Depressurization strategies on OPR1000 during SB/MBLOCA for operation of the passive safety injection system in IPSS

Abstract

The Fukushima accident brought multiple Beyond Design Basis Accidents (BDBA) in nuclear power plants (NPP) to reality. To prevent the next Fukushima, current and future NPPs should retain coping ability for the BDBAs. As a passive supplement to deal with Station Black-out (SBO), the Integrated Passive Safety System (IPSS) was suggested for current generation Ⅲ NPPs. With two elevated water tanks, IPSS possesses capabilities of containment preservation and core cooling with gravitational head under SBO. In a pressur-ized water reactor (PWR), primary pressure needs to decrease substantially under small and medium break loss of coolant accidents (SB/MBLOCA) to initiate a safety injection system. However, many Generation Ⅲ NPPs have difficulties in sufficient depressurization of the reactor coolant system (RCS) for the IPSS opera-tion with their original RCS depressurization system. To overcome the limitation, installation of squib valves on hot legs and indirect RCS depressurization using atmospheric dump valve (ADV) were proposed as an invasive and non-invasive methods. The effectiveness of the proposed methods were evaluated by the MARS -thermal hydraulic evaluation code-. Results show that both methods have successful depressuriza-tion capability to reach the pressure less than 4 bar that IPSS needs. The RCS depressurization strategy of PWR was attained not only for IPSS operation but also for insight on general RCS depressurization during LOCA.