Introductory Chapter: Overview about the Types of Nuclear Reactions and Nuclear Reactors

1. Introduction

1.1 Types of radioactive decay series

The heavy radioactive elements are divided into four series, and the series is usually called the parent element. Three of these chains start with heavy, naturally occurring radioactive elements: thorium, uranium, and actinium. The fourth series begins with neptunium, which is an artificial element.

The reason for having only four groups is that as a result of alpha decay, there is a decrease in A by 4, while beta decay does not change the value of A.

The radioactive elements found in nature, as is known, have a longer half-life than neptunium. The radioactive decay of these elements is carried out by releasing α and β radiations, as they produce radioactive elements and are successively stable until reaching the stable element.

The series is arranged as follows:

• Thorium series (4n)

• Neptunium series (4n + 1)

• Uranium series (4n + 2)

• Actinium series (4n + 3)

1.2 Nuclear reactions

Rutherford showed in 1915 that nuclear reactions can be carried out by bombarding the nuclei of some substances with high-energy particles. Rutherford described the following nuclear reaction:

$${{}_7\mathrm{N}}^{14}{+}_2{\mathrm{He}}^4\to \left({}_9{\mathrm{F}}^{18}\right){\to }_8{\mathrm{O}}^{17}{+}_1{\mathrm{H}}^1+\mathrm{Q}$$E1

This reaction was considered the first nuclear reaction to take place in the laboratory, after which many nuclear reactions were conducted.

1.3 Types of nuclear reactions

1.4 Dispersion elastic

Elastic scattering occurs at all energies and for all particles. The event is called elastic scattering if the energy value of the particles does not change and if the sum of the kinetic energies of the projectile and the target nucleus remains constant.

Elastic scattering of charged particles with energies below the Coulomb barrier to the target nucleus is the famous Rutherford scattering experiment. At high energies, the probability of the projectile approaching the surface of the target nucleus and penetrating the Coulomb barrier increases, and thus, the nuclear forces contribute to elastic scattering. Elastic scattering of neutrons at all energies occurs only as a result of the influence of nuclear forces.

2. Types of energy at nuclear reactions

2.1 Low-energy nuclear reactions

These reactions are less than 40 Mev. The reaction of the disintegration of the nitrogen nucleus with alpha particles represents a series of reactions in which the nucleus captures an alpha particle, forming a compound nucleus that instantly disintegrates into a new nucleus with the emission of a proton. These interactions are known as (p, α) interactions.

Examples of these interactions that have been studied include:

$${\mathrm{B}}^{10}\left(\mathrm{\alpha },\mathrm{p}\right){\mathrm{C}}^{13}$$E2

$${\mathrm{Al}}^{27}\left(\mathrm{\alpha },\mathrm{p}\right){\mathrm{Si}}^{30}$$E3

$${\mathrm{S}}^{32}\left(\mathrm{\alpha },\mathrm{p}\right){\mathrm{Cl}}^{35}$$E4

$${\mathrm{K}}^{39}\left(\mathrm{\alpha },\mathrm{p}\right){\mathrm{Ca}}^{42}$$E5

When beryllium is bombarded with alpha particles, a neutron is emitted, and a new nucleus is formed, Be⁹ (α,n) C¹², and bombardment of beryllium with alpha particles and the subsequent emission of neutrons is one of the nuclear reactions of the type (α, n). Examples of these reactions are:

$${\mathrm{Li}}^7\left(\mathrm{\alpha },\mathrm{n}\right){\mathrm{B}}^{10}$$E6

$${\mathrm{N}}^{14}\left(\mathrm{\alpha },\mathrm{n}\right){\mathrm{F}}^{17}$$E7

$${\mathrm{Al}}^{27}\left(\mathrm{\alpha },\mathrm{n}\right){\mathrm{P}}^{30}$$E8

In many of these artificial nuclear reactions, the resulting nucleus is radioactive. In the last reaction, for example, the isotope P³⁰ is formed, which is unstable and breaks up by the emission of a positron (this reaction was known as the positron preparation experiment by Giulio and Curie in 1934 with a half-life of 2.5 minutes.

Protons can be used as projectile particles, as in the following reaction:

$${}_3{\mathrm{Li}}^7{+}_1{\mathrm{H}}^1\to \left({}_4{\mathrm{Be}}^8\right)\to 2_2{\mathrm{He}}^4+\mathrm{Q}$$E9

That is, the compound nucleus formed as a result of the capture of a proton by the lithium nucleus splits into two bodies of alpha particles, which move in almost opposite directions.

A very large number of reactions were seen, in which the deuterons (deuterium nuclei) were the projectiles. These interactions can be classified according to the type of particle. The particles that are emitted from the compound nucleus are formed as a result of the capture of deuterons. The use of deuterons as shells has led to the processes (d, p), (d, n), (d, α) with all the elements of the periodic table. One of the important interactions is the disintegration of sodium by bombarding deuterons. In this case, the reactions were seen:

$${}_{11}{\mathrm{Na}}^{23}{\left(\mathrm{d},\mathrm{n}\right)}_{12}{\mathrm{Mg}}^{24}$$E10

$${}_{11}{\mathrm{Na}}^{23}{\left(\mathrm{d},\mathrm{p}\right)}_{11}{\mathrm{Na}}^{24}$$E11

The reaction (d, n), the magnesium-24 nucleus formed in a stable state. But in the reaction (d, p), the radioactive sodium-24 is formed, which decays by emission of β-negative particles into Mg-24, which is left in an excited state and decays to the ground state by emission of two gamma-ray photons in two steps, as shown in the following Figure 1:

Figure 1.

Diagram shows how radioactive sodium-24 decays.

Since neutrons are free of electric charge, they are very effective in penetrating nuclei and causing nuclear transformations. Not only high-energy neutrons are able to penetrate the nucleus, but also slow neutrons are also very effective, and the reaction B¹⁰ (n, α) Li⁷ is used as a neutron-sensitive detector.

2.2 High-energy nuclear reactions

The 50 Mev is a boundary between low energy and high energy. The study of nuclear reactions that occur with high-energy missiles revealed several new types of processes, which requires a significant modification of the concept of the formation of the compound nucleus to explain some of these processes.

For example, if the reaction takes place:

With low-energy deuterons, a complex nucleus is formed, and neutrons are emitted from the target in all directions. But if high-energy deuterons are used as projectiles, the dominant direction of neutrons emitted with very high energy is the forward direction.

When precise targets such as beryllium, aluminum, copper, and others were bombarded with high-energy deuterons (more than 100 Mev), a narrow beam of neutrons was seen rushing in the forward direction with very high energy. The phenomenon was explained by the fact that the deuteron does not affect the nucleus but passes close to it, so the proton is extracted from it and the neutron continues to travel at approximately the same speed as the deuteron.

Due to the high speed of the deuteron, the interaction time between it and the nucleus is very small to the extent that the change in the neutron’s motion in this extraction process is very small, and this process represents an interaction (d, n).

We also refer here to a phenomenon related to extraction seen in low-energy cases in which the energy of the deuteron is lower than the Coulomb barrier, but it leads to a process (d, p). This phenomenon occurs when the deuteron passes close to the nucleus, at a distance between 3R–R, where R is the nuclear radius. Oppenheimer and Philips have interpreted it.

These results are arising from the polarization of the deuteron in the Columbian field of the nucleus. In this case, the columbic force arising between the target nucleus and the deuteron proton is sufficient to break the bond between it and the neutron. So the proton is repelled and the neutron is captured in the nucleus, and this process is a special type of (extraction) called the Oppenheimer process. Phillips and the reverse process of extraction is in which a high-energy projectile, such as a proton, neutron, or deuteron, captures another particle while passing near the nucleus [1].

These interactions differ from those in which a compound nucleus is formed in that the particle formed in the “capture” process has a large forward movement, while the particles emitted from the compound nucleus show a symmetrical movement distribution in all directions relative to the center of mass.

An example of a “capture” process is:

$${\mathrm{C}}^{13}\left(\mathrm{d},\mathrm{t}\right){\mathrm{C}}^{12}$$E12

Another important process is seen when high-energy particles collide with the target, and this process includes the emission of several nuclear parts, such as protons, neutrons, or alpha particles, and this process is known as “rupture.” When a target with a medium atomic weight is bombarded with high-energy particles, the products of a rupture usually include a wide range of mass numbers and atomic numbers ranging from 39 to 52 and mass numbers ranging from 124 to 87.

3. Types of nuclear reactions

3.1 Nuclear fission

Whereas, nuclear reactions are of the type of nuclear fission, fusion, or nuclear fusion.

In 1939, after a series of very precise chemical experiments, Hahn and Straßen found that one of the elements formed by bombarding uranium with neutrons is an isotope of the element Ba (Z = 56). Accordingly, the two scientists suggested that it is possible that the process that begins with bombarding uranium with neutrons is a process in which the formed uranium nucleus is considered unstable and splits into two nuclei of medium atomic mass. If one of the two nuclei formed is a barium nucleus (Z = 56), then the other must be a krypton nucleus (Z = 36). This type of fragmentation process in which the heavy nucleus splits into two nuclei close in mass is called nuclear fission. Within 2 years of the discovery of nuclear fission, the range of experiments expanded to include the fission of thorium and protactinium. Much more energy is released in the nuclear fission process than has been seen in any previous nuclear or atomic process.

In addition to the release of energy, the fission of uranium is accompanied by the emission of several neutrons, and it seems clear that it is possible to use those neutrons to create the appropriate conditions to cause nuclear fission of other uranium nuclei and then a chain reaction can be started that can release huge amounts of energy. The following figure represents an illustration of the fission of Uranium-235. The large energy released in such reactions can be used in power plants or nuclear weapons [2].

The fission of uranium-235 can be represented by the following equation:

$${}_{92}{\mathrm{U}}^{235}{+}_0{\mathrm{n}}^1{\to }_{92}{\mathrm{U}}^{236}{\to }_{56}{\mathrm{Ba}}^{191}{+}_{36}{\mathrm{Kr}}^{92}+3_0{\mathrm{n}}^1+\mathrm{Q}$$E13

where Q is the energy released in the reaction and is the difference in mass between the initial and final particles [3].

It can be caused by both slow and fast neutrons, and Bohr and Wheeler’s used the liquid-drop model of the nucleus in their theory of nuclear fission. According to this theory, nuclear fission occurs in two stages:

1. The complex nucleus in which energy is temporarily stored is distributed over the degrees of freedom of nuclear particles in a manner similar to thermal turbulence in a liquid.

2. A sufficient portion of this energy is converted into potential energy to change the shape of the compound nucleus in a way that leads to its fission.

The fission process can be depicted as shown in Figures 2 and 3. Where the drawing (a) depicts the compound nucleus with mass number A and atomic number Z as a liquid droplet in a state of high excitation, as a result of the forces acting between the nucleons, the liquid drop changes its shape as it is in (b). Then reaches a certain stage at which it takes the form shown in (c). This structure is unstable. If the excitation energy is sufficient, then the liquid drop is separated into two drops at some point in the thin neck and two nuclei are formed, as shown in (d).

Figure 2.

Diagram showing how uranium-235 is fission.

Figure 3.

Schematic diagram of nuclear fission.

3.1.1 Serial reaction

After the discovery of uranium and nuclear fission, scientists realized that a new source of energy could be easily available if more than one neutron was emitted for each fission neutron. The average number of neutrons released in fission has been determined by slow or thermal neutrons, and the accepted values are now 2.5 neutrons for each fission of a uranium 235 nucleus. The first generation of neutrons will lead to the production of four neutrons, and the second generation will lead to the fission of four uranium nuclei and produce eight neutrons, thus becoming 64, 32, 16 since these fission are fast. The fission product will be enormous, and if we know that each fission produces 200 Mev, the amount of energy will be enormous.

As a result of the emission of a neutron or more on average in each fission, a chain reaction may occur (i.e., it will continue on its own) in a block of fissionable material.

An atomic bomb is defined as a means designed to produce an increasingly explosive chain reaction modified to the point of explosion.

When the mass of the target sample is very small, a number of neutrons will not be absorbed. This situation leads us to define the so-called critical mass for any fissionable material, which is the smallest mass of fissionable material sufficient to continue the reaction.

In the event that the mass of the diffuse material is very large, i.e., supercritical mass, the number of fissile nuclei will double, and this is what is done in atomic bombs where, if necessary, two separate masses of fissile material can be combined. So that we obtain a supercritical mass, the chain reaction results in the fission of nuclei matter and gets a great deal of energy.

The system or group is called to arrange the fissionable and non-fissionable materials in it so that the serial reaction can proceed in a controlled manner in a nuclear reactor.

The nuclear reactor is a source of the products of the fission process, which are energy, neutrons, and radioactive isotopes. As we mentioned earlier, the energy released, which is in the range of 200 Mev for each fission, occurs for one atom, and this fission is considered a source of atomic energy on an industrial scale. As a neutron source, a nuclear reactor can supply us with a large number of neutrons per unit of time, distributed over a wide range of energies.

Since fission does not occur alone in heavy nuclei but captures the nucleus of the neutron in the reaction (n, α). It can, by capturing the neutron, produce many useful isotopes and convert the elements to each other, for example, the production of plutonium from uranium-238 or the production of C¹⁴ from N¹⁴ or the fissionable production of U²³³ from Th^232, and so on.

3.1.2 Nuclear fission reactor

A nuclear fission reactor is a machine by which a chain reaction can be controlled. In power plants, a nuclear reactor is used to obtain heat that can be used to generate electricity. Inside the reactor, fuel rods and control rods are arranged periodically. The fuel rods contain fissionable materials, and these rods differ from one reactor to another. In the case of a normal water-cooled reactor, the fuel rods consist of uranium oxide manufactured in the form of pellets placed in tubes of zirconium alloy. It is known that the uranium found in nature contains 99.3% of U²³⁸, and the rest is about 0.7% of U²³⁵, and since the latter is fissionable, it is necessary to enrich uranium-235 so that its percentage becomes about 3%. Control rods are usually made of boron, hardened boron, or other materials [4]. The rods absorb neutrons, and thus, the reaction can be calmed and controlled. This is done by changing the extent of the control rods compared to the fuel rods so it is possible to increase or decrease the absorption of neutrons or even stop the reaction, see Figure 4.

Figure 4.

Nuclear fission reactor.

As for the moderator, it is the substance that reduces the speed of neutrons, and high-speed neutrons are mostly absorbed by uranium-238, while slow neutrons are preferentially absorbed by uranium-235. The serial reaction can be kept going even if the U²³⁵ ratio is low. Heavy water, light water, and graphite coal can be used as moderators.

In the light water reactor, the water performs two tasks: cooling and cooling. In this reactor, the water temperature is about (350°) at a pressure of (150 atm). It is known that the water will not boil at this temperature as long as the pressure applied is what was mentioned. The heated water is transferred to a heat exchanger, where the heat is used to generate steam that drives the turbines and for this, we get electricity.

Spent fuel is a major problem for the nuclear industries. Chemical methods can be used to separate the components of spent fuel to obtain the required materials and use them again. However, the treatment plants are considered expensive and have advanced technology and therefore not all countries can obtain them; in addition to the fear of the countries that own, a technology that other countries will obtain materials such as plutonium-239 of the fuel used could be used as a nuclear weapon. This isotope is obtained when uranium-238 is bombarded with neutrons.

Plutonium-239 is similar to uranium-235 in that it is fissionable. Whether the spent fuel is treated or not, this fuel represents a major problem in power plants that rely entirely on nuclear energy because, over time, large quantities of nuclear waste accumulate, which poses a great danger to the environment, and therefore, it must be disposed of in one way or another. One of the proposed methods of disposal is burying it in abandoned mines, far and deep, while taking the necessary precautions to prevent its infiltration into the soil or water sources with approximately the same percentage, in addition to a small amount of energy obtained as a result of the fission of (Pu²³⁹), which was obtained from the previously mentioned reactions.

3.1.3 Breeder reactor

Plutonium-239 is fissionable, such as (U²³⁵), and the generator reactor works by reducing the amount of moderators. So, it produces some fast neutrons that convert (U²³⁵) to (Pu²³⁹), and in this way, the first available in nature can be converted into a source of energy from modern reactors. The generator reactor A fast breeder reactor does not use sedatives so that the largest amount of Pu can be produced. The latter gets to stop the generator reactor from time to time and treat the unspent fuel, and Pu can be used to produce fuel for the other reactors.

3.2 Nuclear fusion

There are serious attempts to obtain nuclear energy through nuclear fusion, which is the process by which two lighter nuclei are fused to obtain a heavier nucleus. This process is similar to the processes occurring in stars such as the sun. Fusion processes release more energy than they obtain from nuclear fission. In fusion reactions, very large activation energy must be provided in order for the merger to take place, and nuclear fission is used to provide such energy. The hydrogen bomb is based on nuclear fusion [5, 6].

The conversion of hydrogen to helium is preferred because the binding energy per nuclear particle is higher in helium than in hydrogen, and the conversion of hydrogen into helium is a fusion reaction, see Figure 5.

Figure 5.

Conversion of hydrogen to helium (carbon cycle).

It is believed that the energy emitted by the sun and other stars results from the conversion of hydrogen into helium in a series of nuclear reactions called the carbon cycle. These reactions take place in the interior of the sun, where the temperature reaches a very high point so that the atoms are completely stripped of their electronics.

3.2.1 Carbon Cycle

$${}_6{\mathrm{C}}^{12}{+}_1{\mathrm{H}}^1{\to }_7{\mathrm{N}}^{13}$$E14

$${}_7{\mathrm{N}}^{13}{\to }_6{\mathrm{C}}^{13}+{\mathrm{\beta }}^{+}$$E15

$${}_6{\mathrm{C}}^{13}{+}_1{\mathrm{H}}^1{\to }_7{\mathrm{N}}^{14}$$E16

$${}_7{\mathrm{N}}^{14}{+}_1{\mathrm{H}}^1{\to }_8{\mathrm{O}}^{15}$$E17

$${}_8{\mathrm{O}}^{15}{\to }_7{\mathrm{N}}^{15}+{\mathrm{\beta }}^{+}$$E18

$${}_7{\mathrm{N}}^{15}{+}_1{\mathrm{H}}^1{\to }_6{\mathrm{C}}^{12}{+}_2{\mathrm{He}}^4$$E19

The net reaction is the conversion of four hydrogen ions into a helium ion and two positrons:

$${}^1{\mathrm{H}}_1{\to }^2{\mathrm{He}}_4+2\mathrm{\beta }+\mathrm{E}$$E20

Since the net reaction belongs to hydrogen as a single reactant, this process has been referred to as (hydrogen combustion).

Nuclear fusion has an advantage over nuclear fission, and the successful development of a fusion reactor will provide a lasting solution to human energy problems. Because he has at his disposal impermeable fusion materials, the rate of energy production in the fusion reaction is higher, and it is not accompanied by the formation of radioactive products that pollute the environment.

Fusion reactions have been carried out in the laboratory using accelerators. For example, deuterons (heavy hydrogen nuclei) can be accelerated and directed toward targets containing H² itself or tritium H³ and the reactions are:

$${}_1{\mathrm{H}}^2{+}_1{\mathrm{H}}^2{\to }_2{\mathrm{He}}^3{+}_0{\mathrm{n}}^1\to$$E21

$${}_1{\mathrm{H}}^2{+}_1{\mathrm{H}}^3{\to }_2{\mathrm{He}}^4{+}_0{\mathrm{n}}^1$$E22

The kinetic energy of the projectile must be high to overcome the forces of repulsion with the positive charges in the nucleus and to interact with a great advantage, which is the availability of deuterium in large quantities in nature, i.e., about (0.015%) of ordinary hydrogen and nuclear accelerators cannot be used to complete fusion reactions for applied purposes due to the presence of many technical problems. At this temperature, the atoms can ionize completely, and we get what is called plasma.

Plasma is a homogeneous mixture of atomic nuclei, and electrons move very quickly and randomly. For this reason, it is necessary to develop fusion technology for the development of plasma research at high temperatures. It is for this reason that a magnetic field is used to prevent plasmas from contacting matter, and in recent years, lasers have been used to cause fusion processes.

4. Types of nuclear reactors

4.1 Graphite reactor

The first graphite reactor was built in the United States in 1943. The reactor core, which has a cubic capacity, consists of 24 feet a number of graphite pieces equipped with more than a thousand nuclear fuel channels. The fuel consists of uranium metal in the form of aluminum-coated rods. The fuel is cooled with air, and the graphite works as a reflector. The core is surrounded by a thick 7-foot concrete shield and equipped with channels for experiments and other control rods. This type of reactor is used to produce plutonium.

$${}_{92}{\mathrm{U}}^{238}{+}_0{\mathrm{n}}^1\to {}_{92}{\mathrm{U}}^{239}–––\mathrm{\beta }{\to }_{93}{\mathrm{Np}}^{239}–––\mathrm{\beta }{\to }_{94}{\mathrm{Pu}}^{239}$$E23

4.2 Uranium heavy water reactors

The reactor core consists of a cylindrical tank with a diameter of 2 meters and contains 7 tons of heavy water D₂O. The cross section of the absorption of neutrons is small, so the moderation of neutrons slows down a few, as this reactor is smaller than the graphite reactor and requires less fuel. The reactor is controlled by plates of cadmium. The reactor is cooled and heavy water is pumped between the reactor core and the heat exchanger.

4.3 Material testing reactors

This type of reactor is mainly used in examining and studying the behavior of residues in high levels of radiation and uses uranium enriched with a percentage of up to (93%) (U²³⁵) as fuel. Ordinary water is used as a lubricant and coolant at the same time. Beryllium is used as the first reflector, and graphite is used as a second reflector, with a thickness of 4 feet. The protective shield is made of concrete, with a thickness of 9 feet. The reactor is equipped with channels for checking the materials, and the control of the nuclear reactor is carried out with rods of cadmium.

4.4 Swimming pool reactors

This type of reactor is similar to material testing reactors with its low price and low complexity. The reactor core is suspended in a pool of water to a depth of 25 feet, and the water acts as a moderator, coolant, reflector, and protective shield. The fuel is composed of uranium enriched with uranium-235 at a rate of (93–20%), and the critical mass of this type of reactor is estimated at 2500 g U²³⁵; the control rods are cadmium and carbon carbide (Figure 6).

Figure 6.

Scheme of the swimming pool reactor.