Chain nuclear reaction. Conditions for the realization of the nuclear chain reaction

The theory of relativity says that mass is a special form of energy. It follows from this that it is possible to convert the mass into energy and energy into mass. At the intra-atomic 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 connect to make a larger one - this is a synthesis reaction. In the universe, such reactions are very common. Suffice it to say that the synthesis reaction is the source of energy for the stars. But the decay reaction is used by mankind on nuclear reactors, as people have learned to control these complex processes. But what is a chain nuclear reaction? How to manage it?

What happens in the nucleus of an atom

A chain nuclear reaction is a process that occurs in the collision of elementary particles or nuclei with other nuclei. Why "chain"? This is a set of successive single nuclear reactions. As a result of this process, there is a change in the quantum state and nucleon composition in the initial nucleus, even new particles appear-reaction products. The chain nuclear reaction, whose physics makes it possible to investigate 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 course of the chain reaction, we must first deal with the single.

What you need for a reaction

In order to carry out such a process as a chain nuclear reaction, it is necessary to approximate 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 the charged particles will be purely Coulomb. In the nuclear reaction all laws are observed: the conservation of energy, momentum, momentum, baryon charge. A chain nuclear reaction is designated by a set of symbols a, b, c, d. The symbol a denotes the original nucleus, b the incident particle, c the new emitted particle, and d denotes the resultant nucleus.

The energy of the reaction

A chain nuclear reaction can take place with both absorption and with the release of energy, which is equal to the difference between the masses of the particles after the reaction and up to it. The absorbed energy determines the minimum kinetic energy of the collision, the so-called threshold of the nuclear reaction, in which it can freely flow. 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.

Reaction

The main source of charged particles, which the core is bombarded with, is the accelerator of charged particles, which gives beams of protons, heavy ions and light nuclei. Slow neutrons are obtained through the use of nuclear reactors. Different types of nuclear reactions, both synthesis and decay, can be used to fix charged particles. Their probability depends on the parameters of the particles that collide. This characteristic is associated with a characteristic of the cross section of the reaction-the effective area that characterizes the nucleus as a target for incident particles and which is a measure of the probability that the particle and the nucleus enter the interaction. If particles with nonzero spin are involved in the reaction, then the cross section directly depends on their orientation. Since the spins of the incident particles are not exactly chaotic, but more or less ordered, all the corpuscles will be polarized. The quantitative characteristic of the oriented beam spins is described by the polarization vector.

The Reaction Mechanism

What is a Chain Nuclear 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 in the collision. Obtaining control over all these mechanisms makes it possible to carry out such a process as a controlled chain nuclear 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 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 the nuclear attraction. For example, such interactions as inelastic neutron scattering, charge exchange, and relate to direct interactions. The contribution of such processes to the characteristic called "total cross-section" is rather meager. However, the distribution of products passing a direct nuclear reaction allows us to determine the probability of emission from the angle of beam direction, quantum numbers, selectivity of populated states and determine their structure.

Pre-equilibrium emission

If a particle does not leave the nuclear interaction region after the first collision, it will be involved in a whole cascade of successive collisions. This is actually what is called a chain nuclear reaction. As a result of this situation, the kinetic energy of the particle is distributed among the constituent parts of the nucleus. The very state of the nucleus will gradually become more complicated. During this process, on some nucleon or a whole cluster (a group of nucleons), the energy sufficient for the emission of this nucleon from the nucleus can be concentrated. Further relaxation will lead to the formation of statistical equilibrium and the formation of a compound nucleus.

Chain Reactions

What is a Chain Nuclear 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 called a chain nuclear reaction? For example, the fission of heavy nuclei, when multiple fission events are initiated by neutrons obtained in previous decays.

Features of the chain nuclear reaction

Among all chemical reactions, chained ones have become very popular. Particles with unused bonds serve as free atoms or radicals. In such a process as a chain nuclear reaction, the mechanism of its flow is provided by neutrons that do not have a Coulomb barrier and excite the nucleus upon absorption. If a 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 the carrier is lost

There are only two reasons for the loss of carrier particles of a continuous chain of reactions. The first is to absorb the particle without the secondary emission process. The second - the departure of a particle beyond the limit of the volume of matter that supports the chain process.

Two types of process

If in each period of the chain reaction only a single carrier particle is born, then this process can be called unbranched. It can not lead to the release of energy on a large scale. If there are many carrier particles, this is called a branched reaction. What is a chain nuclear reaction with branching? One of the secondary particles obtained in the previous act will continue the chain begun earlier, but others will create new reactions that will also branch. The process leading to breakage will compete with this process. The resulting situation will generate specific critical and limiting phenomena. For example, if there are more cliffs than purely new chains, then self-sustaining the reaction will be impossible. Even if you excite it artificially by introducing the right amount of particles into a given environment, the process will still fade with time (usually quite quickly). If the number of new chains exceeds the number of cliffs, then the chain nuclear reaction begins to spread throughout the substance.

Critical condition

A critical state separates the state of matter with a developed self-sustaining chain reaction, and an area where this reaction is impossible at all. This parameter is characterized by the equality between the number of new chains and the number of possible breaks. Like the presence of a free carrier particle, the critical state is the main item on such a list as "the conditions for the realization of a chain nuclear 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 only one neutron. As a result of such a process as a chain nuclear fission reaction, more neutrons appear. Consequently, this process can produce a branched reaction, where neutron carriers will act as carriers. In the case when the rate of neutron capture without fission or departures (the loss rate) will be compensated by the propagation speed of the carrier particles, the chain reaction will proceed in a steady state. 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 the nuclear reaction. If this coefficient is more than one, then the reaction will develop exponentially. Unguided chain reactions are used in nuclear weapons.

Chain nuclear reaction in power engineering

Reactivity of the 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 the change in the temperature of graphite rods, heat carriers or uranium on the reactivity of the reactor and the intensity of the process such as a chain nuclear reaction are characterized by a temperature coefficient (for heat carrier, for uranium, for graphite). There are also dependent characteristics for power, barometric indicators, for 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 chain nuclear reaction to occur-the presence of a substance that is capable of dividing and isolating a number of elementary particles from decay, which, as a consequence, will cause the fission of the remaining nuclei. Uranium-238, uranium-235, plutonium-239 are often used as such a substance. During the passage of the nuclear chain reaction, the isotopes of these elements will disintegrate and form two or more other chemical substances. In this process, so-called "gamma" rays are emitted, intense energy release occurs, two or three neutrons are generated that can continue the reaction events. Distinguish between slow neutrons and fast neutrons, because in order for the nucleus of an atom to disintegrate, these particles must fly at a certain rate.