The management of spent nuclear fuel is one of the most pressing problems of Ukraine’s nuclear energy. To solve this problem, as well as to increase Ukraine’s energy independence, the construction of a centralized spent nuclear fuel storage facility is being completed in the Chornobyl exclusion zone, where the spent fuel of Khmelnytsky, Rivne and South Ukrainian nuclear power plants will be stored for the next 100 years. The technology of centralized storage of spent nuclear fuel is based on the storage of fuel assemblies in ventilated HI-STORM concrete containers manufactured by Holtec International. Long-term operation of a spent nuclear fuel storage facility requires a clear understanding of all processes (thermohydraulic, neutron-physical, aging processes, etc.) occurring in HI-STORM containers. And this cannot be achieved without modeling these processes using modern specialized programs. Modeling of neutron and photon transfer makes it possible to analyze the level of protective properties of the container against radiation, optimize the loading of MPC assemblies of different manufacturers and different levels of combustion and evaluate biological protection against neutron and gamma radiation. In the future it will allow to estimate the change in the isotopic composition of the materials of the container, which will be used for the management of aging processes at the centralized storage of spent nuclear fuel. The article is devoted to the development of the three-dimensional model of the HI-STORM storage system. The model was developed using the modern Monte Carlo code Serpent. The presented model consists of models of 31 spent fuel assemblies 438MT manufactured by TVEL company, model MPC-31 and model HISTORM 190. The model allows to perform a wide range of scientific tasks required in the operation of centralized storage of spent nuclear fuel.

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Institute for Safety Problems of Nuclear Power Plants (NPP) of the National Academy of Sciences of Ukraine at a meeting of the National Commission for Radiation Protection of Ukraine on June 1, 2020. It is described the crisis in the nuclear industry due to approval of the new energy balance for 2020 by the Ministry of Energy and Environmental Protection of Ukraine. According to this balance, the volume of electricity production by NPP decreased and at the same time the share of thermal power plants and “green energy”, which produce much more expensive electricity, increased in the energy market. The situation is analyzed and recommendations are given for the implementation of measures to improve it. It is shown that the main task of the Ukrainian energy sector will be to continue the course of market relations and to abandon excessive market regulation. Market relations will lead to the fact that generating companies with cheaper energy will be able to sell it not at the cost level price, but at tariffs that have formed in the market. The price of nuclear energy will be brought up to the level of the price of energy produced by thermal power plants. Consumption costs will increase, but government generation will receive more funds, and the state will decide where to direct them: to subsidies for the population or to the development of nuclear or renewable energy. It is proposed to make the necessary changes to the legislation on “green energy”; establish a reasonable tariff for energy produced at NPPs; to introduce technical and organizational measures to ensure the operation of NPP power units in shunting modes. It is concluded that the optimal solution for the future from the standpoint of the both environmental and economic policy of Ukraine is the gradual replacement of NPP power units, which have “exhausted” project resource, with modern new ones with a higher level of safety. This will support the share of NPPs in the energy balance of Ukraine at 40−50%, as well as contribute to the fulfillment of its commitments to increase the use of low-carbon technologies in energy. As for solving energy problems of Ukraine on the basis of promising technologies — small modular reactors (SMR), this strategy does not yet have a proper feasibility study and is not confirmed by the necessary proven practice, as in the world such reactors are still being developed. Observations on the construction of the Central Spent Fuel Storage Facility (CSFSF) for long-term storage of spent fuel from the power units of Rivne, Khmelnytskyi and South-Ukraine NPPs were made. The construction of the CSFSF is under threat because already at the final stage of construction it was decided to liquidate the division of NNEGC “Energoatom”, which was engaged in the construction. Such an incomprehensible decision will not lead to anything positive, but will only delay the commissioning period of the storage facility by 1−2 years. It is shown that the construction of a New Safe Confinement (NSC) over the Shelter object of the Chornobyl NPP is just an intermediate stage in the transformation of the Shelter object into an ecologically safe system. In the near future, it is necessary to perform dismantling of building structures of the Shelter object, the term of operation of which ends in 2023. This is prompted by cases of destruction and collapse of building structures of this object, which occurred last year, and therefore this issue requires immediate solution. The critical state of nuclear science in Ukraine is reflected and it is warned that if the financial support does not change properly, in the coming years the country will be left without a unique scientific institution that conducts research on the safety of nuclear facilities, including the most dangerous in the world Shelter object. Emphasis is placed on the complete absence of a safety culture at the highest level of the country’s leadership, the lack of which is manifested in violations of national norms and international standards, the absence of some leaders in nuclear industry and dangerous enterprises, as well as the attitude of government agencies to the nuclear industry.

There is a significant lack of reliable information on the physical characteristics of thermohydraulic processes in emergency heat transfer modes when cooling the surface of fuel rods with light water coolant with supercritical thermodynamic parameters, in particular, on the physics of heat transfer processes and hydromechanics in the critical area. It is shown that in these conditions there is physical uncertainty about the causes of deteriorating heat transfer, which limits the possibility of creating effective calculation techniques for reliable determination of the upper limit of safe forcing of the heat transfer process in the core. At present, the vast majority of theoretical and experimental studies of thermohydraulic processes in the near-critical area have been performed only for the socalled “normal” heat transfer, which corresponds to the heat removal conditions with mixed turbulent convection of superheated to “gas” state of light water coolant in its inertial mode. Attention is paid to the possible appearance of macromolecular ensembles on this surface in the form of pseudo-vapor formations, which are capable of causing an emergency mode of pseudo-film boiling. On the basis of the given experimental data of various authors existence of rather deep physical analogy between processes of heat exchange in supercritical thermodynamic system and unheated boiling at subcritical parameters of the heat carrier is proved. Existence of the pseudo-boiling process in the conditions of supercritical thermodynamic parameters makes it impossible to use in the thermohydraulic calculation the empirical dependences for “hot” gas for the range of active zones operational parameters.

An analytical review of the evolution of attempts to create, schematic and constructive solutions for energycooled nuclear reactors with nuclear steam overheating and supercritical parameters of the working fluid in the conditions of the former USSR and the Russian Federation was made. A comparison of a number of major technical and economic characteristics of main developments of tube and tank reactors is made, the available information on the results of experimental and industrial operation of AMB reactors structures is considered, their advantages and disadvantages are evaluated in terms of technical perfection, reliability, technical and economic performance as well as environmental safety. The expected reduction in capital costs of 40% during the transition of nuclear power units with a capacity of 1,000 MW to single-circuit promising SCWR reactors is achievable only if the steam temperature rises to 625 °C, which has not yet been mastered even in traditional power engineering. The specific energy intensity of reactor’s active zones promising Russian developments under the SCWR program is in most cases extremely high, which will have a negative impact on the characteristics of nuclear safety. The conclusion is made concerning the high probability of a significant increase in the accident rate of the SCWR reactor cores, due to the insufficient study of the heat transfer process characteristics on the heat transfer surface of the TVEL under the conditions of supercritical parameters of the coolant, in particular, such phenomena as pseudo film boiling and thermoacoustic vibrations. In general, insufficient level of completeness of the latest Russian developments and lack of final conceptual projects were noted, which does not provide sufficient grounds for choosing promising schematic and constructive decisions necessary for making reasonable forecasts about the possibility of using supercritical parameters of the coolant in the modern nuclear power industry in the near future.