H-Bomb on

In the years after the Mike shot, the nature of weapons work at Los Alamos began to change dramatically. Design efforts centered on thermonuclear devices, leading to detonation of the largest-yield United States device, codenamed Bravo. The development and testing of thermonuclear weapons by both the United States and the Soviet Union led to increasing concerns about the effects of radioactive fallout. In response to increasing concern about fallout, the Limited Test Ban Treaty was ratified in 1963. Under this treaty, nuclear tests could only be conducted underground. The number of underground tests conducted by the United States declined steadily until testing stopped in 1992. Since then, the United States has relied on stockpile stewardship to insure the safety and reliability of its nuclear weapons.