The Superbomb Programs
Operation Greenhouse was a series of nuclear tests conducted at Eniwetok Atoll in early 1951 to test various design principles that would later become pivotal in the development of thermonuclear weapons. The purpose of these tests was to reduce the size, weight, and amount of fissile material necessary for nuclear weapons, while simultaneously increasing their destructive power.
The most important test during the Greenhouse series came on May 8-9, 1951, when physicists tested a device in which a relatively large fission yield was to be used to ignite a relatively small mass of thermonuclear fuel. This classical design operated on the radiation-implosion principle that had been explored earlier in 1946. The purpose of the experiment, nicknamed the "George" shot, was to demonstrate, as a minimum, that a thermonuclear reaction could under ideal conditions be made to take place in an experimental device. The test worked, and the largest fission explosion to date succeeded in igniting the first small thermonuclear flame ever to burn on earth. According to Edward Teller, the success of the "George" shot was pivotal in the development of the Super and provided scientists with the confidence to proceed along further speculations of thermonuclear design principles.
The Teller-Ulam Breakthrough
While the 1951 "George" shot proved that a large fission explosion could be used to ignite a small mass of thermonuclear fuel in a highly controlled experiment, scientists still grappled with the problem of how to configure a weaponized device that would produce sufficient energy from the exploding mass of fissile material into the thermonuclear fuel in order to achieve the temperature necessary to make it burn. While experimental physicists and engineers tackled that problem, theoretical physicists had already been hard at work thinking about the next step: the ignition of a large mass of thermonuclear fuel by a relatively small fission explosion. The breakthrough came in early March 1951 (two month before the planned tests at Eniwetok), when Edward Teller and Stanislaw Ulam came up with a promising design (the details of which are still highly classified). Calculations based on the new design commenced immediately, most of them done by Los Alamos scientists. In addition, scientists used some of the earliest computing machines to help with calculations.
The new design idea and the calculations supporting it were presented to a wider group of scientists at a meeting held in Oppenheimer's office at the Institute for Advanced Study at Princeton in June 1951. Several members of the AEC and the GAC were also in attendance. Those present immediately concluded that the new design was technically sound and that it probably represented the best chance of succeeding in an experimental test. Shortly thereafter, plans were developed at Los Alamos and approved in Washington for a series of tests of devices that would verify this design.
In the fall of 1951, it was decided that there should be two separate test operations. The first operation, IVY, was to be held as soon as possible and would include a number of tests to verify the Teller-Ulam design in a strictly experimental situation. The second operation, CASTLE, would come as soon as possible after IVY and would test the Teller-Ulam configuration in forms suitable for military use (i.e. weaponized devices).
The first series of thermonuclear tests conducted by the United States took place in November 1952 during Operation IVY. The first test took place on November 1, 1952 on the small pacific island of Eugelab in Eniwetok Atoll. The explosion, nicknamed the "Mike Shot", was very successful. It yielded an energy equivalent of 10 megatons of TNT, an amount roughly 1000 times as large as that released by the Hiroshima bomb (some 13 kilotons) that was dropped on Japan in August 1945. Though the test proved successful, the Mike device was very far from being a practical deliverable weapon. It's thermonuclear fuel consisted of liquid deuterium, a substance which must be cooled to temperatures colder than -250 C in order to liquefy it. As a result, the bomb required a complex refrigerating device (the size of a small laboratory) in order to maintain the fuel in that condition prior to its being exploded.
One of the major reasons physicists decided to test the first thermonuclear device with liquid deuterium as its fuel was because it would be much easier for physicist's to make theoretical calculations from the simple "burning" of deuterium compared to the complex multi-step process involved in the explosion of solid lithium deuteride (LiD). In addition, the United States did not at the time have a production plant capable of producing the high quantities of enriched lithium necessary for use in a thermonuclear device.
The second device tested on November 16, 1952 during Operation IVY was known as the "King Shot". This device was dropped from an aircraft and exploded at an altitude of 1480 feet above its target. The explosion, which derived its energy entirely from fission, was extremely large and probably yielded somewhere around 500 kilotons (the actual yield was never officially released). According to President Dwight D. Eisenhower in a speech made a year after the IVY tests the United States possessed an atomic bomb "25 times" the size of the ones used in Japan, meaning 500 kilotons.
The King Shot was important because it demonstrated that developing thermonuclear weapons was not the only way to build extremely massive bombs. Ever since JOE-1, many politicians and physicists claimed that the only possible way for America to ensure its security was to produce a much bigger bomb than the Soviets had, and that the only way to do that was through a crash development program of the hydrogen bomb. Hans Bethe, in opposing the crash program for the super, suggested that a bigger bomb could readily be produced by a straightforward extrapolation of the current fission technology.
In addition to the work being done on thermonuclear weapons, much of the research done at Los Alamos during the late 1940s investigated new methods to make more efficient, more powerful fission bombs. Some physicists explored ways to improve implosion techniques through experimental laboratory work. Others focused on using mathematical calculations to explore the possibilities of new configurations. By the early 1950s, scientists had determined that very efficient fission bombs in the megaton class could be developed by adopting the latest implosion techniques. The King Shot confirmed these ideas by producing an explosion yielding nearly half a megaton of TNT.
The King Shot
Operation CASTLE was the second thermonuclear test series planned by the Atomic Energy Commission and took place in the spring of 1954. During CASTLE, six variants of the superbomb were tested. The first test in that series, the Bravo test, took place on March 1, 1954. Bravo was a device using Lithium Deuteride as its fuel and the explosion yielded 15 megatons, the largest bomb ever exploded by the United States. The bomb was in a form readily adaptable for delivery by an aircraft and was thus America's first weaponized hydrogen bomb.
The Bravo test explosion yielded more than two and a half times what scientists had expected. The huge explosion released large quantities of radioactive debris into the atmosphere. This resulted in the exposure and contamination of some servicemen, natives, and the crew of a Japanese fishing trawler which had gone noticed in the security zone around the blast. This incident pushed the danger of radioactive fallout from nuclear weapons clearly into the public mind.
Nuclear Tests at Operation CASTLE
- 1. Bravo -- 14.8 Megatons
- 2. Romeo -- 11.0 Megatons
- 3. Koon -- 0.10 Megatons
- 4. Union -- 6.90 Megatons
- 5. Yankee -- 13.5 Megatons
- 6. Nectar -- 1.69 Megatons
The United States has never tested a nuclear device bigger than Bravo. When one such more powerful device was proposed in the mid-1950s by the Livermore laboratory for inclusion in another test series, Eisenhower personally vetoed it on the grounds of its excessive power.
The Bravo Test
- For quoted text and source information, please refer to the Advisers: Oppenheimer, Teller, & the Superbomb by Herbert York
- For more information on the topic, also refer to Richard Rhodes' Dark Sun: The Making of the Hydrogen Bomb