Influence of temperature, initial grain-boundary bubble density and grain structure on fission gas behaviour in UO$_2$: a 3D hybrid multiscale study

TL;DR

This study uses advanced 3D multiscale simulations to investigate how temperature, grain boundary characteristics, and grain structure influence fission gas behavior in UO₂, providing new insights into gas release mechanisms.

Contribution

It extends a hybrid multiscale framework to large 3D polycrystals, incorporating grain boundary migration and heterogeneous diffusion to better understand fission gas evolution.

Findings

At 1600 K, intergranular bubbles coalesce and migrate, affecting gas release.

Bubble density aligns with White's coalescence trend, differing from prior models.

Early gas release is rapid near free surfaces, with bubble collapse forming denuded zones.

Abstract

Fission gas swelling and release in UO$_2$ are governed by the coupled evolution of intragranular clusters and bubbles, migrating grain boundaries (GBs), triple junctions (TJs), and their eventual connection to a free surface (FS). We extend a hybrid multiscale framework that couples cluster dynamics (Xolotl) with a phase-field model (MARMOT) to large 3D polycrystals with heterogeneous GB and surface diffusion and evolving GB networks. We simulate 10- and 100-grain UO$_2$ microstructures at 1200 and 1600 K, with and without a FS, to interrogate bubble growth, coalescence, GB/TJ coverage, gas arrival at interfaces, and fission gas release (FGR). At 1200 K, both GB mobility and gas transport are low, yielding negligible bubble and GB evolution. At 1600 K, intergranular bubbles rapidly become lenticular and coalesce into networks while unpinned GBs migrate; fewer initial bubbles reduce coalescence but enhance GB migration due to less pinning and produce spikes in interfacial gas arrival rate due to GB sweeping. Bubble density versus mean projected area agrees with White's (2004) coalescence trend and remains on the left side of the analytical curve, in contrast to several prior simulations, likely due to the inclusion of GB migration. In domains with a FS, early release is rapid and bubbles near the FS collapse to form a denuded zone, suppressing local network connectivity; GB coverage rises and approaches but does not exceed 50%. TJ coverage remains low without preferential nucleation at TJs. To our knowledge, these are the first large-scale 3D mesoscale simulations of intergranular fission gas behavior that provide mechanistic insight and quantitative metrics to inform engineering-scale FGR models.