Leaching behaviour of non-irradiated and irradiated HTR UO 2 -ThO 2 fuel particles under reducing conditions
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Leaching behaviour of non-irradiated and irradiated HTR UO2-ThO2 fuel particles under reducing conditions Cyrille Alliot1 and Bernd Grambow SUBATECH Laboratory UMR 6457 BP 20722 44307 Nantes cedex 3, France ABSTRACT 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
INTRODUCTION High Temperature Reactors (HTR) are considered as promising medium-term alternatives to present light water reactors. The specific advantages mainly result from inherent safety features, from the better energy conversion efficiency at high temperatures, from the reduction of wastes produced per unit of produced energy and from the very stable fuel design, which consists in embedding in a graphite matrix, the fissile material as actinide oxide in form of sealed-coated particles with particle diameters of about 0.5 mm. This concept allows achieving very high burnups. The fuel concept exhibits also certain advantages for the back-end of the fuel cycle. One of the back-end options is an open cycle with direct disposal of the spend fuel in a geological repository. The behaviour of an HTR disposed spent fuel in a deep repository is based on the integrity of the coated particles. Indeed, it has been shown that no radionuclide can be released as long as the coating layers are intact [1, 2]. However, if the coatings fails due to mechanical or chemical interactions, the long-term behaviour mainly depends on the chemical stability of fuel kernels. Fuel kernel stability is probably a function of the redox conditions in the repository. In European repository concepts, oxidizing conditions may result only in the short term from remaining postclosure oxygen traces or from radiolytic decomposition of water by alpha irradiation, and this only, provided that hydrogen from container corrosion is absent. The high radiation field of fresh spent fuel is known to create local oxidizing conditions at the surface. In contrast, a reducing environment is expected to prevail at the disposal location in the long term. The behaviour of 1
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