Peak power characteristics of postulated criticality in fuel debris

For safe operation at the retrieval of fuel debris in Fukushima Daiichi Nuclear Power Plant, inadvertent criticality should be considered and the prediction of its peak power is important for the evaluation of the shielding between the fission source and operators, while measured data or analytical values in past criticality accidents are quite rare. Characteristics of the peak power under a postulated criticality accident condition have been investigated. One-point kinetics basis analysis of a postulated criticality in the fuel debris in comparison with that of JCO criticality accident showed its dependency on the parameters such as excess reactivity, delayed neutron fraction, neutron generation time, etc. In conclusion, the difference of the peak power values is mainly from heat capacity, and the key factor is the size of fuel debris at criticality. (author)