After its independence, Kazakhstan had to deal with the legacy of being one of the centres of the Soviet Union's huge nuclear arsenal and nuclear weapons industry. There were particular concerns about the former nuclear testing site at Semipalatinsk, a vast swathe of contaminated land where there were tunnels with spent plutonium. When the Soviet Union ended in 1991, the site was left open to scavengers. Louise Hidalgo talks to the former head of America's nuclear weapons laboratory, Dr Siegfried Hecker, about the secret operation by Russian and American scientists to make the site safe; it's been called the greatest nuclear non-proliferation story never told.

PHOTO: The Semipalatinsk site in 1991 (Getty Images)