Waterford - The larger of the two operating nuclear power plants at the Millstone Power Station shut down Friday evening when steam generator water levels were too low to keep the reactor core cool, Ken Holt, spokesman for Millstone owner Dominion, said today.

The 1,200-megawatt Unit 3 remains shut down while plant operators work to fix the problem, which was caused when instruments that measure cooling water levels malfunctioned, he said.

"All safety systems responded normally," he said. "The reactor is stable and shut down."

Neil Sheehan, spokesman for the Nuclear Regulatory Commission, said the automatic shutdown happened as designed.

"The shutdown was uncomplicated, with all safety systems performing as they should," he said.

A resident inspector for the NRC responded to the plant Friday night to observe the operators' response and verify reactor safety, he said. The inspector will continue to follow up on the operators' efforts to diagnose the electrical malfunction, Sheehan said.