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In 1939. Gann and Strassmann discovered the process of uranium fission, in which the uranium nucleus, after capturing a neutron, splits into two parts completely different from the original element.
This process is accompanied by the release of extremely high energy in the form of kinetic energy of fission fragments (162 Mev per act), the energy of B- and y-decay of daughter (fragmentation) nuclei (10 Mev), the energy of fission neutrons 5.0 Mev and instantaneous y-quanta (7.5 Mev), the energy of y-radiation released at neutron capture (8.0 Mev).
A significant part of the energy is carried away by neutrinos (11 Mev). In just one act of fission, the energy of 203.5 Mev is released.
This discovery marked the beginning of the use of nuclear energy for peaceful and military purposes. In addition to the high energy yield, the uranium nuclear fission reaction is characterized by two features underlying its practical use in both peaceful energy and military technology.
Features of the reaction of nuclear fission of uranium.
Firstly, the fission reaction of uranium caused by neutrons, in turn, is accompanied by the release of neutrons; therefore, under suitable conditions, the fission process can be made self-sustaining, regulated with the continuous release of a certain amount of energy. This was done in nuclear reactors.
Secondly, natural uranium experiences spontaneous fission at a certain rate: in 1 g of ordinary uranium, spontaneous fission is experienced on average by about 23 nuclei within an hour; consequently, some “equilibrium concentration” of free neutrons necessary for the fission of uranium nuclei, without any external influence, is supported by the spontaneous fission process.
Under conditions favorable to the use of this spontaneous process, it is possible to obtain a chain reaction of fission with the release of extremely large energy in a short period of time. This was used in the creation of nuclear weapons.
The essence of a nuclear explosion.
Naturally, the fission reaction can be of a chain nature, drawing resources for the development of chains in itself, only if the decay of a fissionable nucleus after the capture of one neutron is accompanied by the release of not one, but several neutrons capable of continuing fission. In fact, it turned out that an unstable uranium nucleus that captured one neutron quickly decays, while unstable fission fragments, in turn, emit 2-3 neutrons.
Thus, the “reproduction” of free neutrons is characterized by an average yield of 2.5 neutrons for each act of fission. This multi-stage process, called chain. The fission reaction is schematically depicted in Fig. 1.
With a sufficient amount of fissile material, the so-called critical mass, the resulting chain reaction leads to a nuclear explosion. In a nuclear bomb, certain amounts of fissile isotopes are separated from each other and there is no chain reaction. For a chain reaction to occur, the separated amounts of fissile matter converge together, forming a critical mass.
On the importance of critical mass
Critical mass is the smallest amount of fissile material in which a chain reaction of fission begins to occur, leading to an explosion if this amount is slightly exceeded. The magnitude of the critical mass depends on the nature of the fissile substance, the degree of its purification from impurities, the nature and thickness of the neutron reflectors, the pressure on the nuclear charge and the geometric shapes of this charge. For uranium-235, the critical mass is about 30 kg, for California-242, the critical mass is about 1.5 g.
Fig. 1. The scheme of the fission chain reaction.
The chain reaction was possible only for one natural isotope of uranium — uranium-235. Another raw material for atomic weapons turned out to be plutonium-239, which was obtained by “bombarding” uranium-238 with neutrons, which at the same time passed into uranium-239, which in turn, emitting electrons, passed into plutonium-239.
Types of nuclear bombs and explosions. Pollution.
In addition to nuclear weapons based on the fission reaction, weapons based on the fusion reaction of light elements were obtained. So, if a hydrogen nucleus merges with a tritium nucleus, then a helium nucleus is formed and a large amount of energy is released.
But to start such a reaction, a high temperature is needed — millions of degrees. Such a temperature develops when a bomb explodes based on a fission reaction. Therefore, using the fission reaction first (the explosion of an atomic bomb), it is possible to obtain a fusion reaction or a thermonuclear reaction. A hydrogen bomb is built on the use of this reaction.
About uranium 238 nuclei.
In addition to these types of reactions, a nuclear reaction using uranium-238 turned out to be possible. Uranium-238 nuclei do not undergo fission when neutrons with an energy of less than 10-14 Mev enter them. Neutrons with such energy are formed during synthesis reactions.
The device using the uranium-238 nuclear fission reaction at the final stage was tested on March 1, 1954 at Bikini Atoll. This type of bomb is called a three-stage bomb: first there is a fission reaction of uranium-235 or plutonium-239, then a fusion reaction and then a fission reaction of uranium-238.
In a nuclear explosion, all or part of the energy is released as a result of a nuclear fission or fusion reaction or a combination of the two. Depending on the amount of fissionable or synthesized substance, an explosion of one or another power occurs, the explosion power is determined by the TNT equivalent.
Thus, the expression “20,000 tons of TNT bomb” means that the nuclear explosion of this bomb is equivalent to the explosion of 20,000 tons of TNT explosive. The power of bombs can vary in the widest range — from hundreds of tons to dozens- thousands of kilotons) and hundreds of millions of tons (hundreds of megatons) of TNT.
Types of nuclear explosions.
There are the following main types of nuclear explosions: air, ground, underground, underwater. In an air explosion, the resulting fireball does not touch the surface of the earth. In a ground explosion, a fireball touches the surface of the earth. A schematic representation of the explosions is shown in Fig. 2. The height at which an explosion can be considered air or ground depends on the TNT equivalent, as shown in Fig. 3.
Fig. 3. The height of air and ground explosions as a function of the TNT equivalent.
The definition of an underground and underwater explosion follows from the names themselves. With an underground explosion, the formation of a funnel on the surface of the earth is possible, with an underwater explosion, the release of a column of water.
The danger for radioactive contamination of the area is represented by a ground explosion of nuclear weapons, since in this case large amounts of soil are captured
a fireball and particles are formed on which radioactive isotopes settle. During air explosions, isotopes condense mainly on the bomb material and do not pose a danger in a combat situation, since they fall out more slowly due to small particle sizes. The phenomena related to a ground-based nuclear explosion will be discussed below.
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From the book:
PROTECTION FROM RADIOACTIVE FALLOUT edited by A.I.Burnazyan
R.V., Petrov, V.N. Pravetsky, Yu.S. Stepanov, M.I. Shalnov
State Publishing House of Medical Literature Moscow -1963