Antares Nuclear, Inc. announced Thursday that its Mark-0 microreactor reached criticality at Idaho National Laboratory, marking the first advanced reactor to achieve this milestone under a U.S. Department of Energy pilot program launched following President Donald Trump’s 2025 executive order. The Torrance, California-based company became the first private entity to bring an advanced reactor to criticality through the Department of Energy’s Reactor Pilot Program. Criticality represents the point when a reactor achieves a self-sustaining nuclear chain reaction, a fundamental milestone in reactor development that validates core physics parameters and generates essential testing data.

Jordan Bramble, CEO of Antares, emphasized the company’s commitment to meeting deadlines in an industry historically plagued by delays. The achievement comes as part of a broader initiative to accelerate nuclear development in the United States. The demonstration validated key reactor physics parameters and produced critical testing data on control system performance that will support future reactor development efforts.

First privately developed non-light-water reactor in four decades

The Department of Energy confirmed the achievement Thursday, describing it as the first privately developed non-light-water reactor to reach criticality in the United States in more than 40 years. Energy Secretary Chris Wright characterized the moment as historic for American nuclear energy. The milestone demonstrates what becomes possible when American innovation operates without excessive regulatory constraints. The achievement required close collaboration between private industry and federal agencies, establishing a new model for nuclear development in the country.

The Mark-0 reactor utilized TRISO fuel fabricated by BWX Technologies and benefited from fuel technology developed through Project Pele, a Defense Department effort focused on building transportable microreactors for military applications. The successful demonstration occurred in partnership with Idaho National Laboratory and BWX Technologies, while the U.S. Army participated as a future end user of the technology.

Trump executive order accelerated reactor testing timeline

The milestone comes just over one year after President Trump signed four executive orders directing the federal government to accelerate reactor testing, expand domestic nuclear fuel production, and streamline pathways for advanced nuclear technologies. Executive Order 14301 specifically directed the Department of Energy to establish a pilot program designed to expedite testing and demonstration of advanced reactor designs. The administration established an ambitious goal of achieving criticality for advanced reactor concepts by July 4, 2026.

• Mark-0 microreactor achieved self-sustaining nuclear chain reaction
• First advanced reactor milestone under DOE Reactor Pilot Program
• TRISO fuel technology from Defense Department Project Pele utilized
• Partnership included DOE, Idaho National Laboratory, and BWXT
• U.S. Army participating as future technology end user

Bramble expressed gratitude to partners at the Department of Energy, Idaho National Laboratory, BWXT, and the U.S. Army, stating the achievement demonstrates what happens when industry and government collaborate effectively on ambitious projects. The accelerated timeline challenged conventional assumptions about how quickly advanced reactor concepts could move from design to demonstration.

Reactor development completed in under 12 months

Assistant Secretary of Nuclear Energy Ted Garrish noted that skeptics doubted President Trump’s Reactor Pilot Program could achieve criticality in less than one year. The successful demonstration proved those doubts unfounded and showcased the dedication of teams who worked to shape the future of nuclear energy in the United States. Engineers gained critical insights into reactor physics, control systems, and supply chain performance during the demonstration phase.

Bramble emphasized that the team progressed from concept to critical reactor safely in less than 12 months by treating the schedule as non-negotiable. The data collected will support future reactor development and eventual commercial licensing processes. The company believes efficient, iterative reactor testing represents a necessary shift from the decade-per-design approach that has characterized American nuclear development in recent decades.

Electricity production and military deployment on schedule

Antares expects to begin producing electricity from the same facility in 2027 and remains on track to deploy electricity-generating microreactors to U.S. military installations by 2028. The company outlined a clear timeline: criticality in 2026, electricity production in 2027, and power delivery to military users in 2028. Thursday’s achievement represents the first of these commitments delivered on the promised schedule.

The successful criticality demonstration positions Antares as a leader in advanced nuclear technology development and validates the effectiveness of the streamlined regulatory approach enabled by the Trump administration’s executive orders. The company’s progress suggests that American nuclear innovation can compete globally when regulatory pathways support rapid iteration and testing. Military applications represent a significant market opportunity for microreactor technology, particularly for installations requiring reliable, portable power generation in remote or strategic locations.