US-based nuclear start-up Deep Fission has announced its emergence from stealth mode and a $4m pre-seed financing round, led by 8VC. Positioned at the forefront of advanced energy technology, Deep Fission is developing a project using standardised pressurised water reactor (PWR) technology for a modular nuclear microreactor (15 MWe) that can be installed at a depth of one mile in a 30-inch borehole.
Deep Fission was founded in 2023 by father-daughter team Elizabeth and Richard Muller. By leveraging natural conditions deep underground, the project eliminates the need for large pressure vessels and containment structures, significantly reducing costs while enhancing safety, sustainability, and operational efficiency. The company says it will use conventional low-enriched uranium fuel and an existing supply chain, “sidestepping a significant source of delay and concern for other advanced reactor designs”.
“Climate change has accelerated the need for clean energy, and nuclear must be cheaper in order to compete with coal and natural gas,” said Elizabeth Muller, co-founder and CEO of Deep Fission. “We’ve innovated beyond other reactor designs and engaged early and often with the Nuclear Regulatory Commission (NRC) to make atomic energy a viable option to power AI, industrial applications, as well as remote communities. We cannot wait to share our findings with the world and do our part to help with the clean energy transition.”
Joe Lonsdale, Managing Partner at 8VC, noted: “With global demand escalating, we need more options. We invested in Deep Fission because they are engineering a way for nuclear power to be exceptionally safe, cost-effective, and reliable – and far enough underground that hopefully neither war nor regulators can turn it off!”
The company has already met several important milestones, including completing a conceptual design, submitting a regulatory engagement plan, a conceptual design white paper, and a conceptual design review meeting with the US NRC.
The Deep Fission reactor will operate at the same pressure (160 atmospheres) as a standard PWR, and at the same core temperatures (about 315°C). As with a standard PWR, the heat is transferred to a steam generator at depth to boil water, and the non-radioactive steam rises rapidly to the surface where a standard steam turbine converts the energy to its electricity.
Deep Fission says the reactor has no moving parts at depth, other than the control rods and the fluid flow of the water coolant. This design minimizes the need for maintenance, although cables attached to the reactor allow it to be raised to the surface (it takes only an hour or two) if inspection of the reactor is deemed necessary.
The major cost of standard PWR is the enormous reactor pressure vessel made of 8 to 12-inch thick steel, the large pressuriser, and the huge containment building with concrete walls 3-6 feet thick. The Deep Fission design replaces all of this with geology. Per kilowatt-hour, a borehole is far cheaper than such large surface structures. The pressure is supplied by the depth: the pressure of water at a mile deep is 160 atmospheres, the same as that found in the thick pressure vessel of the standard PWR.
Safety and security for a borehole reactor are unprecedented and (perhaps equally important) self-evident. The reactor is far out of the reach of tornadoes, floods, hurricanes, tsunamis, airplane crashes, and terrorists.
Deep Fission has begun analysis of credible accidents and will review the safety of the reactor with NRC during pre-application and application review. The reactor has a mile of fresh water above it, so emergency core cooling is simple. It also has a negative temperature coefficient so with any overheating the reactor turns itself off.
