The NRC says the incident occurred at about 9:30 a.m. Sunday at the Waterford, Connecticut plant. There were no injuries or release of radioactivity into the environment.
A Millstone spokesman says the leak was stopped before 10 a.m., and the unusual event was considered over about an hour later.
The incident met the criteria for the lowest of four levels of emergency classifications because the reactor coolant system leaked at a rate of greater than 25 gallons per minute. The NRC says the problem was traced to a relief valve on the shutdown cooling system.
The plant was in the process of being shut down for a planned refueling and maintenance outage.