Antares Nuclear Inc.'s Mark-0 microreactor has successfully achieved zero-power criticality at Idaho National Laboratory’s Reactor and Critical Experiment facility, making it the first advanced reactor to do so under the Department of Energy’s Reactor Pilot Program. Announced on June 4, this milestone is significant as it validates the reactor's design and sets the stage for future electricity generation. The Mark-0 utilizes high-assay low-enriched uranium (HALEU) in TRISO fuel compacts, with a total operational fuel load of less than 120 kilograms. Antares, founded in 2023, has secured over $140 million in funding and aims to deploy the reactor for defense and space applications by 2028. The reactor's design incorporates passive cooling through sodium heat pipes, which reduces safety risks associated with active cooling systems. Antares CEO Jordan Bramble emphasized that achieving criticality is just the beginning, as the company prepares for sustained electricity production following extensive thermal testing. The success of Mark-0 also highlights the collaboration with BWX Technologies, which has played a crucial role in fuel development.