Uranium (chemical symbol U) is a naturally occurring radioactive element. In its pure form it is a silver-coloured heavy metal, similar to lead, cadmium and tungsten. Like tungsten it is very dense, about 19 grams per cubic centimetre, 70% more dense than lead. It is so dense a small 10-centimetre cube would weigh 20 kilograms.
The International Atomic Energy Agency (IAEA) defines uranium as a Low Specific Activity material. In its natural state, it consists of three isotopes (U-234, U-235 and U-238). Other isotopes that cannot be found in natural uranium are U-232, U-233, U-236 and U-237. The table below shows the fraction by weight of the three isotopes in any quantity of natural uranium, their half lives, and specific activity. The half life of a radioactive isotope is the time taken for it to decay to half of its original amount of radioactivity. The specific activity is the activity per unit mass of a particular radionuclide and is used as a measure of how radioactive a radionuclide is. It is expressed in the table in becquerels (Bq) per milligram (1 milligram, mg, = 0.001 grams). An activity of one becquerel (Bq) means that on average one disintegration takes place every second.
The activity concentration arising solely from the decay of the uranium isotopes (U-234, U-235 and U-238) found in natural uranium is 25.4 Bq per mg. In nature, uranium isotopes are typically found in radioactive equilibrium (i.e. the activity of each of the radioactive progeny is equal to the activity of the uranium parent isotope) with their radioactive decay products. Decay products of U-238 include thorium-234 (Th-234), protactinium-234 (Pa-234), U-234, Th-230, radium-226 (Ra-226), radon-222 (Rn-222), polonium-218 (Po-218), lead-214 (Pb-214), bismuth-214 (Bi-214), Po-214 Pb-210 and Po-210. Decay products of U-235 include Th-231, Pa-231, actinium-227 (Ac-227), Th-227,Ra-223,Rn-219, Po-215, Pb-211, Bi-211 and thallium-207 (Tl-207).
Isotopes of natural uranium decay by emitting mainly alpha particles. The emission of beta particles and gamma radiations are low. The table below shows the average energies per transformation emitted by U-238, U-235 and U-234.
Uranium is found in trace amounts in all rocks and soil, in water and air, and in materials made from natural substances. It is a reactive metal, and, therefore, it is not present as free uranium in the environment. In addition to the uranium naturally found in minerals, the uranium metal and compounds produced by industrial activities can also be released back to the environment.
Uranium can combine with other elements in the environment to form uranium compounds. The solubility of these uranium compounds varies greatly . Uranium in the environment is mainly found as a uranium oxide, typically as UO2, which is an anoxic insoluble compound found in minerals and sometimes as UO3, a moderately soluble compound found in surface waters. Soluble uranium compounds can combine with other chemical elements and compounds in the environment to form other uranium compounds. The chemical form of the uranium compounds determines how easily the compound can move through the environment, as well as how toxic it might be. Some forms of uranium oxides are very inert and may stay in the soil for thousands of years without moving downward into groundwater.
The average concentration of natural uranium in soil is about 2 parts per million, which is equivalent to 2 grams of uranium in 1000 kg of soil. This means that the top metre of soil in a typical 10 m ´ 40 m garden contains about 2 kg of uranium (corresponding to about 50,000,000 Bq of activity just from the decay of the uranium isotopes and ignoring the considerable activity associated with the decay of the progeny. Concentrations of uranium in granite range from 2 parts per million to 20 parts per million. Uranium in higher concentrations (50 - 1000 mg per kg of soil) can be found in soil associated with phosphate deposits. In air, uranium exists as dust. Very small, dust-like particles of uranium in the air are deposited onto surface water, plant surfaces, and soil. These particles of uranium eventually end up back in the soil or in the bottom of lakes, rivers, and ponds, where they mix with the natural uranium that is already there. Typical activity concentrations of uranium in air are around 2 µBq per cubic metre. (UNSCEAR 2000).
Most of the uranium in water comes from dissolved uranium from rocks and soil; only a very small part is from the settling of uranium dust out of the air. Activity concentrations of U-238 and U-234 in drinking water are between a few tenths of a mBq per litre to a few hundred mBqs per litre, although activity concentrations as high as 150 Bq per litre have been measured in Finland (UNSCEAR 2000). Activity concentrations of U-235 are generally more than twenty times lower.
Uranium in plants is the result of its absorption from the soil into roots and other plant parts. Typical activity concentrations of uranium isotopes in vegetables are slightly higher than those found in drinking water. The range of activity concentrations of U-238 measured in grain and leafy vegetables is between 1 mBq per kg and 400 mBq per kg and between 6 mBq per kg and 2200 mBq per kg respectively, while activity concentrations of U-235 are 20 times lower. Activity concentrations in root vegetables are generally lower (UNSCEAR 2000).
The uranium transferred to livestock through ingestion of grass and soil is eliminated quickly through urine and feces. Activity concentrations of U-238 measured in milk and meat products around the world are in the range of 0.1 mBq per kg to 17 mBq per kg and 1 mBq per kg to 20 mBq per kg respectively, with activity concentrations of U-235 more than 20 times lower (UNSCEAR 2000).
In order to produce fuel for certain types of nuclear reactors and nuclear
weapons, uranium has to be "enriched" in the U-235 isotope,
which is responsible for nuclear fission. During the enrichment process
the fraction of U-235 is increased from its natural level (0.72% by mass)
to between 2% and 94% by mass. The by-product uranium mixture (after
the enriched uranium is removed) has reduced concentrations of U-235
and U-234. This by-product of the enrichment process is known as depleted
uranium (DU). The official definition of depleted uranium given by the
US Nuclear Regulatory Commission (NRC) is uranium in which the percentage
fraction by weight of U-235 is less than 0.711%. Typically, the percentage
concentration by weight of the uranium isotopes in DU used for military
purposes is: U-238: 99.8%; U-235: 0.2%; and U-234: 0.001%.
The table below compares percentages of uranium isotopes by weight and activity in natural and depleted uranium.
DU is considerably less radioactive than natural uranium because not only does it have less U-234 and U-235 per unit mass than does natural uranium, but in addition, essentially all traces of decay products beyond U-234 and Th-231 have been removed during extraction and chemical processing of the uranium prior to enrichment. The specific activity of uranium alone in DU is 14.8 Bq per mg compared with 25.4 Bq per mg for natural uranium. It takes a long time for the uranium decay products to reach (radioactive) equilibrium with the uranium isotopes. For example it takes almost 1 million years for Th-230 to reach equilibrium with U-234.
Small amounts of natural uranium are ingested and inhaled by everyone every day. It has been estimated (UNSCEAR 2000) that the average person ingests 1.3 µg (1 µg = 1 microgram = 0.000001g) (0.033 Bq) of uranium per day, corresponding to an annual intake of 11.6 Bq. . It has also been estimated that the average person inhales 0.6 µg (15 mBq) every year. Typically, the average person will receive a dose of less than 1 µSv per year from ingestion and inhalation of uranium. In addition, an average individual will receive a dose of about 120 µSv per year from ingestion and inhalation of decay products of uranium, such as Ra-226 and its progeny in water, Rn-222 in homes and Po-210 in cigarette smoke.
Because of the differences in diet, there is a wide variation in consumption levels of uranium around the world, but, primarily, intake depends on the amount of uranium in the water people drink. In some parts of the world, the concentration of uranium in water is very high, and this results in much higher intakes of uranium from drinking water than from food. For example, consumption of uranium in parts of Finland can be tens of micrograms per day.
For information on levels of natural uranium in the human body, see:
For information on average human doses, see:
Uranium's physical and chemical properties make it very suitable for military uses. DU is used in the manufacturing of ammunitions used to pierce armour plating, such as those found on tanks, in missile nose cones and as a component of tank armour. Armour made of depleted uranium is much more resistant to penetration by conventional anti-armour ammunitions than conventional hard rolled steel armour plate.
Armour piercing ammunitions are generally referred to as "kinetic energy penetrators". DU is preferred to other metals, because of its high density, its pyrophoric nature (DU self-ignites when exposed to temperatures of 600° to 700° and high pressures), and its property of becoming sharper, through adiabatic shearing, as it penetrates armour plating . On impact with targets, DU penetrators ignite, breaking up in fragments, and forming an aerosol of particles ("DU dust") whose size depends on the angle of the impact, the velocity of the penetrator, and the temperature. These fine dust particles, can catch fire spontaneously in air. Small pieces may ignite in a fire and burn, but tests have shown that large pieces, like the penetrators used in anti-tank weapons, or in aircraft balance weights, will not normally ignite in a fire.
The vast majority of depleted uranium used by the US Department of Defense comes from the enrichment of natural uranium and is provided by the US Department of Energy. However, between the 1950s and 1970s, the US Department of Energy enriched some reprocessed uranium extracted from spent reactor fuel in order to reclaim the U-235 that did not fission. Unlike natural uranium, the reprocessed uranium contained anthropogenic (man-made) radionuclides including the uranium isotope U-236, small amounts of transuranics (elements heavier than uranium, such as neptunium, plutonium and americium) and fission products such as technetium-99. As a result, the depleted uranium by-product from the enrichment of reprocessed uranium also contained these anthropogenic radionuclides, albeit at very low levels. During the enrichment of reprocessed uranium, the inside surfaces of the equipment also became coated with these anthropogenic radionuclides and as this same equipment was used for the enrichment of natural uranium, these radionuclides later contaminated the DU produced from the enrichment of natural uranium as well. The exact amount is not known. Radiochemical analysis of depleted uranium samples indicate that these trace impurities are in the parts per billion level and result in less than a one percent increase in the radiation dose from the depleted uranium. The US Nuclear Regulatory Commission was aware of the existence of these trace contaminants in DU and determined them to be safe. The presence of U-236 and Pu-239/240 in depleted uranium has been confirmed by analyses of penetrators collected during the UNEP-led mission to Kosovo in November 2000. The activity concentration of U-236 in the penetrators was of the order of 60000 Bq per kg, while the activity concentration of plutonium varied from 0.8 to 12.87 Bq per kg.
Further information on this can be found at:
Since the advent of the nuclear age, there has been widespread use of uranium involving the mining of uranium ore, enrichment, and nuclear fuel fabrication. These industries have employed large numbers of people, and studies of the health of working populations have been carried out. The main risk to miners, and not just those involved in uranium mining, comes from exposures to radon (mainly Rn-222) gas and its decay products. A study of miners who worked in poorly ventilated mines at a time when the hazards of radon were not known and thushad been exposed to high levels of radon, demonstrated that this group had an excess of lung cancers and that the risk of cancer increased with increasing exposure to radon gas. Studies of workers exposed to uranium in the nuclear fuel cycle have also been carried out. There are some reported excesses of cancers but, unlike the miners, no correlation with exposure can be seen. The main finding of these studies has been that the health of workers is better than the average population. This "healthy worker effect" is thought to be due to the selection process inherent in employment and to the overall benefits of employment.
Regarding exposures to DU, there have been studies of the health of military personnel who saw action in the Gulf War (1990-1991) and during the Balkan conflicts (1994-99). A small number of Gulf war veterans have inoperable fragments of DU embedded in their bodies. They have been the subject of intense study and the results have been published. These veterans show elevated excretion levels of DU in urine but, so far, there have been no observable health effects due to DU in this group. There have also been epidemiological studies of the health of military personnel who saw action in conflicts where DU was used, comparing them with the health of personnel who were not in the war zones. The results of these studies have been published and the main conclusion is that the war veterans do show a small (i.e., not statistically significant) increase in mortality rates, but this excess is due to accidents rather than disease. This cannot be linked to any exposures to DU.
For information on doses and risks to miners, see:
For information on the health of people working with uranium, see:
For information on studies of military personnel exposed or potentially exposed to DU see:
Uranium is introduced into the body mainly through ingestion of food and water and inhalation of air.
When inhaled, uranium is attached to particles of different sizes. The size of the uranium aerosols and the solubility of the uranium compounds in the lungs and gut influence the transport of uranium inside the body. Coarse particles are caught in the upper part of the respiratory system (nose, sinuses, and upper part of the lungs) from where they are exhaled or transferred to the throat and then swallowed. Fine particles reach the lower part of the lungs (alveolar region). If the uranium compounds are not easily soluble, the uranium aerosols will tend to remain in the lungs for a longer period of time (up to 16 years), and deliver most of the radiation dose to the lungs. They will gradually dissolve and be transported into the blood stream. For more soluble compounds, uranium is absorbed more quickly from the lungs into the blood stream. About 10% of it will initially concentrate in the kidneys.
Most of the uranium ingested is excreted in feces within a few days and never reaches the blood stream. The remaining fraction will be transferred into the blood stream. Most of the uranium in the blood stream is excreted through urine in a few days, but a small fraction remains in the kidneys and bones and other soft tissue.
In sufficient amounts, uranium that is ingested or inhaled can be harmful because of its chemical toxicity. Like mercury, cadmium, and other heavy-metal ions, excess uranyl ions depress renal function (i.e., affect the kidneys). High concentrations in the kidney can cause damage and, in extreme cases, renal failure. The general medical and scientific consensus is that in cases of high intake, uranium is likely to become a chemical toxicology problem before it is a radiological problem. Since uranium is mildly radioactive, once inside the body it also irradiates the organs, but the primary health effect is associated with its chemical action on body functions.
In many countries, current occupational exposure limits for soluble uranium compounds are related to a maximum concentration of 3 µg uranium per gram of kidney tissue. Any effects caused by exposure of the kidneys at these levels are considered to be minor and transient. Current practices, based on these limits, appear to protect workers in the uranium industry adequately. In order to ensure that this kidney concentration is not exceeded, legislation restricts long term (8 hour) workplace air concentrations of soluble uranium to 0.2 mg per cubic metre and short term (15 minute) to 0.6 mg per cubic metre.
Like any radioactive material, there is a risk of developing cancer from exposure to radiation emitted by natural and depleted uranium. This risk is assumed to be proportional to the dose received. Limits for radiation exposure are recommended by the International Commission on Radiological Protection (ICRP) and have been adopted in the IAEA's Basic Safety Standards. The annual dose limit for a member of the public is 1 mSv, while the corresponding limit for a radiation worker is 20 mSv. The additional risk of fatal cancer associated with a dose of 1 mSv is assumed to be about 1 in 20,000. This small increase in lifetime risk should be considered in light of the risk of 1 in 5 that everyone has of developing a fatal cancer . It must also be noted that cancer may not become apparent until many years after exposure to a radioactive material.
It is possible to estimate how much DU an individual could be exposed to before the above chemical and radiological limits are exceeded. The table below shows how much depleted uranium would have to be inhaled or ingested to lead to a kidney concentration of 3µg per gram of kidney (chemical toxicity limit) or to a dose of 1 mSv (radiation dose limit). These values have been calculated with the biokinetic models currently recommended by the International Commission on Radiological Protection (ICRP). The values have been calculated for two types of uranium compounds: 'moderately soluble' compounds, such as UO3 and U3O8 and 'insoluble' compounds, such as UO2.
It should be borne in mind that the amounts required to give a kidney concentration of 3 µm per gram would be larger if the intake was given over a longer period of time, since it would give the kidneys more time to excrete the DU. The table shows that, for ingestion of DU, the chemical toxicity limit of 3 µg per gram of kidney tissue needs a smaller intake than the radiological limit (for a member of the public) of 1 mSv. For inhalation of a DU aerosol, the reverse is the case.
In addition to the radiological hazard from uranium isotopes, there is also a potential risk associated with other radionuclides that are formed from the radioactive decay of uranium isotopes and that can be found in the food ingested or in the air inhaled. The values in the table above were calculated taking into account the build up of these radionuclides inside the body, but do not include the contribution of these radionuclides in the food ingested or in the air inhaled.
Another potential harmful effect is due to external exposure to the radiation emitted by uranium isotopes. The main radiation emitted by isotopes of uranium is alpha particles (helium nuclei). The range of these alpha particles in air is of the order of one centimetre, while in the case of tissue, they can barely penetrate the external dead layer of the skin. For comparison, beta-particles (electrons) are capable of penetrating about a centimetre of tissue, while gamma-radiation (high energy photons) can pass through the body. Therefore, the potential risk from external exposure to uranium isotopes is exceedingly low, unless the uranium is introduced directly into the body (e.g. through a wound). Moreover, as alpha particles cannot travel very far from the source, an individual can only be exposed by coming in direct contact with uranium isotopes. This is not the case however with natural uranium, where people are also exposed to the more penetrating beta and gamma radiation emitted by the decay products of uranium that are normally found in equilibrium with the uranium isotopes. In the case of DU, the only beta emitting decay products present are Th-234, Pa-234m andTh-231, all of which emit low intensity gamma-radiation, and, thus the risk from external exposure to DU is considerably lower than for exposure to natural uranium.
There have been a number of studies of workers exposed to uranium (see question 8) and, despite some workers being exposed to large amounts of uranium, there is no evidence that either natural uranium or DU is carcinogenic. This lack of evidence is seen even for lung cancer following inhalation of uranium. As a precaution for risk assessment and to set dose limits, DU is assumed to be potentially carcinogenic, but the lack of evidence for a definite cancer risk in studies over many decades is significant and should put the results of assessments in perspective.
Like adults, children are exposed to small amounts of uranium in air, food, and drinking water. However, no cases have been reported where exposure to uranium is known to have caused health effects in children. It is not known whether children differ from adults in their susceptibility to health effects from uranium exposure. In experiments, very young animals have been found to absorb more uranium into their blood than adult animals when they are fed uranium.
lt is not known if exposure to uranium has effects on the development of the human fetus. There have been reports of birth defects and an increase in fetal deaths in animals fed with very high doses of uranium in drinking water. In an experiment with pregnant animals, only a very small amount (0.03%) of the injected uranium reached the fetus. Even less uranium is likely to reach the fetus in mothers exposed to uranium through inhalation and ingestion. There are no available data of measurements of uranium in breast milk. Because of its chemical properties, it is unlikely that uranium would concentrate in breast milk.
The effect of exposure to uranium on the reproductive system is not known. Very high doses of uranium have caused a reduction in sperm counts in some experiments with laboratory animals, but the majority of studies have shown no effects.
The main potential hazard associated with depleted uranium ammunitions is the inhalation of the aerosols created when DU ammunitions hit an armoured target. The size, distribution, and chemical composition of the particles released on impact will be highly variable, but the fraction of the aerosols that can enter the lung can be as high as 96%. A typical composition of these aerosols is about 60% U3O8, 20% UO2, and about 20% other amorphous oxides (Schripsick et al., 1984). Both U3O8 and UO2 are insoluble compounds. The individuals most likely to receive the highest doses from DU ammunitions are, therefore, those near a target at the time of impact or those who examine a target (or enter a tank) in the aftermath of the impact.
A potential exposure pathway for those visiting or living in DU affected areas after the aerosols have settled is the inhalation of DU particles in the soil that have been re-suspended through the action of wind or human activities. The risk will be lower because the re-suspended uranium particles combine with other material and increase in size and, therefore, a smaller fraction of the uranium inhaled will reach the deep part of the lungs. Another possible route of exposure is the inadvertent or deliberate ingestion of soil. For example, farmers working in a field where DU ammunitions were fired could inadvertently ingest small quantities of soil, while children sometimes deliberately eat soil.
In the long term, the exposure pathways that become more important are ingestion of DU incorporated in drinking water and the food chain through migration from the soil or direct deposition on vegetation. The risk from ingestion of food and water is generally low, because uranium is not effectively transported in the food chain.
It has also been estimated that a large fraction of DU ammunitions fired from an aircraft probably miss their intended target. The majority of these projectiles will be buried at various depths under the surface of the ground and even in buildings. Some of them could be lying around on the ground surface in the vicinity of the target. The physical state of these ammunitions will be very variable, depending on the characteristics of the ground, ranging from small fragments to whole intact penetrators.
Individuals, who might find and handle these ammunitions could be exposed to external radiation emitted by DU. For example, a farmer ploughing a field may dig up an intact projectile some time afterwards. Because of the type of radiation emitted by DU, the dose received would be significant only if the person exposed was in contact with DU projectiles. In addition, people could, through handling the penetrators, inadvertently ingest some of the loose friable uranium oxides formed through weathering of the surface of the penetrators.
With time, chemical weathering will cause the metallic DU of penetrators in the ground to corrode and disperse in the soil. The DU in the soil will be in an oxidized, soluble chemical form and migrate to surface and groundwater from where it will eventually be incorporated into the food chain, which then can be consumed. It is difficult to predict how long it would take for individuals to be exposed to DU through this pathway, but it is reasonable to assume that it would take several years before enhanced levels of DU could be measured in water and food.
For information on properties of airborne uranium, see:
The contact dose rate from a DU penetrator is about 2 mSv per hour, primarily from beta particle decay from DU progeny. At this dose rate it is unlikely that prolonged contact with a DU penetrator would lead to skin burns (erythema) or any other acute radiation effect. Nevertheless, the dose that could be delivered from handling of DU ammunitions is such that the exposure and handling time should be kept to a minimum and protective apparel (gloves should be worn A public information campaign may, therefore, be required to ensure that people avoid handling the projectiles. This should form part of any risk assessment and such precautions should depend on the scope and number of ammunitions used in an area.
The environmental impact of depleted uranium depends on the specific situation where DU ammunitions are used and the physical, chemical, and geological characteristics of the environment affected.
However, some general conclusions can still be made. Studies carried out at test ranges show that most of the DU aerosols created by the impact of penetrators against an armoured target settle within a short time (minutes) of the impact and in close proximity to the target site, although smaller particles may be carried to a distance of several hundred metres by the wind.
Once the DU aerosols settle on the ground, the depleted uranium particles combine with other material and increase in size, becoming less of an inhalation hazard. The potential risk from inhalation will be associated with material that is re-suspended from the ground by the action of the wind or by human activities, such as ploughing. With time, the concentrations of depleted uranium on the ground surface will decrease due to wind and precipitation that will transport the depleted uranium away or wash it into the soil. Any risk associated with inhalation of re-suspended material will thus decrease with time.
Depleted uranium present in the soil can migrate to surface and groundwater and flow into water streams. Plants will also uptake DU present in soil and in water. A very small fraction of DU in vegetation and water is the result of direct deposition onto water surfaces. The chemical and physical composition of the soil will determine the solubility and transportability of the DU particles. The DU in water and vegetation will be transferred to livestock through ingestion of grass, soil, and water. Studies have shown that bio-accumulation of uranium in plants and animals is not very high and, therefore, uranium is not effectively transported in the food chain.
Depleted uranium in the soil will be in an oxidized, soluble chemical form and migrate to surface and groundwater and be incorporated into the food chain. It is difficult to predict how long it would take for this to occur. As a result of chemical weathering, DU projectiles lying on the ground or buried under the surface will corrode with time, slowly converting the metallic uranium of the DU penetrators into uranium oxides. The specific soil characteristics will determine the rate and chemical form of the oxidation and the rate of migration and solubility of the depleted uranium. This environmental pathway may result in the long term (in the order of several years) in enhanced levels of depleted uranium being dissolved in ground water and drinking water.
Consumption of water and food is a potential long term route of intake of DU. Given this, monitoring of water sources may be a useful means to assess the potential for intake via ingestion. If the levels were considered unacceptable, some form of filtration/ion exchange system could be implemented to reduce levels of DU.