A nuclear reactor at Idaho National Laboratory achieved criticality for the first time in close to 50 years, marking a significant moment for both the federal research facility and the broader push to advance American nuclear energy development.

The Antares R1 Mark-0 Reactor Project reached the milestone, becoming the first of three advanced microreactors at the site anticipated to achieve criticality. The remaining two are expected to follow ahead of July 4th, which this year marks the 250th anniversary of American independence.

Risch Hails Idaho’s Nuclear Legacy

Sen. Jim Risch (R-Idaho), who founded and co-chairs the Senate Advanced Nuclear Caucus, welcomed the development as validation of Idaho’s long-standing role in nuclear research. “Today’s achievement at the INL adds another chapter to Idaho’s proud leadership in nuclear innovation,” Risch said, while adding that the Antares project represents only an early step in what he sees as a larger expansion of the nation’s nuclear capabilities.

Risch currently chairs the Senate Foreign Relations Committee and serves on the Committee on Energy and Natural Resources. Earlier this year, he introduced legislation known as the ARC Act, aimed at further supporting advanced reactor commercialization. His background includes prior service as Idaho’s Governor, Lieutenant Governor, and State Senator before his tenure in the U.S. Senate.

Significance of the Milestone

Achieving criticality means a reactor has sustained a controlled nuclear chain reaction — a foundational step in demonstrating the viability of a new reactor design. The fact that no new reactor had reached this threshold at INL in nearly half a century underscores how significant the moment is for the laboratory and for federal efforts to modernize the country’s nuclear infrastructure.

Microreactors like those in the Antares project are considered a promising technology for applications ranging from remote power generation to national defense installations, and they have drawn increased attention as policymakers seek to expand carbon-free baseload electricity capacity.

Idaho National Laboratory has historically been one of the country’s premier nuclear research facilities, and the state’s congressional delegation has consistently worked to secure its role as a hub for next-generation energy development. Rep. Mike Simpson recently directed more than $13 million in federal water infrastructure funds to six Idaho communities, reflecting the delegation’s broader effort to channel federal resources into Idaho priorities.

What’s Next

With the first microreactor now critical, attention turns to whether the remaining two Antares reactors will reach the same milestone before the July 4th target date. Success across all three would represent a substantial demonstration of advanced microreactor technology at a nationally significant research site.

Risch and other supporters of expanded nuclear energy in Congress are expected to point to the INL achievement as evidence that investment in advanced reactor development is bearing results, potentially lending momentum to pending legislation and future appropriations debates over nuclear energy infrastructure.