FILE - In this May 21, 1956 file photo, the stem of a hydrogen bomb, the first such nuclear device dropped from a U.S. aircraft, moves upward through a heavy cloud and comes through the top of the cloud, after the bomb was detonated over Namu Island in the Bikini Atoll, Marshall Islands. The hydrogen bomb was never dropped on any targets. It was first successfully tested in the 1950s by the U.S., in bombs called Mike and Bravo. Soviet tests soon followed. (AP Photo, File)