US Nuclear Program Reaches Key Milestone with Second Reactor
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US Nuclear Program Reaches Key Milestone with Second Reactor
- The U.S. Department of Energy's Reactor Pilot Program saw its second advanced reactor achieve "criticality" on June 18, 2026, marking a significant step in developing new nuclear energy technologies.
- This achievement by Valar Atomics' Ward 250 reactor followed Antares Nuclear Inc.'s Mark-0 reactor, which reached criticality on June 4, 2026, at Idaho National Laboratory.
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The U.S. Department of Energy’s (DOE) Reactor Pilot Program recently announced that a second advanced reactor, Valar Atomics’ Ward 250, achieved “zero-power fueled criticality” on June 18, 2026. This milestone occurred at the Utah San Rafael Energy Lab in Emery County, Utah.
Criticality signifies that a nuclear reactor has achieved a self-sustaining chain reaction, capable of a steady release of energy. However, “zero-power criticality” means the reaction is sustained at extremely low heat levels, not yet producing electricity. This achievement is a crucial scientific and engineering validation, proving the system works as designed.
The Ward 250 is the second reactor to reach this milestone under a pilot program established by the Trump administration in May 2025. The program’s goal was to have at least three advanced reactor designs achieve criticality by July 4, 2026, the nation’s 250th anniversary. The first reactor to achieve criticality was Antares Nuclear Inc.’s Mark-0 microreactor on June 4, 2026, at the Idaho National Laboratory. Following Valar Atomics, Deployable Energy’s Unity reactor reached criticality on June 30, and Aalo Atomics’ Critical Test Reactor achieved it on July 4, bringing the total to four.
These advancements are part of a broader effort by the U.S. to accelerate the development and deployment of advanced nuclear technologies, including small modular reactors (SMRs) and microreactors,