By March 1939, groups of scientists working in France and America had proven that for a self-sustaining chain reaction, the release of an average of two to four free neutrons during each fission of a uranium nucleus was sufficient. Growing fears about the possibility of creating an atomic bomb, however, quickly dissipated.
Bohr decided not to waste time. Fission physics, like any other new direction in science, undoubtedly provided a vast field for activity. And, since it was possible to work in Princeton with no less success than in Copenhagen, Bohr turned to Wheeler with a proposal for cooperation. They began to further develop the theory of nuclear fission, relying on new experimental data. They carried out experiments with a device hastily assembled right there in Princeton, in the attic of Palmer's laboratory. The results were initially quite puzzling.
The apparatus mentioned above was needed to study changes in the intensity of fission of the 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 intensity of fission, accordingly, also decreases. Such data were quite expected. However, it soon became clear that when the neutron energy decreases sufficiently, the intensity of nuclear fission increases again.
Placzek, who had previously forced Frisch, who was working in Copenhagen, to look for reliable evidence of nuclear fission, quite unexpectedly ended up in Princeton. “What the hell is this: why is the response the same to 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 energy of 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 develop this hypothesis in detail. 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 nucleus of an atom and the force of 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 to trigger the decay reaction. Consequently, faster neutrons with higher energy are needed to split such a nucleus.
The second of the factors mentioned was the complex composition of the core. An equal number of protons and neutrons is more favorable for it, which is explained by the quantum nature of their subatomic components. Having accepted an additional neutron, U 235 turns into U 236, the nucleus of which has 92 protons and 144 neutrons, that is, an even number of both nucleons. When U 238 accepts an additional neutron, the isotope U 239 is formed with an odd number of neutrons in the nucleus. Uranium-235 “assimilates” the additional neutron and reacts with it much more easily than uranium-238.
The combination of the two factors described above sufficiently explains the significant difference in the behavior of the two isotopes of uranium. Fast neutrons are required to split the stable U 238 nucleus, but the much less stable U 235 nucleus can be split by slow neutrons. Thus, if you make a bomb 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 occur slowly. Then it will fade out, and the bomb will not explode.
Now the chances of creating a bomb in the near future, although not completely eliminated, have decreased significantly. Of course, we must not forget about Bohr’s words, which he repeatedly repeated during discussions with colleagues in April 1939: then he declared that making 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 only possible 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. Moreover, such work on the scale required to create an atomic bomb required unreasonably large efforts - at the then level of development, it required several tons of uranium-235.
Interview with Deputy Director of the All-Russian Research Institute of Nuclear Engineering, Professor Igor Ostretsov
The world is faced with the problem of energy shortage. The habit of burning anything has brought humanity to the brink of survival. People are used to consuming a lot of energy. However, its resources unexpectedly turned out to be exhaustible. There is little heat left for humanity, for about 50 years. And now the only hope is for scientists, who must somehow come up with a way out of the critical situation. We have found such scientists. They figured out how to safely obtain energy even from nuclear waste, and at the same time made sure that future generations could actively explore space.
- Why are modern nuclear technologies bad?
Traditional nuclear energy, which has been developing for a long time, is based on the combustion of uranium-235. This is the only isotope that exists in nature and is fissile. Why is this bad? Firstly, in fact, the Americans have not ordered a single unit since 1978. Europe has practically stopped using nuclear power plants for energy production, and at the legislative level. This is due to the fact that the stations cannot be decommissioned. Today, out of 450 blocks in the world, 99 are worth it, and no one knows what to do with them.
- So they can’t stop the decay reaction?
They are stopped and will remain there forever. Moreover, the spent fuel is inside them. The main problem is nuclear waste, which must be stored for hundreds of years before it can be disposed of.
There is another problem. In the Third World countries, where the energy deficit is greatest, it is necessary to develop nuclear energy in order to prevent an increase in the consumption of fossil fuels. However, Iran has clearly demonstrated that almost any country that has a nuclear power plant is capable of making an atomic bomb. Previously, India, under the control of the IAEA, having a Condu reactor, made such a bomb and entered the nuclear club. Therefore, the situation is a dead end - in developed countries they will not build because the problem with operation and the problem with waste has not been solved, and in developing countries because the problem of the proliferation of nuclear weapons has not been resolved.
However, there is a much more serious thing - humanity’s energy problems cannot be solved without nuclear space programs, without energy and industrial output into space. It will be impossible to launch sufficiently heavy vehicles into orbit using traditional chemical fuel. Using "chemistry" you can launch very small systems into very close space and for a lot of money. The only opportunity that humanity has been given to solve such problems is uranium-235. And it must be treasured like the apple of one’s eye. Burning this isotope in thermal neutron reactors is a crime against humanity greater than the crimes of fascism. The modern program for the construction of 100 units in China, 40 units in our country, is simply a criminal program, because humanity dooms itself to remain in a nuclear dump on Earth and not be able to go into space. This is an extremely serious thing that, unfortunately, is little discussed today. So there is only one option left - to burn uranium-238.
- What are the world’s reserves of “valuable” uranium-235?
It is only 0.7% of the total volume of all mined uranium. This is very little. In terms of energy potential, it has the same amount as oil. That is, if we now begin a program for the widespread deployment of nuclear power plants using thermal neutrons, then by approximately 2040-50 there will be no uranium-235 left.
- Please explain in more detail why it is impossible to use 238 uranium instead of 235?
The fact is that uranium-235 is fissile on its own. With its help, it is possible to organize a so-called “self-sustaining” chain reaction, which is what all nuclear energy, all bombs, are based on. In contrast, uranium-238 does not “burn”; such a reaction cannot be organized on it. But there is a lot of it. Therefore, even at the beginning of the development of nuclear energy, the so-called “breeder” program was proposed. We call it the fast neutron reactor program, which allows us to burn uranium-238. To do this, it is first processed into plutonium, which will produce energy. This idea is now very popular, Bush said this in his last interview, Putin has also repeatedly stated that we invite everyone to cooperate in burning uranium-238. But few people know what a breeder is, although it is also a nuclear power plant, but based on slightly different technologies.
- So how does a breeder work?
Uranium enriched with plutonium is loaded there. The share of the latter is from 18 to 25%. As a result of the operation of this reactor, slightly more plutonium is obtained than was loaded. Moreover, near each such station there should be a radiochemical plant nearby for the separation of plutonium and the fabrication of new fuel rods (a nuclear reactor unit containing fissile material). Each power unit will contain 20 tons of plutonium. Meanwhile, a bomb can be made from just a few kilograms. If the world's energy sector is transferred to this kind of reactors, then up to a million tons of plutonium will be circulating in the world.
A quick question: where will the first such breeder be built? I have already asked this question to Vladimir Asmolov, Deputy for Science of the Atomic Energy Concern. I ask: “I was recently at the Iranian embassy, everyone has the problem of uranium enrichment stuck in their teeth, let’s, instead of them enriching it, we will build a breeder for them. At the same time, you tell the population that there will be 20 tons of plutonium spinning there. I guarantee, that I will persuade them to pay you 5-6 billion dollars for such a station.”
He says: “Ostretsov, you are a provocateur. This can only be within the Russian program.” I say: “What did Putin call us to do? Make nuclear energy aimed at all countries.”
Therefore, proposing breeders as the basis of global nuclear energy is a purely political game based on the circumstances of the moment, nothing more.
- So what way out of this situation do you propose?
That is why we are dealing with a different topic. There is a direct way to burn uranium-238 - the so-called forced fission. And without converting it to plutonium. This produces significantly more energy than in breeders. For such fission, very highly accelerated neutrons are needed, which can only be obtained using an accelerator.
In this regard, two technologies are needed. Firstly, the technology itself for burning such uranium, and the technology for creating an accelerator. Today in Russia there are two patents for both of these technologies. The owner of one is me, the second is Alexey Sergeevich Bogomolov. For example, in America there is an 800 megaelectronvolt accelerator, its length is about a kilometer. We need about 10 times more energy, that is, the accelerator must be approximately 10 km long. Naturally, this doesn’t fit into any corners - it’s expensive. And so Bogomolov came up with an accelerator that would give us the necessary energy, and its length would be only about 50 meters. This is completely acceptable.
We are now “breaking through” these two technologies. Meetings were held in the Federation Council, and at our institute, a public forum was held. First of all, the Kurchatov Institute, which offers a breeder program, strongly opposes this. It opposes it solely on the grounds that a change in the nuclear energy program means a change in elites and funding.
- Tell us about the prospects of your technology.
Sooner or later, humanity will have to burn uranium-238. That is, either enrich it to the state of weapons-grade plutonium, or burn it directly using accelerators. Developing technologies for burning uranium-235 is simply suicide. Therefore, today it is necessary to start two programs - YRT-energy, the one that we propose, and to revive the program to create nuclear rocket engines. And it’s best to do it together with the Americans. Both we and they have serious developments in this area. The fact is that the energy sector of the late 21st and 22nd centuries will be associated with the industrial entry of man into space. The main scientific task of our century is to make neutron accelerators of sufficient power for nuclear radiation radiation. Whoever does them first will control the situation.
- How does YRT energy stand for?
Heavy nuclear relativistic energy. Relativistic because protons generate neutrons, and they divide heavy uranium nuclei.
Now the Americans are showing great interest in these accelerators. They want to get it at any cost, and negotiations are constantly underway with Bogomolov. Meanwhile, we are developing our idea. Experiments were carried out in Dubna, Protvino in 2002. We've been butting heads ever since. The tragedy is that the authorities are incompetent in these matters.
- What needs to be “punched through”? Financing?
No, that's not even the point. We need a government program. And after that, any money from abroad will come, because everyone understands that without this technology you can’t get anywhere. The fact is that in the West it is terribly expensive to conduct experiments, but here the accelerator in Serpukhov is idle. But in order for a state program to be created, this issue must be brought to Putin’s attention. We are very well supported in the Federation Council, and hearings calling on Kiriyenko are due to take place in September. There we expect to receive recommendations, after which our idea will be considered at the scientific and technical council of the president. And after this, we hope, a state program will be created.
- How will such a program work?
To begin with, symbolic funding will be opened, after which foreigners will get involved; we have already secured their consent to participate in the project. We will continue our research. We need about a year and a half for the remaining experiments. After this, you can begin designing the station.
- How long will it take?
It all depends on the pace of work. There are no fundamental barriers to this today.
That is, we can say that in 10 years a prototype of a new generation nuclear power plant can be created?
Undoubtedly.
- How much will such a station cost?
There are no prototypes yet, but it will definitely be cheaper than existing nuclear power plants. This is due to the fact that we will not have a fuel cycle and will not face the problem of decommissioning the station. After all, this is why all these current stations stand and no one knows what to do with them.
So there is no need to talk about price - our price is much lower, because our reactor is much simpler. It will most likely be a concrete body insulated with metal. Micro fuel pins will be filled inside. And there won't be anything special there anymore. The main thing is that there will be no need to enrich anything there. You can use waste uranium and spent fuel. After all, it’s the same as burning there, as long as there are heavy nuclei. And this structure will be bombed by a neutron accelerator. As a result, heavy nuclei will begin to fission, releasing heat.
- How environmentally safe is this system?
Our reactor is subcritical. That is, there is no what was in Chernobyl - a self-sustaining reaction. As soon as any critical situation occurs, the accelerator stops the process.
Tell me, the fact that heavy atoms begin to fission under the impact of neutrons has been known for a long time. Why didn’t scientists think of applying this effect in nuclear energy earlier?
Previously, there were simply no accelerators with high efficiency. Good accelerators appeared only after Chernobyl. Then everyone was frightened by the critical accident and began to use less risky subcritical zones on uranium-235, and supplement the lack of neutrons with accelerators. Thus, the next step was predetermined - take accelerators at higher energy and see what happens next. It so happened that I was the first to come up with it. Moreover, all the processes were described in the scientific literature earlier, I just looked at these possibilities as energy ones. Logically and historically, this step was predetermined. But someone had to do it.
Special for the Centenary
The twentieth century gave so many discoveries to Humanity! For many of them, the goal was to make life easier for the highest being on planet Earth, but reality, as always, is deceptive and human selfishness sometimes exceeds simple concepts of good and evil. Egoism does not allow the feeling of superiority and power over the world to fall asleep, and the greatest discoveries take the path of destruction. The initial stage of 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 during the fission process a huge amount of energy was released per atom of the substance (about 202.5 MeV = 3.24 * 10 -11 J), as well as another 2-3 neutrons that interacted with neighboring nuclei. But it was not possible to use such fuel - the reaction in the uranium sample, for unknown reasons, quickly died out. Later it was found that the course of the reaction is negatively affected by one of the isotopes, namely uranium 238, which, when absorbing a neutron (n), does not emit new neutrons during the fission process. However uranium isotope 235 has the ability to reproduce.
A big discovery was the process of spontaneous fission of the uranium 235 nucleus. In 1 gram of metal per hour, about 20 spontaneous fissions occur, but a chain reaction does not occur, why? The answer to this question is quite banal - neutrons miss in a fairly small volume of matter and come out of the metal without interaction. Through calculations, the minimum mass of the uranium 235 sample was determined, which was about 48 kilograms. In such a sample - a ball with a diameter of 25 cm - the reaction should not die out. But how to isolate the uranium 235 isotope? Let's try to answer this question.
Natural uranium is a silver-colored metal, easy to machine, with 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 toxic, and is a source of hard alpha and beta radiation. Natural uranium consists of several isotopes:
Uranium 235 - 0,7184%;
Uranium 238 - 99.2760%;
Uranium 234 - 0.0056%.
Only the 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 235 passes through. In this way, enrichment of up to 99% can be achieved.
Industrial uranium 235 is primarily used as fuel for power plants, but the 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 fuel makes it possible to reduce the extraction of minerals and move to the level of “clean energy”, provided that reliable emergency systems for reactor operation are designed and the reactor itself is of high quality. A reactor is the main part of a nuclear power plant (nuclear power plant), in which the process of fission of matter nuclei and the transfer of energy to the coolant directly takes place. The coolant transfers energy to the turbine, which, in turn, generates electrical energy. The coolant can be various substances with high heat capacity: water, inert gases, liquid alkali metals.
Currently, the energy of uranium 235 is used to produce 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!
Unlike the other, most common isotope of uranium 238U, a self-sustaining nuclear chain reaction is possible in 235U. Therefore, this isotope is used as fuel in nuclear reactors, as well as in nuclear weapons.
The activity of one gram of this nuclide is approximately 80 kBq.
Uranium-235 is formed as a result of the following decays:
The decay of uranium-235 occurs in the following directions:
In the early 1930s. Enrico Fermi irradiated uranium with neutrons in order to obtain transuranium elements. But in 1939, O. Hahn and F. Strassmann were able to show that when a neutron is absorbed by a uranium nucleus, a forced fission reaction occurs. As a rule, the nucleus splits into two fragments, and 2-3 neutrons are released (see diagram).
About 300 isotopes of various elements were discovered in the fission products of uranium-235: from Z=30 (zinc) to Z=64 (gadolinium). The curve of the relative yield of isotopes formed during irradiation of uranium-235 with slow neutrons on the mass number is symmetrical and resembles the letter “M” in shape. The two pronounced maxima of this curve correspond to mass numbers 95 and 134, and the minimum occurs in the range of mass numbers from 110 to 125. Thus, the fission of uranium into fragments of equal mass (with mass numbers 115-119) occurs with less probability than asymmetric fission, This tendency is observed in all fissile isotopes and is not associated with any individual properties of nuclei or particles, but is inherent in the mechanism of nuclear fission itself. However, the asymmetry decreases with increasing excitation energy of the fissile nucleus and when the neutron energy is more than 100 MeV, the mass distribution of fission fragments has one maximum, corresponding to the symmetric fission of the nucleus.
One of the options for the forced fission of uranium-235 after the absorption of a neutron (diagram)
The fragments formed during the fission of a uranium nucleus are, in turn, radioactive, and undergo a chain of β− decays, during which additional energy is gradually released over a long period of time. The average energy released during the decay of one uranium-235 nucleus, taking into account the decay of fragments, is approximately 202.5 MeV = 3.244·10−11 J, or 19.54 TJ/mol = 83.14 TJ/kg.
Nuclear fission is only one of many processes possible during the interaction of neutrons with nuclei; it is the one that underlies the operation of any nuclear reactor.
During the decay of one 235U nucleus, 1 to 8 (on average 2.5) free neutrons are usually emitted. Each neutron produced during the decay of a 235U nucleus, subject to interaction with another 235U nucleus, can cause a new act of decay; this phenomenon is called a chain reaction of nuclear fission.
Hypothetically, the number of second generation neutrons (after the second stage of nuclear decay) can exceed 3² = 9. With each subsequent stage of the fission reaction, the number of neutrons produced can increase like an avalanche. Under real conditions, free neutrons may not generate a new fission event, leaving the sample before capturing 235U, or being captured either by the 235U isotope itself, transforming it into 236U, or by other materials (for example, 238U, or the resulting nuclear fission fragments, such as 149Sm or 135Xe ).
If, on average, each act of fission generates another new act of fission, then the reaction becomes self-sustaining; this condition is called critical. (see also Neutron multiplication factor)
In real conditions, achieving a critical state of uranium is not so easy, since a number of factors influence the course of the reaction. For example, natural uranium consists of only 0.72% 235U, 99.2745% is 238U, which absorbs neutrons produced during the fission of 235U nuclei. This leads to the fact that the fission chain reaction in natural uranium currently decays very quickly. A continuous fission chain reaction can be carried out in several main ways:
The only known isomer is 235Um with the following characteristics:
The decomposition of the isomeric state occurs through an isomeric transition to the ground state.
uranium 235 50, uranium 235 75, uranium 235 area, uranium 235/75r15
When studying the phenomenon of radioactivity, every scientist turns to such an important characteristic as its half-life. As you know, it says that every second in the world atoms decay, and the quantitative characteristics of these processes are directly related to the number of atoms present. If, over a certain period of time, half of the total number of atoms available decays, then the decay of ½ of the remaining atoms will require the same amount of time. It is this time period that is called the half-life. It varies for different elements - from thousandths of a millisecond to billions of years, as, for example, is the case when it comes to the half-life of uranium.
Uranium, as the heaviest of all elements existing in a 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 discovered quite accidentally at the end of the 19th century by the French chemist A. Becquerel.
Uranium is calculated using the same formula as 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 approximately 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 occur in nature: uranium-234, uranium-235 and uranium-238. uranium is different: 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 is worth noting that the radioactivity of uranium under natural conditions is quite high and allows, for example, photographic plates to be illuminated within just an hour. At the same time, it is worth noting that of all the isotopes of uranium, only U-235 is suitable for making fillings. The thing is that the half-life of uranium-235 in industrial conditions is less intense than its “brothers”, which is why the release of unnecessary neutrons here is minimal.
The half-life of uranium-238 significantly exceeds 4 billion years, however, it is now actively used in the nuclear industry. So, in order to start a chain reaction involving the fission of heavy nuclei of this element, a significant amount of neutron energy is needed. Uranium-238 is used as protection in fission and fusion apparatuses. However, most of the mined uranium-238 is used to synthesize plutonium, used in nuclear weapons.
Scientists use the half-life of uranium to calculate the age of individual minerals and celestial bodies as a whole. Uranium clocks are a fairly universal mechanism for this kind of calculation. 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 to calculate rocks and minerals, it is associated with the so-called spontaneous As is known, as a result of the spontaneous fission of uranium under natural conditions, its particles bombard nearby substances with colossal force, leaving behind special traces - tracks.
It is by the number of these tracks, knowing the half-life of uranium, that scientists draw a conclusion about the age of a particular solid - be it an ancient rock or a relatively “young” vase. The thing is that the age of an object is directly proportional to the quantitative indicator of the uranium atoms whose nuclei bombarded it.
