Voronin A.M., Voronin A.A.

Almaty Technological University, Kazakhstan

Munitions with depleted uranium

From the 1990s, depleted uranium (DU) became an additional source of environmental contamination in areas of local military conflicts, such as the Gulf War (1991), the hostilities in Bosnia and Herzegovina (1994), Serbia (1999), and finally Iraq (2003).

The natural mixture of uranium isotopes includes ²³⁴U (0.0054%), ²³⁵U (0.72%), and ²³⁸U (99.27%). ²³⁴U and ²³⁸U are members of the uranium series; in nature they are usually close to the state of secular equilibrium. ²³⁵U is the ancestor of another family of natural radionuclides, the actinium-uranium series.

It is ²³⁵U that is used to produce nuclear fuel and nuclear weapons. As its concentration in the natural isotopic mixture is low, one has to enrich uranium with this isotope, and DU is a waste of this process. DU was a subject of extensive environmental studies, several scientific conferences as well as numerous publications in the scientific literature.

DU is used in military purposes due to the facts that it is a very dense metal (its density is about 19 g/cm³), it has a high melting point (1132 °C), great pyrophoricity, and tensile strength that is commensurable with that of most types of steel. These features make it ideal for use in armour-piercing ammunition and heavy armour.

DU dust, formed during the collision with a target, may dissipate and pollute the environment. On estimate usually 10–35 % (maximum up to 70%) of DU penetrator is converted into aerosols under a collision or combustion. Most dust particles are lesser than 5 µm in size, so they are suspended in the air for a long time and are carried by the wind. DU dust is of black colour and the target, struck by DU munitions, can often be recognized by the black dusty coating on the surface and around the target. After firing with munitions of DU, the last settles down on the ground and other surfaces as uranium oxide dust. According to researches conducted at the test sites in the U.S., the most part of the settled DU dust falls within 100 m from the target. However, the DU dust can also be transported up to 40 km and stay in the air for a considerable time. Most warheads fallen on soft soil (e.g., sand or clay) penetrate it to a depth of 50 cm, remaining intact for a relatively long time. An impact of DU warhead with soft targets, such as soil or non-armoured vehicles, does not cause significant dust pollution. Weathering of DU warheads depends essentially on the chemical properties of soils and rocks. In quartz sand, granite, or acidic volcanic rocks dissolution rate may be high enough to cause local contamination of groundwater. Wind and water redistribute fine DU dust. Adsorption on soil particles, mainly clay and organic matter, can alter DU mobility and reduce the risk of re-suspension. The main potential hazard to the environment from intact warheads or their large fragments consists in possible contamination of groundwater after weathering.

A use of DU in civilian purposes is mostly limited by production of stabilizers for aircrafts and vessels [1].

It is estimated that by now there are accumulated about 600,000 tons of DU in the U.S. alone. About 320 tons of DU were dispersed in the environment during the Gulf War in the early 1990s, and about 15 tons of DU were used in the Balkans a few years later.

It is believed that, contrary to public belief, a major health hazard is not associated with the radioactivity of DU, but, like other heavy metals, with its chemical toxicity (mainly affecting kidneys). However, the DU, resulting from the reprocessing of irradiated nuclear fuel used in nuclear reactors, contains a wide range of transuranium radionuclides (Table 1), which increases its radiation hazard.

Table 1

Components of 1 g of DU in the anti-tank munitions
made by the Starmet Corp., U.K. [2]

Nuclide	Half-life, years	Mass, g	Specific activity, Bq/g
Uranium-238	4.51·10^9	0.9979	12,448
Uranium-235	7.1·10^8	1.2·10^–3	159.6
Uranium-234	2.47·10^5	1·10^–5	2,310
Uranium-236	2.39·10^7	3·10^–6	7.2
Neptunium-237	2.44·10^6	2.2·10^–7	5.8
Plutonium-238	8.6·10^1	5.2·10^–12	3.3
Plutonium-239 & Plutonium-240	2.44·10^4 6.5·10^3	1.2·10^–9	6.4
Americium-243	7.95·10^3	1·10^–9	7.4
Americium-241	5.58·10^2	1.7·10^–11	2.2

Traces of ²³⁶U and ^239&240U were found in DU warheads collected in Kosovo. It was also reported that DU has trace amounts of Am, Np, and ⁹⁹Tc [3].

Traces of ²³⁶U (<0,003%) are the result of cross-contamination that occurs when the same equipment is used for the non-irradiated and irradiated uranium. This isotope is formed in the capture of neutrons by ²³⁵U, and it is accumulated to high concentrations in the nuclear fuel.

Radioactive components of DU decay primarily by the emission of alpha-particles, which do not penetrate through any clothing or the skin. So the radiation hazard from external exposure by DU is minimal.

The risk to health can however occur through receipts from food or inhalation of aerosols or particles that are formed in combustion of DU shells and armour after an impact or penetration of fragments in the soil or other surfaces. In general the radionuclides may enter the respiratory tract in various forms such as gaseous compounds, aerosols, or particles. They all have different physical and chemical properties and therefore affect the lungs differently. Especial importance should be attached to the size of radioactive particles. Large ones (5–30 µm in diameter) are usually deposited in the upper part of the respiratory tract, whereas small particles (~ 1 µm in diameter) can reach the lower parts of the respiratory system to settle in the alveoli that leads to a high local dose of surrounding cells and tissues. Thus the small particles of DU (less than 2.5 µm) can remain in the lungs for a long time, exposing the lung tissue to radiation, and even go into circulation systems with biological half-life of about 1 year [3].

Pollution of the air and atmospheric precipitation can be estimated either by direct sampling of suspended particles with air filters or by using suitable biological indicators. The former approach provides a quantitative evaluation and information on the transport of pollutants. The latter one provides an inexpensive and reliable means of evaluation of air quality as well as information on the pollution levels in the past [3].

In the summer of 1999, Kerekes et al. [4] applied an air sampler, placed in the southern part of Hungary, to estimate the amount of DU in the air. Isotopic ratios showed values very close to those expected for natural uranium. However, they found slightly higher concentration of ²³⁸U in particles smaller than 2.5 µm, that is connected with scattering of DU dust during the war in Kosovo.

Lichens and mosses are considered to be suitable biological indicators of atmospheric deposition of trace elements. As they get most of their nutrients directly from the atmosphere, they are used for the evaluation and monitoring of air quality in many countries.

Morphology of lichens is not subject to seasonal variations, they usually live long, and accumulation of pollutants lasts years. Due to the absence of roots lichens have no access to the soil nutrients and, consequently, they are capable to accumulate various elements, including uranium, mainly by absorbing suspended particles from the air. However, we know only a few works devoted to uranium accumulation by lichens. It occurs both in wet and dry conditions from airborne particles and dust, and even small pieces of lichen can contain easy detectable concentrations of uranium. Recent international field studies of lichens on the bark of living trees in Kosovo, in areas close to the ‘target area’, determined the presence of DU, hence it previously presented in the air even where soil contamination is not detected. This example highlights the usefulness of lichens as sensitive biological indicators for areas where DU munitions were used. In localities where there are no lichens, similar results were obtained in a study of the surface of tree bark.

One of the sensitive technique for determining the amount of DU, received via the respiratory tract, is connected with measurement of uranium that is excreted with the urine. But there could be very large uncertainties in the estimated income, because there are many assumptions made about the size of aerosol particles, uranium solubility, and the rate of transfer between different parts of the body. An important obstacle lies in natural uranium that is contained in food and water [3].

On average, a person excretes with the urine from 0.01 to 0.4 µg of uranium daily (depending on its income with food). To estimate income of DU, one should measure an isotopic ratio ²³⁵U/²³⁸U. For studies of this type it is often used a mass spectrometer with high resolution that has a detection limit of DU in the urine not less than 0.01 µg per day.

American soldiers who were wounded by DU fragments during the Gulf War had DU concentration in the urine about 100‑fold higher compared with the normal one even 7 years later [3].

References

1. Assimakopoulos P.A. – J. Environ. Radiact., 2003, v. 64, p. 87–88.

2. Trueman E.R., Black S., Read D. Characterisation of depleted uranium (DU) from an unfired CHARM-3 penetrator. – Sci. Total Environ., 2004, v. 327, p. 337–340.

3. Bleise A., Danesi P.R., Burkart W. Properties, use and health effects of depleted uranium (DU): a general overview. – J. Environ. Radiact., 2003, v. 64, p. 93–112.

4. Kerekes A., Capote-Cuellar A., Köteles G.J. Did NATO attacks in Yugoslavia cause a detectable environmental effect in Hungary? – Health Phys., 2001, v. 80, p. 177–178.