TAB M -- Characterizing DU Aerosols

Various impact tests have generated widely varying amounts of aerosols. Characterization of Airborne Uranium from Test Firings of XM-744 (sic) Ammunition, published in 1979, reported on one of the first hard impact tests conducted on DU ammunition. This report concluded that as much as 70 percent (i.e. 2.4 kg airborne out of 3.4 kg total) of the 105mm penetrator would turn into aerosol on impact. Although some have frequently cited this 70 percent figure, it is flawed and misleading -- mainly because it was "back-calculated" from cloud data and represented a worst-case scenario (i.e., an impact against a hard target, which was not penetrated).[536] In 1982 the Ballistic Research Laboratory's report Aerosolization Characteristics of Hard Impact Testing of Depleted Uranium Penetrators contradicted the 1979 test results. In the 1982 test, about 3 percent of the DU was aerosolized 2 to 3 minutes after impact. Allowing for error, it is highly unlikely more than 10 percent of the penetrator was aerosolized in the 1982 test, which further found approximately 44 percent of the aerosolized particles were less than 1.1 micrometers (m m) (Aerodynamic Equivalent Diameter) and 70 percent of the aerosolized particles were less than 7 micrometers, i.e., respirable particles.[537]

Hard-impact testing in 1990 of the M829A1 120mm and XM900E1 105mm cartridges produced somewhat contradictory numbers. This study characterized particulate levels after a hard impact with both complete and partial armor penetration. Besides the M829A1 and XM900E1 rounds, the Laboratory used two non-DU rounds, the M865 and DM13, to evaluate DU resuspension during hard-impact tests. Scientists questioned the sample results when the M829A1 aerosolized only about 0.2 to 0.5 percent of the DU and the XM900E1 0.02 to 0.04 percent, because these values were approximately two orders of magnitude below expected values. After comparing Real-Time Aerosol Monitor (RAM) data with RAM data from a previous test, researchers eventually estimated the percent aerosolized was closer to 18 percent -- substantially less than the 70 percent Battelle cited in the 1979 test. The respirable aerosol fraction (by mass), less than 10 m Aerodynamic Equivalent Diameter (AED), was 91 to 96 percent for the M829A1 and 61 to 89 percent for the XM900E1. Evaluation of the respirable dust fraction indicated 24 to 43 percent was Class "D" material and 57 to 76 percent was Class "Y" material, agreeing with other studies indicating a high percentage of the respirable dust from hard-impact testing was soluble in the lungs. (Note: Class "D" materials have dissolution half-times of less than 10 days; Class "W" materials have dissolution half-times of 10 to 100 days; and Class "Y" materials have dissolution half-times greater than 100 days.)[538] The resuspension tests indicated most of the resuspended dust was non-respirable, consistent with the theory the enclosure's filtering system removed most of the respirable dust. DoD laboratories tested DU munitions many times, including those cited here, to characterize aerosol formation and assess potential exposures. As a result of recommendations in the 1995 report, Health and Environmental Consequences of Depleted Uranium Use in the US Army, Battelle's Pacific Northwest Laboratory evaluated existing test data for predicting aerosol exposures. The Battelle report, Evaluation of DU Aerosol Data: Its Adequacy for Inhalation Modeling, identified some technical problems in estimating exposure under various combat scenarios. The report briefly discusses DU aerosol generation scenarios:[539]

  • Fires. During a munitions "cookoff," the burning propellant does not consume oxygen since the propellant supplies its own. Because combustion is so rapid, little if any DU metal oxidizes. Studies have shown in the absence of violent explosions, few of the particles generated during a fire are small enough to be caught up in the thermal currents. The oxides formed during a fire have very low solubility. Most particles produced in a tank fire adhere to the tank's interior walls, but openings (e.g., hatches, holes created by explosions, etc.) can let particles escape into the surrounding atmosphere.

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  • Vehicles Punctured by Projectiles. As noted in other studies, the quantity of oxides formed during impact depends largely on the target's "hardness." The heavier the armor, the more oxides will form as the DU penetrator expends its kinetic energy piercing the armor. During the Gulf War, numerous DU hits on lightly armored vehicles typically left round, golf ball-sized entrance and exit holes. Because these vehicles offered little resistance, DU aerosolization was limited in these cases unless the round struck the engine or a similar obstruction. Conversely, harder targets (e.g., Abrams Heavy Armor Tanks) tended to produce higher quantities of DU aerosols. Aerosolization is enhanced if the penetrator splits into fragments and those fragments remain inside the vehicle. Aerosol levels inside the vehicle also depend on factors such as the number of open hatches and other ruptures or openings. Eventually, particles from inside the tank either adhere to the tank's inside surfaces or release into the atmosphere through any opening. As they accumulate on interior surfaces, the particles' size and mass change.

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  • Entering Contaminated Vehicles. For emergency rescue personnel who enter a tank shortly after impact, the aerosols generated at impact would be the primary threat. These impact aerosol levels usually will be higher than those of resuspended DU particles remaining after the aerosols in the tank have had time to settle or vent through open hatches, etc. For battle damage assessment teams , recovery personnel, or souvenir hunters entering a damaged vehicle, the primary threat is resuspended DU dust. Resuspension depends on the air turbulence inside a vehicle and other conditions (e.g., oily surface walls minimize resuspension). Physical activity inside the vehicle (e.g., lifting or moving equipment or personnel) would obviously increase the level of resuspension.

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  • Inspecting and Repairing Contaminated Vehicles. Entering contaminated vehicles to inspect and repair them can cause significant DU resuspension. Some repair activities (e.g., cutting and welding) increase resuspension levels. Cleaning operations also can cause resuspension.

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  • Routine Combat Activities. Evaluation of DU Aerosol Data: Its Adequacy for Inhalation Modeling also indicated DU penetrators that did not pierce the target or were deflected potentially could expose personnel. The penetrator would be hot enough to generate aerosols so oxides would continue to form, even after the penetrator buried itself in the soil. The report also cited potential exposure to personnel near the target at impact or exposed to resuspended dust from subsequent activities on, in, or near the target.

Two more recent tests conducted since Battelle's raise questions about the nature and extent of respirable particulates generated during fires and hard-impact testing. In June 1995, the Army fired 120mm and 25mm DU munitions at Soviet armored vehicles. Although technical and procedural difficulties seriously affected the data and limited the conclusions that could be drawn from the test, the Draft report was able to cite several key findings, among them:

The second test was the 1994 burn test of a Bradley Fighting Vehicle (BFV) equipped with TOW anti-tank missiles and 1,125 M919 25mm cartridges, the first time a vehicle with a full DU munition combat load actually was used in a burn test. (Most previous fire data were generated from stack-testing wood or metal shipping crates.) Completely engulfing the BFV, the fire burned vigorously for about an hour, subsided thereafter, but continued to emit a plume over the next five hours; hot spots smoldered into the next day. Of the 1,125 DU penetrators, 625 were accounted for, including 9 live rounds found within a few meters of the test pad. The report indicated a large percentage of the 500 rounds unaccounted for was trapped within the BFV's melted remains and a significant amount of DU oxide mixed with the ash and settled inside and around the vehicle's hull. The researchers detected six DU oxide piles on the vehicle's surface after the fire. Analysis indicated approximately 33 percent of these residual oxide particulates were respirable. However, during the 29 hours of air sampling at various distances the air monitoring filters detected only trace amounts of DU oxide.[541] Although the 33 percent of respirable particulates measured in the piles of DU oxide after the fire is an important consideration in assessing resuspension potential during recovery, further research is needed to determine whether these results are valid.

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