Westinghouse Passive Thermal Shutdown Seal

Westinghouse Electric Company has developed and implemented a passive thermal shutdown seal that protects a nuclear plant’s reactor core.

It does this by preventing loss of reactor coolant system water inventory should an event occur that causes a loss of all reactor coolant pump (RCP) seal cooling.

The SHIELD® passive thermal shutdown 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 Operating Company (SNC) to successfully 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 (USA), during the plant’s fall 2010 refueling outage. SNC has announced plans to apply the product to Farley Unit 2 during its next refueling outage.

The Farley Unit 1 SHIELD® 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® shutdown seal improves Mitigating System Performance Index (MSPI) margin by reducing RCP seal cooling vulnerabilities and decreasing Core Damage Frequency by up to 50 percent. It has been proven reliable for a minimum of 24 hours under station blackout (SBO) conditions and an additional 48 hours after SBO with residual heat removal pumps operational. 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, the SHIELD® response time supports easy-to-implement fire protection strategies for National Fire Protection Association Standard NFPA 805 requirements.

The SHIELD® seal safety evaluation report has been reviewed and approved by the NRC for the Model 93A RCP and is acceptable for referencing in licensing applications for nuclear power plants that install the Westinghouse reactor coolant pump shut down seal.  Approval for the other RCP models is expected in early 2012.  This should 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.

“The SHIELD® shutdown seal offers additional levels of safety and reliability that will be valued by our industry, particularly in light of recent events in Japan.” said Nick Liparulo, senior vice president, Westinghouse Nuclear Services. “Electricity demand is increasing, and nuclear energy will remain a vital part of the world’s energy mix. It is incumbent upon us to provide products and services – such as the SHIELD® seal — that will support plants in the delivery of clean, safe, reliable nuclear energy.”

The current SHIELD® design is applicable to all Model 93A Westinghouse reactor coolant pumps. Westinghouse is currently investing in additional testing to demonstrate survival at extended station blackout durations and to test designs for additional Westinghouse RCP models.