Criticality Safety Study for the Disposal of Damaged Fuels from Fukushima Daiichi Reactors

Abstract

This paper summarizes our previous works on neutronics analysis for the disposal of damaged fuels from Fukushima Daiichi reactors. Three major stages have been identified for the criticality safety assessment after disposal. In order to evaluate the criticality safety for certain repository conditions and engineered barriers designs, neutronics models have been defined for different stages, and numerical results have been calculated by a Monte-Carlo code MCNP. For stages when fissile nuclides in the damaged fuels remains in the vicinity of the engineered barriers, the neutron multiplicity (k eff ) for a canister containing fuel debris surrounded by buffer was calculated over the leaching time. For the stage when fissile nuclides originated from multiple packages deposit in far-field host rocks, the critical masses for uranium depositions were studied for various rock types and geometries. The methodology presented in the present paper could be further improved and utilized to assist the repository system design and criticality safety assessment in the future.