Analysis on criticality properties and control strategies after reflooding of a damaged reactor core

In order to evaluate the crucial physical phenomena in a re-criticality of the reactor corium following a core reflood, a variety of criticality analyses have been performed with the design of a hypothetical core based on a severe accident condition including a core melt-down. This study aims to assess the critical mass of the corium within the core, to analyze the criticality change with the molten level and the coolant void reactivity (CVR), and to establish of practical strategies for a criticality control using H3BO3. The MCNPX 2.5.0 code was used to design the hypothetical accident core and to calculate the effective multiplication factor (keff), and the US Peach Bottom and Japan Fukushima nuclear power plant (NPP) was chosen for a reference model. The critical level was set to 0.95 for a conservative evaluation, and the consequential critical mass was indicated to be about 60 kg only. The 548 fuel assemblies loaded in the core were assumed to be molten up to approximately 77%, the keff value therefore has a range of 1.03241 ± 0.00194 to 1.40801 ± 0.00157. The negative reactivity was increasingly inserted as the coolant density was decreased from 95% to 0%, and the keff value approaches the sub-critical state when the void fraction is above 30%. Finally, according to the boron injection into water, it was found that at least 600 ppm 10B is required to have the sufficient criticality safety and to assure the sub-criticality condition following a severe core damage accident.