Impact of grain boundary and surface diffusion on predicted fission gas bubble behavior and release in UO$_2$ fuel

TL;DR

This study models how grain boundary and surface diffusion influence fission gas bubble behavior and release in UO$_2$ fuel, revealing their significant roles and providing quantitative bounds on diffusivity effects.

Contribution

It introduces a hybrid phase field/cluster dynamics model to quantify the effects of grain boundary and surface diffusion on gas release in UO$_2$ fuel.

Findings

Grain boundary diffusivity significantly affects gas release rate.

Surface diffusivity influences bubble coalescence and mobility.

Estimated diffusivity values are below certain thresholds from prior studies.

Abstract

In this work, we quantify the impact of grain boundary (GB) and surface diffusion on fission gas bubble evolution and fission gas release in UO$_2$ nuclear fuel using simulations with a hybrid phase field/cluster dynamics model. We begin with a comprehensive literature review of uranium vacancy and xenon atom diffusivity in UO$_2$ through the bulk, along GBs, and along surfaces. In our model we represent fast GB and surface diffusion using a heterogeneous diffusivity that is a function of the order parameters that represent bubbles and grains. We find that the GB diffusivity directly impacts the rate of gas release via GB transport, and that the GB diffusivity is likely below 10$^4$ times the lower value from Olander and van Uffelen (2001). We also find that the surface diffusivity impacts bubble coalescence and mobility, and that the bubble surface diffusivity is likely below $10^{-4}$ times the value from Zhou and Olander (1984).