Study on criticality in natural barrier for disposal of fuel debris from Fukushima Daiichi NPS

After fuel debris is removed from the reactor containment vessel at Fukushima Daiichi NPS (1F) and collected in containers in the future, the containers may be disposed of at a deep geological repository. The uranium inventory and fissile U enrichment of the fuel debris can be higher than those of spent fuels and high-level vitrified wastes. In this study, we estimated the quantity of uranium precipitated at the natural barrier using one-dimensional migration analysis, and studied dimension of uranium precipitated in the natural barrier and carried out the criticality analysis. As the result, more than 200 MTU is precipitated at redox front in the natural barrier to reach the criticality. Criticality analysis indicated that keff is lower than 0.98 when all uranium of 250 MTU distributed into longer than 700 m as cuboid shape at the accumulated area.