Supercomputer fitted with new program to analyze workings of Yucca Mountain

Wednesday, July 12, 2000 | 11:51 a.m.

A computer code that did not exist four months ago is a new secret weapon for Department of Energy scientists trying to prove Yucca Mountain will safely contain highly radioactive wastes.

The sophisticated program, completed in February, is being put to work puzzling out how water might behave inside the mountain hundreds of years into the future. Yucca is the only site being studied as a possible permanent repository for 77,000 tons of highly radioactive commercial and defense waste.

Eventually it also will be able to help scientists study underground natural gas and oil at other locations.

While most people think Yucca is a dry desert mound in the middle of nowhere, geochemist Bill Glassley has discovered 20 percent of its bulk contains water clinging inside pores, cracks and earthquake faults.

How water changes in its travels through Yucca's rock as a vapor and a liquid is the question put to a supercomputer called Blue Pacific. A team of scientists led by Glassley in February installed a new program into the supercomputer and packed it with conditions from Yucca Mountain's distant past.

At Yucca the team is using that information to try to answer some critical questions the DOE has not been able to address in its 15 years of research.

Could the water become acidic enough to corrode containers and allow deadly radiation to escape? Could salts dissolved in ground water corrode the containers?

"Nature has done a lot of experiments for us, as long or longer than the proposed repository," Glassley said. The supercomputer allows scientists to push Yucca's limits.

Scientists already know that heat as high as 212 degrees Fahrenheit from the buried nuclear waste containers will turn the trapped water into steam, allowing it to escape from the tunnels, Glassley said. But when the vapor reaches cooler rock, it will condense into liquid again and pick up minerals on its journey back toward the wastes.

The computer model so far has predicted that the dissolved minerals are mostly plain table salt and potassium chloride. Glassley said the computer model so far matches the few water samples taken from the mountain.

The computer also predicts that while the water recycles itself through the mountain over and over again, it grows less salty by spreading itself throughout the rock during the complex process.

"The results are very preliminary," Glassley said in a phone interview, noting that even with the new program, he gets nervous with forecasts beyond 500 years.

One surprise in the model came from results of the water's acid content. "Pure" water registers a 7 on the acid/alkaline scale known as pH. Very alkaline water is a 14. Very acidic water is a 1 or 2 on the scale. Yucca's water ranged between 4 and 8 -- a broad range that causes scientists concern.

"That's a big difference," Glassley said. If nuclear waste containers fail, the pH and the temperature become very important to what happens to the water inside the mountain, he said. Scientists may have to devise several different ways of dealing with water damage to canisters, depending on the alkaline or acidity levels.

Another problem is what happens to the radiation if that nuclear waste containers -- made of stainless steel with corrosion-resistant nickel-chromoly casings wrapped around them -- fail. The buried containers will be further protected by titanium-palladium drip shields to protect them from moving water.

The DOE has predicted a single container will fail in the first 10,000 years.

Livermore scientists plan to simulate 100 separate containers failings. They have already discovered that each tunnel housing the wastes reacts differently to changes in heat and water.

"It is a very challenging project and it is pushing the state of the art," Glassley said.

The computer simulation has not taken into account earthquakes or other natural disruptions, such as a hotter or wetter climate. That is one of the hardest problems to tackle in the new virtual geology, Glassley said.

If a nuclear waste repository is built at Yucca, scientists will have to monitor the buried containers for decades to see how the mountain behaves.

"All of the models, no matter how good, are not going to be good enough," Glassley said. "It is going to be an ongoing process."