By Marsha White, Senior Technical Writer, Westinghouse Technical Communications
Westinghouse Electric Co. and Southern Nuclear Operating Co. (SNC) partnered to successfully install the Shield passive thermal shutdown seal in the reactor coolant pumps (RCPs) at Unit 1 of the 1,776 MW Joseph M. Farley Nuclear Power Plant near Dothan, Ala. This installation was completed during the plant's fall 2010 refueling outage and SNC has since announced plans to install the Shield seal at Farley Unit 2 during its next refueling outage this fall.
The installation at Unit 1 was the first ever in the nuclear industry for the Shield product. The Shield passive thermal shutdown seals were installed without incident and within the duration of a routine seal replacement.
"This installation represents a substantial improvement in the nuclear safety risk profile for Farley and provides the plant operators with significant additional margin to an RCP seal loss of coolant accident (LOCA) in the event of a loss of offsite power," said Richard Wells, Outage and Scheduling manager for Farley.
Installation of the Shield presented no additional challenges to the Westinghouse crew and additional inspection procedures were transparent to the overall outage schedule. At startup, all RCPs performed as intended and had no impact on the expected operation of the pumps and seals.
Westinghouse developed the Shield seal as a passive means of protecting the reactor core by preventing a loss of reactor coolant system (RCS) water inventory should an event occur that causes a loss of all seal cooling. The Shield is a fail-safe protection that will reduce or eliminate leakage from the RCP seal with no operator action or power required. Installation has reduced the estimated risk of core damage by some 40 percent and the overall plant safety margin also has been improved.
|Southern Nuclear Operating Co.'s 1,776 MW Joseph M. Farley Nuclear Power Plant in Alabama. Photo courtesy of Southern Co. and Westinghouse Electric Co.|
The Shield shutdown seal improves the 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 Shieldflow limit of less than 1 gallons per minute (3.8 liters/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 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.
Approval of the Probabilistic Risk Assessment Model for the Westinghouse Shield was received from the Nuclear Regulatory Commission (NRC) in May 2011 for the Model 93A RCP and is acceptable for referencing in licensing applications for nuclear power plants that install the Westinghouse RCP shutdown seal. This should expedite the NRC approval cycle of plant-specific licensing amendment requests to take safety credit for the Shield seal installation.
|Figure 1 Shield schematic|
Development work on Shield seals for the other Westinghouse RCP models used in U.S. plants (Models 93, 93A-1 and 100A) will be completed by the end of 2011. These models require only subtle changes from the Model 93A Shield design and the NRC is expected to complete the safety evaluations for these models by mid-2012.
"We have been working closely with the NRC throughout this entire process, discussing the details, and the timeline for approvals," said Bill Mendez, engineering manager, Westinghouse Rotating Equipment Services.
Mendez recently met with Kansai Electric in Japan to understand the country-specific qualification requirements, and several plants in Europe are also requesting additional information regarding product availability for their fleets.
"Application of the Shield seal to boiling water reactor nuclear steam supply systems is being evaluated as well," Mendez said, "but that is farther off into the future."
Additional Shield seal pressurized water reactor installations are expected to take place in 2012 and could include several plants. Westinghouse is also investing in additional testing to demonstrate survival at extended SBO durations.
The Shield seal has already been tested at the conditions under which it would perform its design basis function of providing a leak-tight seal, up to 2,350 psig and 570 F reactor coolant system conditions, to provide a high reliability level. The Shield seal is designed for a service life of nine years.