Analyses on the recriticality and sub-critical boron concentrations during late phase of a severe accident of pressurized water reactors

The potential for recriticality and sub-critical boron concentrations is analyzed during the relocation of the fuel rods in the assembly, which we call late phase of a severe accident, via coupling between MELCOR and whole-core Monte Carlo analyses by Serpent 2. The recriticality, initiated during the early phase, is found to maintain when the fuel assemblies containing intact fuel rods are submerged by the cooling water. It is also found that the effect of the negative reactivity insertion via remaining fission products in the fuel debris increases as the burnup increases. The sub-critical boron concentrations during the late phase are found to be 76∼544 ppm lower than those during the early phase. Therefore, it can be concluded that the boron concentration that prevents recriticality not only during the early phase but also during the late phase is the sub-critical boron concentration during the early phase. •Recriticality of a degraded reactor core during the late phase of a severe accident.•Sensitivity analyses on the fuel debris for conservative estimation of the criticality.•Whole-core modeling on the relocation of the core materials for Monte-Carlo analysis.•Boron concentration obtained to maintain sub-critical during a severe accident.