Three-dimensional modeling of hydrogen and hydride distribution in zirconium alloy cladding using high-fidelity multi-physics simulations

Localized phenomena within the reactor core, specifically those associated with the nuclear fuel, require high-fidelity simulations to enable accurate physics predictions. One example is that of zirconium cladding, which absorbs hydrogen from the light water coolant during normal reactor operation. This hydrogen is in solid solution in the cladding and its distribution is sensitive to temperature and concentration gradients. At high enough concentrations, the hydrogen will precipitate as a hydride. Thus, the hydrogen distribution as a hydride precipitate in cladding has been identified as a possible ersatz for validating reactor simulation code temperature models. This study reports development efforts of using high-fidelity multi-physics codes to model temperature, hydrogen, and hydride distributions in three dimensions under realistic operating conditions. The Consortium for the Advanced Simulation of Light Water Reactors (CASL) multi-physics code, Tiamat, is used to model select sub-assemblies. Then, a single fuel pin is selected from the sub-assembly and modeled as a three-dimensional BISON problem. The outer cladding temperatures from the Tiamat calculation are used as boundary conditions for the BISON problem in order to obtain hydrogen and hydride distributions. A sub-assembly is also modeled with the commercial CFD code STAR-CCM+, and temperature boundary conditions are again supplied to BISON models. Areas of interest for hydride precipitation include locations along the fuel rod experiencing highest temperatures with significant spatial variation, particularly in the vicinity of the spacer grids and mixing vanes.