The article tells about what nuclear fission is, how this process was discovered and described. Its use as a source of energy and nuclear weapons is revealed.
"Indivisible" atom
The twenty-first century is replete with expressions such as "energy of the atom", "nuclear technology", "radioactive waste". Every now and then in newspaper headlines flash messages about the possibility of radioactive contamination of the soil, oceans, ice of Antarctica. However, an ordinary person often does not have a very good idea of what this field of science is and how it helps in everyday life. It is worth starting, perhaps, with history. From the very first question, which was asked by a well-fed and dressed person, he was interested in how the world works. How the eye sees, why the ear hears, how water differs from stone - this is what worried the wise men from time immemorial. Even in ancient India and Greece, some inquisitive minds suggested that there is a minimal particle (it was also called "indivisible") that has the properties of a material. Medieval chemists confirmed the guess of the sages, and the modern definition of the atom is as follows: an atom is the smallest particle of a substance that is the bearer of its properties.
Parts of an atom
However, the development of technology (inin particular, photography) has led to the fact that the atom is no longer considered the smallest possible particle of matter. And although a single atom is electrically neutral, scientists quickly realized that it consists of two parts with different charges. The number of positively charged parts compensates for the number of negative ones, so the atom remains neutral. But there was no unambiguous model of the atom. Since classical physics still dominated during that period, various assumptions were made.
Atom models
At first, the “raisin roll” model was proposed. The positive charge, as it were, filled the entire space of the atom, and negative charges were distributed in it, like raisins in a bun. The famous experiment of Rutherford determined the following: a very heavy element with a positive charge (the nucleus) is located in the center of the atom, and much lighter electrons are located around. The mass of the nucleus is hundreds of times heavier than the sum of all the electrons (it is 99.9 percent of the mass of the entire atom). Thus, Bohr's planetary model of the atom was born. However, some of its elements contradicted the then accepted classical physics. Therefore, a new, quantum mechanics was developed. With its appearance, the non-classical period of science began.
Atom and radioactivity
From all of the above, it becomes clear that the nucleus is a heavy, positively charged part of the atom, which makes up its bulk. When the quantization of energy and the positions of electrons in the orbit of an atom were well understood, it was time to understandthe nature of the atomic nucleus. The ingenious and unexpectedly discovered radioactivity came to the rescue. It helped to reveal the essence of the heavy central part of the atom, since the source of radioactivity is nuclear fission. At the turn of the nineteenth and twentieth centuries, discoveries rained down one after another. The theoretical solution of one problem necessitated new experiments. The results of the experiments gave rise to theories and hypotheses that needed to be confirmed or refuted. Often the greatest discoveries have come about simply because that is how the formula became easy to calculate (like, for example, Max Planck's quantum). Even at the beginning of the era of photography, scientists knew that uranium s alts light up a photosensitive film, but they did not suspect that nuclear fission was the basis of this phenomenon. Therefore, radioactivity was studied in order to understand the nature of nuclear decay. Obviously, the radiation was generated by quantum transitions, but it was not entirely clear which ones. The Curies mined pure radium and polonium, working almost by hand in uranium ore, to answer this question.
The charge of radioactive radiation
Rutherford did a lot to study the structure of the atom and contributed to the study of how the fission of the atom nucleus occurs. The scientist placed the radiation emitted by a radioactive element in a magnetic field and got an amazing result. It turned out that radiation consists of three components: one was neutral, and the other two were positively and negatively charged. The study of nuclear fission began with the definition of itscomponents. It was proved that the nucleus can divide, give up part of its positive charge.
Structure of the nucleus
Later it turned out that the atomic nucleus consists not only of positively charged particles of protons, but also of neutral particles of neutrons. Together they are called nucleons (from the English "nucleus", the nucleus). However, scientists again ran into a problem: the mass of the nucleus (that is, the number of nucleons) did not always correspond to its charge. In hydrogen, the nucleus has a charge of +1, and the mass can be three, and two, and one. Helium next in the periodic table has a nuclear charge of +2, while its nucleus contains from 4 to 6 nucleons. More complex elements can have many more different masses for the same charge. Such variations of atoms are called isotopes. Moreover, some isotopes turned out to be quite stable, while others quickly decayed, since they were characterized by nuclear fission. What principle corresponded to the number of nucleons of the stability of nuclei? Why did the addition of just one neutron to a heavy and quite stable nucleus lead to its splitting, to the release of radioactivity? Oddly enough, the answer to this important question has not yet been found. Empirically, it turned out that stable configurations of atomic nuclei correspond to certain amounts of protons and neutrons. If there are 2, 4, 8, 50 neutrons and/or protons in the nucleus, then the nucleus will definitely be stable. These numbers are even called magic (and adult scientists, nuclear physicists, called them that). Thus, the fission of nuclei depends on their mass, that is, on the number of nucleons included in them.
Drop, shell, crystal
It has not been possible at the moment to determine the factor responsible for the stability of the core. There are many theories of the model of the structure of the atom. The three most famous and developed often contradict each other on various issues. According to the first, the nucleus is a drop of a special nuclear liquid. Like water, it is characterized by fluidity, surface tension, coalescence and decay. In the shell model, there are also certain energy levels in the nucleus, which are filled with nucleons. The third states that the core is a medium that is capable of refracting special waves (de Broglie), while the refractive index is potential energy. However, no model has yet been able to fully describe why, at a certain critical mass of this particular chemical element, nuclear fission begins.
What breakups are like
Radioactivity, as mentioned above, was found in substances that can be found in nature: uranium, polonium, radium. For example, freshly mined, pure uranium is radioactive. The splitting process in this case will be spontaneous. Without any external influences, a certain number of uranium atoms will emit alpha particles, spontaneously converting into thorium. There is an indicator called the half-life. It shows for what period of time from the initial number of the part about half will remain. For each radioactive element, the half-life is different - from fractions of a second for California tohundreds of thousands of years for uranium and cesium. But there is also forced radioactivity. If the nuclei of atoms are bombarded with protons or alpha particles (helium nuclei) with high kinetic energy, they can "split". The mechanism of transformation, of course, is different from how mother's favorite vase is broken. However, there is a certain analogy.
Atom Energy
So far, we have not answered a practical question: where does the energy come from during nuclear fission. To begin with, it must be clarified that during the formation of a nucleus, special nuclear forces act, which are called the strong interaction. Since the nucleus is made up of many positive protons, the question remains how they stick together, because the electrostatic forces must push them away from each other quite strongly. The answer is both simple and not at the same time: the nucleus is held together by a very fast exchange between nucleons of special particles - pi-mesons. This connection lives incredibly short. As soon as the exchange of pi-mesons stops, the nucleus decays. It is also known for certain that the mass of a nucleus is less than the sum of all its constituent nucleons. This phenomenon is called the mass defect. In fact, the missing mass is the energy that is spent on maintaining the integrity of the nucleus. As soon as some part is separated from the nucleus of an atom, this energy is released and converted into heat in nuclear power plants. That is, the energy of nuclear fission is a clear demonstration of the famous Einstein formula. Recall that the formula says: energy and mass can turn into each other (E=mc2).
Theory and practice
Now we will tell you how this purely theoretical discovery is used in life to produce gigawatts of electricity. First, it should be noted that controlled reactions use forced nuclear fission. Most often it is uranium or polonium, which is bombarded by fast neutrons. Secondly, it is impossible not to understand that nuclear fission is accompanied by the creation of new neutrons. As a result, the number of neutrons in the reaction zone can increase very quickly. Each neutron collides with new, still intact nuclei, splits them, which leads to an increase in heat release. This is the nuclear fission chain reaction. An uncontrolled increase in the number of neutrons in a reactor can lead to an explosion. This is exactly what happened in 1986 at the Chernobyl nuclear power plant. Therefore, in the reaction zone there is always a substance that absorbs excess neutrons, preventing a catastrophe. It is graphite in the form of long rods. The rate of nuclear fission can be slowed down by immersing the rods in the reaction zone. The nuclear reaction equation is compiled specifically for each active radioactive substance and the particles bombarding it (electrons, protons, alpha particles). However, the final energy output is calculated according to the conservation law: E1+E2=E3+E4. That is, the total energy of the original nucleus and particle (E1 + E2) must be equal to the energy of the resulting nucleus and the energy released in free form (E3 + E4). The nuclear reaction equation also shows what kind of substance is obtained as a result of decay. For example, for uranium U=Th+He, U=Pb+Ne, U=Hg+Mg. The isotopes of the elements are not listed here.however, this is important. For example, there are as many as three possibilities for the fission of uranium, in which different isotopes of lead and neon are formed. In almost one hundred percent of cases, the nuclear fission reaction produces radioactive isotopes. That is, the decay of uranium produces radioactive thorium. Thorium can decay to protactinium, that to actinium, and so on. Both bismuth and titanium can be radioactive in this series. Even hydrogen, which contains two protons in the nucleus (at the rate of one proton), is called differently - deuterium. Water formed with such hydrogen is called heavy water and fills the primary circuit in nuclear reactors.
Unpeaceful atom
Such expressions as "arms race", "cold war", "nuclear threat" may seem historical and irrelevant to a modern person. But once upon a time, every news release almost all over the world was accompanied by reports about how many types of nuclear weapons were invented and how to deal with them. People built underground bunkers and stocked up in case of a nuclear winter. Entire families worked to build the shelter. Even the peaceful use of nuclear fission reactions can lead to disaster. It would seem that Chernobyl taught humanity to be careful in this area, but the elements of the planet turned out to be stronger: the earthquake in Japan damaged the very reliable fortifications of the Fukushima nuclear power plant. The energy of a nuclear reaction is much easier to use for destruction. Technologists only need to limit the force of the explosion, so as not to accidentally destroy the entire planet. The most "humane" bombs, if you can call them that, do not pollute the surroundings with radiation. In general, they most often useuncontrolled chain reaction. What they strive to avoid at nuclear power plants by all means is achieved in bombs in a very primitive way. For any naturally radioactive element, there is a certain critical mass of pure substance in which a chain reaction is born by itself. For uranium, for example, it is only fifty kilograms. Since uranium is very heavy, it is only a small metal ball 12-15 centimeters in diameter. The first atomic bombs dropped on Hiroshima and Nagasaki were made exactly according to this principle: two unequal parts of pure uranium simply combined and generated a terrifying explosion. Modern weapons are probably more sophisticated. However, one should not forget about the critical mass: there must be barriers between small volumes of pure radioactive material during storage, preventing the parts from connecting.
Radiation sources
All elements with a nuclear charge greater than 82 are radioactive. Almost all lighter chemical elements have radioactive isotopes. The heavier the nucleus, the shorter its lifetime. Some elements (such as California) can only be obtained artificially - by colliding heavy atoms with lighter particles, most often in accelerators. Since they are very unstable, they do not exist in the earth's crust: during the formation of the planet, they very quickly disintegrated into other elements. Substances with lighter nuclei, such as uranium, can be mined. This process is long, uranium suitable for extraction, even in very rich ores, contains less than one percent. third way,perhaps indicates that a new geological epoch has already begun. This is the extraction of radioactive elements from radioactive waste. After fuel is spent at a power plant, on a submarine or aircraft carrier, a mixture of the original uranium and the final substance, the result of fission, is obtained. At the moment, this is considered solid radioactive waste and there is an acute question of how to dispose of them so that they do not pollute the environment. However, it is likely that in the near future, ready-made concentrated radioactive substances (for example, polonium) will be mined from these wastes.
