When one thinks of underground infrastructure, sewer service, oil and gas pipelines, and transit systems typically come to mind. A new addition to this subterranean set may soon be in the offing: a small modular nuclear reactor.

Deep Fission plans to install its aptly named Gravity reactors one mile underground, exploiting the geological depth as containment to enhance the safety and economics of nuclear power. Measuring 9 m tall, the pressurized water reactors can be installed in 30-inch water-filled boreholes in a modular arrangement that enables multiple 15 MW reactors to be clustered on a single site. An assembly of 10 would total 150 MW; larger groupings are expected to scale to 1.5 GW. Passive shielding and natural containment are afforded by the billions of tons of rocks in the surrounding geology.

Fueled by low-enriched uranium and using steam to transfer heat to the surface for power generation, the reactors are subjected to 160 atmospheres of pressure, the same maintained inside a conventional nuclear reactor at the surface. The steam produced safely circulates in a closed loop system to generate power.

Deep Fission plans to drill the borehole, lower the canister, load the fuel and bring the reactor to criticality underground in 2026. The first commercial-scale projects could be underway by 2028 at selected sites in Utah, Texas and Kansas.