RN Fractional Release of High Burn-Up Fuel: Effect of HBS and Estimation of Accessible Grain Boundary
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RN Fractional Release of High Burn-Up Fuel: Effect of HBS and Estimation of Accessible Grain Boundary F. Clarens1, D. Serrano-Purroy2, A. Martínez-Esparza3, D. Wegen2, E. Gonzalez-Robles4, J. de Pablo1,4, I. Casas4, J. Giménez4, B. Christiansen2 and J.P. Glatz2 1 CTM Centre Tecnològic, Av Bases de Manresa 1, 08242, Spain 2 JRC-ITU, European Commission Joint Research Centre-Institute of Transuranium Elements, Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldshafen, 76344, Germany 3 ENRESA, Emilio Vargas 7, Madrid, Spain. 4 Chemical Engineering Department, Universitat Politècnica de Catalunya UPC, Av Diagonal 647, 08028, Barcelona, Spain ABSTRACT The so-called Instant Release Fraction (IRF) is considered to govern the dose released from Spent Fuel repositories. Often, IRF calculations are based on estimations of fractions of inventory release based in fission gas release [1]. The IRF definition includes the inventory located within the Gap although a conservative approach also includes both the Grain Boundary (GB) and the pores of restructured HBS inventories. A correction factor to estimate the fraction of Grain Boundary accessible for leaching has been determined and applied to spent fuel static leaching experiments carried out in the ITU Hot Cell facilities [2]. Experimental work focuses especially on the different properties of both the external rim area (containing the High Burn-up Structure (HBS)) and the internal area, to which we will refer as Out and Core sample, respectively. Maximal release will correspond to an extrapolation to simulate that all grain boundaries or pores are open and in contact with solution. The correction factor has been determined from SEM studies taking into account the number of particles with HBS in Out sample, the porosity of HBS particles, and the amount of transgranular fractures during sample preparation. INTRODUCTION The data acquired during the previous European Project, Spent Fuel Stability (SFS) [1] points to a lack of experimental data regarding to IRF, which is considered to govern the dose released over a prolonged period of storage. For this reason, the Performance Assessment (PA) exercises are forced to use a conservative approach to integrate the IRF. IRF is defined as the fraction of RN that can be released “Instantly” or, more accurately, faster than the matrix, when the water comes in contact with the SF after the failure of the canister in the repository. Fuels with burnup (BU) higher than 40 MWd/kgU have an external layer called ‘High Burn-Up Structure’ (HBS) characterized by a higher BU than the core as well as higher porosity and plutonium content. The depth of the HBS increases with BU and depends on the fuel irradiation history [3,4]. The dissolution of this layer has not been studied in detail, yet. There is agreement that radionuclides (RN) present at the gap should be considered as part of the IRF. As a conservative approach, the RN located in the GB and on the pores of the HBS are also included into the IRF. On the other hand, there is not a clear consensus
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