DIFFUSIVITIES OF FISSION-GAS SPECIES IN UO2 AND (U,GD)O-2 NUCLEAR-FUELS DURING IRRADIATION

An experiment has been performed to determine fission gas diffusion co efficients in gadolinia-doped UO2 nuclear fuels under irradiation. Sma ll specimens of UO2 containing 0, 4 and 8 wt% Gd2O3 were irradiated in the gas sweep rig of the DIDO reactor and fission gas release rates w ere measured as a function of temperature and sweep gas oxygen potenti al. The fission gas diffusion coefficient in (U,Gd)O-2+/-x under irrad iation increased with temperature above 800 degrees C. Below 800 degre es C, the diffusion coefficient appeared to be independent of temperat ure. Over the range studied here, the fission gas diffusion coefficien t was sensitive to oxygen potential in UO2, less sensitive in 4 wt% Gd 2O3, and independent of oxygen potential in 8 wt% Gd2O3. The fission g as diffusion coefficient in (U,Gd)O-2+/-x was judged to be independent of (GdO3)-O-2 content at the same O/M ratio and also appeared to be i ndependent of O/M in the hypostoichiometric region, whilst it increase d sharply with increasing O/M in the hyperstoichiometric region.