Antares Nuclear Reactor Achieves First Critical Milestone in U.S. Test
A privately developed small modular nuclear reactor called Antares Mark-0 has reached criticality at Idaho National Laboratory on June fourth, marking the first new reactor design to achieve self-sustaining nuclear reactions in over four decades. Criticality means the nuclear chain reaction has become self-sustaining but doesn't mean the reactor produces electricity yet. The milestone fulfills the Trump administration's goal under an executive order to accelerate nuclear development. The test validates reactor physics and safety performance and will inform future commercial reactor designs. Electricity production is planned for 2027. The Antares team achieved this in less than twelve months from concept to criticality. The project involves the Department of Defense, Department of Energy at Idaho National Laboratory, and has received support from NASA. This is one of several advanced reactor projects competing to meet the July fourth deadline for criticality.
Key facts
- Antares Mark-0 reactor reached criticality on June 4, 2026, at Idaho National Laboratory—first new non-light-water reactor design to reach this milestone in over 40 years
- The test is zero-power criticality only; electricity production is planned for 2027, not yet demonstrated
- The Department of Energy Reactor Pilot Program requires at least three reactor designs to reach criticality by July 4, 2026, per Trump's May 2025 executive order
- The reactor uses TRISO fuel containing high-assay low-enriched uranium (HALEU) developed through Department of Defense Project Pele
- Criticality means a self-sustaining nuclear chain reaction; it does NOT confirm safety or commercial viability, according to Union of Concerned Scientists
- Unresolved issues include nuclear waste disposal, cost competitiveness with renewables, and standardized commercial deployment
How different outlets framed it
Left-leaning coverage: Left-leaning sources like PBS and the Union of Concerned Scientists emphasize unresolved challenges, particularly nuclear waste disposal. Critics argue the test is premature and that criticality claims overstate safety—Edwin Lyman from Union of Concerned Scientists called it a 'rudimentary first step' with 'no bearing on whether the Antares reactor will be safe or commercially viable.' Environmental publications note costs remain unproven and waste management remains unsolved.
Right-leaning coverage: Right-leaning and pro-nuclear sources frame this as a major U.S. innovation achievement and proof of concept that government-industry collaboration works. The Department of Energy statement emphasizes it as 'a historic moment for American nuclear energy' and points to the administration's deregulatory approach enabling faster development. These sources highlight strategic benefits for military energy independence and grid resilience.
Limits of this briefing
This story reflects a zero-power criticality test on a demonstration reactor, not a commercial reactor or electricity-generating unit. Major claims about cost-effectiveness and waste management remain unproven. The regulatory approval and licensing process is still ongoing. Long-term commercial viability depends on multiple reactors reaching maturity.
Sources
- PBS NewsHour: Energy Department says advanced nuclear reactor first to reach critical milestone Left
- Bloomberg: US Says Antares' Small Nuclear Reactor Reaches 'Criticality' Centre
- U.S. Department of Energy: Department of Energy Celebrates First Advanced Reactor Criticality Right
- World Nuclear News: First criticality for US microreactor under DOE programme Right
- Environmental Working Group: Why Small Modular Nuclear Reactors Won't Help Counter the Climate Crisis Left