A Seattle-area fusion startup just made a bet that no competitor has tried: selling nuclear fission reactors to pay the bills while it chases the far harder goal of commercial fusion power.
Zap Energy, known for its work on a compact approach to fusion called Z-pinch, announced in June 2025 that it is expanding into fission by developing a modular reactor based on the 4S (Super-Safe, Small and Simple) design, a sodium-cooled fast reactor concept originally created by Toshiba and Japan’s Central Research Institute of Electric Power Industry. The company simultaneously named Zabrina Johal, formerly CEO of uranium enrichment firm Urenco USA, as its new chief executive, and tapped Benjamine A. Cipiti to lead the fission program, according to the company’s press release.
Zap described itself as “evolving into an integrated nuclear platform spanning fission and fusion.” In plain terms, the company wants to generate revenue from deployable fission plants while continuing to develop its fusion technology, which remains years from producing electricity. According to the company’s own framing, no other fusion-focused company has publicly adopted this kind of dual-reactor strategy, though the claim is difficult to independently verify across the entire sector.
Why fission, and why now
The timing is not accidental. Electricity demand in the United States is climbing at its fastest pace in decades, driven largely by the explosive growth of artificial intelligence data centers. Utilities and tech companies are scrambling for carbon-free power sources that can deliver firm, around-the-clock generation, and nuclear is at the top of many wish lists. That demand has created a window for advanced fission startups like Oklo, Kairos Power, and X-energy, all of which are racing to deploy small reactors before the end of the decade.
Zap appears to be positioning itself to compete in that same market. The 4S reactor it has chosen as its foundation is a compact unit originally designed to produce around 10 megawatts of electricity, though later iterations explored output up to 50 MWe. A peer-reviewed paper in Progress in Nuclear Energy describes the 4S as featuring a long-life sealed core engineered to operate for decades without refueling. Because it uses liquid sodium as a coolant instead of pressurized water, the reactor runs at near-atmospheric pressure, eliminating one of the main accident risks in conventional nuclear plants.
The catch: despite decades of study, the 4S has never been licensed or built anywhere in the world. Toshiba explored deploying one in Galena, Alaska, in the mid-2000s, but the project stalled before reaching the licensing stage. Zap says it will adapt the concept into a modular system, though it has not released technical specifications, cost estimates, or a deployment timeline for its version.
A new CEO from the fuel cycle
Johal’s appointment signals that Zap is serious about operating within the regulated nuclear industry, not just publishing research papers. At Urenco USA, she ran one of only two commercial uranium enrichment facilities in the United States, a business that requires navigating stringent Nuclear Regulatory Commission oversight, managing classified technology, and maintaining complex industrial operations. Before Urenco, she held roles at Centrus Energy, another enrichment company, giving her deep experience across the nuclear fuel cycle.
That said, enrichment and reactor development are different disciplines. Building a first-of-a-kind sodium-cooled reactor from a decades-old concept requires reactor physics expertise, supply chain development, and construction management that enrichment operations do not directly provide. Zap has not disclosed the broader technical team behind its fission effort, making it difficult for outside observers to gauge whether the company has the engineering depth to execute.
The regulatory path exists, but it is not simple
The U.S. Nuclear Regulatory Commission has publicly stated that it can accept license applications for microreactors and advanced reactors under its existing frameworks, specifically 10 CFR Part 50 and Part 52, according to the agency’s microreactor guidance page. The NRC is also developing a separate rulemaking for microreactors and low-consequence reactor designs that could streamline future licensing for small units.
None of that means Zap has filed an application, entered pre-application discussions, or received any feedback from the NRC on its specific plans. Whether a sodium-cooled fast reactor based on the 4S would qualify under the microreactor pathway or require a full review depends on design details the company has not made public. For context, Oklo’s application for its Aurora microreactor was denied by the NRC in 2022 before being accepted for a second review, illustrating that even well-funded advanced reactor companies face lengthy regulatory timelines.
The strategic gamble
Fusion startups have historically justified investor patience by arguing that their singular focus on one extraordinarily hard problem is what sets them apart. Helion Energy has a power purchase agreement with Microsoft. Commonwealth Fusion Systems is building a demonstration plant in Virginia. TAE Technologies has attracted more than $1.2 billion. All three remain laser-focused on fusion.
Zap is testing the opposite theory: that a revenue-generating fission business can sustain fusion research through what could be another decade or more of development. The logic has precedent in other industries, where companies balance mature product lines against speculative R&D. But it also carries risk. Splitting engineering talent, management attention, and capital between two reactor programs could slow progress on both, particularly for a startup that, while reportedly having raised north of $200 million through 2024 from investors including Chevron and Lowercarbon Capital, is still far smaller than the utilities and national laboratories that have historically driven reactor development.
Zap has not disclosed how much of its funding is earmarked for fission versus fusion, whether it has identified potential utility or industrial customers for the 4S-derived reactor, or where it might site a first unit. No safety analyses, probabilistic risk assessments, or detailed design documents have been published.
Signals that would turn strategy into substance
For now, Zap Energy’s pivot rests on a narrow set of confirmed facts: a leadership change, a stated commitment to the 4S reactor concept, and the existence of a regulatory framework that could, in theory, accommodate such a design. Everything beyond that, including timelines, costs, technical feasibility, and market demand, remains unproven.
The signals worth tracking in the months ahead are concrete ones: formal engagement with the NRC, publication of reactor design details, partnerships with utilities or data-center operators, and independent validation of progress on both the fission and fusion sides of the business. Until those materialize, Zap’s announcement is best understood as an ambitious strategic declaration, one that could reshape how fusion companies think about their business models, or could quietly fade into the long list of nuclear plans that never reached a construction site.
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*This article was researched with the help of AI, with human editors creating the final content.
