HYDROGEN BOMB

• Gun-type Design
• Implosion Design
• Initiators and Polonium
• High Explosives
• Fuzes and Detonators
• Hydrogen Bomb

While the atomic bombs built during the Manhattan Project used the principle of nuclear fission, the thermonuclear, or hydrogen, bomb was based upon nuclear fusion. While fission is most easily achieved with heavy elements, such as uranium or plutonium, fusion is easiest with light elements. Two isotopes of hydrogen were particularly promising: deuterium, consisting of one proton and one neutron, and tritium, with one proton and two neutrons.

Scientists recognized early on that even more powerful bombs might be built using nuclear fusion. At a meeting of top physicists, including J. Robert Oppenheimer and Edward Teller, at Berkeley in July 1942, a broad range of theoretical issues involving a thermonuclear bomb were discussed, and the possibility of thermonuclear ignition of the atmosphere with a fission device was raised. Priority at the Los Alamos laboratory, however, went to the fission bomb, not the Super, as the hydrogen weapon came to be called, and, in any event, the Super weapon depended on perfecting a nuclear bomb to trigger it. Oppenheimer as head of the lab nonetheless directed a small group of physicists to continue to investigate the Super. With the unknowns still so great, it was difficult to predict what might be useful, and it seemed worthwhile to explore. By February 1944, the theoreticians concluded that heating deuterium to its ignition temperature would be more difficult than anticipated. To find a way would take so much time that they no longer could consider the hydrogen weapon a possibility for the war. Oppenheimer restricted the work, though he did not drop it completely. In June 1944, he made Teller head of an independent group working on the Super and other assignments. Teller reported directly to Oppenheimer, with the two meeting for an hour once a week. Teller's work on the Super did not contribute to the main goal of building an atomic bomb, but it did break ground for the eventual successful work on the thermonuclear bomb.

After the war, Teller continued to pursue the idea of the Super, but it remained a low priority. As tensions with the Soviet Union increased during the early years of the Cold War and the Soviets successfully exploded their own fission device in August 1949, however, an intense, internal governmental debate emerged on the possibility, wisdom, and morality of rapidly developing a thermonuclear weapon. In January 1950, President Harry Truman decided the issue by approving accelerated development of the weapon, although no one was sure that it could be built due to the formidable technical difficulties that remained. Prospects dimmed considerably when calculations and computations, spearheaded by Los Alamos mathematician Stanislaw Ulam, indicated that a fusion reaction could not be sustained in the proposed design.

In early 1951, however, Ulam and Teller proposed a radically new and more promising approach for starting and sustaining a fusion reaction. They proposed using x-rays produced by the fission primary, rather than other attributes from the detonation of the fission primary, to compress the secondary. The process, which allowed a faster and longer-sustained compression of the fusion fuel, became known as staged radiation implosion. What was "a tortured thing that you could well argue did not make a great deal of sense," as Oppenheimer observed, became by mid-1951 "technically so sweet that you could not argue about that."

The United States tested the first full-scale thermonuclear device on November 1, 1952. The result was an explosion that was equivalent to one produced by more than ten million tons of TNT. This was approximately 700 times the power of the uranium (fission) bomb dropped on Hiroshima. In August 1953, the Soviet Union tested its first "boosted fission weapon," which used thermonuclear burning to enhance its yield, and in November 1955 the Soviet Union tested its first true thermonuclear weapon. There was now almost no limit on the size of an explosion either superpower could create. Fission bombs could only be built up to a certain maximum size, because only so much fissile material could be assembled without reaching a critical mass. Fusion bombs had no such limitation.

To view the next Science section of the Manhattan Project, proceed to Radioactivity.

• Gun-type Design
• Implosion Design
• Initiators and Polonium
• High Explosives
• Fuzes and Detonators
• Hydrogen Bomb

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