Antares said Thursday that its test reactor at Idaho National Laboratory reached criticality, putting the company’s Mark 0 unit on a short list of new reactor designs to cross that threshold under the Department of Energy pilot program. The milestone means the nuclear reactions inside the hardware are now self sustaining, but the reactor is still not producing electricity.
That is why the announcement landed now. Under the federal push to speed nuclear development, reaching criticality is the proof point regulators and engineers have been waiting for, and Antares said its test unit is the first new design to do it in the program. The company is using the Mark 0 reactor to validate its modeling of the physical conditions inside the system and to gather safety data for licensing applications.
The design itself is built around TRISO fuel, with tiny pellets that contain a uranium oxide core wrapped in several layers of carbon and a hard ceramic shell. Antares surrounds that fuel with a graphite sheath and uses sodium to pull heat from the reactor to a heat exchanger, where the heat is transferred to pressurized nitrogen that drives a turbine in a closed Brayton cycle setup. The work was carried out at the Department of Energy lab in Idaho.
That progress still leaves a gap between a reactor that can sustain fission and one that can run a generator. The Mark 0 is a test unit and is not yet connected to the power-generation portion, so the next stage will be attempts to run the full system, including electrical generation, next year. Antares is also working with the Defense Department’s Project Pele program on a mobile nuclear reactor and has received support from NASA, while the broader federal target was set just over a year ago when the Trump administration ordered the Department of Energy to have three different reactor designs reach criticality in a bit over a year.
Only one smaller reactor design has been fully licensed so far, and no one plans to actually build it, which is part of the pressure behind the current race. For Antares, Thursday’s result is less a finish line than a gate: the reactor has crossed into sustained nuclear reaction, but the company still has to prove it can turn that heat into power on schedule.
