Since the collapse of the Soviet Union, the threat of a nuclear war between the USA and Russia has diminished, but the threat posed by nuclear weapons and materials on both sides has increased. As nuclear weapons age, they become unstable and begin to behave in unpredictable ways. This film is the first to go behind the scenes in Arzamas-16 - the Russian nuclear city so secret that it has never appeared on any map - and the American nuclear weapons laboratory at Los Alamos, New Mexico, to see Russian and American bomb designers working together to reduce the risk. Exclusive archive material.