About 200 gallons of oil has leaked out of the Monticello Nuclear Generating Plant in southern Minnesota, according to Minneapolis-based Xcel Energy.

The leak was discovered Tuesday morning while crews were testing a motor that helps circulate water through the plant for cooling.

Plant officials say no oil has reached the Mississippi River. Xcel says it has notified federal, state and local officials.

The plant, which Xcel says has been “shut down for maintenance” since Feb. 20, is located about 40 miles northwest — and upriver —of Minneapolis.

“It is our priority to be good stewards of the environment and the communities where our facilities are based,” said Xcel officials in a statement released Monday.

The spill comes more than three years after 400,000 gallons of water containing the radioactive isotope tritium leaked into groundwater underneath the facility. Xcel was criticized for waiting months before alerting the public, and insisted only a small amount reached the river. The company was issued a $14,000 fine from state regulators.