Leaching behaviour of non-irradiated and irradiated HTR UO2-ThO2 fuel particles under reducing conditions

The dissolution of irradiated HTR UO2-ThO2 fuel particle under reducing conditions was studied using a continuous flow-through reactor and compared to the dissolution of unirradiated fuel particle in the same condition. The irradiated fuel particle was leached for more than eight months. A fast 137Cs release was observed, corresponding to a labile fraction ('instant release fraction'). Then, a congruent leaching of 137Cs and 90Sr was measured corresponding to a matrix dissolution rate equal to 1.7 mg/m2/d. A slower release of 238Pu probably due to sorption phenomena is observed. Dissolution rates are 100-2000 times higher than for unirradiated material. We can conclude that alpha-radiolysis is responsible for this increase due to local oxic conditions. However we cannot exclude that due to irradiation the accessible surface area has been increased as well. The coating of this irradiated particle was equally studied to determine the presence of different radionuclides and their leaching properties of leaching. (author)