Insights Into Operating a Low Pressure Feed Pump for Loss of All AC and DC Power

Generic guidance for Pressurized Water Reactors (PWRs) has been developed to address the beyond design basis event of coincident loss of all Alternating Current (AC) and Direct Current (DC) power. The generic guidance included a strategy to use a low pressure feed pump to provide adequate secondary side heat removal via the Steam Generators (SGs) to delay or prevent core uncovery following loss of all AC power with battery depletion, loss of all DC power, seismic initiated events, and/or terrorist initiated events. The purpose of the project was to use thermal hydraulic analyses, operating experience, and other engineering analyses to identify and evaluate technical issues associated with the implementation of the low pressure feed pump strategy at Westinghouse and Combustion Engineering (CE) designed plants. The technical issues that were evaluated are those issues typically addressed in the development of a plant’s Emergency Operating Procedures (EOPs) and Off-Normal Operating Procedures (ONOPs). The thermal hydraulic analyses were performed using the computer code MAAP 4.0.5 and a plant model of a 4-loop Westinghouse designed PWR. The results of the analyses are also applicable to 2-Loop and 3-Loop Westinghouse and CE designed PWRs. The results of the evaluation indicated that the key technical issue potentially impacting the prevention of core uncovery for the implementation of the low pressure feed pump strategy is the potential and consequences of injecting nitrogen into the Reactor Coolant System (RCS) from the cold leg accumulators/Safety Injection Tanks (SITs). The results of the evaluation were used to develop sample instructions for implementing the low pressure feed pump strategy for Westinghouse and CE designed PWRs. The sample instructions were developed for two categories of low pressure feed pumps: (1) low pressure feed pumps with shutoff heads greater than the pressure that nitrogen injects into the RCS and (2) low pressure feed pumps with shutoff heads less than the pressure that nitrogen injects into the RCS. The usefulness of the sample instructions is maximized when the low pressure feed pump has local flow indication, throttling capabilities, deadhead protection, and local SG pressure indication is available. The results demonstrated that the design characteristics of the low pressure feed pump are important to prolonging/preventing the time of core uncovery.