Design of Containment System & Associated Engineered Safety Features in Indian PHWRs

Abstract

The containment systems for the successive PHWRs in India have undergone progressive improvement in various aspects. In particular, the design for the standardised reactor at Narora (NAPP) has seen several notable improvements in its containment and related engineered safety features. The NAPP type containment design has been adopted for the next plant (KAPP), and similar philosophy is being used for our future 235 MWe and 500 MWe reactors of standardised designs. Containment buildings of standardised design are of concrete (PCC/RCC) construction and lined with epoxy/vinyl paint for leaktightness. Complete double containment (except the base slab) philosophy has been adopted with the space between the primary and secondary containment envelopes provided with a purging arrangement to maintain a negative pressure so as to prevent ground level release to the environment during accident conditions involving radioactivity release within the primary containment. The design of containment isolation, penetrations including airlocks and ventilation ducts, is such that the philosophy of double containment extends over them. The containment related Engineered Safety Features include: Reactor Building coolers for rapid post LOCA depressurization; Primary Containment Controlled Discharge System; Primary containment filtration and pumpback system and; Secondary containment filtration, purge and recirculation system. The loads for which the containment structures are designed include normal and construction loads, abnormal (accident) loads and extreme environmental loads including earthquake. Combinations of LOCA and seismic event of level S2 occurring simultaneously are considered. The requirements for provisions to manage and control hydrogen in the containment during postulated accident conditions, are currently under examination. With continuing evaluation and improvements, we believe that the present containment system design in India is an effective barrier in the overall defence in depth strategy for nuclear power plant safety.