Depleted Uranium

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).

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:

• ICRP Publication 23: International Commission on Radiological Protection, Reference Man: Anatomical Physiological and Metabolic Characteristics. ICRP Publication 23, Pergamon Press, Oxford (1975)
• RAND Report: Author(s): Harley N. H, Foulkes E. C., Hilborne L. H, Hudson A., Anthony C., R., A Review of the Scientific Literature as It Pertains to Gulf War Illnesses. Vol. 7, Depleted Uranium. RAND Report MR-1018/7-OSD (1999)

For information on average human doses, see:

• UNSCEAR Reports: UNITED NATIONS, Sources and effects of Ionizing Radiation, Report to the General Assembly with Scientific Annexes, United Nations Scientific Committee On The Effects Of Atomic Radiation, (UNSCEAR), UN, New York (1988, 1993, 1996, 2000).

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:

• http://www.gulflink.osd.mil/du_ii/du_ii_s03.htm#2 and
  http://www.gulflink.osd.mil/du_ii/du_ii_s03.htm#TAB C - Properties and Characteristics of DU.
• http://www.nato.int.
• IAEA Report

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.

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:

• Scripsick, R.C., Crist, K.C, Tillery, M.I., Soderholm, S.C., Differences in in vitro dissolution properties of settled and airborne uranium material, Report presented at Conference on occupational radiation safety in mining, Toronto, Ontario (Canada) 15-18 Oct 1984, Los Alamos National Lab, NM (USA) (1984).

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.