Coolant Circulation Research Articles

Insight into the fretting corrosion behavior of 316L steel in liquid lead-bismuth eutectic at 500°C

Due to flow induced vibration caused by coolant circulation, fretting corrosion inevitably occurs between fuel cladding, heat exchanger tubes and their holders in Gen IV lead-bismuth eutectic (LBE) cooled nuclear reactors. In the current work, fretting corrosion behavior of 316 L steel in oxygen-saturated LBE at 500 °C has been investigated. It is shown that within the gross slip regime, the microstructure under fretting interface presents a stratified structure, composed of third body layer, oxide scales and plastic deformation layer. At the early stage of fretting corrosion, the dominant damage mechanism is fretting wear, showing that the thickness of third body layer is much larger than that of oxide scale. As fretting time increases, the dominant damage mechanism is gradually changed to LBE corrosion and fretting wear together, as the visible oxide scale is formed next to the third body layer. Moreover, the growth rate of oxide scales under fretting interface is accelerated by over an order of magnitude compared to that when 316 L steel exposed to LBE directly. In particular, after 60 h exposure, the thicknesses of oxide scale related to the worn and unworn regions are ∼4.5 μm and ∼0.2 μm, respectively. The occurrence of such phenomenon is thought to be ascribed to the severe plastic deformation under fretting contact interface, since the crystal defects served as perfectional nano-channels can promote the diffusivity of oxygen atoms. In addition, the acceleration diffusivities of oxygen atoms caused by the local high contact stress may also be responsible for this matter.

Implications of nanomaterials reinforcement along with sub-zero temperature cooling on microstructure and mechanical properties of friction stir processed Al7075 alloy

Al 7075 surface composites (SCs) successfully fabricated with additive three different nano-scale particles multi-walled carbon nanotubes (MWCNTs), graphene (GNP), and titanium dioxide (TiO2) by friction stir processing (FSP). The consecutive two-pass FSP was executed under a controlled cooling environment at −20 °C with methanol pumped beneath the workpiece in a specially designed fixture. All of the SCs exhibited homogenous dispersion of filler nanoparticles in their microstructures. The grain size of SCs was remarkably reduced from that of the substrate. The enhancement in hardness and strength was ascribed fundamentally to the grain refinement and Zener (pinning) effect expedited by coolant circulation. The effect of second phase matrix on grain evolution and precipitate-intermetallics formed after FSP was characterized by scanning electron microscope (SEM) and XRD pattern. It has been recorded that there is around 95–110% increase in hardness values in SCs. The wear rate is also dropped in SCs owing to enhanced hardness and the formation of lubricating coating in the case of carbonaceous composites.

Influence of sub-zero coolant circulation and additive nanoparticles on performance characteristics of FSPed Al6061 alloy

ABSTRACT Al6061 alloy has become prevalent in the industrial sector because of its low density, excellent machinability, and corrosion resistance but it lacks properties like hardness and wear. Therefore, surface modifications by friction stir processing (FSP) are done with additive nanosized particles like multi-walled carbon nanotubes (MWCNT), Titanium oxide (TiO2), and Graphene nanoparticles (GNP) To perform 2-pass FSP, a uniquely engineered fixture with coolant circulated underneath the plate at sub-zero temperatures is used. Grain refinement has been observed in micrographs as a consequence of 2-pass FSP. The coolant circulated beneath carries the heat generated and enhances the heat dispersion rate, culminating in finer grains. The calculated mean grain size of the Al6061 alloy was considerably decreased in all the surface composites. There is a substantial improvement in the microhardness profile, which has increased by nearly 70–90%, as well as a significant enhancement in tensile strength of almost 20–30% in all the composites. The ductile type of failure during tensile load is revealed by SEM fractography. The process of dynamic recrystallisation is responsible for the development of equiaxed grains. The wear resistance of all the composites is also enhanced, and the COF of Al6061/GNP composite is found to be lowest at 0.23.

Design of CTP liquid cooling battery pack and thermal Characterization experiments

Range has emerged as a significant barrier to the advancement of electric vehicles, necessitating the development of high-energy–density battery packs. However, the conventional battery integration method, CTM (Cell to Module), has a relatively low space utilization rate, which presents a challenge to the enhancement of vehicle range. Consequently, a novel battery pack integration method, CTP (Cell to Pack), has emerged as a potential solution. In order to enhance the integration degree and effective energy density of the battery pack, a CTP and a symmetric serpentine runner liquid cooling plate are proposed in this paper. A thermal model of a single cell was constructed, and the cooling performance of the battery pack under different discharge conditions was analyzed. The optimal inlet water temperature and coolant mass flow rate were then determined. The findings indicate that the battery pack devised in this study exhibits commendable cooling capabilities and is capable of satisfying the cooling requirements associated with a 2C discharge scenario. Furthermore, when the battery pack is operated under Chinese Light Vehicle Driving Conditions (CLTC), the inlet flow rate of 2 L/min has been observed to reduce the maximum temperature differential by 1.31 °C in comparison to a scenario without internal coolant circulation.

Scale effects on core design, fuel costs, and spent fuel volume of pressurized water reactors

The desire to improve the economic competitiveness and deployment pace of nuclear energy through modularization, manufacturing, and series production had led to the development of smaller size reactors. As the standard 17x17 fuel technology is mainly maintained in the pressurized water reactors (PWRs) category, this translates into a lower number of fuel assemblies in the core and sometimes a reduced fuel height. To assess the impact of such scale change in core design on fuel cycle cost and spent fuel volume, a scoping analysis tool is developed based on infinite lattice calculations, leakage, fuel management reduced models, and levelized unit cost of electricity (LCOE) estimate. As such, cost dynamics driven by fuel specific power, burnup, core leakage, feed, cycle length, fuel assembly height as well as uranium market data are captured with consistent set of assumptions and analysis methods. A selection of 5 reactor designs representative of leading PWR developers is assessed and compared. Pursuing higher specific powers and optimal burnups are highlighted as the main fuel cost reduction drivers, nevertheless, practical limitations and opportunities must be evaluated to establish the feasibility of such enhanced fuel operation. In consequence, a detailed core design is performed using SIMULATE3 code for 5 PWR variations including natural and forced coolant circulation modes, two reactor scales, power densities of 73, 112, and 123 kW/l and higher discharge burnups. Design and optimization are performed at the lattice level, for the reflector, and at the core loading level. Satisfactory steady-state operation including power distribution, coolant operating limits, and reactivity requirements are analyzed and reported in this paper. The fuel economics of the detailed core designs confirm the scoping analysis findings. Despite the unlocked power uprates in small PWRs, the achievable burnup for a given fuel specific power requires more enrichment and shorter fuel height results in higher fabrication costs per mass of fuel, which makes scaling down core size a more expensive endeavor on the fuel cycle front. Spent fuel volumes are reported for the PWRs designed in this paper. These volumes are driven by the core average discharge burnup regardless of the scale in consideration. Additional cost and core performance aspects related to heavy reflector gains, fuel-reflector substitution, and disposal cost policy in the U.S. are examined.

Large-scale Multiphysics Simulations of Small Modular Reactors Operating in Natural Circulation

Thanks to the advancements in high-performance computing, advanced modeling and simulation have become crucial in driving the development and deployment of next-generation nuclear reactors, such as small modular reactors (SMRs). SMRs offer the promise of cost-effective baseload electricity production and improved safety, while addressing some of the challenges associated with large reactor designs, such as high capital costs and extended construction timelines. As part of the Exascale Computing Project, the large-scale multiphysics simulation of an entire SMR primary system has been achieved by combining computational fluid dynamics and neutronics. In addition to the successful demonstration of full-core SMR simulations, the current study integrated the impact of natural circulation into the system. Natural circulation is the primary mechanism driving coolant circulation in SMRs. The mass flow rate in the core depends on the core power, and a numerical model has been developed to predict it. The pressure drop caused by the helical coil steam generator was also accounted for by developing a pressure drop correlation based on high-fidelity large eddy simulation results, further improving prediction accuracy. The results of the study demonstrate that the implemented natural circulation model is effective in predicting the responses of SMR full-core multiphysics simulations.

Performance analysis of the temperature-upgraded flash-driven low-temperature advanced natural circulation heating reactor system

To address the current deficiencies in outlet temperature and thermal power of low-temperature heating reactors while ensuring safety and economic viability, this study introduces the Temperature-Upgraded Flash-driven Low-temperature Advanced Natural Circulation Heating Reactor (TU-FLANC). The FLANC system innovatively utilizes the flashing phenomenon of the coolant in the rising channel to significantly increase the coolant circulation flow rate and thus enhance thermal power at atmospheric pressure. The TU system employs an Absorption Heat Pump (AHP) to upgrade the temperature of the reactor’s output heat. The two systems are interconnected via a Coupled System Heat Exchanger (CSHEX), achieving an upgrade in reactor thermal power and outlet temperature at atmospheric pressure. To evaluate the performance of the TU-FLANC system, a mathematical model of the system was established, and a computational program was developed. The impact of key parameters such as evaporator temperature, condenser temperature, and solution concentration on system performance was analyzed. The results indicate that the evaporator temperature and solution concentration have the most significant impact on the system’s coefficient of performance (COP) and the coefficient of performance considering pump work (COPW). Through Differential Evolution (DE) algorithm optimization, the optimal solution concentration combinations were determined to maximize COP and COPW under different temperature upgrade demands. For a temperature upgrade demand of 50 °C, the optimal solution concentration combinations are 40.02 % and 57.48 %, with corresponding COP and COPW values of 0.5282 and 0.4886, respectively. The research findings highlight the significant innovative potential of the TU-FLANC system in enhancing heat power and outlet temperature.

Corium interface flow dynamics investigation during severe accident in pressurized water reactors using compressive advection interface capturing method

Past nuclear accident occurrences raised strong concerns which led to research on nuclear safety. One of the major causes of nuclear accidents is the impeded circulation of core coolant, leading to decay heat removal cessation and rapid temperature rise. If uncontrolled, this results in critical heat flux, loss of coolant accidents, and core dryout. Detailed melted core relocation (i.e., nuclear fuel, graphite, and zircaloy) needs to be investigated through interface capture and multimaterial flow model coupling, which have not been done in previous studies. This work aims to investigate the impacts of temperature and core material composition on the flow dynamics during core relocation. In this study, mass fraction is discretized using a streamlined upwind Petrov–Galerkin method spatially and a modified Crank–Nicolson method temporally to accurately capture fluid interfaces using a high-order accurate flux-limiter. Two core material composition cases (individual material properties case and bulk material properties case) were considered to assess the impact of temperature and core materials composition on both flow dynamics and computational time. Temperature has a significant impact on core material transport and corium flow dynamics during core relocation. Bulk materials properties case has greater impact of temperature on its corium resulting in faster materials transport, but with higher computation time.

Novel adjustable backflow structure in electric coolant pump for balancing hydraulic performance and reducing temperature rise

An electric coolant pump serves as the source for coolant circulation in the thermal management of electric vehicles. To address the thermal issues for the printed circuit board integrated within the electric coolant pump, a novel backflow structure with an adjustable orifice diameter is designed from the impeller outlet to the printed circuit board to promote convective heat dissipation. Eventually, this flow returns to the impeller inlet, thereby altering the hydraulic performance of pump. The balance between hydraulic performance and temperature rise is achieved by adjusting the orifice diameter. Considering the variations in coolant properties with respect to temperature, numerical simulations of the electric coolant pump are performed using the unsteady compressible Reynolds-Averaged Navier-Stokes Equations. There is good agreement between experimental and numerical simulations results in both cases with and without an orifice. On an average, by increasing the orifice diameter by 1.5 mm which results in a 1.11 % decrease in efficiency, a 0.008 decrease in pressure coefficient, and a reduction of temperature by 2.98 K on printed circuit board. As the orifice diameter increases, leakage losses in the rear side chamber increases with the improvement of heat dissipation. The clash between the backflow jet and the inflow from the suction pipe results in significant entropy generation, forming jet vortices and increasing in pressure fluctuation. However, when the orifice diameter exceeds 3 mm, the cooling advantages no longer justify the trade-off in hydraulic performance. Thus, 3 mm orifice diameter is an optimal compromise between hydraulic performance and temperature control.

Two-phase cooling system for electric vehicles’ battery

The objective of this research project is to design an innovative cooling system for effectively managing the thermal conditions of batteries in electric vehicles. Electric vehicles commonly utilize Lithium-Ion cells as their power source. Despite significant advancements in Lithium-Ion technology from an electrochemical standpoint, the thermal management of these batteries remains a formidable challenge. This is primarily due to the demanding operational conditions that Lithium-Ion cells face during battery discharge, motion, and charging. The power supply unit in electric vehicles often demands high power outputs within short durations, leading to the generation of substantial heat by the batteries. This elevated working temperature poses a risk of decreased battery performance or even malfunction. Therefore, an efficient battery thermal management system is essential to optimize the performance of the batteries. In our research project, we propose and investigate a cooling system that is directly integrated into the power supply unit. Our study introduces an innovative thermal management system that combines two-phase direct liquid cooling with pulsating heat pipes. This system provides a compelling solution by combining high thermal efficiency, passive operation, and cost-effectiveness. Within this configuration, batteries are immersed in a low-boiling dielectric fluid contained in a Plexiglas container, facilitating efficient heat exchange. Simultaneously, the pulsating heat pipe operates to manage heat spikes by promoting vapor recondensation, thereby maintaining safe operational temperatures. The proposed battery thermal management system has demonstrated remarkable efficiency, ensuring that battery temperatures remain within the recommended range even under high load conditions. A notable advantage of this cooling system is its complete passivity, eliminating the need for energy-consuming coolant circulation.

An experimental study on optimization of coolant mass flow rate in a photovoltaic/thermal system

Steady, continuous, and homogeneous cooling of photovoltaic panels that consume less energy for circulation of coolant is extremely important for preventing higher energy loses especially in hotter conditions. In this study, a novel steady continuous active cooling application with water, as the coolant, was applied in a compact photovoltaic/thermal hybrid system in order to find out the effects of coolant mass flow rate and to optimize the required amount of cooling for maximizing electrical efficiency by measuring the real load voltage and current values formed on the resistive load designed for the selected photovoltaic module. It is found that the reduction in panel temperature for the cooled panel can be accepted to be enough, especially for 31.38 kg/h coolant mass flow rate, since the operating temperatures of the cooled panel reached quite near to the ambient temperature. The means of the temperature reduction for cooled panels were between 13.06°C and 13.74°C as compared with the referent panel. The increase rate of average power for the cooled operations were in between 6-8%, while the instantaneous power increase, especially in midday periods, were reached to its highest values between 13% and 15% in comparison to the referent panel. The highest percent increase in power output was derived as 7.62% for the ṁ = 31.38 kg/h coolant mass flow rate at 12:30 pm. The highest daily cumulative electrical energy production was observed for the coolant mass flow rate of 39.66 kg/h, with 6.3% of increase.

Hydraulic and thermal performance enhancement for the cold plate using topology optimization

Liquid-based battery thermal management system (BTMS) is designed to ensure the narrow operating temperature range for the desired performance of Lithium-ion batteries (LIBs), which ensures the reliable operation of electric vehicles (EVs). Among these, GM Volt design circulates the coolant in a multiple serpentine channeled cold plate, whereas the long flow path makes easily thermal saturation as well as higher pressure drop that requires more pumping power for coolant circulation. We present a cold plate that provides enhanced cooling capacity with reduced pressure drop by multi-constraint topologically optimized design. A series of thermal-fluidic numerical validations inclusive of surface temperatures on substrate of cold plate at the fixed pumping power. Compared to the conventional multiple serpentine channels, the pressure drop Δp decreases dramatically by 90 % at the same mass flowrate, and maximum temperature difference ΔTmax decreases by 60.8 % at the fixed pumping power, respectively. The optimized design exhibited a significant improvement in 2- or 3-times higher j/f factor at the same Reynolds number.

Research on the Use of Potassium Formate in the Indirect Contact Freezing System Used in Food Preservation

Low-temperature contact freezing refrigeration solutions available on the market typically contain refrigerants inside the plates, which may result in harmful emissions into the atmosphere and the contamination of food products in case of failure. However, a concept for a refrigeration plant with new coolant—a solution of potassium formate—is introduced in this paper. It comprises two systems: the primary one, where the compressor system is cooling down the coolant in the evaporator (as a heat and mass exchanger), and the secondary one, where the pump system is responsible for coolant circulation. As a coolant in the intermediate system, a 48% aqueous solution of potassium formate was utilized due to its numerous benefits. This liquid is neutral to the pump circuit installation materials and does not pose a threat to the food product in case of leakage, as it is also used as a food preservative additive. The use of liquid coolant ensures that any malfunction results in a visible leak, allowing for prompt intervention, unlike refrigerants that escape unnoticed into the atmosphere. Moreover, a refrigerant with a lower GWP than traditional solutions was chosen, making the system more eco-friendly.

Maximizing electrical output and reducing heat-related losses in photovoltaic thermal systems with a thorough examination of flow channel integration and nanofluid cooling

In recent years, global energy demand has surged, emphasizing the need for nations to enhance energy resources. The photovoltaic thermal (PV/T) system, capable of generating electrical energy from sunlight, is a promising renewable energy solution. However, it faces the challenge of overheating, which reduces efficiency. To address this, we introduce a flow channel within the PV/T system, allowing coolant circulation to improve electrical efficiency. Within this study, we explore into the workings of a PV/T system configuration, featuring a polycrystalline silicon panel atop a copper absorber panel. This innovative setup incorporates a rectangular flow channel, enhanced with a centrally positioned rotating circular cylinder, designed to augment flow velocity. This arrangement presents a forced convection scenario, where heat transfer primarily occurs through conduction in the uppermost two layers, while the flow channel beneath experiences forced convection. To capture this complex phenomenon, we accurately address the two-dimensional Navier–Stokes and energy equations, employing simulations conducted via COMSOL 6.0 software, renowned for its utilization of the finite element method. To optimize heat dissipation and efficiency, we introduce a hybrid nanofluid comprised of titanium oxide and silver nanoparticles dispersed in water, circulating through the flow channel. Various critical parameters come under scrutiny, including the Reynolds number, explored across the range of 100–1000, the volume fractions of both nanoparticle types, systematically tested within the range of 0.001–0.05, and the controlled speed of the circular cylinder, maintained within the range of 0.1–0.25 m/s. It was found that incorporating silver nanoparticles as a suspended component is more effective in enhancing PV/T efficiency than the addition of titanium oxide. Additionally, maintaining the volume fraction of titanium oxide between 4 and 5% yields improved efficiency, provided that the cylinder rotates at a higher speed. It was observed that cell efficiency can be regulated by adjusting four parameters, such as the Reynolds number, cylinder rotation speed, and the volume fraction of both nanoparticles.

Activation analysis of the lead coolant in SUNRISE-LFR

A lumped, zero-dimensional, mass transport model is combined with a depletion matrix solver to study the influence of coolant circulation on radionuclide build-up in a small lead-cooled fast reactor. It is shown that the addition of coolant circulation results in a lower activity for a minority of studied nuclides, and it is thus recommended to consider stagnant coolant when licensing a reactor. Activation analysis of three different lead qualities potentially used in SUNRISE-LFR is performed, and the result shows that a low silver content is desirable to simplify maintenance and decommissioning.

Performance evaluation of additive TiO2, MWCNT and GNP reinforced particles on Mg AZ31 based matrix composites by friction stir processing

Magnesium alloy-based matrix composites are extensively used in transportation industries because of their high specific strength and low weight. Several existing technologies were utilized for preparing these composites in any proportion of constituent elements. However, friction stir processing (FSP), which employs plastic deformation with high-energy input, has proven as the most successful with proper homogenization. This study examines the microstructure and mechanical characteristics evolved by FSP of a novel magnesium alloy grade (AZ31) with Titanium dioxide (TiO2), Multi-walled carbon nanotubes (MWCNT) and Research grade Graphene nanoplatelets (GNP) that have considerable electrical, thermal and mechanical performance. At a parametric combination, tool rotation of 1200 rpm, traverse speed of 80 mm per minute, plunge depth with 0.25 mm, AZ31 alloy was processed on a specifically engineered fixture with subzero coolant circulation through it. The average size of grains of the base alloy reduced from 52.96 µm to 2.07 µm (AZ31/TiO2), 3.19 µm (AZ31/MWCNT) and 2.54 µm (AZ31/GNP) composites at this combination. The hardness and tensile strength were enhanced from 115 to 130% and 25 to 38%, respectively. Micrographs shows that the shortened grain elongation occurred due to coolant circulation resulted in restricted strengthening precipitate dissolution, which usually occur at high temperature.

Design, Modeling, and Analysis of a Compact-External Electromagnetic Pumping System for Pool-Type Liquid Metal-Cooled Fast Reactors

Current pool-type Liquid Metal-Cooled Fast Reactors (LMCFRs), either under development or operational, immerse main reactor components in the primary coolant, (i.e. sodium), that includes heat exchangers, shielding structures, and pumping systems. Proposed main pumping systems, for some reactors that are under development, use electromagnetic pumps (EMPs) for primary coolant circulation. Annular linear induction pumps (ALIPs) are the preferred type since they are known for their advantages over mechanical centrifugal pumps (MCPs) when used to circulate liquid metals. This is due to ALIPs’ absence of moving parts such as shafts and impellers, seals and bearings, auxiliary lubrication systems, and simplicity of flow and pressure control mechanism. The immersion of reactor components in the primary sodium in the reactor vessel minimizes the likelihood of radioactive coolant leakage and loss of coolant accidents. However, since there is only one access point to reactor components, the immersion of ALIPs prevents additional potential advantages. Online pump inspection and replacement, reduction of negative effects on pump components due to the high temperature and radiation environment, an additional heat removal mechanism for self-cooled ALIPs, and simple decommissioning procedures are some possible advantages. This paper discusses a study conducted to investigate the possibility of using large EMPs, ALIP type, that are located outside the reactor vessel and connected in parallel, instead of in vessel sodium immersed ones for pool-type LMCFRs. The large, outside-vessel EMP idea is tested on a liquid metal-cooled test reactor design using an experimentally validated multiphysics finite element analysis tool. Specifically, the steady state reactor’s primary circuit cooling and pressure requirements are used to design two, in-vessel ALIPs and then two outside-vessel ALIPs. The two pumping systems are compared in terms of their geometries, performance characteristics, and impact on overall reactor design. It is found that the outside-vessel EMP design provides the same performance requirements and offers additional advantages compared to the in-vessel pump with only minimal reactor vessel modification and a slight drop in efficiency.

Thermomechanical loads of powerful turbogenerator stator winding insulation in the presence of water cooling defects

Introduction. An analysis of incidents linked to power units’ emergency disconnecting from network as a result of turbogenerators’ malfunction on the NPP of Ukraine is conducted. It is identified, that the reason of the majority of incidents is an insufficient reliability of the stator winding’s direct cooling system. Problem. The most problematic point in winding for today is the frontal parts, where, while cooling is reduced, there are not only thermal, but also thermomechanical loadings on an insulation appearing. The level of these loading depends on structural design of frontal parts and a character of violation of coolant agent circulation in a bar. In some cases they can exceed limit values. The spread and the quality of research on this issue for today are insufficient. Goal. The aim of the completed research is to determine the thermomechanical loading of insulation of stator winding bar in a powerful turbogenerator with a direct liquid cooling under condition when coolant circulation is malfunctioned. Methodology. A complex mathematical model of thermomechanical processes in an insulation of stator winding bar of a powerful turbogenerator is developed. It takes into account the real geometry of the winding bar, variable thermal loading of core elements in radial and axial directions, as well as ways of fixation of slot and frontal winding parts. Studies of thermomechanical processes in an insulation of stator winding bar of turbogenerator are conducted. Results. Values of mechanical displacement and stress for the different modes of malfunction are obtained. Areas of bar, where mechanical loading may exceed the boundaries of mechanical durability of material of insulation of stator winding are identified. With decline of coolant liquid consumption the radial displacement and stress in the winding insulation bar in the area, where the bar exits from the slot are increasing along with that the values of radial stress of insulation of the winding bar in places of frontal parts’ fixation exceed limit values. Practical significance. The offered mathematical models allow to realize calculation experiments and can be used in practice for development and validation of diagnostic systems, analysis, design and investigation of emergency situations during exploitation of turbogenerators on power stations of Ukraine.

Energy efficiency of an electric boiler with indirect surface resistive heating of the heat carrier

Introduction. The share of energy for heating of households makes up to 80 % of the total energy consumption. Works on increase of energy efficiency of HVAC equipment make the biggest contribution to reduction of manmade impact on the environment. Power-to-Heat technology — electric boilers used for heating with water systems with radiators and low-temperature heating devices of the “warm floor” type. The research of energy efficiency of the wall mounted electric boiler equipped with the heat generator of indirect surface resistive heating of the heat carrier is presented.
 
 Materials and methods. The object of the research is an electric boiler ARDERIA E24, with a nominal capacity of 24 kWh, equipped with a circulation pump, an expansion tank with a volume of 6 L, a group of hydraulic safety and heat carrier recharge. Control of the capacity of the boiler is carried out on semistors with cooling radiators located on the reverse line of the circulation circuit. The boiler is equipped with an electric heat generator with indirect resistor surface heating of the heat carrier.
 
 Results. The studied electric heat generators have high specific volumetric power equal to 9.3 kWh/dm3. The average specific power of tubular electric heaters is 25.1 watts/cm2. The coefficient of energy efficiency of the boiler is 0.986. The hydraulic resistance of the unit is 1.25 meters of water column at a flow rate of 1.0 m3/h.
 
 Conclusions. The application of the electric heat generator design consisting of a cast aluminum block with tubular heating elements placed inside and a spiral coolant circulation pipe showed high energy efficiency with coefficients of 0.98–0.986 in the modulation range of heat output. The heat output modulation factor of the electric boiler is 12 (2–24 kW). The energy intensity of the heat generator design is 9.3 kWh/dm3, which is significantly higher than traditional heat generation units with “wet” heating elements. The boiler’s heat output control system eliminates the possibility of the temperature rising above 88 °C on the inner heating medium circulation pipe surface.

Cooldown of a water-cooled reactor during the natural circulation mode using decay heat of the core and a low-power steam turbine

The Fukushima-1 nuclear power plant accident in 2011 confirms that NPP safety is one of the most urgent present-day problems. Moreover, a rapidly growing number of NPP units increases the risk of serious radiation accidents. The authors have developed a novel cooldown method for water-cooled reactors to be used under loss of connection with the power system using decay heat from the core and a low-power steam turbine. A distinctive feature of the method is financial viability of the steam turbine due to its possibility to generate additional electricity at the normal conditions of NPP operation. The aim of the invention is to ensure a high level of NPP safety while maintaining and increasing commercial efficiency and competitiveness of nuclear power plants. To implement the developed method, it is necessary to provide guarantee for reliability of NPP units at natural circulation of the primary coolant, since the primary circulation pumps will be shut off caused by a trip of a reactor. For this reason, the authors carried out an investigation into international practice theory and actual usage of natural circulation of water-cooled reactors. Experiments conducted at Kozloduy NPP and research of a number of scientists demonstrate a possibility to maintain constant decay heat removal that will ensure obtaining steam with stable parameters in the secondary circuit. The second part of the research deals with a new method aimed at beneficial usage of steam obtained by using the decay heat from the core under natural circulation of the primary coolant. The generated steam can be used in the additional low-power steam turbine to power supply the NPP's auxiliary needs. In our earlier research we performed a preliminary assessment of reliability of the developed system. The results of the research showed that a combination of a steam turbine and a standard three-channel emergency power supply system with diesel generators enables a safety level comparable to that of external heat exchangers in the passive heat removal systems. At the same time, passive heat removal systems require significant capital investments and maintenance costs, whereas according to the method developed by the authors, a low-power steam turbine can operate in the normal regime to generate additional electricity, as well as in combination with energy storage systems. This will ensure full cost recovery of the developed installations. To assess a reduction of risks of nuclear accidents in terms of cash equivalents when using the novel method, the authors analyzed the influence of various factors on damage expectancy in case of an accident at NPP, based on the example of Fukushima and Chernobyl NPPs. The investigation showed that a reduction of nuclear accident risk in terms of cash equivalents, when implementing the authors’ method aimed to supply the auxiliary NPP needs in blackout situations, reaches tens of millions of US dollars per year.