The Nuclear Regulatory Commission must explain why the Indian Point nuclear plant north of New York City was exempted from fire safety rules without allowing the public to weigh in, a federal appeals court ruled today.

The U.S. Court of Appeals for the 2nd Circuit found the NRC failed to explain why no public participation was allowed when the agency eased the plant's fire protection standards in 2007.

"The record before us fails to provide any agency explanation for why no public participation was deemed practicable or appropriate with respect to the challenged exemption," the three-judge panel said.

The NRC allowed plant operator Entergy to use a fire barrier that would last about 24 minutes at Unit 3 at the Indian Point plant in Westchester County. Without an exemption, the barrier -- insulation wrapped around wires and other equipment -- must last an hour.

Nuclear operators can apply for exemptions from the NRC's fire safety rules, but they must prove that equipment and wiring in the plants will be safe if a fire breaks out.

The commission will now have a chance to show why it excluded public comment when approving the exemption, NRC spokesman Neil Sheehan said.

But the court rejected claims from various nuclear watchdogs including the Sierra Club that the NRC had violated federal law by granting the exemption.

The NRC carefully vets each request for exemptions and in February denied other exemption requests that Entergy had proposed, Sheehan said.

"We don't approve exemptions in a vacuum, and we do it on a case-by-case basis," he said. "They have to prove that there will be adequate protection."