INFLUENCE OF LOW-TEMPERATURE AIR OXIDATION ON THE DISSOLUTION BEHAVIOR OF UO2 AND MOX SPENT FUEL

The dissolution behaviour of pre-oxidised LWR UO2 and MOX spent fuel w as studied by means of semi-dynamic leaching tests. The fuels, irradia ted to burn-ups of 53 and 39 GWd t(-1) respectively, were thermally tr eated at 250 degrees C in air for various times, up to different oxyge n/metal (O/M) ratios. The leaching tests performed in deionised water at ambient temperature revealed, for both UO2 and MOX, an instantaneou s release strongly dependent on the oxidation degree. After this initi al stage, a very low leaching rate, almost independent of the O/M-rati o, 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 consequ ence of migration upon irradiation or to the inherent higher solubilit y of some of these elements. In contrast to fission products, the frac tional release of Pu from the UO2 fuel was not affected by the oxidati on level of the fuel and was one order of magnitude lower than the rel ease of U. For MOX fuel, the intrinsic heterogeneity of the material d ue to Pu as fissile element being concentrated in agglomerates, led to a stronger release of this element relative to the U-matrix for the n on-oxidised sample. After oxidation, which affects almost exclusively the matrix, the instantaneous release rate of Pu is again significantl y lower in comparison to U. (C) 1998 Elsevier Science S.A.