By MATTHEW L. WALD LAS VEGAS — After spending 14 years and $4.5 billion to figure out whether Yucca Mountain is dry and stable enough to entomb highly radioactive waste for 10,000 years, the Department of Energy is shifting its focus from geology to the protective powers of titanium and steel. What began as an exercise to find dry rock with predictable characteristics in an ancient desert ridge has evolved into a debate about whether the engineers can create materials that will survive the natural environment at Yucca. Nevada officials, who oppose the waste site, argue that 95 percent of the federal plan for safe storage now relies on tough containers, not on the geology that attracted federal interest in the first place. Since a repository with containers that good could go anywhere, said Robert Loux, director of Nevada's Nuclear Waste Project Office, "then you could put it in Central Park." While the alternative to Yucca, near here, is not really Central Park, the federal emphasis on engineered safety features represents a revolution in thinking. To debate that approach, the federal government and the State of Nevada are fielding opposing teams of scientists. When preliminary work on the site began 20 years ago, the emphasis was on the safety afforded by the mountain's geology. At the time, the federal government said the engineered aspects of a "geologic repository" (engineers wince at the word "dump") could not be counted on to keep things safe after the first thousand years, and perhaps not even that long. But as the Energy Department moves nearer to advising President Bush this year on whether to proceed with seeking a license for the site, the "engineered" portion of the design is dominating the discussion. This spring, the department issued a report that said containers could be counted on to hold up for at least 11,000 years after the site was sealed. The wastes will be dangerously radioactive for many millenniums, although the Energy Department need show only that releases will be small for the first 10,000 years. The problem is that Yucca Mountain has turned out to be wetter and its geology more complex than proponents had first thought. "The original perception was that it is drier than it is today," said Dr. Michael D. Voegele, the chief science officer of Bechtel SAIC Company, the Energy Department's chief contractor for investigating the mountain. The site is still extremely dry, Dr. Voegele said. Most of the six or seven inches of annual rainfall evaporates, he said, so a square centimeter of Yucca's surface would generally see a column of water only one centimeter high move through it in a year. Even if climate change triples rainfall at the site, it will be dry enough, they say. But water is more of a consideration than initially thought. One of the characteristics that drew scientists to the site was how slowly water was transmitted through the solid portions of the rock. But water flows through the mountain are about 10 times as large as first thought, largely because the rock, put there by successive volcanic eruptions nearby, turns out to be liberally laced with small fractures that were created as the volcanic ash cooled, and those fractures conduct water quickly. There is no agreement on how fast any escaping radioactive material would move through the dirt and to the nearest well, now about 11 miles away. At this geologic time scale, language tends to change. The state argues that the time for leaking waste to percolate down to those wells would be "essentially instantaneous" — only 300 years or so. Federal officials say that is a misinterpretation of their numerous computer studies and that any leaking waste would move extremely slowly. But the officials agree that their work has shifted into a new area, into how to design a repository consistent with what they have learned about the rock. "There's been an evolution after finding that the site isn't what it's supposed to be," said Mr. Loux of the state's nuclear waste agency. "They're doing engineering to shore it up." The latest design (still described as "conceptual") is to dig about 100 miles of tunnels and fill them with 11,000 to 17,000 containers, each about the size of a tank on a tanker truck, between two faults in the mountain. Each container would be made of stainless steel two inches thick and wrapped in a layer of a durable metal called Alloy 22, then topped with a $500,000 titanium drip shield that would resemble a giant rural mailbox. What water there is will be drawn away down the sides of the tunnel, not drop into the metal, federal engineers predict. A recent innovation is to perch the containers on metal rather than concrete, since concrete can turn the water acidic, encouraging corrosion. Corrosion is very much at issue. Experiments commissioned by the State of Nevada at a laboratory at Catholic University of America showed that Alloy 22 could break down in as little as 15 days — under harsh conditions, with the right recipe of heat, acidic water and trace amounts of lead, arsenic and mercury. One task now is to determine whether heat and time will cook any of those elements out of the rock and create those conditions. Rainwater at the site is not acidic, but state scientists say it will become so in the mountain, by absorbing carbon dioxide there. Scientists working for the federal government doubt that the water can become acidic enough to dissolve and carry lead to the canisters. Difficult decisions lie ahead. Engineers are still trying to determine whether the repository should be "hot" or "cool" — above the boiling point or below it. The peak temperature, which would be reached over a few hundred years, depends on how closely the waste containers are spaced. The "heated halo" around a hot repository would boil water into steam and drive it away, at least for the better part of the first thousand years, protecting the containers from corrosion. But state and federal scientists disagree over whether the steam may also cook trace elements out of the rock and later cause problems when the water flows back to the canisters. Engineers for the Energy Department say, however, that by spacing the emplacement tunnels more widely, they can ensure that the water will not accumulate above the repository but will filter through the cool areas between tunnels. A cool system would avoid the question but would allow water in almost immediately. Federal scientists say Yucca meets one of the early criteria listed for a site: it is a "noncommunicating hydrologic regime," which means that water leaving the site through the ground goes to a dead end, not to major rivers or oceans. In this case, the dead end is Death Valley, which lies just beyond the Funeral Mountains, clearly visible from the summit of Yucca. State scientists said they would be more impressed if the land between Yucca and Death Valley were not the Amaragosa Valley, with pistachio and alfalfa farms. To fill in the blanks in their research, federal scientists are dumping large volumes of water to see where it comes out again and heating areas of the rock the way spent fuel would heat it to measure the physical and chemical effects. New doubts about the site were raised about five years ago, when scientists found a synthetic isotope of chlorine at the depth of the proposed repository, about 1,000 feet below the summit and 1,000 feet above the water table. That was important because the isotope was created by atmospheric testing of nuclear bombs and spread worldwide by weather. The implication was that rainfall at the summit since 1945 had already penetrated to the repository. But when engineers did the tests again recently, they did not find the chlorine isotope, raising the possibility of testing error. "It's all a little bit disconcerting," Dr. Van Luik said. The working assumption is that the water did, in fact, travel the 1,000 feet in about half a century. "Until it's resolved," he said, "we don't want to step away from the conservatism." The "fast path" for water provided by cracks is important for another reason: beneath the repository level, some materials are zeolites, absorbent clays that chemists say would bind up radioactive materials that escaped their containers. But if the area directly beneath the repository is similar to the rock above it, much of the water will travel through cracks and bypass the zeolites. Engineers say they are confident that the zeolites will be beneficial but say they cannot figure that into their official calculations because they cannot demonstrate what will happen. To be confident in Yucca, federal scientists are trying to turn geology and geochemistry — the study of the interaction of water, pressure, heat and soil — into predictive sciences. At the urging of the Nuclear Regulatory Commission, they are also extending their study of volcanism. A line of cinder cones is visible from the mountaintop; scientists disagree about how old they are and when, and where, the next is likely to form. One cone was initially estimated to be half a million years old and later thought to be only 50,000 years old; a joke among project opponents is that when its age is estimated again, the eruption that produced it will not have happened yet. While the questions persist, so does the work at the mountain. About 1,800 people are involved in the project, arriving at the site on air- conditioned buses from Las Vegas. The group is well enough established that it has an employee association that sells coffee mugs and tote bags that say Yucca Mountain. And as the work goes on, the waste piles up at sites around the country. The Yucca Mountain repository is supposed to hold 70,000 metric tons of waste. But the radioactive waste from power plants already exceeds that amount — even before any increase in the number of nuclear plants, as envisioned by the Bush administration. The military also has a lot of waste that needs to be stored permanently somewhere. The alternative to Yucca is probably a purely artificial environment, the dry-cask storage yards now going up at nuclear reactors around the United States. These are thick concrete pads surrounded by razor wire and motion detectors, where enormous steel canisters loaded with spent fuel sit for the indefinite future. Reactor operators say these will operate flawlessly for decades. On the other hand, many sit in places that hardly anybody would have chosen for permanent disposal. Yucca advocates say the site has the advantage of being remote, as opposed to storage sites like Prairie Island, in Red Wing, Minn. That site, on the Mississippi River, is adjacent to a casino and a day care center. Copyright 2001 The New York Times Company