VII.

Thermonuclear (Fusion) Weapons

Even before the first atomic bomb was developed, scientists realized that a type of nuclear reaction different from the fission process was theoretically possible as a source of nuclear energy. Instead of using the energy released as a result of a chain reaction in fissile material, nuclear weapons could utilize the energy liberated in the fusion of light elements. This process is the opposite of fission, since it involves the fusing together of the nuclei of isotopes of light atoms such as hydrogen. It is for this reason that the weapons based on nuclear-fusion reactions are often called hydrogen bombs, or H-bombs.

Of the three isotopes of hydrogen the two heaviest species, deuterium and tritium, combine most readily to form helium. Although the energy release in the fusion process is less per nuclear reaction than in fission, 0.5 kg (1s lb) of the heavier material contains many more atoms; thus, energy liberated from 0.5 kg (1s lb) of hydrogen-isotope fuel is equivalent to that of about 29 kilotons of TNT, or almost three times as much as from uranium. This estimate, however, is based on complete fusion of all hydrogen atoms.

Fusion reactions—the same processes that take place inside the Sun—occur only at temperatures of several millions of degrees, the rate increasing enormously with rising temperature. Such reactions consequently are known as thermonuclear (heat-induced) reactions. Strictly speaking, the term “thermonuclear” implies that the nuclei have a range (or distribution) of energies characteristic of the temperature. This plays an important role in making rapid fusion reactions possible by an increase in temperature.

Development of the hydrogen bomb was impossible before the perfection of A-bombs, for only the latter could yield the tremendous heat necessary to achieve fusion of hydrogen atoms. Atomic scientists used an atomic bomb to act as the trigger of the projected thermonuclear device.

A.

Thermonuclear Tests

Following developmental tests in the spring of 1951 at the Enewetak atoll in the Marshall Islands, a full-scale, successful experiment with a fusion-type device was conducted on November 1, 1952. This test, code-named Mike, produced an explosion with power equivalent to several megatons of TNT. The Soviet Union detonated a thermonuclear weapon in the megaton range in August 1953. As with the Soviet A-bomb, exploded in 1949, this happened many years earlier than expected. On March 1, 1954, the United States exploded a fusion bomb with a power of 15 megatons. It created a glowing fireball more than 4.8 km (3 mi) wide, and a huge mushroom cloud, which quickly rose into the stratosphere.

The March 1954 explosion led to worldwide recognition of the nature of radioactive fallout. The fallout of radioactive debris from the huge bomb cloud also revealed much about the nature of the thermonuclear bomb. Had the bomb been a weapon consisting of an A-bomb trigger and a core of hydrogen isotopes, the only persistent radioactivity from the explosion would have been the result of the fission debris from the trigger and from the radioactivity induced by neutrons in coral and seawater. Testing of radioactive debris that fell on the unfortunate Japanese vessel the Lucky Dragon, which was tuna-fishing about 160 km (100 mi) from the test site, demonstrated that the bomb that dusted the vessel with fallout, with its subsequent casualties, was more than just an H-bomb.