An Analysis of Feed-and-Bleed Cooling of a Pressurized Water Reactor

Operators need to initiate feed-and-bleed (F&B) cooling to depressurize and cool down the reactor coolant system (RCS) of a pressurized water reactor (PWR) in the event of a loss of all feedwater. Long-term responses of the RCS and containment of a PWR in the loss-of-all-feedwater event with and without F&B cooling are analyzed with the Modular Accident Analysis Program (MAAP) computer code. Results of the MAAP analyses are compared with those from the RELAP5/MOD2 code. Results of the MAAP analyses show that the execution of F&B cooling at 48 min, as the steam generator secondary-side water level reaches a 6%-wide range, could depressurize the RCS along the coolant saturation curve with an average cooldown rate of 13 K/h. The conditions of the RCS reach the entry point of the residual heat removal system at ∼7 h.The RCS could still be depressurized if the execution of the F&B cooling operation is delayed to 70 and 100 min, i.e., ∼6 min after steam generator secondary-side dryout and 2 min after core uncovery, respectively. The average RCS cooldown rate, however, is above the limit specified in the technical specifications. Delaying execution of F&B cooling to 133 min can still depressurize the RCS. That, however, is too late to prevent the core from melting. Plant characteristics that are important for the responses of the RCS to F&B cooling are the flow capacity and the setpoints of the pressurizer power-operated relief valves, the flow rate, and the shutoff head of the high-head safety injection system.Results of the MAAP calculations need to be interpreted carefully because of the simplified nature of the MAAP code. Benchmark exercises of the MAAP input deck against the sophisticated system analysis code are essential for the validity of the MAAP results.