Analysis of Impact of Medium Pressure Safety Injection on Severe Accident Process under RRA Connected Mode

Aiming at the loss of coolant accident (LOCA) of China's improved million kilowatt PWR (CPR1000) Nuclear Power Unit losing high and low pressure safety injection and spray in reactor residual heat removal system (RRA) connected mode of intermediate shutdown, the reactor core, reactor coolant system and containment system of the reference unit are simulated and calculated by using MAAP5 program, at the same time the impact of medium pressure safety injection on the severe accident process is analyzed by combining with the calculation results, and the mitigation effect on the accident is studied. The results show that the timely injection of medium pressure safety injection can effectively limit the temperature rising behavior of the core, and play an important role in mitigating the process of severe accidents during the process of core exposure and temperature rise caused by LOCA under RRA connected mode, and even it is possible to avoid core integrity damage when high and low pressure safety injection and spray is lost under accident conditions. Finally, according to the analysis results, the improvement suggestions are put forward for the current operation regulations of nuclear power units. For the administrative isolation behavior of the medium pressure safety injection tank, only the corresponding isolation operation is performed for its electrical switch, while the local part of the valves in the containment building is provided with a tag warning, and the on-site padlock operation is not performed, which can not only avoid the misinjection of the medium pressure safety injection tank under normal operation conditions, the integrity of the reactor core can be also effectively guaranteed after the occurrence of LOCA under RRA connected mode. The normal and safe operation of the power plant can be ensured, and the mitigation ability of severe accidents of the unit under this mode can be improved.