Oklo and Los Alamos National Lab Conduct Fast Spectrum Plutonium Criticality Experiment

Multi-day fast-spectrum reactor experiments at the U.S. Department of Energy’s National Criticality Experiments Research Center generate modern benchmark data.

The data supports Oklo’s plutonium-fueled fast reactor project Pluto under the U.S. Department of Energy’s Reactor Pilot Program and advances work to use surplus plutonium as a bridge fuel for advanced reactors.

NEVADA NATIONAL SECURITY SITE, Nev.--(BUSINESS WIRE)--Oklo Inc. (NYSE: OKLO) (Oklo), an advanced nuclear technology company, today announced it has been conducting a multi-day plutonium fast reactor critical test suite with Los Alamos National Laboratory (LANL) at the U.S. Department of Energy’s (DOE) National Criticality Experiments Research Center (NCERC) under a Strategic Partnership Project (SPP).

LANL—America’s center of excellence for plutonium science—is an essential partner for developing and validating plutonium as an advanced reactor bridge fuel by providing the R&D foundation needed for future fast reactor development and deployment. The campaign marks the first public technical milestone for Oklo’s Pluto reactor—a plutonium-fueled fast test reactor project selected under DOE’s Reactor Pilot Program (RPP)—and the first step in a larger collaborative program with LANL to qualify surplus plutonium as fuel for commercial advanced reactors.

Using plutonium materials already held within its inventory, LANL carried out a series of low-power experiments on the Flattop fast-spectrum critical assembly with Oklo. Over several days, the system was taken critical and through power maneuvers and transients to capture detailed reactivity feedback and power response measurements, demonstrating the inherent safety features of a plutonium-fueled fast-spectrum system. Tests included taking the system critical and then increasing power and temperature in the reactor’s core, which led to negative reactivity feedback that shut the system down. The resulting data constitutes a modern set of benchmark measurements in a fast-spectrum configuration, helping build data to use surplus plutonium as commercial reactor fuel and informing Oklo’s work on Pluto and future systems.

The experiments were carried out at NCERC, a unique national asset dedicated to critical experiments and reactor physics testing located at the LANL-operated Nevada National Security Site (NNSS), under the oversight of DOE and the National Nuclear Security Administration (NNSA). NCERC provides specialized test assemblies, diagnostic systems, and expert staff to perform rigorous, highly controlled criticality and reactivity experiments under DOE and NNSA oversight. The work builds on a growing collaboration between LANL and Oklo spanning SPPs and a Cooperative Research and Development Agreement focused on advanced fuels, such as plutonium, and advanced reactors.

“This campaign is part of a larger plan to turn America’s surplus fuel stockpiles into bridge fuel for advanced reactors,” said Jacob DeWitte, co-founder and CEO of Oklo. “By working with LANL—the worldwide leader in plutonium science—at DOE’s NCERC facility, we are generating the modern benchmark data needed to qualify surplus plutonium as a bridge fuel for advanced reactors, strengthening U.S. energy dominance, supporting the near-term deployment of clean, reliable capacity, and eliminating material that would otherwise remain in long-term storage.”

The U.S. holds a sizable legacy surplus of Cold War-era materials, including plutonium, that has been managed for decades at significant cost to taxpayers. In May 2025, federal policies directed DOE to halt the dilute-and-dispose program and begin establishing pathways to make around 34 metric tons of surplus plutonium available to industry in forms suitable for the fabrication of advanced reactor fuel. That evolution opens the door to convert material previously treated as a liability into a source of useful energy.

“One of the challenges and gates to deploying new nuclear is the shortfall in domestic nuclear fuel production across the fuel supply chains. The government has reserves of usable fuel materials, like plutonium, that can be used as bridge fuel for new reactors as the industry expands its supply chains. This bridge fuel can help us build more reactors more quickly and bring clean power online sooner in larger quantities while also helping alleviate strains on power supplies that are increasing energy costs,” said DeWitte. “This is material that the government would otherwise spend billions of dollars of taxpayer money to ultimately bury. Instead, we can use it to build more power plants sooner. Greater fuel supplies also help us bring stronger orders to our developing fuel supply chains, which helps scale those sooner. Legacy materials like these surplus stockpiles of plutonium are significant enablers in producing more clean power sooner.”

Fast reactors also offer a productive disposition pathway for these materials: fissioning surplus plutonium destroys the material while generating clean energy, creating a bridge fuel that can enable early deployment of advanced reactors while high-assay low-enriched uranium (HALEU) and recycling-based fuel supply chains scale. The surplus plutonium can be fabricated into fuel that is interchangeable with HALEU-based fuel and the fuel produced from recycling.

Oklo’s work with DOE and LANL is a clear demonstration of DOE’s capabilities in handling and testing advanced nuclear systems and fuels, the first in a planned series of fast-spectrum experiments Oklo expects to conduct at NCERC. The recent test campaign builds on a large data set of similar tests and experiments run by LANL and NCERC, and future work will include additional benchmark measurements for transuranic-bearing recycled fuels, HALEU-based fuels, and other configurations that will advance both near-term reactor designs and longer-term use of recycled fuel.

Pluto, one of Oklo’s selections under DOE’s RPP, is a fast test reactor being developed by Oklo. Data from this project provides an early contribution to Pluto’s design and safety basis. The data also helps build the technical foundations for using fuel fabricated from surplus plutonium in future Aurora powerhouses, which are intended to deliver clean, reliable power to the U.S. grid.

About Oklo Inc.: Oklo Inc. is developing fast fission power plants to deliver clean, reliable, affordable energy at global scale; establishing a domestic supply chain for critical radioisotopes; and advancing nuclear fuel recycling to convert used nuclear fuel into clean energy. Oklo was the first to receive a site use permit from the U.S. Department of Energy for a commercial advanced fission plant, was awarded fuel from Idaho National Laboratory, and submitted the first custom combined license application for an advanced reactor to the U.S. Nuclear Regulatory Commission. Oklo is also developing advanced fuel recycling technologies in collaboration with the U.S. Department of Energy and U.S. National Laboratories.

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