Even before the nuclear bomb was completed, some physicists at Los Alamos started work on what they called the “super,” a weapon based on fusion. After the war was over there were heated discussions, both scientific and political, about the wisdom of developing a new, even more destructive weapon. Due to tensions in the Cold War, both the United States and the Soviet Union embarked on programs to create these weapons, informally called hydrogen bombs. The United States tested such a weapon in 1952; the Soviet Union in 1955. Britain, China, and France are known to have tested fusion bombs.

Some aspects of fusion weapons are known while others are still secret. What is known is that the bomb starts with a uranium or plutonium implosion bomb with deuterium and tritium gases inside its core. The neutrons released by the fusion increase the fission of plutonium or uranium, boosting the efficiency of the fission device. The energy released is transferred to a lithium-dihydride fusion fuel in a process that remains secret. That undergoes fusion, and the energy released causes fission in a surrounding layer of natural uranium. The Soviet Union exploded a fusion weapon that released the equivalent of 50 million tons of TNT. The most recent effort on weapons development is to decrease the size of the weapons so they can be deployed in smaller delivery rockets.