A hydrogen gas leak shut down the Millstone Unit 3 nuclear reactor and the Nuclear Regulatory Commission (NRC) is now investigating.
The NRC said the leak occurred Sunday morning in a turbine building at the Waterford, CT-based plant, and workers responded by manually shutting down the reactor.
The leak occurred after a sealant plug for hydrogen in the main generator came lose. Workers successfully vented out the hydrogen, said Neil Sheehan, a spokesman for the NRC.
The reactor recently returned to operation after a scheduled outage for refueling.
The NRC says it is trying to determine if the leak may have been the result of work done during that refueling.