Investigation of the recriticality potential during reflooding phase of Fukushima Daiichi Unit-3 accident

The paper presents an investigation of the potential for the recriticality phenomenon occurrence during core reflooding at the time of early in-vessel phase of the Fukushima-Daiichi Unit 3 (1F3) accident. A Monte Carlo criticality analysis was performed using the SERPENT code for STEP-3 BWR fuel type. The analysis was conducted for a representative core unit cell composed of four assemblies in three dimensions. The MELCOR computer code was used as the source of the accident progression and thermal-hydraulic input. A data exchange framework between MELCOR and SERPENT was developed and employed. The calculations reveal that for the applied MELCOR model, recriticality occurs due to simultaneous control blades loss, fuel rods intactness and non-borated water injection. It suggests that during reflooding even a relatively small fraction of the core covered with water and without control blades is sufficient to lead to the critical condition. Additionally, a sensitivity analysis for various control blade survival fractions was performed.