Chain reaction of division.
The division of kerns of uranium occurs at interaction to prompt or sluggish neutrons, but the irradiation by sluggish neutrons is more effective. The practical interest is represented by reactions, at which the major energy is oozed and there are neutrons, which give in magnification of number of the acts of division. For realization of chain reaction the maintenance of fixed number of neutrons is necessary. For an estimation of number of neutrons is inlet coefficient of manifolding of neutrons k. It determines the ratio of number of neutrons, which arise at reaction, to number of neutrons, which participated in reaction. If k >= 1 , the reaction will be explicated.
The minimum sizes of a fissile region, at which the realization of a chain reaction is possible, term as the critical sizes. The minimum quantity of a fissionable material necessary to sustain a nuclear chain reaction is called the critical mass.
Controllable reaction of division.
The controllable reactions occurs in a spacial nuclear reactors. Reactors can be used for research or for power production. A research reactor is designed to produce various beams of radiation for experimental application; the heat produced is a waste product and is dissipated as efficiently as possible. In a power reactor the heat produced is of primary importance for use in driving conventional heat engines; the beams of radiation are controlled by shielding.
Practical fissionable nuclei for atomic bombs are the isotopes uranium-235 and plutonium-239, which are capable of undergoing chain reaction. If the mass of the fissionable material exceeds the critical mass (a few pounds), the chain reaction multiplies rapidly into an uncontrollable release of energy. An atomic bomb is detonated by bringing together very rapidly (e.g., by means of a chemical explosive) two subcritical masses of fissionable material, the combined mass exceeding the critical mass. An atomic bomb explosion produces, in addition to the shock wave accompanying any explosion, intense neutron and gamma radiation, both of which are very damaging to living tissue. The neighborhood of the explosion becomes contaminated with radioactive fission products. Some radioactive products are borne into the upper atmosphere as dust or gas and may subsequently be deposited partially decayed as radioactive fallout far from the site of the explosion.