Influence of low temperature air oxidation on the dissolution behaviour of UO2 and MOX spent fuel

The dissolution behaviour of pre-oxidised LWR UO2 and MOX spent fuel was studied by means of semi-dynamic leaching tests. The fuels, irradiated to burn-ups of 53 and 39 GWd t-1 respectively, were thermally treated at 250 C in air for various times, up to different oxygen/metal (O/M) ratios. The leaching tests performed in deionised water at ambient temperature revealed, for both UO2 and MOX, an instantaneous release strongly dependent on the oxidation degree. After this initial stage, a very low leaching rate, almost independent of the O/M-ratio, was measured. For both fuel types, the fission products considered showed a fractional release normalised to that of uranium higher than 1, due to either the larger inventory at grain boundaries as a consequence of migration upon irradiation or to the inherent higher solubility of some of these elements. In contrast to fission products, the fractional release of Pu from the UO2 fuel was not affected by the oxidation level of the fuel and was one order of magnitude lower than the release of U. For MOX fuel, the intrinsic heterogeneity of the material due to Pu as fissile element being concentrated in agglomerates, led to a stronger release of this element relative to the U-matrix for the non-oxidised sample. After oxidation, which affects almost exclusively the matrix, the instantaneous release rate of Pu is again significantly lower in comparison to U. (orig.)