Consequences of the Anticipated Long-Term Evolution of Spent Nuclear Fuel for the Assessment of the Release Rate of Radi
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Consequences of the Anticipated Long-Term Evolution of Spent Nuclear Fuel for the Assessment of the Release Rate of Radionuclides. Christophe POINSSOT1, Patrick LOVERA1, Cécile FERRY1, Jean-Marie GRAS2 Commissariat à l’Energie Atomique, CEA Saclay, Nuclear Energy Division, Department of Physics and Chemistry, Service for the studies of the Radionuclides behaviour, BP11, F-91191 Gif-sur-Yvette Cedex 2 Electricité de France, R&D Division, Les Renardières,77 Moret-sur-Loing, France
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ABSTRACT The research conducted in the framework of the French research project on spent nuclear fuel (SNF) long - term evolution (PRECCI Project) has enlightened the potential significance of spent nuclear fuel intrinsic evolution in closed system for the assessment of radionuclide (RN) source term in long-term storage or geological disposal. Beyond others, alpha self-irradiation enhanced diffusion and evolution of the grain boundaries cohesion are two major processes which have to be accounted for in view of the RN source term models development. Accounting for these processes, operational models are developed, the aim of which is to quantitatively define the RN release rates from SNF in long-term storage or geological disposal. They distinguish basically an instantaneous contribution (IRF in geological disposal) and a timedependent contribution (matrix oxidation or alteration). RN inventories associated to these two different processes have to be modeled since they are time-dependent due to the RN diffusion within the pellet. The present paper details the models that are developed in France in terms of assumptions, conservatism and robustness. It comes out from this work that for the instant release fraction, we have to consider a much higher instant release fraction than classically assumed (5-6% in geological disposal) in particular for geological disposal. CONTEXT AND APPROACH 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 (SNF) is reprocessed in order 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. In this context, it is of prime importance to determine the spent fuel long-term evolution under conditions prevailing in long-term interim storage (0 to 300 y.) and geological disposal (0 to > 10 000 y.); so that the political and industrial decision on the ultimate fate of spent fuel will be robust [1]. In particular, models predicting the potential release of radionuclides (RN) as a function of time and environment conditions (the so-called RN source terms) are mandatory to assess the safety and risk of any scenario. We demonstrated in previous papers the necessity for developing new RN source term for geological disposal [1;2;3]. This paper extends this new concept to any scenario (p
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