The utility said the situation was not an emergency and no radiation was released, the (Minneapolis) Star Tribune reported.
The 600 megawatt plan was idled Friday night after staff noticed more-than-usual amounts of water in a system that cools plant components. Excel spokesman Tom Hoen said the leakage occurred in pipes that are inside the reactor's containment structure.
The repairs are expected to take several days.
Xcel completed a $665 million project to extend the plant's life and increase its production of electricity last July. Hoen said the heat exchanger that failed was not replaced at that time.
Xcel is buying electricity from other sources or increase production at its other power plants as needed to make up the difference while the Monticello plant is out of commission.