HIGH-TEMPERATURE STEAM OXIDATION OF UO2 FUEL PELLETS

In order to evaluate the behavior of steam oxidation of UO2 pellets af ter the occurrence of primary failure, steam oxidation tests were cond ucted from 800-1200 degrees C in various atmospheres of H2O, H-2/H2O o r H2O2/H2O, using unirradiated and irradiated UO2 pellets. The change in O/U ratio was analyzed by means of a surface reaction controlled mo del and the oxidation rate was formulated in terms of exposure time, s urface-to-volume ratio and temperature. The effects of liberated hydro gen and hydrogen peroxide on the steam oxidation of UO2 fuels were eva luated. The presence of small amounts of hydrogen in the steam decreas ed only the equilibrium O/U ratio, due to the low oxygen potential in this atmosphere, but had no effect on the surface exchange coefficient . On the other hand, when the concentrations of hydrogen peroxide in w ater were more than 10 vol.%, the surface exchange coefficient was one order of magnitude higher than in pure steam. The steam oxidation pro cess of the irradiated pellets was also controlled by a reaction at th e solid/gas interface. The surface-to-volume ratio of irradiated UO2 p ellets, which was evaluated from the measured oxidation curves, was ap proximately 2.6 times that of unirradiated fuel pellets. (C) 1997 Else vier Science B.V.