Anchor
Antares reactor reaches criticality under federal pilot program
Built WorldA startup's microreactor was first to fire up under a federal sprint — four more reactors have since followed
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Overview
Updated Aug 8On June 4, 2026, Antares Nuclear's Mark-0 reactor at an Idaho desert lab started a self-sustaining nuclear reaction. It was the first privately built non-light-water reactor to do that in the U.S. in more than 40 years, and four rival startups matched the feat within nine weeks.
Valar Atomics and Deployable Energy also reached criticality before the government's July 4 deadline. Aalo Atomics followed on deadline day, and Oklo's test reactor went critical on August 7. Antares raised $470 million in July to build a reactor that actually makes electricity, still aiming to power a military base 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.
5
Reactors critical under the pilot program
Antares, Valar Atomics, Deployable Energy, Aalo Atomics and Oklo have each reached criticality since June 2026.
$470M
Antares Series C funding
Raised in July 2026, led by Paradigm and Caffeinated Capital, pushing Antares' total funding past $604 million.
6 days
Ahead of the federal deadline
Deployable Energy's Unity reactor met the program's three-reactor target on June 30, 2026, six days before the July 4 deadline.
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Antares reactor reaches criticality under federal pilot program
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People Involved
Jordan Bramble
Founder and CEO, Antares Nuclear
Leading Antares after a $470 million Series C to build its first electricity-producing reactor
Chris Wright
U.S. Energy Secretary
Overseeing the reactor pilot program
Jeff Waksman
Principal deputy assistant secretary of the Army
Coordinating the Army's microreactor effort
Organizations Involved
Antares Nuclear Inc.
Nuclear startup
Raised $470 million to build its first electricity-producing reactor after leading the pilot program's first criticality
A startup building small, factory-made nuclear reactors for civilian and military power.
DOE Reactor Pilot Program
Federal program
Met its three-reactor criticality goal ahead of schedule and expanded into a longer-running Nuclear Energy Launch Pad program
An Energy Department program to push at least three advanced reactors to criticality by July 4, 2026.
VA
Valar Atomics
Private Company
Second company in the pilot program to reach criticality
A startup building the Ward 250, a small helium-cooled reactor for industrial and military customers.
Deployable Energy Inc.
Nuclear startup
Third company in the pilot program to reach criticality, meeting the federal three-reactor target
A startup building the Unity microreactor for fast, low-cost demonstration.
Aalo Atomics
Private Company
Fourth company in the pilot program to reach criticality
A Microsoft-backed startup building sodium-cooled microreactors aimed at AI data centers.
Oklo Inc.
Nuclear reactor developer
Fifth company in the pilot program to reach criticality
A publicly traded startup testing a small isotope-production reactor under the pilot program.
Timeline
May 2025 August 2026
12 events
Latest: August 3rd, 2026 · 1 month ago
Showing 8 of 12
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-
Antares details military base plans
Latest MilitaryAntares says its microreactors are designed to run more than six years without refueling, aimed at giving bases off-grid power during outages or attacks.
-
Antares raises $470 million Series C
FundingA round led by Paradigm and Caffeinated Capital funds Antares' Mark-1, its first reactor meant to produce electricity, targeted for 2027.
-
Aalo Atomics reaches criticality on deadline day
MilestoneAalo Atomics' test reactor goes critical at Idaho National Laboratory just after midnight, the fourth under the pilot program, on the program's own July 4 deadline.
-
Deployable Energy meets the three-reactor goal
MilestoneDeployable Energy's Unity reactor reaches criticality at Idaho National Laboratory, the third under the pilot program and the one that meets the executive order's three-reactor target four days early.
-
Valar Atomics reaches criticality
MilestoneValar Atomics' Ward 250 reactor goes critical at Utah's San Rafael Energy Lab, the second reactor under the pilot program and the first built outside a national lab.
-
Mark-0 reaches criticality
MilestoneAntares' Mark-0 starts a self-sustaining chain reaction at Idaho National Laboratory, a month before the federal deadline.
-
Air Force eyes Antares for a base deployment
MilitaryThe Department of the Air Force selects Antares for a proposed microreactor at Joint Base San Antonio.
-
DOE launches Nuclear Energy Launch Pad
ProgramThe Energy Department expands the pilot program into a longer-running effort called the Nuclear Energy Launch Pad, letting existing projects continue without reapplying.
-
Antares raises $96 million
FundingA Series B round led by Shine Capital pushes the company's total funding past $140 million.
-
Eleven reactor projects selected
ProgramThe department names 11 projects from 10 companies, including Antares, Oklo, Radiant and Valar Atomics.
-
Energy Department opens pilot applications
ProgramThe department issues its call for projects, aiming for three criticalities outside the national labs by July 4, 2026.
-
Executive order targets faster reactor testing
PolicyA 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.
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