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WASHINGTON - A few years ago, the plan to store the nation's nuclear waste in Nevada seemed all but certain.

Congress decided that highly radioactive waste from commercial nuclear-power plants, which takes centuries to decay, needed to be stored underground. And it reaffirmed by wide margins in 2002 that Yucca Mountain, 100 miles from Las Vegas, was the place to build such a repository.

But now that's being rethought, for a variety of reasons. And the Nov. 7 elections, which propelled Democrats into power on Capitol Hill, are likely to accelerate that thinking despite strong bipartisan support for Yucca Mountain in Congress.

* The incoming majority leader of the Senate, Nevadan Harry Reid, long has pledged that Yucca Mountain will never open. The incoming chairman of the Senate Environment and Public Works Committee, Californian Barbara Boxer, agrees. Both voted against the Yucca repository.

They think that nuclear waste should stay right where it is - at the nation's nuclear power plants - at least until better waste technology comes along.

''There's no rush to put it someplace that's dangerous,'' Boxer said.

* There are questions about how safe the Yucca Mountain facility would be, and others about whether transporting radioactive waste on roads and rail lines would pose unacceptable risks of accidents or terrorist attacks.

* More than 100 national and state environmental groups - including the Sierra Club, Greenpeace and the Natural Resources Defense Council - coalesced in September behind a set of principles that include permanent storage of used fuel at the reactor sites.

''The problem is the concept that the public wants the waste moved,'' said Michele Boyd, the legislative director and nuclear expert at Public Citizen. ''That's a 20-year-old concept.''

* Even the nuclear-power industry is giving ground. It still wants Yucca Mountain opened, but it's willing to allow taxes that plant operators pay into a fund for Yucca Mountain to be used for interim storage, a kind of euphemism for aboveground storage until there's a way to reprocess old fuel assemblies safely into new fuel. Nuclear Energy Institute President Frank L. Bowman told the Senate environment committee in September that surface-level interim storage could ''instill public confidence in the waste-management program.''

That confidence may have eroded because the Energy Department is eight years late in responding to a federal mandate to open an underground repository. Deputy Energy Secretary Clay Sell said recently that it could be ''decades'' before Yucca Mountain opens. Because of the long delay, plants already are turning to surface storage. At facilities such as Pacific Gas and Electric Co.'s Diablo Canyon, near San Luis Obispo on California's scenic central coast, construction is well under way on thick concrete pads that eventually will hold concrete-encased steel containers where fuel assemblies would be entombed.

PG&E spokesman Shawn Cooper said the company was still hopeful that Yucca Mountain would open someday. But as long as the Nuclear Regulatory Commission licenses cask storage, the waste could be there well into the next century, venting heat from the decaying fuel into the brisk Pacific Ocean winds.

''It's called temporary dry-cask storage, but the canisters can hold the waste 100 years,'' he said.

Jill ZamEk, a leader of San Luis Obispo Mothers for Peace, was one of the signers of the environmentalists' principles in September. Mothers for Peace is fighting to force a rearrangement of the dry casks so that they'd better survive a terrorist attack, and the Supreme Court will decide soon whether to hear that case.

''We want Diablo Canyon plants shut down,'' ZamEk said. When it comes to the plant's waste, however, she said, ''the risk of transporting it is so great it needs to stay where it is.''

Rep. Lois Capps, D-Calif., whose district includes Diablo Canyon, agrees that the waste should stay put but with more security to protect it.

''I believe that we should actually be beefing up security against potential terrorism and improving safety to prevent accidents at all nuclear facilities around the country,'' she said in a prepared statement.

Among Boxer's biggest concerns about Yucca Mountain is that it's not as impervious to water as initially thought. Sophisticated testing has shown that water percolates through its caverns and heads toward the Colorado River.

''Sixteen million Californians drink from that river,'' Boxer said.

Jon Summers, Reid's spokesman, said the senator would do all that he could to make sure Yucca Mountain never opened because the site was unsuitable. He said Reid had introduced legislation a year ago directing the Energy Department to take possession of the waste at the nation's nuclear plants and store it on site.

The bill drew a sharp rebuke last January from the nuclear energy industry, which said the legislation would only further undercut Yucca Mountain, which is exactly what Reid wants to do.

The bill went nowhere this year. The chairman of the Senate environment committee, James Inhofe, R-Okla., favors a Yucca Mountain repository. When the bill is reintroduced next year, however, Boxer will be heading the committee - and she likes it, or something like it.

She leans toward on-site storage but with the possibility of constructing regional or state gathering places for some of it, such as that at Rancho Seco near Sacramento, Calif., where a reactor closed in 1989.

Although she's a skeptic, Boxer also favors research into reprocessing, something that environmentalists oppose.

Boxer said that if a way to reprocess nuclear waste safely could be found, it would help with the waste issue, produce new fuel for reactors and ''make me feel more positive about nuclear power'' as a pollution-free alternative for lowering greenhouse-gas emissions from oil-, natural gas- and coal-burning power plants.

Growing interest in building a new generation of nuclear plants since the enactment of an energy bill that offers generous government subsidies is driving the industry's shifting attitude about waste storage.

Since Congress began working on the energy bill, nearly three dozen applications for new reactors have been planned. The bill was signed into law in August 2005, touching off what Sen. Pete Domenici, R-N.M., called a ''nuclear renaissance.''