Phase-field simulations of the impact of bimodal pore size distributions on solid-state densification

The densification behavior of a porous body subjected to elevated temperature is relevant to many nuclear fuel performance metrics. The diffusional processes that govern such densification occur on microscopic length scales, and depend on the type of material and the nature of the porosity. In this study, we explore how bimodal pore size distributions impact diffusion-based densification relative to monomodal pore sizes for a given overall value of porosity. We utilize a Cahn-Hilliard phase-field model implemented with the MOOSE framework to simulate densification evolution with a range of overall porosities and a variety of porosity distributions between small and large pores. The results demonstrate that bimodal porosity can resist densification to a much greater extent than monomodal porosity. These findings have implications for the microstructural design of metallic nuclear fuels, in which an initial, bimodal porosity may resist early-stage densification and therefore provide collection sites for fission gases, thus reducing in-pile fission gas swelling.