In a big win for the future of nuclear power, four US microreactor companies just hit a key technical milestone—criticality—right on the Fourth of July deadline. The Trump administration had set a goal to see three new microreactors achieve a sustained chain reaction by the nation’s 250th birthday. But Antares Nuclear, Valar Atomics, Deployable Energy, and Aalo Atomics all got there, with Aalo literally meeting the deadline on July 4th. These microreactors are tiny compared to traditional nuclear plants, but they’re being fast-tracked through a special Department of Energy program that gives them land and lab support. Still, hitting criticality isn’t the same as powering your home. These reactors achieved “zero-power criticality”—basically proving the chain reaction works, but not generating any electricity yet. The real engineering hurdles—like cooling systems to extract heat—are still ahead. The companies project commercial power in the mid-2030s, but don’t expect quick results. Why it matters: This success shows that nimble, startup-driven nuclear innovation is possible, but the gap between a technical demo and clean energy on the grid is still enormous.