Criticality assessment assuming spent fuel failure at Fukushima Daiichi Nuclear Power Plant Unit 1

Removal of spent fuel stored in a spent fuel pool (SFP) at the Fukushima Daiichi Nuclear Power Plant Unit 1 (1F1) was planned. In the future removal process, spent fuel will be lifted from the spent fuel racks and then moved to the transportation container at the location of cask pit. Some of the fuel assemblies have been confirmed to be damaged and those fuels must be handled appropriately according to the form of damage. In this study, we assumed that external factors occur during the transportation of the damaged fuels causing the scattering of fuel rods and/or pellets onto the SFP floor. Simulating a situation of scattering of fuel rods and/or pellets on the SFP floor, the possibility of criticality was examined. The computational domain consists of two areas, one was the scattered fuel pellets and the other was the fuel rods which keeps mechanical integrity. In order to take into account the heterogeneous effects of the fuels and moderator, a simple cubic lattice model which contains a sphere as fuel region was applied for both regions. The heterogeneous effects were considered by changing the sphere's diameter and lattice pitch. In this study, evaluation uncertainty caused by the fuel sphere location within the computational domain was also considered. As a result, our calculation model indicated the risk of criticality was low enough even if considering the heterogeneous effects. (author)