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1 Introduction and Technical Background Depleted uranium (DU) is a weakly radioactive, chemically toxic heavy metal derived from natural uranium. It is used commercially as an industrial catalyst, as a counterweight for airplane control surfaces, and for radiation shielding, particularly for particle accelerators and radioactive sources used in industrial radiography. The U.S. military uses DU for munitions and for armor on some tanks. DU is well suited as a munition because of its very high density and âself-sharpeningâ nature, both of which help it to penetrate armor. Its high density also makes it an effective shield. It has been used by all branches of the U.S. military since the 1980s, and it has been used on the battlefield in the Per- sian Gulf War, the Balkans, and the Iraq War. When used as an antitank armor-piercing munition, a DU penetrator can create an airborne spray of uranium with particles of various sizes that can be inhaled by the tank crew or escape into the environment. Many think that DU may be responsible for some of the illnesses noted among veterans of the con- flicts and civilians living near the battlefields. Because of the concern about health effects, the U.S. Army commissioned the report Depleted Uranium Aero- sols Doses and Risks: Summary of U.S. Assessmentsâhereafter referred to as the Capstone Reportâwhich evaluates the health risks associated with exposure to DU. NATURAL URANIUM Uranium is a silvery, dense, weakly radioactive metal. When finely di- vided, it is pyrophoric (it burns spontaneously) and forms various oxides. It is the heaviest commonly occurring natural element and is found in virtually all geologic materials and, as a consequence, in virtually all natural waters, plants, and animals, as shown in Table 1-1. Natural uranium consists of three isotopes with atomic masses of about 234, 235, and 238. The three isotopes are chemically indistinguishable, behave 9
10 Risks to Military Personnel from Exposure to Depleted Uranium TABLE 1-1 Natural Uranium Concentrations Location Uranium Concentration Reference Seawater 3.2 ppb Faure and Mensing 2005 Earthâs crust 2.8 ppm Eisenbud and Gesell 1997 Bulk earth 0.01 ppm Faure and Mensing 2005 Human blood 0.074-0.94 ppm Hamilton 1970 similarly in the body and the environment, and decay to stable lead isotopes through a series of radioactive decays. Table 1-2 lists the progeny, decay modes, half-lives, and other relevant characteristics of the two main isotopes, 235U and 238 U. The half-life of 238U is much longer than that of 234Th, and relatively few atoms of 238U will decay on a human timescale, so 238U acts effectively as an inexhaustible source of 234Th. Eventually, an equilibrium at which the rate of production of 234Th will equal its decay rate will be reached (that is, secular equilibrium will be reached). That situation can be generalized to the entire de- cay chain, but reaching such an equilibrium takes a few million years. DEPLETED URANIUM AND POSSIBLE CONTAMINANTS THAT AFFECT ITS CHEMICAL AND RADIOLOGIC TOXICITY Of the two main uranium isotopes, only the less abundant 235U is fissile, that is, capable of supporting a self-sustained chain reaction under particular circumstances. Natural uranium consists largely of 238U, and it must be proc- essed to increase the percentage of 235U. The process is called uranium enrich- ment, and it has two end products: enriched uranium, in which the 235U concen- tration has been increased; and depleted uranium, in which the 235U concentration is lower than 0.72%. Spent reactor fuel can be reprocessed and reintroduced into the uranium- enrichment process. Contaminants that remain in the uranium after chemical processingâsuch as fission products,1 transuranic elements,2 and other trace contaminantsâcan therefore enter the enrichment process. Minor amounts of fission products and transuranic elements were introduced into the uranium- enrichment system in the 1960s and 1970s when the United States reprocessed 1 Uranium fission produces two atoms with unequal atomic mass known as fission products. Most fission products have relatively short half-lives, but a few have half-lives long enough for them to be present in measurable quantities for many years after the initial reaction. 2 Uranium fission produces neutrons that can be absorbed by 238U or other decay prod- ucts; this absorption leads to the production of elements that have atomic numbers greater than that of uranium. Those elements are known as transuranic elements and share some chemical similarity to uranium.
Introduction and Technical Background 11 235 TABLE 1-2 U and 238U Decay Seriesa Decay Energy Decay Energy Nuclide Half-life Mode (MeV) Nuclide Half-life Mode (MeV) 238 235 U 4.5 Ã 109 y Alpha 4.198 U 7.0 Ã 108 y Alpha 4.596 234 231 Th 24 d Beta 0.199 Th 26 h Beta 0.390 234m 231 4 Pa* 1.2 min Beta 2.271 Pa 3.3 Ã 10 y Alpha 5.059 234 5 227 U 2.5 Ã 10 y Alpha 4.775 Ac* 22 y Beta 0.045 230 4 227 Th 7.5 Ã 10 y Alpha 4.688 Th 19 d Alpha 6.038 226 223 Ra 1,600 y Alpha 4.784 Ra 11 d Alpha 5.871 222 219 Rn 3.8 d Alpha 5.490 Rn 4.0 s Alpha 6.819 218 215 Po* 3.1 min Alpha 6.002 Po* 1.8 ms Alpha 7.386 214 211 Pb 27 min Beta 1.023 Pb 36 min Beta 1.373 214 211 Bi* 20 min Beta 3.272 Bi* 2.1 min Alpha 6.623 214 207 Po 160 Âµs Alpha 7.687 Tl 4.8 min Beta 1.423 210 207 Pb* 22 y Beta 0.064 Pb Stable 210 Bi* 5.0 d Beta 1.163 210 Po 140 d Alpha 5.304 206 Pb Stable a Each decay series has multiple branches, each with its own probability. The pathways outlined in this table are the most common. Note: Asterisks indicate branching points from most common pathways. Source: Firestone and Shirley 1996. Reprinted with permission; copyright 1996, John Wiley and Sons. spent reactor fuel. And the United States purchased highly enriched uranium from Russia after the dissolution of the Soviet Union for the purpose of âblend- ing downâ weapons-grade uranium. The Russian uranium may have contained fission and activation products as trace contaminants, and they may have been introduced into the uranium process stream. Those practices have inevitably led to the presence of very small amounts of the contaminants in both enriched ura- nium and DU. Many of the nuclides found in spent reactor fuel and possibly present as contaminants in DU are listed in Table 1-3. Thus, DU does not consist of pure uranium. At least three processes can introduce contaminants: ingrowth of radioactive progeny nuclides due to series decay of 238U (see Table 1-2), the presence of fission products from reprocessed reactor fuel (see Table 1-3), and the presence of transuranic elements from re- processed reactor fuel (see Table 1-3). The presence of radionuclides from all those sources is noted in environmental-monitoring reports from uranium- enrichment facilities (see, for example, DOE 2004).
12 Risks to Military Personnel from Exposure to Depleted Uranium TABLE 1-3 Important Fission Products and Transuranic Elements Found in Spent Reactor Fuel Activity in Spent Light- Water Reactor Fuel, Radiation Energy 180 Days after shutdown Nuclide Half-life Emitted (MeV) (GBq/tonne) 90 Sr 28.76 y Beta 0.546 3.03 Ã 106 2.28a 91 Y 59 d Beta 1.543 5.55 Ã 106 95 Zr 64 d Beta 0.366, 0.399 8.87 Ã 106 Gamma 0.724, 0.757 99 Tc 2.1 Ã 105 y Beta 0.294 525 106 Ru 1.02 y Beta 0.0394 1.28 Ã 107 129 7 I 1.6 Ã 10 y Beta 0.152 1.19 Gamma 0.30 131 I 8.04 d Beta 0.606 7.04 Gamma 0.364 134 Cs 2.06 y Beta 0.658 8.39 Ã 106 Gamma 0.605, 0.796 137 Cs 30.2 y Gamma 0.662b 4.01 Ã 106 144 Ce 284 d Beta 0.318, 0.185 2.89 Ã 107 239 Pu 24,000 y Alpha 5.16 1.21 Ã 104 240 Pu 6570 y Alpha 5.17 1.76 Ã 104 241 Pu 14.4 y Beta 0.0208 3.90 Ã106 241 Am 432 y Alpha 5.49 6034 Gamma 0.595 a From 90Y progeny. b From 137mBa progeny. Source: Knief 1992. Reprinted with permission from the author; copyright 1992. MILITARY USES OF DEPLETED URANIUM Modern tanks are protected by heavy armor designed to safeguard their occupants against injury by most weapons. Modern antitank warfare requires the ability to penetrate that armor. The efficacy of armor-penetrating munitions de- pends primarily on two factors: the length of the penetrator and its density com- pared with that of the armor (Marshall 2005). The density of DU (18.95 g/cm3) is about 1.6 times that of lead (11.7 g/cm3) and is similar to that of tungsten (19.3 g/cm3). However, unlike tungsten, DU is self-sharpening: as it penetrates armor, the outer layers peel away. Other materials tend to become dull and blunt because of âmushroomingâ as they pass through armor, and this limits their penetrating ability. DUâs other advantage as a weapon is that it is pyrophoric: the small particles created when a DU weapon penetrates a vehicle spontane- ously ignite and can cause secondary explosions of onboard fuel and munitions (AEPI 1995). DU penetrators were used in combat by U.S. forces for the first
Introduction and Technical Background 13 time in the 1990-1991 Gulf War and are now the most effective antitank weapon in the U.S. arsenal. DU munitions are fired by a number of weapons platforms that saw ser- vice in Iraq, Kosovo, Saudi Arabia, and Kuwait. Airborne weapons platforms include the A-10 Warthog, the A-16 (a ground-attack version of the F-16 fighter aircraft), and the Marine Corpsâs AV8 Harrier. The aircraft fire 30-mm, 30-mm, and 25-mm penetrators, respectively, that contain about 280 g (about 0.6 lb) of DU each. Most DU used in the battlefield was fired by A-10 aircraft using the GAU-8 Gatling gun. Table 1-4 summarizes the use of DU penetrators in the Persian Gulf War, the Balkans, and the Iraq War. The other primary weapons platforms that fired DU penetrators were the M1A1 Abrams tank and the UK Challenger tank, both of which fire a 120-mm, 4,700-g (about 10-lb) penetrator. In addition, some variants of the M1A1 use DU as armor in the forward part of the turret. The only weapon that is able to penetrate DU armor is a DU penetrator, which potentially exposes crews struck by friendly fire to even more DU. COMBAT EXPOSURE TO DEPLETED URANIUM When a DU penetrator strikes a tank, its kinetic energy is directed toward the part of the tank that is struck. The armor around the crew compartment is the portion of most interest for the purposes of this report. That armor comprises metal and ceramic set into a resin, possibly with other layers to add protection and durability. The inside of the turret may be lined with insulation, wiring, and piping that contains a variety of fluids; the turret is painted inside and outside. Thus, as the DU round is entering the crew compartment, it penetrates various materials, including polymers, paints, ceramics, and metals; and the spray of TABLE 1-4 Amount of DU Used in Recent Wars Mass of DU Fired (kg) Persian Gulf War Balkans Iraq War Weapons Platform (Marshall 2005) (DOD 2001) (USAF 2003) M1 tank 1,930 0 0 M1A1 tank 37,293 0 0 U.K. Challenger tank 408 0 0 A-10 aircraft 236,319 11,480 75,282a A-16 aircraft 302 0 0 AV8B aircraft 9,881 0 0 Total 286, 133 11,480 75,282 a USAF (2003) reports that a total of 311,597 30-mm rounds were fired by A-10 aircraft during Operation Iraqi Freedom. The Federation of American Scientists (FAS 2000) re- ports that in combat 80% of the 30-mm rounds fired by A-10 aircraft are DU armor- piercing rounds. Marshall (2005) reports that each 30-mm armor-piercing round contains 0.302 kg of DU.
14 Risks to Military Personnel from Exposure to Depleted Uranium particles that emerges in the compartment contains those materials. Many DU particles themselves ignite spontaneously (they are referred to as fireflies) and may burn or ignite other materials in the compartment; all of them emit fumes. The air in the crew compartment will therefore contain a complex mixture of fumes and particles from a number of sources, and the composition of the mix- ture will change according to the location struck by the penetrator and the mate- rials incorporated in the plume that reaches the crew compartment. The sub- stances may be toxic or carcinogenic, and their inhalation by the tank crew is inevitable. Their presence complicates the task of determining the effects that can be attributed to DU in this environment. Concern about the health effects of combat exposure to DU in the Gulf War developed in response to incidents of so-called friendly fire. About 115 U.S. soldiers in six Abrams tanks that contained DU armor and 14 Bradley fighting vehicles were caught in friendly fire that involved the use of large- caliber munitions containing DU penetrators (OSAGWI 2000). Some of the sol- diers in or on the Abrams tanks and Bradley fighting vehicles when they were hit were injured by DU fragments. Most of the large metal fragments in the sur- viving 104 soldiers were removed during treatment for their injuries, but many small fragments remain embedded in their muscle tissue (AEPI 1995; Hooper et al. 1999). Soldiers involved in the friendly-fire events were also exposed to DU by inhalation, oral, and dermal pathways. Aerosols are created when DU particles ignite, and the aerosolsâwhich vary in particle size, chemical composition, and solubilityâfill a struck vehicle. Soldiers in or on the tanks and fighting vehicles and those who entered immediately after the vehicles were struck may have in- haled fine DU oxide particles or ingested DU oxide dust because of hand-to- mouth contact or by swallowing dust that was coughed up. Exposure may also have occurred by contamination of open wounds, burns, or breaks in the skin with DU oxide dust (AEPI 1995; OSAGWI 2000). Several scenarios in addition to friendly fire led to exposure of soldiers to DU during the 1991 Gulf War. A number of fires involving Abrams tanks and an ammunition explosion and fire at Camp Doha, Kuwait, that burned, oxidized, and fragmented many DU rounds created potential exposure of soldiers operat- ing in the vicinity and involved in cleanup operations. Other military personnel may have been exposed to DU by inhalation or ingestion of DU residues resus- pended by their activities when they entered Iraqi vehicles damaged by DU mu- nitions. EXPOSURE SCENARIOS In 1996, the Office of the Special Assistant for Gulf War Illnesses (OSAGWI) in the Department of Defense (DOD) assumed responsibility for investigating specific DU exposure scenarios in which U.S. soldiers may have been involved. The scenarios were developed through interviews with Gulf War
Introduction and Technical Background 15 combatants and eyewitnesses, reconstruction of military operations, and consul- tations with experts (OSAGWI 2000). An outcome of the investigation was the classification of DU-exposure scenarios based on the predicted risk of health effects as understood at the time. Three exposure levels were established to de- scribe the military activities that gave rise to particular types of exposures; they are briefly described below (OSAGWI 2000). â¢ Level 1. Soldiers in, on, or near combat vehicles when vehicles were struck by DU munitions and soldiers who entered vehicles immediately after they were struck by DU munitions. These soldiers could have been struck by DU fragments, could have inhaled DU aerosols, and could have ingested DU residues, and DU particles could have landed on open wounds, burns, or other breaks in their skin. â¢ Level II. Soldiers and civilian employees who worked in or around ve- hicles that contained DU fragments and particles and soldiers who were in- volved in cleaning up DU residues from Camp Dohaâs North Compound after the July 1991 explosion and firesâthey may have inhaled DU residues that were stirred up by their activities on or in the vehicles, ingested DU after trans- ferring it from hand to mouth, or spread contamination on their clothing. â¢ Level III. Soldiers downwind of burning DU-contaminated equipment, personnel who entered DU-contaminated Iraqi equipment, and personnel who were present at Camp Doha during and after the motor-pool fire but did not take part in cleanup operations in the North Compound. These people could have inhaled airborne DU particles but are unlikely to have received enough to cause health effects. OSAGWI exposure levels I, II, and III were used as the framework for the human health risk assessment in the Capstone Report. The report describes five scenarios (A-E) for level I exposures. Scenarios A-D estimate the DU mass in- take by inhalation for level I soldiers who were in a tank at the time of impact by a DU round. The scenarios vary with the length of time that a soldier remained in the tank. Scenario E involves a level I soldier who entered a tank immediately (up to 5 min) after it was hit to perform a rescue operation, and remained in the vehicle for 10 min. The Capstone Report also included exposure estimates for level II and level III activities that involved Abrams tanks and Bradley fighting vehicles destroyed by DU munitions. COMMITTEE TASK AND APPROACH Because of exposure of many soldiers in Operation Desert Storm and the Balkan War and because of concern about potential future exposures of soldiers, civilian contractor workers, and civilian residents who reoccupy battlefield ar- eas, the Army asked the National Research Councilâs Committee on Toxicology (COT), in collaboration with its Nuclear and Radiation Studies Board, to review
16 Risks to Military Personnel from Exposure to Depleted Uranium the Capstone Report. As a result of the Armyâs request, the National Research Council convened the Committee on Toxicologic and Radiologic Effects from Exposure to Depleted Uranium During and After Combat, which prepared the present report. (see Appendix A for biographic information on the committee members.) The committee was asked specifically to review the toxicologic, ra- diologic, epidemiologic, and toxicokinetic data on DU and to assess the Cap- stone Reportâs estimates of toxic and radiologic risks to soldiers posed by expo- sure to DU. In preparing its report, the committee considered health-hazard and environmental reports prepared by such organizations as the World Health Or- ganization, the UN Environment Programme (for the postconflict Balkans), the International Atomic Energy Commission, the Agency for Toxic Substances and Disease Registry, and the UK Royal Society. It identified relevant data deficien- cies and offers recommendations for future research. To prepare its report, the committee held several meetings and independ- ently reviewed a large body of written material on health effects of DU. The available data included numerous research articles, literature reviews, and un- published data submitted by various sources. Each paper or submission was evaluated on its own merits. The committee used a general weight-of-evidence approach to evaluate the literature, which included an evaluation of in vitro as- says, animal research, and human studies and involved assessing whether multi- ple lines of evidence indicate a human health risk. The collective evidence was considered in perspective with exposures to DU that are likely in combat. Over- all, the committee found the literature to be consistent with findings of other organizations, such as the Institute of Medicine and the Royal Society, that iden- tified the primary health concerns associated with DU exposure to be toxic ef- fects on the kidneys and cancer. The committee therefore focused its evaluation and report on those health end points. The committee thoroughly evaluated exposures that were most likely en- countered by U.S. military personnel during the Gulf War and verified the expo- sure estimates by performing its own independent calculation. It also reviewed and evaluated methods used for generating and measuring aerosols produced by the firing of DU projectiles into Abrams tanks and Bradley vehicles; the mathe- matical models used to calculate exposures to DU through inhalation, ingestion, and skin contact; and the methods used to assess noncancer and cancer risks to exposed military personnel. REPORT ORGANIZATION The remainder of this report is organized into seven chapters. Chapter 2 reviews the toxicokinetics of DU and the biokinetic models related to it. Chap- ters 3 and 4 discuss the toxic effects of uranium on the kidney and lung, respec- tively, and Chapter 5 the possible effects on other organ systems. Chapter 6 evaluates radiologic effects of DU. Chapter 7 evaluates studies on the carcino-
Introduction and Technical Background 17 genicity and genotoxicity of uranium compounds, focusing on studies exploring the evidence for a chemical contribution to the carcinogenic potential of DU. Chapter 8 provides the committeeâs evaluation of the exposure assessment and health risk assessment presented in the Capstone Report. Appendix A provides biographic information on the committee, and Appendix B a detailed description of the epidemiologic studies mentioned in the report.