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

High burn-up (53.1 GWd/t(U)) light water reactor (LWR) spent fuel (UO2), pr eviously oxidised in flowing air to O/M ratios varying from 2.0 to 2.44 (U4 O9+x), was sequentially leached in deionised water at room temperature. A c lear 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 lea ching 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 x 1 0(-7) g cm(-2) d(-1)) was measured independent of the O/M ratio. In compari son to the matrix, all the fission products have higher release rates, sugg esting a non-congruent leaching of elements segregated to the grain boundar ies. (C) 2001 Elsevier Science B.V. All rights reserved.