Deep Fission is a Berkeley-based nuclear startup developing a small modular reactor designed to operate one mile underground, inside a drilled borehole. The company was founded in 2023 by Liz Muller, who serves as CEO, and her father Richard Muller, an emeritus physics professor at UC Berkeley, MacArthur Fellow, and former advisor to the U.S. Secretary of Energy. Both previously co-founded Deep Isolation in 2016, a company focused on disposing of nuclear waste in deep boreholes, which gave the pair direct experience with borehole geology and drilling engineering that now underpins Deep Fission's reactor siting approach. The company exited stealth in 2024, raised $80 million in February 2026 from investors including Joe Lonsdale's 8VC, and filed an S-1 registration statement for a Nasdaq IPO in May 2026, targeting a $1.66 billion valuation and seeking to raise approximately $150 million. No reactor has been built. The company began drilling data acquisition wells in Parsons, Kansas in March 2026.
Deep Fission's Gravity reactor is a 15-megawatt pressurized water reactor narrow enough to be lowered into a drilled borehole and designed to run at one mile of depth where water pressure alone maintains the 160 atmospheres the reactor requires, replacing the thick steel pressure vessel that is one of the costliest and most time-consuming components of a conventional nuclear plant. The company claims the approach can reduce nuclear construction cost by 70 to 80 percent relative to surface plants and achieve a levelized cost of electricity between $0.05 and $0.07 per kilowatt-hour. Both claims are theoretical. The Gravity reactor has not been licensed, constructed, or operated. The path from borehole drilling to commercial power generation involves regulatory questions, fuel qualification, and engineering challenges that have not yet been addressed.
The Gravity reactor is 9 meters tall and approximately 0.75 meters wide — small enough to be manufactured off-site and lowered into a borehole drilled using techniques adapted from the oil and gas industry. At one mile of depth, the weight of the water column above the reactor creates roughly 160 atmospheres of hydrostatic pressure, which is the operating pressure of a standard pressurized water reactor. A conventional PWR requires a massively thick steel pressure vessel to contain that pressure at the surface; the borehole geometry makes the surrounding water and rock do the same job for free. The reactor uses standard low-enriched uranium fuel, requiring no exotic fuel types or new fuel fabrication infrastructure. Emergency cooling relies on the mile of water in the borehole above the core; decay heat dissipates into the surrounding rock. Deep Fission claims this geological shielding also provides radiation containment and security against surface-level incidents that conventional plant designs must address through engineered systems.
In December 2025, Deep Fission announced Parsons, Kansas as the site for its DOE Reactor Pilot Program demonstration. The Trump administration's Reactor Pilot Program set a July 4, 2026 target for achieving first reactor criticality at selected pilot sites, an aggressive timeline intended to accelerate advanced nuclear deployment. Deep Fission began drilling the first of three planned data acquisition wells in Parsons on March 10, 2026 — test wells to characterize the geology and confirm drilling feasibility before any reactor is installed. Within two weeks, the company had pulled back from the July 4 criticality commitment, with Liz Muller clarifying that the company had committed to the drilling program but not to the scale or timeline of any commercial project. A Kansas state law that prohibits direct sale of power to industrial customers such as data centers created additional uncertainty about the commercial path; the company was in discussions with Evergy, the local utility, about a solution. As of mid-2026, the data acquisition drilling is ongoing and the regulatory and commercial structure of the first operational reactor remains unresolved.
The insight at the center of Deep Fission's design is worth examining carefully, because it is genuinely clever and the engineering case for it is real, even though the commercial case is unproven. A pressurized water reactor requires high-pressure coolant — roughly 160 atmospheres — to prevent the reactor coolant water from boiling at operating temperature. At the surface, maintaining that pressure requires a reactor pressure vessel: a steel cylinder 20 to 30 centimeters thick, typically around 4 to 5 meters in diameter and 12 meters tall, forged from special low-alloy steel and requiring years to manufacture at one of the few global facilities capable of producing it. The pressure vessel and its associated primary coolant piping represent a major fraction of a conventional nuclear plant's capital cost and schedule.
A column of water one mile tall exerts approximately 160 atmospheres of pressure at its base. If you put a reactor at the bottom of a water-filled borehole one mile deep, the water column provides the required operating pressure through hydrostatics alone, with no engineered pressure vessel required. The reactor still needs structural containment — a tube strong enough to hold its geometry and prevent external rock pressure from crushing it — but this is a much simpler engineering problem than building a pressure vessel rated to 160 atmospheres from the outside. Deep Fission's tube is roughly the diameter of a large oil well casing, manufacturable with existing oilfield pipe supply chains. The same water column provides emergency cooling: if the reactor trips, the water above it absorbs decay heat passively, without pumps or operator action. The surrounding rock provides radiation shielding that a surface plant must achieve with multiple meters of concrete and steel.
The approach borrows from two engineering fields with well-established cost curves. Deep oil and gas wells reach one mile routinely; the drilling industry has developed the rigs, drill bits, and wellbore management techniques to do this in days to weeks at costs that have declined dramatically over decades. Geothermal energy developers drill boreholes to extract heat from the Earth and transfer it to surface systems, working out the heat transfer physics that Deep Fission's cooling approach draws on. Deep Fission is not inventing borehole drilling or PWR physics; it is combining them in a configuration that has not been attempted before for power generation.
The open questions are real. Getting a reactor into a borehole and retrieving it for refueling requires handling fuel assemblies at depth — a process with no prior art in the nuclear industry. The NRC has no existing license framework for a borehole reactor; the entire regulatory pathway must be developed from scratch, including safety analysis methods that account for one-mile-deep operations. Heat extraction from the bottom of a borehole to the surface power systems involves thermal losses along the entire mile-long column. And the 15-megawatt output of the Gravity reactor is small enough that clustering many units is necessary to serve a data center or industrial load at commercial scale, which multiplies the permitting and drilling effort proportionally. Deep Fission's cost and LCOE projections are pre-engineering estimates with no construction data behind them. They may prove accurate; they may not.
Deep Fission raised a $4 million seed round when it exited stealth in 2024. An additional round closed in September 2025 at an undisclosed size, led by Ed Eisler of EE Holdings and Mark Tompkins of Montrose Capital. In February 2026, the company closed an $80 million financing round with investors including 8VC (the fund co-founded by Joe Lonsdale, who also co-founded Palantir), Deep Future, and Wave Function. In May 2026, Deep Fission filed an S-1 registration statement with the SEC to list on the Nasdaq Global Market under the ticker FISN, seeking to raise approximately $150 million and carrying a target valuation of approximately $1.66 billion. The IPO filing is for a pre-revenue company that has drilled test wells but has not built, licensed, or operated a reactor.
The company has also signed a strategic partnership with Endeavour Energy to supply up to 2 gigawatts of power for AI data center applications. At 15 megawatts per reactor, 2 gigawatts of installed capacity would require approximately 133 individual units. The partnership is a commercial intent agreement rather than a contracted order; it indicates market demand for the product if it works, not certainty that it will.
Deep Fission is at the earliest stage of any company profiled here: no reactor built, no operating license, no commercial revenue. The borehole PWR concept has first-principles merit, but merit and buildability are different questions. The NRC must develop an entirely new licensing framework for a reactor configuration it has never reviewed, at a depth it has no direct safety analysis experience with. The fuel handling process — inserting and retrieving fuel assemblies one mile underground — has no procedural precedent in U.S. commercial nuclear operations. Heat extraction efficiency over a one-mile borehole depends on thermal resistance calculations that have not been validated at any scale. Each of these is a solvable engineering problem in principle; none of them has been solved yet.
The $1.66 billion IPO valuation is a bet on the concept rather than on demonstrated execution. For context, TerraPower has raised over $3.4 billion in total capital, has a $2 billion DOE cost-share, has a construction permit from the NRC, and began nuclear construction in April 2026 — and its first plant will not generate power until 2030 or 2031 at earliest. Deep Fission has $84 million, a test well in Kansas, and a regulatory pathway that has not yet been scoped with the NRC. That gap is not disqualifying — every eventual success was once at this stage — but it is the honest picture of where the company stands.
This profile was compiled from publicly available information including:
Deep Fission corporate website — Technology description, Gravity reactor specifications, press releases.
$80M financing announcement (February 2026); Deep Fission S-1 IPO filing (May 2026, Renaissance Capital and World Nuclear News coverage); Parsons, Kansas site announcement (December 2025); borehole drilling start press release (March 2026).
KCUR Kansas City reporting on Parsons community concerns (March 2026) and company commercial-path uncertainty (March 2026); IEEE Spectrum deep-dive on borehole reactor design; NRC pre-application white paper (ML24172A286); Neutron Bytes on DOE Reactor Pilot Program; New Atlas and Interesting Engineering on $30M raise and borehole design.
This profile is for informational purposes only and does not constitute investment advice, a recommendation, or a solicitation to buy or sell any security.