[Significance] Aiming at carbon neutrality, energy structure transformation and upgrading has become a trend for global energy system progress. Nuclear energy can effectively fill the power and heat supply gap during coal substitution. It has the advantages of a flexible layout, wide application, and insensitivity to climate change and the global market, which ensures national energy security. A heat pipe (HP) is a passive and efficient heat exchange element with a wide temperature range, stable and reliable performance, and high security. It is ubiquitously applied in the aerospace, energy and chemical industries, as a solar collector, for electronic cooling, and in other fields. HPs are irreplaceable in advanced nuclear energy with multi-domain, multi-scale, and multi-section applications. Therefore, existing studies on HPs must be summarized for advanced nuclear technology.[Progress] According to operation temperature, HP applications in nuclear technology are classified into three parts:nuclear power/propulsion systems, unclear safety facilities, and nuclear urban service. First, heat pipe-cooled reactors (HPRs) use alkali metal high-temperature HPs to passively export the core heat, which has the advantages of inherent safety and storage and transportation. Because of a long phase transition during startup and the unraveling alkali metal dynamic and heat transfer process in the steady state and the transitory state, the startup characteristic and heat transfer performance of alkali metal high-temperature HPs have been the difficult part of HPRs development. To adapt to different energy needs, the designs of HPRs ranging from kilowatts to megawatts and the corresponding thermoelectric conversion schemes have been proposed. HPRs will have broad prospects in aerospace, ship power, deep sea exploration, land-based power supplies and other fields. Second, with passive characteristics, an HP is a better technical choice for safety facilities. In nuclear power plants, separated HPs have been applied to passive heat removal systems, passive emergency core cooling systems, passive containment cooling systems, and passive spent fuel pool cooling systems. In nuclear spacecraft cooling, an HP space radiator composed of an HP and a heat sink is a more promising space radiator, having good thermal properties, temperature conversion characteristics, environmental adaptability, anti-debris impact performance, and anti-single point failure characteristics. In a thermonuclear reactor, HP is also used in first-wall cooling. Third, HPs are mainly used in waste-heat recovery and low-temperature heat transfer to improve energy efficiency and safety in nuclear industry applications and urban services. Researchers have developed several desalination systems based on HP systems and waste heat from steam power plants and generators. Districted heating and nuclear power generation, hydrogen production, and heating triple production systems are promoted and have become popular in China. Finally, challenges in HP performance, adaptive design in HPRs, and HP operation and maintenance were discussed.[Conclusions and Prospects] The HP is perfectly in line with the advanced nuclear safety design concept. Currently, although HPs are widely used in nuclear power/propulsion systems and reactor safety facilities, their practical applications in the nuclear industry and urban service remain relatively scarce, and there is almost no participation in the intermediate temperature segment. At last, we propose the prospects of advanced HP technology.

[Significance] Two-phase flow instability is a classic problem in the field of steam generators and other two-phase flows. Therefore, it has been studied extensively. In nuclear reactor steam generators, two-phase flow instability may occur on the secondary side and interfere with the control system, causing fatigue-induced damage to the equipment. While two-phase flow instability can have different complex mechanisms and many influencing factors, there are various methods to research and analyze this phenomenon.[Progress] The phenomenon of two-phase flow instability can be classified into two types. The mechanisms of flow excursion (LE), density wave oscillation (DWO) and pressure drop oscillation (PDO) are introduced. LE and PDO can occur in conditions corresponding to the region of negative slope in the hydrodynamic characteristic curve (mass flow rate vs. pressure drop curve) in the heated tube and can be avoided by eliminating the negative slope region. However, DWO can also occur in the positive slope region due to the phase difference between two transient processes. One of these is the mass flow rate variation caused by variation in the driving pressure difference, which is controlled by the rate of momentum transfer. The other is the transient variation of the subcooled water region length and the density of saturated two-phase region fluid, which is caused by heat transfer. Changes caused by heat transfer are slower than changes in flow and pressure. Various research methods of two-phase flow instability are systematically summarized, including the theoretical time-domain method (nonlinear and linear methods), theoretical frequency-domain method, and discrete numerical method, starting from the conservation equations. The mathematical criterion obtained from the theoretical time-domain model can analyze the parameters' influence exactly over a wide range. The spatial distribution of density, enthalpy, and other physical parameters in the frequency domain can be obtained using the theoretical frequency-domain method, and the stability boundary it predicts is more accurate than that predicted by the theoretically simplified linear time-domain method. In addition, the research status of LE, DWO, and PDO is systematically summarized, with a particular focus on the work of our research group. New dimensionless numbers (two-phase number, superheated number, dimensionless pump number, and dimensionless bypass number) are proposed to describe the stability of the complex, superheated, two-phase flow boiling systems. A law unifying the influence of the Froude number, friction number, and geometric parameters (tube length, tube diameter, etc.) on DWO was developed. Previous contradictory conclusions are explained. A rigorous theoretical derivation and proof of the effects of model simplification and boundary conditions are presented. The requirements for conservatively modeling a real nuclear power plant steam generator and secondary loop system using a test section consisting of a single or multiple parallel small-scale heated tubes and a simplified engineering verification test loop in the laboratory are clarified. Finally, methods to avoid LE and DWO in the steam generator of the high-temperature gas-cooled reactor are introduced based on reactor design. To predict the stability of the high-temperature gas-cooled reactor-pebble bed module (HTR-PM) engineering test facility-steam generator (ETF-SG), theoretical time-domain method, theoretical frequency-domain method, RELAP5 model, and one-dimensional transient program are developed, which are in good agreement with the experiments.[Conclusion and Prospects] The results from the ETF-SG can conservatively predict the stability boundary of the steam generator and secondary loop of the HTR-PM nuclear power plant. The conditions for the occurrence of in-phase and out-of-phase DWO in ETF-SG are revealed, and methods for eliminating them are recommended. The above achievements are applied in the design, commissioning, and operation of the HTR-PM steam generator.

[Objective] A highly reliable energy-dense power source is the critical core component of the facilities involved in space and ocean exploration. The heat pipe reactor is equipped with a compact solid-state core, which provides benefits such as high reliability, passive heat transfer, and long life, rendering it the ideal solution for the current multipurpose power supply. However, the technical feasibility, reliability, and performance of the alkali metal high-temperature heat pipe, which is the critical component of the heat pipe reactor, remains to be verified and tested. Furthermore, the integrated heat pipes and thermoelectric converter technology need to be validated. In addition, the coupled operating characteristics of heat pipes and thermoelectric converters under high-temperature operating conditions is an important issue that remains to be addressed, both theoretically and experimentally. The principle design of the heat pipe nuclear reactor power supply requires efficient support based on a sufficient amount of experimental data. This study verifies the principle feasibility of the heat pipe nuclear reactor used as a power supply. The operating state features of the alkali metal high-temperature heat pipes and thermo-acoustic power generators under startup and steady states are studied experimentally.[Methods] This paper studies the coupled operating characteristics of sodium heat pipes and thermo-acoustic power generators experimentally. A multihole stainless steel cube assembled with ten electric heating elements and four sodium heat pipes is used to simulate the heat pipe nuclear reactor. A specially designed thermo-acoustic power generator is fitted with two symmetrical generator units and used as the thermoelectric converter. Stimulate and activate the thermo-acoustic power generator while the heat pipes are in fully startup state is the critical operation of thermo-acoustic power generator coupling sodium heat pipe system startup and stable running.[Results] The key technology of sodium heat pipes and thermo-acoustic power generator-integrated system startup method was obtained in this study. The sodium heat pipe was of prominent thermal response, and the temperature buffer could effectively improve the reliability of the thermoelectric conversion system. The coupling characteristics featured by the temperature field evolution of sodium heat pipes and thermoelectric converters were obtained at steady states. The thermoelectric conversion efficiency of the system and the output power of the thermo-acoustic power generator increased as the heat pipe operating temperature rised. In the long-time operation test, with a system heating power of 1 900 W, the thermo-acoustic power generator output power and thermoelectric conversion efficiency were 360 W and 19.00%, respectively. In the limit test of the operating temperature, with a heating power of 2 300 W, the thermo-acoustic power generator output power and thermoelectric conversion efficiency were 463 W and 20.13%, respectively.[Conclusions] The integrated technique of high-temperature sodium heat pipes and thermo-acoustic-electric energy converter effectively simulates the processes of heat transfer and thermoelectric conversion of the thermoelectric conversion system based on a heat pipe reactor. The coupling characteristics of heat pipes and thermo-acoustic power generators are obtained. This study verified the energy conversion principle feasibility for a heat pipe reactor coupled with a thermo-acoustic power generator. The results of this study can provide support for the prototype design of a nuclear power facility based on a heat pipe reactor.

[Objective] In high-temperature gas-cooled reactors (HTGR), experimental results show that thermal radiation plays a significant role in the heat transfer process and it is highly related to the inherent safety of the nuclear reactor. The HTGR core is a dense pebble bed of single-sized fuel spheres. It is a fundamental and very challenging task to discuss the particle-scale radiative heat transfer in a pebble bed. In this study, using a machine learning approach, an artificial intelligent regression model is developed and trained on a large dataset to predict the obstructed view factor between fuel spheres in a nuclear pebble bed.[Methods] Comparing the numerical and experimental results, the radiative transfer equation (RTE) method significantly underestimates the radiative heat flux in dense pebble beds. The Lagrangian discrete element method (DEM) is often applied in pebble flow simulations. Thus, particle-particle interactions in the DEM framework are used in this present study. The view factor in the base model is calculated using an explicit analytical expression with the elliptic integral function. The model reasonably describes the effect of the distance between spheres and the average obstruction contribution of the surroundings. The obstruction function is proposed by fitting the numerical results. The analytical base model is used to efficiently obtain the dominant parts of the view factor. Furthermore, for the machine learning model, a large dataset of the view factor is established by the particle-scale DEM packing of the HTR-PM, an HTGR demonstration plant, and the thermal ray tracing method. Following preprocessing, the dataset contains a total of 16.6 million records under various conditions in a pebble bed. The gradient boosting decision tree (GBDT) model is used to learn the rules for view factor regression. The model input is the Cartesian coordinates of the sphere and its surrounding ones. The model output is the difference between the ground truth of the view factor and the analytical base model's prediction. 80% of the dataset is used for training, and 20% is left for validation. The mean square error (MSE) and coefficient of determination are selected to evaluate the machine learning model. The GBDT model was trained using the open-source software LightGBM, and the hyperparameter tuning was performed in the FLAML platform to find the best model parameters.[Results] The MSE of the trained machine learning model decreases gradually as the model complexity increases. The analytical base model provides a generally satisfactory forecast of the view factor in the pebble bed, and the gradient boosting decision tree model trained by big data greatly improved the prediction accuracy. With the base model, the coefficients of determination of the trained machine learning model are greater than 0.999.[Conclusions] This study presents an efficient artificial intelligent model for obstructed view factor prediction for heat transfer research, parameter optimization, and thermal hydraulic analysis of the nuclear pebble bed. The trained machine learning model can also be used in effective thermal conductivity analysis, and it is feasible to be coupled with the CFD-DEM simulations of conduction and heat convection in large-scale nuclear pebble beds.

[Objective] The helium circulator of the high-temperature gas-cooled reactor (HTGR) is advanced core equipment independently developed by Tsinghua University and is highly important for normal reactor operation. The shutdown of the helium circulator will lead to an emergency shutdown of the reactor, directly affecting the operation of the nuclear power plant and possibly causing safety problems. Therefore, it is necessary to evaluate the reliability of the helium circulator and study the preventive maintenance strategy to ensure the high-quality operation of the HTGR demonstration project (HTR-PM).[Methods] First, we used the failure mode, effects and criticality analysis (FMECA) method to analyze the failure modes, causes, effects, and degree of severity of the components of the helium circulator and list the usage guarantee recommendations. Since FMECA has not been performed on HTGR thus far, we referred to the national military standard to specify the severity degree of the helium circulator's failure consequences. Through FMECA, we can also identify its key components, the parts that must be emphasized during the design and maintenance of the circulator. Then, we used the general component data to determine the failure rate of the circulator and the failure rate proportion of each component. Finally, we used the reliability-centered maintenance analysis (RCMA) method to plan the preventive maintenance strategy of the circulator and put forward preventive maintenance plan suggestions. Preventive maintenance is mainly performed through condition monitoring, function test, etc., which will not affect the normal operation of nuclear power plants. According to RCMA, the preventive maintenance measures of the helium circulator mainly include condition-based maintenance (CBM), usage inspection, function test, and so on. CBM can be performed online, and other preventive maintenance measures can be completed during the overhaul; thus, these measures can effectively improve system availability and reduce financial losses. In addition, the maintenance interval is mainly based on the severity degree and the proportion of the failure rate of components, as well as the corresponding maintenance measures. A more accurate maintenance interval must be updated after receiving the monitoring data feedback.[Results] The calculated failure rate of the helium circulator was 0.18 times/year, which met the design criteria that the helium circulator shut down due to failure should occur less than once a year. However, a more accurate failure rate evaluation needs to be further updated after accumulating actual operation data. The calculation results showed that the drive motor in the helium circulator exhibited the highest failure rate of 88.57%, while that of the frequency converter in the drive motor was 60.82%. Therefore, the reliability of these components should be increased to improve that of the helium circulator. The reliability prediction results can provide a reference for improving the design and then the operation reliability of the helium circulator.[Conclusions] The research process of this paper is significant as a reference for conducting reliability analysis, improving the design quality, and planning maintenance strategy of newly developed nuclear power equipment, and it can also provide insights for relevant analyses of equipment in other nuclear power plants.

[Significance] China's strategic goals of "carbon peaks" and "carbon neutrality" will have a significant impact on the country's economic and social development despite the challenges along the way. The energy and power industry is an important player in carbon dioxide emissions in China and the main battlefield for constructing new energy systems and initiating relevant industrial revolutions. Despite the increasing maturity of carbon capture, utilization, and storage(CCUS) technologies, their deployment faces strong resistance from the industry because of the high cost and energy consumption. For example, the cost of carbon capture alone ranges between 260 and 280 RMB/t, corresponding to an increase in utility cost of 57.51% to 93.38%, depending on the region. More importantly, the planet earth has a physical limit for carbon storage, and an alternative technical route is needed to achieve cost-effective zero-carbon emissions. Nonetheless, despite the importance of constructing new energy systems, China's energy resources determine that we will continue to rely on traditional fossil fuels for decades to come.[Progress] Therefore, this study analyzes the feasibility of state-of-the-art technologies, such as catalytic conversion, carbon material and hydrogen utilization, and hydrogen-fired power generation. This study proposes the use of coal, gasoline, natural gas, and biomass as chemicals rather than fuels. The "fuels" are first converted into hydrogen-carbon chemicals and then decomposed into elemental carbon and hydrogen by catalytic conversion. The resultant elemental carbon is upgraded into high-value carbon materials, such as carbon nanotubes, graphene, and carbon fibers, which can be used for battery production. Meanwhile, hydrogen is used for energy production through combustion and fuel cells. The batteries produced using carbon materials can also support decentralized energy and energy storage from power plants. Regarding hydrogen-based energy production, developed countries, such as the USA, and Japan, have developed hydrogen-fired power generation aimed at commercialization in 2030 or earlier. We also conduct a feasibility study by pilot testing and techno-economic analysis. State-of-the-art experimental studies show that the key technical elements include (1) the production of carbon-hydrogen feedstock from coal, which is ready for deployment to the market; (2) the catalytic decomposition of hydrogen-carbon, e.g., CH₄ and C₃H₆, into carbon nanotube and hydrogen, which is proven feasible at the pilot scale but requires further research and development in catalysis and fluidized bed reactor system for upscaled production; (3) the separation and purification of downstream products for high-purity carbon materials and hydrogen, where catalytic removal or recycling is essential to the pure carbon product, and membrane separation needs to be developed for pure hydrogen production; and (4) the most challenging, but essential, technology is the hydrogen-based gas turbine for power generation, with pilot plants built in the USA, Australia, and China for testing with 5% to 10% of hydrogen. Nonetheless, only catalytic conversion of CH₄ can provide the amount of hydrogen needed in a power plant in real time. Thus, we conducted a techno-economic analysis by retrofitting a natural gas-fired power plant, where part of the natural gas is converted into hydrogen and the hydrogen is mixed with the incoming natural gas for power generation. The proposed pathway has been proven to be economically feasible, provided all of the technologies are ready.[Conclusions and Prospects] In conclusion, we propose a novel pathway to efficient and clean utilization of fossil fuels as resources to produce high-efficiency, low-carbon, and low-cost hydrogen and high-value-added carbon materials, as well as zero-emission power generation. Admittedly, it takes decades to reach the final goal, but this pathway is expected to tackle the economic challenges to achieving the "carbon peaks" and "carbon neutrality" goals (or "double carbon" goals) of the energy and power industry.

[Objective] High-temperature gas-cooled reactors (HTGRs) have promising applications in the face of current environmental and energy problems due to their inherent safety and high reactor outlet temperature. They can be used not only for power generation but also for large-scale hydrogen production. Hydrogen can be used as a direct reducing agent in steelmaking, contributing to carbon reduction in the steel industry. It is necessary to study the coupling of HTGRs and the steelmaking system.[Methods] In this study, an HTGR-based steelmaking system is proposed, which includes five submodules:reactor module, reactor intermediate loop module, hydrogen production module, power generation module, and steelmaking module; then, a multi-generation energy system was investigated. In the reactor module, two HTGRs are connected in parallel as heat source, their thermal power is 250 MW, and the reactor outlet temperature is 950 ℃. The heat from the reactor module is transferred to the hydrogen generation module and the power generation module through an intermediate heat exchanger. The hydrogen generation module uses hydrothermal decomposition based on the iodine-sulfur process to generate hydrogen. The heat required for the iodine-sulfur process is provided by helium in the intermediate heat exchanger circuit and by the extracted steam from the power generation module. The hydrogen produced by the hydrogen production module is routed to a shaft furnace (SF) as the reductant and fuel for direct reduction ironmaking, and the oxygen produced by the hydrogen production module and the electricity produced by the power generation module are routed to an electric arc furnace (EAF) for steelmaking. The iodine-sulfur process efficiency, the power ratio of the power generation module to the reactor, the percentage of direct reduction iron in raw materials on the EAF system capacity, and the carbon emissions of the system are analyzed.[Results] In a steelmaking system with heat supplied by two 250 MW HTGRs, 1.35 t of iron ore is required to produce 1 t of steel when the power ratio of the power generation module and the hydrogen generation module is 1:1, the proportion of direct reduction iron in the raw material is 90%, and the iodine-sulfur process efficiency is 37.8%. Simultaneously, the system can deliver 63.0 MW (4.97 GJ for 1 t of steel) of electric energy to the power grid, and the steel production rate is 45.6 t/h. The parameter analysis shows that increasing the hydrogen production efficiency of the iodine-sulfur process can significantly increase the steel yield; however, the power consumption of the iodine-sulfur process module increases simultaneously, which reduces the output to the power grid. The steelmaking system proposed in this paper has very low CO₂ emissions. When the proportion of directly reduced iron in the EAF is 90%, only 17.2 Nm³ (33.8 kg) of CO₂ is emitted in producing 1 t of steel.[Conclusions] Therefore, coupling the HTGR hydrogen production with the steelmaking system has great application potential for significantly reducing the CO₂ emissions of the steelmaking industry and eliminating the dependence on coke.

[Objective] The green energy system revolution is accelerating, and hydrogen plays an increasingly important role in energy systems. However, the "green hydrogen production technique" with low carbon emissions evokes increasing concern.[Methods] This article suggests using a high-temperature electrolysis hydrogen production system coupled with a high-temperature gas-cooled reactor (HTGR), which has a heat power of 250 MW and a helium exit temperature of 750℃ or 950℃. An ASPEN simulation module of the full hydrogen production process was constructed, and the effects of the heat and electricity ratio on the hydrogen production rate and energy costs were analyzed. Based on the results, the hydrogen production costs were estimated, and cost reduction methods were discussed.[Results] The key results were as follows:1) a higher HTGR helium exit temperature resulted in a larger hydrogen production rate and lower energy costs. At 750℃, the maximum hydrogen production rate, electricity cost, heat cost, and total energy conversion efficiency of the HTGR hydrogen production system were 28 108 m³/h, 3.73 kW·h/m³, 0.49 kW·h/m³, and 40.1%, respectively. However, at 950℃, the maximum hydrogen production rate increased to 35 160 m³/h, the electricity cost fell to 3.11 kW·h/m³, the heating cost increased to 0.56 kW·h/m³, and the total energy conversion efficiency rose to 50.2%. These data fitted a system with 7 013 solid oxide electrolysis cell (SOEC) modules, as designed in this article. 2) Increasing the current density of SOEC would significantly decrease the cost of investment in the hydrogen production system and, therefore, the hydrogen production costs. For a 950℃ HTGR hydrogen production system, if the current density rose from 1 A/cm² to 5 A/cm², the power density of SOEC would increase five times, and the number of SOEC modules would drop to one-fifth; therefore, the investment cost of the module would be low. Following upgrades and breakthroughs in SOEC stack integration technology, each module would contain 150 SOEC cells instead of 30, and the number of modules would fall to 281, and this would be followed by a proportional drop in the balance of plant and repairing costs. However, the calculation results also showed that when the current density increased from 1 A/cm² to 1.5 A/cm², the hydrogen production cost dropped from 26.1 yuan/kg to 19.4 yuan/kg, which could fulfill the demand for hydrogen energy in transportation. 3) The production of valuable chemicals from the anode is another option for increasing the value of the anode product and expanding the application of high-temperature electrolysis via nuclear energy. In addition, the cost of the hydrogen would be apportioned. We also estimated the hydrogen production cost when ethane was added to the anode to produce ethene. When the conversion rate of ethane was higher than 20%, the production cost of both systems dropped significantly. Based on the 950℃ system, the hydrogen production cost would be lower than 20 yuan/kg with an ethane conversion rate above 28%; if the ethane conversion rate approaches 100%, the hydrogen production cost would be lower than 3.8 yuan/kg.[Conclusions] The proposed HTGR high-temperature electrolysis hydrogen production system has a high hydrogen production rate, low energy costs, and huge potential for further cost reduction, meaning that this technique is eco-friendly and can be employed on a large scale.

[Objective] Rectangular channels are widely used in energy power, petroleum, and chemical systems due to their compact structure and high heat exchange efficiency. In heat exchangers and reactor cores that use rectangular flow channels, the channels are separated from each other, which could result in instability phenomena under certain working conditions. Existing research shows that the coupling structure can distribute heat among the channels based on their respective heat transfer characteristics. Heat conduction through the wall can reduce the wall temperature fluctuations, reduce peak wall temperatures in the dried-out state, and improve the stability of the system. Given the fact that wall coupling heat transfer between parallel channels improves system stability, this study aims to explore the influence of coupled heat transfer on the flow instability of parallel rectangular channels, which has high research value.[Methods] In this paper, the thermal-hydraulic program REALP5/MOD3.3 was used to analyze the flow instability characteristics of the parallel channel, and the independent and coupled heating conditions were realized by changing the thermal components. The objects used in this paper are parallel rectangular channels with a heating length of 1 000 mm and a cross-section of 40.0 mm×2.0 mm; a coupled heat transfer wall with 40.0, 2.0, and 1 000.0 in width, thickness, and height, respectively; an axial grid size of 40 mm in size; and a grid size of 0.5 mm in the direction of the thickness of the coupled heat transfer wall. The influence of the coupled heat transfer on the flow instability of parallel channels was studied based on the ratio of heat transfer of the coupling wall and the heat transfer inside the fluid medium during a flow oscillation cycle. Accordingly, the influence of thermal parameters such as system pressure, mass flow rate, and inlet subcooling of the parallel-channel system coupled with heat transfer on the flow instability boundary parameters was studied.[Results] (1) The heat transfer through the coupling heat transfer wall was less than that of the fluid in the unstable process, making it difficult to eliminate the flow instability between channels. (2) The instability boundary of the coupled rectangular channel was slightly higher than that of the separation channel due to the influence of heat transfer through the wall, and the stability of the system was higher before instability occurred. (3) The boundary power increased almost linearly as the mass flow rate increased. This was primarily because the length of the single-phase section and the proportion of frictional pressure drop increased with an increase in the mass flow rate, enhancing the overall stability of the system. (4) Given the same pressure and flow rate, the difference in the density of the fluid at the inlet and outlet of the rectangular channel and the accelerated pressure drop decreased, and the stability of the parallel rectangular channel was enhanced with an increase in the inlet subcooling degree. (5) Given the same inlet subcooling degree and flow, with the increase in system pressure, the density and kinematic viscosity differences and frictional pressure drop of the vapor and liquid phases decreased, and the overall stability of the system was enhanced with an increase in the system pressure.[Conclusions] The instability boundary parameters of the coupled and separated rectangular channels are similar; however, the system stability of the coupled rectangular channels is higher before instability occurs. The influence of thermal parameters on the instability boundary is similar for coupled heat transfer parallel rectangular channels and separated channels. Furthermore, increasing the system pressure, mass flow rate, and inlet subcooling can enhance the stability of the system.

[Objective] The unintended leakage that occurs on the interface between the static seal and the flange leads to potential environmental pollution, economic loss, and safety accidents. The evaluation of leakage rates is a major concern in the industrial field. However, due to the problem's complexity, the leakage rates are usually calculated by a simplified or equivalent model, which deviates from the actual state of static seals.[Methods] In this study, a semi-analytical model is proposed to evaluate the leakage rates for static seals using the contacting model and seepage theory. The leakage rate is expressed as a function of working conditions, fluid properties, and leakage channel permeability. Working conditions, such as fluid pressure and fluid properties, including viscosity, are input parameters, while permeability needs to be inferred. Therefore, the deformation of asperities on the surfaces of static seals is calculated in advance by the elastic-plastic contact model. The porosity and permeability are calculated afterward. Several assumptions are made to simplify the calculations. First, the contacting process between the flange and the static seal is equivalent to the process of a rigid and rough surface. The distribution of asperity heights is then described using a Gaussian function, which is suitable for common materials. Finally, the leakage channel is assumed to be self-similar since the observation scale is much smaller than the characteristic scale of leakage channels. This assumption enables the relationship between permeability to porosity to be expressed explicitly.[Results] The deformation of asperities under different loads was investigated using this model. It was shown that the reduction of the dimensionless height of the leakage channel results in a lower porosity, and a larger load was required. During this process, the portion of the asperities subjected to plastic deformation also increased. The porosity of the leakage channel decreased rapidly under a lower load. However, the porosity decreased at a slower rate as the load further increased because the asperities became difficult to deform. The total deformation of the rough surface was less relative to the plasticity index compared to the pure elastic model because the former considers the rearrangement of local stress by interacting asperities and the displacement of the reference plane. The plasticity index was used to examine the deformation of the rough surface in relation to the surface topology. Although asperities with a higher plasticity index were more likely to undergo plastic deformation, the total surface deformation was reduced in this case. Several relationships between porosity and permeability were presented and validated. The relation under the assumption of self-similarity agreed well with the literature results, whereas the Kozeny-Carman relation was less accurate because it was based on an assumption deviating from the actual geometry of the leakage channel. When the load increased, the permeability of the leakage channel decreased, and thus the leakage rates reduced.[Conclusions] The value of leakage rates can be inferred by the model proposed in this study as long as the working condition and material properties are given. The model can explain the effects of load and surface topology. The model can be applied to various types of static seals, such as gaskets and rings, since it focuses on the contacting process, which is irrelevant to the structural design. However, the model's predictability decreases for seals with uneven stress or heavily worn surfaces.

[Objective] Condensing heat exchange is a crucial process in the primary circuit of small modular reactors and passive safety systems that rely on natural circulation as the driving force. With the higher requirements for heat exchange efficiency and reactor safety, in-depth research and an understanding of the condensation heat exchange process are needed. Therefore, numerical simulations of the condensing heat exchange process have attracted increasingly more interest. However, due to the complex phase change, the condensing heat exchange process is difficult to model using analytical equations. Traditional numerical simulation methods use the empirical equations summarized in experiments, and their universalities are controversial. In contrast, the lattice Boltzmann method is a mesoscopic-level numerical simulation method that tracks particle clusters and uses probability density functions to describe their distribution, resulting in a simple and clear structure that appropriately ignores the details of molecular motion. Moreover, it allows direct iterative solving of the probability density distribution function without relying on empirical equations. In previous studies, the feasibility of using the lattice Boltzmann pseudopotential model in condensation process simulation was verified. Subsequently, numerous researchers have used this model to analyze the condensation mechanism.[Methods] This study is based on the lattice Boltzmann method and uses a dual distribution function to simulate the condensation process of stationary saturated vapor within a vertical channel. To analyze the fluid flow characteristics, a pseudopotential model is used to simulate the density field variations during the vapor condensation. Additionally, a temperature distribution function is employed to simulate the temperature field changes during the vapor condensation, allowing for an examination of heat transfer efficiency. Throughout the simulation, we analyze the effects of channel width and the hydrophilicity and hydrophobicity of wall conditions on the condensate flow and heat transfer rate.[Results] The results showed that:1) When saturated vapor encountered a hydrophilic wall, it first condensed to form a thin liquid film covering the entire wall surface and then formed a steady liquid film from the top of the vertical channel, gradually expanding downward. Due to the pressure difference caused by the vapor condensation, the saturated vapor flowed down into the channel from the inlet at the top of the channel. 2) Under hydrophilic wall conditions, decreasing the channel width from 500 to 150 decreased the steady-state average mass flow rate at the inlet by approximately 20% and decreased the steady-state average heat flux density on the wall by approximately 6.5%. 3) The simulation results under different hydrophobic and hydrophilic characteristics were consistent with the theoretical analysis, indicating that the stronger the wall hydrophobicity was, the later the starting time of droplet nucleation and the lower the starting point of the vertical liquid film. On the ordinary hydrophobic wall surface, the droplet condensation was difficult to sustain, and after this surface was covered by a liquid film, the heat transfer rate was slower compared to the hydrophilic wall surface. Before the liquid film slipped out of the computational domain, the maximum average wall heat flux at an angle of 127° was approximately 75.8% of that at an angle of 51°.[Conclusions] The lattice Boltzmann pseudopotential model can simulate the condensation process of stationary saturated vapor within a vertical channel. During the simulation process, the effects of channel width and the hydrophilicity and hydrophobicity of wall conditions on condensate flow and heat flux density are important. In general, wider channel widths lead to higher wall heat flux density, and a higher inlet mass flow rate of steam is achieved at a steady state for saturated vapor initially in a stationary state within the channel. However, ordinary hydrophobic wall surfaces cannot sustain droplet condensation and do not demonstrate enhanced heat transfer. These findings have certain reference values for designing and optimizing heat transfer systems involving condensation in vertical channels.

[Objective] Long lifespan, compact, high-energy density, and efficient power systems are necessary to achieve future space exploration goals. The space reactor coupled Brayton cycle is high in energy conversion efficiency, small in volume, light in weight, and stable in operation, which is optimal for megawatt space power systems. The power control features are the key to the safe and efficient operation of a Brayton space nuclear power system. The reactor reactivity, inventory, and bypass valve are effective means of system power control. The bypass valve can change the local mass flow rate of a Brayton system and is expected to rapidly control system power to meet the frequently changing load of a space vehicle.[Methods] In this paper, a model of a Brayton space reactor system is established. A system power control simulation program is compiled based on the idea of modular modeling, each component of the system is solved independently, and the mass, momentum, and energy are transferred through data transmission between components. The calculation results of the model in this paper are compared with the simulation results of the startup process in the references, and the accuracy of the program and model is verified. The power-on and power-off transient performance of the system under the control of the bypass valve is investigated, and the effects of the bypass valve opening on system performance are studied.[Results] The power-on and power-off transient results of the system under bypass valve control indicated that bypass valve control could quickly change the pressure and distribution of mass flow rates in the system, the working conditions of the turbine and compressor, and the output power of the system, which could quickly respond to the power demand and load changes of a space vehicle. The change in the load led to a torque unbalance of the shaft, which could further induce rotating shaft overspeed accidents. The strong centrifugal force may damage the blades of the turbine and compressor. The bypass control adjusted the mass flow rate, pressure ratio, and output power of the turbine and compressor, controlled the shaft speed to operate near the rated value and simultaneously avoided the overspeed risk of the rotating shaft. Furthermore, the effect results of the bypass valve opening on system performance showed that the low-pressure side of the system and the radiant heat reject loop were sensitive to the parameter disturbance caused by the bypass valve control. The high-pressure gas at the compressor outlet mixed with the low-pressure gas at the turbine outlet through the bypass valve, and the pressure of the low-pressure side pipes and components increased. The elevated heat rejection power of the radiator increased the temperature of the heat reject loop, and the radiator needed greater heat rejection capacity.[Conclusions] Therefore, bypass valve control is an effective means to control the power and prevent shaft overspeed in a Brayton space nuclear power system. This study provides a reference for operating a Brayton space reactor system.

[Objective] Continuous enhancement of energy efficiency is an essential element of China's green development plan to meet the peak carbon dioxide emission target by 2030 and carbon neutrality objective by 2060. Boiling heat transfer, being one of the most effective methods for phase-change heat transfer, is crucial for heat-energy transfer and conversion in various industries. Therefore, improved boiling performance can increase the efficiency, safety, and cost-effectiveness of energy systems. Graphene, a novel material discovered at the turn of the 21st century, has exceptional properties in numerous fields and can be used as nanoparticles for enhancing the heat transfer of base fluids. This research study aims to enhance boiling heat transfer by examining the effects of the heating surface and working fluids, specifically with graphene nanofluids.[Methods] Reduced graphene oxide (RGO) nanofluid, which is a product of graphene preparation via the redox method, was utilized as the working fluid. The experimental investigation aimed to examine the heat-transfer characteristics of RGO-nanofluid saturated pool boiling at atmospheric pressure. The experimental data were collected and analyzed using a high-speed camera to record the morphology of vapor bubbles during boiling. The research study also used a pool-boiling experiment with a pure copper heating surface and distilled water as the working fluid to provide benchmark data to compare with the RGO nanofluid experiment.[Results] The results indicated that the RGO nanofluids had a significant impact on the critical heat flux (CHF) of pool boiling, which reached 1 684.22 kW/m², increased by 49.2% compared to distilled water. However, the nanofluids did not significantly affect the heat transfer coefficient (HTC) of pool boiling, which reached 73.87 kW/(m²·K), only increased by 2.3% compared to distilled water. At a constant heat-flow density, the effect of RGO nanofluids on the superheated wall was insignificant. Further analysis of the experimental data revealed that the RGO deposition layer formed by the RGO nanofluids on the heated surface during boiling was the core factor contributing to the increase in CHF. The deposition layer changed the wettability and vaporization core number of the surface, reduced the detachment diameter of vapor bubbles on the heated surface, and increased the detachment frequency, which delayed the appearance of CHF. This was supported by the measurement of the contact angle of the heated surface, surface observation of the heated surface after boiling, and analysis of the vapor bubble visualization images.[Conclusions] In conclusion, this study demonstrated that the use of RGO nanofluids can significantly improve the critical heat flux of pool boiling, which can contribute to the efficiency, safety, and cost-effectiveness of energy systems. The results also provide insights into the mechanism of heat transfer enhancement through the use of RGO nanofluids, specifically through the formation of a deposition layer on the heated surface during boiling. These findings can have practical implications in various industrial applications, including nuclear reactors, electronic cooling systems, and heat exchangers. However, further research is necessary to optimize the use of graphene nanofluids in various industrial applications and to assess their long-term effects on energy systems.

[Objective] The preparation of SiC materials is significant in nuclear fuel research. Presently, SiC materials are applied as a key material in coating layers of tri-structural isotropic (TRISO)-type coated particles and as fully ceramic microencapsulated accident-tolerant fuel (FCM-ATF) matrix materials. The matrix SiC materials of FCM-ATF are sintered from SiC powder or nanoparticles. The SiC nanocoated particles are a kind of important SiC nanoparticles. The study of the sintering behavior of SiC nanocoated particles aims to develop a framework for optimizing the sintering preparation process of the FCM-ATF matrix material.[Methods] In this paper, the melting points of pure SiC nanoparticles of different sizes were first investigated. The feasibility of the Tersoff potential function for molecular dynamics simulations of SiC materials was confirmed using the nanoparticle melting point variation law. Then, the sintering evolution processes of three typical SiC nanoparticles, pure SiC, SiC@Si, and SiC@C, were examined to investigate the influence of the coating layer structure on the sintering behavior of SiC. The sintering process was quantitatively described using variables such as the sintering neck width, atomic number in the neck region, shrinkage ratio, and degree of system densification. The sintering mechanism was described by the ratio of grain boundary energy to surface energy, mean square displacement, atomic displacement vector, and atomic diffusion coefficient.[Results] The study of the structure of SiC nanocoated particles showed that SiC@Si particles were more prone to sintering than SiC@C particles. Vulnerability to sintering was mainly reflected in the faster neck growth and higher densification during the sintering process. The results were closely related to energy evolution and atomic diffusion phenomena. Regarding energy evolution, the grain boundary energy of SiC@Si particles was rapidly converted to surface energy during the sintering process, but the conversion of grain boundary energy to surface energy of SiC@C particles was very slow. According to classical sintering theory, the sintering driving force was mainly provided by the surface energy of the particles. High surface energy catalyzed the surface diffusion of particle atoms during the sintering process. The evidence was corroborated by an analysis of the atomic diffusion aspect. The coating layer had as high surface energy as the surface of the coated particles. Thus, the overall atomic diffusivity of the particles was partially affected by the atomic diffusivity of the coating layer. The overall sintering behavior of the particles was catalyzed by the high atomic diffusivity of the coating layer. The atomic diffusivity of the silicon coating layer was better than that of the carbon coating layer, and the coated layer of the SiC@Si particles was more prone to atomic diffusion than that of the SiC@C particles; hence, sintering and atomic diffusion were more probable in the SiC@Si than in the SiC@C. The study of the heating rate showed that a lower heating rate was somewhat beneficial for sintering but did not affect the atomic diffusion pattern of the coated particle.[Conclusions] The results give a quantitative explanation of the sintering mechanism of SiC nanoparticles. It helps to understand the laws of the SiC sintering preparation process for FCM-ATF matrix materials and also provides a good reference for raw material design, sintering regime, and process optimization of SiC materials preparation.