Non-isothermal or non-isobaric kinetic modelling of the reduction of U3O8 into UO2 by hydrogen

A method for the kinetic modelling of non-isothermal and / or non-isobaric successive reactions in powders is presented, and illustrated with the reduction of U3O8 into UO2 by hydrogen. This transformation involves two intermediate compounds, U3O8-x and U4O9. The principle of the method was based on the kinetic modelling of the three reactions by means of the rate equations dalpha(i)/dt = phi(i)(T,P) E-i (alpha), for i=1 to 3, with phi(i)(T, P), in mol.m(-2).s(-1), and F, in m(2).mol(-1). A mechanism involving several elementary steps (interface, diffusion) was proposed for each reaction; then, the determination of the rate-limiting step, due the study of the influence of the hydrogen partial pressure, gave the expressions of the functions phi(i)(T, P) and E-i. Moreover, it was taken into account a particle size distribution for the theoretical expression of the rates. The resulting theoretical rate was obtained by means of a linear combination of the rates dalpha(i)/dt, and successfully compared to the experiments performed in non-isothermal or non-isobaric conditions.