But by the time the water reached the lake, the level of radioactivity had been diluted to the point where it did not represent a health or safety risk, a spokeswoman for the NRC said.

"There was no danger to the public. It did occur. It is not anything to be alarmed about," said the NRC's Viktoria Mitlyng. Palisades does planned releases of diluted radioactive water into the lake at regular intervals, she said.

Over the weekend, the water, which leaked from a 300,000-gallon storage tank, went down a drain and into a basin, where what Mitlyng characterized as "an extreme dilution factor" occurred.

This weekend's unplanned event occurred after a leak in a safety injection/refueling water tank increased from one gallon a day on Thursday to 90 gallons either Friday night or Saturday, Mitlyng said. That crossed the 38-gallon threshold agreed to by the plant and the NRC as allowable in 2012. Palisades staff had been monitoring the leak daily, in accord with its agreement with the NRC, Mitlyng said.

The cause of the increase in volume of leaking water is unknown, according to an event report Palisades filed with the NRC.

"The licensee has been operating with SIRW leakage at a rate of less than 34 gallons per day. The leakage has increased for unknown reasons to a calculated value of approximately 90 gallons per day," the report stated.

The plant began the shutdown at 1:12 a.m. Sunday after the tank was declared inoperable.

The nuclear power plant was offline Monday as inspectors and Palisades staff worked on finding and repairing the leak. The tank will have to be drained to determine the location of the leak.

 

A leak in the same tank, resulting in seepage in the control room, caused Palisades to be shut down last year, according to the NRC.

Leaks have been an ongoing issue at Palisades, owned by New Orleans-based Entergy Corp., which shut down four times in 2012 and twice so far this year. Most recently, in February, the plant shut down for six days to repair a component cooling water heat exchanger and replace a damaged switch.

The NRC resident inspectors monitored the shutdown and are closely watching repairs, Mitlyng said, and the NRC is sending an additional inspector. There is no current timeline for when Palisades might resume service, she said.

Palisades has been under extra scrutiny after a series of safety issues in 2011. In September, the NRC conducted an 11-day inspection of the plant and determined that those problems had been "adequately addressed" by operators, but that additional monitoring was warranted. The Nuclear Regulatory Commission has scheduled an extra 1,000 hours of inspection at Palisades during 2013.