On October 29, 2026, U.S.-based Radiant announced that its Kaleidos microreactor, currently under development, had received approval from the U.S. Department of Energy (DOE) for a safety analysis document covering first criticality and power operation. The company plans to run the tests at a demonstration facility at Idaho National Laboratory (INL). Next comes an operational readiness review, which checks whether equipment, personnel, operating procedures and other preparations are in place. Once the reactor passes that review, the company plans to load fuel and begin a five-phase test program. The approval is an important milestone toward actual reactor operation, but it does not mean operation itself has been authorized.
Radiant plans to operate the reactor for "60 effective full-power days" over several months of testing. In the final phase, it aims for more than 150 hours of continuous full-power operation with no human intervention. The former is a cumulative metric combining power output and operating time, while the latter measures how long the reactor can run continuously under set conditions. Each target is meant to verify a different aspect of performance. Looking at the terms of use for the shared test facility and the plan to deliver commercial units in 2028, it becomes clearer what kind of verification is needed between this approval and practical deployment.
From safety analysis approval to confirming equipment and personnel readiness
According to Radiant, the DOE approved Kaleidos's Preliminary Documented Safety Analysis (PDSA) in February 2026. The newly approved Documented Safety Analysis (DSA) covers not only "first criticality," when a fission chain reaction first becomes self-sustaining, but also power operation, the company says. With the safety review advancing a stage, the next step is to confirm that the actual equipment and operating organization are ready. However, approval of a safety analysis document and completion of an operational readiness review are separate steps, and as of this announcement the latter had not yet taken place.
Based on the October 29 announcement, the steps leading up to the start of Kaleidos testing and their current status are as follows.
| Step | Status at time of announcement | Details / conditions |
|---|---|---|
| Documented Safety Analysis (DSA) | Radiant announced DOE approval | According to the company, covers first criticality and power operation |
| Operational readiness review | Next stage | Checks readiness of equipment, personnel, operating procedures and more |
| Fuel loading and five-phase testing | Planned after the readiness review is completed | Aims for 60 effective full-power days and 150+ hours of continuous full-power operation |
According to the DOE's 2025 environmental assessment of the DOME test facility (Section 3.11.4), operational readiness evaluations include safety management systems, operator training and qualification, equipment operability, operating procedures and emergency response. In other words, backing the safety of operations with a safety analysis document and confirming that equipment and personnel are actually ready to operate must each be verified separately. These are, however, general evaluation items at DOME, and do not mean that Kaleidos has passed these reviews.
Radiant has not disclosed an expected completion date for the operational readiness review or a start date for reactor operation. What matters next is whether the reactor can pass the review and proceed to fuel loading, and whether it can then carry out tests that meet the planned power output and operating time.
What will 60 effective full-power days and 150 hours of continuous operation verify?
The "60 effective full-power days" target for Kaleidos testing is a metric that accumulates reactor power and operating time and converts them into the equivalent number of days at rated power. For example, running at half of rated power for two days equals one effective full-power day. Radiant plans to conduct tests over several months, but this does not mean 60 consecutive days of full-power operation. The metric is used to evaluate how much power the reactor actually sustained, and for how long.
By contrast, continuous operation of more than 150 hours is a separate goal for the final stage of testing. It requires the reactor to keep running at full power for more than 150 hours without human intervention. The aim is not only to confirm cumulative operating output but also to check whether a steady power level can be maintained over a long period.
According to Radiant, the tests will use a design close to that of reactors for commercial customers, along with dedicated fuel. The company plans to evaluate the core, instrumentation and integrated systems under actual operating conditions, offering a chance to verify the behavior of the reactor as a whole, not just its individual components. The tests will also be important for comparing design-stage analysis results with real operating data.
However, the company's explanation does not make clear whether the test unit and the reactors for customers will have fully identical specifications. Moreover, both 60 effective full-power days and 150+ hours of continuous operation are future goals, not achieved results. Assessing actual performance will require reviewing operating data published after the tests.
DOME's facilities and terms of use as a shared test site
DOME, where Kaleidos testing is scheduled, is a shared demonstration facility for microreactors at INL. According to the DOE, it repurposes the containment structure of the former experimental reactor EBR-II, and developers can use it for reactor criticality tests and validation of design analyses. It also aims to support validation of digital twins and the collection of operating data useful for future design and licensing. Rather than building a dedicated test facility from scratch, companies can use existing infrastructure to demonstrate their reactors.
As a shared facility, however, it comes with certain constraints. DOME can accept reactors with a thermal output of up to 20 megawatts, which is a limit set by the facility and not an indication of Kaleidos's own output. Only one reactor can be operated at a time, and each experiment is expected to last up to about six months. Because multiple developers use the site, the order of tests and the duration of facility use are also important conditions.
Each developer bears its own testing costs. In deciding the order of experiments, the DOE says it will consider technology and fuel readiness as well as regulatory plans and procurement schedules. While using an existing facility has advantages, the pace of testing depends not only on a company's development progress but also on the shared facility's operating plan.
Toward 2028 commercial deliveries, establishing an operating track record comes next
According to Radiant, Kaleidos was shipped to INL in August 2026. The company plans to manufacture units at its "R-50" facility and aims to deliver its first commercial units to Buckley Space Force Base and Fort Benning in 2028. In October, it also announced that it had been selected for the U.S. Navy's Crane. It is advancing plans for manufacturing and delivering commercial units in parallel with preparing the test unit for operation.
Still, judging the feasibility of the delivery plan first requires confirming that the reactor completes the operational readiness review and moves on to fuel loading and the start of testing. After that, the focus will be on whether tests using a near-commercial design and dedicated fuel can achieve both the cumulative 60 effective full-power days and more than 150 hours of continuous, unattended full-power operation. Approval of the safety analysis is an important step forward, but it does not demonstrate the reactor's performance or operational stability. The operating data obtained in upcoming tests will be the next basis for judging Kaleidos's path to practical use.