Antares Nuclear is preparing its electricity-producing Mark-1 reactor for a planned 2027 demonstration using a full-scale reactor core, moving the company from nuclear-criticality testing into power generation in a single step. The timeline rests on a design that pairs a graphite-based core containing TRISO fuel with sodium heat pipes and a nitrogen closed Brayton-cycle power conversion system.

Mark-0 established the nuclear basis

Antares achieved initial criticality with its Mark-0 reactor in June 2026 using fuel produced by BWXT, providing a milestone for the reactor's nuclear performance ahead of the Mark-1 electricity-generation demonstration. That campaign effectively validated the core physics and the fuel supply route before the company committed to a full-scale core.

HALEU is the enabling input

The design uses high-assay low-enriched uranium, which contains more uranium-235 than conventional light-water reactor fuel but remains below 20% enrichment, to support the neutron behaviour required by its compact reactor core. Compactness is the commercial premise of microreactors: a smaller core with higher fissile density allows a transportable unit, but it also makes the reactor dependent on a HALEU supply chain that is only now taking shape in the United States.

Fuel supply as a de-risking step

The multi-year TRISO agreement with BWXT Advanced Fuels, reserving an initial one metric tonne of uranium with quantities set by future task orders, gives Antares a manufacturing pathway at the Lynchburg facility in Virginia rather than a one-off batch. For a developer on a 2027 demonstration clock, an order-driven framework matters as much as the fuel itself.

What the 2027 demonstration will show

The Mark-1 campaign is intended to demonstrate electricity production from a full-scale core, not merely criticality. If it proceeds on schedule, Antares would be among the first advanced reactor developers to close the loop from HALEU-fuelled TRISO fabrication through reactor operation to power conversion — evidence that the demand side of the HALEU market is maturing alongside enrichment and fabrication capacity.

Antares achieved initial criticality with its Mark-0 reactor in June 2026 using fuel produced by BWXT.

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