IDAHO FALLS, Idaho—History was made with the flip of a switch at 12:30 a.m. on June 4, under partly cloudy skies and a waning three-quarter moon in Idaho’s Arco Desert, when a prototype reactor sustained a nuclear chain reaction, becoming the first new design to achieve “criticality,” or viability, in the United States since 1973. In that midnight milestone’s wake, the future is following fast. Since Antares Nuclear’s Mark-0 design was validated in early June, three other novel reactor designs have met the U.S. Department of Energy’s criticality requirements and, according to Energy Secretary Chris Wright. Up to four more could do so by year’s end....