Effect of additives (Cr2O3, Al2O3, SiO2, MgO) on diffusional release of 133Xe from UO2 fuels

The effect of small amounts of additives on the diffusional release of Xe-133 from UO2 fuel has been examined in the temperature range of 1100-1600 degrees C by means of a post-irradiation annealing technique. The fuel specimens were lightly irradiated to 4 MWd/tU in JRR-4 reactor. Four kinds of additives, i.e. 0.085 wt% Cr2O3, 0.076 wt% Al2O3, 0.085 wt% SiO2 and 0.50 wt% MgO, were used. The Cr2O3, SiO2 and MgO dissolved in the UO2 matrix; their dissolved concentrations were 0.012, 0.045 and 0.08 wt%, respectively. The diffusion coefficient of Xe-133 for the Cr2O3-doped UO2 was about three times larger than that for the undoped UO, at high temperatures of 1500-1600 degrees C, while the value for the SiO2-doped UO2 was about one order of magnitude smaller than that for the undoped UO2 over the whole temperature range. The diffusional fission gas release was not affected by the addition of MgO, although MgO in the matrix was partly present as a large number of small MgO precipitates of 9 nm uniformly dispersed in it. The added Al2O3, which did not dissolve in the matrix, had no effect on the diffusion coefficient. The effect of the soluble additives on fission gas diffusion was discussed in terms of the lattice defect equilibrium model. (C) 1998 Elsevier Science B.V.