The government’s 17-year effort to bury nuclear waste at Yucca Mountain in Nevada suffered a significant setback today when a federal appeals court said that the rules on radiation leaks could not be limited to the site’s first 10,000 years, as the Environmental Protection Agency had decided.

The Court of Appeals for the District of Columbia, ruling in a case brought by the State of Nevada and environmental groups, did not say what the planning period should be, but it quoted a National Academy of Sciences report that said a million years was possible. A 1992 law that committed the country to burying the waste required the government to follow the advice of the National Academy, the court ruled.

The government has predicted that Yucca Mountain, which is 100 miles northwest of Las Vegas, could contain nearly all the radioactivity for the first 100,000 years, but it has also said that by about 300,000 years, the dose to people at the site’s boundary would be many times higher than the legal maximum.

A lawyer at the Natural Resources Defense Council, Geoffrey Fettus, said that for the Energy Department to argue now that Yucca would function acceptably for hundreds of thousands of years, “they will have to contradict things they’ve already said in their earlier dose assessments.”

His group, one of the plaintiffs in the case, favors burying the waste, but Mr. Fettus said that it remained to be seen whether Yucca could be scientifically demonstrated to be appropriate.

“We want a repository based on science, not on political weakness in the late 1980’s, which is what happened here,” he said. Congress picked Yucca as the lead candidate for burying radioactive waste in 1987.

The dose estimates are based on several factors, including how long the metal containers holding the waste would stay intact and how radioactive materials would be carried through the soil by underground water flows.

In a 100-page decision, the three-judge panel rejected other arguments made against Yucca by Nevada and the environmentalists, including that it was unconstitutional for Congress to force a project like Yucca on an unwilling state.

But the decision gave Nevada the right to challenge the site’s environmental impact statement done by the Energy Department. Nevada wants to argue that the Energy Department gave insufficient attention to an alternative, leaving the waste where it is, mostly in the spent fuel pools of nuclear reactors, or in concrete casks nearby.

Michael A. Bauser, the associate general counsel of the Nuclear Energy Institute, the trade association of reactor operators, said the decision was a victory for his side because it “validated the overall process that led to the recommendation and selection of Yucca Mountain.” He acknowledged, though, that making predictions beyond 10,000 years was harder. “Uncertainties clearly increase with greater periods of time,” he said.

Mr. Bauser said that he did not know what the next steps would be, but that they could include asking for a rehearing, asking the Supreme Court to take the case or going to Congress to ask for a change in the law.

The Nuclear Energy Institute had argued in the case that the Environmental Protection Agency rules were too restrictive. The E.P.A. rules set a maximum permissible radiation dose for people outside the boundary of the Yucca project and set a second standard for the maximum dose through contamination of well water. The industry said there was no basis for a separate water standard, but the court disagreed.

Over the long term, rainwater percolating through the mountain and then flowing underground to wells is the most likely way that the public would be exposed. Because nuclear waste breaks down over time, eventually becoming harmless, predicting doses requires calculating the rate at which different radioactive materials will decay and how fast each would flow through the soil, based on its chemical and physical characteristics.

Nuclear materials are measured according to their half-life, or the time it takes for half the radiation to die away. Half-lives for the isotopes reaching Yucca vary from decades to millions of years, periods that the judges called “beyond human comprehension.”

At the very least, the decision makes it even less likely that the Energy Department can begin accepting waste at the site, on the edge of the Nevada Test Site, where the Energy Department and predecessor agencies tested nuclear bombs for decades. The department was planning to file an application for a license later this year with the Nuclear Regulatory Commission, which is supposed to make a licensing decision using standards set by the E.P.A. The Nuclear Regulatory Commission expected to begin hearings next spring, but a major element of the rules has now been thrown out.

A lawyer for Nevada, Joseph Egan, said, “As a practical matter, that means the licensing proceeding is completely on hold.”

The Energy Department did not return phone calls seeking comment.

Even before today’s decision, the 2010 target date for opening the site was regarded by people in the nuclear industry as highly suspect, because of the unprecedented nature of the legal proceedings that would be required before the Nuclear Regulatory Commission could grant a license.

The Energy Department has spent about $8 billion on the Yucca project, most of it collected from nuclear utilities, who signed contracts to pay for waste disposal at the rate of one tenth of a cent per kilowatt-hour generated at their reactors. The department faces damage suits that will run at least into the hundreds of millions of dollars for its failure to keep its end of the deal, accepting waste beginning in 1998.