It is bad for humanity to have plenty of energy. Both Paul Ehrlich and Amory Lovins used to say that, but I think they have soft pedalled the point recently. I have so little intellectual contact with this idea that I think I'll pass by the point until someone brings it up in a reaction to this page. Actually, Ehrlich continues the point to some degree in his 1995 book. Here's a review of The Stork and the Plow .
Nuclear reactors produce plutonium, and plutonium is terrible because it can be used to make bombs.
Perhaps I have caricatured this argument, but I haven't seen it in a coherent form. Safeguards are indeed needed. There is no policy including a ban on nuclear energy that can prevent a determined country from launching a nuclear war. So far deterrents have sufficed, and a total ban would give an enormous advantage to the first country to violate it. Plutonium is the most toxic substance in existence.