|Year : 2019 | Volume
| Issue : 4 | Page : 123-127
Depleted uranium: Properties and health effects
Anilkumar S Pillai
Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
|Date of Submission||10-Jan-2020|
|Date of Acceptance||11-Jan-2020|
|Date of Web Publication||27-Jan-2020|
Anilkumar S Pillai
Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Pillai AS. Depleted uranium: Properties and health effects. Radiat Prot Environ 2019;42:123-7
| Introduction|| |
Uranium is a naturally occurring, ubiquitous, heavy metal found in various chemical forms in all soils, rocks, seas, and oceans. The earth crust contains an average concentration of 3 ppm of uranium, whereas in seawater, it is 3 ppb. On an average, 90 μg of uranium exists in the human body from natural intake of water, food, etc., It is the heaviest metal that occurs in nature with a density of 18.9 g/cm3. In nature, uranium exists in three major isotopes of238 U,235 U, and234 U. All these isotopes are radioactive. All these isotopes are alpha-emitters. Among the isotopes,238 U and235 U are the parents of two natural radioactive series called uranium and actinium series with so many alpha- and beta-active progenies up to the stable206 Pb and207 Pb isotopes. The radiological properties of uranium isotopes and its isotopic compositions in nature are shown in [Table 1]. The specific alpha-activity of natural uranium due to all the isotopes is 25,278 Bq/g. In nature, uranium mainly occurs in oxidized form. Among the various sources of uranium, phosphate rocks contain a very large concentration of the order of 50–200 ppm. It is also present in other minerals such as lignite, monazite sands, and phosphate fertilizers. In uranium ore deposits, secular equilibrium exists between238 U and its daughter products as well as between235 U and its daughter products. The equilibrium may be sometimes disturbed by geochemical migration processes. When uranium is separated from its ores, the decay chain is broken. Only thorium (234 Th) and protactinium (234 Pa) reach equilibrium with238 U within about 6 months and are the major contributors to the radioactivity of the purified uranium. The uranium ore concentrate produced in the milling process contains a mixed oxide usually referred to as uranium octaoxide (U3O8) (UO2. 2UO3). Due to the presence of number of radioactive and nonradioactive impurities, it needs further refining before it can be used for nuclear fuel production. Typically, one ton of U3O8 is equivalent to 0.848 tons of uranium.
For the use in light-water nuclear reactors, the fissile isotope of235 U contained in natural uranium has to be enriched. For the enrichment process, uranium is needed in the form of uranium hexafluoride (UF6). This is obtained from the uranium ore concentrate by refining and conversion. One ton of UF6 is equivalent to 0.676 tons of uranium. At room temperature, the UF6 is crystalline and solid, but it sublimes at 56.4°C. After enrichment, the waste product from the process is again UF6 where235 U has been depleted significantly and the product is referred to as depleted uranium (DU). It is so named because it has been depleted of its fissile component. Accordingly, it cannot be used to produce either nuclear energy or weapons. Furthermore, DU is 40% less radioactive than natural U, which is itself categorized as being weakly radioactive by international standards. The typical concentration of235 U in DU is about 0.2% by weight that is about 30% of its concentration in natural uranium. As the enrichment process is based on isotopic mass separation, the isotope234 U is depleted to an even lower ratio than235 U because of its lower atomic weight. The light-water reactors require uranium in which the235 U content is enriched typically from 0.72% to about 3%–4% by weight usually called low enriched uranium. Uranium enriched above these concentrations is called highly enriched uranium and commonly used in strategic applications. The DU contains isotopes in typical composition of 99.8%238 U, 0.2%235 U, and 0.0006%234 U. For the same mass, the DU has about 60% of the radioactivity of natural uranium (~14,800 Bq/g of DU).
During the reactor operation, the consumption of235 U will be limited and a sufficient amount of this isotope will be available in the spent fuel. It is possible to extract this fissile isotope of uranium from the spent fuel during the reprocessing stage. The uranium separated from reprocessing of spent fuel is called reprocessed DU or RepDU. The concentration of235 U in RepDU may vary and depends on the type of the reactor and burnup of the fuel from where the RepDU is extracted. The typical concentration of235 U in RepDU obtained during the reprocessing of PHWR spent fuel is about 0.3%. Unlike natural uranium, the reprocessed uranium contained anthropogenic (human-made) radionuclides including the uranium isotope236 U,232 U, small amounts of transuranics (elements heavier than uranium, such as neptunium, plutonium, and americium), and long-lived fission products such as technetium-99. Because of this, there is also a name for this reprocessed DU as dirty uranium. However, in early days of nuclear fuel cycle program, many countries had enriched some reprocessed uranium extracted from spent reactor fuel in order to reclaim the235 U present in the reprocessed uranium. As a result, the DU by-product from the enrichment of reprocessed uranium also contained these anthropogenic radionuclides but at very low levels and traces of fission products also will come as contaminants in the DU because of their production in nuclear reactor. The exact concentration of these contaminants is not reported or known. Radiochemical analysis of many DU samples indicates that these trace impurities are in the parts per billion level and result in less than a 1% increase in the radiation dose from the DU. The presence of236 U and239 Pu/240 Pu in DU has been confirmed by analyses of penetrators collected during the United Nations Environment Program (UNEP)-led mission to Kosovo. The activity concentration of236 U in the penetrators was of the order of 60,000 Bq/kg, whereas the activity concentration of plutonium (239+240 Pu) varied from 0.8 to 12.87 Bq/kg.
| Uses Of Depleted Uranium|| |
DU has a number of peaceful civilian applications. It is employed as counterweights or ballasts in aircraft, as radiation shields in medical equipment, as containers for the transport of radioactive material and as chemical catalysts. DU has also been used in glassware and ceramics (as cooking and serving containers) and dentistry. Most of the applications of DU are based on its material properties such as high density and resistance along with its coloring property of its salts. Uranium has been extensively used for coloring of ceramics and glass. The production of such glasses continued until the middle of the 20th century. DU has been and is still used as the basis of yellow enamel powder used in the manufacture of badges and jewelry. DU has replaced the natural uranium in many such applications. Until 1980, natural and DU have been used for dental porcelains to obtain a natural color, florescence of dentures, and the superficial parts of crowns. Later, uranium has been replaced by rare earth elements such as cerium, terbium, dysprosium, and samarium. Uranium has also been used as a catalyst in certain specialized chemical reactions in chemical industries. The high density and high atomic number of DU make it suitable for the shielding of gamma-radiation. DU has been extensively used in medical, research, and transport sectors as a radiation beam collimators and containers to transport nuclear sources. It is used as a shield for radioactive sources in teletherapy units used in various treatments. Currently, DU has been in use for radioactive waste management as a shielding material. Vessels and equipment such as boats and satellites require a large amount of weight to be carried in the form of ballast. The high density and relative availability of DU make it a potentially suitable material for this use by fulfilling the weight requirements while minimizing the amount of space taken up by the ballast material. Industries in which the use of DU has been cited for this purpose are the aircraft, military, and aerospace industry. DU counterweights have been used in commercial aircrafts on rudders, wing assembly, and tail assembly. Typically, 680–1000 kg of DU has been used in domestic aircrafts such as Boeing, Lockheed, and McDonnell Douglas. Since 1980, many manufactures have replaced DU with tungsten. In civilian life, these uses are generally outside the purview of most people and there is a little name recognition of this metal. It is only in untoward circumstances that the public may become aware of its utilization. This has no impact at all on human health except for those occasions when an airplane crashes and burns. High temperature causes rapid oxidation of DU and spread into local environment. This occurred outside Amsterdam in October 1992 when a Boeing 747-258F cargo plane crashed into two apartment buildings near the runway. The plane carried 282 kg of DU as ballast, but only 130 kg was recovered. It was assumed that the remainder was consumed in the fire which created the possibility that uranium oxide particles could have been produced and dispersed in dust and smoke.
Uranium's physical and chemical properties make it very suitable for military applications, particularly in defense armoring for tanks and other vehicles. DU is used in the manufacturing of ammunitions used to pierce armor plating, such as those found on tanks, in missile nose cones and as a component of tank armor. Armor made of DU is much more resistant to penetration by conventional anti-armor ammunitions than conventional hard-rolled steel armor plate. Armor-piercing ammunitions are generally referred to as “kinetic energy penetrators.” DU is preferred to other metals, because of its high density (19 g/cm3), high melting point (1130°C), pyrophoric nature (DU self-ignites when exposed to temperatures of 600°–700° and high pressures), and its property of becoming sharper, through adiabatic shearing, as it penetrates armor 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, such as the penetrators used in anti-tank weapons, or in aircraft balance weights, will not normally ignite in a fire. DU weapons are regarded as conventional weapons. Ammunition containing DU was used by US forces in the Gulf War (1991), NATO air strikes at Bosnia and Herzegovina (1995), and Kosovo (1999). Health hazard due to inhalation and ingestion of DU aerosol and retention of DU fragments after the firing of munitions has been discussed recently by international organizations such as IAEA and WHO. Postconflict environmental assessment has been initiated by UNEP.
| Detection And Measurement Of Depleted Uranium|| |
Determination of DU in environmental and biological samples requires the measurement of the concentrations of the two isotopes238 U and235 U. For the detection of DU, it is required to use the emission radiation from the different isotopes of uranium in the material. All uranium isotopes of interest in DU are alpha active and associated very low-yield gamma-rays. Only235 U is having some gamma-emission of significant abundance. Furthermore, the immediate progenies of238 U,234 Th, and234m Pa have gamma-emissions of low abundance. It is a common practice of using the gamma-emission of234m Pa at 1001 keV for the estimation of238 U in various uranium-bearing samples. Although this gamma-ray does not interfere with other emissions and is practically free from self-absorption effects, its low emission probability (0.835%) makes it inappropriate for the analysis of environmental samples in which uranium activity is very low. High-resolution gamma-ray spectrometry which is a popular nondestructive analytical technique can be applied to analyze the isotopic composition of uranium isotopes based on the gamma-rays from uranium and its immediate progenies. The method requires the knowledge regarding the buildup of activities of these daughter products (234 Th and234m Pa) after processing of uranium. In the case of235 U, its most probable gamma-ray at 185.7 keV (57.2%) is commonly used. Other gamma-photons from235 U such as 143.76 keV (10.96%), 163.3 keV (5.08%), and 205.3 keV (5.01%) keV also can be used. Because of the high density and effective atomic number in the sample matrix, the self-attenuation effects for the photons will be prominent in these samples. Special emphasis is required in making thin samples for bringing down the attenuation effects in the gamma-spectrometric measurements. Typically, a gamma-dose rate of 2 mSv/h is observed on the surface of the DU material due to these gamma-rays. It is also possible to utilize the beta-emissions from the daughter nuclides for checking the contamination due to isotopes of uranium in various matrices.
Another most common analytical technique which gives information of uranium isotopic composition is alpha-spectrometry. Although these methods give the most accurate results, they are sophisticated and take more time for sample preparation and analysis. They cannot be used as a nondestructive tool to handle the problem. Stringent sample preparation procedures are essential for accurate results in these systems. However, for accurate estimation of the isotopic content in uranium samples, mass spectrometry techniques are widely used. The concentrations of the different uranium isotopes can be accurately measured by inductively coupled plasma mass spectrometry, thermal ionization mass spectrometry, or secondary-ion mass spectrometry.
| Hazards Due To Depleted Uranium|| |
There were reports of the international organization, UNEP, focused on environmental and health impact of DU as a part of postconflict environmental assessment in Bosnia, Kosovo, and Kuwait. DU concentration levels in soil exceeding background levels of uranium were reported close to locations of DU shrapnel or remains of tanks left after military operations. Over time, the DU concentration is dispersed into the wider natural environment by wind and rain. People living or working in affected areas may inhale resuspended contaminated dust.
The average annual normal intake of uranium by an adult is estimated to be about 500 μg from ingestion of food and water and 0.6 μg from inhaling air. Ingestion of small amounts of DU-contaminated soil by small children may occur while playing in postconflict zones. Occasional exposure of DU through the skin contact does not result in any ascertainable health effect. Because DU is only weakly radioactive, chemical toxicity is the prevailing concern. The kidneys are the main site of potential damage from chemical toxicity of uranium. Individuals can be exposed to DU in the same way they are routinely exposed to natural uranium, i.e., by inhalation, ingestion, and dermal contact (including injury by embedded fragments). Inhalation is the most likely route of intake during or following the use of DU munitions in conflict or when DU in the environment is resuspended in the atmosphere by wind or other forms of disturbance. Accidental inhalation may also occur as a consequence of a fire in a DU storage facility, an aircraft crash, or the decontamination of vehicles from within or near conflict areas. Ingestion could occur in large sections of the population if their drinking water or food became contaminated with DU. Dermal contact is considered a relatively unimportant type of exposure since little of the DU will pass across the skin into the blood. However, DU could enter the systemic circulation through open wounds or from embedded DU fragments. Gulf War veterans having embedded DU shrapnel in their bodies were found to excrete elevated levels of urinary uranium even 7 years after their first exposure. It was shown that the DU metal slowly solubilizes in the body fluids and that, several years after the war, blood and urine levels of uranium are elevated by up to two orders of magnitude.
| Body Retention|| |
Most (>95%) uranium entering the body is not absorbed but is eliminated throughout the feces. Of the uranium that is absorbed into the blood, approximately 70% will be filtered by the kidney and excreted in the urine in 24 h. Typically, between 0.2% and 2% of the uranium in food and water is absorbed by the gastrointestinal tract. Soluble uranium compounds are more readily absorbed than those which are insoluble. The most soluble compounds, such as uranyl fluoride and UF6, are absorbed relatively quickly from the lungs and are also absorbed from the gastrointestinal tract into the blood and then cleared through the kidneys. The least soluble compounds are uranium dioxide, uranium peroxide, and U3O8 which may take years before they become solubilized and absorbed into the blood. It has been found that the more soluble compounds are most toxic to the kidneys because they quickly reach higher blood and kidney concentrations, whereas the less soluble oxides produce a larger radiation dose to the lungs and lymph nodes because of their longer exposure. Those that are soluble clear the body quickly, whereas those retained, such as in shrapnel, expose the surrounding tissue over much longer periods and in some cases years.
| Health Effects|| |
Potentially, DU has both chemical and radiological toxicities with the two important target organs being the kidneys and the lungs. Health consequences are determined by the physical and chemical nature of the DU to which an individual is exposed and to the level and duration of exposure. Insoluble inhaled uranium particles, 1–10 μm in size, tend to be retained in the lung and may lead to irradiation damage of the lung and even lung cancer if a high enough radiation dose results over a prolonged period. Respirability refers to the size of the particles that are inhaled. Larger particles do not reach the lungs, but very small dust particles can reach deep into the lungs where they have the capacity to do more damage. Direct contact of DU metal with the skin, even for several weeks, is unlikely to produce radiation-induced erythema (superficial inflammation of the skin) or other short-term effects. Follow-up studies of veterans with embedded fragments in the tissue have shown detectable levels of DU in the urine but without apparent health consequences. The radiation dose to military personnel within an armored vehicle is very unlikely to exceed the average annual external dose from natural background radiation from all sources.
| Assessment Of Intake And Treatment|| |
For the general population, it is unlikely that the exposure to DU will significantly exceed the normal background uranium levels. When there is a good reason to believe that an exceptional exposure has taken place, the best way to verify this is to measure uranium in the urine. The intake of DU can be determined from the amounts excreted daily in urine. DU levels are determined using sensitive mass spectrometric techniques; in such circumstances, it should be possible to assess doses at the mSv level. Fecal monitoring can give useful information on intake if samples are collected soon after exposure. External radiation monitoring of the chest is of limited application because it requires the use of specialist facilities, and measurements need to be made soon after exposure for the purpose of dose assessment. Even under optimal conditions, the minimum doses that can be assessed are in the tens of mSv. There is no suitable treatment for highly exposed individuals that can be used to appreciably reduce the systemic content of DU when the time between exposure and treatment exceeds a few hours. Patients should be treated based on the symptoms observed.
Ten years after first exposure, a small group of Gulf War veterans wounded with DU-containing shrapnel continue to excrete elevated concentrations of uranium in their urine. Urine U concentrations in this group of soldiers are clearly above normal concentrations present in the general population, which occur from exposure to natural U through dietary and drinking sources. The highest urine uranium concentrations in soldiers with fragments are similar to levels reported for a cohort of uranium mill workers in 1975. Other than the frequency of battle injury, which is the method by which shrapnel fragments were inflicted, there is clear absence of a “signature-” specific medical problem shared by this cohort of Gulf War vets. Long-term follow-up studies on military personnel wounded during military operations and living with DU-containing fragments embedded in soft tissue show an elevated level of DU in urine., Most of them have not developed any abnormalities due to uranium chemical toxicity (kidney malfunction) or radiotoxicity (leukemia or cancer). Although uranium released from embedded fragments may accumulate in the central nervous system tissue, and some animal and human studies are suggestive of effects on central nervous system function, it is difficult to draw firm conclusions from the few studies reported. Further research should be performed in relevant areas that would provide a better understanding of DU hazards and health effects.
| References|| |
United Nations Environment Programme. UNEP Final Report: Depleted Uranium in Kosovo. Post-Conflict Environmental Assessment, United Nations Environment Programme; 2001.
Betti M. Civil uses of depleted uranium. J Environ Radioact 2003;64:113-9.
Bleise A, Danesi PR, Burkart W. Properties, use and health effects of depleted uranium (DU): A general overview. J Environ Radioact 2003;64:93-112.
Hooper FJ, Squibb KS, Siegel EL, McPhaul K, Keogh JP. Elevated urine uranium excretion by soldiers with retained uranium shrapnel. Health Phys 1999;77:512-9.
McDiarmid MA, Engelhardt S, Oliver M, Gucer P, Wilson PD, Kane R, et al
. Health effects of depleted uranium on exposed Gulf War veterans: A 10-year follow-up. J Toxicol Environ Health A 2004;67:277-96.
United Nations Environment Programme. Depleted Uranium in Bosnia and Herzegovina. Post-Conflict Environmental Assessment. United Nations Environment Programme; March, 2003.
United Nations Environment Programme. Depleted uranium in Serbia and Montenegro: Post-Conflict Environmental Assessment in the Federal Republic of Yugoslavia. United Nations Environment Programme; 2002.