One of the most potentially dangerous legacies of the collapse of the Soviet Union was its huge nuclear arsenal and nuclear weapons industry. There were particular concerns about the Soviets' former nuclear testing site at Semipalatinsk in Kazakhstan, a vast swathe of contaminated land where there were tunnels with spent plutonium. When the Soviet Union ended, the site was left open to scavengers. Louise Hidalgo has been hearing from the former head of America's nuclear weapons laboratory, Dr Siegfried Hecker, about the long secret operation by Russian and American scientists to make the site safe in what's been called the greatest nuclear non-proliferation story never told.Photo: the first historic visit by American nuclear scientists to the secret Soviet city of Sarov where Moscow developed nuclear weapons, February 1992. First on the left is the great Russian physicist, Alexander Pavlovsky. Next, looking down, is Yuli Khariton, the father of the Soviet atomic bomb. Opposite, with a white turtle-neck jumper, is Dr Siegfreid Hecker, then director of Los Alamos Laboratory where America developed the world's first nuclear bomb (Credit: Dr Siegfreid Hecker)