Reduction of mixed NiO-U3O8 oxides by hydrogen and the effect of gamma radiation on the process

Abstract

The reduction of mixed NiO-U3O8 oxides of various composition by hydrogen was studied in the temperature range 350-590 °C using the thermogravimetric method. An addition of nickel oxide in the amount of few per cent increases the reduction rate of U3O8 and substantially decreases the initial temperature of its reduction. The reduction of pure U3O8 proceeds with a maximum and steeply decreasing rate in the region of the oxide composition U3O8 → UO2.59 and with a constant rate in the region UO2.59 → UO2.25. In the heterogeneous two-component system the reduction rate increases with the increasing content of nickel oxide. The reduction of the stoichiometric NiO is retarded by a small homophaseous addition of U3O8. Radiation oxidation of U4+ ions proceeds during the pre-irradiation of the samples with gamma radiation at a dose of 1 MGy and the reduction rate of samples with a surplus of U3O8 is decreased. With the increasing content of nickel oxide the negative effect of radiation is continuously changed into a positive effect (an increase of the reduction rate). The reduction kinetics of irradiated samples is in both homogeneous extreme regions of composition governed by the adsorption processes and in the heterogeneous region it depends on the radiation-induced defects with strongly temperature-dependent stability.