UPDATE: Story has been updated to include comment from Cook Nuclear Plant spokesman.

BRIDGMAN, MI -- Donald C. Cook Nuclear Plant in Bridgman has leaked an estimated 2,000 gallons of oil into Lake Michigan since Oct. 25, according to an event notification posted on the Nuclear Regulatory Commission website.

Plant officials notified the NRC, plus state and local officials, on Dec. 20 about the leak, which is from the Unit 2 main turbine lube oil cooler.

The notification said that "no visible oil or oil sheen is present on Lake Michigan or the shore line."

The leak was isolated as of 10:30 a.m. Dec. 20, the notice said. "Leak repairs will be made to the cooler prior to placing back in service."

Plant officials notified federal, state and local officials as soon as they realized there was a leak, said Bill Schalk, the spokesman for Cook.

He said the company noticed a slight drop in its oil levels starting on Oct. 26, but "there are lots of things that can cause that."

In addition, the plant was taken offline for two days at the end of October after excessive debris from large waves from Lake Michigan damaged several water screens.

"Because of that, we didn't have a good steady stream of date to recognize that we were losing (oil)," Schalk said.

It wasn't until Dec. 16 that it was determined a leak might exist and on Dec. 19, it was traced to one of four heat-exchange tanks, where hot oil runs through hundreds of tubes and is cooled by lake water in the tank.

"We've shut off that tank, and now we need to develop a plan to find the leak," he said.

The oil was leaking at a rate of about a third of cup, or 5 ounces, an hour into the plant's lake-water discharge of 1.5 million gallons an hour.

"We take samples of lake water every day, but the leak was small enough that we didn't pick it up," Schalk said.

The estimate of 2,000 gallons of oil leakage overall is based on a calculation that the leak began Oct. 26.

"That the worse-case scenario," he said. "It wouldn't be more than that, and it's probably less."

The incident has not affected the plant's electrical production, Schalk said, and the leak was small enough that there is no cleanup involved.

"We just need to fix the tank and make sure it doesn't happen again," he said.

In August, plant officials issued a report that 8,700 gallons of diesel fuel were suspected to have been released from a buried fuel oil tank, but later said there was no leak and the report resulted from "instrumentation error."

The plant is located near Bridgman, 11 miles south of St. Joseph along the Lake Michigan shoreline. Unit 2 went into operation in 1978.

The plant is owned by American Electric Power and operated by Indiana Michigan Power, an AEP subsidiary.

Julie Mack covers K-12 education and writes a column for Kalamazoo Gazette. Email her at jmack1@mlive.com, call her at 269-350-0277 or follow her on Twitter at @kzjuliemack.