The theory of relativity says that mass is a special form of energy. It follows from this that mass can be converted into energy and energy into mass. At the intra-atomic level, such reactions take place. In particular, a certain amount of mass of the atomic nucleus itself may well turn into energy. This happens in several ways. Firstly, the nucleus can decay into a number of smaller nuclei, this reaction is called "decay". Secondly, smaller kernels can easily connect to make a larger one - this is a fusion reaction. In the universe, such reactions are very common. Suffice it to say that the synthesis reaction is a source of energy for stars. But the decay reaction is used by mankind in nuclear reactors, as people have learned to control these complex processes. But what is a nuclear chain reaction? How to manage it?
What happens in the nucleus of an atom
A nuclear chain reaction is a process that occurs when elementary particles or nuclei collide with other nuclei. Why is it chain? This is a collection of sequential single nuclear reactions. As a result of this process, a change in the quantum state and nucleon composition of the initial nucleus occurs, even new particles appear - reaction products. A nuclear chain reaction, the physics of which allows us to study the mechanisms of interaction of nuclei with nuclei and particles, is the main method for obtaining new elements and isotopes. In order to understand the chain reaction, you must first deal with single people.
What you need for a reaction
In order to carry out such a process as a nuclear chain reaction, it is necessary to bring together the particles (nucleus and nucleon, two nuclei) by a distance of the radius of strong interaction (approximately one Fermi). If the distances are large, then the interaction of charged particles will be purely Coulomb. In a nuclear reaction, all laws are observed: conservation of energy, momentum, momentum, baryon charge. A nuclear chain reaction is indicated by the character set a, b, c, d. The symbol a denotes the original nucleus, b denotes an incoming particle, c denotes a new emitted particle, and d denotes the resulting nucleus.
Reaction energy
A nuclear chain reaction can occur both with absorption and with the release of energy, which is equal to the difference in mass of particles after the reaction and before it. The absorbed energy determines the minimum kinetic energy of the collision, the so-called threshold of a nuclear reaction at which it can flow freely. This threshold depends on the particles that participate in the interaction, and on their characteristics. At the initial stage, all particles are in a predetermined quantum state.
The implementation of the reaction
The main source of charged particles by which the nucleus is bombarded is the charged particle accelerator, which produces beams of protons, heavy ions and light nuclei. Slow neutrons are obtained through the use of nuclear reactors. To fix charged charged particles, different types of nuclear reactions can be used - both synthesis and decay. Their probability depends on the parameters of the particles that collide. This probability is associated with such a characteristic as the reaction cross section — the effective area value that characterizes the nucleus as a target for incident particles and which is a measure of the probability of a particle and nucleus entering into interaction. If particles with a nonzero spin value participate in the reaction, then the cross section directly depends on their orientation. Since the spins of the incident particles are oriented not quite randomly, but more or less orderly, all the corpuscles will be polarized. The quantitative characteristic of oriented beam spins is described by a polarization vector.
Reaction mechanism
What is a nuclear chain reaction? As already mentioned, this is a sequence of simpler reactions. The characteristics of an incident particle and its interaction with the nucleus depend on mass, charge, and kinetic energy. The interaction is determined by the degree of freedom of the nuclei, which are excited during the collision. Taking control of all these mechanisms allows a process such as a controlled nuclear chain reaction.
Direct reactions
If a charged particle that hits a target nucleus only touches it, then the duration of the collision will be equal to that necessary to overcome the distance of the radius of the nucleus. Such a nuclear reaction is called direct. A common characteristic for all reactions of this type is the excitation of a small number of degrees of freedom. In such a process, after the first collision, the particle still has enough energy to overcome nuclear attraction. For example, interactions such as inelastic neutron scattering, charge exchange, and are direct. The contribution of such processes to the characteristic called “full cross-section” is rather meager. However, the distribution of the products of the passage of the direct nuclear reaction makes it possible to determine the probability of departure from the beam direction angle, quantum numbers, the selectivity of the populated states, and determine their structure.
Pre-equilibrium emission
If a particle does not leave the nuclear interaction region after the first collision, then it will be involved in a cascade of successive collisions. This is actually just what is called a nuclear chain reaction. As a result of this situation, the kinetic energy of a particle is distributed among the constituent parts of the nucleus. The very state of the nucleus will gradually become much more complicated. During this process, energy sufficient for the emission of this nucleon from the nucleus can be concentrated on some nucleon or a whole cluster (group of nucleons). Further relaxation will lead to the formation of statistical equilibrium and the formation of a compound nucleus.
Chain reactions
What is a nuclear chain reaction? This is the sequence of its components. That is, multiple consecutive single nuclear reactions caused by charged particles appear as reaction products in the previous steps. What is called a nuclear chain reaction? For example, fission of heavy nuclei, when multiple fission events are initiated by neutrons obtained during previous decays.
Features of a nuclear chain reaction
Among all chemical reactions, chain reactions are very widespread. Particles with unused bonds act as free atoms or radicals. In a process such as a nuclear chain reaction, the mechanism of its occurrence is provided by neutrons that do not have a Coulomb barrier and excite the nucleus upon absorption. If the necessary particle appears in the medium, then it causes a chain of subsequent transformations, which will continue until the chain breaks due to the loss of the carrier particle.
Why media is lost
There are only two reasons for the loss of a carrier particle in a continuous chain of reactions. The first is to absorb the particles without the secondary emission process. The second is the departure of a particle beyond the limit of the volume of the substance that supports the chain process.
Two types of process
If only a single carrier particle is born in each period of the chain reaction, then this process can be called unbranched. It cannot lead to the release of energy on a large scale. If many carrier particles have appeared, then this is called a branched reaction. What is a branched chain nuclear reaction? One of the secondary particles obtained in the previous act will continue the chain that was started earlier, while the others will create new reactions that will also branch. Clipping processes will compete with this process. The resulting situation will generate specific critical and extreme phenomena. For example, if there are more breaks than purely new chains, then self-support of the reaction will be impossible. Even if you excite it artificially by introducing the right amount of particles into a given medium, the process will still fade over time (usually quite quickly). If the number of new chains exceeds the number of breaks, a nuclear chain reaction will begin to spread throughout the substance.
Critical situation
The critical state separates the region of the state of matter with a developed self-sustaining chain reaction, and the region where this reaction is impossible at all. This parameter is characterized by the equality between the number of new circuits and the number of possible breaks. Like the presence of a free carrier particle, the critical state is the main item in such a list as “conditions for the implementation of a nuclear chain reaction”. The achievement of this condition can be determined by a number of possible factors. The fission of the nucleus of a heavy element is excited by just one neutron. As a result of a process such as a nuclear chain fission reaction, more neutrons appear. Therefore, this process can produce a branched reaction, where neutrons will act as carriers. In the case when the rate of neutron capture without fission or take-off (loss rate) will be compensated by the rate of reproduction of the carrier particles, the chain reaction will proceed in a stationary mode. This equality characterizes the reproduction rate. In the above case, it is equal to one. In nuclear energy, due to the introduction of negative feedback between the rate of energy release and the multiplication factor, it is possible to control the course of a nuclear reaction. If this coefficient is greater than one, then the reaction will develop exponentially. Uncontrolled chain reactions are used in nuclear weapons.
Chain nuclear reaction in the energy sector
The reactivity of a reactor is determined by a large number of processes that occur in its core. All these influences are determined by the so-called reactivity coefficient. The effect of changes in the temperature of graphite rods, coolants or uranium on the reactivity of a reactor and the intensity of a process such as a nuclear chain reaction are characterized by a temperature coefficient (for the coolant, for uranium, for graphite). There are also dependent characteristics for power, for barometric indicators, for steam indicators. To maintain a nuclear reaction in a reactor, it is necessary to convert some elements to others. For this, it is necessary to take into account the conditions of the nuclear chain reaction — the presence of a substance that is capable of fission and excrete from itself during decay a certain number of elementary particles, which, as a result, will cause fission of the remaining nuclei. Uranium-238, uranium-235, plutonium-239 are often used as such a substance. During the passage of a nuclear chain reaction, the isotopes of these elements will decay and form two or more other chemicals. In this process, the so-called "gamma" rays are emitted, intense energy is released, two or three neutrons are formed that can continue the reaction. There are slow and fast neutrons, because in order for the atomic nucleus to decay, these particles must fly at a certain speed.