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 safety approval 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. 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 significant portion of their cost and the time it takes to build them. But independent engineers say it is unclear how successful the approach will be. 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. US to fire up small reactors in 2026 as part of ‘nuclear renaissance’ The company’s Gravity reactor uses one of the most common nuclear technologies: a pressurised water reactor, which runs on standard low-enriched uranium fuel and is water-cooled. It is 9 metres tall, but less than a metre in diameter, and is designed to produce 15 megawatts of electricity.
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. While the components of Deep Fission’s model aren’t novel, combining them is. 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 – 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. Michael Brasel, Deep Fission’s chief operating officer, says the reactor design also cuts the need for “engineered active emergency core cooling systems with pumps and backup power”. However, she adds, the trade-off is a lack of easy access to the reactor, which will make inspection and maintenance “both more difficult and more important”.
Todd Allen, 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 – for example, by cooling based on natural circulation rather than pumps – it is impossible to prepare for all eventualities. New NASA head wants to build a nuclear reactor on the moon – but why?
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