ASME Code guidelines for PWR low temperature overpressure protection system limits

As reactor pressure vessels become more highly irradiated, pressure relieving devices installed to mitigate low temperature overpressure transients (LTOP systems) in pressurized water reactors (PWRs) can impose significant burden on plant operations. The burden imposed by the LTOP system can have a significant economic impact by extending plant start-up time (slower heat-up) and restricting plant operation. Further, LTOP systems can have an adverse safety impact because there is a possibility that once a pressure relieving device is activated it will fail to close. The ASME Section XI Working Group on Operating Plant Criteria (WGOPC) considered the burden and safety impact imposed by licensing guidelines for LTOP systems, and recommended Code guidelines for establishing pressure limits for LTOP systems. These guidelines relieve some operational restrictions, yet provide adequate margins against failure for the reactor pressure vessel; these margins are consistent with Sections III and XI procedures for ensuring vessel integrity. Further, by relieving the operational restrictions, these guidelines result in a reduced potential for activation of the LTOP system pressure relieving devices, thereby improving plant safety. These guidelines recently have been incorporated into Section XI of the ASME Code