Kaleidos Microreactor Clears Safety Review for Testing
The reactor must still pass a DOE Readiness Review before fuel loading and nuclear operations begin.
Updated on Nov. 2, 2026 in Nuclear
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The Department of Energy has approved a safety analysis allowing Radiant's Kaleidos microreactor to proceed to initial criticality and operation at power, though it must first pass a DOE Readiness Review before fuel loading and nuclear operations begin. The development is at the testing stage, not yet a commercially available product.
Why it matters
The approval moves a small modular reactor design from paper toward hands-on demonstration at a federal test site. DOME test results may support future commercial licensing applications for microreactors.
The planned Radiant campaign specifies 60 effective full-power days and 150 hours of uninterrupted full-power operation, while Last Energy's PWR-5 first phase is limited to 100 kilowatts thermal. Whether Kaleidos meets these targets during testing has not yet been reported.
The players
Department of Energy
The U.S. federal agency that oversees nuclear safety authorizations and approved both the Kaleidos and PWR-5 safety analyses.
Radiant
A developer of advanced microreactors whose Kaleidos design is now cleared for a DOE-supervised test campaign.
Last Energy
A developer of small modular reactors whose PWR-5 pilot received DOE safety analysis approval with phased, low-power limits.
Idaho National Laboratory
A federal research lab hosting the DOME test facility where Radiant's Kaleidos will be evaluated.
The details
A microreactor is a small nuclear reactor designed to produce a modest amount of power, often for remote sites or industrial use. Radiant's Kaleidos will be tested in a five-phase campaign at the DOME facility, a test site for experimental reactors at Idaho National Laboratory. Each phase of Last Energy's PWR-5 pilot likewise needs separate DOE authorization before advancing, which means the department reviews each step rather than granting a single blanket approval.
Timeline
February 2026: DOE approved Radiant's Preliminary Documented Safety Analysis.
August: Radiant shipped Kaleidos to Idaho National Laboratory.
October 7: DOE approved Last Energy's PWR-5 pilot safety analysis.
Thursday, Oct. 29: Radiant announced DOE approval of the Kaleidos safety analysis.
The Tech Race
Radiant and Last Energy are both advancing microreactor designs through DOE's staged authorization process at the same time. Both projects show the federal path from safety analysis to phased testing that U.S. microreactor developers must now follow.
The approval does not yet put a usable reactor into service, since testing must first clear a Readiness Review and proceed through five phases. Readers will not see power from Kaleidos until those steps are completed and the DOME test results are reported.
The takeaway
Watch for the DOE Readiness Review outcome, which must be passed before Kaleidos loads fuel. DOME test data on the 60 full-power days and 150-hour continuous run will be the next concrete measure of whether the design performs as planned.
What happens next
Radiant's Kaleidos must pass a DOE Readiness Review before fuel loading and nuclear operations can begin, and the results of that review will determine when testing starts.
Further reading
For more on the reactor designs and approval pathways behind this news, see our Nuclear coverage.
Source note: This article includes information reported by TokenPost.
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