Radionuclides Release Model for Performance Assessment Studies of Spent Nuclear Fuel in Geological Disposal
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Radionuclides Release Model for Performance Assessment Studies of Spent Nuclear Fuel in Geological Disposal Christophe Poinssot, Patrick Lovera, Cécile Ferry CEA Saclay, Nuclear Energy Division, Department of Physics and Chemistry, Service for the Studies of Radionuclides Behavior, BP.11, F-91191 Gif-sur-Yvette Cedex, France; [email protected] ABSTRACT In the framework of the research conducted on the long term evolution of spent nuclear fuel in geological disposal conditions, a source term model has been developed to evaluate the instantaneous release of RN (Instant Release Fraction IRF) and the delayed release of the RN which are embedded within the matrix. This model takes into account all the scientific results currently available in the literature except the hydrogen effect. IRF was assessed by considering the evolution with time of the RN inventories located within the fuel microstructure to which no confinement properties can be allocated on the long term (rim, gap, grain boundaries). It allows to propose some reference bounding values for the IRF as a function of time of canister breaching and burnup. The matrix radiolytic dissolution was modeled by a simple kinetic model neglecting the radiolytic species recombination and the influence of aqueous ligands and radiolytic oxidants were supposed to completely react with the fuel surface. Spent fuel performance was therefore demonstrated to deeply depend on the reactive surface area. INTRODUCTION Direct disposal is being studied in many countries as a possible way to manage Spent Nuclear Fuel (SNF). Although the reprocessing of spent fuel is clearly the reference scenario for the management of spent fuel in France, only two thirds of the total annual budget of spent nuclear fuel is reprocessed in order to equilibrate the Pu mass recovered by reprocessing and recycled in the MOX fuel fabrication: ~350 tHM y-1 are currently stored waiting for further decision, delayed reprocessing, long-term storage or ultimate disposal. These stored fuels are either MOX or relatively high burnup UOX fuels (≥ 47.5 GWd/t). It is therefore of prime importance to determine the spent fuel long-term evolution under conditions prevailing in long-term interim storage (from 0 to 300 y.) and geological disposal (0 to more than 100,000 y.), so that the political and industrial decision on the ultimate fate of spent fuel will be robust [1,2,3]. In particular reliable source terms for SNF has to be assessed to be used in any performance assessment calculations. Performance assessment has specific requirement in the sense that they need to be robust towards the current uncertainty or lack of knowledge on all the relevant mechanisms and parameters. This paper will focus on the case of geological disposal. The SNF source term is normally described as the combination of two terms [4]: (i) an instantaneous release of radionuclides, often referred to as the Instant Release Fraction (IRF). This fraction is usually considered as due to the radionuclides (RN) located within the zones
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