Eleven U.S. nuclear power plants may have used computerized accident models that underestimated how much heat older nuclear fuel retains during severe accidents, according to federal regulators.

The Nuclear Regulatory Commission on Friday asked plant operators in six states to evaluate the heat-release phenomenon known as thermal conductivity degradation that affects older fuel.

Although there is no threat to public safety, NRC is concerned that older fuel rods at the plants could exceed the federal safety threshold of 2,200 degrees Fahrenheit during an accident that wipes out power to the sites.

Such high temperatures could damage the fuel rods' outer layer and eventually the reactor, NRC said.

"There are margins of safety built into everything, but the 2,200 [degree] limit is the part where we start to get uncomfortable," said NRC spokesman David McIntyre. "Anything over that ... would make it more difficult to cool the fuel."

The review could result in some plants being "dialed back" until the reactors meet federal requirements, McIntyre said.

Of particular concern are models that Westinghouse Electric Co. used at 34 of its pressurized water reactors to gauge the performance of nuclear fuel during disasters that wipe out electricity.

The commission said the models may not account for thermal conductivity degradation, and 11 plants reported high temperatures that raised concerns.

Those plants are Beaver Valley 1 and 2 in Pennsylvania, Braidwood 2 and Byron 2 in Illinois, Catawba 1 and 2 in South Carolina, the Donald C. Cook Units 1 and 2 in Michigan, Kewaunee in Wisconsin, and McGuire Units 1 and 2 in North Carolina. Operators of the units must respond to NRC's request for information by March 19.

"We need information from a few nuclear power plant licensees to maintain assurance that they can continue to operate safely with sufficient margin," said Eric Leeds, director of the NRC's Office of Nuclear Reactor Regulation, in a statement.

NRC initially alerted the industry to the issue in 2009 but did not require immediate action because earlier test data were inconclusive about the problem's significance.

Westinghouse notified the agency in December 2011 that analysis at one plant indicated the phenomenon could cause temperatures to increase by more than 100 degrees during a severe accident at the company's pressurized water reactors.

NRC notified 23 additional plants that used the Westinghouse accident models of the problem, and those plants are also expected to analyze the issue, McIntyre said.

Scott Shaw, a spokesman for Westinghouse, said the company is working with clients at specific sites to analyze the fuel and generate information NRC needs. Shaw said the issue won't shut down any plants and does not affect public health and safety.