Passive measure for preventing recriticality of fuel debris dust for defueling at Fukushima Daiichi nuclear power station

Abstract

The drilling or cutting of resolidified fuel debris required to defuel the Fukushima Daiichi nuclear power station is certain to generate debris dust. This paper focused on drilling resolidified fuel debris in water and conservatively confirmed by criticality calculations that neutron multiplication effect is higher if debris dust is suspended separately from the debris rather than if it is suspended closely around the debris. No use of vacuuming of debris dust, borated water, and active components was assumed in this study. Also, this paper confirmed that the use of a debris dust guide effectively and passively limited the increase in neutron multiplication by debris dust because the guide distributes dust particles so flatly that sufficient neutron leakage limits neutron multiplication even if the volume fraction of the particles in water reaches the optimum condition. In the actual defueling operation at the nuclear power station, the use of a flatter debris dust guide will be more effective to prevent local recriticality concurrently with the careful control of the mass of debris dust. The physics and ideas in this paper should be applicable to other defueling technologies such as laser cutting as long as debris dust is generated and suspended in water.