Spent fuel radionuclide source term model for assessing spent fuel performance in geological disposal. Part II: Matrix alteration model and global performance Christophe Poinssot a, * , Ce ´cile Ferry a , Patrick Lovera a , Christophe Jegou b , Jean-Marie Gras c a Commissariat a ` l’Energie Atomique (CEA), Nuclear Energy Division, CEA Saclay, Department of Physics and Chemistry, Service for the Studies of the Radionuclides Behavior, DPC/SECR, BP.11, F-91191 Gif-sur-Yvette cedex, France b Commissariat a ` l’Energie Atomique (CEA), Nuclear Energy Division, Department of Waste Treatment and Conditioning, Service for the Studies of the Conditioning Material Behavior, DTCD/SECM/LMPA, CEA Marcoule, France c Electricite ´ de France (EDF), R&D Division, Les Renardie `res, France Abstract In the framework of the research conducted on the long term evolution of spent nuclear fuel under geological dis- posal conditions, a source term model has been developed to evaluate the instantaneous release of radionuclides (RN) (instant release fraction, IRF) and the delayed release of the RN which are embedded within the matrix. This model takes into account most of the scientific results currently available except the effect of hydrogen and the current knowl- edge of the uncertainties. IRF was assessed by considering the evolution with time of the RN inventories located within the fuel microstructure to which no confinement properties can be allocated over the long term (gap, rim, grain bound- aries). This allows for bounding values for the IRF as a function of time of canister breach and burnup. The matrix radiolytic dissolution was modeled by a simple kinetic model neglecting the recombination of radiolytic species and the influence of aqueous ligands. The oxidation of the UO 2 matrix was assumed not to be kinetically controlled. Spent fuel performance was therefore demonstrated to mainly depend on the reactive surface area. Ó 2005 Elsevier B.V. All rights reserved. PACS: 28.41.Kw; 89.60.Ec 1. Introduction Direct disposal is being studied in many countries as a possible way to manage commercial spent nuclear fuel (CSNF). 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 are currently reprocessed in order to meet the need for the Pu recovered by repro- cessing and recycled as MOX fuel: 350 tHM y 1 are currently stored waiting for further decision, delayed reprocessing, long-term storage or ultimate disposal. These stored fuels are either MOX or relatively high 0022-3115/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2005.05.021 * Corresponding author. Tel.: +33 1 6908 3260; fax: +33 1 6908 5254. E-mail address: christophe.poinssot@cea.fr (C. Poinssot). Journal of Nuclear Materials 346 (2005) 66–77 www.elsevier.com/locate/jnucmat