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The first hydrogen bombs

Origins of the “Super”

U.S. research on thermonuclear weapons was started by a conversation in September 1941 between Fermi and Teller. Fermi wondered if the explosion of a fission weapon could ignite a mass of deuterium sufficiently to begin nuclear fusion. (Deuterium, an isotope of hydrogen with one proton and one neutron in the nucleus—i.e., twice the normal weight—makes up 0.015 percent of natural hydrogen and can be separated in quantity by electrolysis and distillation. It exists in liquid form only below about −250 °C, or −418 °F, depending on pressure.) Teller undertook to analyze thermonuclear processes in some detail and presented his findings to a group of theoretical physicists convened by Oppenheimer in Berkeley in the summer of 1942. One participant, Emil Konopinski, suggested that the use of tritium be investigated as a thermonuclear fuel, an insight that would later be important to most designs. (Tritium, an isotope of hydrogen with one proton and two neutrons in the nucleus—i.e., three times the normal weight—does not exist in nature except in trace amounts, but it can be made by irradiating lithium in a nuclear reactor.)

As a result of these discussions, the participants concluded that a weapon based on thermonuclear fusion was possible. When the Los Alamos laboratory was being planned, a small research program on the Super, as the thermonuclear design came to be known, was included. Several conferences were held at the laboratory in late April 1943 to acquaint the new staff members with the existing state of knowledge and the direction of the research program. The consensus was that modest thermonuclear research should be pursued along theoretical lines. Teller proposed more intensive investigations, and some work did proceed, but the more urgent task of developing a fission weapon always took precedence—a necessary prerequisite for a thermonuclear bomb in any event.

In the fall of 1945, after the success of the atomic bomb and the end of World War II, the future of the Manhattan Project, including Los Alamos and the other facilities, was unclear. Government funding was severely reduced, many scientists returned to universities and to their careers, and contractor companies turned to other pursuits. The Atomic Energy Act, signed by President Truman on August 1, 1946, established the Atomic Energy Commission (AEC), replacing the Manhattan Engineer District, and gave it civilian authority over all aspects of atomic energy, including oversight of nuclear warhead research, development, testing, and production.

From April 18 to 20, 1946, a conference led by Teller at Los Alamos reviewed the status of the Super. At that time it was believed that a fission weapon could be used to ignite one end of a cylinder of liquid deuterium and that the resulting thermonuclear reaction would self-propagate to the other end. This conceptual design was known as the “classical Super.”

One of the two central design problems was how to ignite the thermonuclear fuel. It was recognized early on that a mixture of deuterium and tritium theoretically could be ignited at lower temperatures and would have a faster reaction time than deuterium alone, but the question of how to achieve ignition remained unresolved. The other problem, equally difficult, was whether and under what conditions burning might proceed in thermonuclear fuel once ignition had taken place. An exploding thermonuclear weapon involves many extremely complicated, interacting physical and nuclear processes. The speeds of the exploding materials can be up to millions of metres per second, temperatures and pressures are greater than those at the centre of the Sun, and timescales are billionths of a second. To resolve whether the classical Super or any other design would work required accurate numerical models of these processes—a formidable task, especially as the computers needed to perform the calculations were still under development. Also, the requisite fission triggers were not yet ready, and the limited resources of Los Alamos could not support an extensive program.

Policy differences, technical problems

On September 23, 1949, President Truman announced, “We have evidence that within recent weeks an atomic explosion occurred in the U.S.S.R.” This first Soviet test (see below The Soviet Union) stimulated an intense four-month secret debate about whether to proceed with the hydrogen bomb project. One of the strongest statements of opposition against proceeding with the program came from the General Advisory Committee (GAC) of the AEC, chaired by Oppenheimer. In their report of October 30, 1949, the majority recommended “strongly against” initiating an all-out effort, believing “that the extreme dangers to mankind inherent in the proposal wholly outweigh any military advantages that could come from this development.” “A super bomb,” they went on to say, “might become a weapon of genocide” and “should never be produced.” Two members went even further, stating: “The fact that no limits exist to the destructiveness of this weapon makes its very existence and the knowledge of its construction a danger to humanity as a whole. It is necessarily an evil thing considered in any light.” Nevertheless, the Joint Chiefs of Staff, State Department, Defense Department, Joint Committee on Atomic Energy, and a special subcommittee of the National Security Council all recommended proceeding with the hydrogen bomb. On January 31, 1950, Truman announced that he had directed the AEC to continue its work on all forms of nuclear weapons, including hydrogen bombs.

In the months that followed Truman’s decision, the prospect of building a thermonuclear weapon seemed less and less likely. Mathematician Stanislaw M. Ulam, with the assistance of Cornelius J. Everett, had undertaken calculations of the amount of tritium that would be needed for ignition of the classical Super. Their results were spectacular and discouraging: the amount needed was estimated to be enormous. In the summer of 1950, more detailed and thorough calculations by other members of the Los Alamos Theoretical Division confirmed Ulam’s estimates. This meant that the cost of the Super program would be prohibitive.

Also in the summer of 1950, Fermi and Ulam calculated that liquid deuterium probably would not “burn”—that is, there would probably be no self-sustaining and propagating reaction. Barring surprises, therefore, the theoretical work to 1950 indicated that every important assumption regarding the viability of the classical Super was wrong. If success was to come, it would have to be accomplished by other means.