Assessment of the Evolution with Time of the Instant Release Fraction of Spent Nuclear Fuel in Geological Disposal Condi
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Assessment of the Evolution with Time of the Instant Release Fraction of Spent Nuclear Fuel in Geological Disposal Conditions Christophe POINSSOT, Patrick LOVERA, Marie-Hélène FAURE (✝) Commissariat à l’Energie Atomique, Nuclear Energy Division, CEA-SACLAY, Department of Chemical Physics, Service for Physical and Analytical Chemistry, Laboratory for the study of the radionuclides Behavior in their Environment, BP11, F-91191 Gif-sur-Yvette Cedex ABSTRACT Under the geological disposal conditions, spent nuclear fuel (SNF) is expected to evolve during the first thousands years while being maintained isolated from the biosphere before water comes in. Under those circumstances, several driving forces would lead to the progressive intrinsic SNF transformations within the rod which would basically modify the physical and chemical state of the fuel and the subsequent release of radionuclides in solution. In this paper, we briefly summarize the mechanisms we estimate to be significant and propose a new framework for the quantitative assessment of the radionuclide (RN) inventory we estimate to be associated to the classically referred to “Instant Release Fraction” (IRF). We hence demonstrate that in this framework, significantly high IRF values have to be expected for the long term due mainly to the presence of athermal diffusion processes. STRATEGIC CONTEXT AND EVOLUTION OF RESEARCH PROGRAMS In France, the reprocessing of spent fuel is clearly the reference scenario for the management of spent fuel since it allows both to recover some valuable matter and to optimize the waste conditioning in a long term perspective (glass canister). However, only two thirds of the total annual budget of spent nuclear fuel (SNF) is reprocessed since the industrial policy of the French utility Electricité de France (EDF) is to equilibrate the Pu mass recovered by reprocessing and recycled in the MOX fuel fabrication. In this context, roughly 350 tHM.y-1 are currently stored waiting for further decision which can be either reprocessing, or long term storage or ultimate disposal. This new situation leads to increasing concerns regarding the long term evolution of spent fuel in long term storage or geological disposal. Consequently, CEA support since 1998 a wide research program (the acronym of which is PRECCI) the objectives of which are to determine the spent fuel long term evolution under generic conditions prevailing in long term interim storage and/or geological disposal [1]. This program is supported by EDF, the producer of SNF, and is being conducted in close interaction with. One of the major objective of this program is to build the radionuclides (RN) spent fuel source terms for long term storage and geological disposal. In particular, CEA demonstrated in a previous paper the necessity for developing new RN source terms which can intrinsically accounts for the actual spent fuel evolution before the release of radionuclides [2]. This paper aims to detail the conceptual framework which can be used to derive quantitative assessment of the in
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