Development of fuel performance analysis code, BISON for MOX, named Okami: Analyses of pore migration behavior to affect the MA-bearing MOX fuel restructuring

Fuel composition, i.e. Pu content, minor actinide (MA) content, oxygen-to-metal (O/M) ratio, etc., affects the irradiation behavior of MA-bearing mixed oxide (MOX) for fast reactor. To evaluate the O/M dependence of pore migration regarding fuel restructuring at the beginning of irradiation, we are developing BISON for MOX in cooperation with Idaho National Laboratory (INL) and have installed a pore migration model considering vapor pressure of vapor species and thermal conductivity for MOX. The O/M dependence of fuel restructuring observed in MA-bearing MOX irradiation in the experimental fast reactor Joyo was evaluated by a two-dimensional (2-D) analysis. To solve partial differential equations for energy conservation and advection diffusion equations to calculate temperature and pore migration, respectively, a correlation of thermal conductivity as a function of porosity, temperature, deviation from stoichiometry, Am content and Np content was used. In addition, the pore migration velocity was calculated by using a correlation depending on vapor pressure applying Rand-Markin's model. Four MA-bearing MOX test pins with different O/M ratio and pellet/cladding gap size were irradiated in B14 test in Joyo to study the effects of O/M ratio and gap size on irradiation behavior at the beginning of irradiation. In post-irradiation examinations (PIE), a remarkable restructuring was observed in stoichiometric fuels, and a discrepancy in central void diameter among various O/M ratios. This restructuring behavior could be evaluated by considering the influence of O/M ratio on vapor pressure. Also, a central void is assumed to offset from the geometric center to the larger-gap side due to the pellet eccentricity, but the PIE results were inconsistent in this regard. It is expected that the irradiated pellet might displace to another side during sample preparation in PIE from a reverse offset location during irradiation by comparing the result of 2-D analysis and radial profile of fuel density.