Radionuclides Release From the Spent Fuel Under Disposal Conditions: Re-evaluation of the Instant Release Fraction
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Radionuclides Release From the Spent Fuel Under Disposal Conditions: Re-evaluation of the Instant Release Fraction Cécile Ferry1, Jean-Paul Piron2, Arnaud Poulesquen1 and Christophe Poinssot1 1 Department of Physico-chemistry, Commissariat à l’Energie Atomique, CEA-Saclay, Gif-surYvette, 91191, France 2 Department of Fuel Studies, Commissariat à l’Energie Atomique, CEA-Cadarache, Saint-Paul Lez Durance, 13108, France ABSTRACT The so-called Instantaneous Release Fraction (or IRF) corresponds to the rapid release of radioactivity by the spent fuel rod at the canister breaching time in repository and depends on the spent fuel evolution in a closed system. The effect of He accumulation on the spent fuel pellet microstructure and diffusion processes are here re-assessed in the light of the recent results issued from the projects Near-Field PROcesses and PRECCI. It allows diminishing the conservatism of the former IRF values of key safety relevant radionuclides for PWR UO2 fuels. INTRODUCTION In repository, a fraction of the radionuclides (RN) inventory will be rapidly released from the spent fuel rod. In the framework of the European “Spent Fuel Stability under Repository Conditions” project [1] the so-called Instantaneous Release Fraction (or IRF) has been defined as the fraction of the inventory of safety-relevant radionuclides, which is located in fuel microstructures for which no confinement properties is anticipated or can be demonstrated at the time of canister breaching [2]. Hence, the IRF depends on (1) the intrinsic evolution of the spent fuel pellet microstructure before the canister breaching (i.e. during~10 ky.); (2) RN distribution within the spent fuel rod after discharge from the reactor and (3) its evolution with time before canister breaching. IRF best and pessimistic estimates have been proposed by Johnson et al. [2] for PWR UO2 fuels. These values take into account uncertainties on the spent nuclear fuel (SNF) evolution during a confinement phase of 10,000 years prior to the canister breaching. Hereafter the IRF values of the key-safety-relevant radionuclides are updated considering the recent results issued from the European NF-PRO and French PRECCI projects on (1) the diffusion processes in spent fuel, (2) evolution of the pellet microstructure with helium accumulation and (3) leaching data. DIFFUSION PROCESSES IN SPENT FUEL Due to diffusion processes, part of the inventory, which is initially located within grains after irradiation, may reach the fuel surface or grain-boundaries and contribute to the IRF. RN mobility processes in spent fuel have been detailed in a recent review [3]. Due to the low temperature expected during disposal, thermal diffusion is not relevant in repository conditions even in the long-term [4]. Regarding athermal diffusion, various modelling approaches have been proposed to estimate the α self-irradiation enhanced diffusion [5]. The upper estimate is
based on extrapolation of the measured in-reactor athermal diffusion coefficient of U atoms under irradiation and yields a r
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