First criticality for US microreactor under DOE programme

The demonstration took place at Idaho National Laboratory (INL) under US Department of Energy (DOE) authorisation, with Antares saying it is the first private company to bring an advanced reactor to criticality under the DOE Reactor Pilot Program. Antares was one of five companies selected last year by the DOE for support under the programme to expedite the testing of advanced reactor designs. The pilot programme is part of the Reforming Nuclear Reactor Testing at the Department of Energy executive order signed by Trump in May last year, with a goal to "to construct, operate, and achieve criticality of at least three test reactors using the DOE authorisation process by July 4, 2026".

Criticality means that the reactor has achieved a sustained nuclear chain reaction, with each fission event - when an atom of uranium in the fuel is split - releasing a sufficient number of neutrons to sustain an ongoing series of reactions. In a nuclear power reactor, the heat energy from those fission reactions is used to produce steam and generate electricity.

The Mark-0 is a demonstration reactor, validating key reactor physics parameters for Antares' sodium heat-pipe cooled microreactor technology, which uses tri-isostructural isotropic - or TRISO - fuel containing high assay low-enriched uranium (known as HALEU).

The DOE described the criticality test of the 53rd reactor to be built at INL since 1951 as a "tremendous accomplishment" validating the safety and operational performance of Antares Nuclear’s fission reactor, and one of the most significant technological achievements in nuclear energy in more than 40 years. "When commercialised after further tests and licensure by the Nuclear Regulatory Commission, microreactors like those that Antares makes are anticipated to be used in a variety of terrestrial and space applications and to ensure readiness at military installations requiring reliable energy," the Department said.

The demonstration was conducted in partnership with DOE, INL, and BWX Technologies, Inc. (BWXT) - supplier of the TRISO fuel used to power the reactor - and with integration and observation support from the US Army, which is seen as a future end user of the technology.

As well as meeting the administration's objectives to reform how the federal government tests advanced reactors, the demonstration establishes a replicable licensing pathway that DOE and industry can use to accelerate future reactor demonstrations on commercial timelines, Antares said.

"Hitting our commitments is everything to us. Nuclear in America has been defined for too long by delays, by companies that said they would and then didn't," said Antares CEO Jordan Bramble.

The fuel used by Antares is modelled on the TRISO fuel compacts delivered by BWXT for Project Pele, a 1.5 MW transportable microreactor BWXT is building for the US Army's Strategic Capabilities Office. Building on a proven fuel specification and manufacturing expertise matured through Project Pele directly underpins the criticality milestone, Joe Miller, BWXT's president for Government Operations, said.

TRISO fuel compacts produced for the Mark-0 by BWXT (Image: BWXT)

The HALEU feedstock material used to manufacture the Antares TRISO fuel compacts comes from scrap materials provided by the DOE's National Nuclear Security Administration. BWXT said it will continue to support Antares with ongoing TRISO fuel manufacturing, reinforcing the company’s readiness to meet customer timelines and the growing national demand for advanced reactor fuel.

"We're grateful for a partnership that continues as we move from neutrons to electrons," Bramble said.

Antares' timeline envisages electricity production in 2027, with the first customer deployments of electricity-producing microreactors the following year. The criticality demonstration, and the licensing pathway it establishes, represent a key step toward deploying electricity-producing microreactors for US military installations by the end of September 2028, Antares said.

"We said criticality in 2026, electricity production in 2027, and power to the warfighter in 2028. Today is the first of those commitments delivered on the schedule we set. The President and DOE set an ambitious timeline for reactor testing, and we met that challenge," Bramble said. "I want to thank our partners at the Department of Energy, Idaho National Lab, BWXT, and the US Army. This is what happens when industry and government work together to accomplish big things."

"We went from concept to a critical reactor, safely, in less than 12 months," he added. "That doesn't happen by accident. … It also doesn't happen without decades of DOE investment in the AGR-2 TRISO specification and the Project Pele fuel supply chain at BWXT. Our partners at Idaho National Laboratory and DOE-ID provided the design, regulatory, and facilities support that enabled this schedule."