On Thursday, February 16, 2012, the Nuclear Regulatory Commission (NRC) staff issued 10 CFR 50.54(f) letters to plants that use Westinghouse realistic emergency core cooling system (ECCS) methods (ASTRUM or CQD) and have a peak cladding temperature (PCT) greater than 2000 degrees F. The NRC is concerned that aging nuclear fuel used in Westinghouse reactors in Beaver County and elsewhere could have reduced ability to transfer heat and therefore exceed the 2,200-degree Fahrenheit limit in loss-of-coolant accidents.
Westinghouse informed the NRC in early December that a worst-case loss-of-coolant accident at a Westinghouse pressurized water reactor could raise the peak cladding temperature by more than 100 degrees Fahrenheit “
The NRC said there was a fundamental flaw in a computer program Westinghouse used in determining how reactor fuel loses the ability to conduct heat, a phenomenon known as “thermal conductivity degradation.” Because of that error there is a possibility that plants could underestimate how hot their fuel could get in an accident.
The NRC’s review could result in some nuclear plants being ordered to “dial back” their power-generating capacity to meet safety standards, NRC spokesman Scott Burnell said.
“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 letters required that the plants listed below provide information regarding the effect of a potentially significant error, as defined in 10 CFR 50.46(a)(3)(i), associated with thermal conductivity degradation (TCD), on peak cladding temperature in the Westinghouse Electric Company-furnished realistic ECCS evaluation models, to enable the NRC staff to determine whether the affected plant licenses should be modified, suspended, or revoked.
The 11 reactors are located at FirstEnergy’s Beaver Valley in Pennsylvania, Exelon’s Byron in Illinois, Duke Energy’s Catawba in South Carolina and McGuire in North Carolina, American Electric Power’s Cook in Michigan, and Dominion’s Kewaunee in Wisconsin.
The plants have until March 19th 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 press release states that:
“NRC regulations set a fuel thermal limit of 2200 degrees Fahrenheit for ‘peak cladding temperature’ under predicted loss-of-coolant accident conditions. Above that limit, the fuel rod is considered susceptible to damage. 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.”
BACKGROUND
Licensees operating nuclear power stations employ a series of computer codes to analyze plant behavior in the safety analyses they perform to demonstrate compliance with the Commission’s regulations. The computational approach models various physical processes to predict transient and accident events. These models simulate reactor conditions for postulated events and compare predicted plant performance to applicable regulatory criteria.
The simulation of the fuel element is an integral part of the safety analysis. Within the analysis, the fuel pellet thermal conductivity model determines the rate at which heat is transferred from the fuel pellet, first to the gas gap, then to the fuel cladding, and subsequently to the coolant.
A lower fuel pellet conductivity results in higher fuel temperatures at a given linear heat-generation rate. Therefore, the analytical prediction of the fuel thermal conductivity will affect the results of several types of safety analyses.
Any codes used for safety analyses that incorporate data starting at the fuel rod level and generated by the pre-1999 models may mischaracterize the expected plant performance.
A list of plants that received the 10 CFR 50.54(f) letters and ADAMS Accession number is below:
Beaver Valley, Units 1 and 2 – ML120400672
DC Cook, Units 1 and 2 – ML12041A384
Kewaunee – ML120410195
Byron, Unit 2 / Braidwood, Unit 2 – ML120410134
Catawba, Units 1 and 2 – ML12044A018
McGuire, Units 1 and 2 – ML12044A019
The NRC said it also sent copies of the request for information to an additional 23 plants that use the Westinghouse performance models to ensure that they too are aware of their obligations to address this error.
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