Another Advanced Reactor Goes Critical

On Thursday, Valar Atomics announced that it had taken its Ward 250 reactor critical. In doing so, it became the second advanced reactor to do so under the DOE’s Reactor Pilot Program (the other being Antares Nuclear’s Mark-0), and the first DOE-authorized reactor built and operated outside the national laboratory system. As Valar put it, the reactor went critical as a complete, fully integrated system configured for power operations and has now begun non-commercial power ascension. Founder and CEO Isaiah Taylor noted that Ward 250 is Valar’s second criticality as a company, following the NOVA core’s zero-power criticality at Los Alamos in November 2025. The company still has its eye on a harder target: power operations by July 4, the deadline set by President Trump’s executive order for at least three advanced reactors to achieve criticality.

This is the second major milestone in nuclear this month. As we wrote a few weeks ago about Antares, the persistent knock on advanced reactor startups has been a pattern of missed deadlines, and the value of these milestones lies as much in proving execution as in the underlying physics. Two more contenders remain in the running for the July 4 deadline: Aalo Atomics and Oklo’s Atomic Alchemy subsidiary, both still working through DOE authorization reviews. With two of the three required reactors now critical within six months, the challenge now shifts from proving feasibility to generating electricity at scale.