Safety margins improvement by means of the passive heat removal system and the HA-2 in VVER-1000/V320 reactors

One of the key functions of the safety systems is the removal of decay heat during an accidental sequence. Conventional VVER reactor designs have safety systems capable of removing residual heat in an emergency scenario, but these systems will be challenged to operate under station blackout conditions. As a result, some of the Gen III/Gen III + VVER designs incorporate passive safety systems capable of maintaining the decay heat removal in the event of a total loss of AC power. The present study focuses on the analysis of the air-cooled Passive Heat Removal System (PHRS) incorporated in some Gen III/Gen III + designs such as the VVER-1000/V412, the VVER-1200/V392M, V509, V523 or the VVER-TOI. For this purpose, a PHRS model has been developed in the TRACE system code, which has been incorporated into a model of a VVER-1000/V320 reactor which also includes another Gen III/Gen III + VVER reactors common feature; the Second Stage Hydroaccumulators (HA-2), subsystem of the Emergency Core Cooling System. Subsequently, a Station Blackout (SBO), a SBO along with an SBLOCA and a SBO along with an LBLOCA sequences have been analyzed with the air-cooled PHRS operating. ... mehrThe results show that in some scenarios, the PHRS performance is critical to lead the sequence to a safe state by transporting the decay heat to the atmosphere heat sink for at least 24 h.