Full Breakdown
Antares Nuclear’s Mark-0 Microreactor Reaches Zero-Power Criticality at Idaho National Laboratory
6/5/2026, 10:28:11 PM
Mark-0 Achieves Zero-Power Criticality
On June 4 2026 the Mark-0 microreactor, designed by Antares Nuclear, completed a zero-power criticality demonstration at the Department of Energy’s Idaho National Laboratory. The test produced a self-sustaining fission chain reaction in a privately developed, non-light-water reactor—the first such achievement in the United States in more than four decades and the inaugural criticality under the DOE Reactor Pilot Program.
Program Background
The Reactor Pilot Program was created by President Donald Trump’s Executive Order 14301 in May 2025 to accelerate advanced-reactor development. It set a July 4 2026 deadline for at least three startups to achieve criticality, selecting ten firms—including Antares, Oklo, Radiant and Valar Atomics—to receive DOE authorization and funding.
Timeline & Milestones
May 2025 – Executive Order establishes the July 4 2026 target.
January 2026 – DOE approves Mark-0’s Preliminary Documented Safety Analysis.
June 4 2026 – Zero-power criticality at INL.
2027 – Planned Mark-1 full-power test and electricity production.
2028 – Targeted deployment of an R1 microreactor at Joint Base San Antonio under the ANPI initiative.
Technical Overview
Mark-0 is a sodium heat-pipe-cooled microreactor using HALEU TRISO fuel (<120 kg). R1 design targets 100 kW–1 MW and six-plus years between refuelings without grid connection. The test unit is the 53rd reactor built at INL since 1951 and the first novel design to achieve criticality there in 50 years.
Significance
The demonstration validates advanced non-light-water reactor technology, supports DOE’s Nuclear Energy Launch Pad, and expands the U.S. advanced-nuclear supply chain. It also offers off-grid power for military bases, remote sites and space missions, meeting Pentagon goals for resilient, independent energy.
Official Statements
Energy Secretary Chris Wright called the event “a historic moment for American energy.” Assistant Secretary Ted Garrish noted the program’s success despite early skeptics. DOE Director John Wagner said the test achieved zero-power criticality, a proof-of-concept step, not electricity generation. Antares CEO Jordan Bramble called it first step toward commercial electricity and warfighter power.
Verbatim Quotes
- “It is fitting that on the eve of our nation’s 250th anniversary, we are witnessing a historic moment for American energy,” — Chris Wright, U.S. Energy Secretary
- “The skeptics didn’t believe President Trump's Reactor Pilot Program could achieve criticality in less than a year.” — Ted Garrish, Assistant Secretary of Nuclear Energy
- “Now that Mark-0 is critical, the real work is just beginning,” — Jordan Bramble, CEO, Antares Nuclear
- “Our skilled workforce, advanced manufacturing technologies and nuclear-qualified supply chain are driving a new generation of reactor demonstrations across the country,” — Rex D. Geveden, President and CEO, BWX Technologies
Next Steps
Antares will run a sustained electricity-production test with Mark-1 in 2027 at INL, then deploy an R1 microreactor at Joint Base San Antonio by 2028. DOE’s Nuclear Energy Launch Pad will keep accelerating advanced-reactor designs, targeting more criticalities before the July 4 deadline.
