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Review of the Toxicologic and Radiologic Risks to Military Personnel from Exposures to Depleted Uranium During and After Combat
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 “self-sharpening” 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 Persian 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 conflicts and civilians living near the battlefields. Because of the concern about health effects, the U.S. Army commissioned the report Depleted Uranium Aerosols Doses and Risks: Summary of U.S. Assessments—hereafter referred to as the Capstone Report—which evaluates the health risks associated with exposure to DU.
NATURAL URANIUM
Uranium is a silvery, dense, weakly radioactive metal. When finely divided, 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
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Review of the Toxicologic and Radiologic Risks to Military Personnel from Exposures to Depleted Uranium During and After Combat
TABLE 1-1 Natural Uranium Concentrations
Location
Uranium Concentration
Reference
Seawater
3.2 ppb
Faure and Mensing 2005
Earth’s 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 238U. 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 decay 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 processed to increase the percentage of 235U. The process is called uranium enrichment, and it has two end products: enriched uranium, in which the 235U concentration 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 processing—such as fission products,1 transuranic elements,2 and other trace contaminants—can 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 products; 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.
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Review of the Toxicologic and Radiologic Risks to Military Personnel from Exposures to Depleted Uranium During and After Combat
TABLE 1-2 235U and 238U Decay Seriesa
Nuclide
Half-life
Decay Mode
Energy (MeV)
Nuclide
Half-life
Decay Mode
Energy (MeV)
238U
4.5 × 109 y
Alpha
4.198
235U
7.0 × 108 y
Alpha
4.596
234Th
24 d
Beta
0.199
231Th
26 h
Beta
0.390
234mPa*
1.2 min
Beta
2.271
231Pa
3.3 × 104 y
Alpha
5.059
234U
2.5 × 105 y
Alpha
4.775
227Ac*
22 y
Beta
0.045
230Th
7.5 × 104 y
Alpha
4.688
227Th
19 d
Alpha
6.038
226Ra
1,600 y
Alpha
4.784
223Ra
11 d
Alpha
5.871
222Rn
3.8 d
Alpha
5.490
219Rn
4.0 s
Alpha
6.819
218Po*
3.1 min
Alpha
6.002
215Po*
1.8 ms
Alpha
7.386
214Pb
27 min
Beta
1.023
211Pb
36 min
Beta
1.373
214Bi*
20 min
Beta
3.272
211Bi*
2.1 min
Alpha
6.623
214Po
160 μs
Alpha
7.687
207Tl
4.8 min
Beta
1.423
210Pb*
22 y
Beta
0.064
207Pb
Stable
210Bi*
5.0 d
Beta
1.163
210Po
140 d
Alpha
5.304
206Pb
Stable
aEach 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 “blending down” 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 uranium 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 reprocessed 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).
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Review of the Toxicologic and Radiologic Risks to Military Personnel from Exposures to Depleted Uranium During and After Combat
TABLE 1-3 Important Fission Products and Transuranic Elements Found in Spent Reactor Fuel
Nuclide
Half-life
Radiation Emitted
Energy (MeV)
Activity in Spent Light-Water Reactor Fuel, 180 Days after shutdown (GBq/tonne)
90Sr
28.76 y
Beta
0.546
3.03 × 106
2.28a
91Y
59 d
Beta
1.543
5.55 × 106
95Zr
64 d
Beta
0.366, 0.399
8.87 × 106
Gamma
0.724, 0.757
99Tc
2.1 × 105 y
Beta
0.294
525
106Ru
1.02 y
Beta
0.0394
1.28 × 107
129I
1.6 × 107 y
Beta
0.152
1.19
Gamma
0.30
131I
8.04 d
Beta
0.606
7.04
Gamma
0.364
134Cs
2.06 y
Beta
0.658
8.39 × 106
Gamma
0.605, 0.796
137Cs
30.2 y
Gamma
0.662b
4.01 × 106
144Ce
284 d
Beta
0.318, 0.185
2.89 × 107
239Pu
24,000 y
Alpha
5.16
1.21 × 104
240Pu
6570 y
Alpha
5.17
1.76 × 104
241Pu
14.4 y
Beta
0.0208
3.90 × 106
241Am
432 y
Alpha
5.49
6034
Gamma
0.595
aFrom 90Y progeny.
bFrom 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 depends primarily on two factors: the length of the penetrator and its density compared 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 “mushrooming” as they pass through armor, and this limits their penetrating ability. DU’s other advantage as a weapon is that it is pyrophoric: the small particles created when a DU weapon penetrates a vehicle spontaneously 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
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Review of the Toxicologic and Radiologic Risks to Military Personnel from Exposures to Depleted Uranium During and After Combat
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 service 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 Corps’s 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
Weapons Platform
Mass of DU Fired (kg)
Persian Gulf War (Marshall 2005)
Balkans (DOD 2001)
Iraq War (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
aUSAF (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) reports 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.
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Review of the Toxicologic and Radiologic Risks to Military Personnel from Exposures to Depleted Uranium During and After Combat
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 mixture will change according to the location struck by the penetrator and the materials incorporated in the plume that reaches the crew compartment. The substances 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 soldiers 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 surviving 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 aerosols—which vary in particle size, chemical composition, and solubility—fill a struck vehicle. Soldiers in or on the tanks and fighting vehicles and those who entered immediately after the vehicles were struck may have inhaled 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 operating in the vicinity and involved in cleanup operations. Other military personnel may have been exposed to DU by inhalation or ingestion of DU residues resuspended by their activities when they entered Iraqi vehicles damaged by DU munitions.
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
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Review of the Toxicologic and Radiologic Risks to Military Personnel from Exposures to Depleted Uranium During and After Combat
combatants and eyewitnesses, reconstruction of military operations, and consultations 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 describe the military activities that gave rise to particular types of exposures; they are briefly described below (OSAGWI 2000).
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.
Level II. Soldiers and civilian employees who worked in or around vehicles that contained DU fragments and particles and soldiers who were involved in cleaning up DU residues from Camp Doha’s North Compound after the July 1991 explosion and fires—they may have inhaled DU residues that were stirred up by their activities on or in the vehicles, ingested DU after transferring it from hand to mouth, or spread contamination on their clothing.
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 intake 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 areas, the Army asked the National Research Council’s Committee on Toxicology (COT), in collaboration with its Nuclear and Radiation Studies Board, to review
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Review of the Toxicologic and Radiologic Risks to Military Personnel from Exposures to Depleted Uranium During and After Combat
the Capstone Report. As a result of the Army’s 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, radiologic, epidemiologic, and toxicokinetic data on DU and to assess the Capstone Report’s estimates of toxic and radiologic risks to soldiers posed by exposure to DU. In preparing its report, the committee considered health-hazard and environmental reports prepared by such organizations as the World Health Organization, 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 deficiencies and offers recommendations for future research.
To prepare its report, the committee held several meetings and independently reviewed a large body of written material on health effects of DU. The available data included numerous research articles, literature reviews, and unpublished 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 assays, animal research, and human studies and involved assessing whether multiple lines of evidence indicate a human health risk. The collective evidence was considered in perspective with exposures to DU that are likely in combat. Overall, the committee found the literature to be consistent with findings of other organizations, such as the Institute of Medicine and the Royal Society, that identified the primary health concerns associated with DU exposure to be toxic effects 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 encountered by U.S. military personnel during the Gulf War and verified the exposure 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 mathematical 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. Chapters 3 and 4 discuss the toxic effects of uranium on the kidney and lung, respectively, and Chapter 5 the possible effects on other organ systems. Chapter 6 evaluates radiologic effects of DU. Chapter 7 evaluates studies on the carcino-
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Review of the Toxicologic and Radiologic Risks to Military Personnel from Exposures to Depleted Uranium During and After Combat
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 committee’s 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.
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