Eleven U.S. nuclear plants received a Request for Information (RFI) from the Nuclear Regulatory Commission (NRC) on Friday asking for analyses of the effects of “thermal conductivity degradation” for nuclear fuel developed by Westinghouse under certain postulated accident conditions.

“Thermal conductivity degradation” refers to a phenomenon whereby older fuel has a reduced capacity to transfer heat. This could change its performance during various accident scenarios, including loss-of-coolant accidents, the regulatory agency warned. “The NRC is concerned that this phenomenon may not have been accounted for in realistic performance models for nuclear fuel developed by Westinghouse Electric Co.,” it said.

NRC regulations set a fuel thermal limit of 2,200F for “peak cladding temperature” under predicted loss-of-coolant accident conditions. Above that limit, the fuel rod is considered susceptible to damage.

The 11 plants named in the RFI are all Westinghouse clients with currently reported peak cladding temperatures above 2,000F: Beaver Valley 1 and 2, Braidwood 2, Byron 2, Catawba 1 and 2, Donald C. Cook 1 and 2, Kewaunee, and McGuire 1 and 2. An additional 23 plants that use the Westinghouse performance models also received informational copies of the RFI, to ensure that they are aware “of their obligations to address this error,” the NRC said.

“Thermal conductivity must be accounted for in realistic computer models used to evaluate a reactor’s emergency core cooling system. An error in the models may underestimate the fuel’s calculated peak cladding temperature. An error is considered significant if it would result in a difference of 50 degrees F or more in the predicted peak cladding temperature during the worst postulated loss-of-coolant accident scenario,” the agency warned.

“The NRC alerted the industry to this problem in 2009, and Westinghouse needs to do more to account for thermal conductivity degradation in its fuel performance codes,” said Eric Leeds, director of the NRC’s Office of Nuclear Reactor Regulation. “We need information from a few nuclear power plant licensees to maintain assurance that they can continue to operate safely with sufficient margin.”

The RFI stems from a December notification by Westinghouse to the NRC that an analysis it had conducted for a power plant indicated that thermal conductivity degradation could cause peak cladding temperature to increase by more than 100F during a worst-case loss-of-coolant accident at a Westinghouse pressurized water reactor.

The NRC staff responded by issuing an Information Notice on Dec. 13 (IN 2011-21) alerting industry that the error could cause a number of plant-specific evaluations to exceed the 2,200F limit.

The 11 plants have until March 19 to provide the requested information to the NRC staff. “If the information received does not demonstrate that NRC regulations are met, the staff will recommend imposing restrictions on reactor operating limits until acceptable action has been taken,” the NRC said.
 
Sources: POWERnews, NRC, Westinghouse