 This prompted preliminary research in the United States, beginning in late 1939.Progress was slow in the United States, but in Britain, Otto Frisch and Rudolf Peierls, two refugee physicists from Germany at the University of Birmingham, examined the theoretical issues involved in developing, producing and using atomic bombs.They considered what would happen to a sphere of pure uranium-235, and found that not only could a chain reaction occur, but it might require as little as of uranium-235 to unleash the energy of hundreds of tons of TNT.