"Our site is not operational at this time. The Nuclear Regulatory Commission has taken the action to shut down its web site," the NRC said in a statement posted on the site (http://www.nrc.gov).

"In support of our mission to protect public health and safety, we are performing a review of all material on our site. We appreciate your patience and understanding during these difficult times."

Among information previously shown on the Web site was the plants' locations, including longitude and latitude, and general design specifications for each facility.

Several electricity traders, who look daily to the federal agency's plant status report for fundamental market supply data, told Reuters not knowing whether a plant was operating raised uncertainties that would be reflected in higher prices.

"It's amazing how Sept. 11 has affected things you would never expect," one Houston-based trader said.

The daily plant status report lists the operating status of each of the 103 U.S. nuclear reactors, which provide about 20 percent of the country's electricity.

"If they find some reason this information would be dangerous in the hands of a terrorist, then I'm all for keeping it off the Web site," the trader said, echoing the views of all the power traders Reuters surveyed.

They warned, however, that keeping the information from the marketplace would give reactor owners and the local utilities they supply a big advantage over energy marketers who have no power plants in the area.

Nuclear reactors are among the lowest cost sources of electricity in the United States.

When a nuclear plant shuts, the region's grid operator tells generating companies to fire up more expensive oil-and gas-fired plants to cover the shortfall.