Thermonuclear Explosions

Because of the high temperatures required to initiate a nuclear fusion reaction, such devices are often called thermonuclear devices. A thermonuclear explosion can be created only by producing the required temperature, about a hundred-million Kelvins, and by forcing the material together so quickly that it will fuse rapidly. This is typically done with the isotopes of hydrogen, deuterium and tritium. This led to the term "hydrogen bomb" to describe the deuterium-tritium fusion bomb.

To obtain the two parts of the fuel, pellets were made from lithium hydride, LiD, made with the deuterium isotope. The only way which was found to produce the ignition temperature was to set off a fission bomb such that it would heat and compress the lithium hydride. In the process, the lithium was bombarded with neutrons, breeding tritium. Then the deuterium-tritium fusion reaction could take place.

Hydrogen Bomb

Because the thermonuclear explosive devices used hydrogen isotopes, (deuterium-tritium fusion), the resulting bombs were often called "hydrogen bombs". The first hydrogen bomb was detonated on November 1, 1952 at the small island Eniwetok in the Marshall Islands. Its yield was several megatons of TNT. The Soviet Union detonated a fusion bomb in the megaton range in August of 1953. The U.S. exploded a 15 megaton fusion bomb on March 1, 1954. It had a fireball 4.8 km in diameter and created a huge characteristic mushroom-shaped cloud. Analysis of the radioactive fallout from this bomb revealed it to be a fission-fusion-fission weapon, a "hydrogen bomb" with an outer sheath of natural uranium to increase the yield.

Uranium Bomb

Using the energy release from the nuclear fission of uranium-235, an explosive device can be made by simply positioning two masses of U-235 so that they can be forced together quickly enough to form a critical mass and a rapid, uncontrolled fission chain reaction. That is not to say that this is an easy task to accomplish. First you must obtain enough uranium which is highly enriched to over 90% U-235, whereas natural uranium is only 0.7% U-235. This enrichment is an exceptionally difficult task, a fact that has helped control the proliferation of nuclear weapons. Once the required mass is obtained, it must be kept in two or more pieces until the moment of detonation. Then the pieces must be forced together quickly and in such a geometry that the generation time for fission is extremely short. This leads to an almost instantaneous buildup of the chain reaction, creating a powerful explosion before the pieces can fly apart. Two hemispheres which are explosively forced into contact can produce a bomb such as the one detonated at Hiroshima.

Plutonium Bomb

Plutonium-239 is a fissionable isotope and can be used to make a nuclear fission bomb similar to that produced with uranium-235. The bomb which was dropped at Nagasaki was a plutonium bomb. Not enough Pu-239 exists in nature to make a major weapons supply, but it is easily produced in breeder reactors. In the U.S., there are reactors at Savannah River Plant, S.C., and at Hanford, Washington which are classified as plutonium production reactors. They breed plutonium by surrounding a fission reactor with a uranium-238 "blanket" to make use of the breeding reaction between neutrons and U-238. Once the plutonium is produced, it is easily separated from the other fission products by chemical means, so that less technology is needed to produce a nuclear weapon if you have a breeder reactor. This makes plutonium a greater source of concern for weapons proliferation, because reactors which appear to be just electric power generators can be breeding plutonium for weapons along with the power production.

The type of bomb which was dropped on Nagasaki on August 9, 1945 had been tested at Alamagordo, New Mexico on July 16. It developed from the Manhattan Project after Fermi demonstrated in 1942 that a sustained nuclear chain reaction was possible.

Hiroshima

On August 6, 1945, a uranium fission bomb was detonated over the Japanese city of Hiroshima. The bomb, called "Little Boy" was a "gun-type" device which used an explosive charge to force two sub-critical masses of U-235 together. It was 28 inches in diameter and 120 inches long, a relatively small package to deliver an explosive force of some 20,000 tons of TNT by converting about 1 gram of matter into energy. This could be accomplished with a sphere of U-235 about the size of a baseball. This kind of device had never been tested, in contrast to the plutonium bomb which was dropped on Nagasaki three days later. No device like this has been used since, making the estimates of radiation exposure at Hiroshima very difficult. Casualties included both direct blast victims plus those who died from radiation-induced cancer in subsequent years.

The bomb was triggered to explode at a height of 550 meters (1800 ft), a height calculated to cause the widest area of damage.

In the detonation of the uranium fission bomb over Hiroshima, about 130,000 people were reported killed, injured, or missing. Another 177,000 were made homeless.

Nagasaki

On August 9, 1945 a plutonium fission bomb was detonated over the Japanese city of Nagasaki, three days after a uranium fission bomb was dropped on Hiroshima. The bomb, called "Fat Man", was 128 inches long and had a diameter of 60.5 inches. It used implosion to compress the sub-critical assembly of plutonium. This kind of device had been tested less than a month before the drop, and was the subject of several other weapons tests after World War II. The explosive yield was about 20,000 tons of TNT, generated in about a microsecond.

The bomb was triggered to explode at a height of 550 meters (1800 ft), a height calculated to cause the widest area of damage.