The EPA proposal would put the project to build a massive underground storage depot beneath Yucca Mountain back on track.

Last year, a federal appeals court said the Bush administration wrongly ignored a recommendation from the National Academy of Sciences to ensure safety from leaks for well beyond 10,000 years. Radioactive releases could peak in an estimated 300,000 years and the administration must assure safeguards on that scale, it said.

The EPA said its proposal would set a maximum annual radiation dose of 15 millirem for the first 10,000 years of the project -- twice as long as recorded human history to date. The agency proposed a separate 350 millirem limit based on natural background radiation for the following period stretching to 1 million years.

"It is an unprecedented scientific challenge to develop proposed standards today that will protect the next 25,000 generations of Americans," said EPA assistant administrator Jeffrey Holmstead.

Nevada state officials, who oppose the project, said the million-year standard is too lax compared to protections required for repositories planned in Sweden, Germany and France.

The 350 millirem limit indicates that Yucca Mountain will rely almost entirely on containment from metal waste casks rather than the mountain's geology, said Robert Loux at the State of Nevada Agency for Nuclear Projects.

"We always thought the site was bad but it appears to be worse than even we thought it was," Loux said.

Nevada Gov. Kenny Guinn, a Republican, has repeatedly tried to block the project and may challenge the proposed radiation limits, Loux said.

The Energy Department had planned to apply to the Nuclear Regulatory Commission by the end of 2004 for a permit, but has indefinitely delayed that plan.

The administration hoped to open the site in 2010. However, recent delays call into question the timetable for the plan to store 77,000 tons (70,000 metric tons) of waste from 103 US nuclear power reactors.