Exelon Generation, a unit of Chicago-based Exelon Corp. EXC.N , filed at the Nuclear Regulatory Commission for an early site permit to possibly build a nuclear plant on land next to an existing Exelon reactor in Clinton, Illinois.

Dominion Resources, of Richmond, Virginia, also asked for the permit that could lead to a third reactor at its twin-unit North Anna station in Mineral, Virginia.

Each company said it had no immediate plans to build new plants.

The permit doesn't authorize plant construction but is the first of a two-part licensing process under NRC rules on site suitability, environmental impact and emergency planning.

The utilities said the NRC's review and approval process could take up to 33 months or more. If granted, the permit would be good for 20 years and renewable for another 20, allowing Exelon and Dominion to "bank" the land for future use.

If the companies wanted to proceed with construction, they would have to go through a second process to get a license to build and operate a reactor.

No new nuclear power plants have been put in service in the U.S. since 1996, when the Tennessee Valley Authority's Watts Bar 1 plant went on the grid in Tennessee.

New construction was halted by safety worries in the wake of the near meltdown of the reactor core at the Three Mile Island plant in Pennsylvania in 1979 and the deadly explosion at the Chernobyl reactor in 1986 in Ukraine, then part of the former Soviet Union.

Opponents of nuclear power also point to hazards raised by long-term storage of spent radioactive fuel.

But new nuclear plant construction is gaining some support in utility and government circles due to rising prices for fossil-fueled plants and concerns about the impact of their emissions on the environment.

Exelon, which operates a fleet of nuclear stations in the Midwest, said the Clinton site originally was designed for two units but only one was built, and it is close to the Midwest region's transmission grid.