Actual problems of the thermal hydraulic reliability ensuring of prospective nuclear reactors with supercritical parameters

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.

The Work Results of the Institute for Safety Problems of Nuclear Power Plants of the NAS of Ukraine in 2020

The year 2020 has become a real challenge for almost all aspects of life all over the world. Under these conditions, Ukrainian science, which has been in a state of crisis for more than a year, has been forced to overcome additional difficulties. However, due to the efforts of scientists, scientific work not only did not stop, but also received a large development effort in new directions and formats. In 2020, the Institute for Safety Problems of Nuclear Power Plants was one of the scientific institutions that under the new conditions made every effort to carry out the planned scientific researches and perform works in accordance with its activities. The main results of scientific and scientific-organizational activities of the Institute for Safety Problems of Nuclear Power Plants in 2020 are presented in the article. Despite the difficult economic situation due to the epidemic, the staff of the Institute obtained important results in studies of nuclear and radiation safety of the Shelter object, and in works aimed at improving the reliability and safety of existing Ukrainian and foreign nuclear power plants. The results of works in the field of the Shelter object transformation into an ecologically safe system, safe operation of nuclear facilities, decommissioning of nuclear facilities, spent nuclear fuel and radioactive waste management are presented in the article. The results of the work performed under the International Atomic Energy Agency (IAEA) grant and the Ukrainian-Japanese Science and Technology Research Partnership for Sustainable Development (SATREPS) project are described. The implementation of the results of these studies in practice is indicated. Scientific and expert activities in the interests and at the request of public authorities, cooperation with national and foreign scientific organizations as well as scientific and organizational activities in 2020 are described. Information on internal certification of employees, participation in international and national events, publishing and etc. is presented.

Estimation of the Contribution of Dust Storm on April 16, 2020 to Radioactive Contamination of the Atmosphere During Forest Fires in the Exclusion Zone

On April 16, 2020, a strong dust storm was observed in the northern regions of Ukraine, which coincided with the period of intense wildland fires in the Chornobyl exclusion zone. The activity of 137Cs in aerosol particles released into the atmosphere as a result of resuspension from burned areas in the meadow biocenoses in the exclusion zone is evaluated in the article. Resuspension of radioactively contaminated particles from burned areas formed after fires in meadow biocenoses of the exclusion zone can be a powerful source of air contamination in the zone itself, as well as increase of the radionuclides transport outside it. The total 137Cs activity that entered the atmosphere during the dust storm was estimated to be about 162 GBq, i. e. up to 20% of the total activity emitted in the air during the entire period of forest fires on April 3–20, 2020. The 137Cs emission from burned areas during the dust storm on April 16 and 17 amounted to 0.24% of the total stock of 137Cs activity in this territory. According to the results of modeling, the relative contribution of wildland fires and resuspension due to the dust storm on April 16 and 17 significantly depends on the distance to the emission sources. It was found that the resuspension of radioactive particles from burned areas during the dust storm determined 80–95% of the 137Cs activity concentration in the surface air near Chornobyl nuclear power plant and in Chornobyl city and the rest was due to the continuing forest fires in neighboring territories. The maximum 3-hour averaged value of the 137Cs activity concentration in the air due to resuspension from the burned areas was obtained for the location of the monitoring post VRP-750 of SSE “Ecocenter” to be about 28 mBq/m3 for the period 9–12 hours on April 16. In Kyiv, the 3-hour averaged 137Cs activity concentration due to the dust storm in the Exclusion Zone was calculated as 44 μBq/m3 in the period from 9 to 12 hours on April 17, 2020. This value was only about 4% of the total 137Cs activity in the air in this period.

Influence of Geopolymer Binder Components on Properties of Compounds during Cementation of Boron-Containing Liquid Radioactive Waste

The paper presents an analysis of the experimental results of the use of geopolymer binders for cementing boron-containing liquid radioactive waste (LRW). The dependence of the properties of compounds on the component composition of binders has been studied. The following components are considered: liquid glass with a silicon modulus of 2.9, a mixture of ash of Darnytsya TPP with slag of the Mariupol metallurgical plant in a ratio of 1: 1 and potassium hydroxide. To perform a factor analysis of the effect, the mass of these substances was taken as three factors in the analysis at two levels. For the manufacture of compounds imitation LRW was mixed with zeolite in a ratio of 10:1 at a temperature of about 60 оC. Subsequently, the above components were added to the mixture, the weight of which varied by ±17% relative to the weight of the base compound. To study the properties, samples of different sizes 5×5×5 cm, 1.5×1.5×1.5 cm and rectangular samples with an outer surface from 96 cm2 to 104 cm2 were made. Each property was studied in 8 samples. The obtained results allowed to construct linear equations that quantitatively link the corresponding characteristic of the compound with the composition of the binders. The correlation coefficients between the experimental and the data calculated by the equation are estimated. The average values of the correlation coefficients may indicate that not all factors were taken into account. The obtained regularities show that in the conditions of the experiment slag and ash increase, and liquid glass and potassium hydroxide reduce the rate of setting of the samples. The density of the samples is increased by ash and slag, while liquid glass and potassium hydroxide are reduced. The compressive strength of liquid glass and potassium hydroxide is reduced, while the mixture of ash and slag is increased. The leaching rate Сs of liquid glass and potassium hydroxide is increased, the mixture of ash and slag is reduced. At the same time, the leaching rate of Sr increases the ash/slag mixture, while liquid glass and potassium hydroxide decrease. The time during which the leaching of Cs reaches the normative values, slag and ash are reduced, and liquid glass and potassium hydroxide are lengthened. The obtained results can be taken into account when optimizing the composition of geopolymer binders for cementing LRW.

Peculiarities of 137Cs Sorption/Desorption by Bentonite Clays of Cherkasy Deposit from Groundwater Model Solutions of Radioactive Waste Disposal Facilities at the “Vector” Production Complex

The main feature of bentonite clays is their high sorption capacity with respect to various radionuclides. The study of sorption kinetics of 137Cs was performed in the static mode by natural and industrial soda modified (PBA-20) samples of bentonite clays of Cherkasy deposit of bentonite and paligorskite clays from groundwater model solutions of radioactive waste disposal facilities of “Vector” production complex under various pH and solution mineralisation. The desorption of occluded samples was studied in distilled water and acetateammonium buffer solution. The value of the degree of sorption (S) for 137Cs on the modified samples exceeds 90%, for natural bentonite this indicator is lower (about 83–85%). On both types of bentonite with increasing time of their contact with aqueous solution and pH, there is a redistribution of water-soluble, ion-exchange and fixed forms of radionuclide and the share of the latter, that is not participating in migration processes increases, indicating the ability of bentonites to immobilize effectively for a long time. It is shown that Na-modified bentonite has higher proportion of sorption in fixed form compared to natural one and its application increases the probability of irreversible fixation of migrating radionuclides under non-optimal conditions of sorption (high pH (>11) of water after prolonged contact with cement-concrete components of engineering barriers) and thus increases the environmental safety of the storage facility. It is shown that bentonite clays of the Cherkasy deposit can serve as an effective material for creating anti-migration barriers of I and II stages of surface/near-surface storage facilities for radioactive waste disposal at the “Vector” production complex. At the same time, the issue of practical application of bentonite clays of Cherkasy deposit for accurate predictions of securing radioactive waste disposal of Chornobyl origin requires further study of sorption-desorption properties of bentonite clay with respect to other fission products and actinides

To a Burst of 137Cs Activity in the Air under the “Arch” Object on October 17, 2019

The construction of a New Safe Confinement (NSC) is one of the main stages in the transformation of the Chornobyl nuclear power plant 4th power unit into an ecologically safe system. On the one hand, this system is designed to ensure the safety of personnel, the population and the environment, on the other — to allow safe work with the remnants of the unit. At the same time, the project allows quite high levels of bulk activity of radioactive aerosols inside the NSC — up to 210 Bq/m3 for total β-activity. The sharp increase in the volumetric air activity, observed on October 17–19, 2019 in a number of rooms of the Shelter object and in the space under the “Arch” object in the absence of a burst of activity outside these objects, can be considered as a successful kind of spontaneous test of the NSC isolation systems. The chronology of the dynamics of volume activity according to the act of official investigation of the precedent is given. The results of studies of aerosol filters exposed in the near zone of the Chornobyl NPP before and after the burst of activity in the air under the “Arсh” object are presented. Autoradiograms of fragments of these filters demonstrate a significant (≈2 orders of magnitude) increase in the number of hot particles immediately after the fall of the concrete fragment of the floor in the room 402/3. The analysis of features of the hot particles after the collapse indicates a significant contribution to the total activity of a large number of low-activity particles (0.005÷5 Bq), and the low 137Cs/241Am ratio that indicates their fuel nature. Based on the assumption about the fuel nature of hot particles, their minimum possible aerodynamic diameters were calculated, which are in the range 2.91<d<36.7 μm, which does not exclude the presence of smaller hot particles on the filter that are not visible using this method. The contribution of particles with aerodynamic diameters less than 10 μm, determined using the impactor, did not exceed 20% of the total beta activity of the aerosol.

Experimental research of the air permeability of concrete in the compressed zone of the containment for power units no. 1, 2 of the South-Ukrainian NPP

The determination of the air permeability of concrete in the compressed zone of NPP’s containment under conditions of excessive air pressure in the subshell space of the reactor compartment under a maximum design basis accident is necessary to confirm the localizing functions of the containment when extending the lifetime of power units. Approaches to determining the localizing ability (air permeability) of concrete in the compressed zone of the containment of NPPs with WWER-1000 type reactors under the impact of excessive air pressure under conditions of maximum design basis accident are discussed in the article. The designed testing installation for experimental studies of the air permeability of concrete by the method of stationary radial filtration is described and the results of experimental researches of the air permeability on the installation are presented for samples simulating the composition of concrete used under construction of the containment for Power Units No. 1, 2 (series B-302 and B-338, respectively) SD “South-Ukrainian NPP”. A comparative analysis of the results of abovementioned experimental researches with the results of experimental and theoretical work on the study of air permeability of the similar samples-imitators of concrete by the method of stationary linear filtration, as well as the results of measurements for samples of concrete sampled directly from the compressed zone of containment for Power Unit No. 2 of SD “South-Ukrainian NPP” was fulfilled. The results are explained by processes of compaction of the concrete structure under a complex stress state that occurs under specific hardening conditions and during longtime compression by pre-stressed reinforcing ropes, which takes place under conditions of real containment operation. It was concluded that the simulators made according to the currently accepted technology cannot provide a correct determination of the localizing ability of the NPP’s containment, since the conditions and time of containment concrete hardening, as well as the longtime constant compressive action of reinforcing ropes, cannot be correctly modeled in full under the formation of samples-imitators. The issue of the containment concrete samplesimitators creating in laboratory conditions requires additional study, improvement of technology and the development of new approaches with maximum physical modeling of the conditions characteristic of the operation of the NPP containment.

Radioactive Aerosols within Conditions of the New Safe Confinement in 2017–2020

The results of monitored behavior of radioactive aerosols within the conditions of the New Safe Confinement (NSC) in 2017–2020 are presented. Maximum “unorganized” flux of beta-emitting products of the Chornobyl accident from the Shelter object through the process openings and leakages of light roofing in the NSC basic volume observed in 2017 made 7.9 kBq/(m2 ∙day). Mean annual density of radionuclide deposition onto the light roofing during four years has been changing within the range of 1.7–2.2 kBq/(m2 ∙day). Over these years, mean annual volumetric activity of sum of long-lived beta-emitting nuclides coming in the “Bypass” system dropped from 0.84 to 0.17 Bq/m3 . Their carriers were aerosol particles with activity median aerodynamic diameter, mainly, from 4.3 to 10 μm. The data are given on radioactive contamination of near-surface air layer in the NSC basic volume.

Analysis of Seismic Resistance of Large-Sized Electrical Equipment by the Method of Computational Analysis

In the framework of this article, the problem of confirming the seismic resistance of large-sized electrical equipment is raised using the example of large asynchronous electric motors. The analysis of regulatory documents for the assessment of the seismic resistance of such equipment showed that confirmation of seismic resistance should be performed by an experimental method and other methods can be applied only with sufficient justification of the correct functioning of products. At the same time, rather stringent requirements are established for the test conditions, which in many cases are rather difficult to implement or cannot be met at all. The analysis of the possibility of testing the ASVO 15-23-34M1 electric motor, which is used as fan drives for seismic resistance by an experimental method, under the conditions of specialized organizations accredited in Ukraine, has been carried out. It was found that the existing test installations, due to their characteristics of carrying capacity, cannot be used to carry out tests for seismic resistance of electric motors weighing more than 600 kg. Considering the above, we can conclude that the justification of the seismic resistance of such equipment by experimental methods in Ukraine today is impossible. The analysis of current standards and special normative documents for seismic testing has been carried out. In these documents, clarifications are established on the separation of electrical equipment according to their design features, which make it possible to confirm the seismic resistance of certain groups of equipment by calculation methods. The requirements for the assessment of seismic resistance by the computational method and the evaluation criteria are established. The main purpose of such an assessment is to confirm that the engine retains its structural integrity and performance during and after the passage of seismic action. An assessment of the seismic resistance of the ASVO 15–23–34M1 electric motor was carried out by the method of computational analysis using the method of limiting seismic resistance. A finite element model of an electric motor has been built taking into account all the necessary operational and seismic loads. Strength analysis was carried out using the APM Structure 3D code. Taking into account the results of the performed calculations, it can be concluded that the seismic resistance of the engine during an earthquake with an intensity of up to 8 points according to the DSTU BV.1.1–28:2010 scale is confirmed. The engine withstands seismic loads and remains operational during and after the passage of an earthquake. The minimum value of the ultimate seismic resistance of the engine is determined by the seismic resistance of the stator HCLPF = 0.142 g. Based on the results of the analyses carried out, it can be concluded that the use of the computational method for assessing the seismic resistance of large-sized electrical equipment does not contradict the requirements of the current regulatory documents and can be used as a replacement for the experimental method in cases where tests are impossible or impractical. Modern computer simulation and calculation technologies allow for a comprehensive assessment of the equipment seismic resistance and obtain high reliability results.

Modeling of the Phase Formation Processes on the Steel Surface Contacting with Water Medium under Conditions of Deep Geological Repository

The experimental modeling of the corrosive phase formation processes was performed under conditions approaching the initial and transitional stages of evolution of the deep geological repository (the hydrogen index of the medium lied in the range of 9–12 and a temperature was 50–70 °C). The specificity of the system of rotating disk electrode made it possible to determine the phase composition of corrosion products formed under oxidative conditions (the near surface layer, NSL) and, conditionally, reductive — on the steel surface (SL) covered by NSL, that significantly complicated the access of oxygen into the reaction area. It was determined that phase composition of the corrosion products at the pH0 values 9–11 was identical and it is regulated by the compensative action of cathodic half-reaction of oxygen reducing on the steel surface. Green Rust and magnetite or non-stoichiometric spinel ferrite characterized by coagulative type of the structure and spherical particle shape were determined as the main phases of SL. Iron oxyhydroxides — goethite and lepidocrocite were presented as the phases of NSL. Increase in the pH0 to 12 changes the chemical mechanism of the corrosion process and it leads to the formation of weak crystallized iron oxyhydroxide phases. It was proved the main phase formed under corrosion of steel at 50–70 °C was spinel ferrite. Its morphology is presented as the cubic shaped particles that evidences about condensingcrystallizing mechanism of their formation. Generally, the presence of Co2+ and Mn2+ cations does not influence on the phase formation process and the phase composition as well, whereas the iron oxyhydroxides with admixture of Mn2+ and Mn3+ oxygen compounds are dominant in the sediment compositions when they are formed in the presence of Mn7+.