Integration of the H₂ Inhibition Effect of UO₂ Matrix Dissolution Into Radiolytic Models

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 ones is the Matrix Alteration Model (MAM), which is an Alteration/Dissolution source term model based on the oxidative dissolution of spent fuel. Oxidant and reducing species can be naturally or radiolytically-generated. The experimentally-observed inhibition of matrix dissolution by H₂, was integrated into MAM by considering that H₂ is able to consume the oxidant species responsible for UO₂ oxidation, e.g. H₂O₂. As a consequence, MAM predicts lower H₂O₂ concentrations for systems containing larger amounts of dissolved H₂. Radiolysis experiments carried out by Pastina and coworkers have shown that under specific conditions such as high linear energy transfer (LET) radiation and absence of solid phase, dissolved H₂ has a negligible effect on the H₂O₂ concentration, thus suggesting that the H₂ inhibition effect catalyzed by the matrix surface has not been properly implemented in MAM. Modelling exercises performed in this work confirm such point and reveal the necessity of considering H₂-activation when modelling this kind of systems. In addition, it has been demonstrated that, for high LET radiation, a clear dependence exists between the extent of the H2 activation and the integral LET of the radiation. The integration of the function describing such dependence allows improving the implementation of the H₂ inhibition effect in MAM.