A Privately Built Advanced Reactor Reaches Criticality in Idaho

The startup Antares achieved initial criticality of its Mark-0 microreactor at Idaho National Laboratory on June 4, the United States Department of Energy announced, describing it as the first privately developed advanced reactor to reach criticality under its pilot program. Criticality is the point at which a reactor sustains a controlled nuclear chain reaction.

It is the first American non-light-water privately developed reactor to reach that point in more than 40 years, the company said. Antares, founded in 2023 and backed by more than $140 million, says it aims to produce electricity from an advanced reactor in 2027, with early deployments to military installations beginning in 2028. The work was carried out with the Department of Energy, the national laboratory and the contractor BWX Technologies.

The milestone has a clear economic cause. Demand for reliable, high-density power to run artificial-intelligence data centers has revived private interest in nuclear technology, and a small, factory-built reactor that can be placed near where the power is used is the type of product buyers want. The Department of Energy described the moment as a revival of the country's nuclear industry.