Atomic Bomb

Atomic Bomb, also known as the atom bomb or fission bomb, a weapon whose explosive power originates from the fission of atomic nuclei. When the nucleus of a heavy atom, such as uranium-235, is split, a certain amount of mass disappears and an equivalent amount of energy is released. This is the energy that powers an atom bomb. On a pound-for-pound basis, the U-235 in an atomic bomb can release on the order of one million times as much energy as TNT.

Late in 1938, shortly before the outbreak of WORLD WAR II , two German physicists observed that a neutron can cause the nucleus of a uranium atom to break in two (fission). Soon thereafter others showed that the energy released by fission was about 100 million times larger per atom than in a chemical reaction. Within a few months scientists in Europe and America made two more important discoveries. They showed that the fissions had occurred in the uranium isotope U-235, which makes up only 1 part in 140 of natural uranium. The rest of the uranium, consisting of U-238, had not taken part in the reaction. They also showed that each fission event released 2 or 3 neutrons. This made it possible to envision a chain reaction, in which the neutrons released from one fission would lead to more than one subsequent fission. In such a reaction, neutrons (and energy) would increase exponentially--that is, at a rate proportional to the number of neutrons present. Near the end of such a reaction, energy would be released at a tremendous rate.

In a chain reaction the material would heat up, liquefy, vaporize--and then explode. As it blew apart, it would expand quickly to a stage where most of the neutrons released would escape without causing additional fissions, thus stopping the reaction. By the time the uranium was vaporized it would have an energy content comparable to that of an equal amount of a chemical high explosive such as TNT. But a chain reaction initiated in pure U-235 might escalate so quickly that a much greater amount of fission energy could be released before the reaction was stopped by expansion of the vaporized uranium. In this case the explosion might be of unprecedented power.

Such intriguing possibilities fascinated all those who were aware of the new findings. A flood of technical papers reported on new research as individual scientists and small laboratory groups worked to determine whether the speculations might have a basis in reality. By mid-1940 the accumulating evidence became so promising (or ominous) that senior British and American scientists judged it imprudent to continue to publish new results. Under a self-imposed blackout on publication, further work in the United States and Britain was done in secret to prevent German scientists from using the findings to develop an atomic bomb of their own for use in the war then under way.

There was reason to fear that Germany might win the race to produce the bomb. Fission had been discovered in Germany, and German scientists were at least as able as anyone else to assess its significance. Moreover, it seemed ominous that Germany had stopped the sale of uranium ore from the rich mines in Czechoslovakia. Up until mid-1941, concern over a German bomb had been stronger in Britain than in the United States. About that time, however, the sense of urgency began to pervade U.S. nuclear scientists.

About this time, too, an alternative atomic bomb material was proposed: plutonium (Pu-239). Scientists realized that if a chain reaction in natural uranium could be established (by slowing down the neutrons emitted by fission), it might be possible to transmute part of the U-238 into the plutonium isotope Pu-239. This material was expected to be superior to U-235 for use as an explosive. Although large-scale production of either material would be very difficult, the difficulties were different in nature. Thus it seemed prudent to set up programs to produce both materials, in case one program should fail.

In mid-1942 an all-out program to develop the bomb was initiated by the United States. Further research was still needed since key information was still missing, but the truly formidable requirement was to engineer and build two very large plants, each of a type radically different from anything previously constructed, which would be required for separation of U-235 and production of Pu-239 on the necessary scale. Responsibility for the entire program--research as well as construction--was assigned to the U.S. Army Corps of Engineers under the code name "Manhattan Project. The project, unprecedented in scale, involved the assembly of a work force in excess of 100,000 persons and took nearly three years to complete.

On July 16, 1945, at a site called Trinity in the desert near Alamogordo, N. Mex., an atomic bomb using plutonium was tested. A tremendous explosion resulted. Its energy was equivalent to that released by 20,000 tons (20 kilotons) of TNT. This is an amount of energy equal to all the energy produced and consumed in the whole United States in about half a minute. However, this energy was released in a few millionths of a second, and in a volume only a few inches across. The sturdy 100-foot (30-meter) steel tower on which the bomb was mounted was completely vaporized. For hundreds of yards (meters) around the zero point the surface of the desert sand was fused to glass. The ball of incandescent air formed by the explosion boiled up rapidly to a height of 35,000 feet (10,700 meters). The speculations of 1939-1940 had had a basis. The anxiety experienced in 1941-1942 had been well-grounded--even though, as it turned out, Germany had not pursued the development of atomic weapons.

On Aug. 6, 1945, an atomic weapon of about 15 kilotons--carried by a U.S. bomber named Enola Gay--was exploded about 1,800 feet (550 meters) over the Japanese city of Hiroshima. The result was similar to that of the first test, but this bomb used enriched uranium. On August 9, a Trinity-type weapon of about 20 kilotons was exploded about 1,800 feet (550 meters) above Nagasaki. The bombs devastated both cities. About 70,000 people died at Hiroshima and about 40,000 at Nagasaki, and many thousands more were injured. Within days the war with Japan was over.

The devastation of Hiroshima and Nagasaki resulted from three main types of effects: blast, thermal radiation, and nuclear radiation. Of these, only the blast effect is significant for chemical high explosives. The blast effect of an atomic bomb is similar to that of a conventional explosive but much more intense and far-reaching. Thermal radiation, which results from the extremely high temperatures created by an atomic explosion, causes serious burns on exposed parts of the body and may ignite fires over a wide radius. Nuclear radiation, which results from the neutrons and gamma rays associated with fission, causes death and injury as a result of damage to living tissue.

Among the survivors of the attacks on Hiroshima and Nagasaki, roughly equal numbers of injuries were caused by blast and thermal radiation but considerably fewer by nuclear radiation. Each of the three types of effects posed serious hazards to unprotected persons out to a distance of about a mile (1.6 km) from a point directly below the explosion.

The toll of death and injury at Hiroshima and Nagasaki--appalling as it was--was not the most meaningful measure of the significance of the new weapon. In the massive fire-bomb raid on Tokyo on March 9, 1945, for example, the Japanese suffered more fatalities than at Hiroshima. But the attack on Tokyo engaged a fleet of many hundreds of bombers for many hours. The awesome difference was that damage on this scale could be inflicted by a single bomb carried in a single plane.

Since 1945 the design of atomic weapons has advanced greatly. The explosive yields now available range from less than a kiloton to many tens of kilotons. The relative importance of the effects of weapon explosions depend on yield. For explosions larger than 20 kilotons the radius of hazard for each of the three types of effects would be larger than for a 20-kiloton weapon, of course, but the range of thermal hazard would exceed that for blast, and the range of blast damage in turn would considerably exceed that for nuclear radiation injury. For explosions smaller than 20 kilotons, this order would be reversed, with nuclear radiation reaching out farther than blast, and thermal radiation still more limited.

The weights of fission bombs now range down to less than 100 pounds (45 kg), compared with about 5 tons for the weapons exploded over Hiroshima and Nagasaki. Heavy bombers are no longer required as carriers: small missiles, light aircraft, and artillery shells can be used instead.

Most significantly, by the mid-1980s atomic weapons were in the arsenals of at least five countries: the United States, the Soviet Union, France, the United Kingdom, and China. India has performed one test explosion of an atomic bomb, and several other countries could make atomic weapons if they choose to expend the effort required to obtain the necessary materials.

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