Two advanced reactors reach criticality under Energy Department pilot program

Two advanced nuclear reactors participating in a Department of Energy pilot program have reached criticality, the agency announced in June. The first reactor, developed by Antares Nuclear and known as Mark-0, achieved zero-power fueled criticality at the Idaho National Laboratory on June 4.

The second, called the Ward 250 and developed by Valar Atomics, reached the same milestone on June 18 at the Utah San Rafael Energy Lab.

program background Criticality occurs when a reactor sustains a self-sustaining nuclear chain reaction. Reactors pass through five stages, from subcritical with control rods fully inserted to full-power operation. The pilot program, launched last year, selected 10 companies and 11 projects to test simplified federal approvals for research and demonstration reactors.

The goal is to supply data to the Nuclear Regulatory Commission for future commercial licensing.

remaining projects President Donald Trump directed that at least three reactors reach criticality by July 4. Several additional companies in the pilot program, including Aalo Atomics, are working toward the same date. Antares Nuclear plans to begin electricity production in 2027 and deploy reactors at military sites in 2028.

The Department of Energy stated the program aims to support rising electricity demand linked to artificial intelligence and defense operations.