Antares Mark-0 microreactor reaches criticality at INL

Antares Nuclear’s Mark-0 microreactor achieved criticality at Idaho National Laboratory (INL) on June 4. The demonstration used high-assay low-enriched uranium (HALEU) fuel supplied by BWX Technologies and was the first privately developed advanced reactor to reach criticality under the Department of Energy’s Reactor Pilot Program. It was also the first novel reactor design to go critical at INL in more than 40 years.

Criticality indicates the reactor sustained a self-sustaining fission chain reaction, with neutron production balancing losses. The Mark-0 test produced essentially no measurable thermal power or electricity. The system was configured as a low-power physics test bed and did not include power conversion equipment, full heat removal systems or balance-of-plant infrastructure.

BWX Technologies manufactured and processed the HALEU feedstock and fabricated the reactor fuel at its Lynchburg, Virginia, facility. BWXT’s fuel work built on prior fabrication experience for the Army’s Project Pele microreactor program. The company supplied the qualified fuel used in the Mark-0 demonstration.

Antares completed the transition from design to a fueled criticality demonstration in roughly nine months. The focused, low-power configuration was planned to test core physics and fuel behavior without the added complexity of full-scale power systems.

Aalo Atomics, another participant in the DOE Reactor Pilot Program, has received DOE approval to begin reactor startup preparations at INL and is targeting a criticality demonstration with its Aalo-X test reactor in the coming weeks. Aalo has a strategic partnership with Flowserve to supply pumps, valves and flow-control equipment for its modular designs aimed at data center cooling and power applications. Flowserve components will be used in Aalo’s reactor hardware at INL.

HALEU refers to uranium enriched above conventional commercial fuel levels but below 20 percent U-235. Advanced reactor designs commonly use HALEU because higher enrichment permits greater fuel performance in smaller reactor cores.

The Mark-0 test provided experimental data on core physics and fuel behavior for a HALEU-fueled microreactor design. The results add technical information for regulators, reactor developers and equipment suppliers involved in advanced reactor demonstrations.