NON-ISOTHERMAL OR NON-ISOBARIC KINETIC MODELLING OF THE REDUCTION OF U_3O_8 INTO UO_2 BY HYDROGEN

摘要

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 U_3O_8 into UO_2 by hydrogen. This transformation involves two intermediate compounds, U_3C_(8-x) and U_4O_9. The principle of the method was based on the kinetic modelling of the three reactions by means of the rate equations dα_i/dt=φ_i(T,P)E_i(α), for i=1 to 3, with φ_i(T, P), in mol.m~(-2).s~(-1), and E_i, 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 φ_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 dα_i/dt, and successfully compared to the experiments performed in non-isothermal or non-isobaric conditions.