YUCCA MOUNTAIN, Nev. - Deep beneath the desert floor, within a long, artificially lighted chamber, a crowd of helmet-clad visitors huddled over a grayish spot on the floor: a dried water stain.
Was groundwater leaking from overhead rocks? Isn't this chamber - which may soon become part of the Taj Mahal of dumpsites, a proposed high-tech burial site for the world's deadliest nuclear wastes, managed by the U.S. Department of Energy (DOE) - supposed to be dry as a bone?
Official representatives from a DOE contractor spent the next several minutes insisting that the water stain had not dribbled from groundwater in overhead rocks. Rather, they explained, it resulted from the condensation of vapor from outside air.
Such seemingly mundane questions could decide the fate of the U.S. nuclear industry. It might also affect the environmental future of this spectacular desert landscape, located northwest of Las Vegas.
Only months remain before Energy Department scientists expect to advise Energy Secretary Spencer Abraham whether Yucca Mountain is safe enough to become America's nuclear cemetery: the last home for superhot, murderously radioactive fuel rods discarded by the nation's 104 nuclear reactors.
Two decades ago, the Three Mile Island nuclear accident in Pennsylvania soured the U.S. nuclear industry's growth prospects. Now, emboldened by the energy crisis in California, President Bush and others advocate building more nuclear power plants as a partial solution.
But the nuclear industry's growth is stymied partly by the cost of maintaining discarded nuclear reactor rods at reactor sites. U.S. reactors have accumulated some 42,000 tons of fuel rods. At present, they're stored either in reactor pools or in "dry storage" casks nearby.
The United States has to put all that spent nuclear fuel somewhere: "Something has to be done with it," says Allen Benson, director of institutional affairs at the Energy Department's Yucca Mountain office in Las Vegas.
Officially, Energy Department experts say they haven't made up their minds how to advise Abraham. But it looks like their recommendation will be favorable, judging by their bubbly optimism during a guided tour of the Yucca Mountain site earlier this month.
Even if Bush goes along, Yucca Mountain faces tough going. Since Nevada's congressional delegation vigorously opposes the repository, its fate is likely to be decided by Congress, including a newly Democrat-controlled Senate.
Two decades of study, proponents say, show that water creeps extremely slowly through the mountain - so slowly that by the time it reaches the repository, the radioactivity will have decayed to a much safer level.
"We have found nothing that would disqualify this as a site," declared one of the two tour guides, mining engineer Jim Niggemeyer, who works for an Energy Department contractor.
The proposed repository would cost $49 billion - or much higher, critics suggest. It would consist of some 50 parallel underground tunnels, containing nickel-alloy chambers in which the fuel rods would rest.
Construction could begin by 2005, assuming it gets the go-ahead. The first nuclear waste shipments could begin by 2010. Designed to hold about 70,000 metric tons of waste, the dumpsite would likely be filled by the year 2035.
Afterwards it might be expanded, said mining engineer Patrick Rowe, who also works for a DOE contractor.
Antinuclear activists have opposed the repository for decades. They warn that if it is breached - by anything from a volcanic eruption to 25th-century gold prospectors to groundwater seepage - the result could be an environmental disaster of Hollywood dimensions.
Rowe said, "We've never found a single location of (rocks) dripping water ... We have yet to find anything in 20 years (of research) that would show Yucca Mountain is not suitable (for a nuclear repository)."
That draws a skeptical laugh from one of the site's relentless opponents, Bob Loux, head of the State of Nevada's Agency for Nuclear Development.
"DOE really doesn't know how much water there is in the mountain, doesn't know where it moves, and how it moves," Loux says.
To meet federal specifications, the dumpsite must remain intact longer than any other complex human artifact - 10,000 years. Groundwater typically travels slowly - about one millimeter a year - through the rock, Niggemeyer said. To illustrate, he held his arms apart and said: "It goes about that far every 1,000 years."
How do Energy Department researchers know this? They've wired the mountain with scientific instruments that show water moves at an agonizingly slow pace through the mountain, a mile-high pile of solidified volcanic ash left over from a volcanic eruption 12 million to 13 million years ago.
However, Niggemeyer adds, water can flow faster through cracks and faults in the mountain. But how fast? That's where scientific debate turns fierce.
Energy Department scientists' self-confidence was badly shaken two years ago, when researchers made a shocking discovery inside a Yucca Mountain tunnel: traces of water containing the isotope chlorine-36. The isotope had only one conceivable source: rainwater bearing radioactive debris from nuclear weapons tests in the Pacific Ocean after World War II.
How could the rainwater have then percolated through 1,000 feet of Yucca Mountain rock in less than 50 years?
But Rowe and Niggemeyer said the chlorine-36 study didn't worry them. The original study was conducted by a lab that lacked the right scientific equipment, Niggemeyer said. "We have never been able to reproduce the original results."
Significantly, he and Rowe noted, the water sample lacked traces of the radioactive substance tritium, which is routinely found in rainwater contaminated by atomic debris.
Yet the Energy Department continues to use the chlorine-36 data in its computer models of Yucca Mountain hydrology.
"Even though we've refuted it, we won't walk completely away from it. I don't quite understand it," Niggemeyer acknowledged.
Soon after the chlorine-36 study was reported, the Energy Department proposed adding an $8 billion titanium shield. Its purpose is to provide extra protection against rainwater sinking through the mountain.
The last-minute addition of the shield was a coincidence unrelated to the chlorine-36 study, Niggemeyer and Rowe insisted.
However, adding the shield, Loux suggests, reflects the department's dawning realization that the repository would be much more vulnerable to groundwater seepage than it admits.