Ethical and political reflection on nuclear power was initially stimulated by the dangers of nuclear weapons. Even as the possibility of the atomic bomb began to be imagined in the 1930s, physicists became worried about its social, political, and ethical implications. By the time the first bomb was exploded in 1945, and even more as the nuclear arms race took hold in the 1950s, scientists, engineers, military professionals, politicians, and the attentive public became increasingly concerned about nuclear research and development, testing, and deterrence policy. As much as any other science and technology during the twentieth century, nuclear weapons have challenged ethical reflection. Although such weapons present major benefits—otherwise they would not have been invented, produced, and used—they also have built-in disadvantages that are not always easy to assess. As Albert Einstein remarked in 1946, the problem created by nuclear weapons is "not one of physics...