Existing reactors use uranium or plutonium—the stuff of bombs. Uranium reactors need the same fuel-enrichment technology that bomb-makers employ, and can thus give cover for clandestine weapons programmes. Plutonium is made from unenriched uranium in reactors whose purpose can easily be switched to bomb-making. Thorium, though, is hard to turn into a bomb; not impossible, but sufficiently uninviting a prospect that America axed thorium research in the 1970s. It is also three or four times as abundant as uranium. In a world where nuclear energy was a primary goal of research, rather than a military spin-off, it would certainly look worthy of investigation. And it is, indeed, being investigated.