Oxidation of UO2to U3O8in dry air and oxygen at temperatures of 160 to 350deg;C proceeds by a twohyphen;step reaction. In the first step the tetragonal oxide of composition UO2.34plusmn;0.03is 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 coefficientDisDequals;5.5times;10minus;3explsqb;minus;lpar;26 300plusmn;1500rpar;sol;RTrsqb;,where the activation energy is 26.3plusmn;1.5 kcal/mole. The second stage of the reaction proceeds by a process of nucleation and growth.
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