New Delhi, July 28 -- a working prototype that did exactly what it was designed to do. Antares Nuclear's Mark-0 demonstration reactor reached criticality at Idaho National Laboratory - the controlled chain reaction that means a nuclear system is sustaining its own power output - delivering on a milestone that has eluded far larger and better-funded programs before it.

Six weeks later, the company is closing a $470 million Series C to turn that proof point into deployed military infrastructure.

The round, led by Paradigm and Caffeinated Capital with participation from Industrious Ventures, Point72 Ventures, and Shine Capital, brings Antares' total raised to $604 million since its founding. The $470 million breaks into $370 million in equ...