A Nuclear Regulatory Commission posting says that the Donald C. Cook Nuclear Power Plant near Bridgman, MI, leaked about 2,000 gallons of oil into Lake Michigan over a two month period, starting about October 25.
The oil was not involved in the plants radioactive process, plant officials said.
It was undetected for two months because the level of leakage was tiny compared to the millions of gallons of water that carried it into the lake, plant officials said.
The oil likely got into the water through a leak in tubes of a heat exchange system that draws water from Lake Michigan to cool the oil that lubricates plant turbines. But the turbines are not a part of the nuclear reaction process, so the oil was not exposed to radioactivity, said a plant spokesman.
Communication manager Bill Schalk said circumstantial factors contributed to the delay in its the leak's discovery. The oil level can be affected by factors other than a leak in a heat exchange tube, including changes in lake water temperatures, Schalk explained.
Workers have shut down the offending cooling system tank. The plant can operate at full capacity, however, because there are three other cooling tanks that keep the lubricant from overheating.
The NRC, the Michigan Department of Environmental Quality and the Environmental Protection Agency are all expected to monitor the lake for continued problems.
The plant estimated the oil leaked into the water at a rate of 0.04 gallons per minute compared to water discharge rate of 1.5 million gallons of water per minute.