133 Mater. Res. Soc. Symp. Proc. Vol. 1518 © 2013 Materials Research Society DOI: 1 1557/op 2013. Modelling the Activation of H 2 on Spent Fuel Surface and Inhibiting Effect of UO 2 Dissolution L. Duro 1 , O. Riba 1 , A. Martínez-Esparza 2 and J. Bruno 1 1 Amphos 21 Consulting, S.L., P. Garcia Faria 49-51, 1-1, Barcelona, E-08019, Spain 2 ENRESA C/ Emilio Vargas, 7 Madrid, E-28043 Spain. ABSTRACT The dissolution of spent nuclear fuel is defined in two different time steps, i) the Instant Release Fraction (IRF) occurring shortly after water contacts the solid spent fuel and responsible of the fast release of those radionuclides that have been accumulated in the zones of the spent fuel pellet with low confinement, such as gap and grain boundaries and ii) the long term release of radionuclides confined in the spent fuel matrix, much slower and dependent on the conditions of the water that contacts the spent fuel. Several models have been developed to date to explain the dissolution behavior of spent nuclear fuel under disposal conditions. The Matrix Alteration Model (MAM) is one of the most evolved radiolytic models describing the dissolution mechanism in which an Alteration/Dissolution source term model is based on the oxidative dissolution of spent fuel. Under deep repository conditions and at the expected of water contacting time (after 1000 years of spent fuel storage), α radiation will be the main contributor to water radiolysis. In the current study, simulations evaluating the effect of surface area on the alteration/dissolution of spent fuel matrix are performed considering different particle sizes of spent fuel and simulations integrating the actinides dissolution have been performed considering the precipitation of secondary phases. INTRODUCTION Different models describing the dissolution mechanism of spent nuclear fuel under repository conditions have been developed in the last years. One of the most evolved is the Matrix Alteration Model (MAM). This source term model was developed within the frame of “Spent Fuel Stability under Repository Conditions” (SFS) project [1]. Since then, MAM has been improved under the umbrella of different European projects: “Understanding and Physical and Numerical Modelling of the Key Processes in the Near-Field and their Coupling for Different Host Rocks and Repository Strategies” (NF-PRO), “Model uncertainty for the mechanism of dissolution of spent fuel in nuclear waste repository” (MICADO) and also thanks to several projects framed in the ENRESA R&D programs. MATRIX ALTERATION MODEL MAM is a radiolytic source term model based on the oxidative dissolution of spent fuel under repository conditions. Radiolysis accounts for the generation of oxidising and reducing species in the vicinity of the spent fuel surface. The presence of oxidants produces an enhancement of the dissolution of the matrix, although the generation of molecular reductants, mainly hydrogen has been observed to inhibit importantly the dissolution of the matrix. In this work we present the latest updates incorporated to the MAM to account for the 76 l. 0.