IT SOUNDS impossible: a nuclear reactor that generates electricity from nuclear waste. Yet that is what Claudio Filippone, a nuclear scientist and director of the Centre for Advanced Energy Concepts at the University of Maryland, proposed a few years ago. He has now devised an improved design, called CAESAR (“clean and environmentally safe advanced reactor”) that is even more counterintuitive. As well as being environmentally friendly—it can produce electricity without causing any extra pollution—his new design could also help prevent nuclear proliferation. Conventional nuclear reactors run on fuel rods made largely of uranium-238, enriched with about 4% uranium-235. If you...