The US Department of Energy (DOE) has approved the Preliminary Documented Safety Analysis (PDSA) for the Mark-0 reactor being developed by California-based nuclear start-up Antares reactor. Antares received an approved Nuclear Safety Design Agreement (NSDA) from DOE in October 2025.
Antares, founded in 2023, is one of 10 companies participating in the DOE’s Reactor Pilot Program established following President Donald Trump’s May 2025 Executive Order 14301 with the ambitious target of having at least three reactors achieve criticality by 4 July 2026. Under the programme, companies generally bear their own costs but benefit from an expedited DOE authorisation process that bypasses traditional Nuclear Regulatory Commission (NRC) licensing for the initial testing phase.
“The DOE approval of our PDSA is an important step on our way to criticality and ultimately producing power for defence and space applications,” said Antares CEO Jordan Bramble. “Safety is at the core of everything we do, and we thank the DOE for their continued partnership. We are excited to turn on our low-power demonstration unit before 4 July, and we look forward to learning valuable lessons on fuelling, reactor controls, and the physics of our core from our Mark-0 operations.”
In August 2925, Antares secured an allocation of High-Assay Low-Enriched Uranium (HALEU) fuel feedstock from DOE. This specific batch of HALEU was designated for the Mark-0 demonstration reactor. Antares is also targeting a first-of-a-kind, electricity-producing demonstration unit (Mark-1) for 2027. The production-ready commercial R1 design of the microreactor, capable of generating 100 kW to 1 MWe is targeted for initial deployment as early as 2028.
Antares will test Mark-0 at Idaho National Laboratory (INL) in Building 793 of the Materials and Fuels Complex. The Antares Mark-0 will use a full-scale core. The same facility and fuel used in Mark-0 will support Antares’s planned electricity-producing demonstration in 2027. Earlier the Building 793 housed ML-1, the US Army’s first mobile nuclear reactor. Since 2024, Antares says it has worked to establish this facility as an enduring testbed, enabling rapid progress without the need for groundbreaking construction. The company has invested over $40m in Idaho and maintains a growing presence in Idaho Falls.
Since its launch in 2023, Antares has rapidly expanded. In October 2024 it secured $30m in Series A funding to support early R&D. In February 2025 opened its Torrance manufacturing and R&D facility, known as Antares Prime, which is designed to produce up to 10 microreactor units a year. A further $96m was raised in Series B funding in December 2025 to accelerate reactor production.
In addition to its corporate headquarters in Redondo Beach and Torrance facility in California, Antares maintains an office and testbed facility near INL to support the 2026 reactor demonstration.
In Aiken, South Carolina it has manufacturing partnerships with the Savannah River National Laboratory (SRNL). These are facilitated through a Cooperative Research and Development Agreement (CRADA), not direct manufacturing at the lab site. This leverages SRNL’s expertise and the resources of its new Advanced Manufacturing Collaborative (AMC), which provides a platform for industry partners to collaborate on R&D to accelerate the technological maturity and development of the Antares reactors for both defence and commercial applications.
Antares does not manufacture the High-Assay Low-Enriched Uranium (HALEU) TRISO fuel intended for its R1 commercial microreactor in-house at its Torrance facility or any of its other sites. Instead, fuel is being fabricated by BWX Technologies (BWXT) at its facilities in Lynchburg, Virginia. BWXT is also fabricating the fuel for the Mark-0 and Mark-1 reactors.
Antares has secured a portfolio of military and federal contracts, positioning itself as a primary supplier for the Department of Defense (now Department of War) and NASA. The company holds more than $13m in active government contracts.
In April 2025, Antares was selected by the Defense Innovation Unit for the Advanced Nuclear Power for Installations (ANPI) programme. This aims to deploy microreactors at permanent military installations within the 50 US states to provide power for critical infrastructure.
Antares is a leading competitor for the US Army’s JANUS programme launched in October 2025, which targets reactors up to 20 MWe for defence applications. Antares has received multiple Small Business Innovation Research (SBIR) Phase I and Phase II contracts from the Air Force Research Laboratory (AFRL). These projects focus on: rapidly deployable microreactors for Department of the Air Force use cases; and nuclear-powered spacecraft and high-power space propulsion in partnership with ExLabs.
In 2025, Antares completed an electrically heated demonstration of its pipe-cooled reactor prototype at NASA’s Marshall Space Flight Center. Antares is also proposing systems for NASA’s Fission Surface Power (FSP) initiative, which aims to put a 100 kWe reactor on the Moon’s surface by 2030.
