Nuclear Fission
Nuclear Fission:
Nuclear Fission was first discovered in 1938 by Otto Hahn and Fritz Strassmann. Nuclear Fission is the process in which a nucleus of an atom splits into several smaller fragments. These fragments, or fission products, are about equal to half the original mass. Two or three neutrons are also emitted. The sum of the masses of these fragments is less than the original mass. This 'missing' mass (about 0.1 percent of the original mass) has been converted into energy according to Einstein's equation. Fission can occur when a nucleus of a heavy atom captures a neutron, or it can happen spontaneously. A chain reaction is a process in which neutrons released in fission produce an additional fission in at least one further nucleus. This nucleus in turn produces neutrons, and the process repeats. The process may be controlled (nuclear power) or uncontrolled (nuclear weapons).
In a nuclear reaction, the average number of neutrons released is 2.5. This makes a chain reaction possible, provided at least one neutron per fission is captured by another nucleus and causes another fission. In a controlled reaction, the ratio of neutrons striking another nucleus has to be exactly one. If the ratio is less than one then the reaction will die out; and if it is greater than one it will grow uncontrolled (an atomic explosion). Nuclear reactions are controlled by a neutron-absorbing material, such as graphite.
The spontaneous fission rate is the probability per second that a given atom will fission spontaneously that is, without any external intervention. If a spontaneous fission occurs before the bomb is fully ready, it could fizzle. The released neutron travels at speeds of about 10 million meters per second, or about 3% the speed of light. The characteristic time for a generation is roughly the time required to cross the diameter of the sphere of fissionable material. The complete process of a bomb explosion is about a microsecond.
Uranium-235, which occurs naturally as one part in 140 in a natural mixture of uranium isotopes, is not the only material fissionable by thermal neutrons. Uranium-233 and plutonium-239 can also be used but must be produced artificially. Uranium-233 is produced from thorium-232, which absorbs a neutron and then undergoes beta decay (the loss of an electron). Plutonium-239 is produced in a similar manner from uranium-238, which is the most common isotope of natural uranium. The average energy released by the fission of uranium-235 is 200 million electron volts, and that released by uranium-233 and plutonium-239 is comparable. Fission can also occur spontaneously, but the time required for a heavy nucleus to decay spontaneously by fission (10 million billion years in the case of uranium-238) is so long that induced fission by thermal neutrons is the only practical application of nuclear fission.
The Detonation Sequence:
1. The high explosive surrounding the fissile material is ignited.
2. A compressional shock wave begins to move inward. The shock wave moves faster than the speed of sound and creates a large increase in pressure. The shock wave impinges on all points on the surface of the sphere of the fissile material in the bomb core at the same instant. This starts the compression process.
3. As the core density increases, the mass becomes critical, and then supercritical (where the chain reactions grows exponentially).
4. Now the initiator is released, producing many neutrons, so that many early generations are bypassed.
5. The chain reaction continues until the energy generated inside the bomb becomes so great that the internal pressure due to the energy of the fission fragments exceed the implosion pressure due to the shock wave.
6. As the bomb disassembles, the energy released in the fission process is transferred to the surroundings.
The development of nuclear energy from fission reactions began with the program to produce atomic weapons in the United States. Early work was carried out at several universities, and the first sustained nuclear chain reaction was achieved at the Univ. of Chicago in 1942 by a group under Enrico Fermi. Later the weapons themselves were developed at Los Alamos, N.Mex., under the direction of J. Robert Oppenheimer.