Toss a dart at a map of the United States and, more often than not, it will land where shale can be found underground. A drab, relatively featureless sedimentary rock that historically attracted little interest, shale (as used here, the term includes clay and a range of clay-rich rocks) is entering Americans’ consciousness as a new source of gas and oil. But shale may also offer something entirely different—the ability to safely and permanently house high-level nuclear waste.
More than 70,000 metric tons of commercial spent nuclear fuel currently sit aboveground at 75 power-generation facilities across the United States. Surface storage of this growing inventory of long-lived waste is both a financial and security burden, and as the 2011 tsunami at Fukushima graphically showed, threats can be hard to anticipate. Recognizing this, plans in the United States and other nuclear nations, however vague, call for the eventual isolation of high-level waste underground. The devil is in the details of where and how to accomplish this.
Because spent nuclear fuel remains dangerous for hundreds of thousands of years, it is helpful (and arguably necessary) for the rock housing an underground repository to provide long-term containment as a backup to the waste packaging and repository engineering itself. Globally, considerable research is being devoted to determining what types of rock can do this. Sweden and Finland are almost entirely underlain by granite and well along with plans for granite-hosted repositories. Germany, once focused on underground salt formations, is considering its options. France, Switzerland, and Belgium are planning or considering repositories in shale.
So there is reason to believe shale can isolate spent nuclear fuel underground as long as needed. Shale also has what amounts to built-in containment redundancies, including a tendency for clay particles to sorb, or grab, dissolved waste. Then there is the sheer abundance of shale, which greatly expands geographic options for repository siting, even when oil- and gas-rich or otherwise unsuitable formations are ruled out.
On the flip side, concerns about shale include the possible instability of excavated repository shafts and rooms, and permeability increases caused by excavation. Shale is also a poor conductor of heat, which means a shale-based repository could get hotter than a facility sited in a different type of rock.
Shale’s virtues and deficits should be evaluated in the context of what has been learned about other rock types. Salt has extremely low permeability and is so deformable that fractures quickly heal, and even repository access shafts would close over time. But salt is wetter than first supposed, and heat from spent nuclear fuel may draw corrosive brine into a repository. (The salt-hosted Waste Isolation Pilot Plant facility in New Mexico, which until a recent incident was the world’s only operating geologic repository for long-lived radioactive wastes, accepts only transuranic wastes producing little heat.) Tuff at Yucca Mountain is above the water table and in a location where rainfall is scant. Nevertheless, there is evidence that water may pass through the tuff relatively quickly, perhaps along fractures. Excavations in granite are quite stable, and granite handles heat from spent fuel with little difficulty, but groundwater flow is affected by fractures and difficult to predict.
Read more at Shale: an overlooked option for US nuclear waste disposal