Four advanced microreactors achieved initial criticality by the early hours of July 4, 2026, at Idaho National Laboratory, surpassing a federal goal of bringing at least three test reactors to criticality by that date, according to World Nuclear News. Initial criticality marks the point at which a reactor sustains a controlled nuclear chain reaction, a key milestone on the path to operation.

The reactors advanced under the Department of Energy's Reactor Pilot Program, created to speed advanced reactor testing using DOE authorization rather than the standard NRC licensing route. Antares Nuclear's Mark-0 reached criticality first, on June 3, becoming the first privately developed advanced reactor to do so under the program. Valar Atomics' Ward 250 followed, then Deployable Energy's Unity, and finally Aalo Atomics' Critical Test Reactor completed the milestone as the fourth unit.

All four reactors were built at the National Reactor Innovation Center at Idaho National Laboratory, a federal site that has hosted reactor development since the 1950s. The program aims to demonstrate that small advanced designs can move from concept to operation faster than conventional large reactors, which typically take a decade or more to license and build.

The achievement signals momentum for the US advanced reactor sector, which has drawn interest from data center operators and industrial users seeking compact, reliable power. Reaching criticality is an early step, and developers still face testing, scale-up, and commercial deployment before the designs can supply power to customers. The milestone nonetheless showed that the pilot approach can accelerate reactor timelines.

Source: World Nuclear News - https://www.world-nuclear-news.org/articles/criticality-for-fourth-us-microreactor-meets-deadline