Fission and Fusion Bombs

Fission weapons were developed first in the U.S., then in the Soviet Union, and later in Britain, France, China, India, and Pakistan. By the first decade of the twenty-first century, there were seven countries that announced that they had nuclear weapons, and another three or four suspected of developing them.

The first atomic weapon tested was Fat Man, the plutonium weapon designed at Los Alamos and detonated at the Trinity test, Alamogordo, New Mexico, 16 July 1945. A weapon on the Fat Man design was dropped 8 August 1945 on the city of Nagasaki Japan, two days after the dropping of the uranium-fueled weapon, Little Boy, on Hiroshima.

The Soviets developed their first nuclear weapon utilizing information gathered through espionage on the American program, much of it supplied by the German-born British physicist, Klaus Fuchs. The Soviets tested a duplicate of Fat Man in a test dubbed ‘‘Joe 1’’ by the American press in August 1949.

The British cabinet decided to build nuclear weapons in January 1947, and the first British test was held in Australia in 1952. In the British program of testing weapons above ground, altogether there were 21 devices detonated between 1952 and 1958, 12 in or near Australia and 9 at Christmas Island. In addition to announced tests, the British tested many weapons components at Maralinga, the Australian test range.

Several designs of hydrogen bombs or thermonuclear bombs that relied on nuclear fusion rather than on fission for the majority of their energy release were considered in the period 1946-1955 by both the U.S. and the Soviet Union. In some designs, tritium, an isotope of hydrogen, was used to create a fusion effect that would more thoroughly cause fission in the plutonium core of the weapon, an effect known as ‘‘boosting.’’

In the late 1940s, American weapons designers thought through and abandoned the Alarm Clock design in which a fission weapon would ignite a deuterium fusion fuel. The device could be known as a fission-fusion-fission weapon, with layers of fissionable material, including some uranium-238 that would fission in the intense neutron environment. The upper limit of such a weapon would be several hundred kilotons. That design was not pursued or built.

The U.S. pursued the design of a cooled device using liquid deuterium, in the Ivy-Mike test of October 26, 1952. Later, the Teller-Ulam design worked out by Edward Teller and Stansilaw Ulam was far smaller and could be transported by aircraft. Teller and some others called the later design the ‘‘Classical Super.’’

The Soviets pursued a weapon similar to the U.S. Alarm Clock design, called the ‘‘Sloyka’’ or ‘‘Layer Cake’’ design, first conceived by Yakov Zel’dovich as the ‘‘First Idea’’ and improved by Andrei Sakharov. Sakharov’s ‘‘Second Idea’’ involved surrounding the deuterium with a uranium-238 shell, in the layers or ‘‘sloyka,’’ as Sakharov called it, to increase the neutron flux during the detonation. In a Russian pun, colleagues referred to the Layer Cake design as having been ‘‘sugarized,’’ as the name Sakharov means ‘‘of sugar.’’

The Soviet scientists tested the Layer Cake with the Joe 4 test of 12 August 1953 that yielded 400 kilotons, with 15 to 20 percent of its power derived from fusion. The U.S. already had larger boosted weapons, and American analysis of the Joe 4 test concluded that the device in the Joe 4 test was simply a variation on a boosted fission weapon. At the Russian weapons museum at Arzamas, the Joe 4 weapon is labeled the world’s first hydrogen bomb, while U.S. analysts have continued to regard it as a boosted fission weapon. Thus the question of whether a boosted weapon can be regarded as a fusion weapon is crucial to the issue of priority of invention.

The U.S. tested a new design that could be delivered as a weapon, with the Teller-Ulam design in the Castle/Bravo test held in 1954. That device yielded an estimated 15 megatons. The ‘‘Third Idea’’ developed by Igor Tamm and Sakharov, like the Teller-Ulam device, could go much higher in yield than the limits imposed by the Layer Cake idea, and the later Soviet tests of hydrogen bombs followed the Third Idea, with true, two-stage detonations.

The first Soviet test of the Third Idea weapon came 22 November 1955 with a yield estimated at 1.6 megatons. Later models of this weapon had a yield of 3 megatons. The accuracy of some of this information cannot be confirmed from official sources, as the material is derived from generally scholarly but unauthorized open literature.

Some observers believed that Joe 4 represented a thermonuclear weapon, just as the Soviets asserted. If that had been true, it would appear that the Soviets developed a deliverable nuclear weapon about seven months before the U.S. did so with the 1954 Castle/Bravo test of the huge low-temperature fusion device, too large to carry as a weapon aboard an aircraft. Careful evaluation of the Joe 4 test revealed that it should be regarded as a boosted fission weapon, somewhat along the lines of that detonated in the Greenhouse/Item test by the United States in 1952.

By such logic, American scientists could claim that the Classical Super test of Ivy/Mike in 1952 was the first true thermonuclear detonation, and that the Castle/Bravo Teller-Ulam weapon tested in 1954 was the world’s first deliverable thermonuclear weapon. That remains the American view of invention priority.