Last Energy and Radiant cleared a key DOE safety step for their first reactors

The Department of Energy approved a documented safety analysis for Last Energy’s pilot reactor on Oct. 7 and for Radiant’s Kaleidos demonstration unit on Oct. 29. Each approval moves a company’s first reactor to the last stop before startup, a DOE readiness review. A third company, Antares, cleared a lesser milestone for its second design on Oct. 23.

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ANS Nuclear Newswire grouped the three announcements in a Nov. 2 roundup of the Reactor Pilot Program, DOE’s route for testing advanced reactors outside the Nuclear Regulatory Commission licensing process.

Five steps from agreement to startup

ANS lays out the DOE authorization pathway as five milestones: an other transaction agreement, a nuclear safety design agreement, a preliminary documented safety analysis, the documented safety analysis, and a readiness review. The ANS summary describes the documented safety analysis, or DSA, as the document explaining how a reactor and facility will operate, with hazards, accident scenarios and safety controls worked through in detail.

Companies reach those steps through the Reactor Pilot Program or the Nuclear Energy Launch Pad, which was created to extend and expand the pilot pathway. Terminology differs by source. ANS calls the DSA the penultimate step, while Last Energy’s own release calls it the final and most comprehensive safety analysis, and both descriptions fit if the readiness review that follows is treated as an operational check rather than a further analysis.

The program itself stems from a 2025 executive order on reforming reactor testing at DOE. The Nuclear Regulatory Commission remains responsible for licensing commercial power plants, so a DSA clears a reactor to run at a DOE-authorized test site and does not amount to a commercial license.

Last Energy’s pilot reactor at Texas A&M’s RELLIS campus

Last Energy’s pilot sits on the RELLIS campus of Texas A&M University in Bryan, Texas, where Terrestrial Energy has a second Reactor Pilot Program project, according to World Nuclear News. The reactor is a pressurized water design previously called PWR-5. It uses the same full-scale core geometry as the company’s 20-megawatt commercial PWR-20 but runs lower-enrichment fuel scaled to 5 megawatts of electrical output.

In a release distributed on Oct. 7, the company said the Texas project is a direct bridge to its commercial PWR-20 deployment model. The DSA builds on a preliminary version DOE approved in May, and the pilot facility has already been built. The pilot would show low-power criticality before any electricity generation. No criticality date has been published.

Last Energy was selected for the Reactor Pilot Program in August 2025 under an executive order on reforming reactor testing at DOE.

Radiant’s Kaleidos and 60 full-power days at Idaho National Laboratory

Radiant announced its DSA approval on Oct. 29, saying DOE had cleared the Kaleidos demonstration unit for full-power operation. Rita Baranwal, Radiant’s chief nuclear officer, said the approval for a full-power demonstration is a first within the microreactor industry. That claim is Radiant’s own, and DOE has not made a matching statement in the sources reviewed.

The testing plan runs at Idaho National Laboratory’s Demonstration of Microreactor Experiments test bed, known as DOME. Radiant says the campaign targets 60 effective full-power days, which it calls the industry’s only full-power, extended-duration microreactor demonstration to date, as carried in its Business Wire release of Oct. 29. Kaleidos is a helium-cooled, TRISO-fueled design rated near 1 megawatt electric.

The DOME campaign has five phases, running from first criticality to at least 150 hours of continuous full-power operation without operator intervention. Radiant built the unit in El Segundo, California, and shipped it to Idaho National Laboratory in August, so the hardware is already on site while paperwork and the readiness review remain.

Radiant plans to deliver similar units to Buckley Space Force Base, Fort Benning and Naval Weapons Station Crane after the DOME run. Reporting on the approval notes the schedule has slipped from an earlier plan to fuel in May and reach criticality in June 2026, and that a DSA is no substitute for an NRC license on commercial units.

Antares and what still stands between the companies and criticality

Antares announced on Oct. 23 that DOE approved a nuclear safety design agreement for Mark-1, its first electricity-producing reactor. ANS says no company had previously earned that agreement for a second reactor design under the framework. Mark-1 builds on Mark-0, which reached zero-power criticality at Idaho National Laboratory in June, and Antares plans a six-month full-power run at the lab in 2027.

The longer-term Antares design, called R1, is a reactor in the 100-kilowatt to 1-megawatt electric range that uses TRISO fuel, sodium heat pipes for cooling and a nitrogen-based Brayton cycle for power conversion. Antares plans to place several R1 units at Fort Bragg under the Army’s Janus Program, with the first in 2028.

For Last Energy and Radiant the open item is the readiness review, which assesses whether the reactor and its operating team are prepared to begin operations. DOE has not published dates for either review, so the first criticality at Texas A&M and in Idaho remains unscheduled.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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