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Model for Simulation of Hydride Precipitation in Zr-Based Used Fuel Claddings: A Status Report on Current Model Capabilities

The report demonstrates a meso-scale, microstructural evolution model for simulation of zirconium hydride precipitation in the cladding of used fuels during long-term dry-storage. While the Zr-based claddings (regarded as a barrier for containment of radioactive fission products and fuel) are manufactured free of any hydrogen, they absorb hydrogen during service in the reactor. The amount of hydrogen that the cladding picks up is primarily a function of the exact chemistry and microstructure of the claddings and reactor operating conditions, time-temperature history, and irradiation conditions); it is known to have performance consequences in reactor and during post-service used fuel storage. The objective of this work is to develop computational capability for the prediction of hydride formation in a stockpile of cladding of used fuel (using recently developed hybrid Potts-phase field model that combines elements of Potts Monte Carlo and the phase-field model to treat coupled microstructural-compositional evolution to simulate the evolution of microstructure along physically realistic paths). This document demonstrates a basic hydride precipitation model that accounts for (microstucture) grain texture and orientation, and includes free energy of two Zr phases. The chemical potential that drives the compositional and phase changes are enhanced in the model for future simulations. This work assumes precipitates grow along particular crystallographic directions that are thermodynamically favored growth directions; it examines nucleation sites in the microstructure, and hydride precipitate growth is simulated in stress-free claddings and in one with a constant uniaxial stress. More rigorous models with evolving micromechanics will be developed in the future; confidence in these models will further increase with expanding our current understanding of hydride precipitate formation.