"In the case of plutonium, how much plutonium did they produce? Well, they produced between 140 and 162 tons. If they can’t find a tenth of 1 percent, that means there is 310 to 360 pounds of plutonium lying around somewhere," Reed, co-author of the new book The Nuclear Express: a Political History of the Bomb and its Proliferation, told the paper.

The United States and Russia are "hard at work looking for" the material, said Reed, a former nuclear-weapon designer at the Lawrence Livermore National Laboratory in California.