The Mark-0 sodium heat pipe-cooled microreactor developed by California-based Antares was recently brought to zero-power fueled criticality at the U.S. Idaho National Laboratory’s Reactor and Critical Experiment facility.

As part of the U.S. Department of Energy’s Reactor pilot Program, the high-assay low-enriched uranium (HALEU) tri-structural isotropic (TRISO) fuel compacts-fueled reactor was brought to the minimum power level required to start a nuclear chain reaction. The goal is to validate the reactor's computational physics models, core geometry, control rod performance and initial neutronic behavior without generating significant thermal energy or requiring active coolant flow.

Zero-power criticality testing confirms that the nuclear core can sustain a controlled chain reaction at very low power levels. It validates the reactor’s basic nuclear behavior, instrumentation and control systems before any higher-power testing begins.

This achievement represents a key step toward deploying electricity-producing microreactors for U.S. military installations by September 30, 2028. The data obtained will be used to infuse development of Antares’ commercial product, the R1 microreactor, a modular, transportable unit rated at 100 kWe to 1-Mwe and designed to operate for six or more years between refueling without connection to the commercial grid.