A federal appeals court threw the future of proposed nuclear waste repository at Yucca Mountain, Nev., into doubt yesterday by demanding that the federal government devise a new plan to protect the public against radiation releases beyond the next 10,000 years.

Although the three-judge panel rejected Nevada's constitutional challenge to the project, which has been under consideration for more than two decades, it required the government to meet a radiation standard some Energy Department officials have described as unrealistic.

Still, Energy Secretary Spencer Abraham welcomed the decision, and said he is confident the government could move forward, now that the court has "dismissed all challenges to the site selection."

"Our scientific basis for the Yucca Mountain project is sound," Abraham said. "The project will protect the public health and safety."

He said energy officials would work with the Environmental Protection Agency and Congress to "determine appropriate steps to address" the call for a new radiation standard.

Nevada officials, however, said the ruling means the end of the Yucca project, which is slated to accept as much as 70,000 metric tons of nuclear waste -- an amount that would fill a football field 19 feet deep. The waste consists of highly radioactive used fuel rods and other waste from national defense activities and commercial power reactors.

"We think the project's effectively dead," said Robert Loux, Nevada's state nuclear project director.

In recent years, federal officials have been pressing aggressively for construction of the radioactive waste repository at the site, 90 miles northwest of Las Vegas. The Bush administration determined in 2001 that it was technically and scientifically feasible to build the facility at Yucca Mountain, and Congress approved the site in 2002 over Nevada's objections.

Without question, the ruling means more legal obstacles for the Energy Department, which was planning to seek a license from the Nuclear Regulatory Commission this fall to begin construction. In addition to setting a new radiation standard, the federal government may have to defend itself against new challenges to the project's environmental impact.

Joseph Egan, Nevada's lead attorney for the Yucca Mountain Project, said the state could sue on matters ranging from transportation to whether the government should store nuclear waste on site. "There's all sorts of things now that are fair game for us," he said.

The court dismissed Nevada's assertion that locating the site at Yucca is unconstitutional and that other measures violate the nuclear waste law. But in a separate section of the 100-page decision, the judges took aim at the EPA for disregarding findings by the National Academy of Sciences, which called for a storage system that would protect against radiation releases beyond the next 10,000 years. Congress in 1992 approved legislation requiring federal officials to take the academy's recommendations into account.

The three-judge panel wrote that the EPA "unabashedly rejected NAS's findings, and then went on to promulgate a dramatically different standard, one that the Academy had expressly rejected."

Geoffrey Fettus, a Natural Resources Defense Council staff attorney who challenged EPA's standard, said the court ruling means that the government must show its repository can "rely on geology to isolate waste" rather than human-made barriers.

"It certainly raises serious questions about the adequacy of Yucca's geology to isolate waste for the requisite period of time," Fettus said.

The ruling quotes a former energy official, Lake H. Barrett, who wrote in 1999 that devising a radiation standard beyond the next 10,000 years "would be unprecedented, unworkable, and probably unimplementable."

But Energy Department spokesman Joe Davis said the agency is confident it could come up with a plan to protect the public against radiation for longer than 10,000 years. He also said the court indicated Congress could set a new radiation requirement despite the academy's study.

"We're confident in our science and we're moving forward," Davis said.

Either side could appeal yesterday's ruling to the Supreme Court, but it is unclear whether either will. These same groups -- in addition to Nevada authorities and environmental organizations, the Nuclear Energy Institute, which is the nuclear energy industry's policy organization, also disputed aspects of the plan -- are likely to challenge the Energy Department's license application before the NRC.

The department has already filed 5.6 million pages of scientific material as part of its application, Davis said, and the process could take three to four years to complete. He added that federal officials are reevaluating whether to seek a license this fall in light of the court's ruling.

"We still have to take our case before the NRC," Davis said.

Staff researcher Meg Smith contributed to this report.

A three-judge panel ruled that the government must change a radiation standard for Yucca Mountain, Nev., site of a proposed national nuclear waste dump.