While some of the current facilities and systems "are very important to safety," others provide "minimal or a negligible contribution to safety," the NRC said in a statement.

The revised rules would let plant owners "more precisely determine the safety significance" of the thousands of systems and pieces of equipment in their plants, the NRC said.

Nuclear power accounts for 20 percent of the electricity generated in the U.S.

The new regulations are being weighed as the industry grapples with heightened security and safety concerns in the wake of the Sept. 11, 2001 attacks on New York and the Pentagon and worries about the safety of the thick steel lids that cap pressurized water reactors.

Cracks in reactor vessel heads and leaks have been discovered at a handful of nuclear plants, prompting lengthy shutdowns and costly repairs.

The NRC has issued new orders to tighten security at nuclear plants to thwart attacks and has stepped up reactor safety inspections.

An NRC spokesman wasn't available to discuss the proposed revisions, but the agency has been working on them since 1999, said David Lochbaum, a nuclear safety engineer at the Union of Concerned Scientists, a nuclear industry watchdog group.

"The industry is trying to define what is safety equipment and non-safety equipment. This will enable them to save time and money on things that don't need a high safety regulation," Lochbaum said.

It was not clear when the NRC will issue the revised regulations.