After the Fukushima accident, the interest of the scientific community in severe accident research has been renewed. One of the severe accident research issues that needs to be further investigated is the potential for recriticality of the fuel debris, which is formed after the core meltdown. In this study, a conservative criticality evaluation of the Fukushima Daiichi Unit 1 debris bed has been carried out. Parameters, such as debris size, porosity, particle size, fuel burnup and the coolant conditions, including the water density and the content of boron, were considered. The effect of these parameters on the neutron multiplication factor was analysed and safety parameter ranges, i.e. zones where the recriticality can be totally excluded, have been identified. The content of boron in water required to secure the subcriticality was calculated for those zones with recriticality potential. It was found that recriticality is achievable for a wide range of fuel debris conditions. 1600 ppm B would ensure subcriticality under any conditions.

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