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Antares reactor reaches criticality under federal pilot program

Antares reactor reaches criticality under federal pilot program

Built World

A startup's microreactor is the first to fire up under the Energy Department's race to build advanced reactors fast

Yesterday: Mark-0 reaches criticality

Overview

On June 4, 2026, a small reactor at an Idaho desert lab started a self-sustaining nuclear chain reaction. It was the first privately built non-light-water reactor to reach criticality in the United States in more than 40 years.

Antares Nuclear got there a month before a federal deadline. The test is the first hard result from a 2025 program betting that startups can build reactors fast enough to power military bases by 2028.

Why it matters

If Antares hits its next targets, U.S. military bases could draw power from factory-built reactors by 2028, cutting reliance on the public grid.

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Key Indicators

40+ years
Since the last comparable U.S. reactor
Time since a privately developed non-light-water reactor last reached criticality in the country.
11
Projects in the pilot program
Reactor projects from 10 companies selected by the Energy Department in August 2025.
$96M
Antares Series B funding
Raised in December 2025, led by Shine Capital, bringing total funding past $140 million.
1 month
Ahead of the federal deadline
The program set a July 4, 2026 target for the first criticalities.

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People Involved

Organizations Involved

Timeline

May 2025 June 2026

6 events Latest: Yesterday
Tap a bar to jump to that date
  1. Mark-0 reaches criticality

    Latest Milestone

    Antares' Mark-0 starts a self-sustaining chain reaction at Idaho National Laboratory, a month before the federal deadline.

  2. Air Force eyes Antares for a base deployment

    Military

    The Department of the Air Force selects Antares for a proposed microreactor at Joint Base San Antonio.

  3. Antares raises $96 million

    Funding

    A Series B round led by Shine Capital pushes the company's total funding past $140 million.

  4. Eleven reactor projects selected

    Program

    The department names 11 projects from 10 companies, including Antares, Oklo, Radiant and Valar Atomics.

  5. Energy Department opens pilot applications

    Program

    The department issues its call for projects, aiming for three criticalities outside the national labs by July 4, 2026.

  6. Executive order targets faster reactor testing

    Policy

    A presidential order directs the Energy Department to speed up advanced reactor testing and set criticality deadlines.

Historical Context

3 moments from history that rhyme with this story — and how they unfolded.

December 1951

EBR-I lights four bulbs (1951)

At the same Idaho desert site, the Experimental Breeder Reactor-I produced the first usable electricity from nuclear fission. It powered four light bulbs, then the building that housed it. The reactor was a government science project, not a product.

Then

It proved fission could generate electricity and that a reactor could breed new fuel.

Now

EBR-I became a National Historic Landmark and anchored Idaho's role as the country's reactor proving ground.

Why this matters now

Antares reached criticality at the same site, but as a private company on a deadline rather than a federal lab. The contrast shows how the model for building reactors has shifted.

January 1961

SL-1 reactor explosion (1961)

An Army experimental reactor at the Idaho site exploded during maintenance, killing three operators. It remains the only fatal reactor accident in U.S. history. The reactor was a prototype meant to power remote military outposts.

Then

The military's small-reactor field program lost momentum and faced years of added scrutiny.

Now

The Army largely abandoned compact reactors for decades, a retreat only now being reversed.

Why this matters now

Today's Janus and Air Force programs revive the same idea SL-1 chased: reactors for military bases. It is a reminder that safety, not just speed, decides whether the effort lasts.

November 2023

NuScale's Idaho project canceled (2023)

NuScale held the first U.S. design approval for a small modular reactor and planned to build one in Idaho with a utility group. Rising costs pushed the projected power price up, and subscribers backed out. The flagship project was scrapped before construction.

Then

NuScale shelved its lead deployment and cut staff.

Now

The collapse showed that regulatory approval does not guarantee a buildable, affordable plant.

Why this matters now

Antares cleared a physics milestone, not a market one. NuScale's failure marks the harder test ahead: building reactors people will actually pay to run.

Sources

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