An initial assessment of a mechanistic model, GRASS-SST, in U–Pu–Zr metallic alloy fuel fission-gas behavior simulations

Abstract

A mechanistic kinetic rate theory model originally developed for the prediction of fission gas behavior in oxide nuclear fuels under steady-state and transient conditions has been assessed to investigate its applicability to model fission gas behavior in U–Pu–Zr metallic alloy fuel. In order to capture and validate the underlying physics for irradiated U–Pu–Zr fuels, the mechanistic model was applied to evaluate fission gas release, fission gas and fission product induced swelling, and detailed gas bubble size distributions in three different fuel zones: the outer α-U, the intermediate, and the inner γ-U zones.Due to its special microstructural features, the α-U zone in U–Pu–Zr fuels is believed to contribute the largest fraction of fission gas release among the different fuel zones. It is shown that with the use of small effective grain sizes, the mechanistic model can predict fission gas release that is in reasonable consistence with (though slightly lower than) experimentally measured data. These simulation results are comparable to the experimentally measured fission gas release since the mechanism of fission gas transport through the densely distributed laminar porosity in the α-U zone is analogous to the mechanism of fission gas transport through the interconnected gas bubble porosity utilized in the mechanistic model.Detailed gas bubble size distributions predicted with the mechanistic model in both the intermediate zone and the high temperature γ-U zone of U–Pu–Zr fuel are also compared to experimental measurements from available SEM micrographs. These comparisons show good agreement between the simulation results and experimental measurements, and therefore provide crucial guidelines for the selection of key physical parameters required for modeling these two zones. Material properties such as fuel grain size and thermal diffusivity of gas and model parameters such as di-atom nucleation probability and gas bubble re-solution constant are predicted by these comparisons. In addition, the results of parametric studies for several parameters are presented for both the intermediate zone and the γ-U zone simulations in order to clarify the sensitivities of simulation results on these parameters.