Xcel Energy said Monday that the Monticello Nuclear Power Plant could remain shut down for several days as workers repair a leaking heat exchanger on the reactor’s coolant system.

Operators shut the plant down Friday night after detecting greater-than-normal levels of water in a system that cools plant components, Xcel said. The problem turned out to be a leak inside a heat exchanger.

Xcel spokesman Tom Hoen said the leakage was into other pipes, all of which are inside the boiling-water reactor’s containment structure, and no radiation was released. The event is not considered an emergency.

“It was an operational issue that required a plant shutdown,” Hoen added.

In July, the Minneapolis-based utility completed a $665 million project to extend the life and boost output of the plant, which is 45 miles northwest of the Twin Cities.

Hoen said the failed heat exchanger was not one of the components replaced during the project. He said workers on Monday were determining the best way to repair the unit.

While workers make repairs, Xcel said it will purchase power from the midcontinent power grid or other utilities, or increase electricity production at its other generating plants.

The five-year upgrade project, which replaced pumps, filters and other equipment, ended up costing more than double the original estimate. State utility regulators are investigating whether the expense was prudent. Xcel has defended the investment.

The reactor generates 600 megawatts of electricity. The output is expected to increase by 12 percent once the U.S. Nuclear Regulatory Commission approves the new power rating.