Whole-core analyses on recriticality of conventional high power pressurized water reactor in Korea during early phase of severe accidents

In this paper, first, MELCOR calculations for a conventional high power pressurized water reactor are performed to obtain the geometrical configurations and remaining fraction of the fission products in the degraded reactor core during the early phase of a severe accident. Then, based on the coupling between MELCOR and Serpent 2 code, sensitivity analyses on the criticality are performed for various water levels, burnup, and release of fission products with whole-core modelling on the degraded reactor. Sub-critical boron concentrations are obtained on the configuration of the degraded reactor core showing the largest criticality. The sub-critical boron concentrations are compared to critical boron concentrations of the intact reactor core to quantify additional boron concentrations required to make the reactor sub-critical during the early phase of the severe accident. Effects of control rod degradation and fission product release are also presented in terms of boron concentrations from the results of sensitivity analyses.