This essay compares “expert” attempts, commissioned by the US government, to imagine future nuclear risk with the attempt made by Lydia Millet in her novel Oh Pure and Radiant Heart (2005). During the Cold War, nuclear attack was conceptualized primarily as a matter of instantaneous, remainderless destruction—a destruction so total that it would annihilate any position from which it could be assessed or measured—and that conception has a lingering hold even now, a generation later. Recent attention has shifted, however, to the equally problematic issue of how to imagine the long-term, even posthuman risks posed by radioactive waste. Conventional models of risk and threat are inadequate to thinking through the problems of temporality raised by nuclear materials: those problems may well require the speculative resources of fiction if we are even to begin conceptualizing them. Millet's novel is read as an attempt to imagine these threats and problems and to conceive of how we might think our way beyond them.