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This article deals with the problem of optimizing composition and structure of the ignition zone of the fast reactor operating in the self-sustaining mode of nuclear burning wave with the purpose of its smooth start-up and reducing fissile material amount in initial assembly. The cylindrical homogeneous reactor with the ignition zone placed in the center or near the cylinder end is considered. The analysis has been performed basing on solving the non-stationary neutron diffusion equation together with the fuel burn-up equations and the equations of nuclear kinetics for precursor nuclei of delayed neutrons, with using the radial buckling approximation. An optimized structure of the ignition zone has been proposed, which ensures a smooth transition of the reactor to the self-sustaining nuclear burning wave mode, avoiding an excessive energy release, which is observed when using a simplified scheme of ignition zone. Comparison of the startup variants with the ignition zone at the cylinder end and at its center shows the benefits of the second one.

After two major nuclear power plant accidents in Chernobyl (1986) and Fukushima (2011), one of the main requirements for the nuclear power engineering is the safety of the nuclear reactors in operation, as well as new nuclear power plants of the fourth generation, which are being developed now. One of such requirements is presence of the so-called “inherent safety” mechanism, which renders the uncontrolled reactor runaway impossible under any conditions, moreover, the implementation of such a mechanism should be ensured on the level of physical principles embedded in the reactor design. Another important problem of the nuclear power engineering is the need of the transition to the large-scale use of the fast-neutron breeder reactors, with which it would be possible to set up expanded reproduction of the nuclear fuel and by that means solve the problem of supplying humanity with relatively cheap energy for thousands of years. Moreover, at present an unresolved problem is the disposal of spent nuclear fuel containing radioactive nuclides with long half-lives, which presents a long-term danger to the ecology. One of the promising conceptions of the fast-neutron breeder reactor, which can, in the case of successful implementation, partially or even entirely solve the problems of the nuclear power engineering mentioned above, is the reactor that operates in the nuclear burning wave mode, which is also known as “Traveling wave reactor”, CANDLE and by some other names. This paper presents a short review of the main theoretical approaches used for description of such a physical phenomenon as slow nuclear burning (deflagration) wave in the neutron multiplication medium initially composed of the fertile material 238U or 232Th. A comparative analysis of the possibilities of different mathematical models for describing this phenomenon is performed, both for those based on the deterministic approach (i.e. solving neutron transport equations) and for models that use Monte Carlo methods. The main merits of the fast breeder reactor, working in the nuclear burning wave mode, and problems related to the practical realization of the considered concept are discussed.

By calculation methods, the dose rate of the radioactive waste, behind concrete protection, was evaluated in current work. Parameters, which were taken in account in the calculations, are geometry of the protection shell, size of the source and its isotopic composition. As model geometrical parameters the spent fuel assembly's size and thickness of the concrete wall of the ventilated storage container (VSC)-VVER were taken. The computer program that does numerical calculation was composed in the Wolfram Alpha environment. The program takes into account change of the isotopic composition and spectra of gamma-radiation with time. Calculation results were compared to the known data on the spent nuclear fuel heat dissipation. Approach described in this work can be used for fast estimation of change in the quality of radioactive waste (RAW) in the long-term storage without recycling, for different initial isotopic composition. Obtained results were analyzed on the matter of change in gamma-radiation of RAW. В данной работе численными методами рассчитывалась мощность дозы излучения радиоактивных отходов за бетонной защитой. При расчётах учитывались геометрия защитной оболочки, размер источника и его изотопный состав. В качестве модельных геометрических параметров были взяты размеры отработанной тепловыделяющей сборки (ОТВС) и толщина стенки бетонного вентилируемого контейнера хранения (ВКХ)-ВВЭР. Для проведения численных расчётов, в среде Wolfram Alpha была составлена программа. В программе учитывается изменение изотопного состава отработанного ядерного топлива со временем и изменение энергетического спектра гамма-излучения. Проведено сравнение результатов вычисления с известными данными по тепловыделению отработанного ядерного топлива (ОЯТ). Используемая методика позволяет проводить быструю оценку изменения качества радиоактивных отходов (РАО) при долговременном хранении без переработки, для различных изотопных составов топлива. По результатам расчётов был проведён анализ изменения гамма-излучения РАО. КЛЮЧЕВЫЕ СЛОВА: радиоактивные отходы, мощность дозы, долгосрочное хранение, бетонная защита, изотопный состав KEYWORDSProduction and accumulation of the radioactive waste (RAW) are one of the major problems associated with the use of nuclear energy in any of its forms. According to the existing data, there is about 300 thousand tons of accumulated spent nuclear fuel (SNF) with total activity of ~10 20 Bk, and by year 2030, this quantity is predicted to be 500 thousand tons. In particular, importance of this problem is reflected by the fact, that in the last years several monographs [1][2][3][4][5] on the problems of dealing with RAW were published.Leaving aside the cosmic origin and associated with uranium mining radionuclides, in this paper we will consider 2 forms of RAW: 1) contained in SNF; 2) the so-called operational waste of nuclear plant. According to the established terminology, in countries, where radiochemical or other recycling is not intended (for example -Ukraine), it belongs to the high-level RAW (HLW).

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