Landmark part of federal push to achieve criticality for at least three reactors by 4 July 2026
Valar Atomics has achieved first criticality and completed zero-power testing at Ward 250, its Generation IV high-temperature gas reactor (HTGR) at the Utah San Rafael Energy Lab in the US.
The project is the second advanced reactor to go critical under the Department of Energy’s (DOE’s) Reactor Pilot Program and the first DOE-authorised reactor built and operated outside the national laboratory system.
The milestone, confirmed by the DOE’s Office of Nuclear Energy at about 16:30 MDT on 18 June (00:30 on 19 June CET), involved a “zero-power fuelled criticality demonstration”, the DOE said.
Criticality is the point at which a nuclear fission chain reaction becomes self-sustaining. The Ward 250’s zero-power criticality demonstrates reactor physics at essentially no measurable energy output before power is increased.
“Ward 250 reached these milestones as a complete, fully integrated system configured for power operations,” Valar said in a statement.
Valar said the reactor had begun non-commercial power ascension and it will share further milestones as they are achieved.
“Moments ago, Valar Atomics took Ward 250 critical for the first time,” Valar founder and chief executive officer Isiah Taylor said on social media network X.
“This fulfills president Trump’s EO 14301, which called for 3 advanced reactors to go critical by July 4th. This is our second criticality as a company, and an important step toward our goal of power by July 4.”
Mission Is Abundant Energy
Taylor said: “Our mission at Valar Atomics is to make abundant energy for all mankind. The best way to make energy 10x cheaper today is to mass-scale nuclear fission. We began our mission by creating WardZero, our fully functional thermal prototype. Next, we took the Nova core critical.
“Today, we took the next step: criticality in the Ward 250 test reactor.”
The Nova core reached zero-power criticality in November 2025 at Los Alamos National Laboratory’s (LANL) National Criticality Experiments Research Center. Nova, however, was a zero-power plant that ran under National Nuclear Security Administration oversight at LANL rather than under the DOE Reactor Pilot Program.
Ward 250 went critical as a complete, fully integrated system configured for power operations at a privately built site under DOE Reactor Pilot Program authorisation.
The Ward 250 HTGR, a microreactor slightly larger than a minivan, is designed to generate up to 5 MW of electricity – theoretically enough to power around 5,000 homes.
It is powered by Triso (TRi-structural ISOtropic) nuclear fuel, a highly robust, advanced fuel form of fuel that consists of a uranium-based fuel kernel surrounded by three distinct layers of ceramic and carbon materials, which form an individual containment system for each particle.
This tri-structural design is said to provide exceptional resistance to corrosion, high temperatures and irradiation, making it impossible to melt under extreme conditions and containing radioactive fission products effectively.
According to the International Atomic Energy Agency, interest in HTGRs is increasing because they can provide efficient and cost-effective electricity and produce high-temperature process heat usable for various industrial applications.
The Operation Windlord Reactor Airlift
In February, the Ward 250 reactor was transported to Utah aboard three C-17 Globemaster III aircraft in Operation Windlord – the first military airlift of a nuclear reactor – as part of a joint operation between DOE and the Department of War.
The US military wants to deploy mobile nuclear microreactors to ensure, resilient, carbon-free and independent power for remote bases and critical infrastructure.
US energy secretary Chris Wright accompanied the flight, describing the operation as a milestone for nuclear logistics and base energy resilience.
The Ward 250 criticality marks the second advanced reactor criticality under Executive Order 14301, which directed the DOE to achieve criticality for at least three advanced reactors by 4 July 2026.
On 4 June Antares announced that its Mark-0 microreactor achieved initial criticality at Idaho National Laboratory (INL), making Antares the first private company to bring an advanced reactor to criticality under the DOE’s push to deploy advanced plants.
Antares is developing what it calls factory-produced, deployable fission microreactors of up to 1 MW to provide strategic energy for mission-critical systems across Earth, space and underseas.
In April energy secretary Wright said the US was on track to meet its objective of having multiple nuclear test reactors reach criticality by the country’s 250th anniversary on 4 July.
In February, the Ward 250 reactor was transported to Utah aboard three C-17 Globemaster III aircraft. Courtesy DOE.
