Antares Nuclear achieved zero-power criticality with its Mark-0 microreactor at Idaho National Laboratory on June 4, 2026, marking the first time a novel reactor design has reached criticality at INL in more than 50 years. The milestone makes the Mark-0 the 53rd reactor to achieve criticality at the national laboratory.
The Mark-0 is the first advanced reactor to complete criticality testing under the Department of Energy's Reactor Pilot Program, a pathway established to demonstrate new reactor technologies at smaller scale before commercial deployment. Antares has raised more than $140 million in private capital to fund development of the microreactor.
The company plans to begin electricity production from the Mark-0 in 2027 and is targeting military deployment applications by 2028. Microreactors in the one to ten megawatt range are being evaluated by the U.S. military as portable power sources for forward operating bases and remote installations.
The Mark-0 criticality occurred under Presidential Executive Order 14301, which directed federal agencies to accelerate advanced nuclear reactor demonstrations at national laboratories. The Antares milestone is the first criticality achieved under that directive.
Microreactors represent a distinct category from the small modular reactors under development by companies such as NuScale and Kairos Power. Where SMRs aim to compete in the commercial electricity market at 50 to 300 megawatts, microreactors target off-grid and remote applications where diesel generation is currently the only option.
The achievement at INL positions Antares as one of the first advanced reactor developers to move from design to operating hardware in the current nuclear revival.
Source: ANS Nuclear Newswire -- https://www.ans.org/news/2026-06-05/article-8099/antares-achieves-zeropower-criticality-at-inl/