US-based Oklo has been conducting plutonium fast reactor critical tests with Los Alamos National Laboratory (LANL) at the Department of Energy’s (DOE’s) National Criticality Experiments Research Center (NCERC) under a Strategic Partnership Project (SPP).
NCERC is located at the LANL-operated Nevada National Security Site (NNSS), under the oversight of DOE and the National Nuclear Security Administration (NNSA). It provides specialised test assemblies, diagnostic systems, and expert staff to perform rigorous, highly controlled criticality and reactivity experiments. 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.
Oklo says LANL 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-fuelled fast test reactor project selected under DOE’s Reactor Pilot Program (RPP). It is the first step in a larger collaborative programme 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 manoeuvres and transients to capture detailed reactivity feedback and power response measurements, demonstrating the inherent safety features of a plutonium-fuelled 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.
“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… 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 US energy dominance, supporting the near-term deployment of clean, reliable capacity, and eliminating material that would otherwise remain in long-term storage.”
The US 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 programme and begin establishing pathways to make around 34 tonnes of surplus plutonium available to industry in forms suitable for the fabrication of advanced reactor fuel.
“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.
Plutonium is seen as a bridge fuel that can enable early deployment of advanced reactors while high-assay low-enriched uranium (HALEU) and recycling-based fuel supply chains are developed. The surplus plutonium can be fabricated into fuel that is interchangeable with HALEU-based fuel and the fuel produced from recycling.
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 Oklo’s future Aurora powerhouses.
The Aurora powerhouse is a sodium-cooled fast reactor that uses metal fuel and builds on the design and operating heritage of the Experimental Breeder Reactor II (EBR-II), which ran in Idaho from 1964 to 1994. Oklo was awarded fuel recovered from EBR-II by the DOE in 2019 and aims to fabricate its initial core at the Aurora Fuel Fabrication Facility (A3F) at the Idaho National Laboratory (INL).
DOE’s Idaho Operations Office in December 2025 approved the Preliminary Documented Safety Analysis (PDSA) for the A3F, marking the start of the assembly of the facility. The PDSA for the A3F, which will fabricate fuel for Oklo’s first commercial-scale powerhouse, is the first facility to be approved under DOE’s Fuel Line Pilot Program. The previous procedural milestone, the Nuclear Safety Design Agreement, was approved in just under 2 weeks, demonstrating a new authorisation pathway aimed at accelerating a reproducible framework for scaling production capacity following the Executive Orders signed by President Donald Trump in May 2025.
The PDSA is the second of three safety-basis documents to be approved for authorisation of the A3F, with the next step being a Documented Safety Analysis (DSA), an update to the PDSA based on final design and construction. The DSA will be submitted during construction and updated to reflect the final build once the facility is completed. The final step includes a readiness review for startup of the facility. The A3F will fabricate fuel for Oklo’s first full-scale powerhouse, the Aurora-INL, which was also selected for the DOE’s Reactor Pilot Program.
“By advancing modern fuel fabrication and recycling, we’re addressing fuel-supply constraints, improving the economics of our powerhouses, and opening new long-term revenue streams for the business,” noted Jacob DeWitte. “We’re moving swiftly toward full deployment of this fuel facility where we will repurpose fuel from the legacy Experimental Breeder Reactor-II (EBR-II) for use in Oklo’s Aurora-INL.”
Oklo was granted access to EBR-II fuel material through a competitive DOE process launched in 2019. The same year, the company also received a site-use permit at INL for its first powerhouse. Oklo recently broke ground for the Aurora-INL reactor which can be built and begin operating under a DOE authorisation pathway, potentially fast-tracking future commercial licensing by the Nuclear Regulatory Commission. Oklo says it is on track to bring its first plant online at INL before the end of the decade.
