The article talks 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 such expressions as “atomic energy”, “nuclear technology”, “radioactive waste”. Every now and then in newspaper headlines flicker messages about the possibility of radioactive contamination of soil, oceans, ice of the Antarctic. However, an ordinary person often does not very well imagine what kind of science this is and how it helps in everyday life. Perhaps it’s worth starting with history. From the very first question asked by a well-fed and well-dressed man, he was interested in how the world works. As the eye sees, why it hears the ear, how the water differs from the stone - this is what anciently worried the sages. Even in ancient India and Greece, some inquisitive minds suggested that there is a minimal particle (it was also called "indivisible"), which has the properties of a material. Medieval chemists have confirmed the wisdom of the sages, and the modern definition of an atom is as follows: an atom is the smallest particle of matter that carries its properties.
Atom parts
However, the development of technology (in 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: 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 an unambiguous atom model did not exist. Since classical physics still dominated at that time, various assumptions were made.
Atom models
Initially, the model was proposed "roll with raisins." The positive charge seemed to fill the entire space of the atom, and negative charges were distributed in it, like raisins in a bun. Rutherford's famous experience determined the following: a very heavy element with a positive charge (nucleus) is located in the center of the atom, and much lighter electrons are located around it. 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, the planetary model of the Bohr atom was born. However, some of its elements contradicted classical physics accepted at that time. 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 the heavy, positively charged part of the atom, which makes up its bulk. When the quantization of energy and electron positions in an atom’s orbit was well studied, it was time to understand the nature of the atomic nucleus. Brilliant and unexpectedly open radioactivity came to the rescue. She 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 century, discoveries rained down one after another. The theoretical solution of one problem made it necessary to set new experiments. The experimental results generated theories and hypotheses that needed to be confirmed or disproved. Often the greatest discoveries appeared simply because it was in this way that the formula became convenient for calculations (such as, for example, the Max Planck quantum). At the beginning of the era of photography, scientists knew: uranium salts illuminate the photosensitive film, but they did not suspect that the basis of this phenomenon is nuclear fission. Therefore, radioactivity was studied to understand the nature of nuclear decay. Obviously, the radiation was generated by quantum transitions, but it was not completely clear which ones. The Curie couple mined pure radium and polonium, processing uranium ore almost manually to get an answer to this question.
Radiation charge
Rutherford did a lot to study the structure of the atom and contributed to the study of how nuclear fission occurs. The scientist placed the radiation released by the 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 its components. It was proved that the core can divide, give away part of its positive charge.
Core structure
Later it turned out that the atomic nucleus consists not only of positively charged proton particles, but also neutral neutron particles. All together they are called nucleons (from the English "nucleus", nucleus). However, scientists again ran into a problem: the mass of the nucleus (i.e., the number of nucleons) did not always correspond to its charge. In hydrogen, the core has a charge of +1, and the mass can be three, two, or one. The helium next to it in the periodic table has a +2 nucleus charge, while its nucleus contains from 4 to 6 nucleons. More complex elements can have a much larger number of different masses with the same charge. Such variations of atoms are called isotopes. Moreover, some isotopes turned out to be quite stable, while others quickly decayed, since nuclear fission was characteristic of them. What principle did the number of nucleons of nuclear stability correspond to? Why did the addition of just one neutron to a heavy and quite stable nucleus lead to its split, to the release of radioactivity? Oddly enough, the answer to this important question has not yet been found. It was experimentally found out that stable configurations of atomic nuclei correspond to certain quantities 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 they were called by adult scientists, nuclear physicists). Thus, the fission of nuclei depends on their mass, that is, on the number of nucleons entering them.
Drop, shell, crystal
At the moment, it has not been possible to determine the factor that is responsible for the stability of the nucleus. There are many theories of atomic structure models . The three most famous and developed often contradict each other in different issues. According to the first, the core is a drop of special nuclear liquid. Like water, it is characterized by fluidity, surface tension, coalescence and decay. In the shell model in the nucleus, there are also certain energy levels that are filled with nucleons. The third states that the core is a medium that is capable of refracting specific waves (de Broglie), while the refractive index is potential energy. However, no model has yet been able to fully describe why, with a certain critical mass of this particular chemical element, nuclear fission begins.
What is the breakdown
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 transforming into thorium. There is an indicator called the half-life. It shows how much time from the initial number of the part will remain approximately half. For each radioactive element, its half-life is from fractions of a second for California to hundreds of thousands of years for uranium and cesium. But there is also forced radioactivity. If atomic nuclei are bombarded with protons or alpha particles (helium nuclei) with high kinetic energy, then they can “crack”. The transformation mechanism, of course, is different from how the beloved mother's vase breaks. However, a certain analogy can be traced.
Atom energy
So far, we have not answered a practical question: where does energy come from when fissioning a nucleus? First you need to clarify that when a nucleus is formed, special nuclear forces act, which are called strong interactions. Since the nucleus consists of many positive protons, the question remains how they stick together, because the electrostatic forces should push them apart quite strongly. The answer is both simple and no: the nucleus is supported by a very fast exchange between nucleons of special particles - pi mesons. This connection lives incredibly little. As soon as the exchange of pi-mesons ceases, the nucleus decays. It is also known for sure that the mass of the nucleus is less than the sum of all its nucleons. This phenomenon is called 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 is converted into heat at nuclear power plants. That is, the nuclear fission energy is a clear demonstration of the famous Einstein formula. Recall that the formula says: energy and mass can turn into each other (E = mc 2 ).
Theory and practice
Now we will tell how this purely theoretical discovery is used in life to produce gigawatts of electricity. First, it should be noted that in controlled reactions, forced fission of nuclei is used. Most often it is uranium or polonium, which is bombarded by fast neutrons. Secondly, one cannot fail 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 grow very quickly. Each neutron collides with new, still whole nuclei, splits them, which leads to an increase in heat generation. This is a chain reaction of nuclear fission. 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. This is graphite in the form of long rods. The speed of nuclear fission can be slowed by immersing the rods in the reaction zone. The nuclear reaction equation is compiled specifically for each active radioactive substance and its bombarding particles (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 the particle (E1 + E2) should be equal to the energy of the resulting nucleus and the energy released in free form (E3 + E4). The nuclear reaction equation also shows which substance is produced by decay. For example, for uranium, U = Th + He, U = Pb + Ne, U = Hg + Mg. The isotopes of chemical elements are not shown here, but this is important. For example, there are three possibilities for uranium fission, in which various isotopes of lead and neon are formed. In almost a 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, one to actinium, and so on. Both bismuth and titanium can be radioactive in this series. Even hydrogen containing two protons in the nucleus (normally one proton) is called differently - deuterium. The water formed with such hydrogen is called heavy and fills the first circuit in nuclear reactors.
Non-peaceful atom
Such expressions as “arms race”, “cold war”, “nuclear threat” to modern man may seem historical and irrelevant. But once every news release almost around the world was accompanied by reports on how many types of nuclear weapons were invented and how to deal with them. People built underground bunkers and stockpiled in the event of a nuclear winter. Entire families worked to create shelter. Even the peaceful use of nuclear fission reactions can lead to disaster. It would seem that Chernobyl taught mankind neatness 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, most often they use an uncontrolled chain reaction. What nuclear forces are striving to avoid with all their might are achieved in bombs in a very primitive way. For any naturally radioactive element, there is a certain critical mass of pure substance in which the chain reaction nucleates 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 according to this principle: two unequal parts of pure uranium simply connected and gave rise to a terrifying explosion. Modern weapons are probably more complex. However, you should not forget about the critical mass: between small volumes of pure radioactive material during storage there should be barriers that do not allow the parts to connect.
Radiation sources
All elements with an atomic nucleus charge greater than 82 are radioactive. Almost all of the lighter chemicals have radioactive isotopes. The heavier the core, the shorter its lifetime. Some elements (such as California) can only be obtained artificially - by colliding heavy atoms with lighter particles, most often at accelerators. Since they are very unstable, they do not exist in the earth's crust: when the planet was formed, they very quickly broke up into other elements. Substances with lighter nuclei, such as uranium, may well be mined. This process is long, and uranium mining, even in very rich ores, contains less than one percent. The third path, perhaps, indicates that a new geological era has already begun. This is the extraction of radioactive elements from radioactive waste. After the fuel is exhausted at a power plant, on a submarine or aircraft carrier, a mixture of the initial uranium and the final substance, the result of the division, is obtained. At the moment, it 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 extracted from these wastes.