The ZiaCore advanced nuclear microreactor developed at Los Alamos National Laboratory (LANL) has achieved a major milestone by reaching high-temperature, zero-power criticality during a four-week testing window. This validates the fundamental reactor physics and viability of using commercially available, off-the-shelf materials for next-generation nuclear technology.
“The execution of our ZiaCore experiment will support an entire category of nuclear power microreactors,” said Christopher Stanek, director of the Nuclear Energy Program Office at Los Alamos. “The experiment will provide valuable data for low-enriched reactor technologies, including components developed at Los Alamos, at representative reactor temperatures.”
The test was conducted on the Deimos testbed at the National Criticality Experiments Research Center (NCERC) at the Nevada National Security Sites. ZiaCore uses low-enriched uranium dioxide fuel. It is designed to use whatever is commercially available on the market, while remaining fully adaptable to high-assay low-enriched uranium (HALEU) as domestic production scales up.
Los Alamos researchers began developing the technology in 2021, spurred by the goal of a microreactor that would efficiently use low-enriched uranium and be readily buildable. The team focused on the design of the reactor and components, using zirconium hydride moderator and passive heat pipe cooling. In addition to the primary components, a vacuum chamber was designed to hold the fuel assembly from the ZiaCore reactor, as well as the assembly’s heat pipes, zirconium hydride moderator components, fuel and custom electric heaters.
It was the ZiaCore fuel assembly in its vacuum chamber that was deployed on the Deimos testbed. This allowed researchers to glean key data about the operations and characteristics of the full-scale system, including temperature coefficients of reactivity. These are needed insights for this system and for the larger category of microreactor development. The core physics were tested in a high-temperature environment exceeding 800°C.
Zero-power criticality means a sustained fission chain reaction was achieved without generating electricity or producing measurable heat. This allows engineers to safely gather data on operations, including the temperature coefficients of reactivity. Zero-power experiments are practical because negligible concentrations of fission products are created, meaning the radioactivity is sufficiently low that work in the laboratory space can continue quickly following the experiment. The nuclear fuel is not expended and remains available for future experiments.
The system was developed on a lean budget with $5.5m in funding over three years through the LANL Laboratory Directed Research and Development (LDRD) programme. It features a highly modular, compact design engineered to scale up or down to fit inside standard shipping containers. It targets an operational lifespan of eight years without needing to refuel, providing a clean, quiet alternative to large diesel generators. Future deployment is aimed at remote communities, critical infrastructure, data centres, and Department of War needs.
ZiaCore joins our other microreactor concepts reaching initial criticality. Antares, Valar Atomics, Aalo Atomics, and Deployable Energy recently demonstrated critical milestones as participants in the Department of Energy’s (DOE’s) Reactor Pilot Program (RPP) driven by the Trump administration’s 2025 executive orders, to include “Deploying Advanced Nuclear Reactor Technologies for National Security,” which positions the government to “accelerate the secure and responsible development, demonstration, deployment, and export of United States designed advanced nuclear technologies to bolster readiness and enhance American technological superiority.”
“This project draws together experimentalists, physicists, engineers as well as production and fabrication experts,” said Ellen Cerreta, associate Laboratory director for Physical Sciences. “The team saw a pressing need in our energy landscape and was able to move quickly to build on Los Alamos’ strengths in heat pipes, materials development and criticality testing. The experiments proved a success not just on the terms of zero-power criticality, but how Los Alamos is able to pivot to important, practical applications in the national interest.”
Pat Fitch, Deputy Laboratory Director for Science, Technology and Engineering, noted: “This experiment truly highlights the Laboratory’s ability to draw upon a wide range of disciplines to tackle complex technological challenges,” said “Los Alamos research and development is critical to achieving our mission goals in energy security, science dominance and national security.”
Topher Matthews was the principal investigator of the project, and together with co-principal investigators Samantha Lawrence and Nick Thompson, and NCERC crew chief Theresa Cutler, led the design, planning and execution of the experiment. “With more than 100 people contributing to this project, the sense of shared success became very real,” Matthews said. “It was incredibly rewarding – and honestly joyful – to see so many people’s hard work come together.”
