There’s one thing every planned permanent repository for spent nuclear fuel has in common: They're all underground mines.

Like any mine, a mined repository for nuclear waste is a complex feat of engineering. It must be excavated by blasting or a boring machine, it must keep the tunnels stable using rock supports, and it must have ventilation, seals, and pumps to handle groundwater and make it safe for people and machinery. Unlike a mine, however, a repository must also transport and entomb canisters of radioactive waste, and it must be engineered to exacting standards that ensure the tunnels will keep the canisters safe for many millennia.

There is an alternative idea that dispenses with most of those downsides: disposal in deep boreholes. But can they be both feasible and safe?

Going deeper underground

At first blush, deep borehole disposal sounds entirely feasible.

The US Department of Energy was planning to drill a vertical borehole 4 to 5 kilometers (2.5 to 3 miles) to gain experience with the process, but the project was canceled in 2017. This borehole would have been about 10 times deeper than a mined repository, but such depths are not unusual for oil and gas boreholes.

Governments aren’t the only ones interested in the approach. Deep Isolation, a company founded in 2016 and headquartered in California, aims to offer nuclear waste disposal in deep boreholes as a commercial service anywhere in the world. “Depending on your geology, we can design a borehole for it,” said John Midgley, a geologist with Deep Isolation. The company’s designs could be anything from deep vertical boreholes to shallower J-shaped holes with horizontal disposal sections. Again, the oil and gas industry has gotten there first, drilling around 160,000 boreholes with horizontal sections in the USA alone.

“There are lots of oil and gas wells that deep, so the problem is going to be how hard the rocks are and how often your drill bits wear out, things like that, but in general… I don't think [depth] presents any additional problems,” said Sherilyn Williams-Stroud of the University of Illinois, an expert on geological disposal of nuclear waste and CO₂.

Since several disposal holes can be drilled and splayed out underground from one point on the surface, costs and environmental impact can be minimized, and there would be much less rock to remove and dump than with a mine. In theory, therefore, every nuclear plant could have its own disposal borehole, eliminating the need to transport spent fuel across the country.

Deep boreholes should also be able to take hotter waste than mined repositories because the canisters would be placed end to end and cooled by the surrounding rock. That means spent fuel wouldn’t need to spend as long as it does now in cooling pools at power plants. Proponents also claim that because deep boreholes would take up less space, be far deeper, and not be occupied, they would need far less and far simpler investigation of the site’s geology, saving even more time and money.

Boreholes should also be able to receive waste quicker. “We could complete the first borehole in less than two months,” said Rod Baltzer, chief operating officer of Deep Isolation. That’s in stark contrast to the decade or two needed to develop a mined repository. Baltzer also told me that Deep Isolation's initial calculations suggest the company could dispose of nuclear waste for “less than half the cost of a mined repository.”

As a bonus, the method is especially attractive for disposing of some nuclear weapon wastes. Putting weapons-grade plutonium at the bottom of a 4-kilometer borehole is intrinsically more secure than putting it in an accessible mined repository, and the extremely radioactive, heat-generating “Hanford Capsules,” which contain cesium-137 and strontium-90, could all be disposed of in a single oil-industry-standard borehole.

But the same attributes that make deep boreholes attractive also limit their practicality.