Westinghouse completes zero-power criticality test for eVinci microreactor at federal research center

Westinghouse announced on August 25 that it successfully completed zero-power criticality testing for its eVinci microreactor at the National Criticality Experiments Research Center. The test was a partnership with Los Alamos National Laboratory and Idaho National Laboratory. It validated the reactor’s models and core design assumptions—a key milestone for a technology still working its way through the development pipeline.

Westinghouse completes cold criticality test for eVinci at NCERC

The August 25 announcement marks a concrete step forward for the eVinci program. Westinghouse confirmed the zero-power criticality test took place at the National Criticality Experiments Research Center—NCERC—with technical support from Los Alamos National Laboratory and Idaho National Laboratory, two of the country’s most prominent nuclear research institutions.

That kind of federal-industry collaboration has become fairly standard in advanced nuclear development. National labs bring decades of experimental expertise and specialized infrastructure that private companies simply can’t replicate on their own.

Confirming they accurately predict real-world neutron behavior gives the development team a reliable basis to move forward—and that’s not a small thing.

The test validated the reactor’s computational models and core design assumptions—a specific and meaningful outcome. It means the physics predictions built into the eVinci design held up against real experimental data. For a program still in the prototyping phase, that confirmation carries real weight.

Why zero-power criticality testing is conducted

Zero-power criticality testing—sometimes called cold criticality testing—is a standard but technically demanding step in reactor development. The goal is to confirm that a reactor core can sustain a controlled nuclear chain reaction without generating significant heat.

At zero power, the fission reaction is real, but energy output stays low enough to produce no meaningful thermal result. That’s intentional. Engineers can study neutron behavior inside the core without the complications that come with heat generation.

The central value of these tests is verification. Reactor designers build computational models to predict how neutrons move through the core, how they interact with fuel and structural materials, and whether the chain reaction stays stable. Zero-power tests check whether those predictions hold.

NCERC is purpose-built for exactly this kind of experiment. Operated under the Nevada National Security Site, it’s a specialized federal facility with the infrastructure and safety systems required to run criticality experiments—and not many places in the country can do this work safely, at the scale a microreactor test article requires. Running tests at low power also minimizes risk while still producing data directly relevant to how the full reactor will behave, making it an effective way to stress-test a design’s theoretical foundation before committing to higher-power phases.

What the results mean for the eVinci program

A successful zero-power criticality test doesn’t mean the eVinci is ready to deploy. What it does mean is that the design’s technical foundation cleared an important hurdle.

Validating the computational models matters because those models underpin nearly every other engineering decision in the program. If the models are wrong, the design could be wrong too. Confirming they accurately predict real-world neutron behavior gives the development team a reliable basis to move forward—and that’s not a small thing.

Westinghouse said it plans to continue prototyping the eVinci system following this milestone. Upcoming testing phases will focus on performance, operability, and manufacturability—questions that go well beyond core physics and into whether the reactor can actually be built, operated, and maintained under real-world conditions.

Those are harder questions in some ways. A reactor that works in theory still has to be assembled reliably, fueled safely, and operated by people who aren’t nuclear physicists. Demonstrating that a microreactor can meet those practical standards is a distinct challenge from proving the neutronics are sound. Even so, completing the cold criticality test moves the program meaningfully closer to a deployable system.

Background: The eVinci microreactor and its intended applications

The eVinci is a heat-pipe-cooled microreactor developed by Westinghouse. Unlike conventional reactors that use water or gas to transfer heat from the core, the eVinci relies on heat pipes—passive devices that move thermal energy without pumps or active cooling systems. That design choice supports reliability and simplicity, which matters considerably for the applications Westinghouse has in mind.

The eVinci targets remote, off-grid, and defense use cases—places where grid power isn’t available or dependable, and where a compact, self-contained power source would be valuable. Military forward operating bases, remote mining operations, and isolated communities are among the scenarios microreactor developers frequently cite.

Microreactors are generally defined as nuclear fission systems rated below 20 megawatts electric, designed to be factory-fabricated and rapidly deployable. That sets them apart from large conventional reactors requiring years of on-site construction. The U.S. Department of Energy and national laboratories have supported multiple microreactor development programs as part of a broader advanced nuclear strategy, and Westinghouse is one of several companies pursuing microreactor licensing and demonstration in the United States—competing and collaborating in a space that’s drawn significant federal attention in recent years.

Models and core design assumptions are sound

The completion of zero-power criticality testing at NCERC is a verified technical milestone for the eVinci program. Conducted with Los Alamos and Idaho National Laboratory, the test confirmed that the reactor’s models and core design assumptions are sound. Westinghouse plans to continue prototyping, with further testing targeting performance, operability, and manufacturability. The eVinci remains in development, but this result advances the program toward an eventual deployable microreactor demonstration.

Kelly Lippke

Kelly is an experienced writer with 15 years of experience exploring the big stories that shape our world, from tech breakthroughs and space exploration to climate, energy, and the fascinating quirks of science. She has a talent for turning complex ideas into sharp, memorable insights that stay with readers long after they’ve finished reading.