Sandia LabNews

Drilling for answers: Sandia pioneers nuclear waste disposal techniques

PAT BRADY AND BOB McKINNON look over a state-of-the-art drill bit used in deep boreholes. (Photo by Randy Montoya)

 This month, amid growing concerns over what to do about the nation’s high-level nuclear waste, Nature highlighted a promising concept being developed by Sandia, and the idea could soon become a reality. Early indications are that the DOE Office of Nuclear Energy’s proposed budget request for FY15 might include $3 million to start a deep borehole disposal demonstration project.

Sandia’s deep borehole disposal design is relatively straightforward: Using existing oil and gas drilling technology, drill a hole 17 inches wide and five kilometers deep in crystalline basement rock, line it with steel, and lower canisters of waste down into a stack two kilometers high. Finally, seal the top three kilometers of the hole with concrete and other materials. Each borehole could store about 400 canisters of waste and all that would be left at the surface is a mound of concrete.

Every year, US nuclear power plants generate roughly 2,000 tons of high-level nuclear waste, and the federal government is obligated by law to find workable storage and disposal solutions. "Right now, that waste is sitting in temporary storage, but Sandia researchers estimate the current and projected inventory of spent commercial fuel from the existing reactor fleet in the US could be stored in fewer than 800 boreholes, and more storage could quickly be drilled as new waste is generated.”

“Unlike a single mined repository that serves the entire US, boreholes are modular, so you could start storing waste in a matter of months, and could do it around the country, perhaps at the site of nuclear power plants, which would reduce transportation costs,” says Pat Brady (6910) who led a three-year LDRD project for further study of deep borehole disposal.  

Sandia researchers say deep boreholes could also be an option for disposing of other high-level radioactive waste that meets the size restrictions. For example, nearly 40 percent of the total radioactivity at DOE’s Hanford Site is concentrated in fewer than 2,000 capsules of cesium and strontium salts, separated from reprocessing wastes during the 1970s and 1980s. Each capsule is less than 9 centimeters across and less than 56 centimeters long, so potentially, a single deep borehole could dispose of all of Hanford’s waste.

Proponents of the concept say it’s permanent, more cost effective, more flexible, and more secure than other available options, and is immune to surface effects, such as water movement and climate change. Furthermore, waste stored at that depth is virtually inaccessible, since only a drill rig could access the canisters, and malicious activity could be spotted by satellite.

Andrew Orrell (6100), former director of Sandia’s Nuclear Energy and Fuel Cycle Programs, has two decades of technical and managerial experience supporting both Yucca Mountain Project and the Waste Isolation Pilot Plant. He knew that Sandia had the capacity to lead the nation’s discussion on the growing problem of waste disposal, and in 2009, met with Pat, Peter Swift (6220), Bill Arnold (6224), and a few others to discuss further study of deep boreholes and asked the team to produce the first performance analysis of the deep borehole disposal concept, which eventually led to the three-year LDRD. Sandia researchers formed collaborations with the University of Sheffield in England and MIT, which were instrumental to how well the analysis was received. “The team took on the challenge and produced a pivotal report that set the stage for enthusiastic discussions among the waste management community, in the US and abroad,” Andrew says.

An engineered solution

He says one major advantage of boreholes is that they can be engineered, which avoids the costly and time-consuming task of characterizing imperfections found in a natural setting such as a mountain. “The US invested heavily in Yucca, and one third of that spending went to site characterization of a complex natural system. That’s more than the GDP of some countries that have nuclear waste, so we need to find a solution offering high-confidence isolation that can be developed faster and at less cost” Andrew says. “Deep boreholes allow us to design a disposal system that should be faster, cheaper, and better performing.”

In January 2012, with Yucca Mountain essentially off the table and Sandia’s LDRD wrapping up, the Blue Ribbon Commission on America’s Nuclear Future recommended further study of Sandia’s deep borehole disposal design. DOE subsequently invested more than $2 two million in Sandia and its partners to continue studies and develop a full-scale demonstration project. 

The team has studied ways to overcome such technical obstacles as how thermal, mechanical, and chemical processes control borehole stability and fluid transport at 5 km depth. In particular, Sandia scientists had to show that the seals would remain intact over a million years. 

            Peter Swift is the national technical director of DOE’s Office of Nuclear Energy Used Fuel Disposition program. DOE is looking at four options as feasible alternatives: mined repositories in crystalline rock, mined repositories in salt, such as WIPP, mined repositories in clay or shale, and deep borehole disposal. “Sandia’s LDRD really did go after the major questions short of testing a demonstration, which is the next step,” Peter says. “The confidence that it’s safe long term will come from a good understanding of material properties at that depth and the behavior of the seal systems installed. We’re also relying on smaller-scale experimental work and modeling studies.”

            Since the initial DOE funding for further study, Sandia received an additional $400,000 to design a borehole demonstration project, which is under consideration by DOE for implementation. In addition, DOE has provided $850,000 per year for Sandia’s partner MIT through a competitively awarded Nuclear Energy University Programs three-year grant.

            “People have been talking about boreholes for the past 30 or 40 years, but nobody has actually done it, so we wanted to provide the technical basis to allow this to be one of the options for consideration,” Pat says, adding that given approval by DOE, work on a demonstration in the US could start as early as a year from now.