The Nuclear Regulator Commission on Thursday said that one of the Limerick nuclear units in Montgomery County will receive additional oversight following a determination that the operators followed inadequate safety procedures.

An NRC inspection found that two main feedwater system valves failed to fully close during the startup of Unit 2 in April, resulting in a safety system being inoperable for a month.

The partially open valves created a path that would have prevented the majority of water from the reactor core isolation cooling system from reaching the reactor during an accident, the commission said.

"Because nuclear power plants are equipped with multiple safety systems, the reactor still could have been cooled and the plant safely shut down despite the condition," the NRC said. The unavailability of the system represented "a reduction in the plant's safety margins."

The NRC, which uses a color-coded ranking of inspections, said the "white" citation was of "low to moderate safety significance."

The NRC said the plant's owner, Exelon Corp., opted not to provide a response to the agency's finding, but the company had informed the agency of corrective actions it has taken, including checks of other valves that might be similarly affected, revisions to the operating procedure for the valves, and revisions to maintenance and testing procedures.

The NRC said that it would conduct a supplemental inspection to evaluate Exelon's analysis of the problems and its corrective actions.