US Switches On First New Reactor in 40+ Years

What happened at Idaho National Laboratory in June 2026?

Antares Nuclear's Mark-0 reactor achieved criticality at Idaho National Laboratory on June 4, 2026. It is the first non-light water reactor to go critical in the United States in over 40 years, according to CNBC's interview with Energy Secretary Chris Wright.

Criticality is the point at which a nuclear reactor can maintain a self-sustaining chain reaction. The Mark-0 completed a zero-power criticality demonstration inside the NRIC DOME on INL's campus in Idaho Falls.

What is the DOE's Reactor Pilot Program?

The Reactor Pilot Program is a Department of Energy initiative created by a presidential executive order signed in May 2025. Its explicit goal: get three advanced nuclear reactors to criticality by July 4, 2026.

An advanced nuclear reactor uses innovative technology — more efficient fuel, new coolant types, passive safety features, or smaller modular sizes — compared to conventional light-water reactors.

Which reactors have gone critical so far?

Two reactors reached criticality ahead of Wright's June 25 visit to INL. Here's the timeline:

The Ward 250 is notable for several reasons. It is the first reactor built outside a national laboratory. It is also the first reactor transported by flight — moved between Air Force bases in California and Utah aboard a C-17. The minivan-sized microreactor was not loaded with nuclear fuel during transport, as reported by the Idaho Statesman.

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Will the third reactor meet the July 4 deadline?

Wright said yes — twice. "We are gonna meet that," he told CNBC on June 25. He announced that Aalo Atomics had received approval to move forward on tests, making it the third reactor in the program.

INL Director John Wagner also confirmed the lab was on track for three reactors to go critical by July 4. Wagner added that "several more will follow shortly thereafter." He noted that Radiant Nuclear was already moving components for its Kaleidos reactor into the DOME at INL for testing.

What makes these reactors different from existing ones?

All commercially operated US reactors currently use water as a coolant and rely on electrically operated pumps. The Mark-0 uses a different approach entirely.

Antares Nuclear's Mark-0 uses sodium heat pipes that passively transfer heat away from the reactor core without electricity. Liquid sodium near the core heats up, turns to vapor, rises to the top of the pipes, releases heat, then condenses back to liquid and repeats the cycle.

The zero-power criticality tests completed so far have not yet put these new cooling systems under load. Future tests at energy-generating levels will generate heat, and that's when the cooling systems face their real test.

When could small modular reactors produce electricity?

Wright told CNBC he believes the first electricity from a small modular reactor could arrive "before the end of next year, and definitely within 2028, within the Trump administration."

The long-term goal is larger. INL Director Wagner stated: "The goal was never just criticality. The goal is 400 gigawatts of nuclear capacity by 2050." That aligns with the administration's stated aim to quadruple US nuclear capacity by 2050.

Here's what we know so far: the criticality milestones are real and confirmed, but commercial electricity generation from these specific reactors remains a future step — the cooling systems and energy-output tests are still ahead.

Why is Idaho significant to US nuclear history?

Nuclear fission first produced electricity for civilian applications at what is now Idaho National Laboratory on December 20, 1951. Idaho Lt. Gov. Scott Bedke, who attended the June 25 event, noted: "This is the ground where America first lit a lightbulb with the atom."

Wright echoed that history at the event. He reminded the audience that within two to two-and-a-half decades after that first civilian application, the US launched construction of over a hundred nuclear reactors, which quickly provided 20% of the country's electricity.

The energy investment surge happening now parallels broader trends in power demand. Goldman Sachs projects AI capex hitting $800B in 2026, with data centers driving much of the electricity demand that nuclear advocates say these new reactors could serve. The scale of AI infrastructure spending is one reason the administration has framed nuclear capacity as a national priority.

Wright's remarks at INL on June 25 closed the event. He was the final speaker in a series that included Kyle Haustveit, undersecretary of energy, and INL Director John Wagner. The next confirmed milestone is the third reactor — from Aalo Atomics — reaching criticality by July 4, 2026.