Safety Analysis on Mitigation Overpressure ATWS in VVER-1000 NPP

Safety analysis on operating nuclear power plant (NPP) plays an important role on nuclear energy application, especially after the severe accident of Fukushima plants. This paper focuses on one of the operating NPPs in China, the TianWan NPP Unit1&2. TianWan Unit1&2 belong to VVER-1000 NPP type, which have special design characteristics and different safety migrating methods comparing with other domestic PWR NPPs in China. Calculations and analyses were made to give thermal hydraulics support to Level-1 PSA of this VVER type PWR on the anticipated transient without scram (ATWS) accidents. The calculation of loss of main feed water ATWS was carried out using the RELAP5 code to evaluate the intrinsic safety mainly impacted by fuel and moderator. The model also considers reactivity introduction caused by the change of boron concentration to find out the influence of the emergency boron injection system (JDH) on mitigating the coolant system over pressure. The paper gives out the success criteria of the safety valves of pressurizer (PRZ) with the critical moderator temperature coefficient (MTC) value according to the condition of coolant system pressure being under the pressure limit. It is indicated that the JDH system can play an important role on mitigating the over pressure of coolant system in the late phase of the transient.