The U.S. nuclear sector reached a historic technical milestone earlier this month when Antares Nuclear’s Mark-0 microreactor achieved criticality at Idaho National Laboratory (INL). The June 4 demonstration marked the first time a privately developed advanced reactor reached this state under the Department of Energy’s (DOE) Reactor Pilot Program (RPP). It also represented the first novel reactor design to go critical at INL in more than 40 years.
This event provides concrete validation that reactor concepts can move from design and analysis into physical testing on accelerated timelines. It also underscores the supporting role in the nuclear supply chain of established companies that deliver the specialized fuel and components required for these tests.
Key Takeaways
- Antares Nuclear’s microreactor became the first advanced design to complete a criticality test under the DOE RPP.
- BWX Technologies (BWXT) manufactured and supplied the reactor fuel that powered the successful test.
- The demonstration clears the way for Aalo Atomics to pursue its own criticality milestone later this summer with support from Flowserve (FLS), another established constituent of the VettaFi Nuclear Renaissance Index (NUKZX).
What Happened at INL
Antares conducted the test at INL using the Mark-0, a microreactor fueled with high-assay low-enriched uranium (HALEU). The reactor reached a self-sustaining nuclear chain reaction, but produced essentially no measurable thermal power or electricity.
The Mark-0 configuration was built specifically as a low-power physics test bed. It lacks the power conversion equipment, full heat removal systems, and balance-of-plant infrastructure that a commercial or higher-power prototype would require. This focused setup allowed Antares to complete the fueled test safely on an accelerated schedule; the company transitioned from chalkboard to splitting atoms in only nine months.
Criticality is the condition in which a nuclear reactor achieves a self-sustaining fission chain reaction. In practical terms, it means the number of neutrons produced by fission exactly balances the number lost to absorption or leakage. The reactor is neither increasing nor decreasing in power on its own; it holds steady at whatever level the operators set through control systems.
BWXT’s Fuel Role and the Supply Chain Connection
BWXT played a direct, enabling role by manufacturing the fuel used in the Mark-0 test. The company also processed the HALEU feedstock into the form needed for advanced reactors. This work built on BWXT’s prior fuel production experience for the Army’s Project Pele microreactor program.
The successful use of this fuel in the first RPP criticality test highlights how established manufacturers reduce technical risk for newer reactor developers. BWXT’s Lynchburg facility has decades of specialized nuclear fuel fabrication expertise that newer entrants can leverage rather than recreate from scratch.
Aalo Atomics Positioned to Follow Quickly
Aalo Atomics, also participating in the DOE RPP, is targeting its own criticality demonstration with the Aalo-X critical test reactor in the coming weeks, with the goal of meeting the July 4 executive order timeline. Aalo has already secured DOE approval to begin reactor start up preparations at INL.
Aalo maintains a strategic partnership with Flowserve (FLS) focused on pumps, valves, and critical flow-control equipment for its modular reactor designs aimed at data center applications. Flowserve’s involvement provides Aalo with proven nuclear-grade components and engineering support as it moves toward its own criticality milestone.
Implications for Investors and the Nuclear Value Chain
Early criticality demonstrations like the one completed by Antares de-risk advanced reactor concepts and compress development timelines. They demonstrate today’s nuclear industry is ready, willing, and able to build capacity at speeds not seen since the 1950s/1960s.
The VettaFi Nuclear Renaissance Index (NUKZX) captures exposure to these developments through companies such as BWXT, which supplied the enabling fuel, and Flowserve, which is already partnered with the next company expected to reach criticality. NUKZX includes a diversified mix of fuel cycle, component, engineering, and service providers positioned across the nuclear value chain. NUKZX serves as the underlying index for the Range Nuclear Renaissance Index ETF (NUKZ ).
These milestones illustrate how progress on advanced reactors creates tangible opportunities for established public companies long before any single reactor developer reaches commercial revenue. For investors seeking broad participation in the nuclear renaissance without concentrating risk in pre-revenue reactor developers, the diversified approach embedded in NUKZX offers a practical path to capture value from the full ecosystem supporting these projects.
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