The Time Travelling Mountain |
July 1, 2000 |
Fancy a visit to the catastrophic earthquake of AD 6000? Or perhaps you'd prefer to watch nuclear waste dumps crack open during the tenth millennium. Larry O'Hanlon takes you on a trip into the far future
BILL GLASSLEY wasn't looking for trouble, but he could be facing mountains of it. He and his team at Lawrence Livermore National Laboratory in California only wanted to create some tools for geologists like himself to study the Earth's anatomy. His aim was to build a giant computer model that would travel in time and reveal the changing structures of rocks and mountains. But Glassley's foray into virtual geology might be about to make his life a little uncomfortable. With his first look into the geological future he has foreseen some rather serious surprises in store for an underground nuclear waste dump planned by the government.
Glassley's prototype is a model of Yucca Mountain, a flat-topped volcanic ridge that rises some 360 metres out of the Nevada desert. The mountain is the proposed site for dumping nearly 80 000 tonnes of radioactive waste from American nuclear reactors and military facilities. Glassley's simulations show that drilling tunnels and storing the waste will produce some harmless-and possibly even advantageous-changes to the mountain's structure. The bad news is that, according to Glassley's model, the waste will create corrosive waters that could threaten waste canisters. There will also be some dramatic changes in the chemistry of the rock below the tunnels. And Glassley has not yet begun to factor in the effects of geological forces that grind, shake and split the region's rocks.
He never meant the software to be a political bombshell. The idea was for it to help geologists understand everything from groundwater supplies to earthquake prediction, by creating virtual landforms such as deep-sea vents, volcanoes and geological fault lines. But whatever Glassley's intentions, his Yucca Mountain results seem certain to be seized on by government officials and environmental campaigners. The Department of Energy-which also happens to be Glassley's employer-has already poured billions of dollars into checking the geology of Yucca Mountain. And as it has no alternative plan if the mountain is deemed unfit for use as a waste repository, there is a lot of pressure on the government to approve the disposal site.
Glassley's program is not the sort of application you'd want to try running on your desktop PC. The virtual Yucca Mountain exists inside more than 1400 parallel microprocessors, controlled by the Blue Pacific supercomputer. At its full potential the mountain will be split into 40 million cells of various sizes, each one described by 120 geological parameters such as stress, heat, chemistry, grain size, porosity and permeability. In key areas, the cells are just centimetres across. The computing power, fed with the best geological data available, can take the rocks forwards and backwards through millions of years, simulating every conceivable process going on within this portion of the Earth's crust. The whole thing has grown from equations developed in the late 1980s to describe the movement of water through rock pores and fissures-an essential part of modelling geological features. At that time no one had the software or the computing power to crunch the complex coupled equations, but now Glassley and mathematician John Nitao have done the necessary programming and proved that it works.
Once Glassley and Nitao were over that obstacle, they hit another one: too little geological data. To get credible results from a simulation, everything needs to be broken down to the scale of centimetres. Lawrence Livermore's supercomputing facility and the virtual mountain software make that possible; the problem, says Glassley, is that it needs to be given data of comparable accuracy. And, despite extensive surveying with boreholes, gravimetric surveys and detailed mapping of the main tunnel already drilled into the mountain, there still wasn't quite enough data to build its digital doppelgÄnger.
To compensate, the researchers generated their own data. They decided what a younger version of Yucca Mountain would probably have been like, grew it into a high-resolution structure and used the computer-generated data to make up for the missing geology. It might seem like a fudge, but it is surprisingly accurate: the virtual data even produced features that geologists had observed on the real mountain but had never been able to explain.
Once they were satisfied with their simulation, Glassley and his team drilled miles of tunnels in their virtual mountain, loaded them with canisters of hot nuclear waste, and sent the mountain on some preliminary voyages through time. It did not take long before the surprises started to appear. The tunnels themselves are a major disturbance to the mountain, Glassley says. "Then you add these hot things and it just goes nuts," he says.
The radioactive decay of spent fuel rods-the primary waste targeted for Yucca Mountain-gives off a lot of heat in the tunnels, often raising the temperature to over 100 °C. That's hot enough to drive water out of the 14-million-year-old compressed volcanic ash known as "tuff", which is the stratum geologists favour for tunnelling into the mountain. Some 20 per cent of the mass of the tuff is water, bound to the rocks' crystalline minerals.
According to the simulation, the combination of heat and water causes physical processes and chemical reactions that change the structure of the rock around the tunnels. The water that boils off moves up and away from the heat through fissures and pores in the rocks. On reaching cooler rocks, however, this vapour condenses and starts to run back down, dissolving some minerals out of the rock as it goes. Eventually this mineral-laden solution reaches the hot rocks again, where it vaporises, deposits its load of dissolved minerals and starts the cycle again (see Diagram).
Just a hundred years into the simulation, the heat and the movement of carbon dioxide has caused changes in the pH of rocks below the tunnels. Long, keel-shaped regions of more alkaline rock begin to appear, stretching towards the groundwater below.
After 300 years, the leaching cycle has created dome-like regions in the rock above and to the sides of the tunnels. The pores and fissures that initially carried the water have become clogged like household pipes encrusted with hard-water deposits, and this eventually prevents the water from moving through the rocks. It is a neat self-sealing trick, and since water percolating down from the surface is the main vehicle for spreading radioactive waste at Yucca Mountain, these relatively impermeable domes are probably a good thing, Glassley believes.
Chemical attacks
This benefit comes at a cost, however. Once the initial heat of the waste materials abates, and water is able to seep back into the tunnels, local variations in the chemistry of the rock mean this water is more acidic in some areas and more alkaline in others. This creates zones of highly variable acidity and alkalinity just centimetres apart within the tunnels and surrounding rock. That's a problem, says Glassley, because it means that waste canisters have to be resistant to a variety of chemical attacks.
Matters aren't improved by another surprise that the simulation revealed. Some waste packages produced more heat than others, and water evaporating from around the hot packages was continually condensing on their cooler neighbours. The result was dripping-wet canisters that, thanks to the mineral chemistry in the tunnels, also had high concentrations of chloride salts on their surfaces. And salt plus metal equals corrosion, as anyone who has seen the effect of seawater on steel will know.
The simulation also threw up a perplexing result: no matter how carefully the researchers tried to make the virtual tunnels the same as each other, each one developed differently. After puzzling over this for a while, the modellers realised that the difference was a matter of location. The mountain rock provides a huge heat sink, but each tunnel has a different amount of rock around it, depending on its position in the mountain. This affects the way the heat and water behave in each of the tunnels. It's a factor that probably would never have occurred to the repository's developers if the virtual mountain had not been built.
"This is an unbelievably powerful learning tool," Glassley says. Almost every time he runs the simulation it triggers new ideas and provides new insights into the challenges facing the Yucca Mountain project-and this, he insists, is good news for the DoE. By discovering these problems at the simulation stage, it may be possible to devise methods of arranging different types of waste with different heats to minimise the pH changes and moisture problems. At the very least, the designers will know where to place monitoring devices during the planned 100-year "confirmation period": a century of checks to ensure that nothing unexpected is going to happen in the repository.
But the timescale that really matters is much longer: the 10 000 years for which the government stipulates that the mountain must safely contain the waste. During that time Yucca Mountain could experience massive earthquakes or even the beginnings of an ice age. Neither situation has yet been simulated in the virtual mountain.
Turn the clock back 10 000 years and Yucca Mountain was a greener, wetter place. If another ice age should begin within a similar length of time, rainfall might again increase, sending extra rainwater percolating down through the mountain, and raising the water table beneath it. This would make it more likely that radioactive waste might escape into the water supply. Drier weather, on the other hand, would probably help keep the waste isolated from water. Either scenario can be easily applied to the virtual mountain to see how it responds, says Glassley.
Modelling what happens deep underground is rather more difficult. Yucca Mountain is set in a tectonic province called the Basin and Range. It has arisen because the North American continental crust is being stretched eastward and westward, causing blocks of crust to tilt and slide diagonally to fill the widening gap. This stretching has created a series of long north-south ranges, with equally long basins in between-each bounded by long faults.
According to preliminary measurements by geologist Brian Wernicke of the California Institute of Technology, the Yucca Mountain region moves about 2 millimetres a year. That may not seem like much, but it translates into 20 metres over the 10 000-year life of the disposal site. Even a remote possibility of a 20-metre crack in a nuclear waste dump is hardly reassuring.
Unfortunately, Wernicke's data won't necessarily help make the virtual Yucca Mountain more accurate. No one knows exactly where and along which faults that annual 2 millimetres of movement is taking place, and there's no way yet to determine how it will affect Yucca Mountain. The virtual mountain does take fault lines into account, Glassley says, but only as conduits or barriers to fluid movement, depending on the minerals they contain.
Ultra-secure designs
Uncertainties like these will not go unnoticed by the Yucca Mountain project's opponents who have already picked scores of holes in previous geological assessments of the mountain. Despite the DoE's strenuous efforts to prove that the mountain will never leak waste, its case is far from proven and many of the problems thrown up by Glassley's preliminary simulation had never been anticipated before. The DoE is now trying to counter these worries with ultra-secure designs for the waste canisters: stainless-steel cases wrapped in corrosion-resistant nickel-chromoly casings and titanium-palladium drip shields. Paradoxically, if these leak-proof containers prove to be a success they will give the protesters another argument against storing the waste underground: if the canisters are so safe, why do they have to go inside a mountain?
Like every other state, Nevada doesn't want to be a dumping ground for nuclear waste. Unlike most states, however, the majority of Nevada, including Yucca Mountain, is under federal control-so the state doesn't have much say in its use. Local politicians claim that Nevada was chosen simply because it has fewer people and votes than other states.
The virtual mountain won't settle the issue: Glassley and his team recognise that their work is just a scientific drop in a very large political bucket. But he remains optimistic that virtual landscapes will help to resolve similar issues in the future. "I would like to be in the position where this model can be used to arbitrate," he says. "I think in a lot of cases it's not going to provide the ultimate answer, but it's going to provide a next step."
He envisions three-dimensional projection systems that would someday enable decision-makers and scientists to "walk" into a time-travelling virtual mountain at any moment they choose. If they disagree on a scenario they could tinker with the model's parameters as they see fit, and immediately test the results.
Glassley is confident that the virtual Yucca Mountain is worth the effort-even given the controversy it is likely to stir up-because of the multitude of other potential applications. The same equations and coding could be applied to test various rock-fluid dynamics theories along the San Andreas Fault, explore for oil, and investigate damaging intrusions of seawater in coastal areas. It could even help determine where Mars is hiding its water, he says.
"This sort of computing power is going to revolutionise science," Glassley predicts. Until now, geologists have only been able to look backwards, but that's all going to change, he says. Thanks to the time-travelling mountain, geologists will from now on be keeping a wary eye on the future, too.
Larry O'Hanlon is a freelance writer based in California and Nevada
| 