Reduction by hydrogen of U3O8 into UO2: Nucleation and growth, influence of hydration

The reduction of U3O8 by hydrogen has been studied by thermogravimetry in t he range 510-600 degrees C and 1-40 kPa hydrogen partial pressure. The kine tic curves have been interpreted by a new model involving steps of nucleati on and growth in which the rate limiting step of growth is located at the e xternal interface. This allowed us to determine the variations in the areic nucleation frequency and the areic reactivity of growth as a function of h ydrogen pressure and temperature. Mechanisms have been proposed which accou nt for the variations in the areic nucleation frequency and reactivity of g rowth as a function of the partial pressure of hydrogen and the temperature of reduction. The influence of the hydration of UO3 prior to its reduction by hydrogen on the kinetics could also be explained by the same model. It was found to lower the areic nucleation frequency without changing the arei c growth reactivity.