WASHINGTON — The owner of a small nuclear reactor in Wisconsin said Monday that it would close the Kewaunee Power Station early next year because it was unable to find a buyer and the plant was no longer economically viable.

The decision was viewed as an early sign that the wave of retirements of old generating stations across the Midwest is now stretching from the coal industry into nuclear power, driven by slack demand for energy and the low price of natural gas.

After receiving a 20-year extension from the Nuclear Regulatory Commission in February 2011 to continue operating Kewaunee, Dominion, based in Richmond, Va., put the power station up for sale. At 556 megawatts, it is about half the size of the largest plants now operating and is the only reactor at the Carlton site, rendering costs higher per unit of power than sites with two reactors. Dominion had hoped to buy several reactors in the Midwest that could share some overhead expenses with Kewaunee, but did not succeed.

“This was an extremely difficult decision, especially in light of how well the station is running and the dedication of the employees,” said Thomas F. Farrell II, Dominion’s chairman, president and chief executive, in a statement. “This decision was based purely on economics.”