California Startup Claims Historic ‘First’ in Fission Reactor Milestone

In May, a series of executive orders from the Trump administration signaled sweeping changes to nuclear power in the United States. Despite some bumps along the way—namely major upheavals to nuclear regulation—the nuclear industry does appear to be delivering progress “at speeds not seen since the Manhattan Project,†according to industry observers. Now, one of the newest players in nuclear energy claims to have taken a small but significant first step toward making those ambitions real.

California-based startup Valar Atomics announced that it had achieved zero-power criticality at 2:45 p.m. ET on November 17. Criticality refers to a stage for a nuclear reactor in which each fission event—the splitting of a heavyweight atom—releases enough neutrons to sustain chain reactions. The result was a collaboration between Valar Atomics and the Los Alamos National Laboratory (LANL), which provided the operational capacity and other key technologies for the experiment.

Valar claims it is the first nuclear startup to reach criticality, as well as the first to do so among the startups participating in a Department of Energy pilot program to bring at least three startups to criticality by July 4, 2026.

“Zero power criticality is a reactor’s first heartbeat, proof the physics holds,†Isaiah Taylor, Valar’s CEO and founder, said in the statement. “This moment marks the dawn of a new era in American nuclear engineering—one defined by speed, scale, and private-sector execution with closer federal partnership.â€

Some caveats

To be clear, reaching criticality doesn’t mean the reactor is ready to generate power. Valar has achieved what is called cold criticality or zero-power criticality, which helps validate the reactor design but doesn’t necessarily create the heat for actually creating power.

The feat is still impressive, considering the company only started to take off earlier this year. As if to make up for its late entry to the industry, Valar has set aggressive goals in its attempt to get ahead of the curve, including the delivery of a fully operational reactor by the July deadline set by the executive order in May.

So far, they appear to be on track. Just last week, Valar reported securing $130 million in funding. In September, they broke ground at their first physical location for a reactor in Utah.

The latest criticality experiments tested the firm’s NOVA core, which builds upon previous structural designs from LANL. The experiments themselves were conducted by LANL and closely model real-life conditions planned for Valar’s upcoming reactor, the company explained.

A nuclear renaissance?

The developments illustrate major industry shifts since the Trump administration’s nearly militant approach to nuclear regulation. Prior to this series of executive orders, startups received stringent regulatory oversight from the Nuclear Regulatory Commission (NRC), which companies claimed obstructed innovation.

Valar, along with Last Energy and Deep Fission, had actually filed a lawsuit against the NRC late last year, criticizing the agency for “blocking†nuclear innovation on smaller scales. The suit has yet to be resolved, although Valar has since consistently praised the current administration’s approach to nuclear reform, calling it the “most comprehensive nuclear policy reform in decades†and “America’s nuclear renaissance.â€

The executive order has shifted the mindset behind interpreting the legal framing for nuclear power tests, Adam Stein, director of the Nuclear Energy Innovation program at the Breakthrough Institute, told Wired. “It basically asserted that if [a reactor] hasn’t been deployed commercially yet, then it is inherently a research system,†he added.

Indeed, the reforms have allowed for quick advancements, such as Valar’s criticality tests. At the same time, others have expressed concerns about loosened safety regulations based on several decades of research and experiments. For better or worse, it remains to be seen whether the changes will end up being a longer-term net positive for the U.S. nuclear industry.

In the meantime, Valar plans to continue testing criticality for its NOVA core at LANL facilities.