Nuclear power, always controversial, has been under an especially dark cloud since Japan’s Fukushima disaster five years ago.

And in the United States, few new nuclear plants have been ordered since the 1979 partial meltdown at Three Mile Island in Pennsylvania, thanks to ongoing safety concerns, high capital costs, and the availability of lower-cost energy sources. But nuclear engineer Leslie Dewan believes that a safe, environmentally friendly, next-generation nuclear reactor isn’t just feasible—it's commercially viable.

As cofounder and CEO of Boston-based startup Transatomic Power, Dewan and fellow Massachusetts Institute of Technology grad Mark Massie are working on commercial-scale development of a molten salt reactor first prototyped in the 1960s at the Oak Ridge National Laboratory.

“We’ve changed the design to make it more compact, power dense, and able to run on spent nuclear fuel,’’ says the 31-year-old Dewan, a National Geographic Emerging Explorer whose energy and hip style belies the public image of a nuclear scientist as a lab-coated, pocket protector–wearing middle-aged man.