(Changes time frame to late February instead of late January in paragraph 2)
By Timothy Gardner
WASHINGTON, March 1 (Reuters) - A group of engineers within the U.S. nuclear power regulator is concerned that a design flaw in nearly all U.S. nuclear plants could endanger emergency core cooling systems. The group has urged the regulator to order power station operators to either fix the problem or face mandatory shutdowns.
Seven engineers in late February petitioned the Nuclear Regulatory Commission to order immediate enforcement actions against licensees of U.S. nuclear power plants, in a little-noticed, but public move.
The petition, filed under a standard NRC process, urges the agency to respond by March 21.
The engineers are concerned that a design flaw in nearly all U.S. nuclear facilities leaves them vulnerable to so-called open phase events in which an unbalanced voltage, such as an electrical short, could cause motors to burn out and reduce the ability of a reactor's emergency cooling system to function. If the motors are burned out, backup electricity systems would be of little help, the petition said.
In early 2012 an unbalanced voltage event forced Exelon Corp's Byron 2 reactor in Illinois to shut down automatically. The unit was shut for about a week.
Later that year, the NRC alerted nuclear power plant operators in a bulletin to a potential design vulnerability concerning open phase and collected feedback from the operators.
But the agency never ordered the plants to make changes to reduce any open phase vulnerabilities. The petition said 13 open phase events have occurred at U.S. and international nuclear plants over the last 14 years.
Dave Lochbaum, a nuclear expert at nonprofit group the Union of Concerned Scientists, said it was encouraging that the engineers stepped forward without fear of retribution. But he said those concerns show "something is not right with the safety culture at the agency." The NRC could have eased concerns years ago by forcing plants to take action, he said.
"Why the NRC snatched defeat from the jaws of victory, I don't know," Lochbaum said.
The nuclear industry played down the petition. "This is not a matter of safety significance that merited interruption of the safe operations of our facilities, in 2012 or now," said John Keeley, a spokesman at the Nuclear Energy Institute.
Scott Burnell, an NRC spokesman, said the engineers' petition "will be considered under our normal process." Based on responses to the bulletin the agency issued in 2012, the NRC is "confident that plants are safe to continue operating," Burnell said. (Editing by Matthew Lewis)