Deep Fission Plans Underground Nuclear Reactors to Cut Costs

July 20, 2024 – A Berkeley-based startup named Deep Fission is pursuing a radical vision for nuclear power: burying reactors a mile underground to harness natural pressure and rock for containment, potentially slashing construction costs by up to 80%. Founded in 2023 by father-daughter duo Liz and Rich Muller, the company aims to redefine nuclear energy economics by leveraging existing oil-and-gas drilling technology and off-the-shelf components, with a prototype project underway in Kansas.

The core innovation replaces traditional concrete domes and steel vessels with depth. A standard pressurized water reactor requires about 160 atmospheres of pressure to keep water liquid while transferring heat from the core. On the surface, this demands heavy engineered structures. Deep Fission’s design, called “Gravity,” lowers a reactor down a 30-inch diameter borehole to approximately one mile deep, where the weight of the water column above naturally provides the necessary pressure, and billions of tons of surrounding bedrock act as containment. Each unit generates up to 15 megawatts of electricity from 45 megawatts of thermal energy, with steam piped to the surface to drive turbines.

This approach diverges sharply from the small modular reactor (SMR) trend focused on compact, factory-built units for surface installation. While SMRs like Canada’s recent project occupy significant land area, Deep Fission emphasizes vertical scalability, claiming that 100 boreholes on a single site could yield 1.5 gigawatts—equivalent to a large traditional plant—on a fraction of the footprint. The company argues that going underground is more cost-effective, relying on gravity rather than custom megastructures.

The concept originated from the founders’ previous work at Deep Isolation, a nuclear waste disposal firm using deep boreholes. During safety assessments, they realized that fresh fuel accidentally dropped into such a hole would encounter conditions ideal for a pressurized water reactor. “Gravity is one of the most reliable forces in nature,” said CEO Liz Muller, highlighting the design’s reliance on natural elements over human-engineered solutions. Rich Muller, a Professor Emeritus of Physics at UC Berkeley and MacArthur Fellow, provides scientific oversight.

Financially, Deep Fission banks on integrating established supply chains. By combining standard PWR components from the nuclear sector, deep-borehole drilling expertise from oil and gas, and heat-transfer methods from geothermal energy, the company estimates construction cost reductions of 70–80% compared to conventional plants. It targets a levelized cost of electricity between $50 and $70 per megawatt-hour, with a single reactor potentially moving from groundbreaking to operation in six months after design validation.

Current progress centers on the Great Plains Industrial Park in Parsons, Kansas. In March 2024, drilling began for an 8-inch-wide, 6,000-foot-deep data-acquisition borehole to gather geological, thermal, and hydrological data for final design and safety cases. This precedes plans to drill a full-scale commercial borehole and deploy a prototype reactor, with a commercial license application to the Nuclear Regulatory Commission slated for early 2027.

Funding has advanced rapidly. In February 2024, Deep Fission raised $80 million in private financing, followed by a Nasdaq IPO in June under the ticker FISN, which raised about $40 million more at a share price of $16. The company has secured non-binding letters of intent with data centers and industrial partners, covering up to 18.5 gigawatts of potential demand, though these agreements do not guarantee purchases and are purely exploratory.

Deep Fission is among ten participants in the Department of Energy’s Reactor Pilot Program, launched via a May 2025 executive order to fast-track test reactors through DOE authorization instead of standard NRC licensing. The program set a symbolic goal of achieving criticality for at least three reactors by July 4, 2026—the nation’s 250th Independence Day. While Deep Fission initially targeted this deadline, its current milestones, with only a data well completed, make compliance unlikely. Energy Secretary Chris Wright has publicly noted that only one or two reactors might succeed; so far, Antares Nuclear is the sole participant to reach criticality, achieving it in June 2024 with a 500-kilowatt heat-pipe test reactor.

Critics, including the Union of Concerned Scientists, question the safety trade-offs of bypassing NRC review. They argue that rigorous regulatory oversight should not be compromised for speed, emphasizing that novel designs require thorough evaluation. Physically, the concept is elegant, but practical challenges include ensuring long-term reactor integrity underground, managing heat transfer over mile-long distances, and developing protocols for maintenance or decommissioning.

Industry experts point to growing interest in deep geology for energy applications. For instance, Britain’s recent drilling into Cornish granite taps natural radioactivity for geothermal heat, showcasing subterranean potential. Deep Fission’s application to active fission is a novel extension, but its viability hinges on overcoming unique engineering hurdles. Independent analyses, such as those from the Electric Power Research Institute, suggest that while the physics is sound, real-world validation is critical for credibility and scalability.

Looking ahead, Deep Fission’s journey will test whether depth can truly disrupt nuclear power economics. If the Kansas prototype succeeds, it could offer a faster, cheaper path to carbon-free energy. However, with regulatory, technical, and financial obstacles remaining, the road to commercialization is uncertain. As the U.S. seeks scalable clean energy solutions, this underground ambition represents a high-stakes experiment in innovation.