Valar Atomics' advanced reactor design completed a criticality demonstration during an experiment at the Utah San Rafael Energy Lab, according to a statement by the US Department of Energy on Thursday.
Ward 250 achieved zero-power criticality, proving that the reactor "can sustain a controlled nuclear chain reaction," the DOE said, a step required for power generation.
Valar Atomics is a participant in the DOE's Reactor Pilot Program, which sets a July 4 deadline for advanced reactors to demonstrate criticality.
Antares Nuclear's Mark-0 reactor was the first to reach the milestone at Idaho National Laboratory earlier this month.
The DOE said Ward 250 is the first reactor authorized by the agency to be built outside of a national laboratory.