Antares reactor achieves criticality in US first for new nuclear design
The Mark 0 reactor utilises TRISO fuel and a sodium-cooled cycle, with full power generation tests expected next year.
A test reactor developed by nuclear startup Antares has reached criticality at the Idaho National Laboratory, marking the first time a new nuclear design has achieved this milestone in the United States. The achievement follows an executive order issued by the Trump Administration approximately 18 months ago, which directed the Department of Energy to have three different reactor designs reach criticality in roughly that timeframe.
The reactor currently operates in a 'Mark 0' phase, meaning it is not connected to power generation. This stage is focused on validating physical models and generating safety data rather than producing electricity. Full system tests, including electrical generation, are scheduled to commence next year. Criticality indicates that the nuclear reactions within the hardware have become self-sustaining, a technical threshold distinct from commercial power output.
Antares is one of several startups developing small modular reactors, yet it remains the only entity with a fully licensed design to date, although there are no immediate plans to construct commercial instances of that design. The company’s technology relies on Tristructural-Isotropic (TRISO) fuel, which consists of uranium oxide cores encased in carbon layers and a hard ceramic shell. This design aims to contain fission products within the fuel pellets, theoretically preventing meltdowns or the release of dangerous isotopes as long as the pellets remain intact.
The reactor employs a sodium-cooled closed Brayton cycle, where heat is transferred from sodium to pressurised nitrogen to drive a turbine. To mitigate non-radioactive risks, the design uses a graphite sheath to slow neutrons that escape the fuel pellets. While the TRISO fuel enhances safety, neutrons can still convert surrounding material into unstable isotopes, a factor the company is monitoring during this validation phase.
The project has received support from NASA and is collaborating with the Department of Defence’s Project Pele program for mobile reactor development. The work was conducted at a Department of Energy facility commonly used for nuclear research and testing, underscoring the government’s continued interest in accelerating advanced nuclear capabilities.
