A Physics-Based Description of Inter-Granular Helium Behaviour in SCIANTIX for Application in Fuel Performance Codes

In this work, we propose a new mechanistic model for the treatment of helium behaviour at grain boundaries in oxide nuclear fuel. The model pairs rate-theory description of helium intra-granular behaviour (diffusion towards grain boundaries, trapping in spherical bubbles, thermal re-solution), developed in a first step, with rate-theory description of helium inter-granular behaviour (diffusion towards grain edges, trapping in lenticular bubbles, thermal re-solution). The proposed model has been implemented in SCIANTIX (meso-scale software designed for coupling with fuel performance codes) and validated against thermal desorption experiments performed on doped UO2 samples annealed at different temperatures. The overall agreement of the new model with the experimental data is satisfactory, both in terms of integral helium release and of helium release rate, showing an improvement compared to previous mechanistic models, which do not consider the behaviour of helium at grain boundaries. By considering the contribution of helium at grain boundaries it is possible to represent the kinetics of helium release rate at high temperature. Given the uncertainties involved in the initial conditions for the inter-granular part of the model (initial helium concentrations in inter-granular bubbles, and in solution at grain boundaries) and the uncertainties associated to some model parameters for which limited lower-length scale information is available (helium diffusivity at the grain boundaries in particular) the results are complemented by a dedicated sensitivity analysis. This analysis demonstrates that the initial conditions, if chosen in a reasonable range, have limited impact on the results, and confirms that it is possible to achieve satisfying validation results using sound values for the uncertain physical parameters.