A US nuclear microreactor has cleared a key safety hurdle ahead of a planned test campaign targeting 60 effective full-power days of operation and at least 150 continuous hours without operator intervention.California-based nuclear technology developer Radiant announced that the US Department of Energy (DOE) has approved the Documented Safety Analysis (DSA) for its Kaleidos microreactor.The approval brings the reactor closer to full-power testing at Idaho National Laboratory (INL), where Radiant plans to evaluate its performance under operating conditions.Unlike preliminary tests that establish whether a reactor can sustain a controlled nuclear chain reaction, the upcoming campaign will examine how Kaleidos performs during extended operation.Radiant says the demonstration will be the industry’s only full-power, extended-duration test of a nuclear microreactor to date.However, the reactor must first pass DOE’s Readiness Review before nuclear operations can begin.Testing beyond criticalityThe planned testing campaign will take place at the National Reactor Innovation Center’s Demonstration of Microreactor Experiments (DOME) facility at INL.The five-phase program will begin with initial criticality, the point at which a reactor achieves a self-sustaining nuclear chain reaction.Testing will then progress through increasingly higher power levels, allowing engineers to evaluate the reactor core, instrumentation, and integrated systems under operating conditions.The campaign targets 60 effective full-power days (EFPD) over several months. This represents the equivalent of 60 days of operation at full power, rather than necessarily 60 consecutive days.Another major objective is to demonstrate at least 150 hours of uninterrupted operation at full power without operator intervention.The test would help establish whether the reactor can maintain sustained operation without requiring constant human adjustments.Radiant’s chief nuclear officer, Rita Baranwal, said the approval covers a broader demonstration than initial criticality alone.“This approval for full power demonstration is a first within the microreactor industry. It moves us forward to test our complete reactor system under sustained operating conditions.”The latest approval follows DOE’s acceptance of Radiant’s preliminary safety analysis in February 2026.Toward commercial deploymentKaleidos is a transportable nuclear microreactor designed to provide electricity for military installations, industrial facilities, and locations requiring reliable power.The reactor has a planned electrical output of 1 megawatt and is designed to be transported by land, sea, or air.Radiant built the unit at its El Segundo, California, facility and shipped it to Idaho National Laboratory in August 2026.The company plans to test a reactor design and custom fuel similar to those intended for commercial customers.This approach is intended to generate operating data directly relevant to future deployments, rather than relying solely on smaller experimental systems.Radiant plans to manufacture Kaleidos units at its R-50 facility in Oak Ridge, Tennessee.The company is targeting its first commercial deliveries to Buckley Space Force Base in Colorado and Fort Benning in Georgia in 2028.Earlier in October, the US Navy also selected Radiant to deploy Kaleidos at Naval Weapons Station Crane in Indiana.These projects reflect growing military interest in compact nuclear reactors that could provide power independently of conventional electricity grids.Before testing begins, DOE’s Readiness Review must determine whether the reactor, supporting systems, and operating personnel are prepared for nuclear operations.If the review is successful, Radiant can proceed with fueling Kaleidos and beginning its planned test campaign.The reactor has not yet demonstrated the full-power endurance or unattended operation targeted in the program.