Re: DU scare story from Los Alamos

["Michael S Ford" <MFORD@pantex.com>]

Andrew, the following is from DOE-HDBK-3010-94 (I can email you a .pdf version of the document if you would like.):

4.2.1.2 Uranium

Mishima, et al. (March 1985) reviewed the published literature on uranium behavior under
fire conditions. For natural or depleted uranium or uranium with 235 U enrichment <10%,
the toxic hazard of uranium as a heavy metal is of greater concern than the radiological
hazard. The toxicological hazard from uranium results from transport of inhaled, soluble
uranium compounds to the kidneys. For non-volatile (soluble and non-soluble) materials to
be an inhalation hazard, the size of the particles/aggregates must be 10 µm AED (more
probably 3 µm AED) or less. For normal and depleted uranium, the materials must be
soluble. For uranium with enrichments >10%, the radiological hazard is of concern and the
solubility of the uranium in interstitial lung fluids determines the critical organ. Fire is a
phenomenon that could subdivide uranium metal by conversion to the oxides.

Due to the similarity in matrix spacing, hyperstoichiometric uranium dioxide formed at the
metal-atmosphere interface is adhering and limits oxygen availability. At temperatures
<200 deg C, the hyperstoichiometric dioxide, UO2 + x , is the principal product. At slightly higher temperatures, a mixture of various suboxides (e.g. U3 O7 , U3 O8 , etc.) are found. At
temperatures >275 deg C, UO2 and predominantly U3 O8 are produced. In the temperature
range of 350 deg C to 600 deg C, the UO2 formed rapidly oxidizes to U3 O8 that falls away as a black, fine powder. In the temperature range of 650 deg C to 850 deg C, the UO2 forms a protective layer that at some point breaks away. At temperature >900 deg C, the UO2 is adherent and protective. The presence of water vapor accelerates oxidation in air at
temperature <300 o C and in carbon dioxide at temperatures <350 o C to 500 o C. Uranium
reacts with hydrogen, nitrogen and carbon at elevated temperatures and the presence of
surface inclusions accelerates oxidation.

The presence of some additive used to phase-stabilize uranium (e.g., aluminum, titanium)
may change the first- or second-stage oxidation rates or the break weight (plateau) or prevent transition to protective oxide formation that may result in a single, accelerated oxidation rate. Some of the factors that affect oxidation rates are listed in Table 4-9 taken from the reference document. Measured oxidation rates in air, carbon dioxide and oxygen are available in the reference document, but the oxidation rate during a fire will be the sum of a variety of rates dependent upon local conditions at many sites on the metal surface and is difficult to predict.

Unlike plutonium, uranium is difficult to ignite. The presence of an adherent, protective
layer of hyperstoichiometric dioxide at the interface limits oxygen availability. Also, the
heats of reaction are lower. Figure 4-8 reproduced from the reference document shows the
ignition temperature for uranium as a function of surface area/mass ratio. At surface to mass ratios <1.0 cm2/g, the ignition temperature exceeds 500 deg C and is increasing rapidly indicating that large pieces of uranium are very difficult to ignite as large amounts of
external heat must be supplied and serious heat loss prevented.

The particle size distributions of residual oxides produced under a variety of conditions have also been measured and are shown in the reference document (see Figures 4.8, 4.10 and 4.11). The distribution becomes coarser and the solubility in simulated lung fluid decreases as the temperature increases. Oxidation of the metal at <450 deg C generated a fine, black non-adherent powder. At temperatures around 535 deg C, the oxide was a fine, black powder sintered into lumps. At temperatures >700 deg C, the oxide appeared to be a hard, black scale.

The ARF and RF for three potential accident configurations for thermal stress (airborne
release during the oxidation of uranium at elevated temperatures, airborne release from
disturbed molten uranium surfaces, and airborne release during explosive release of fine
molten metal drops) are covered below.

4.2.1.2.1 Oxidation at Elevated Temperatures. 

Mishima et al. (March 1985) characterized the oxide generated by the April 1983 burn test involving munitions containing depleted uranium (DU) penetrators and reviewed the literature on airborne release.  Tests subjecting munitions to rigorous fire conditions are performed prior to deployment to ascertain the thermal and blast hazards during transport and storage. Twelve 120-mm rounds containing 48 kg of DU as rods ~ 1 in. in diameter by 30" long were subjected to a wood and diesel fuel fire. The rounds cooked-off (i.e., the propellant used flared) and the DU rods were retained in the burning mass at temperatures from 800 to 1100 deg C range for ~ 3 hours. *** No detectible airborne DU was collected by air samplers surrounding the burn at distances <100 m.*** [emphasis added]  Samples of the oxides generated were collected and the particle size distribution, morphology and solubility in simulated interstitial lung fluid were measured.  The fraction of the oxide generated by the burn <10 µm AED ranged from 0.2 to 0.65 wt/o. The fraction of the residual oxide <10 µm AED were predominantly U3 O8 and all in the "Y" class (dissolution halftimes in simulated interstitial lung fluids of >100 days).

The ARF x RF values for uranium during oxidation at elevated temperatures found in the
literature were:

Elder and Tinkle (December 1980):
Air, up to 3.2 m/s, fire: 5E-3
Air/Air-CO2 , 3.2 m/s, 500 deg C: 1E-7
                                       900 deg C: 4E-6

Carter and Stewart (September 1970)
Air, static, molten metal:  4E-4
Free-fall molten drops:    6E-3

'Hope this helps.
v/r
Michael
mford@pantex.com 

"Someday we'll look back on all this and plow into a parked car." 
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