Kenneth Glad
The test moves a compact energy project from planning into live reactor operation. Officials now face the harder task of turning the system into a reliable source of electricity.
Antares, a U.S. nuclear start-up founded in 2023, has achieved initial criticality with its Mark-0 microreactor at Idaho National Laboratory, according to a press release from the company
Initial criticality means the reactor sustained a nuclear chain reaction. It is a central technical threshold in reactor testing, though the Mark-0 is not yet producing electricity for users.
A private reactor passes its first major trial
The demonstration was authorized under the U.S. Department of Energy’s Reactor Pilot Program, which is designed to speed up testing for advanced reactor projects. Idaho National Laboratory, BWX Technologies and the U.S. Army were involved in the work.
U.S. Energy Secretary Chris Wright, quoted in a statement carried by the outlet Morningstar, described the result as “a historic moment for American nuclear energy.”
Jordan Bramble, the chief executive of Antares, said the company had delivered on its first target. “We said criticality in 2026, electricity production in 2027, and power to the warfighter in 2028.”
Military power needs shaped the design
The Mark-0 used TRISO fuel supplied by BWXT. The fuel work is connected to Project Pele, a U.S. defense program aimed at demonstrating transportable nuclear power.
The technology has attracted interest from the U.S. military because it could reduce dependence on fuel deliveries and vulnerable power infrastructure. Remote bases, training sites and isolated installations often need steady energy in places where conventional supply lines can be difficult to protect.
Bramble said speed was a defining part of the project. “We went from concept to a critical reactor, safely, in less than 12 months.”
That pace is central to the company’s pitch, but reaching criticality is not the same as proving day-to-day usefulness. The next stage will test whether the system can move from reactor physics data to usable power.
Electricity production is the next deadline
Antares says it plans to produce electricity in 2027. Its longer target is to deploy electricity-generating microreactors at U.S. military installations by September 30, 2028.
Before the reactor can be offered more broadly, it would need approval from the U.S. Nuclear Regulatory Commission, according to Digi24. That regulatory step remains separate from the special authorization used for the Idaho test.
The demonstration also generated data on core behavior, control systems and fuel performance, according to the company statement.
Such information can help engineers refine future versions of the reactor before full-power operation.
Romania follows a separate SMR path
The U.S. test comes as other small reactor projects continue to develop internationally.
Digi24 noted that Romania is pursuing a NuScale-based small modular reactor project at Doicești, on the site of a former coal plant.
That plan begins with a proposed 77 MW module and could later expand to six units.
According to the Romanian newssite, NuScale remains the only small modular reactor design so far approved by the NRC.
Sources: Digi24, Morningstar, Antares press release
