Model Development in MARMOT for High Burn-up Structure Formation

Abstract

A phase field model for the formation of High Burn-up Structure (HBS) was developed and implemented in MARMOT. The model takes into consideration the chemical energy of gas atoms, interfacial energies of grain boundaries and bubble surfaces, and strain energy associated with dislocations, which renders the model capable of simulating the concurrent formation and growth of both bubbles and sub-grains. Moreover, in contrast to existing phase-field models of recrystallization, the current model makes no ad hoc or a priori assumptions for the nucleation rate or recrystallized grain size and shape. All the model parameters were quantified in terms of thermodynamic and kinetic properties. The model predicts a strong effect of magnitude and distribution of dislocation density, grain boundary energy, and bubble surface energy on the formation of sub-grains. The model results are shown to be consistent with theoretical predictions. For polycrystalline UO 2 at 1200K, preliminary results suggest an averaged recrystallized grain size of 0.6-1.3 microns at a critical dislocation density in the range of ρ = 9 × 10 15 − 1.2 × 10 16 m-2 , which corresponds to a burn-up of 98-105 GWD/tU. These values lie well within the range of reported data in literature. The model will be further refined to perform mesoscale simulations for HBS formation with the purpose of developing materials models to be used in the engineering scale fuel performance modeling. This work is supported by the DOE Nuclear Energy Advanced Modeling and Simulation (NEAMS) program.