Effect of HBS Structure in Fast Release Fraction of 48 GWd/tU PWR Fuel
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Effect of HBS Structure in Fast Release Fraction of 48 GWd/tU PWR Fuel J. de Pablo1,2, D. Serrano-Purroy3, E. Gonzalez-Robles1, F. Clarens1, A. Martinez-Esparza4, D.H. Wegen3, I. Casas2, B. Christiansen 3, J.-P. Glatz3, J. Gimenez2 1 CTM Centre Tecnologic, Av Bases de Manresa 1, 08242, Spain 2 Chemical Engineering Department, Universitat Politecnica de Catalunya UPC, Av Diagonal 647, 08028, Barcelona, Spain 3 JRC-ITU, European Commission Joint Research Centre-Institute of Transuranium Elements, Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldshafen, 76344, Germany 4 ENRESA, Emilio Vargas 7, Madrid, Spain ABSTRACT Two important issues in Performance Assessment exercises regarding the alteration of Spent Nuclear Fuel (SNF) are the contribution of Instant or better called Fast release Fraction (FRF) and the effect of High Burn-Up Structure (HBS). Therefore this paper focuses on the effect of HBS in FRF of a PWR irradiated in a commercial reactor with a mean Burn-Up (BU) of 48 GWd/tU. Additionally, determined FRF are compared with previous experiments performed with PWR with a mean BU of 60 GWd/tU, in order to evaluate the effect of BU on FRF. In order to study the HBS contribution, static leaching experiments are performed with two samples prepared from different radial positions, labeled Core, central region and Out, enriched with HBS. Two synthetic leaching solutions, bicarbonate and bentonitic granitic groundwater are used under oxic conditions. The estimated FRF are calculated from the determined Fraction of Inventory in Aqueous Phase (FIAP) taking into account the inventory determined experimentally for each fraction. Higher release is observed for Core sample, which can be an indicative that HBS does not increase the RN release. With the exception of Rb and Cs, which release is higher at lower BU, no clear effect of BU is observed within the studied range. INTRODUCTION In Performance Assessment (PA) exercises, the Spent Nuclear Fuel (SNF) source term is normally described as the combination of two terms: the Instant Release Fraction (IRF) and the Radionuclide (RN) released congruently with the matrix dissolution processes. The so called IRF represents the fraction of the inventory of safety-relevant RNs that may be rapidly released from the fuel and fuel assembles at the time of canister breaches. The experiments focused on the IRF measurement are scarce, especially for high burn-up UOX fuels or in the case of MOX type fuel. For this reason, published values for IRF are often based on correlation with Fission Gas Release [1]. With increasing burn-ups, neutron capture of U-238 produces Pu-239 generating an external layer with a higher burn-up (BU), increased porosity and fuel grain subdivision resulting on the formation of the so-called High Burn-Up Structure (HBS). The width of this layer, observed for BU’s higher than 40 GWd/tU [2], increases with the BU and depends on the irradiation history. HBS will be affected by two contradictory effects: on the one hand the restructuration will increase the available surf
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