A New Approach to the RN Source Term for Spent Nuclear Fuel under Geological Disposal Conditions
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A New Approach to the RN Source Term for Spent Nuclear Fuel under Geological Disposal Conditions Christophe Poinssot1, Christophe Jegou2, Pierre Toulhoat1, Jean-Paul Piron3 and Jean-Marie Gras4 1 Commissariat à l’Energie Atomique CEA, Fuel Cycle Division, Department for Waste Storage and Disposal, CEA-Saclay, BP11, F-91191 Gif-sur-Yvette Cedex 2 Commissariat à l’Energie Atomique CEA, Fuel Cycle Division, Department for Reprocessing and Vitrification, CEA-Marcoule, BP171 30207 Bagnols sur Cèze 3 Commissariat à l’Energie Atomique CEA, Nuclear Reactor Division, Department for Fuel Studies, CEA-Cadarache, 13107 Saint Paul lez Durance 4 Electricité de France EDF, R&D Division, EDF, Les Renardières, Route de Sens, F-77250 Moret-sur-Loing, France. ABSTRACT Under the geological disposal conditions, spent fuel (SF) is expected to evolve during the 10,000 years while being maintained isolated from the biosphere before coming in contact with water. Under these circumstances, several driving forces would lead to the progressive intrinsic transformations within the rod which would modify the subsequent release of radionuclides. The major mechanisms are the production of a significant volume of He within the UO2 lattice, the accumulation of irradiation defects due to the low temperature which avoids any annealing, the slow migration of radionuclides (RN) within the system by (i) the α self-irradiation-induced athermal diffusion and (ii) locally the building-up of internal mechanical stresses which could turn the pellets into powder. However, the current RN source terms for SF have never accounted for this evolution as they are based on existing knowledge of the fresh SF. Two major mechanisms were considered, the leaching of the readily available fraction (one which was supposed to be instantly accessible to water), and the release of RN through alteration of the UO2 grains. We are now proposing a new RN source term model based on a microscopic description of the system in order to also take account of the early evolution of the closed system, the amplitude of which increases with the burnup and is greater for MOX fuels. 1. Strategic context and evolution of research programs In France, research on nuclear waste management has been ongoing since the end of 1991 in the context of the legislative framework set forth by law. The law identifies the year 2006 as the deadline and three main study areas : (i) partitioning and transmutation (coordinated by CEA) [1], (ii) retrievable or non-retrievable geological disposal (coordinated by ANDRA) [2], and (iii) the design of optimized conditioning processes as well as the study of the waste packages long term behavior and their long term interim storage (coordinated by CEA) [1]. The recent implementation by the French utility company EDF of its new policy, “not to reprocess all of the annual spent nuclear fuel (SNF) production in order to balance the reprocessed and recycled amounts of Pu”, leads to increasing concerns regarding the long term evolution of spent fuel. In 1998 and in line with
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