Westinghouse’s eVinci Core Went Critical in Nevada, at a Power Level That Produces No Usable Heat

Westinghouse Electric Company took the core of its eVinci microreactor critical on August 24, 2026, at 10:39 a.m. Pacific time. The experiment ran at the National Criticality Experiments Research Center in Nevada. Los Alamos National Laboratory and Idaho National Laboratory ran it with the company.

The commercial eVinci design carries a rating of 5 MW of electricity, and it is built to run more than eight years without refueling. The core tested in Nevada produced no electricity and no usable heat.

A zero-power criticality test assembles enough fissile material in the intended geometry to sustain a fission chain reaction. Power stays low enough that decay heat and radioactive inventory stay negligible. Engineers run the test to check the core against the computer models. It confirms neutron behavior, control element worth and core geometry. Westinghouse said the experiment validated the models and core design assumptions behind the technology.

The test leaves the unusual parts of eVinci unproven. The design moves heat with sealed sodium heat pipes instead of pumps and coolant loops. It converts that heat through an open-air Brayton cycle. Neither system operates during a zero-power experiment, because the core makes no heat to move. POWER called the milestone a physics validation experiment rather than a fueled power demonstration.

eVinci burns TRISO fuel made with high-assay low-enriched uranium inside a solid steel core block. Westinghouse markets the unit as a nuclear battery. The company puts its footprint near half a hockey rink and its industrial heat at up to 600 C.

The published thermal rating is not consistent. The American Nuclear Society reported 15 MW thermal against 5 MW electric. POWER cited 13 MW thermal, then noted that Westinghouse product materials also carry a 15 MW thermal core rating and a separate research variant at 6 MW thermal.

Westinghouse is not the first developer to reach this point in 2026. Antares Nuclear took its Mark-0 test reactor critical at Idaho National Laboratory on June 4. Valar Atomics reached criticality with Ward 250 at the San Rafael Energy Lab in Utah on June 18. POWER reports that both followed the Department of Energy Reactor Pilot Program, which Executive Order 14301 created in June 2025.

The Nevada National Security Site holds the only general-purpose critical experiments facility in the United States. Los Alamos operates it for the National Nuclear Security Administration. Westinghouse said it will now move to prototype testing, targeting performance, operability and manufacturability.

Why It Matters

Microreactor developers now hold several criticality announcements, and the press releases give buyers little help telling them apart. A zero-power test validates neutronics. It says nothing about whether heat pipes survive years of thermal cycling, or whether the Brayton loop holds its efficiency at scale.

It says nothing about price either. Westinghouse still has to run a fueled prototype at power. Anyone sizing an eVinci into a data center or a base energy plan should read August 24 as a modeling checkpoint, not a delivery date.

Critical Perspective

None of the announcements say whether the core tested in Nevada carried the same fuel form, enrichment and control drum hardware that Westinghouse plans to sell. That distinction decides how much of the commercial design this result validates.

The inconsistent thermal rating is a second open question. A unit quoted at 13 MW thermal in one account and 15 MW thermal in another differs by roughly 15 percent in heat output. That gap changes the economics for any industrial buyer sizing eVinci against a gas boiler.

The eight-year refueling claim rests on how TRISO fuel behaves across eight years of operation. A zero-power test runs for hours and produces no burnup. Nothing in this milestone touches the claim that sells the machine.

Sources

• Westinghouse: eVinci Microreactor Achieves Zero-Power Criticality
• American Nuclear Society: eVinci completes cold criticality test at NCERC
• POWER: Westinghouse eVinci Reaches Criticality Milestone at Nevada Test Site
• Power Engineering: Westinghouse achieves zero-power criticality with its eVinci microreactor
• World Nuclear News: Criticality testing milestone for Westinghouse’s eVinci microreactor
• Los Alamos National Laboratory: National Criticality Experiments Research Center

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