WASHINGTON -- The Department of Energy disclosed Monday it is dramatically scaling back at Yucca Mountain, laying off dozens of workers and virtually shutting down activity at the nuclear waste site in response to deep budget cuts.

"The tunnel is closed for all intents and purposes," DOE spokesman Allen Benson said.

Barriers may be placed at the mouth of the 25-foot-diameter exploratory tunnel that represented more than a decade of activity at the nuclear waste site 100 miles northwest of Las Vegas.

DOE officials described a new round of layoffs as a "restructuring" forced by budget shortfalls and a way to conserve money. They told members of Congress they will continue to work in Las Vegas offices to move forward in building a construction license for a waste-handling complex and underground repository.

But officials did not rule out other layoffs and delays, including possible decisions to put off plans to file for a license by the end of June.

"We are still evaluating what this latest reduction does to us," said Jason Bohne, a spokesman for Bechtel SAIC, the project's managing contractor. "The site is the first real impact and we are looking to see what else."

The Yucca repository, envisioned to store 70,000 metric tons of highly radioactive material, was originally planned for a 1998 opening. But a long series of DOE missteps, budget cuts engineered in Congress by Sen. Harry Reid, D-Nev., and legal challenges by the state have contributed to years of delay.

Sixty-three workers on the mountain ridge were told Monday they were being laid off, project officials said. They include 27 union metalworkers, electricians, miners and pipefitters, and another 36 people in administrative, environmental safety and health and property management positions.

DOE officials maintained research needed for licensing has been completed at the site, and the remaining workers were performing largely maintenance tasks. The job cuts are DOE's first response to budget cuts totalling more than $100 million for 2008.

More than 80 jobs were eliminated in earlier rounds of budget cuts last year.

DOE was given $386.5 million for this fiscal year, a 22 percent reduction from the Bush administration's request to stay on target. Managers say they are marshalling whatever money they have left to prepare a construction license application.

Bohne said the site is not being closed "but it is being greatly reduced."

Fewer than 10 DOE caretakers will monitor the site's access to water and electricity and take air quality readings as required by state permits. Security guards will be provided by the Nevada Test Site.

"We are not just locking the gate and walking away," Bohne said.

Reid said the cutbacks signal a continuing downhill spiral for Yucca Mountain.

"I am disappointed that people have to lose their jobs, but this is what this is all about," Reid said. "The only number that will make me happy will be zero" as far as Yucca Mountain spending.

Reid said the Bush administration "is playing a game" by characterizing the layoffs as a measure to conserve money for licensing. DOE should shore up safety studies with whatever money it has left, he said.

Bob Loux, a leading repository critic and executive director of Nevada's Agency for Nuclear Projects, said DOE had little choice but cut back, "and I don't think things get any better for them."

In its upcoming session, Congress is likely to be distracted by presidential politics and unlikely to pass an Energy Department budget, Loux said. The failure to pass a budget until the final week of this year's session contributed to DOE's woes.

"Until DOE makes up its mind what it is going to do about filing and defending a license application, this was probably the only thing they can do in the short run," Loux said of the layoffs.