Civilian nuclear power plants came on line in the mid-1950s before most of us were born. Yet in all the years since then, there has never been a permanent place to keep their highly radioactive wastes. The resulting technological constipation was supposed to be solved with a repository carved into the volcanic rocks of Yucca Mountain in southern Nevada. As geologists got into the details, the worrisome part of Yucca Mountain turned out to be not rock, but water leakage.
We think of rock as strong, permanent material, and we tend to think the best rock should be the strongest. The U.S. and other nations have sought to keep their wastes in deep, crystalline rocks like granite. But we are misled by the presence of rocky mountains and the impregnable facades of bank buildings. The hardest rock, in geological reality, is highly fractured stuff because with strength comes brittleness. Tests at Yucca Mountain showed that some of the cracks in its volcanic rocks could have allowed rainwater to trickle down to the storage levels in a mere 50 yearsthis in a facility that by law is supposed to keep things safe not just for 10,000 years, but a million. That was all the scientific evidence needed to reinforce the stiff political opposition to Yucca Mountain, and by the time the Obama administration told Congress in 2009 that the project was off the table, it was all over but the shouting.
Now a government hydrologist, Christopher Neuzil, uses his knowledge of groundwater to make a case instead for shale. The best metric for a waste repository rock is not its strength, he says, but its impermeabilityresistance to water. This argument changes the game dramatically.
Shale, for our purposes, is rock that is primarily clay. Because geologists are choosy about what they call shale, Neuzil uses "shale" as shorthand for "argillaceous formations" (mudrocks and claystones). Water has a very difficult time moving through it. A lot of specialists don't like shale: water-well drillers find it unproductive; geotechnical engineers hate to deal with its poor strength; miners consider it waste rock; road builders must take special care in it. On the other hand, shale is good for municipal landfills and for wetlands precisely because it's so impermeable. Thick beds of shale are what keeps "fracked" oil and gas separate from drinking-water supplies.
Neuzil's publically readable paper, "Can Shale Safely Host U.S. Nuclear Waste?" is in this week's issue of Eos, the house organ of the American Geophysical Union. In it he points out an interesting peculiarity of shale: in many places it has different groundwater pressure than the surrounding rocks. In some places the shale has excess groundwater pressure; in others it's actually negativethe rock would suck up more water if it could but it's too impermeable. Imagine a waste repository in a place like that where water moves inward rather than outward.
The deeper significance of these anomalies is that they represent really, really long-term natural experiments. For instance, Neuzil describes a shale formation at the Bruce Nuclear Complex in Ontario where the hydraulic head is 400 meters lower than in the rock just 200 meters above it. The explanation is that the Ice Age glaciers there squeezed groundwater out of the shale, but in the roughly 10,000 years since the glaciers melted the groundwater has barely been able to start moving back. An overpressured shale body, on the other hand, could be holding its groundwater from a time millions of years earlier before erosion removed the rocks above it. These examples suggest how well this kind of rock can seal things off.
Similarly, Neuzil notes, shale blocks the movement of chemicals. Drill cores show that natural chemicals like chloride ions and hydrogen isotopes move through shale only a few centimeters per thousand years. Pure molecular diffusion, without any movement of water at all, is enough to account for that. These lines of evidence show that shale could very well meet the million-year criterion for waste repositories.
Is there enough shale to handle our nuclear waste? Neuzil says that "the United States is in an enviable position with respect to the scale and sheer diversity in age, history, composition, and thickness of argillaceous formations within its borders." It opens up the possibility of smaller, more localized repositories that share the national burden more widely. Politically, it means the states wouldn't need to gang up on Nevada again. Scientifically, it means that more research into the details of shale would be a very fruitful public investment.