US utilities order Westinghouse nuclear shutdown seal
London – In the US, Westinghouse Electric Co. has installed its reactor coolant pump (RCP) passive thermal shutdown seal in one dual-unit nuclear plant, and has received orders from five more utilities for a total of 37 reactor coolant pump (RCP) installations.
The company’s Shield passive thermal shutdown seal protects a nuclear plant’s reactor core by preventing loss of reactor coolant system water inventory should an event occur that causes a loss of all reactor coolant pump seal cooling.
The seal is a fail-safe protection that requires no operator action, power or control logic. It is activated by heated reactor coolant and provides an extremely tight seal if cooling for the RCP seals is lost.
Westinghouse partnered with Southern Nuclear Co (SNC) to install the ’first-of-its-kind’ Shield passive thermal shutdown seal in each RCP at the Joseph M. Farley Nuclear Power Plant (Unit 1) near Dothan, Alabama, during the plant’s fall 2010 refueling outage and in Farley 2 during the fall 2011 refueling outage.
The Farley Unit 1 seal installation garnered a 2011 Nuclear Energy Institute (NEI) Top Industry Practice (TIP) Award for Southern Nuclear. The installation of the SHIELD shutdown seal reduced the estimated risk of core damage by some 40 percent, and the overall plant safety margin also has been improved.
The Shield unit is said to improve the mitigating system performance Index (MSPI) margin by reducing RCP seal cooling vulnerabilities and decreasing Ccore damage frequency by up to 50%.
Currently, the US Nuclear Regulatory Commission (NRC) has approved 24-hour survivability of the Shield seal under station blackout (SBO) conditions for regulatory applications. Testing is in progress to extend the mission time for the SHIELD shutdown seal to 7 days, and it is expected that NRC approval will be requested in the first quarter of 2012.
The Shield flow limit of less than 1 gpm (3.8 L/min) also addresses the need for supplemental makeup for compliance with the NRC’s 10 CFR Part 50 Appendix R - Fire Protection regulation. Additionally, its response time supports easy-to-implement fire protection strategies for National Fire Protection Association Standard NFPA 805 requirements.
With the shutdown seal installed, operators do not need to implement an immediate cooldown to address RCP seal leakage at the onset of a station blackout event, and thus, are able to focus their efforts on other critical tasks, such as power recovery and maintaining a heat sink, said Westinghouse.
The seal safety evaluation report supporting the installation of the SHIELD seal at Farley Units 1 & 2 has been reviewed and approved by the NRC and is acceptable for referencing in licensing applications for nuclear power plants that install the Westinghouse reactor coolant pump shutdown seal.
This will expedite the NRC approval cycle of plant-specific licensing amendment requests to take safety credit for the SHIELD seal installation. In addition, the SHIELD seal does not require periodic surveillance and maintenance, and was designed to require no plant and no onsite equipment design modifications. This supports short installation lead time.