Nuclear fission is the splitting of a heavy atom into two fragments of approximately equal mass, accompanied by the release of a large amount of energy.
The discovery of nuclear fission began a new era - the "atomic age." The potential of its possible use and the ratio of risk to the benefit of its use not only gave rise to many sociological, political, economic and scientific achievements, but also serious problems. Even from a purely scientific point of view, the nuclear fission process has created a large number of puzzles and complications, and its full theoretical explanation is a matter of the future.
Sharing is profitable
The binding energies (per nucleon) of different nuclei differ. Heavier ones have lower binding energy than those located in the middle of the periodic table.
This means that it is advantageous for heavy nuclei with an atomic number greater than 100 to divide into two smaller fragments, thereby releasing energy, which is converted into kinetic energy of the fragments. This process is called fission of the atomic nucleus.
In accordance with the stability curve, which shows the dependence of the number of protons on the number of neutrons for stable nuclides, heavier nuclei prefer a larger number of neutrons (compared to the number of protons) than lighter ones. This suggests that along with the fission process, some "spare" neutrons will be emitted. In addition, they will also take on some of the energy released. The study of fission of the nucleus of the uranium atom showed that in this case 3-4 neutrons are released: 238 U β 145 La + 90 Br + 3n.
The atomic number (and atomic mass) of the fragment is not equal to half the atomic mass of the parent. The difference between the masses of atoms formed as a result of cleavage is usually around 50. True, the reason for this is not yet entirely understood.
The binding energies of 238 U, 145 La, and 90 Br are 1803, 1198, and 763 MeV, respectively. This means that as a result of this reaction, the fission energy of the uranium nucleus is released, equal to 1198 + 763-1803 = 158 MeV.
Spontaneous division
Spontaneous cleavage processes are known in nature, but they are very rare. The average lifetime of this process is about 10 17 years, and, for example, the average lifetime of alpha decay of the same radionuclide is about 10 11 years.
The reason for this is that in order to split into two parts, the core must first undergo deformation (stretch) into an ellipsoidal shape, and then, before final splitting into two fragments, form a βneckβ in the middle.
Potential barrier
In a deformed state, two forces act on the core. One of them is increased surface energy (the surface tension of a liquid droplet explains its spherical shape), and the other is the Coulomb repulsion between fission fragments. Together they produce a potential barrier.
As in the case of alpha decay, in order for spontaneous fission of the nucleus of the uranium atom to occur, the fragments must overcome this barrier using quantum tunneling. The value of the barrier is about 6 MeV, as is the case with alpha decay, but the probability of tunneling of the Ξ± particle is much greater than the much heavier atom cleavage product.
Forced Cleavage
Much more likely is the induced fission of the uranium nucleus. In this case, the mother nucleus is irradiated with neutrons. If the parent absorbs it, then they bind, releasing the binding energy in the form of vibrational energy, which can exceed 6 MeV, necessary to overcome the potential barrier.
Where the energy of the additional neutron is not enough to overcome the potential barrier, the incident neutron must have minimal kinetic energy in order to be able to induce atom splitting. In the case of 238 U, the binding energy of additional neutrons is not enough about 1 MeV. This means that fission of the uranium nucleus is induced only by a neutron with a kinetic energy greater than 1 MeV. On the other hand, the 235 U isotope has one unpaired neutron. When the core absorbs the additional, it forms a pair with it, and as a result of this pairing, additional binding energy appears. This is enough to release the amount of energy necessary for the nucleus to overcome a potential barrier and fission of the isotope occurs in a collision with any neutron.
Beta decay
Although three or four neutrons are emitted during the fission reaction, the fragments still contain more neutrons than their stable isobars. This means that cleavage fragments are generally unstable with respect to beta decay.
For example, when 238 U uranium fission occurs, the stable isobar with A = 145 is the 145 Nd neodymium, which means that the 145 La lanthanum fragment decomposes in three stages, emitting an electron and an antineutrino each time, until a stable nuclide is formed. Zirconium 90 Zr is a stable isobar with A = 90; therefore, the 90 Br bromine cleavage fragment decomposes in five stages of the Ξ²-decay chain.
These Ξ²-decay chains release additional energy, which is almost completely carried away by electrons and antineutrinos.
Nuclear Reactions: Uranium Fission
Direct neutron emission from a nuclide with too many of them to ensure nuclear stability is unlikely. The point here is that there is no Coulomb repulsion, and therefore surface energy tends to confine the neutron due to the parent. However, this sometimes happens. For example, a 90 Br fission fragment in the first beta decay stage produces krypton-90, which can be in an excited state with sufficient energy to overcome surface energy. In this case, neutron emission can occur directly with the formation of krypton-89. This isobar is still unstable with respect to Ξ² decay, until it becomes stable yttrium-89, so that krypton-89 decays in three stages.
Uranium Fission: Chain Reaction
The neutrons emitted in the fission reaction can be absorbed by another parent nucleus, which then undergoes induced fission itself. In the case of uranium-238, the three neutrons that arise, come out with an energy of less than 1 MeV (the energy released during the fission of the uranium nucleus - 158 MeV - mainly goes into the kinetic energy of the fission fragments), so they cannot cause further fission of this nuclide. Nevertheless, at a significant concentration of the rare 235 U isotope, these free neutrons can be captured by 235 U nuclei, which can actually cause fission, since in this case there is no energy threshold below which fission is not induced.
This is the principle of chain reaction.
Types of Nuclear Reactions
Let k be the number of neutrons produced in a sample of fissile material at stage n of this chain, divided by the number of neutrons produced at stage n - 1. This number will depend on how many neutrons obtained at stage n - 1 are absorbed by the nucleus, which may undergo forced division.
β’ If k <1, then the chain reaction will simply expire and the process will stop very quickly. This is exactly what happens in natural uranium ore, in which the concentration of 235 U is so low that the probability of absorption of one of the neutrons by this isotope is extremely negligible.
β’ If k> 1, then the chain reaction will grow until all the fissile material is used (atomic bomb). This is achieved by enriching natural ore to obtain a sufficiently high concentration of uranium-235. For a spherical sample, the value of k increases with increasing probability of neutron absorption, which depends on the radius of the sphere. Therefore, the mass U must exceed a certain critical mass so that the fission of uranium nuclei (chain reaction) can occur.
β’ If k = 1, then a controlled reaction takes place. It is used in nuclear reactors. The process is controlled by the distribution of cadmium or boron rods among uranium, which absorb most of the neutrons (these elements have the ability to capture neutrons). The fission of the uranium nucleus is automatically controlled by moving the rods so that the value of k remains equal to unity.