One of the new features of the ABWR-II safety design is the adoption of passive safety systems. The passive heat removal system (PHRS) consists of two dedicated systems, namely the passive reactor cooling system (PRCS: the same as Isolation condenser) and the passive containment cooling system (PCCS), that use a common heat sink pool above the containment allowing a one-day grace period (Figure I-2). These passive systems not only cover beyond DBA condition, but also provide in-depth heat removal backup for the RHR, and practically eliminate the necessity of containment venting before and after core damage as a means of overpressure protection.

With regard to ABWR-II development, the horizontal type PCCS has been designed with a focus on anti-seismic structure and decreasing PCCS pool depth instead of conventional vertical type PCCS. Figure I-3 shows PCCS functional schematic and an example of containment pressure transient following typical low pressure core melt scenario. Containment venting for overpressure protection under severe accident condition is practically excluded by adopting PCCS.