Sandia at Yucca Mountain Project: Providing data for hard decisions
Sandia scientists — in the lab and inside Yucca Mountain — are conducting important experiments for the proposed geologic repository in Nevada that will permanently store America’s high-level radioactive waste.
"Sandia’s strength is in testing," says Cliff Howard, manager of the Labs’ Yucca Mountain Project (YMP) Repository Test and Analysis Dept. 6855. "We’re good at it, both in the field and in the lab. And the data we are developing are fed to modelers, who are working on the project’s performance assessment."
Cliff, who spent 14 years in Carlsbad working in the test group on the Waste Isolation Pilot
Project, knows whereof he speaks. Although he is quick to point out that YMP is an altogether different project (see "Different waste, different regulators . . ." on page 6), Cliff notes that Sandia’s reputation and respect have grown steadily at the Nevada site by dint of its quality testing results.
Project managers Bechtel-SAIC have come to value Sandia’s work on the project as they move forward with a complex effort to provide needed evidence to back up a repository license application. The application is due at the Nuclear Regulatory Commission by late 2004.
"Because natural systems are inherently uncertain, the tests and analyses we conduct are designed to support a risk-informed decision process," Cliff explains, driving north on US Highway 95 from Las Vegas toward the Nevada Test Site, where Yucca Mountain is located. "We are asking ourselves ‘What are the chances of a certain scenario occurring?’ and then ‘What are the consequences of that?'"
Sandia has done experiments that will answer parts of the puzzle when fitted together with the work of other experimenters at the Experimental Studies Facility, the name given to the cluster of experimental alcoves along the main tunnel drift and to the support structures at the two tunnel portals. Sandia’s testing has focused on the mechanical and thermal response of the rock to heat and pressure, Cliff explains.
Much of the work in the project for managers like Cliff and his boss Andrew Orrell (6850) involves meeting with other team members to make sure experiments are on track, quality assurance issues are being addressed, and the architecture of having multiple teams feeding data for modeling repository performance and providing design information continues to be robust.
To make such a meeting later this day, Cliff gets started before 5 a.m., picking up a visitor and heading for Yucca Mountain. As the desert flows past and the sun peeks over a range of mountains to the east, he is discussing the unique challenges posed by volcanic tuff, a dense, welded ash that showered down on the site 12 million years ago.
"The plan for YMP is that it will be in an active operating mode for about 75 years. We have to be able to answer questions about mechanical stability and worker protection in the mine. How many rock bolts will we need to use? What about wire mesh? What other structures and components are needed to operate in the underground environment to emplace radioactive waste packages weighing in excess of 90,000 pounds? In order to build a safety case you’d normally go to civil project experience from the mining of tunnels and other similar work. But mining isn’t done in volcanics and there are few civil projects to turn to for analogs. There’s almost nothing out there that’s comparable, so you can’t go to a textbook to consult."
Instead, experiments must provide the values to plug gaps in the knowledge base.
At a building just outside the north portal of the Yucca Mountain drift, nearly 100 miners, experimenters, and support staff have gathered for the 7 a.m. safety and operational briefing. To get this far — to the verge of a trip into the tunnel — visitors must have been trained in general underground and radon safety and the operation of a self-rescue device, to be used in the event of a fire inside the tunnel. It’s clear at the briefing that safety is important in this operational environment. "Radon readings are high near the south portal and may require respirators," a foreman informs the assemblage. A radioactive gas that is found in many places, radon can be encountered in higher-than-normal concentrations in the YMP tunnels, requiring precautions.
After strapping on a belt with the self-rescuer and a battery-powered lamp, which attaches to his hardhat, Cliff is ready for a trip inside the mountain. Visitors carry a card, showing they have received safety briefings and a radon dosimeter. Paperwork to ensure accountability for all workers is also a part of the process. Soon in a diesel-powered, open mining car, Cliff joins other workers rolling into the five-mile-long drift.
The main drift is 25-feet in diameter, generally horizontal, and carves a U-shape through the mountain. A 16-foot cross-drift has also been bored off the main tunnel into the rock horizon where most of the wastes will be stored. At the top of the tunnel bore runs a large-diameter air duct. A conveyor belt, used to carry "muck" or rock debris from the boring machine during the three-year drilling effort, runs down one wall. Along the other wall, thick high-voltage cables snake their way into the drift.
Just beyond the cross drift, the train comes to a stop at Alcove 5. Here two key Sandia experiments were conducted. To determine how heat will influence the rock, experimenters placed a heater, powered by several thousand watts, into a small bore and measured heat flow in the surrounding rock. "This helps us calculate the thermal conductivity of the rock," says Cliff.
Further down into the alcove, technicians are at work near a group of small bores that surround a large heated room. A bank of high-voltage control panels stands nearby. The boreholes allow for sampling water and studying the temperature and physical response of the rock to the large cylindrical heater. This is the drift-scale test, where according to a read-out on a large signboard, the rock received more than 6.09 million kilowatt- hours of heating. Heated from December 1997 to January 2002, the room is now in the cooling phase. However, a visitor, looking into the room through thick glass windows, can still feel the heat. "It will take several years to cool down, and measurements will continue from time to time to understand that process," Cliff says.
The water samples will be critical for modeling of another phenomenon associated with the repository, possible corrosion of the waste packages. "We need to know the chemical environment inside the drift," says Cliff. In fact the rock has about 10 percent pore space. The water, which fills about 80 percent of the pore space, gets hot and moves away from heat sources. (The high-level waste containers will generate heat.) After the rock cools, the water condenses again.
Later in the morning, Cliff walks past a giant boring machine, now in storage outside the south portal, and through an unlighted portion of the main drift to examine another large-scale Sandia test. In this case, workers cut two five-foot-tall slots into the wall of the drift, about a yard apart. Experimenters placed flat jacks into each slot to essentially squeeze the rock together using hydraulics and measure its stiffness. Midway between them, a borehole filled with instruments measured changes in the rock.
"We were able to map the fractures on the surface and in the borehole between the two flat jacks. Ron Price does similar work with samples that range from one inch to 12-inches. We’re taking a cubic-meter-size sample here and extending his work to get the engineering properties that will help design and support systems and help us assess stability during seismic events."
Before returning to Las Vegas, Cliff takes a radio, clears with ranch control, and starts up toward the crest of Yucca Mountain in a government pickup. From the top, the classic basin and range topography spreads in all directions. There is still a lot of work ahead for Sandia on the application to NRC, he says, between bites of a rye-bread sandwich, specially made for him by his daughter, Nalin. "We have to try to finish some experiments before we lose our craft help in the tunnel. There are real hard decisions yet to be made."