Waterford - The Unit 3 reactor at the Millstone Power Station came within one hour of a mandatory shutdown Thursday, but repairs to safety equipment were made in time to keep it operating uninterrupted.

The Nuclear Regulatory Commission on Friday issued a notice of a non-emergency event at the plant involving 12 main steam line pressure transmitters on one of the two operating units at the nuclear power plant. During an accident in which a main line carrying steam broke, the transmitters would initiate essential safety functions, said Neil Sheehan, spokesman for the NRC.

The problem with the equipment involved seals known as O-rings that were not replaced on schedule, he said. It was discovered by Millstone operators and reported to the NRC, and crews worked for about five hours until 10 p.m. Thursday to make the repairs, he said. The problem had existed for about a week, he added.

According to NRC requirements, Millstone would have been required to shut the plant down if the repairs had not been made within six hours after the discovery of the problem.

"It's an important safety function," Sheehan said. "If the transmitters aren't available during an accident, it would impede the operator's ability to monitor conditions in the plant affecting the steam lines."

Ken Holt, spokesman for Millstone owner Dominion, said the company is conducting a "human performance review" to determine why the O-rings were not replaced on schedule. "It was certainly serious, and we take it seriously," he said.

Sheehan said the NRC will do a follow-up analysis of the incident to determine whether Millstone would be required to take further actions or whether penalties against Dominion are warranted.

j.benson@theday.com