Nuclear fission: the process of splitting an atomic nucleus. Nuclear reactions. Fission of the uranium nucleus. Chain reaction. Process description
All this confusion is now quite clear. It turned out that a new type of nuclear transformation can occur in uranium under the action of neutrons. This transformation, discovered in 1938 by Hahn and Strassmann and made known at the beginning of 1939, consists in the fact that, having captured a neutron, the uranium nucleus can split into two halves.
In all other nuclear reactions, at the most, an alpha particle flies out of the nucleus. Here, two nuclei of average atomic weight are obtained from uranium, for example, krypton and barium:
(uranium) 2|| + neutron ->. (uranium) U (krypton) ^ -[- (barium)'|?.
The binding energy of the fragments, i.e., the nuclei of krypton and barium, is much greater than that of uranium. Therefore, during the fission of uranium, an enormous energy of 170 million volts is released, i.e., 10 times more than during the destruction of ligium by protons. The energy released during fission passes into the kinetic energy of the uranium fragments, i.e., these fragments acquire an enormous speed.
The fission of uranium, by the way, is similar to the fission of Lithium:
(lithium) - (- proton) (beryllium) ® - ". (helium) 2+ (helium) *.
In both cases, the nucleus is divided into two halves, and the reasons for the release of energy are also the same. However, nuclei heavier than lithium always emit at most an alpha particle; when lithium is destroyed, only alpha particles are also obtained. Therefore, the fission of uranium is a very special phenomenon.
Let's see how this fission of uranium occurs. The uranium nucleus, consisting of more than two hundred particles, is like a small round charged drop and has a spherical shape (Fig. 16, a). If we begin to change the shape of the nucleus, then exactly the same thing will happen as with the droplet. With a small
When the nucleus is stretched, it tends to return to its original spherical shape, since in this case the surface of the nucleus is the smallest; increasing the surface is not beneficial, it requires energy.
But if we change a lot - the shape of the nucleus - as shown in Fig. 16, in - then you will already be the core
It is better to fall apart into two halves, because both parts of the nucleus are repelled from each other by electric forces, and this repulsion becomes significant.
No, than the loss of energy associated with an increase in surface.
Thus, in order for the fission of the uranium nucleus to occur, it is necessary to cause strong movements in the nucleus, which would lead to the desired change in its shape.
4 V. L. Ginzburg 49
A neutron entering the nucleus of uranium can just excite strong movements and thereby lead to the fission of this nucleus. During fission, various fragments are obtained, for example, krypton and barium, or rubidium and cesium (from case to case, either one pair of nuclei or another can be obtained).
Fragments can be observed in the cloud chamber (Fig. 17).
For all the fragments resulting from the fission of uranium, however, one feature is characteristic - they turn out to be very overloaded with neutrons. The thing is
The fact that in heavier elements the ratio of the number of neutrons to the number of protons is greater than in light elements.
For example, in uranium 2!! there are 146 neutrons and 92 protons, and in oxygen there's an equal number of neutrons and protons.
The naturally occurring isotopes of krypton and barium have at most 50 and 82 neutrons, respectively, or a total of 132 neutrons. Meanwhile, in a uranium nucleus with a weight of 239, decaying into krypton and barium, there are 147 neutrons; therefore, the nuclei of krypton and barium, formed during the fission of uranium, together will have 50
15 extra neutrons. This circumstance leads to the fact that in the fragments resulting from the fission of uranium, excess neutrons are converted into protons, i.e., these fragments turn out to be radioactive and emit beta particles. Krypton, for example, decays like this:
(krypton) 3(G> (rubidium) 37-- (electron) (strontium) 38-)- (electron).
Thus, during the fission of uranium, a lot of elements are produced, most of which are radioactive.
But the overload of fragments by neutrons is so great that the matter is not limited to one radioactivity, and several neutrons simply fly out in a free form.
Consequently, during the fission of uranium caused by neutrons, new neutrons are released, the number of which is equal to two or three per one collapsing nucleus (Fig. 18).
This fact plays a decisive role in the use of nuclear energy.
The fission of uranium turns out to be a nuclear transformation of just this type, in which one neutron leads to the emission of several new neutrons. At the same time, a lot of energy is released. If neutrons produced by fission can successfully cause new fission of nuclei, then the number of neutrons and broken nuclei will increase all the time, and the reaction will not stop.
Moreover, if special measures are not taken, then this reaction will grow so violently that an explosion will result. Such a reaction, growing without any external sources, as we have already said, is called a chain reaction.
It turned out that in uranium such a chain reaction at certain conditions can be carried out.
This is how nuclear energy was first released.
Release of energy during nuclear fission. As in other nuclear reactions, the energy released during fission is equivalent to the difference in the masses of the interacting particles and the final products. Since the binding energy of a nucleon in uranium and the binding energy of one nucleon in fragments, during the fission of uranium, energy must be released
Thus, during the fission of the nucleus, huge energy is released, the overwhelming part of it is released in the form of the kinetic energy of the fission fragments.
Mass distribution of fission products. The uranium nucleus in most cases is divided asymmetrically. Two nuclear fragments have respectively different speeds and different weights.
The fragments fall into two groups according to their masses; one near krypton with the other near xenon. The masses of the fragments are related to each other on average as From the laws of conservation of energy and momentum, it can be obtained that the kinetic energies of the fragments should be inversely proportional to their masses:
The fission product yield curve is symmetrical with respect to the vertical straight line passing through the point. The significant width of the maxima indicates the diversity of fission paths.
Rice. 82. Mass distribution of uranium fission products
The listed characteristics refer mainly to fission under the action of thermal neutrons; in the case of fission under the action of neutrons with an energy of several or more, the nucleus breaks up into two fragments more symmetrical in mass.
Properties of fission products. During the fission of a uranium atom, very many electrons of the shell are cut off, and the fission fragments are approximately -fold ionized positive ions, which, when passing through the substance, strongly ionize the atoms. Therefore, the paths of the fragments in the air are small and close to 2 cm.
It is easy to establish that the fragments formed during fission must be radioactive, prone to emitting neutrons. Indeed, for stable nuclei, the ratio of the number of neutrons and protons varies depending on A as follows:
(see scan)
Nuclei produced by fission lie in the middle of the table and therefore contain more neutrons than is acceptable for their stability. They can be freed from excess neutrons both by decay and by directly emitting neutrons.
delayed neutrons. In one of options fission produces radioactive bromine. On fig. 83 shows a diagram of its decay, at the end of which are stable isotopes
An interesting feature of this chain is that krypton can be freed from an excess neutron either due to -decay, or if it was formed in an excited state due to the direct emission of a neutron. These neutrons appear 56 seconds after fission (the lifetime is relative to the transition to an excited state, although it itself emits neutrons almost instantly.
Rice. 83. Scheme of the decay of radioactive bromine formed in an excited state during the fission of uranium
They are called delayed neutrons. Over time, the intensity of delayed neutrons decreases exponentially, as in normal radioactive decay.
The energy of these neutrons is equal to the excitation energy of the nucleus. Although they make up only 0.75% of all neutrons emitted in fission, delayed neutrons play an important role in the implementation of a chain reaction.
Prompt neutrons. Over 99% of the neutrons are released within an extremely short time; they are called prompt neutrons.
When studying the fission process, the fundamental question arises, how many neutrons are produced in one fission event; this question is important because if their number is large on average, they can be used to divide subsequent nuclei, i.e., it becomes possible to create a chain reaction. Over the resolution of this issue in 1939-1940. worked in almost all major nuclear laboratories in the world.
Rice. 84. Energy spectrum of neutrons obtained from the fission of uranium-235
Fission energy distribution. Direct measurement of the energy of fragments and the energy carried away by other fission products gave the following approximate energy distribution
1.8. Nuclear fission
1.8.1. Fission reactions of heavy nuclei. Nuclear fission mechanism and activation energy. Composition of nuclear fission products and fission energy. Elementary theory of fission
Nuclear fission- a nuclear reaction in which two (rarely three) fragment nuclei are formed. The process is accompanied by the emission of secondary neutrons, quanta and the release of a significant amount of energy.
Historical reference. In 1938, in Germany, O. Gann and F. Strassmann showed by precise radiochemical analysis that when uranium is irradiated with neutrons, the element barium is formed in it, which is in the middle of the periodic table. The reaction looked like
, (Q≈ 200 MeV). (1.82)
There are over 30 uranium-235 fission output channels. F. Joliot-Curie with collaborators in France and E. Fermi with collaborators in Italy discovered the emission of several neutrons in the output channel. O. Frisch and L. Meitner in Germany noted the enormous amount of energy released during fission. This served to put forward the idea of a self-sustaining nuclear fission reaction. In 1940, spontaneous nuclear fission was also discovered in Russia. The basis of modern nuclear energy is the fission of uranium and plutonium nuclei under the action of neutrons. The nuclear age began in 1938.
Nuclear fission can also occur under the action of protons, γ-quanta, α-particles, etc. Forced fission of an excited nucleus by a neutron ( n, f) competes with other processes: with radiative neutron capture ( n, γ ), i.e., the emission of a γ-quantum and the scattering of a neutron on a nucleus ( n, n).
The probability of nuclear fission is determined by the ratio of the fission cross section σ f nucleus to the total neutron capture cross section.
Isotopes , , are divided by neutrons of all energies, starting from zero. In the course of the fission cross sections of these isotopes, resonances appear corresponding to the energy levels of the fissile nucleus (see Fig. 1.13).
Nuclear fission mechanism and activation energy
The process of nuclear fission is explained as the division of a homogeneous charged liquid drop under the action of Coulomb forces (Frenkel Ya. M, Bor N., Wheeler, 1939). To separate, the nucleus must acquire a certain critical energy, called the activation energy. After the capture of a neutron, a compound excited nucleus is formed. The excited nucleus begins to oscillate. The volume of the nucleus does not change (nuclear matter is practically incompressible), but the surface of the nucleus increases. The surface energy increases, therefore, the surface tension forces tend to return the core to its original state. The Coulomb energy decreases in absolute value due to the increase in the average distance between protons. Coulomb forces tend to break the nucleus. The nucleus transforms from a spherical shape into an ellipsoidal one, then a quadrupole deformation of the nucleus occurs, a constriction is formed, the nucleus turns into a dumbbell, which breaks, forming two fragments, and “splashes” - a pair of neutrons.
A characteristic of the ability of the nucleus to fission is the ratio of the Coulomb energy to the surface energy, taken from the semi-empirical formula for the binding energy of the nucleus
Where - divisibility parameter.
Nuclei with a fissility parameter >17 can fission, with a critical fission parameter ()cr = 45 they immediately fission (the condition for spontaneous nuclear fission). For a nucleus to split, it must overcome an energy barrier called the fission barrier. In the case of forced fission, the nucleus receives this energy when a neutron is captured.
Composition of fission products
Fission Shards . The main type of nuclear fission is fission into two fragments. The fragments are divided by mass asymmetrically in a ratio of two to three. The yield of fission products is defined as the ratio of the number of fissions producing a fragment with the given A To full number divisions. Since each fission produces two nuclei, the total yield per fission for all mass numbers is 200%. The fragment mass distribution during nuclear fission is shown in fig. 1.14. The figure shows a typical two-hump curve for the distribution of the total fission yield by thermal neutrons. The momenta of the fragments are equal and opposite in sign. The fragment velocities reach ~107 m/s.
Fig.1.14. Dependence of the yields of fission products of uranium-235 and plutonium-239 under the action of thermal neutrons on the mass number A.
fission neutrons . At the moment of formation, the fragments of the original nucleus are strongly deformed. The excess potential energy of deformation transforms into the excitation energy of the fragments. Fission fragments have a large charge and are enriched with neutrons, like the original nucleus. They pass into stable nuclei, throwing out secondary neutrons and γ-quanta. The excitation of fragment nuclei is removed by "evaporation" of neutrons.
Prompt fission neutrons are neutrons emitted by excited fragments in a time less than 4 10-14 sec. They evaporate from the fragments isotropically.
IN laboratory coordinate system(l.s.c.) the energy spectrum of fission neutrons is well described by the Maxwellian distribution
Where E is the neutron energy in l. s.k..gif" width="63 height=46" height="46"> – average spectrum energy.
Number v secondary neutrons per 1 act of fission by thermal neutrons is for uranium-235 v= 2.43, plutonium-239 v= 2.89. (for example, 289 secondary neutrons are produced simultaneously for 100 fission events).
Emission of γ-quanta . After the "evaporation" of neutrons from fragments, they still have excitation energy, which is carried away by prompt γ-quanta. The process of emission of γ-quanta occurs in a time of ~ 10-14 s after the emission of neutrons. Total effective radiant energy per division E total = 7.5 MeV..gif" width="67" height="28 src="> MeV. Average number of γ-quanta per division.
delayed neutrons – neutrons that appear after the fission of the original nuclei (from 10-2 sec to 102 sec). Number of delayed neutrons< 1% от полного количества нейтронов деления. Механизм испускания связан с β - decay of fission fragments of the type , , whose energy β -decay more than the binding energy of the neutron. In this case, there is a ban β -transition to the ground state and low neutron separation energy. The excitation energy of the nucleus is greater than the binding energy of the neutron. The neutron flies out instantly after the formation of an excited nucleus from a fragment nucleus as a result of its β -decay. However, in time, this occurs only after the half-life of the fragment nucleus.
The distribution of energy per act of fission of a heavy nucleus by thermal neutrons is shown in Table. 1.4.
Energy of nuclear fission products Table 1.4
Kinetic energy of a light fragment T osk l, MeV | ||
Kinetic energy of a heavy fragment T osc t MeV | ||
Kinetic energy of fission neutrons En MeV | ||
Energy of prompt γ-quanta Еγ m MeV | ||
Energy β - particles of fission products Еβ MeV | ||
Energy of γ-radiation of fission products Еγ pr MeV | ||
Fission product antineutrino energy Ev MeV | ||
Energy of γ-radiation due to neutron capture Еγn MeV | ||
The total energy released during nuclear fission QΣ MeV | ||
Thermal fission energy
QT = T osk l + T osc t + En+ Еγ m + Еβ + Еγ etc + Еγ = 204 MeV.
The energy carried away by the antineutrino is not released in the form of thermal energy; therefore, ~ 200 MeV falls on 1 act of fission of the nucleus by a thermal neutron. With a thermal power of 1 W, 3.1.1010 divisions / sec occur. IN chemical reactions one atom has an energy of ~ 1 eV.
Elementary theory of fission
Suppose that in the process of dividing https://pandia.ru/text/78/550/images/image028_18.gif" width="31" height="27 src="> the mass number is preserved A and charge Z. This means that we only consider shards:
A 1+ A 2 = A , Z 1+ Z 2 = Z,
the nucleus is divided in a ratio of 2 to 3:
A 1 / A 2 = Z 1 / Z 2=2/3.
The reaction energy is equal to the energy of fragments Q = T ok
Q = c2 [M – (M1 + M2 ) ],
Q= Esv1+ Esv2– ESt., (1.85)
Where ESt. is the total binding energy of the nucleus with respect to all its constituent nucleons
likewise E sv1 , Esv2 are the binding energies of the first and second fragments.
Substituting (1.86) and both formulas for E sv1, E s2 in (1.85) and neglecting the last term, we obtain
Assuming according to (1.15) = 17.23 MeV, https://pandia.ru/text/78/550/images/image026_22.gif" width="31" height="20"> we obtain the kinetic energy of fragments Tock ≈178 MeV , which exceeds the table value by only 10 MeV.
1.8.2. Chain reactions of fission of uranium nuclei. Formula for reproduction in a chain reaction. reproduction rates. Formula of four factors
Nuclear fission chain reactions heavy nuclei by neutrons are nuclear reactions in which the number of neutrons increases and a self-sustaining process of nuclear fission of matter occurs. Chemical and nuclear branched chain reactions are always exothermic. Chain reaction fission is feasible practically on three isotopes and is possible only because during the fission of a nucleus by a primary neutron, more than two secondary neutrons fly out in the output channel.
multiplication factor TO- the main characteristic of the development of a nuclear chain reaction.
Where Ni is the number of neutrons produced in i-generation, Ni–1 is the number of neutrons produced in ( i–1)-generation.
The theory of nuclear chain reactions was also created in 1939 by analogy with the theory of chemical chain reactions (1934). A self-sustaining nuclear chain reaction is possible when K>1 – supercritical reaction, K=1 – critical reaction. If K<1 – реакция подкритическая, она затухает.
Formula for multiplying neutrons in a chain reaction
If at the beginning of the reaction there is n neutrons, then in one generation their number will become
i.e..gif" width="108" height="48">,
where τ is average lifetime of one generation of neutrons
If we separate the variables and integrate, we get
using the formula , we finally obtain that the number of neutrons increases with time t exponentially with a positive exponent
https://pandia.ru/text/78/550/images/image027_18.gif" width="37" height="23"> slow neutrons and nuclear fission by fast neutrons.
reproduction rates. Formula of four factors
Let the uranium + moderator system have infinite dimensions. Let us assume that, at the moment of the birth of a generation of neutrons, n thermal neutrons, each of which forms https://pandia.ru/text/78/550/images/image058_8.gif" width="126" height="37">, (1.91)
where σU is the absorption cross section of slowed thermal neutrons by uranium,
σ3 is the absorption cross section of slow thermal neutrons by the moderator,
ρU is the concentration of uranium nuclei, ρ3 is the concentration of moderator nuclei.
Thus, the number of thermal neutrons captured by nuclear fuel is ( nηεрf). Neutron multiplication factor in an infinite medium(formula of four factors)
Neutron multiplication factor in the final medium
Kef=, (1.93)
Where - total probability that a neutron will escape core leakage.
For a stationary nuclear chain reaction to occur in the final system, it is enough Kef=1. This corresponds critical(smallest for the reaction) the size of the active zone. (For pure uranium, this is a ball with a radius of 8.5 cm and a mass of 47 kg)..gif" width="25 height=23" height="23">>1.
The first controlled nuclear chain reaction was carried out by E. Fermi in Chicago in 1942. The nuclear reactor had η = 1.35, ε ≈ 1.03, ε pf≈ 0.8, = 1.08, for TO eff needed θ0.93, which corresponds to a size of 5÷10 m. The nuclear reactor built in Moscow in 1946 had similar parameters.
>> uranium fission
§ 107 FISSION OF URANIUS NUCLEI
Only the nuclei of some heavy elements can be divided into parts. During the fission of nuclei, two or three neutrons and -rays are emitted. At the same time, a lot of energy is released.
Discovery of uranium fission. The fission of uranium nuclei was discovered in 1938 by the German scientists O. Hahn and F. Strassmann. They found that when uranium is bombarded with neutrons, elements of the middle part arise. periodic system: barium, krypton, etc. However, the correct interpretation of this fact precisely as the fission of a uranium nucleus that captured a neutron was given at the beginning of 1939 by the English physicist O. Frisch together with the Austrian physicist L. Meitner.
The capture of a neutron destroys the stability of the nucleus. The nucleus is excited and becomes unstable, which leads to its division into fragments. Nuclear fission is possible because the rest mass of a heavy nucleus more than the amount the rest masses of the fragments produced during fission. Therefore, there is a release of energy equivalent to a decrease in the rest mass that accompanies fission.
The possibility of fission of heavy nuclei can also be explained using the dependency graph specific energy connections from the mass number A (see Fig. 13.11). The specific binding energy of the nuclei of atoms of elements occupying the last places in the periodic system (A 200) is approximately 1 MeV less than the specific binding energy in the nuclei of elements located in the middle of the periodic system (A 100). Therefore, the process of fission of heavy nuclei into nuclei of elements in the middle part of the periodic system is energetically favorable. After fission, the system goes into a state with minimal internal energy. After all, the greater the binding energy of the nucleus, the greater the energy must be released when the nucleus arises and, consequently, the lower the internal energy of the newly formed system.
During nuclear fission, the binding energy per nucleon increases by 1 MeV, and the total energy released should be huge - about 200 MeV. No other nuclear reaction (not related to fission) releases such large energies.
Direct measurements of the energy released during the fission of the uranium nucleus confirmed the above considerations and gave a value of 200 MeV. Moreover, most of this energy (168 MeV) falls on the kinetic energy of the fragments. In Figure 13.13 you see the tracks of fissile uranium fragments in a cloud chamber.
The energy released during nuclear fission is of electrostatic rather than nuclear origin. The large kinetic energy that fragments have arises due to their Coulomb repulsion.
mechanism of nuclear fission. division process atomic nucleus can be explained on the basis of the drop model of the nucleus. According to this model, a bunch of nucleons resembles a drop of a charged liquid (Fig. 13.14, a). The nuclear forces between nucleons are short-range, like the forces acting between liquid molecules. Along with the strong forces of electrostatic repulsion between the protons, which tend to tear the nucleus apart, there are still large nuclear forces of attraction. These forces keep the nucleus from disintegrating.
The uranium-235 nucleus is spherical. Having absorbed an extra neutron, it is excited and begins to deform, acquiring an elongated shape (Fig. 13.14, b). The core will stretch until the repulsive forces between the halves of the elongated core begin to prevail over the attractive forces acting in the isthmus (Fig. 13.14, c). After that, it is torn into two parts (Fig. 13.14, d).
Under the action of the Coulomb repulsive forces, these fragments fly apart at a speed equal to 1/30 of the speed of light.
Emission of neutrons during fission. The fundamental fact of nuclear fission is the emission of two or three neutrons during fission. It was thanks to this that the practical use of intranuclear energy became possible.
It is possible to understand why free neutrons are emitted from the following considerations. It is known that the ratio of the number of neutrons to the number of protons in stable nuclei increases with increasing atomic number. Therefore, in the fragments that arise during fission, the relative number of neutrons turns out to be greater than is permissible for the nuclei of atoms located in the middle of the periodic table. As a result, several neutrons are released in the fission process. Their energy is various meanings- from several million electron volts to very small, close to zero.
The fission usually occurs into fragments, the masses of which differ by about 1.5 times. These fragments are highly radioactive, as they contain an excess amount of neutrons. As a result of a series of successive -decays, stable isotopes are eventually obtained.
In conclusion, we note that there is also spontaneous fission of uranium nuclei. It was discovered by Soviet physicists G. N. Flerov and K. A. Petrzhak in 1940. The half-life for spontaneous fission is 10 16 years. This is two million times longer than the half-life of uranium decay.
The nuclear fission reaction is accompanied by the release of energy.
Lesson content lesson summary support frame lesson presentation accelerative methods interactive technologies Practice tasks and exercises self-examination workshops, trainings, cases, quests homework discussion questions rhetorical questions from students Illustrations audio, video clips and multimedia photographs, pictures graphics, tables, schemes humor, anecdotes, jokes, comics parables, sayings, crossword puzzles, quotes Add-ons abstracts articles chips for inquisitive cheat sheets textbooks basic and additional glossary of terms other Improving textbooks and lessonscorrecting errors in the textbook updating a fragment in the textbook elements of innovation in the lesson replacing obsolete knowledge with new ones Only for teachers perfect lessons calendar plan for a year guidelines discussion programs Integrated LessonsThe fission of uranium nuclei occurs in the following way: first, a neutron hits the nucleus, like a bullet in an apple. In the case of an apple, a bullet would have made a hole in it, or would have blown it to pieces. When a neutron enters the nucleus, it is captured by nuclear forces. The neutron is known to be neutral, so it is not repelled by electrostatic forces.
How does uranium fission occur?
So, having got into the composition of the nucleus, the neutron breaks the balance, and the nucleus is excited. It stretches to the sides like a dumbbell or an infinity sign: ∞ . Nuclear forces, as is known, act at a distance commensurate with the size of the particles. When the nucleus is stretched, the action of nuclear forces becomes insignificant for the extreme particles of the "dumbbell", while electrical forces act very powerfully at such a distance, and the nucleus simply breaks into two parts. In this case, two or three neutrons are also emitted.
Fragments of the nucleus and the emitted neutrons fly apart at great speed in different sides. Shards decelerate pretty quickly environment, but their kinetic energy is enormous. It transforms into internal energy environment that is heated. In this case, the amount of energy released is enormous. The energy obtained from the complete fission of one gram of uranium is approximately equal to the energy obtained from burning 2.5 tons of oil.
Chain reaction of fission of several nuclei
We have considered the fission of one uranium nucleus. During fission, several (most often two or three) neutrons were released. They scatter to the sides at great speed and can easily fall into the nuclei of other atoms, causing a fission reaction in them. This is the chain reaction.
That is, the neutrons obtained as a result of nuclear fission excite and force other nuclei to fission, which in turn themselves emit neutrons that continue to stimulate further fission. And so on until the fission of all uranium nuclei in the immediate vicinity occurs.
In this case, a chain reaction can occur like an avalanche, for example, in the event of an explosion atomic bomb. The number of nuclear fission increases in geometric progression for a short period of time. However, a chain reaction can occur with damping.
The fact is that not all neutrons meet nuclei on their way, which they induce to fission. As we remember, inside the substance the main volume is occupied by the void between the particles. Therefore, some neutrons fly through all matter without colliding with anything along the way. And if the number of nuclear fission decreases with time, then the reaction gradually fades.
Nuclear reactions and the critical mass of uranium
What determines the type of reaction? From the mass of uranium. The larger the mass, the more particles the flying neutron will meet on its way and it has more chances to get into the nucleus. Therefore, a "critical mass" of uranium is distinguished - this is such a minimum mass at which a chain reaction is possible.
The number of neutrons formed will be equal to the number of neutrons that have flown out. And the reaction will proceed at approximately the same rate until the entire volume of the substance is produced. This is used in practice for nuclear power plants and is called a controlled nuclear reaction.
