The theory of relativity says that mass is a special form of energy. It follows that it is possible to convert mass into energy and energy into mass. At the intraatomic level, such reactions take place. In particular, some of the mass of the atomic nucleus itself may well turn into energy. This happens in several ways. First, the nucleus can decay into a number of smaller nuclei, this reaction is called "decay". Secondly, smaller nuclei can easily combine to make a larger one - this is a fusion reaction. In the universe, such reactions are very common. Suffice it to say that the fusion reaction is the source of energy for stars. But the decay reaction is used by mankind in nuclear reactors, since 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 "chain"? This is a set of successive single nuclear reactions. As a result of this process, a change in the quantum state and nucleon composition of the original nucleus occurs, even new particles appear - reaction products. The nuclear chain reaction, whose physics allows one to study the mechanisms of interaction of nuclei with nuclei and with particles, is the main method for obtaining new elements and isotopes. In order to understand the flow of a chain reaction, one must first deal with single ones.
What is needed for the reaction
In order to carry out such a process as a nuclear chain reaction, it is necessary to bring particles (a nucleus and a nucleon, two nuclei) closer together at a distance of the strong interaction radius (about 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 denoted by the symbol set a, b, c, d. The symbol a denotes the original nucleus, b the incoming particle, c the new outgoing particle, and d the resulting nucleus.
Reaction energy
A nuclear chain reaction can take place both with absorption and with the release of energy, which is equal to the difference in the masses 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 freely proceed. This threshold depends on the particles involved in the interaction and on their characteristics. At the initial stage, all particles are in a predetermined quantum state.
Reaction implementation
The main source of charged particles that bombard the nucleus is the particle accelerator, which produces beams of protons, heavy ions and light nuclei. Slow neutrons are obtained through the use of nuclear reactors. To fix incident charged particles, different types of nuclear reactions, both fusion and decay, can be used. 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 value of the effective area, which characterizes the nucleus as a target for incident particles and which is a measure of the probability that the particle and the nucleus will enter into interaction. If particles with a nonzero spin take part in the reaction, then the cross section directly depends on their orientation. Since the spins of the incident particles are not completely randomly oriented, but more or less ordered, all corpuscles will be polarized. The quantitative characteristic of the oriented beam spins is described by the polarization vector.
Reaction mechanism
What is a nuclear chain reaction? As already mentioned, this is a sequence of simpler reactions. The characteristics of the incident particle and its interaction with the nucleus depend on the mass, charge,kinetic energy. The interaction is determined by the degree of freedom of the nuclei, which are excited during the collision. Gaining control of all these mechanisms allows for a process such as a controlled nuclear chain reaction.
Direct reactions
If a charged particle that hits the target nucleus only touches it, then the duration of the collision will be equal to the distance necessary to overcome the distance of the nucleus radius. Such a nuclear reaction is called a direct reaction. 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 the nuclear attraction. For example, such interactions as inelastic neutron scattering, charge exchange, and refer to direct. The contribution of such processes to the characteristic called "total cross section" is quite negligible. However, the distribution of the products of the passage of a direct nuclear reaction makes it possible to determine the probability of escape from the beam direction angle, quantum numbers, the selectivity of the populated states, and determine their structure.
Pre-equilibrium emission
If the particle does not leave the region of nuclear interaction after the first collision, then it will be involved in a whole 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 the particle is distributed amongconstituent parts of the nucleus. The state of the nucleus itself will gradually become much more complicated. During this process, a certain nucleon or a whole cluster (a group of nucleons) can concentrate energy sufficient for the emission of this nucleon from the nucleus. 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 constituent parts. That is, multiple successive single nuclear reactions caused by charged particles appear as reaction products in the previous steps. What is a nuclear chain reaction? For example, the 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 widely used. Particles with unused bonds play the role of free atoms or radicals. In a process such as a nuclear chain reaction, the mechanism of its occurrence is provided by neutrons, which 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 that will continue until the chain breaks due to the loss of the carrier particle.
Why the carrier is lost
There are only two reasons for the loss of the carrier particle of a continuous chain of reactions. The first is the absorption of the particle without the process of emissionsecondary. The second is the departure of the 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 there are many carrier particles, then this is called a branched reaction. What is a nuclear chain reaction with branching? One of the secondary particles obtained in the previous act will continue the chain started earlier, while the others will create new reactions that will also branch. This process will compete with the processes leading to the break. The resulting situation will give rise to specific critical and limiting phenomena. For example, if there are more breaks than purely new chains, then self-sustaining of the reaction will be impossible. Even if it is excited artificially by introducing the required number of particles into a given medium, the process will still decay with time (usually rather quickly). If the number of new chains exceeds the number of breaks, then a nuclear chain reaction will begin to spread throughout the substance.
Critical condition
The critical state separates the area of the state of matter with a developed self-sustaining chain reaction, and the area where this reaction is impossible at all. This parameter is characterized by equality between the number of new circuits and the number of possible breaks. Like the presence of a free carrier particle, the criticalstate is the main item in such a list as "conditions for the implementation of a nuclear chain reaction." The achievement of this state 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 fission chain reaction, more neutrons are produced. Therefore, this process can produce a branched reaction, where neutrons will act as carriers. In the case when the rate of neutron captures without fission or escapes (loss rate) is compensated by the rate of multiplication of carrier particles, then the chain reaction will proceed in a stationary mode. This equality characterizes the multiplication factor. In the above case, it is equal to one. In nuclear power, due to the introduction of a 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.
Nuclear chain reaction in energy
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 the reactor and the intensity of such a process as a nuclear chain reaction are characterized by a temperature coefficient (for coolant, for uranium, for graphite). There are also dependent characteristics in terms of power, in terms of barometric indicators, in terms of steam indicators. To maintain a nuclear reaction in a reactor, it is necessary to convert some elements into others. To do this, it is necessary to take into account the conditions for the flow of a nuclear chain reaction - the presence of a substance that is able to divide and release from itself during decay a certain number of elementary particles, which, as a result, will cause the fission of the remaining nuclei. As such a substance, uranium-238, uranium-235, plutonium-239 are often used. 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, an intense release of energy occurs, two or three neutrons are formed, capable of continuing the reaction acts. There are slow and fast neutrons, because in order for the nucleus of an atom to disintegrate, these particles must fly at a certain speed.
