Elimination of BWR Mark I Program Primary Containment Drywell-to-Wetwell Differential Pressure

Abstract

The Boiling Water Reactor (BWR) Mark I (Mk I) Containment Program was implemented by the Owner’s Group circa 1970 to mitigate the effects of small, intermediate and design bases, Loss of Coolant Accident (LOCA), pipe break accidents on the torus, supports, internal structures and attached piping [1 & 2]. One of the significant mitigation attributes implemented during the Mk I Program is Normal Operation (NO) with a drywell-to-wetwell Differential Pressure (dP) maintained by the plant Nitrogen Make-up System used to inert the Primary Containment (PC) [3]. The differential pressure served to reduce the downcomer water leg in the vent system resulting in significantly reduced vent system thrust and pool swell loads. The vent system is designed to transport the mixture of drywell atmosphere, steam, and water forced from the drywell by the increasing pressure post-accident, quenching it in the pool of water maintained in the suppression chamber (torus). Based on plant experience, to sustain PC dP requires frequent venting of the Torus or nitrogen make-up to the drywell requiring Standby Gas System operation and cycling of PC vent and purge valves which places additional burden on the charcoal filters. The venting also places burden on Control Room Operators to perform the required actions and maintain Technical Specification (TS) limits. Elimination of the dP benefits plant operation by minimizing cycling of PC Isolation Valves, reduction of burden on Control Room operators due to the wider pressure control envelop, reduction in Standby Gas System cycles extending charcoal filter life and a reduction of the nitrogen make-up [4]. Overall, the Utility estimates that elimination of the dP will provide an estimated cost savings of $0.5 M per year.