A Modelling Methodology for the Description of Helium Behaviour Accounting for the Grain-Size Distribution

Physics-based meso-scale models of fission gas behaviour for fuel performance codes currently consider only the average grain size as physical parameter to describe the fuel microstructure. Nevertheless, information on the grain-size distribution is available for several metallographically characterized fuel samples. To extend the current modelling approach, we present new experimental data and develop a methodology to treat the fuel grain-size distribution based on multi-grain analysis. This is applied to describe helium behaviour from infused and annealed polycrystalline UO2 samples. The methodology consists of three steps: (1) Acquisition of the empirical grain distribution from sections of the polycrystals, (2) Calculation of the gas distribution factor pertaining to each grain size class after helium infusion, and (3) Simulation of the experimental annealing histories with input parameters weighted by the gas distribution factors. To perform the multi-grain analysis, we used the SCIANTIX code, which allows calculating the helium kinetics in a single fuel grain. The application of this methodology is promising because it can represent the dynamics of helium release more consistently with the physics of the fuel microstructure compared to the state-of-art approaches, and it can be suitable for application in fuel performance codes for different enhanced accident tolerant fuel concepts as outlined in our conclusions.