Deep Fission has developed an innovative way to speed up deployment and meet some of the safety, environmental and cost issues around nuclear energy: placing small modular reactors in boreholes one mile underground.
The California-based startup, which is currently participating in the U.S. Department of Energy’s Nuclear Reactor Pilot Program, is targeting criticality by July 4, 2026, with commercialization expected to follow by 2027 or 2028, depending on regulatory approvals. It already has letters of intent for off-take of 12.5 gigawatts of electricity from Silicon Valley hyperscalers eager to use its underground reactors to address the massive energy demand from AI data centers.
“Data centers need electricity now, they can’t wait for new nuclear power plants to be built in the 2030s,” says Deep Fission CEO Liz Muller, who co-founded the company in 2023 with her father, Richard Muller, a Professor Emeritus of Physics at the University of California, Berkeley and a prolific inventor with over 80 issued patents. “The question is whether we can build this fast enough.”
Deep Fission says it is poised to move faster than other nuclear energy companies building small modular reactors (SMRs). Its proprietary design combines proven methods from the nuclear, oil and gas, and geothermal industries, while utilizing off-the-shelf parts and readily available, low-enriched uranium (LEU) to simplify supply chains. By siting the reactor one mile underground, the surrounding geology offers natural shielding and protection with billions of tons of bedrock providing passive safety and containment, says Muller. The approach reduces the surface footprint, strengthens security, and is designed to enable a faster, more affordable path to deployment.
The company says it can shorten construction timelines to six months from groundbreaking to operation.“ Whats more “We expect we can take 80% of the cost out of producing nuclear power,” says Muller.
Deep Fission’s pressurized water reactor – called Gravity- gets lowered a mile deep in a borehole. A pipe in the hole gets filled with water that cycles through the reactor, where it instantly becomes steam that goes back up to the surface into a turbine and turns a generator. Fifteen megawatts of electricity are produced, enough to power about eleven thousand homes. A typical data center might require 100-200 megawatts, which could be achieved by drilling multiple boreholes, Muller says.
Deep Fission’s reactor is about the size of a small car and costs – about half of the cost of a traditional nuclear power plant.
It can do this because “the actual specification for nuclear reactor safety is that you have to build so much cement around it that a 747 could kamikaze into it without damaging the core,” says Pablos Holman,managing director of the venture firm Deep Future, an investor in Deep Fission. “That containment vessel is one of the biggest expenses in building nuclear power plants. The Deep Fission reactor gets this containment for free. There’s ten billion tons of rock between this reactor and anyone’s backyard so you can put that reactor anywhere- under your house, under a skyscraper, under a data center – and it is safe.”
Most of today’s nuclear reactors require pumps to push water through the reactor core for cooling. Chernobyl and Fukushima are examples of what happens if the pumping stops: the reactor overheats and causes a meltdown. The Deep Fission reactor solves this problem in a very clever way, says Holman. A mile-deep hole full of water creates the exact amount of pressure needed, just from gravity, meaning there is no risk of a meltdown.
Storing nuclear waste in boreholes is also safe, says Muller. She previously ran a company that specializes in doing just that and her co-founder father has authored more than 100 peer-reviewed scientific articles, including key work on the safety of deeply buried nuclear waste. “When you store nuclear waste a mile underground underneath billions of tons of rock the containment is so good that there is no risk of it ever impacting humans or the environment,” she says.
Since its reactors are water-pressurized, just like many existing nuclear power plants, the regulatory path is straightforward, says Muller.
Deep Fission is targeting a levelized cost of electricity of 5-7 cents per kWh from its first-of-a-kind commercial implementation. The first locations will be in Texas, Utah and Kansas.
“We are excited about building our reactors first in the U.S. but fully expect that once we demonstrate that we can do it and at a substantially lower cost that there will be strong demand from around the world,” says Muller.
The company, which says it is the only one globally to attempt to operate a nuclear power plant deep underground, currently has 24 pending patent applications with one U.S. application granted and over 40 unique innovations.
