Deep Fission, Inc. (Nasdaq: FISN) (“Deep Fission” or the “Company”), an advanced nuclear energy company developing small modular pressurized water reactors a mile underground, today announced that it has successfully installed and retrieved a full-size prototype reactor canister from a borehole using standard commercial drilling equipment operated by a commercial drilling and rigging crew. Nothing about the demonstration required equipment that had to be invented, custom-built, or adapted for nuclear service.

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On Thursday, September 3, 2026, the crew lowered the 20-foot canister to 100 feet depth inside a 34-inch-wide borehole, aligned it, and brought it back to the surface. This is the emplacement and retrieval sequence that Deep Fission’s deployment model depends on. The equipment used is available today from commercial drilling contractors.

“The most important thing about this demonstration is what we did not have to do,” said Liz Muller, CEO and Co-Founder of Deep Fission. “We did not have to develop new technology. We used a rig and rigging that is commercially available in the field today, and our reactor uses pressurized water technology that has been operating in the nuclear industry for decades. Our innovation is in how we put proven pieces together, not in inventing something that has never been built. That is the difference between a science project and something you can deploy.”

Deep Fission’s approach rests on two mature industrial bases. The first is commercial drilling. Boreholes at the required diameter and depth, and the equipment to place and recover loads inside them, are routine work for the commercial drilling industry, and Deep Fission draws on that existing supply chain, workforce, and equipment fleet rather than building a parallel one. The second is pressurized water reactor technology. The Gravity™ Nuclear Reactor is a pressurized water reactor, the most widely deployed reactor type in the world, running on standard low-enriched uranium fuel. Deep Fission changes where the reactor sits, not the underlying reactor physics or fuel cycle.

Placing the reactor a mile underground in a water-filled borehole allows the water column and surrounding rock to provide containment, operating pressure, heat transfer, and emergency cooling. That simplifies the plant and removes much of the extensive surface infrastructure required by conventional nuclear facilities, which the Company expects to improve safety margins, shorten schedules, and reduce construction cost.

The prototype canister is a full-size, non-nuclear replica of the canister designed to house the Gravity™ reactor core. It contained no nuclear fuel. The demonstration is one step in a broader commercial validation program that also includes drilling, system integration, regulatory approvals, and ultimately commercial operation.

The Company recently received approval from the U.S. Department of Energy for its Nuclear Safety Design Agreement (NSDA) for the Gravity™ Nuclear Reactor, as it advances its deployment program under the Department of Energy Reactor Pilot Program.

About Deep Fission

Deep Fission is developing technology that places a small modular pressurized water reactor in a borehole approximately one mile underground. The Company’s Gravity™ Nuclear Reactor approach combines established pressurized water reactor technology with a novel underground deployment model designed to simplify construction, enhance safety, and support scalable commercial deployment. Deep Fission is focused on delivering reliable, low-carbon baseload power to meet growing electricity demand from utilities, industrial customers, and data centers. The Company is currently advancing the development of its first reactor project in Parsons, Kansas, and was selected for the U.S. Department of Energy’s Reactor Pilot Program.

Forward-Looking Statements

This press release contains forward-looking statements within the meaning of the federal securities laws. Forward-looking statements include, but are not limited to, statements regarding Deep Fission’s business strategy, technology development plans, potential commercial deployments, potential demand represented by non-binding LOIs, expected regulatory activities, planned project milestones, potential commercialization, potential revenue recognition, and the timing, feasibility, scalability, safety, and performance of the Company’s technology. These statements are based on current expectations and assumptions and are subject to risks and uncertainties that could cause actual results to differ materially from those expressed or implied.

Important factors that may cause actual results to differ materially include, among others, risks related to the Company’s early stage of development; the non-binding nature of the LOIs; the Company’s ability to negotiate and enter into definitive commercial agreements; technical, engineering, drilling, construction, regulatory, licensing, financing, supply chain, and deployment risks; the Company’s ability to obtain required approvals from the NRC, DOE, and other governmental authorities; market adoption of the Company’s technology; and the other risks described under “Risk Factors” and “Cautionary Note Regarding Forward-Looking Statements” in Deep Fission’s filings with the Securities and Exchange Commission.

Forward-looking statements speak only as of the date of this press release. Deep Fission undertakes no obligation to update any forward-looking statements, except as required by law.

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