Table 4-9. Possible Factors Influencing Uranium Oxidation

DOE-HDBK-3010-94

4.0 Solids; Metals

<200 oC, the hyperstoichiometric dioxide, UO2 + x, is the principal product. At slightly higher

temperatures, a mixture of various suboxides (e.g. U3O7, U3O8, etc.) are found. At

temperatures >275 oC, UO2 and predominantly U3O8 are produced. In the temperature

range of 350 oC to 600 oC, the UO2 formed rapidly oxidizes to U3O8 that falls away as a

black, fine powder. In the temperature range of 650 oC to 850 oC, the UO2 forms a

protective layer that at some point breaks away. At temperature >900 oC, the UO2 is

adherent and protective. The presence of water vapor accelerates oxidation in air at

temperature <300 oC and in carbon dioxide at temperatures <350 oC to 500 oC. 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.

Table 4-9. Possible Factors Influencing Uranium Oxidation

(Table 4.1 - M ishim a et al. M arch 1985)

Step

Possible Factors Influencing Rate

Metal surface of unit area oxidizing in air

Metal purity

Metallurgical condition (grain size, strains, etc.)

Temperature

Time

Type of oxide film formed (protective or not)

Loss of heat of reaction by conduction to

Thermal conductivity of metal

the surroundings

Thermal conductivity of oxide coating

Cross-sectional area at right angles to direction of heat flow

Temperature gradient

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 oC and is increasing rapidly

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