Oklo’s test reactor reaches criticality in under a year

On private land south of Austin, Texas, Oklo has reached a threshold that every nuclear reactor must cross: its Groves Isotope Test Reactor sustained a chain reaction. The low-power unit reached criticality after less than one year of construction, giving the high-profile startup its first reactor to split atoms.

This is not yet an electricity plant. Groves is a demonstration reactor for Oklo’s medical-isotope business, which the company hopes will provide early revenue. The work is also meant to support its broader plan for 75-megawatt liquid-sodium-cooled power reactors. Criticality is necessary for commercialization, but it is far from sufficient.

The Texas site gives Oklo a place to rehearse the industrial details that a larger reactor will require. The company completed the project on a greenfield site on private land, bought low-enriched uranium from the commercial fuel supplier Framatome, and developed its operating programs in-house. Oklo says the project is distinct from four other startups that reached criticality this year at federal facilities or on public land.

The harder step remains ahead. The Nuclear Regulatory Commission rejected Oklo’s reactor-license application in 2022, and the company has not yet resubmitted the paperwork for its power-reactor design. It is preparing a site near Idaho National Laboratory, but does not yet have permission to work on that reactor. The agency did grant Oklo its first license to operate a medical-isotope reactor in March.

So what, concretely? Oklo now has a demonstration reactor intended to support its medical-isotope business, plus experience with construction, buying low-enriched uranium from Framatome, and developing operating programs in-house, according to Canary Media. That can reduce uncertainty around its next projects and help build a supply chain before a commercial power reactor is authorized. But the immediate result is a prototype milestone—not electricity on the grid, and not yet a full-scale reactor under construction.