Influence of low-temperature air oxidation on the dissolution behaviour of high burn-up LWR spent fuel

High burn-up (53.1 GWd/t(U)) light water reactor (LWR) spent fuel (UO2), previously oxidised in flowing air to O/M ratios varying from 2.0 to 2.44 (U4O9+x), was sequentially leached in deionised water at room temperature. A clear increase in the leaching rate of the matrix and fission products with a high redox sensitivity, like technetium and molybdenum, was observed as a function of the degree of oxidation. This effect is limited to the initial step of leaching and was not observed for plutonium and for subsequent leaching periods, no relevant influence of the O/M ratio on the leaching rate could be detected. For the uranium matrix, a very low leaching rate (<3×10−7 g cm−2 d−1) was measured independent of the O/M ratio. In comparison to the matrix, all the fission products have higher release rates, suggesting a non-congruent leaching of elements segregated to the grain boundaries.