Radiolytic Modelling Intercomparison Exercise: Influence of Alpha Radiation on Spent Fuel Alteration Process

Radiolytic models have been usually considered useful tools for studying the behaviour of complex chemical systems under the presence of a radiation field. However, they still don’t have a wider acceptance due to limited availability of kinetic data, difficulty to handle heterogeneous systems, and a lack of model validation. We carry out an intercomparison exercise between two kinetic codes, “Chemsimul” and “Maksima”. The objective of the work is to model the influence of an alpha radiation field in the spent fuel alteration process under repository conditions. The experimental system to be modelled is the dissolution of UO₂ α-doped pellets under initial anoxic conditions. These experiments have been carried out under well-controlled conditions, and good quality data are available to us for this purpose. The uranium concentration in solution will be predicted by considering the presence of an α-radiation field and its influence due to radiolysis of water on the pellet surface oxidation and subsequent dissolution. In this paper, a kinetic model for the alpha doped pellet under these conditions is presented. Both codes use the same initial fixed parameters in order to reproduce the pellet alteration process: system geometry, chemical composition of the solution, physicochemical characteristics of the solute and the oxidation conditions of the pellet surface (expressed in terms of U(VI)/U(IV) ratio). The last one is the key parameter in the model for simulating the initial quick dissolution process. The differences in the input data are related to intrinsic restrictions of each code. A review of the differences and the capacities of both codes is also included. The predictions obtained using the two codes are compared and the results obtained are in good agreement, having a small difference related to the way of solving the differential equations by each code. However, this difference is not considered to be of significance when comparing modelled and experimental data, leading to the conclusion that both codes can confidently be used in the radiolytic modelling of this kind of systems.