Nuclear Weapons | Research & Encyclopedia Articles

Nuclear Weapons

The first nuclear explosion on earth occured on July 16, 1945, in Alamagordo, New Mexico, as part of a test for the Manhattan project initiated by President Franklin Delano Roosevelt. The first nuclear weapon used in war was deployed less than a month later, when the United States dropped a Uranium 235 bomb on the Japanese city of Hiroshima on August 6, 1945, near the end of World War II. It killed about 140,000 men women and children. The United States managed to maintain a monopoly on nuclear weapons only for a few years. On August 29, 1949, the Soviet Union exploded its first nuclear bomb, escalating an arms race that would heavily influence international politics for the rest of the century.

This state of world affairs had its origins about 50 years earlier in the seemingly innocent studies of radioactivity which was first discovered by Henri Bequerel in 1896. In 1898, Ernst Rutherford characterized simple radioactivity as consisting of beta rays, later identified as electrons, and alpha rays, later identified as helium nuclei made up of two protons and two neutrons. However even as of the 1920s, no one seriously thought that the phenomenon of heavy nuclei ejecting radiation could be exploited to harness large amounts of energy. It was in 1934 that Leo Szilard, an eminent Hungarian physicist, first applied for a patent on the extraction of energy from the nucleus. His patent described the notion of a nuclear chain reaction which in spirit embodies the mechanism behind the first nuclear weapons. Szilard's ideas soon became appreciated by the physics community. Many physicists, including Albert Einstein and Niels Bohr, alerted Roosevelt about the possibility of nuclear weapons. These warnings eventually lead to the establishment of the Manhattan Project which in turn culminated in the first successful explosion of an atomic bomb.

The physical process underlying the first nuclear explosions is known as fission. The atom consists of a cloud of electrons surrounding a relatively tiny, densely packed nucleus consisting of protons and neutrons. Most chemical reactions, including the ones in conventional explosives such as TNT, involve a release of energy due to changes in the configurations of the electrons. Electromagnetic binding energy is converted into the light and heat energy of the explosion. Nuclear weapons employ the same principle, only with the nuclear force rather than the electromagnetic force. In a fission reaction a heavy nucleus, such as uranium-235, which has 92 protons and 143 neutrons, is bombarded by a neutron, which splits the nucleus into two lighter nuclei plus extra neutrons. The sum of the masses of the fission products is less than the mass of the original nucleus, and the extra mass is converted into energy according to Einstein's famous equivalence between mass and energy: E = mc2 . Each fission event releases about 200 MeV of energy (1 MeV = 1.6x10-13 Joules). This is a microscopic amount of energy but there are typically many fission events in a nuclear explosion. Just 1.99 oz (57 g) of uranium, when fissioned, can convert approximately 0.00199 oz (0.057 g) of mass into energy, yielding an energy release of 1 kiloton.

In order to sustain a large nuclear reaction in a bomb, one needs a critical mass of fissionable material. In such a mass the density of material is high enough the two or more neutrons released in each fission event have a high probability to induce further fission events which in turn release more neutrons. One then gets an exponential proliferation of fission events which yield the energy of the nuclear explosion. The uranium-235 isotope is a good choice for fissionable material because it has a high probability to undergo fission upon capture of a neutron. In nature uranium consists mostly (99.3%) of uranium 238 which absorbs neutrons without fissioning and hence is unsuitable for attaining a critical mass. Through a complicated process of isotopic enrichment one must obtain 90 percent pure uranium-235. This difficult and expensive process was a main obstacle in the creation of the first bomb. However a baseball sized piece of enriched uranium-235 weighing about 22.1 lb (10 kg) is sufficient for critical mass.

Another technical difficulty in creating an atomic bomb is the triggering mechanism that starts the nuclear fission reaction. Fissionable material at critical mass is highly unstable and could explode without warning. Thus atomic bombs must consist of two or more pieces of fissionable material at subcritical mass that are brought together immediately before the bomb is to be detonated. In a gun-type bomb this so called critical assembly is achieved by a gun that fires one subcritical mass of uranium-235 at another. The two masses must be brought together very quickly, before the whole assembly has a chance to fission prematurely. The gun system moves the two masses together at a few millimeters per microsecond which is sufficiently fast. However, in plutonium-239 bombs, the rate of plutonium fission is faster than that of uranium, and the gun assembly is not fast enough to bring the subcritical pieces together. An alternate mechanism can be used known as implosion. In the implosion bomb a subcritical configuration of plutonium is surround by carefully arranged layers of explosive, which is then symmetrically detonated. The resulting shockwave compresses the plutonium so that it becomes critical and can then undergo a nuclear chain reaction. The bomb that was dropped on Hiroshima was a gun-type uranium 235 bomb, whereas the bomb tested in Los Alamos was an implosion type plutonium bomb.

Besides the fission based bombs there exists a class of more powerful nuclear weapons based upon the principle of nuclear fusion, the same process that powers our sun. The first fusion bomb, the Mike shot, was detonated on the Eniwetok Atoll in the South Pacific in 1952. The basic principle giving rise to the energy release in a fusion bomb is the fusion of two isotopes of hydrogen, deuterium with two neutrons, and tritium with three neutrons, into a helium-4 nucleus. One neutron is then left over and 17.6 MeV is released. Although 17.6 Mev is much less than the 200 MeV released in a fission event, hydrogen is much lighter than uranium, so many more fusion events can occur than fission events for a given weight of explosive material. To obtain an energy release of 1 megaton, 130 lb (57 kg) of uranium-235 or plutonium-239 would have to be fissioned whereas only 31 lb (14 kg) of tritium and deuterium would have to be fused.

The difficult part of the hydrogen bomb is giving the hydrogen nuclei enough energy to overcome the coulumb repulsion of their protons in order to fuse into helium. The phenomenon of quantum mechanical tunnelling through this barrier helps in the process of fusion (as it does in the sun as well) but still considerable heat and pressure must be supplied. Usually the source of this heat and pressure is a separate fission bomb called the primary which produces a flood of radiation. The radiation hits the thermonuclear part of the bomb, also known as the secondary which consists of lithium deuteride. A neutron hits the lithium splitting it into tritium and helium-4. The tritium then fuses with the deuterium in the lithium deuteride yielding energy. Lithium deuteride is used in this indirect fashion in order to avoid the cryogenic apparatus that would be required to store the isotopes of hydrogen. In order to enhance the yield of the bomb, it is encased in uranium 238. Excess neutrons hit the uranium 238 which may fission yielding even more neutrons which can subsequently induce further fusion. This combination of fission and fusion forms the mechanism behind the most powerful weapon known to man and can yield up to 100 megatons of energy, compared to the 15 kilotons of energy released by the atomic bomb dropped on Hiroshima.

The effects of a one megaton hydrogen bomb can be severe. Temperatures at the beginning of the nuclear explosion are comparable to the interior of the sun, about 1x107 degrees celsius. The surrounding air immediately turns into a fireball which also creates a high pressure shockwave that spreads out from the explosion. The shockwave consists of an overpressure front that exceeds atmospheric pressure by 100 psi 0.5 mi (0.8 km) from the blast. An overpressure of 5 psi (which exists about 4 mi [6.4 km] from the blast) is enough to destroy brick houses. Behind this shockwave comes hurricane force winds. The fireball also rises up into the air and cools off in the process creating the famous mushroom cloud. After about a minute the cloud can be 6 mi (9.6 km) high and still rises at 220 mi (354 km) per hour. It eventually reaches a maximum height of 12 mi (19.3 km). If the bomb is detonated high enough in the air, radioactive fallout from the fission events in the casing of the bomb can reach the stratosphere and from there can reach any part of the world, resulting in global fallout.