Risk Reduction in 700 MWe Indian PHWR—A Case Study with Passive Decay Heat Removal System Using Level-1 PSA

The design of 700 MWe Indian PHWR has evolved based on the operating experience of Indian PHWRs. India’s first 700 MWe PHWR with innovative features has attained criticality and recently started commercial operation. Some features of the 700 MWe PHWR design are similar to those of standardized Indian PHWRs. A water-filled Calandria vault and two diverse and fast-acting shutdown systems are among these features (Bianchi et al. in Nuclear Energy in Central Europe, pp. 1–8, 2001 [1]). Many First of a Kind (FOAK) design features have been incorporated in the 700 MWe PHWR (NPCIL in Status report 105-Indian 700 MWe PHWR (IPHWR-700) Mumbai, 2011 [2]). Passive Decay Heat Removal System (Bianchi et al. in Nuclear Energy in Central Europe, pp. 1–8, 2001 [1]) (PDHRS) is one of such systems (NPCIL in Status report 105-Indian 700 MWe PHWR (IPHWR-700) Mumbai, 2011 [2]). PDHRS ensures continued decay heat removal throughout a Station Blackout (loss of both off-site and on-site power) by recirculating the inventory on the secondary side of the steam generators via natural circulation. Level-1 Probabilistic Safety Assessment (PSA) (Griffiths in Risk Manag 5:63–64, 2003 [3]) at full power was performed as part of the licensing requirement before the first approach to the criticality of 700 MWe Indian PHWR. A comprehensive safety assessment is presented here, including design intent, operational practices, component reliability, dependencies, and human reliability considerations. Level-1 PSA for External Events was also performed. PDHRS provides the ultimate heat sink to ensure the core cooling and reduce the potential core damage due to internal and external Events. Thus, overall safety enhancement of 700 MWe Indian PHWR is achieved with this FOAK system which is seen in the CDF results.