Postwar World

Fission Bomb Research

As soon as World War II ended, the Los Alamos laboratory demobilized. Senior scientists, including Oppenheimer, Bethe, and Fermi, returned to their prewar university positions, and younger staff members left to enter graduate school. As a result, nuclear weapons work languished until the summer of 1946, when two slightly improved Fat Man bombs were tested in Operation Crossroads at Bikini Atoll in the Marshall Islands.

By 1947, atomic bombs had become a key part of America's defense posture. New types of fission weapons, designed to meet military requirements, were tested by Los Alamos during the 1948 Sandstone tests. These new designs significantly improved and expanded the nuclear stockpile. Los Alamos continued to design fission bombs into the 1950s, primarily working to make such bombs smaller and more efficient.

Hydrogen Bomb Research

Along with fission bomb development, Los Alamos also conducted research on the hydrogen bomb. The idea for a hydrogen bomb came from the thermonuclear study of stars conducted in the 1930s by Hans Bethe. Unlike fission weapons, which derive their energy from splitting atoms of the heavy elements uranium and plutonium, hydrogen bombs derive their power from fusing atoms of the light element hydrogen. Since fusion can only be achieved with stellar temperatures, hydrogen bombs were not possible until such a heat source (fission bombs) became available. After intensive research, conducted primarily with computers, and the discovery of radiation implosion, the first hydrogen bomb was detonated in October 1951. The success of the first thermonuclear test ushered in a new era in nuclear weaponry with significantly increased destructive power.

Just as fission weapons started with crude designs and were continually improved, so too were thermonuclear weapons. The first hydrogen bomb, Mike, relied primarily on the use of liquid deuterium, a cryogenic material. Three stories tall and weighing over a million pounds, Mike was a bomb only in the explosive sense. It was not a deliverable weapon. From 1951 until 1956, hydrogen bomb research focused on using dry fuel. This change reduced the size of thermonuclear bombs and made them deliverable by aircraft, including the Navy's smaller, carrier-based planes.

Miniaturization

Beginning in the late 1950s, delivery systems, such as ballistic missiles, governed design changes in nuclear weapons. Nuclear weapons had to be reduced in size to fit into the much smaller spaces of nose cones. Although nuclear weapons had always been constrained in size by the lift capacity of bombers, the constraints imposed by missiles were much more stringent. Miniaturization of weapons became a dominant theme in nuclear weapons development throughout the 1960s.

Stockpile Stewardship

By the early 1970s the number of new weapons designed and built began to decline. Increasing emphasis was placed on improving and upgrading weapons already in the stockpile, as well as enhancing safety. It became increasingly important to know that nuclear weapons would only detonate on command and not by accident. Weapon accidents at Palomares, Spain, and Thule, Greenland, underscored this need.

Many of the underground nuclear tests in the 1980s and early 1990s conducted by the United States were safety tests of stockpile weapons. With the current ban on nuclear weapon testing, other methods, primarily computer simulation, are now used in ensure the safety of the United States stockpile. This activity, called Science Based Stockpile Stewardship, is of critical importance as the age of individual weapons increases. As the cessation of nuclear weapon testing continues, the safety and reliability of the stockpile will remain the key weapon concern well into the 21st century.