First Small Nuclear Reactor Reaches Criticality in Idaho

After years of promise, next-generation nuclear power just took its biggest leap forward yet.

Antares, a nuclear energy startup, announced Thursday that its test reactor at Idaho National Laboratory reached criticality. That means the nuclear reactions inside became self-sustaining, proving the design actually works in the real world.

This marks the first time a new reactor design has crossed this crucial threshold, despite an entire startup ecosystem racing to reinvent nuclear power. Only one other design has earned full licensing, and nobody plans to build it.

The Antares reactor uses a revolutionary fuel system called TRISO that makes nuclear power significantly safer. Instead of giant fuel rods, it relies on tiny uranium pellets wrapped in layers of protective carbon and sealed in hard ceramic shells.

These pellets can withstand the highest temperatures the uranium inside can produce. As long as the reactor keeps them contained, there's virtually no risk of meltdown or dangerous radiation leaks.

The design goes even further with safety. A graphite shield surrounds the TRISO pellets to slow down escaping neutrons. Sodium transfers heat to a separate system using pressurized nitrogen, keeping the radioactive and power-generation parts isolated.

Right now, Antares is testing what it calls a Mark 0 reactor without the power-generation components attached. The team is validating their computer models and gathering safety data for licensing applications.

Next year, they plan to fire up the complete system and actually generate electricity. The reactor is being developed with the Department of Defense's Project Pele program, which wants mobile nuclear reactors. NASA is supporting the work too.

The Ripple Effect

This breakthrough arrives at a perfect moment. The world desperately needs clean, reliable energy that works when the sun sets and wind stops blowing. Traditional nuclear plants take over a decade to build and cost billions of dollars.

Small modular reactors like this one could be manufactured in factories and installed in months instead of years. They're designed to power military bases, remote communities, or complement renewable energy grids.

The safety improvements matter just as much as the size. By baking protection into the fuel itself rather than relying entirely on massive containment buildings and backup systems, these reactors could finally overcome public fear that has stalled nuclear power for decades.

Other companies are watching closely. This successful test proves the regulatory pathway exists and the technology delivers on its promises.

One small reactor in Idaho just lit the way toward a cleaner energy future.