Static dissolution tests of α-doped UO₂ under repository relevant conditions: Influence of Boom Clay and α-activity on fuel dissolution rates

Since reprocessing is no longer the reference policy in Belgium, studies on the direct disposal of spent fuel in a clay formation have gained increased interest in the last years. In order to determine to what extent the clay properties and the α-activity influence the dissolution kinetics of spent fuel for the long term disposal, static dissolution tests have been performed on 5 different types of α-dbped UO₂, representing a PWR fuel with a burn-up of 45 or 55 GWd · tHM^-1 and fuel ages ranging between 150 and 90,000 years, in different Boom Clay (BC) media at room temperature and in an anoxic atmosphere for 90 to 720 days. The uranium activity in the clay fraction over time was found to be much higher than the U-activity in the leachates, which has been mainly ascribed to the high retention capacity of the BC. The average dissolution rate between 0 and 90 days obtained for the 5 types of α-doped UO₂ were all found to be high and quite similar at ∼263 μg ·^-2 · d^-1 and a "longer-term" rate (181 to 720 days) ranging between zero and 15 μg · m^-2 · d^-1. These results suggest that the activity of the fuels does not seem to have an effect on the UO₂ dissolution rates under the considered test conditions. In order to study the partition/redistribution of U during UO ₂ dissolution, sequential extraction experiments were performed. Results of the latter have provided a better mechanistic understanding of BC/spent fuel interaction processes as well as information on the role of the different minerals controlling the U-retention/immobilization.