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.
This study investigates the criticality characteristics of debris beds that may have been formed through the molten core - concrete - interaction (MCCI) at the pedestal floor of the damaged reactors in Fukushima Daiichi Nuclear Power Station. These were modeled as UO2-concrete systems submerged in water. Firstly, a conservative model was used to evaluate the impact that the presence of concrete has on the neutron multiplication factor (keff ) of debris beds. The good moderation capacities of concrete were proved and it was found that recriticality would be possible under the considered conservative assumptions. Secondly, a more realistic model was used to perform an uncertainty and sensitivity analysis of a wide range of debris parameters (debris porosity,core meltdown grade, debris size, debris composition,concrete erosion factor, etc.). In this case, the results indicate that the probability of a recriticality event is very remote. It was also found that the presence of boron (B4C) from the control rods within debris has by far the highest influence by far on keff.
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