and WHO fact sheet on DU.
Sources:
BNFL, Cogema, JNFL, SKB and ANSTO publications and papers.
Bulletin of Atomic Scientists, Nov-Dec 1999.
New Scientist 5 & 26/6/99, AFP 29/10/01.
UNEP/UNCHS, 1999, Balkans Task Force report, Appendix 4.
OECD NEA 2001, Management of Depleted Uranium.
Burchall & Clark, Depleted Uranium, NRPB Bulletin #229, March 2001.
Sandia report 2005.
Appendix:
Statement by Australasian Radiation Protection Society
Potential Health Effects of Depleted Uranium in Munitions
(February 2001)
Some military personnel involved in the 1991 Gulf War have complained of continuing stress-like symptoms for which no obvious cause has been found. These symptoms have at times been attributed to the use of depleted uranium in shells and other missiles, which are said to have caused toxic effects. Similar complaints have arisen from the more recent fighting in the Balkans, particularly the Kosovo conflict about a year ago.
Depleted uranium (DU) is natural uranium which is depleted in the rarer U-235 isotope (see below). It is a heavy metal and, in common with other heavy metals, it is chemically toxic. It is also slightly radioactive and there is therefore said to be a hypothetical possibility that it could give rise to a radiological hazard under some circumstances, e.g. if dispersed in finely divided form so that it is inhaled.
However, because of the latency period for the induction of cancer by radiation, it is not credible that any cases of radiation-induced cancer could yet 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.
From National Radiation Protection Board (UK) Bulletin Editorial
(March 2001)
Uses and Risks of DU
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.