US private microreactor hits criticality first under Trump DOE pilot

Antares Nuclear's Mark-0 sodium heat-pipe microreactor achieved zero-power criticality at Idaho National Laboratory on June 4, 2026, becoming the first privately developed advanced reactor to reach that milestone under the Department of Energy's Reactor Pilot Program - a Trump executive-order initiative targeting at least three advanced reactor criticalities by July 4. The Mark-0 is fueled by HALEU TRISO compacts and uses a heat-pipe-cooled, fast-spectrum design initially targeting U.S. military bases, with commercial electricity sales targeted for late 2027 and full deployment by end of 2028 - at the same Idaho National Laboratory campus where Oklo is constructing its Aurora powerhouse under an NRC-issued combined construction and operating license. The DOE has selected eleven advanced reactor projects under the RPP, and the Antares milestone validates the regulatory testing infrastructure and fast-fission design path that underpins the broader U.S. advanced nuclear build-out.