Nuclear fission: the process of splitting the atomic nucleus. Nuclear Reactions

The article tells how nuclear fission is, how this process was discovered and described. It reveals its use as a source of energy and nuclear weapons.

"Indivisible" atom

The twenty-first century is replete with expressions such as "energy of the atom", "nuclear technology", "radioactive waste". Every now and then newspaper headlines show messages about the possibility of radioactive contamination of the soil, oceans, ice of the Antarctic. However, an ordinary person often does not very well imagine what kind of area of science and how it helps in everyday life. It is worth starting, perhaps, with history. From the very first question asked by a well-fed and dressed man, he was interested in how the world works. As the eye sees, why he hears the ear, than the water differs from the stone - that's what worried the sages from the beginning. Back in ancient India and Greece, some inquisitive minds assumed that there is a minimal particle (it was also called "indivisible") that possesses the properties of the material. Medieval chemists have confirmed the guess of the wise, 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 the atom

However, the development of technology (in particular, photography) led to the fact that the atom ceased to be considered the smallest possible particle of matter. And although a single atom is electrically neutral, the scientists quickly realized that it consists of two parts with different charges. The number of positively charged parts compensates for the number of negative particles, so the atom remains neutral. But there was no single-valued model of the atom. Since at that time classical physics still dominated, various assumptions were made.

Models of the Atom

At first, a "raisin-roll" model was proposed. The positive charge seemed to fill the entire space of the atom, and in it, like raisins in a loaf, negative charges were distributed. The famous experience of Rutherford determined the following: in the center of the atom is a very heavy element with a positive charge (nucleus), and around it there are much lighter electrons. 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 the classical physics accepted at that time. Therefore, a new, quantum mechanics was developed. With her appearance, the nonclassical period of science began.

Atom and radioactivity

From all that has been said 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 the energy and positions of the electrons in the orbit of the atom were well studied, it was time to understand the nature of the atomic nucleus. To the aid came the ingenious and unexpectedly open radioactivity. It helped to reveal the essence of the heavy central part of the atom, since the source of radioactivity is the fission of nuclei. At the turn of the nineteenth and twentieth centuries, the discoveries fell one after another. The theoretical solution of one problem caused the need to set new experiments. The results of the experiments 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 (as, for example, the Max Planck quantum). Even at the beginning of the era, scientists knew that uranium salts light a photosensitive film, but they did not suspect that nuclear fission was at the heart of this phenomenon. Therefore, radioactivity was studied in order to understand the nature of the decay of the nucleus. Obviously, the radiation was generated by quantum transitions, but it was not completely clear what exactly. The Curie couple mined clean radium and polonium, processing almost manually the uranium ore to get an answer to this question.

The charge of radioactive radiation

Rutherford did much to study the structure of the atom and contributed to the study of how the fission of the atomic nucleus takes place. The scientist placed the radiation emitted by the radioactive element in a magnetic field and received an amazing result. It turned out that the radiation consists of three components: one was neutral, and two others - positively and negatively charged. The study of nuclear fission began with the determination of its components. It was proved that the nucleus can divide, give away some of its positive charge.

The structure of the nucleus

Later it was found out that the atomic nucleus consists not only of positively charged particles of protons, but also neutral particles of neutrons. All together they are called nucleons (from the English "nucleus", the nucleus). However, scientists again came across 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. The next charge is followed by a charge of the nucleus +2 in the periodic table of helium, while its core 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. And some isotopes proved to be quite stable, while others quickly disintegrated, since nuclear fission was characteristic for them. To what principle did the number of nuclear stability nucleons correspond? Why was the addition of just one neutron to a heavy and completely stable nucleus led to its split, to the release of radioactivity? Strangely enough, the answer to this important question has not yet been found. It has been experimentally established that stable configurations of atomic nuclei correspond to certain amounts of protons and neutrons. If in the nucleus 2, 4, 8, 50 neutrons and / or protons, then the nucleus will be unambiguously stable. These numbers are even called magical (and they were named so by adult scientists, nuclear physicists). Thus, the fission of nuclei depends on their mass, that is, on the number of nucleons entering into them.

Drop, shell, crystal

Determine the factor that is responsible for the stability of the kernel, at the moment it was not possible. There are many theories of the model of the structure of the atom. The three most famous and developed ones often contradict each other in different matters. According to the first, the nucleus is a drop of a special nuclear liquid. Like water, it is characterized by fluidity, surface tension, fusion and decay. In the shell model in the core, too, there are certain energy levels that are filled with nucleons. The third assertion that the core is an environment that is capable of refracting special waves (de Broglie), while the refractive index is the potential energy. However, no model has so far been able to fully describe why, at a certain critical mass of this particular chemical element, the splitting of the nucleus 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, for what time interval from an initial number of a part there will be 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 the atomic nuclei 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 the beloved mother's vase is broken. However, some analogy is traced.

The energy of the atom

So far we have not answered the practical question: where does the energy take the fission of the nucleus? To begin with, we need to clarify that when a nucleus forms, special nuclear forces operate, which are called strong interactions. Since the core consists of a set of positive protons, the question remains as to how they stick together, because electrostatic forces should strongly repel them from each other. The answer is both simple and not: the nucleus is kept at the expense of a very fast exchange between the nucleons by special particles-pi-mesons. This connection lives incredibly little. As soon as the exchange of pions is stopped, the nucleus decays. It is also known that the core mass is smaller than the sum of all its constituent nucleons. This phenomenon was called the defect of the masses. In fact, the missing mass is the energy that is spent on maintaining the integrity of the core. As soon as a part of the atom nucleus is separated, this energy is released and converted to heat in nuclear power plants. That is, the energy of nuclear fission is a visual demonstration of Einstein's famous formula. Recall, the formula says: energy and mass can be converted into each other (E = mc ² ).

Theory and practice

Now let's talk about how this purely theoretical discovery is used in life to generate gigawatts of electricity. First, it must be noted that in controlled reactions, forced nuclear fission is used. Most often this is uranium or polonium, which is bombarded by fast neutrons. Secondly, one can not fail to understand that the fission of nuclei is accompanied by the creation of new neutrons. As a result, the number of neutrons in the reaction zone is able to grow very rapidly. Each neutron collides with new, still whole nuclei, splits them, which leads to an increase in the release of heat. This is the chain reaction of nuclear fission. Uncontrolled growth in the number of neutrons in the reactor can lead to an explosion. This is 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 fission rate of the nuclei can be slowed down by immersing the rods in the reaction zone. The nuclear reaction equation is made specifically for each active radioactive substance and its 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 initial nucleus and the particle (E1 + E2) should be equal to the energy of the resulting nucleus and the energy liberated in the free form (E3 + E4). The nuclear reaction equation also shows what substance is produced as a result of the decay. For example, for uranium U = Th + He, U = Pb + Ne, U = Hg + Mg. It is not shown isotopes of chemical elements, but this is important. For example, there are as many as three possibilities for fission of uranium, at which different isotopes of lead and neon are formed. In almost a hundred percent of the cases, the nuclear fission reaction yields radioactive isotopes. That is, the decay of uranium produces a radioactive thorium. Thorium is able to dissolve to protactinium, that - to actinia, and so on. Radioactive in this series can be both bismuth and titanium. Even hydrogen, which contains two protons in the nucleus (at the rate of 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 can seem historical and irrelevant. But at one time, every issue of news almost all over the world was accompanied by reports about how many types of nuclear weapons were invented and how to deal with it. People built underground bunkers and made supplies in the event of a nuclear winter. Whole families worked to establish asylum. Even the peaceful use of nuclear fission reactions can lead to catastrophe. It would seem that Chernobyl taught humanity the accuracy in this area, but the elements of the planet turned out to be stronger: the earthquake in Japan was damaged by the very reliable strengthening of the Fukushima nuclear power plant. The energy of the nuclear reaction is much easier to use for destruction. Technologists need only limit the power of the explosion, so as not to destroy the entire planet inadvertently. The most "humane" bombs, if they can be called so, do not pollute the neighborhood with radiation. In general, they most often use an uncontrolled chain reaction. What they are trying to avoid at nuclear power stations is bombarded in a very primitive way. For any naturally radioactive element, there is some critical mass of pure matter in which the chain reaction is generated by itself. For uranium, for example, it's 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 precisely on this principle: two unequal parts of pure uranium were simply connected and gave rise to a terrifying explosion. Modern weapons are probably more complex. However, it should not be forgotten about the critical mass: between small volumes of pure radioactive material during storage there must be barriers that do not allow to connect to parts.

Sources of radiation

All elements with an atomic nucleus charge greater than 82 are radioactive. Almost all lighter chemical elements have radioactive isotopes. The heavier the core, the less its life time. Some elements (such as california) can only be obtained artificially - by hitting heavy atoms with lighter particles, most often on accelerators. Since they are very unstable, they do not exist in the earth's crust: when they formed the planet, they quickly disintegrated into other elements. Substances with lighter nuclei, such as uranium, can be extracted. This long process, suitable for mining uranium even in very rich ores, contains less than one percent. The third way, perhaps, indicates that the new geological epoch has already begun. This is the extraction of radioactive elements from radioactive waste. After working off the fuel at a power station, on a submarine or aircraft carrier, a mixture of the initial uranium and the final substance, the result of the fission, is obtained. At the moment it is considered solid radioactive waste and it is an urgent question how to dispose of them so that they do not pollute the environment. However, it is likely that in the near future already prepared concentrated radioactive substances (for example, polonium) will be extracted from these wastes.