By March 1939, teams of scientists working in France and America had proven that an average of two to four free neutrons were released for a self-sustaining chain reaction with every fission of a uranium nucleus. The growing fears about the possibility of creating an atomic bomb, however, were quickly dispelled.

Bohr decided not to waste time. The physics of fission, like any other new direction in science, undoubtedly provided an immense field for activity. And, since Princeton could work with no less success than in Copenhagen, Bohr approached Wheeler with an offer of cooperation. They set about further development of the theory of nuclear fission, relying on new experimental data. They carried out the experiments with a device that was hastily assembled right there, in Princeton, in the attic of the Palmer laboratory. The results were puzzling at first.

The apparatus mentioned above was needed to study changes in the intensity of fission of a uranium nucleus under the influence of neutrons, each time carrying different amounts of energy. It was found that the greater this energy, the more intense the fission occurs, and with its decrease, the fission intensity, accordingly, also decreases. Such data was quite expected. However, it soon became clear that with a sufficient decrease in the neutron energy, the intensity of nuclear fission increases again.

Placzek, who had previously forced Frisch, who worked in Copenhagen, to seek reliable confirmation of nuclear fission, quite unexpectedly found himself in Princeton. "What the hell is this: why the response is the same for both fast and slow impacts ?!" - he was indignant, sitting at breakfast with Rosenfeld and Bohr.

Returning soon to his office, Niels Bohr already knew the answer to this question. Apparently, the reason for the high intensity of nuclear fission at low energies of the acting neutrons is the rare isotope uranium-235 (U 235), which makes up a negligible percentage of the total amount of this element found in nature. Bohr and Wheeler now began to elaborate on this hypothesis. And in the new theory, two fundamental factors were established.

In the U 235 isotope, the balance between the repulsive force of protons in the atomic nucleus and the surface tension that keeps the nucleus from decay is much more fragile than in the U 238 isotope. Three additional neutrons from uranium-238 stabilize the nucleus and increase the energy barrier that must be overcome in order to trigger the decay reaction. Consequently, for the splitting of such a nucleus, faster neutrons with higher energy are needed.

The second of the factors mentioned was the complex composition of the nucleus itself. For him, an equal number of protons and neutrons is more favorable, which is explained by the quantum nature of their subatomic constituents. Having received an additional neutron, U 235 turns into U 236, in the nucleus of which there are 92 protons and 144 neutrons, that is, an even number of both nucleons. When U 238 receives an extra neutron, the isotope U 239 is formed with an odd number of neutrons in the nucleus. Uranium-235 "assimilates" an additional neutron and reacts with it much easier than uranium-238.

The combination of the above two factors sufficiently explains the significant difference in the behavior of the two isotopes of uranium. Fission of the stable U 238 nucleus requires fast neutrons, while the much less stable U 235 nucleus can be fissioned slowly. Thus, if a bomb is made consisting of a mixture of U 235 and U 238, the action of which will be based on the fission of uranium-235 under the influence of slow neutrons, then the chain reaction in it will proceed slowly. Then it will die out, and the bomb will never detonate.

Now the chances of creating a bomb in the near future, although not completely disappeared, have significantly decreased. Of course, we must not forget about the words of Bohr, which he repeatedly repeated during discussions with colleagues in April 1939: then he said that to make a bomb can provided that it is made on the basis of pure uranium-235. However, U 235 is a rare isotope and its share in relation to natural uranium is 1: 140, that is, an insignificant 0.7%. In addition, U 235 and U 238 are identical in chemical properties, and therefore cannot be separated using a chemical reaction. This is possible only with the use of special physical methods that make it possible to separate isotopes from each other using an almost imperceptible difference in their mass. At the same time, such work on the scale necessary to create an atomic bomb required unreasonably large efforts - at the then level of development, it required several tons of uranium-235.

The twentieth century has given so many discoveries into the hands of Mankind! For many of them, the goal was to make life easier for a higher being on planet Earth, but reality, as always, is deceptive and human egoism sometimes surpasses simple concepts of good and evil. Selfishness does not let the feeling of superiority, power over the world fall asleep, and the greatest discoveries are on the path of destruction. The initial stage in the discovery of the fission of the most destructive substance on Earth was the rapid development of industry, which required huge amounts of energy - and this energy was found! German scientists Otto Hahn and Fritz Strassmann discovered an amazing phenomenon: the fission of a uranium nucleus (U) when bombarded with neutrons (n), while in the process of fission a huge amount of energy was released per atom of matter (about 202.5 MeV = 3.24 * 10 -11 J), as well as another 2-3 neutrons, which interacted with neighboring nuclei. But it was not possible to use such fuel - the reaction in the uranium sample was rapidly dying out for unknown reasons. Later it was found that one of the isotopes, namely uranium 238, which, upon absorption of a neutron (n), does not emit new neutrons during fission, has a negative effect on the course of the reaction. but uranium isotope 235 has the ability to reproduce.
A great discovery was the process of spontaneous fission of the uranium 235 nucleus. About 20 spontaneous fissions occur in 1 gram of metal per hour, but a chain reaction does not occur, and why? The answer to this question is quite trivial - neutrons miss in a sufficiently small volume of matter and leave the metal without interaction. By means of calculations, the minimum mass of the uranium 235 sample was found, which was about 48 kilograms. In such a sample - a ball 25 cm in diameter, the reaction should not have died out. But how to isolate the isotope of uranium 235? Let's try to answer this question.
Natural uranium is a silvery metal that is easy to machine and has a melting point of 1130 degrees Celsius. Uranium oxidizes well in air and ignites in the atmosphere at a temperature of 100 degrees Celsius, is very poisonous, and is a source of hard alpha and beta radiation. Natural uranium is composed of several isotopes:
Uranus 235 - 0,7184%;
Uranium 238 - 99.2760%;
Uranium 234 - 0.0056%.
Only isotope with mass number 235 is suitable for industrial use, the rest are "garbage". It is not so easy to isolate the required isotope: the main way to obtain enriched uranium 235 is to pump uranium fluoride through a system of centrifuges, in which the heavier isotope settles on the walls, and the 235th passes through. In this way, an enrichment of up to 99% can be obtained.
Industrial uranium 235 is mainly used as a fuel for power plants, but this metal was originally used for military purposes as the most powerful explosive on Earth. The consequences of the military use of uranium 235 made a great contribution precisely to the peaceful development of the energy of the atomic nucleus. The energy released by 1 gram of uranium is comparable to burning 2.5 tons of oil. The benefit is obvious - the use of metal as a fuel makes it possible to reduce the extraction of minerals and move to the level of "clean energy", provided that reliable emergency systems for the operation of the reactor are designed and the reactor itself is of high quality. The reactor is the main part of a nuclear power plant (nuclear power plant), in it the process of fission of the nuclei of a substance and the transfer of energy to the coolant directly take place. The heat carrier transfers energy to the turbine, which, in turn, generates electrical energy. The heat carrier can be various substances with high heat capacity: water, inert gases, liquid alkali metals.
At present, the energy of uranium 235 is used to generate electrical energy, but the reserves of the metal on Earth are limited and, according to scientists, they will only last for 50 years of intensive use. And it is in our interests to save electrical energy, which is so difficult for us to get from Nature!

Studying the phenomenon of radioactivity, each scientist turns to such an important characteristic as the half-life. As you know, it says that every second in the world there is a disintegration of atoms, while the quantitative characteristics of these processes is directly related to the number of atoms available. If, over a certain period of time, half of the total available number of atoms decays, then the decay of ½ of the remaining atoms will take the same amount of time. It is this time period that is called the half-life. It is different for different elements - from thousandths of a millisecond to billions of years, as, for example, when it comes to the half-life of uranium.

Uranium, as the heaviest element in the natural state on Earth, is generally the most excellent object for studying the process of radioactivity. This element was discovered back in 1789 by the German scientist M. Klaproth, who named it in honor of the recently discovered planet Uranus. The fact that uranium is radioactive was completely accidentally discovered at the end of the 19th century by the French chemist A. Becquerel.

Uranium is calculated using the same formula as for similar periods of other radioactive elements:

T_ (1/2) = au ln 2 = frac (ln 2) (lambda),

where "au" is the average lifetime of an atom, "lambda" is the main decay constant. Since ln 2 is about 0.7, the half-life is only 30% shorter on average than the total lifetime of the atom.

Despite the fact that today scientists know 14 isotopes of uranium, only three of them are found in nature: uranium-234, uranium-235 and uranium-238. uranium is different: so for U-234 it is "only" 270 thousand years, and the half-life of uranium-238 exceeds 4.5 billion. The half-life of uranium-235 is in the "golden mean" - 710 million years.

It should be noted that the radioactivity of uranium in natural conditions is quite high and allows, for example, to light up photographic plates for only an hour. At the same time, it should be noted that of all uranium isotopes, only U-235 is suitable for making filling for the whole thing is that the half-life of uranium-235 under industrial conditions is less intense than its "counterparts", therefore, the yield of unnecessary neutrons here is minimal.

The half-life of uranium-238 is well over 4 billion years, but it is now actively used in the nuclear industry. So, in order to start a chain reaction for the fission of heavy nuclei of this element, a significant amount of neutron energy is needed. Uranium-238 is used as a shield in fission and fusion apparatus. However, most of the uranium-238 mined is used to synthesize plutonium for use in nuclear weapons.

Scientists use the length of the half-life of uranium in order to calculate the age of individual minerals and celestial bodies in general. A uranium clock is a fairly versatile mechanism for this kind of calculations. At the same time, in order for the age to be calculated more or less accurately, it is necessary to know not only the amount of uranium in certain rocks, but also the ratio of uranium and lead as the final product into which uranium nuclei are converted.

There is another way of calculating rocks and minerals, it is associated with the so-called spontaneous. As you know, as a result of spontaneous fission of uranium in natural conditions, its particles with tremendous force bombard nearby substances, leaving behind special traces - tracks.

It is by the number of these tracks, knowing at the same time the half-life of uranium, that scientists conclude about the age of this or that solid body - be it an ancient breed or a relatively "young" vase. The thing is that the age of an object is directly proportional to the quantitative index of uranium atoms, the nuclei of which bombarded it.

Mr. Winsor's main field of activity was public lectures, for which he traveled throughout the country, speaking on the radio and even shooting small films in which he tried to educate Americans about a global conspiracy in the global nuclear industry.

The task of the conspiracy is to intimidate people with radiation as much as possible so that a small group of unknown persons can freely dispose of the most valuable energy resource in the world. And so that the word does not differ from the deed, Mr. Winsor shot an amazing film reflecting his lecture, recorded in 1986.

In addition to such demonstrations, during the construction of his house, Mr. Winsor poured such an amount of radioactive material into the concrete that the Geiger counter broke from overload on the approach to the building. And despite all this, Galen Winsor lived to an old age in good health, dying at 82 from natural causes for his age, which had nothing to do with radiation.

“Why is there a worldwide uranium conspiracy? “- asks Galen Winsor. And he answers it:

At its core, the federal nuclear material control law is about maintaining power and controlling the masses by rejecting self-sufficient sources of energy. Obviously, if someone had a small source of energy that was cheap and efficient, this person would be independent, he would not need to be connected to some kind of "power grid". The power grid is not a power supply system, but a veritable control network that our rulers use to keep us in check.

Galen Winsor gave an example of simple nuclear waste, which have accumulated in the world in kilotons. Moreover, each ton costs $ 10 million at 1986 prices. They can be used to re-produce fuel uranium from them, but precious isotopes are deliberately kept underground, thereby creating exorbitant prices for nuclear fuel on the world market.

To this we must add the planetary cut of the dough into the "disposal", "transportation" and "storage" of nuclear materials, which could well be transported by ordinary transport and stored in ordinary buildings. But instead, the government drills basalt holes for them, in the process of which a lot of money evaporates.

However, the most important thing is that not even highly enriched uranium, but all this nuclear waste through thermionic conversion back in the mid-1950s could be turned into relatively eternal portable sources of free energy, one of which would be enough for the entire life of an average American family. Such sources have been powering the navigation network of US submarines for years, which would be enough for 50 years of lighting and heating a house, washing machines, televisions and refrigerators. But then people would become independent from energy companies, which is unacceptable for any government.

The Big The One's editorial commentary: Mr. Galen Winsor's fantastic experiences on himself are really impressive, but one cannot fail to notice that radiation sickness as a nosological unit still exists, although it is impossible to say for sure whether it is really caused by uranium? Nevertheless, the very topic of a conspiracy in nuclear energy is very interesting, since there are certain conspiracy theories on this subject.

In particular, many people believe that nuclear power plants built everywhere are nothing more than dummies. In reality, there is some small secret device inside the power unit, which produces electrical energy, part of which is allowed to boil the "cooler" and spin the fake "generators" and "turbines". Nobody can prove that this is exactly how it is at a nuclear power plant, since there even personnel only go where they are allowed according to the admission rules, which gives rise to even more suspicion among conspiracy theorists, but it is possible that Mr. Galen Winsor sheds light on this situation as well. ...

The second interesting point in the light of the uranium conspiracy is the state control over such a resource as mercury, the danger of which is not just exaggerated, but sucked out of thin air. Mercury can be drunk, which was actively used in the 19th century to treat intestinal obstruction. Nevertheless, free circulation of this metal is prohibited even in North Korea and Honduras. The question arises: why?

We do not know the answer to this question. But what we know, and what Mr. Galen Winsor said quite rightly, is that the state needs complete control over the herd and the main instruments of control are energy and food.

In the late 1940s and early 1950s, a group of Leningrad biologists who survived the blockade set about creating a special culture of yeast that could, like plants, synthesize carbohydrates and proteins from the air when small amounts of light and electricity were fed into the substrate. And although it was almost a private project, that is, people took only equipment and reagents from the state, there were no investments, biologists successfully solved the problem.

The microorganism they produced was producing almost tons of free protein, the prospect of which was breathtaking. It was the ideal feed for poultry and livestock, which would fill a starving post-war country with meat. And with continued development, it would be possible to obtain more advanced products, to release the necessary sugars, lipids and amino acids, creating almost free food briquettes for people.

The delighted scientists ran with the results to report to the Politburo, imagining that now they would be given the Lenin Prize directly and allowed to build factories for the country. But ... for the initiative, the laboratory was dispersed and the topic was buried. Again the question arises: why?

The answer to it is the same as to the question why in the huge USSR, which occupies 1/6 of the land, people were given land plots of 10 acres (6 in the southern regions). The land is the source of food, and food is the basis of independence, which is unacceptable for the “herd”.

Thus, although we can neither refute nor confirm the nutritional value of enriched uranium, we know one thing for sure: there have been no portable sources of cheap energy in this world for a long time, no, and, most likely, it will not be for a very long time. And what is most surprising: all the governments of all countries are at the same time on this issue, as if another government of some kind controls all these governments.

Fission of uranium nuclei was discovered in 1938 by German scientists O. Hahn and F. Strassmann. They were able to establish that when uranium nuclei are bombarded with neutrons, elements of the middle part of the periodic system are formed: barium, krypton, etc. This fact was correctly interpreted by the Austrian physicist L. Meitner and the English physicist O. Frisch. They explained the appearance of these elements by the decay of uranium nuclei, which captured a neutron, into two approximately equal parts. This phenomenon is called nuclear fission, and the resulting nuclei are called fission fragments.

see also

1. Vasiliev A. Fission of uranium: from Klaproth to Ghana // Kvant. - 2001. - No. 4. - S. 20-21.30.

Drip core model

This fission reaction can be explained based on the droplet model of the nucleus. In this model, the nucleus is considered as a drop of an electrically charged incompressible liquid. In addition to the nuclear forces acting between all the nucleons of the nucleus, the protons experience additional electrostatic repulsion, as a result of which they are located at the periphery of the nucleus. In an unexcited state, the forces of electrostatic repulsion are compensated, therefore the core has a spherical shape (Fig. 1, a).

After the capture of a neutron by the nucleus \ (~ ^ (235) _ (92) U \) an intermediate nucleus \ (~ (^ (236) _ (92) U) ^ * \) is formed, which is in an excited state. In this case, the neutron energy is evenly distributed between all nucleons, and the intermediate nucleus itself is deformed and begins to vibrate. If the excitation is small, then the nucleus (Fig. 1, b), freeing itself from excess energy by emission γ -quantum or neutron, returns to a steady state. If the excitation energy is high enough, then the deformation of the nucleus during oscillations can be so large that a constriction is formed in it (Fig. 1, c), similar to the constriction between two parts of a splitting liquid droplet. Nuclear forces acting in a narrow waist can no longer withstand the significant Coulomb force of repulsion of parts of the nucleus. The constriction breaks, and the nucleus splits into two "fragments" (Fig. 1, d), which fly away in opposite directions.

uran.swf Flash: Fission of uranium Enlarge Flash Fig. 2.

At present, about 100 different isotopes with mass numbers from about 90 to 145 are known, arising from the fission of this nucleus. Two typical fission reactions of this nucleus are:

\ (~ ^ (235) _ (92) U + \ ^ 1_0n \ ^ (\ nearrow) _ (\ searrow) \ \ begin (matrix) ^ (144) _ (56) Ba + \ ^ (89) _ ( 36) Kr + \ 3 ^ 1_0n \\ ^ (140) _ (54) Xe + \ ^ (94) _ (38) Sr + \ 2 ^ 1_0n \ end (matrix) \).

Note that neutron-initiated fission produces new neutrons that can trigger fission reactions in other nuclei. Fission products of uranium-235 nuclei can also be other isotopes of barium, xenon, strontium, rubidium, etc.

During the fission of nuclei of heavy atoms (\ (~ ^ (235) _ (92) U \)), a very large energy is released - about 200 MeV during the fission of each nucleus. About 80% of this energy is released in the form of the kinetic energy of the fragments; the remaining 20% ​​is accounted for by the energy of radioactive radiation from fragments and the kinetic energy of prompt neutrons.

The energy released during nuclear fission can be estimated using the specific binding energy of nucleons in the nucleus. Specific binding energy of nucleons in nuclei with mass number A≈ 240 of the order of 7.6 MeV / nucleon, while in nuclei with mass numbers A= 90 - 145 the specific energy is approximately equal to 8.5 MeV / nucleon. Consequently, fission of a uranium nucleus releases an energy of the order of 0.9 MeV / nucleon, or approximately 210 MeV per uranium atom. With the complete fission of all the nuclei contained in 1 g of uranium, the same energy is released as in the combustion of 3 tons of coal or 2.5 tons of oil.

see also

1. Varlamov A.A. Drip model of the nucleus, Kvant. - 1986. - No. 5. - P. 23-24

Chain reaction

Chain reaction- a nuclear reaction in which the particles causing the reaction are formed as products of this reaction.

When the uranium-235 nucleus fission, which is caused by a collision with a neutron, 2 or 3 neutrons are released. Under favorable conditions, these neutrons can enter other uranium nuclei and cause their fission. At this stage, from 4 to 9 neutrons will appear, capable of causing new decays of uranium nuclei, etc. Such an avalanche-like process is called a chain reaction. The scheme of the development of the chain reaction of fission of uranium nuclei is shown in Fig. 3.

reakcia.swf Flash: chain reaction Enlarge Flash Fig. 4.

Uranium occurs in nature in the form of two isotopes \ [~ ^ (238) _ (92) U \] (99.3%) and \ (~ ^ (235) _ (92) U \) (0.7%). When bombarded with neutrons, the nuclei of both isotopes can split into two fragments. In this case, the fission reaction \ (~ ^ (235) _ (92) U \) proceeds most intensively on slow (thermal) neutrons, while the nuclei \ (~ ^ (238) _ (92) U \) enter into the reaction fission only with fast neutrons with energies of the order of 1 MeV. Otherwise, the excitation energy of the formed nuclei \ (~ ^ (239) _ (92) U \) turns out to be insufficient for fission, and then instead of fission, nuclear reactions take place:

\ (~ ^ (238) _ (92) U + \ ^ 1_0n \ to \ ^ (239) _ (92) U \ to \ ^ (239) _ (93) Np + \ ^ 0 _ (- 1) e \ ).

Uranium isotope \ (~ ^ (238) _ (92) U \) β - radioactive, half-life 23 min. The isotope of neptunium \ (~ ^ (239) _ (93) Np \) is also radioactive, with a half-life of about 2 days.

\ (~ ^ (239) _ (93) Np \ to \ ^ (239) _ (94) Pu + \ ^ 0 _ (- 1) e \).

The isotope of plutonium \ (~ ^ (239) _ (94) Np \) is relatively stable, with a half-life of 24,000 years. The most important property of plutonium is that it fissions under the influence of neutrons in the same way as \ (~ ^ (235) _ (92) U \). Therefore, with the help of \ (~ ^ (239) _ (94) Np \) a chain reaction can be carried out.

The chain reaction scheme discussed above is an ideal case. Under real conditions, not all neutrons produced during fission participate in the fission of other nuclei. Some of them are captured by the non-fissioning nuclei of foreign atoms, while others fly out of the uranium (neutron leakage).

Therefore, the chain reaction of fission of heavy nuclei does not always occur and not for any mass of uranium.

Neutron multiplication factor

The development of a chain reaction is characterized by the so-called neutron multiplication factor TO, which is measured by the ratio of the number N i neutrons, causing the fission of nuclei of matter at one of the stages of the reaction, to the number N i-1 neutrons that caused fission at the previous stage of the reaction:

\ (~ K = \ dfrac (N_i) (N_ (i - 1)) \).

The multiplication factor depends on a number of factors, in particular on the nature and amount of fissile matter, on the geometric shape of the volume it occupies. The same amount of a given substance has a different meaning. TO. TO maximum, if the substance has a spherical shape, since in this case the loss of prompt neutrons through the surface will be the smallest.

The mass of fissile material in which the chain reaction proceeds with the multiplication factor TO= 1 is called the critical mass. In small pieces of uranium, most of the neutrons, without hitting any nucleus, fly out.

The value of the critical mass is determined by the geometry of the physical system, its structure and external environment. So, for a ball of pure uranium \ (~ ^ (235) _ (92) U \), the critical mass is 47 kg (a ball with a diameter of 17 cm). The critical mass of uranium can be reduced many times by using so-called neutron moderators. The fact is that neutrons produced during the decay of uranium nuclei have too high velocities, and the probability of capturing slow neutrons by uranium-235 nuclei is hundreds of times greater than that of fast ones. The best neutron moderator is heavy water D 2 O. When interacting with neutrons, ordinary water itself turns into heavy water.

Graphite, the nuclei of which does not absorb neutrons, is also a good moderator. In elastic interaction with deuterium or carbon nuclei, neutrons are slowed down to thermal velocities.

The use of neutron moderators and a special beryllium shell that reflects neutrons makes it possible to reduce the critical mass to 250 g.

With a multiplication factor TO= 1 the number of fissile nuclei is kept constant. Such a regime is provided in nuclear reactors.

If the mass of nuclear fuel is less than the critical mass, then the multiplication factor TO < 1; каждое новое поколение вызывает все меньшее и меньшее число делений, и реакция без внешнего источника нейтронов быстро затухает.

If the mass of nuclear fuel is greater than the critical value, then the multiplication factor is TO> 1 and each new generation of neutrons causes an increasing number of fissions. The chain reaction grows like an avalanche and has the character of an explosion, accompanied by a huge release of energy and an increase in the ambient temperature up to several million degrees. A chain reaction of this kind occurs when an atomic bomb explodes.

Nuclear bomb

In its normal state, a nuclear bomb does not explode because the nuclear charge in it is divided into several small parts by partitions that absorb the decay products of uranium - neutrons. The nuclear chain reaction that causes a nuclear explosion cannot be sustained under such conditions. However, if the fragments of a nuclear charge are combined together, then their total mass will become sufficient for the uranium fission chain reaction to begin to develop. The result is a nuclear explosion. In this case, the power of the explosion, developed by a nuclear bomb of a relatively small size, is equivalent to the power released during the explosion of millions and billions of tons of TNT.

Rice. 5. Atomic bomb