Since the dawn of the Atomic Age, nuclear experts have labored to imagine the unimaginable and prevent it. They confronted a deceptively simple question: When is a reactor "safe enough" to adequately protect the public from catastrophe? Some experts sought a deceptively simple answer: an estimate that the odds of a major accident were, literally, a million to one. Far from simple, this search to quantify accident risk proved to be a tremendously complex...