Paul Golan, an acting director at the Department of Energy, did not tell the Senate Environment and Public Works Committee when the department will send its proposal to the Nuclear Regulatory Commission. That step was originally planned for 2004.

But Golan said the department will publish a schedule of when it intends to make such a submission "later this summer."

"We believe that submission of our license application should not be driven by artificial dates," Golan said.

The NRC must sign off on the plan before Yucca Mountain can begin accepting waste from the nation's 103 nuclear power plants.

Spent fuel from US nuclear plants -- which supply about 20 percent of US electricity -- is piling up. More than 50,000 tons (45,500 metric tons) of it is stored at over 100 temporary locations in 39 states.

The administration hoped to open the site in 2010 and allow 77,000 tons (70,000 metric tons) of waste to be stored deep underground.

On another front, the Environmental Protection Agency hopes to issue a proposal by year-end that would assure safe radiation doses from the site for 1 million years, which would satisfy a court order that threatens to derail the project.

Bill Wehrum, acting assistant administrator for air and radiation for the Environmental Protection Agency, told the committee his agency hopes to finish its proposal by year end.

Sen. Harry Reid, a Nevada Democrat and ardent opponent of the site for safety reasons, told the panel that the repository "will never be built because the project is mired in scientific, safety and technical problems."

Reid proposed handling nuclear waste through "dry cask storage," a process that would allow nuclear reactors to store waste on-site. He and Senator John Ensign have introduced a bill requiring nuclear utilities to use the casks.