Nuclear power start-up Deep Fission wants to put a reactor where no reactor has gone before: a mile underground. The California-based company received聽聽for its reactor design from the US Department of Energy earlier this month, and is preparing the site for its commercial pilot outside Parsons, a small city in south-east Kansas.聽
The company hopes that placing a reactor deep underground will cut installation and operational costs, particularly since the bedrock can provide natural containment and pressure for the reactor. This eliminates the need for a heavy-duty concrete containment dome like those that enclose conventional nuclear reactors, which account for a聽 of their cost and the time it takes to build them. But independent engineers say it is unclear how successful the approach will be.聽
Deep Fission has already drilled a 1830-metre data-collection well, and will soon begin drilling a 762-metre proof-of-concept borehole. Its next phase will involve installing the reactor canister, heat exchanger and other components into the borehole, with a nuclear demonstration targeted for 2027.聽
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The company鈥檚聽 uses one of the most common nuclear technologies: a pressurised water reactor, which runs on standard low-enriched uranium fuel and is water-cooled.聽It is聽,聽but less than a metre in diameter, and is designed to produce 15 megawatts of electricity.聽
Drilling deep boreholes is routine in the oil and gas industry, and the heat exchanger technology that will create electricity is standard in geothermal energy production: the heat created by the reactor converts water to steam, which rises to the surface and turns a turbine.聽
While the components of Deep Fission鈥檚 model aren鈥檛 novel, combining them is. 鈥淭he biggest technical challenge is proving that all of these individually familiar technologies will work together in this unfamiliar configuration,鈥� says聽, a nuclear engineer at independent consulting firm Neutronic Designs and a specialist in advanced reactor technologies.聽
The company thinks its design will not only work, but improve safety and reduce costs compared with conventional nuclear power projects. Putting the reactor a mile underground means the surrounding rock will serve as a built-in containment structure, and the weight of the water column above will create a pressure of 160 atmospheres 鈥� high enough to keep water in liquid form at ultra-high temperatures and eliminating the need for a pressuriser that is a necessary component of ground surface reactors.
, Deep Fission鈥檚 chief operating officer, says the reactor design also cuts the need for 鈥渆ngineered active emergency core cooling systems with pumps and backup power鈥�.
鈥淲e anticipate that could translate into real capital and operating savings,鈥� he says.聽
鈥淭hey鈥檙e using the environment itself to perform functions that we normally have to engineer into a nuclear power plant,鈥� says Dewan. However, she adds, the trade-off is a lack of easy access to the reactor, which will make inspection and maintenance 鈥渂oth more difficult and more important鈥�.
, a nuclear engineer at the University of Michigan, sees the potential for maintenance problems, too. Even if Deep Fission designs the Gravity reactor for minimal maintenance, he says 鈥� for example, by cooling based on natural circulation rather than pumps 鈥� it is impossible to prepare for all eventualities. 鈥淚鈥檇 be more concerned about something happening that you didn鈥檛 anticipate,鈥� says Allen.聽
Brasel says the company isn鈥檛 planning to raise the reactor to the surface for routine maintenance, but will be able to do so if it is needed.聽聽
No reactor has ever operated at this depth; the closest is Norway鈥檚聽, which was installed聽inside a cavern excavated into a mountainside聽about 100 metres underground, also for containment purposes. It is uncertain how Deep Fission鈥檚 borehole, casing, reactor vessel and surrounding geology will hold up over potentially decades of operation. 鈥淛ust due to the fact that you鈥檙e putting a reactor underground, there will be a learning curve,鈥� says Allen.
It is a curve the company hopes to overcome quickly. Its modular approach would ultimately group multiple Gravity reactors at one site, scaling the 15-megawatt capacity to 150 megawatts or even over a gigawatt.聽
Deep Fission isn鈥檛 alone on its quest to make nuclear power smaller,聽cheaper and聽easier to build. Small modular reactors are intended to replace conventional nuclear power with smaller standardised units that can be manufactured and deployed in series. Several companies have made headway in their efforts: the US Nuclear Regulatory Commission issued TerraPower a construction permit in March for its Natrium reactor in Wyoming. Kairos Power and X-Energy are also moving reactors towards construction.聽
A 聽demonstrated the capacity to run stable and self-sustaining reactors by 4 July 2026 鈥� a 鈥渃riticality deadline鈥� set by the US Department of Energy鈥檚聽Reactor Pilot Program聽that increases the odds of the companies in question finding a path to commercial deployment.
Small modular reactor technology could challenge long-standing assumptions within the nuclear industry. For instance, for many companies working on small modular reactors, 鈥渢heir business bet is that the changes in technology and improvements in safety will mean you won鈥檛 need containment鈥�, says Allen. 鈥淭he question with the Deep Fission idea, where you鈥檙e going to the work of drilling a very deep borehole, is: is it necessary?鈥澛�
For now, Deep Fission is betting that it is, and will continue to work towards showing that a reactor can work at the bottom of a hole. If it succeeds, it could offer a radically different way to build small nuclear plants.聽
The most valuable experimental results to look forward to, says Dewan, will be 鈥渄emonstrating the installation and retrieval聽of the prototype reactor equipment, measuring thermal performance, and quantifying their ability to inspect and maintain the equipment over its operating lifetime鈥�.聽
