Mesoscale Benchmark Demonstration Problem 1: Mesoscale Simulations of Intra-granular Fission Gas Bubbles in UO2 under Post-irradiation Thermal Annealing

Abstract

SUMMARYA study was conducted to evaluate the capabilities of different numerical methods used to represent microstructure behavior at the mesoscale for irradiated material using an idealized benchmark problem. The purpose of the mesoscale benchmark problem was to provide a common basis for assessing several mesoscale methods to identify the strengths and areas of improvement in the predictive modeling of microstructure evolution. In this work, mesoscale models (phase-field, Potts, and kinetic Monte Carlo) developed by Pacific Northwest National Laboratory, Idaho National Laboratory, Sandia National Laboratory, and Oak Ridge National Laboratory were used to calculate the evolution kinetics of intragranular fission gas bubbles in UO 2 fuel under post-irradiation thermal annealing conditions. The benchmark problem was constructed to include important microstructural evolution kinetics of intragranular fission gas bubble behavior, such as the atomic diffusion of Xe atoms, U vacancies, and O vacancies; the effect of vacancy capture and emission from defects; and the elastic interaction on nonequilibrium gas bubbles. An idealized set of assumptions and a common set of thermodynamic and kinetic data were imposed on the benchmark problem to simplify the mechanisms considered. The modeling capabilities of different methods are compared against selected experimental and simulation results. These comparisons find that while the phase-field methods and Potts kinetic Monte Carlo methods are able to incorporate several of the mechanisms that influence intra-granular bubble growth and coarsening, the Potts model is challenged by the low solubility and long-range diffusion necessary to simulate this problem correctly. The statistical-mechanical nature of Potts kMC requires large ensembles with long simulation times to treat this problem. Future efforts are recommended to construct increasingly more complex mesoscale benchmark problems to further verify and validate the predictive capabilities of the mesoscale modeling methods used in this study.

(Project 13-5292) Correlating thermal and mechanical coupling based multiphysics behavior of nuclear materials through in-situ measurements

“…Continuum-level mechanisms, such as fission gas release to the fuel rod environment and volumetric swelling due to bubble growth, are included in nuclear fuel performance assessments. The behavior of fission gas atoms in UO2 has been extensively studied using experiments performed on single crystal and polycrystalline material under a variety of temperature and irradiation conditions for more than 50 years Yulan Li (2012). From these studies a general picture of the mechanisms that influence the transport and release of fission gas atoms has been developed.…”