Radiolytic Modelling: Application To Spent Fuel Dissolution Experiments
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Radiolytic Modelling: Application To Spent Fuel Dissolution Experiments Juan Merino, Esther Cera and Jordi Bruno Enviros Spain Pg. de Rubí, 29-31 08197 Valldoreix, Spain ABSTRACT In this work we describe our first efforts to simulate the chemical evolution of the spent fuel/water system in dissolution experiments using a radiolytic model. These experiments were carried out with spent fuel fragments in deionized water [1]. In order to build the radiolytic model, a set of reactions were selected, together with their kinetic constants, including the catalytic decomposition of H2O2 in the UO2 surface and a reaction mechanism for UO2 oxidation and dissolution. Averaged alpha and beta G-values were used, neglecting gamma radiation. Diffusion to the gas phase was treated thermodynamically assuming Henry’s law. The computer code used in the simulations was CHEMSIMUL [9]. First attempts to simulate the experimental results were not successful and a sensitivity analysis was carried out. As a result of this, the kinetic constant of the reaction OH• + H2 → H• + H2O, the only reaction that consumes hydrogen in the system, was identified as a key parameter, together with the kinetic constants of the UO2 oxidation and dissolution reactions. Varying these parameters we were able to reproduce the experimental data. Although more simulations are needed, these results are encouraging because they give us more confidence in the use of radiolytic models as useful tools to predict the longterm alteration rate of the spent fuel under repository conditions, which is our ultimate goal. INTRODUCTION It is generally acknowledged that one of the main factors influencing the stability of the spent fuel under repository conditions is the radiolytic generation of oxidants in the water surrounding the spent fuel pellets. Under the presence of a radiation field, the aqueous chemistry of the system becomes even more complex, and therefore the use of radiolytic models has become a very useful tool to study these systems. These models consist in a set of chemical reactions representing the system under study together with the radiation field characterised by a dose rate and the radiolytic yields for the different species and type of radiation. Depending on code capabilities, the models can include several processes like diffusion, gas equilibrium and dose rate decay. One of the main drawbacks of radiolytic models is that the codes are usually designed to solve only homogeneous systems. Thus, in order to apply these codes to heterogeneous systems, such as the spent fuel in contact with water, some approximations have to be made (see below). Several authors have developed and applied radiolytic models in order to understand the behaviour of experimental systems, ranging from spent fuel dissolution experiments [3], [4], radiolysis in brine solutions [5], and even radiolysis induced by He-atom beams in UO2 targets in contact with water [6]. Models have also been built to try to predict the long-term alteration rate of spent fuel under repository
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