Four privately built reactors go critical in a month under DOE authorization

July 2026 — Four privately developed reactors reached zero-power criticality under DOE authorization, the last on July 4, 2026, one more than the goal President Trump set in a May 2025 executive order. Here's what happened, what it cost, and what went wrong.

Cost and outcome (as of July 6, 2026)

Outcome: four DOE-authorized advanced reactors reached criticality against a goal of three by July 4. (DOE, July 6)
First: Antares' Mark-0 at Idaho National Laboratory, announced June 4, 2026. (DOE, June 4)
Fourth: Aalo Atomics' Aalo-X at INL, at 12:20 a.m. Mountain time on July 4, 2026. (POWER)
Cost: not stated by DOE.
Status: zero-power tests. DOE says they establish a basis for later reactors to produce electricity "in 2027 and beyond." (DOE, June 4)

The first one

Antares Nuclear's Mark-0 completed a zero-power fueled criticality demonstration at INL. DOE said the test "confirms that the reactor can operate safely" and called it the first of several reactors expected to go critical by the July 4 deadline. (DOE, June 4)

"For the first time in more than four decades, a new privately developed non-light-water reactor has reached criticality in the United States," Energy Secretary Chris Wright said. DOE said Mark-0 is the 53rd reactor built at the INL site since 1951. (DOE, June 4)

Antares CEO Jordan Bramble said the demonstration would keep generating lessons "in a safe, rapid setting to improve our reactor design." Any commercial version would still need further tests and Nuclear Regulatory Commission licensing, DOE said. (DOE, June 4)

What DOE expects of microreactors

Once commercialized, DOE said, microreactors like Antares' are expected to serve "a variety of terrestrial and space applications" and military installations that need reliable energy. Assistant Secretary for Nuclear Energy Ted Garrish said skeptics "didn't believe" the program could reach criticality in under a year. (DOE, June 4)

Bramble said that beyond reactor physics, the company had gained "a profoundly greater mastery of our regulatory pathway and our supply chain." (DOE, June 4)

Two more in June

In June, Valar Atomics' Ward 250 and Deployable Energy's Unity also reached criticality, DOE said. (DOE, July 6) POWER reports Valar's Ward 250 went critical June 18 at the Utah San Rafael Energy Lab in Emery County, and Deployable's Unity June 30 at INL through the Nuclear Energy Launch Pad. (POWER)

The fourth, and the goal

Aalo-X went critical early Saturday, July 4, making it the fourth DOE-authorized advanced reactor to do so and "exceeding the July 4th goal," DOE said. Wright said President Trump "asked for three advanced reactors to be authorized and achieve criticality by the 250th anniversary of our great country," adding that DOE and its partners "have surpassed that ask and delivered four!" (DOE, July 6)

DOE's Reactor Pilot Program, which Aalo-X was part of, uses DOE's own authority, rather than NRC licensing, to "expeditiously certify and construct first-of-a-kind advanced reactor designs for demonstration," according to DOE. (DOE, July 6)

Aalo's account

Aalo president Yasir Arafat said the company went "from breaking ground to a sustained chain reaction in just eight months." (DOE, July 6) Aalo also told POWER the reactor building took 36 days and the reactor was built in its factory in 28 days. Those are company figures. (POWER)

POWER, reviewing Aalo's livestream, reported a core of 13 uranium dioxide fuel assemblies enriched to about 5%, graphite moderation, four safety rods and six control rods. Arafat described the approach to criticality as "a very slow and deliberate process," with control rods withdrawn in small steps and neutron counts checked against the company's physics model at each one. (POWER)

What went wrong

No electricity. These were zero-power tests; DOE points to power production "in 2027 and beyond" for later reactors. (DOE, June 4)
Not NRC-licensed. Commercial versions need further testing and NRC licensing. (DOE, June 4)
Speed claims are self-reported. Aalo's eight-month figure is its own. (DOE, July 6)
What we don't know yet: what the four reactors cost. DOE doesn't say.

Our follow-up question for the Department of Energy

What have the four reactors that went critical under DOE authorization cost the government so far, and when is the first of them expected to generate electricity?

Sources

U.S. Department of Energy, “Department of Energy Celebrates First Advanced Reactor Criticality,” June 4, 2026. energy.gov
U.S. Department of Energy, “Department of Energy Celebrates Fourth Criticality Ahead of July 4th Goal,” July 6, 2026. energy.gov
POWER magazine, “Aalo Atomics' Test Reactor Reaches Criticality at INL, Fourth DOE-Authorized Advanced Reactor by July 4,” July 2026. powermag.com

Disclosure: [not in copy] Editor placeholder