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In the latest assessments of the strategic prospects for the development of nuclear power, one can note the tendency of a condescending arrogant attitude towards thermonuclear power, which, unfortunately, in large part corresponds to the real state of affairs. At the same time, an analysis of the problems and potential of two nuclear technologies based on nuclear reactions of fusion of light nuclei and fission of heavy ones shows the following. Independent large-scale development of each of these areas will inevitably lead to the need to overcome the still unsolved problems of technological, materials science, environmental and economic nature, which will raise the question of the expediency of further development of these energy sectors. At the same time, the physical features of the fission and fusion processes objectively indicate the advisability of combining them within a single nuclear power system, which causes a large synergistic effect that suppresses their negative aspects, developing nuclear technologies independently.
The article presents the calculations of the multiplication of thermonuclear neutrons in the blanket of a hybrid thermonuclear reactor, which confirm the physical validity and reliability of the choice of the strategic direction of development in the form of a united nuclear power system.
Introduction
Now, in the assessments of the strategic path of development of nuclear energy, serious reassessments of the seemingly established provisions are taking place. The two-component concept for the development of nuclear power, in which fast and thermal fission reactors operate in concert, has recently undergone a serious revision. Previously, it was assumed that the structural development of nuclear power would be based at the initial stage, on capacity building at the expense of thermal reactors. In the future, there will be fast reactors with a high breeding ratio of the order of 1.5 and higher. This will make it possible, with the growing shortage of natural uranium, to organize a closed fuel cycle with efficient reprocessing of irradiated spent nuclear fuel and to satisfy the need for fissile isotopes by producing them in fast reactors. It was assumedthat in the nuclear power system the share of thermal reactors will be about 60%, and the share of fast reactors will be about 40%. Thermal reactors will take on the inconveniences of working in the power system (power range adapted to the requirements of the consumer, work in a variable load curve, provide non-electrical needs of the system, etc.). Fast reactors will operate predominantly on the basis, and produce fuel from raw isotopes for themselves and for thermal reactors.and to produce fuel from raw isotopes for itself and for thermal reactors.and to produce fuel from raw isotopes for itself and for thermal reactors.
Modern tendencies
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However, the severe accidents that occurred at nuclear power plants led to the need to significantly tighten safety requirements for nuclear power plants. For this reason, significant adjustments were made to fast reactor designs focused on intensive fuel production, and new conceptual designs of fast reactors are already being considered with a breeding ratio close to unity, with a low energy intensity of the core. In this situation, adherents of new projects of fast reactors have found another way to maintain their significance. They began to propagate a scenario that assumes that in the long term the abandonment of thermal reactors is inevitable, that in any development of events fast reactors will replace thermal ones.
People have different assessments of the future and many believe that the proposed direction for the development of nuclear power may not be realized, and the new concept of the dominance of fast reactors will turn out to be wrong. And this position is largely justified. The available alternatives allow us to speak about the options for the development of the nuclear power system in a much more attractive configuration.
The most noticeable systemic drawbacks in the construction of nuclear power, predominantly based on fast reactors, are obvious. Even if we assume that the fast reactor itself is made perfectly and has no flaws that would raise doubts about its absolute superiority over any other projects, there are unavoidable systemic difficulties.
First. The bulk of the newly produced fissile isotope (plutonium) in fast reactors will be produced in the core, where energy will be produced and the bulk of radioactive fission products will be formed. This highly active fuel must be chemically processed quickly. Reprocessing will release all radioactive isotopes from the irradiated fuel. A large amount of radioactivity will leave the sealed fuel element and be distributed throughout the working room. Despite the fact that they will try to keep all this radioactivity under control, it will determine the main risk of potential radioactive incidents, for various reasons, from the notorious human factor to planned sabotage.
Second. Fast reactors will have to replace thermal ones, almost completely. Considering that the required prototype of fast reactors is not yet available, that such a replacement will take place gradually, that it will begin no earlier than the middle of the century, and even if everyone in the world agrees to support it, the procedure will last for at least two centuries. During this time, among those who live after us, there will probably be people able to come up with and implement a more attractive profile of the nuclear industry. And efforts to create the ideal fast reactor will be in vain.
Third. Multiple recycling of plutonium will lead to the formation of a significant amount of minor actinides, isotopes absent in nature, with which mankind, for various reasons, does not intend to put up, and requires their destruction. It will also be necessary to organize the transmutation of these isotopes, a process with a high risk of an accident also capable of leading to significant radioactive contamination of the environment.
One could accept these shortcomings as an inevitable evil, but such a position can be justified only in the absence of an alternative, but it does exist.
Fusion energy
An alternative to the dominance of fast reactors can be the development of a nuclear power system based on fusion and fission reactors. Proposals for the use of thermonuclear reactors in the structure of nuclear power, providing a significant increase in the neutron potential of the system were made by I. V. Kuchatov Later, the concept of a hybrid thermonuclear reactor appeared, in the blank of which a new fissile isotope was produced and energy was produced. In recent years, the development of this concept has continued. The new version of the nuclear system assumes that fusion reactors (thermonuclear reactors) operate to produce nuclear fuel from raw isotopes for fission reactors, and fission reactors, as now, produce energy.
In a recently published article "Nuclear Problems of Fusion Energy", the authors concluded that fusion, for a number of reasons, should not be considered as a large-scale energy technology. But this conclusion is completely unfair when considering an integrated system in which nuclear power technologies (fusion and fission) complement each other and provide more efficient performance of functions that are difficult for the other.
Creation of a reliable nuclear power system with fission and fusion reactors is most preferable within the framework of the thorium fuel cycle. In this case, the share of thermonuclear reactors in the system will be minimal (less than 10%), the artificial fissile isotope uranium-233, obtained from the feed isotope thorium-232 is the best option for thermal neutron reactors, in the united nuclear system the problem of minor transurans simply will not exist. The amount of Am, Cm, etc. produced in the system. will be negligible. Such a system will have a fuel cycle in which the risk of radioactive contamination of the environment will be the lowest.
The natural criterion for the implementation of this concept is the neutron balance. The nuclear reaction on which the production of neutrons in a fusion reactor will be based is the reaction of fusion of tritium and deuterium
D + T = He + n +17.6 MeV
As a result of the reaction, a neutron with an energy of 14.1 MeV and an alpha particle with an energy of 3.5 MeV are obtained, which remains to heat the plasma. A high-energy neutron flying through the wall of the vacuum chamber enters the blanket of a thermonuclear reactor, in which it multiplies; when it is captured by a raw isotope, a new fissile isotope is obtained. The multiplication of a thermonuclear neutron occurs as a result of the reactions (n, 2n), (n, 3n) and (n, fission) - the fission reaction of heavy nuclei, in this case, a raw isotope. All these reactions are of a threshold nature. Figure 1 shows the graphs of the indicated cross sections. To ensure the maximum neutron multiplication, it is important that the blanket fuel composition contains a minimum amount of light nuclei and, of course, neutron absorbers.
Fig. 1 Microsections of neutron multiplication in Th-232.
To assess the potential for the production of new fissile isotopes in a thermonuclear reactor, a series of calculations was performed for different variants of blanket fuel compositions with thorium as a feed isotope. Calculations were performed using various programs and nuclear data libraries. The programs used were MCU library ENDF / B-6, MCNP, library ENDF / B-6, LUKY group library. The table presents the results of calculations of neutron capture by thorium-232 per one fusion neutron source for a fuel composition with the specified ratio of nuclear isotope concentrations. In some embodiments, it was assumed that the indicated ratio of isotopes was obtained not as a chemical compound, but constructively, when a certain amount of thorium was stirred with the appropriate amount of the desired isotope.
Table 1 Multiplication of thermonuclear neutrons (E = 14.1 MeV) in the blanket of a hybrid reactor with a thorium fuel composition.
The last column lists the values characterizing the multiplication of neutrons due to the fission reaction of the raw isotope. The values of neutron production due to fission are given, i.e. ν∑f. In the LUKY group program, the cross section matrices for the reaction (n, 2n) and (n, 3n) are integrated with the cross sections for inelastic scattering. This does not allow obtaining the values of the rates of these reactions separately.
On the whole, the presented calculated data are in good agreement with each other, which gives reason to count on the effective multiplication of thermonuclear neutrons in the blanket of a hybrid reactor. The calculation results presented in the table show the theoretical multiplication potential of thermonuclear neutrons (14.1 MeV). In an infinite medium of thorium, it is approximately 2.6, i.e. one neutron multiplies due to reactions (n, 2n) and reactions (n, 3n) approximately 2 times, and due to the fission of thorium-232 in 1.5 times. Calculations for different programs and different libraries differ by about 10%. These differences are due to the use of several nuclear data libraries. Taking into account the indicated error, the presented results can serve as a conservative guideline for assessing the parameters of the breeding of fissile isotopes in the blanket of a thermonuclear reactor. They show that the determining factor that leads to a decrease in the multiplying ability of the blanket is the presence in it of light scattering isotopes, including O-16, F-19, which also have a reaction of inelastic neutron scattering at high energies. Calculations show that the use of S-12 for the manufacture of claddings for fuel cells filling the blanket is quite promising. The use of graphite can be considered as one of the design options. Even in the case when there are two and a half times more carbon nuclei than thorium, the multiplication factor of thermonuclear neutrons is close to 2. This means that with the correct organization of the neutron balance, one nucleus of a new fissile isotope uranium-233 can be obtained in a blanket, and one nucleus tritium.which leads to a decrease in the multiplying ability of the blanket is the presence of light scattering isotopes in it, including O-16, F-19, which also have a reaction of inelastic scattering of neutrons at high energies. Calculations show that the use of C-12 for the manufacture of claddings for fuel cells filling the blanket is quite promising. The use of graphite can be considered as one of the design options. Even in the case when there are two and a half times more carbon nuclei than thorium, the multiplication factor of thermonuclear neutrons is close to 2. This means that with the correct organization of the neutron balance, one nucleus of a new fissile isotope uranium-233 can be obtained in a blanket, and one nucleus tritium.which leads to a decrease in the multiplying ability of the blanket is the presence of light scattering isotopes in it, including O-16, F-19, which also have a reaction of inelastic scattering of neutrons at high energies. Calculations show that the use of S-12 for the manufacture of claddings for fuel cells filling the blanket is quite promising. The use of graphite can be considered as one of the design options. Even in the case when there are two and a half times more carbon nuclei than thorium, the multiplication factor of thermonuclear neutrons is close to 2. This means that with the correct organization of the neutron balance, one nucleus of a new fissile isotope uranium-233 can be obtained in a blanket, and one nucleus tritium. F-19, which also have a reaction of inelastic scattering of neutrons at high energies. Calculations show that the use of C-12 for the manufacture of claddings for fuel cells filling the blanket is quite promising. The use of graphite can be considered as one of the design options. Even in the case when there are two and a half times more carbon nuclei than thorium, the multiplication factor of thermonuclear neutrons is close to 2. This means that with the correct organization of the neutron balance, one nucleus of a new fissile isotope uranium-233 can be obtained in a blanket, and one nucleus tritium. F-19 also have a reaction of inelastic scattering of neutrons at high energies. Calculations show that the use of S-12 for the manufacture of claddings for fuel cells filling the blanket is quite promising. The use of graphite can be considered as one of the design options. Even in the case when there are two and a half times more carbon nuclei than thorium, the multiplication factor of thermonuclear neutrons is close to 2. This means that with the correct organization of the neutron balance, one nucleus of a new fissile isotope uranium-233 can be obtained in a blanket, and one nucleus tritium. The use of graphite can be considered as one of the design options. Even in the case when there are two and a half times more carbon nuclei than thorium, the multiplication factor of thermonuclear neutrons is close to 2. This means that with the correct organization of the neutron balance, one nucleus of a new fissile isotope uranium-233 can be obtained in a blanket, and one nucleus tritium. The use of graphite can be considered as one of the design options. Even in the case when there are two and a half times more carbon nuclei than thorium, the multiplication factor of thermonuclear neutrons is close to 2. This means that with the correct organization of the neutron balance, one nucleus of a new fissile isotope uranium-233 can be obtained in a blanket, and one nucleus tritium.
Of course, in practice, there will be losses of neutrons and additional neutrons will be required to compensate for them. Such neutrons can be produced in a variety of ways. For example, some of the tritium, which is required for the fusion reaction, can be produced in the core of a fission reactor. The potential of this neutron replenishment method is very high. In thermal fission reactors for the uranium-233 fuel cycle, the breeding ratio is about 0.8, i.e. for one burned uranium-233 nucleus, 0.8 tritium nuclei can be obtained. This value will more than cover all neutron losses. It is possible to reduce the carbon content of the blanket of a fusion reactor, i.e. to make the cladding of the fuel cell thinner, the potential of this proposal is 0.2.-0.3 additional neutrons. Another way to allow a small fission of uranium-233 accumulated in the blanket. Reasonable potential of this option,which will not lead to a significant increase in the fission products of heavy nuclei in the blanket is more than 0.5 neutrons.
Conclusion
The importance of efficient neutron multiplication in the blank of a hybrid reactor is all the more important because it makes it possible to abandon the reprocessing of spent nuclear fuel from fission reactors. There will be enough neutrons in the system to completely compensate for the loss of fissile isotopes during the production of energy in fission reactors by their production from the feed isotope in the blanket of a thermonuclear reactor.
It doesn't matter at all what type of fission reactors are in the system, fast or thermal, large or small.
The extraction of the newly produced uranium-233 from the blanket fuel composition will be accompanied by the release of radioactivity by about two to three orders of magnitude less, in comparison with the option when the fissile isotopes will have to be separated from the SNF of fission reactors. This circumstance will ensure the minimum risk of radioactive contamination of the environment.
Based on the calculations performed, it is easy to estimate the proportion of hybrid thermonuclear reactors. It will be less than 10% of the thermal power of the entire system, and, therefore, the economic burden of the entire system will not be great, even if hybrid thermonuclear reactors are more expensive than fission reactors.
Thermonuclear technologies embedded in the nuclear power system and their future development should be considered as the general direction of the strategic development of the nuclear industry, capable of solving key problems of energy supply for a long time, practically of any scale, with a minimum risk of negative radioactive impact on the environment.