Updated Phase Calculation Method for Metallic Nuclear Fuel in BISON

Zirconium based metallic nuclear fuel undergoes significant redistribution of its constituents, resulting in several different phases present within a single fuel pin. Primarily dominated by the movement of zirconium in U-Pu-Zr and UZr fuels, phase changes result in a three (or more) zoned fuel structure clearly visible during post-irradiation examination (PIE). Each of these regions may posses differing material properties, requiring knowledge of the relative concentration of each phase in order for fuel performance codes to capture the relevant fuel behavior. The model described here was implemented into the fuel performance code BISON, similar to previous work for modeling zirconium redistribution in U-Pu-Zr fuel. However, the modules have since been updated with several key assumptions for easier use in future zirconium redistribution work and other phase-dependent material properties implementation, as discussed in the following sections. A new material model has been implemented in BISON to calculate the fractional phase concentrations as a function of temperature and zirconium concentration. Instead of smoothing the material properties to ensure numerical convergence, the new model smoothes the phase contributions directly, allowing easier implementation of material properties. By comparing the zirconium distribution for different smoothing parameters, it was shown that the smoothing parameters utilized here was adequate for the parameters of interest, while decreasing computational expense. The calculated phase information has been used to determine phase-dependent diffusivities, and is available for other phase-dependent properties. (authors)