WASHINGTON, July 4 -- When engineers at the National Reactor Innovation Center brought Deployable Energy's Unity reactor to self-sustaining criticality on July 1, the compact machine completed the journey in roughly 150 days from construction start. It was a pace that would have seemed implausible to anyone familiar with the past four decades of American nuclear stagnation.

Unity was the third advanced microreactor to go critical in the United States within a single month. Antares Nuclear's Mark-0 had done it first, at Idaho National Laboratory on June 4. Valar Atomics' Ward 250 followed on June 18, at a privately operated facility in Emery County, Utah. It was the first time a Department of Energy-authorized reactor had been built and b...