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Oxidation of UO2 to U3O8 in dry air and oxygen at temperatures of 160 to 350°C proceeds by a two-step reaction. In the first step the tetragonal oxide of composition UO2.34±0.03 is formed. In the second step this tetragonal phase is converted to orthorhombic U3O8. The rate controlling process in the first stage is the diffusion of oxygen through the uranium dioxide lattice. The value found for the diffusion coefficient D is D=5.5×10−3exp[−(26 300±1500)/RT],where the activation energy is 26.3±1.5 kcal/mole. The second stage of the reaction proceeds by a process of nucleation and growth.

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The rate of oxidation of high purity zirconium was determined for the temperature r,~nge of 575 ~ to 950~ at 1 arm pressure. Data can be fitted to a cubic law and the rate constant in (ml/cm~)3/sec has been calculated to be k = 3.9 X 106 e -47,2~176 where 47,200 • 1,000 cal/mole is the activation energy for the reaction.