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• A novel concrete shielding scheme for SPLTHR was Introduced. • The efficacy of the thermal insulation strategy was demonstrated through the design and implementation of principled experiments. • An analysis of the internal temperature distribution and thermal stratification within the water layer has been conducted. • A discussion has been undertaken regarding the factors influencing the thermal insulation effectiveness of the water layer insulation scheme. Elevating the operational parameters of Swimming Pool-type Low-Temperature Heating Reactors to augment heating capacity raises concerns about heightened water temperatures within the pool. Elevated water temperatures pose a significant challenge to the integrity and durability of the concrete shielding, necessitating innovative solutions for effective thermal insulation. Existing approaches, such as insulation coatings and static insulation water layers, have shown limitations in maintaining prolonged cooling efficiency under reactor operating conditions. In response to this challenge, our research introduced a novel approach: the implementation of a flowing water layer within the concrete structure. A specialized insulation water layer is integrated within the reactor to efficiently isolate high-temperature pool water from the surrounding concrete shielding. A comprehensive experimental setup is established to assess the insulation effectiveness, yielding insights into the water layer's insulation performance. Results demonstrate a significant radial temperature difference of up to 14.31 °C between the cold and hot walls of the water layer within the experimental parameters. Investigation into critical experimental variables indicates that inlet positioning minimally affects insulation efficiency. This study not only verifies the viability of the insulation water layer approach but also furnishes crucial experimental data for future research endeavors about the cooling scheme of the Swimming Pool-type Low-Temperature Heating Reactor. [ABSTRACT FROM AUTHOR]

• A new measuring method for the length of LZVV is introduced. • A numerical approach for simulating the separator is developed. • The effect of flowrate and split ratio on the length of LZVV is investigated. • Larger LPO ratio leads to a longer LZVV. The bubble separator using centrifugal technology is the key device to remove the fission gas from the molten salt in the molten salt reactor. The Locus of Zero Vertical Velocity (LZVV) is an important characteristic of swirling flow. In this paper, a new measuring method is introduced to obtain the length of the LZVV of the bubble separator by tracking the droplet trajectories and analyzing the particular droplets' location at the single-phase condition. The effects of the flowrate, split ratio and the flowrate distribution of the two Light Phase Outlets (defined as LPO ratio) on the length of LZVV are studied. Moreover, numerical simulation at single-phase condition is carried out using the Reynolds stress model and it shows a good agreement with the experimental result. The experimental and numerical results show that the velocity distribution does not change with the flowrate and the larger split ratio leads to a longer length of LZVV and stronger swirl strength. [ABSTRACT FROM AUTHOR]

Highlights: [•] The HTC decreases more sharply compared with Nusselt analysis. [•] Helium stratification phenomenon can be ignored during the experiment. [•] New empirical correlations have been developed. [ABSTRACT FROM AUTHOR]

• A one-dimensional steady-state model of the separate heat pipe containing non-condensable gas for SPLTHR is developed. • The effects of initial system pressure and external parameters on the separate heat pipe performance are investigated. • The heat transfer rate of the system decreases by 23 % and 79 % for initial system pressures of 6 kPa and 12 kPa respectively. • The heat pipe shows good heat transfer performance for the residual heat removal of SPLTHR. The application of separate heat pipe technology in the Passive Residual Heat Removal System (PRHRS) of Swimming Pool-type Low-Temperature Heating Reactor (SPLTHR) can efficiently transfer heat without concerns of working medium leakage and installation difficulties. Since the reactor pool is at atmospheric pressure, the separate heat pipe is required to operate at sub-atmospheric pressure. In this paper, a one-dimensional program code to evaluate the heat transfer performance of a separate heat pipe loop containing non-condensable gas (NCG) is established. On this basis, the effects of some parameters on the system performance are investigated. The results show that the NCG occupies part of the condenser space, weakening the heat transfer performance of the system significantly. Within the calculated range, the presence of NCG can lead a maximum 79% decrease in system's heat transfer rate. For the system's external conditions, the heat transfer rate exhibits greater sensitivity to the water temperature of the pool as compared to the air temperature. Moreover, an increase in the air velocity tends to diminish the strengthening effect in system heat transfer capacity gradually. The obtained analytical results can provide reference for practical engineering application. [ABSTRACT FROM AUTHOR]

Experimental investigations have been conducted to study the steam heat removal capacity with noncondensable gases (e.g. air, helium) under low wall subcooling over a vertical tube external surface. The effect of the wall subcooling on the steam condensation heat transfer coefficients has been researched by experiments when the pressure and the air mass fractions are stable. At the air/steam cases, condensation heat transfer coefficient has been obtained under the wall subcooling degree ranging from13 to 25 °C, total pressure ranging from 0.4 MPa to 0.6 MPa and air mass fraction ranging from 0.07 to 0.52. Under the same pressure and noncondensable gases mass fraction, the effect of wall subcooling on condensation heat transfer coefficient with noncondensable gases is stronger than that with pure steam. The empirical correlation is developed for the heat transfer coefficient which covered all data points within 15%. Under air/helium/steam cases, the effect of helium (simulating hydrogen) on the heat transfer coefficient is investigated under the wall subcooling degree ranging from 18 to 27 °C, total pressure ranging from 0.53 MPa to 0.6 MPa, steam mass fraction ranging from 0.6 to 0.92 and helium volume fraction in noncondensable gases keeping 0.3. None of the experimental conditions is found the helium stratification. The condensation heat transfer coefficient that got from steam/air/helium condition is about 20% lower than that got from air/steam cases. [ABSTRACT FROM AUTHOR]

• An experimental study on SHP PRHRS under sub-atmospheric has been carried out. • The effect of boundary conditions on natural circulation has been studied. • The process and causes of system instability have been analyzed. • The thermal stratification in the pool has been analyzed. The use of separate heat pipe in Passive Residual Heat Removal System (PRHRS) for Swimming Pool-type Low-Temperature Heating Reactor (SPLTHR) has a good application prospect. An experimental study on miniaturized separate heat pipe-type PRHRS has been carried out. The heat transfer medium in the separate heat pipe is water and is at sub-atmospheric. The boundary conditions of the experiment refer to the operating range of SPLTHR. The two-phase natural circulation characteristics of the separate heat pipe-type PRHRS and the influence of boundary conditions on the system heat transfer capacity, flow instability and heat stratification of pool water are studied. Within the experimental range, the heat transfer rate is from 72.0 kW to 459.2 kW and the steam mass flowrate is from 0.031 kg/s to 0.197 kg/s. During the operation of the system, there is long-term flow instability. The process and causes of system instability are analyzed from the perspective of heat transfer. It is believed that the natural convective heat transfer outside the tube bundle hinders the heat transfer, resulting in instability. The heat transfer capacity of the system under three filling ratios of 30%, 50% and 70% was studied, and the results showed that the filling ratio within this variation range had little effect on the heat transfer capacity of the system. Furthermore, the thermal stratification in the pool has been analyzed. [ABSTRACT FROM AUTHOR]

• Designed a passive residual heat removal system which is suitable for swimming pool-type low temperature heating reactor. • The thermal stratification in the reactor pool. • Factors affecting the power of the passive residual heat removal system. • Fitting correlation of natural convection outside PRHR HX vertical tube bundle. The Passive Residual Heat Removal System (PRHRS) of Swimming Pool-type Low Temperature Heating Reactor (SPLTHR) runs at low temperature and pressure condition, which is completely different from the PRHRS of other reactors. According to the previous calculation results, an experimental facility with a power of 240 kW was built. Experimental data shows that after PRHRS is running, the temperature of the reactor pool water drops rapidly. The power of PRHR heat exchanger (HX) will gradually stabilize at about 1500 s, and both the pool water temperature and the air temperature have a significant effect on the power. The experimental Nusselt number (Nu) are all bigger than the calculated values of correlations, indicating that these correlations are not applicable to the natural convection outside the PRHR HX tube bundle. After analyzing the 30 groups of natural convection data outside the PRHR HX vertical tube bundle, the fitting correlation is in good agreement with the experimental data, which can provide important reference for engineering applications. [ABSTRACT FROM AUTHOR]

In Swimming Pool-type Low-Temperature Heating Reactor (SPLTHR), the Passive Residual Heat Removal System (PRHRS) is an indispensable part of the reactor safety system. In the heat transfer process of the PRHRS system, the natural convection heat transfer outside the Passive Residual Heat Removal Heat Exchanger (PRHR HX) in the reactor pool has a great influence on the flow and heat transfer characteristics of the system. In this study, an experimental facility was built to study the natural convection heat transfer outside the tube bundle under low temperature difference. According to the experimental results, a PRHRS for SPLTHR was designed. And a double-coupled natural circulation system analysis code was developed to investigate the start-up characteristics of the system under different initial conditions. The research results showed that the correlation of Yang predicts the experimental heat transfer results well. The calculation results indicated that the designed system met the heat removal requirements after reactor shutdown. If the temperature in the pipeline between the Passive Residual Heat Removal Heat Exchanger (PRHR HX) and the air-cooler was linearly distributed, the system would quickly establish a natural circulation and reduced the number of oscillations before the system reached stability. At the air-cooler, when the temperature of the heat transfer fluid in the water side changed, the temperature and mass flow rate of the air loop would quickly follow the change. The designed system and calculation results can provide a design basis for engineering applications. [ABSTRACT FROM AUTHOR]

• Chrome-plated tube and polished tube were used to enhance heat transfer. • Effects of air mass fraction, surface subcooling and total pressure are analyzed. • Condensation of steam on chrome-plated tube and polished tube was analyzed. • Chrome-plated tube has the potential to enhance PCS heat transfer. In this article, a series of steam condensation experiments were performed on the outer surface of chrome-plated tube and polished tube under steam and steam–air mixed conditions to study their heat transfer characteristics and whether they could be used to optimize Passive Containment Cooling System in advanced nuclear reactors. The effects of total pressure, air mass fraction and subcooling on condensation heat transfer of the chrome-plated tube and polished tube were analyzed. Moreover, basing on the existing experiment, a smooth tube that has the same size as the polished tube and chrome-plated tube was tested under the same conditions. The experimental phenomena indicated that filmwise condensation and dropwise condensation co-exist on the tube exterior surface, and more droplets were observed on the exterior surface of the chrome-plated tube. Under the steam condition, with the fixed wall subcooling, condensation heat transfer coefficient of the polished tube and chrome-plated tube shows different change rules under different pressure: the condensation heat transfer coefficient of the polished tube increases with the increase of pressure, while the chrome-plated tube decreases with the increase of pressure. Compared with the smooth tube, the heat transfer performance can be improved by using the chrome-plated tube and polished tube. Under the steam–air condition, the existence of air deteriorates heat transfer performance seriously. The heat transfer performance of the chrome-plated tube and the polished tube are both improved compared with the smooth tube when the air mass fraction is below 60℃. When the air mass fraction is over 60℃, the heat transfer ability of these tubes are similar to the smooth tube. Besides, within the measurement parameters of our experiment, the increase in total pressure has a positive effect on steam condensation with air. [ABSTRACT FROM AUTHOR]