Westinghouse Electric Company has completed zero-power criticality testing for its eVinci microreactor in partnership with Los Alamos and Idaho National Laboratories at the National Criticality Experiments Research Center (NCERC), a National Nuclear Security Administration (NNSA) facility at the Nevada National Security Site (NNSS).
The eVinci microreactor is a 15MWt/5MWe nuclear battery designed for remote communities, mining sites, data centres, military applications, and deep-space missions. It functions like a thermal battery by locking its components in a solid block and pulling heat out passively.
The core comprises HALEU (high-assay low-enriched uranium) encapsulated into TRISO (Tristructural Isotropic) fuel particles embedded in a solid graphite block, which serves as the neutron moderator and provides thermal inertia. Instead of pumping liquid water or gas through the core to cool it, the eVinci uses an array of 24-foot-long liquid sodium heat pipes. Capillary action and phase changes drive this system: heat from the core vaporises the sodium, which flows to the cold end of the pipe, releases its energy to a heat exchanger, condenses back into liquid, and cycles back. This eliminates pumps, mechanical valves, seals, and the risk of a loss-of-coolant accident.
Unlike traditional nuclear plants that boil water to create steam, the eVinci uses an open-air Brayton cycle gas turbine. Atmospheric air is compressed, passed through the reactor’s heat exchanger to absorb energy directly from the heat pipes, and then expanded through a gas turbine to drive an electrical generator.
The eVinci design is engineered to be a “walk-away safe” reactor that relies on physics rather than operator intervention or mechanical systems to prevent accidents. As the reactor core gets hotter, its structural physics naturally reduce its reactivity. This passive negative feedback loop automatically dampens the nuclear chain reaction if temperatures spike.
To adjust power output, the reactor uses rotating control drums lining the periphery of the core rather than traditional insertable control rods. In the event of a power loss, gravity or mechanical springs passively rotate these drums to their “off” positions, safely shutting down the system. Using a logic-based instrumentation and control platform (the Westinghouse ALS v2), the system operates autonomously, minimising the required onsite personnel and allowing for remote monitoring.
However, the zero-power criticality experiment did not use the full-scale commercial solid monolith block core, but a specialised scale mock-up configuration designed to test the nuclear physics of a smaller segment. The test was conducted using the eDeimos experimental assembly at NCERC. The experiment swapped out the central core of an existing, flexible laboratory testbed with a “unit cell” array. This mock architecture combined the real materials (TRISO fuel, graphite moderator materials, and heat pipes) arranged in the exact physical spacing and dimensions of the eVinci core, but without using the massive, final monolithic block framework.
The purpose of a zero-power criticality test is to study neutronics and material behaviours, not heat transfer or power production. Because the experiment it generated virtually zero heat, standard temperature gauges could not track the core’s status. Instead, the team relied on a specialised suite of high-sensitivity radiation detectors within the eDeimos assembly to monitor the approach to criticality.
Before the core could sustain itself, researchers used a small, temporary radioactive source to kickstart a baseline neutron flow. The detectors tracked the exact point where the fuel material began multiplying these source neutrons on its own, marking the official transition into self-sustaining criticality. The zero-power criticality test verified negative thermal feedback, the fundamental physics-based safety mechanism of the eVinci core, which prevents a runaway nuclear chain reaction without needing any human intervention or computer-controlled rods.
However, as no heat was generated that needed to be passively pumped away by the heat pipes, the structural solid block, which acts as the main thermal conductor for heat extraction, was not required to gather this physics data.
The physical, structural integrity of the solid block core and its manufacturing tolerances will be tested in the next phase. Westinghouse is currently building a 1/5 scaled, 1 MWe prototype reactor, at the eVinci Microreactor Accelerator Hub in Etna, Pennsylvania, that will feature the integrated solid block architecture. This prototype is slated to be installed and powered up inside Idaho National Laboratory’s (INL’s) National Reactor Innovation Center (NRIC) Demonstration of Microreactor Experiments (DOME) testbed.
NRIC structures DOME as a single-occupancy, repeating assembly-line environment where advanced reactors are run one at a time. Radiant Industries was selected to run its 1-MW Kaleidos unit first, kicking off its physical installation and testing campaign in mid-to-late 2026. Westinghouse is scheduled to move its prototype into the DOME facility immediately following Radiant in the 2027–2028 timeframe.
Unlike the zero-power tests, the DOME trials will run the reactor at high temperatures and thermal capacities for up to six months. This will be the first time the solid monolith block and liquid sodium heat pipes are subjected to real, sustained fission heat to test material expansion, vacuum sealing, and long-term passive heat extraction.
Following its operational run inside the DOME facility, the eVinci prototype will enters a highly structured post-irradiation examination (PIE) phase at INL. After the reactor is shut down, it will remain inside the DOME containment structure for 30-60 days. This allows short-lived radioactive isotopes to decay and gives the liquid sodium heat pipes time to cool and solidify into solid metal.
The reactor module will then be loaded into a heavily shielded shipping cask and transported across the INL site to the Hot Fuel Examination Facility (HFEF) for examination. Scientists will use precision gamma scanning and high-energy x-ray imaging to inspect the reactor without cutting it open. This tracks how neutrons changed the graphite material structure and checks if any micro-warping occurred in the drilling channels.
The core will eventually be disassembled to examine individual components. Engineers will slice open the sodium heat pipes to check for internal corrosion or chemical degradation and analyse the TRISO fuel pellets to confirm how well the ceramic silicon-carbide shells contained the radioactive fission products under real-world stress.
Westinghouse is aiming to begin high-rate fleet assembly at the Etna production hub in Pennsylvania in 2029-30 focusing heavily on defence, space, and initial specialised civilian rollouts aiming to produce 1-2 commercial-scale 5 MWe units a year by 2029. Etna features specialized equipment tasked exclusively with fabricating and vacuum-sealing the proprietary nuclear-grade sodium heat pipes, alongside precision machining for the thermal graphite core monoliths.
The empty graphite monolith blocks, structural steel casings, control-rod mechanisms, and sealed sodium heat pipes are fully assembled at Etna. Because there is no radioactive material present, these modules can be transported via standard commercial freight trucks or rail cars under normal transport regulations. The un-fuelled module will be shipped directly to a specialised nuclear facility equipped to handle HALEU. For early prototypes and first commercial units, this will be the Materials and Fuels Complex (MFC) at INL or a commercial partner facility such as Standard Nuclear.
Inside a secured nuclear environment, robotic fuelling machines will insert the millimetre-sized TRISO fuel pellets into the designated fuel channels within the monolith block. Once filled, the access ports are permanently welded shut, and the reactor containment canister is vacuum-sealed. At this point the eVinci module becomes a regulated Class 7 Radioactive Material. It is locked into a rugged, accident-tested shipping package designed to withstand high-impact crashes and firesand transported in a dedicated heavy-transport vehicle with armed security escorts directly to its final destination.
The complexity of the eVinci monolithic solid block core could lead to production delays. Hundreds of deep channels must be drilled through the entire length of this block to hold the fuel pellets and sodium heat pipes. At these depths, even a fraction of a millimetre of drill-bit drift or microscopic material warping will ruin the entire block. Also, the heat pipes must maintain tight, continuous contact with the monolith walls to transfer heat effectively. Because sodium heat pipes expand and contract at different rates to the surrounding structural block during startup and shutdown, any micro-gaps that form will create thermal insulation pockets. This could cause localised core hot-spots.
The eVinci has no commercial orders as yet. The Saskatchewan Research Council (SRC) originally signed an agreement to become Westinghouse’s first “customer”, backed by a CAD $80m provincial grant to build a microreactor by 2029. However, in late 2025, Westinghouse cancelled the contract due to the logistical and security challenges of transporting highly radioactive used fuel blocks to and from remote commercial sites. Instead, Westinghouse pivoted to focus entirely on institutional government use cases.
In April 2026, Westinghouse Government Services announced that the microreactor had been selected by the Department of the Air Force and the Defense Innovation Unit (DIU) for installation at Malmstrom Air Force Base in Montana under the Advanced Nuclear Power for Installations (ANPI) programme. It is intended to provide resilient, off-grid power for critical defence infrastructure. Westinghouse also holds government research contracts to develop specialised, scaled-down variants of the technology (AstroVinci) for satellite propulsion and lunar base power systems in partnership with NASA and the Department of War.
