TL;DR
Nuclear power has usually meant plants built on site over years; even China's small Linglong One has taken about five years. A young company, Deployable Energy, is betting on the opposite: a reactor built and fueled in a factory, then hauled whole on a truck to wherever the power is needed. It says its Unity microreactor reached a self-sustaining chain reaction in Idaho on June 30, about 150 days, or five months, after the project began. If its full-power test in 2027 works and regulators later license reactors like it, towns, mines and bases beyond the reach of any power line could one day get steady, carbon-free electricity delivered like freight. Reactors this small are also the scale a Moon base would need, though nothing yet links this one to space. For now it is one prototype and a test with no month attached.
Below the line, for paid subscribers: The full story: where it stands, how we got here and what's dated next; our Odds board on 5 milestones, starting with "Deployable Energy runs its Unity microreactor at full power by 2027"; our labeled speculation on what it adds up to; and every source.