I, who had been in favour of nuclear energy for generating electricity … I suddenly realised that anybody who has a nuclear reactor can extract the plutonium from the reactor and make nuclear weapons, so that a country which has a nuclear reactor can, at any moment that it wants to, become a nuclear weapons power. And I, right from the beginning, have been terribly worried by the existence of nuclear weapons and very much against their use.

On potentials for misuse of nuclear power, p. 31 - Portraits in Science interviews (1994)