Conference Agenda

The most critical task in developing microreactors is investigating the thermal-hydraulics of long-length sodium heat pipes. Although research on sodium heat pipes has increased in recent years, the manufacturing and testing of long (~4 meter) heat pipes remain significant challenges. Given their importance as a key milestone in microreactor development, this paper aims to validate the thermal similarity between sodium and water heat pipes using scaling laws. These laws are applied by matching dimensionless numbers related to pressure distribution, such as pressure drops in the wick and vapor, with the geometry and boundary conditions of the heat pipes determined through a 1D pressure distribution analysis code. The results indicate that the largest discrepancy between sodium and water heat pipes arises from differences in vapor inertia drops due to thermal properties. To verify this similarity, experiments are conducted on 900 mm sodium-water pipes. Based on these experimental results, we aim to predict the thermal distribution of a 4000 mm sodium pipe. Additionally, by conducting experiments with heat pipes of varying lengths, the study seeks to analyze how length impacts heat transfer behavior and to further validate thermal similarity under different conditions. Leveraging these insights, this study will assess the potential application of long (~4 meter) heat pipes in microreactors.

Heat pipes play a crucial role in microreactor cooling systems, valued for their high efficiency and compact design. However, optimizing their performance remains a complex challenge, particularly when considering critical factors such as filling ratio and inclination angle, both of which can significantly influence heat transfer efficiency. These variables can substantially affect heat transfer efficiency. This study addresses the gap in existing numerical investigations of filling ratio effects on heat pipe performance, with a focus on conduction-based models. While most existing codes primarily solve vapor and liquid flow dynamics to evaluate heat pipe performance, these calculations are computationally expensive and highly unstable. In contrast, conduction-based models offer a faster and more efficient alternative but have lacked proper implementation of filling ratio effects. In this work, we develop a new heat pipe performance code that incorporates filling ratio into a twodimensional conduction model. This approach provides a more practical and efficient solution for analyzing heat pipe behavior, making it well-suited for applications where computational resources are limited. The model was tested under both steady-state and startup conditions, with system coupling to computational fluid dynamics (CFD) programs to evaluate its performance. The preliminary validations results show good agreement with the experimental data.

Solid state heat pipe reactors have a broad prospect in the fields of sea, land, air and space. The high temperature heat pipe, as the most critical heat transfer component in a heat pipe reactor, has a high priority on the wick. However, the influence of key parameters such as permeability, porosity, and capillary force of the wick structure on the working fluid distribution inside the heat pipe is difficult to measure experimentally. In this study, firstly, a mechanistic experiment with a wick is used for performance testing, followed by a three-dimensional CFD model of a heat pipe with a wick structure, which can predict the heat transfer characteristics under different steady state conditions. Based on Star-CCM+ numerical simulation software, the effects of fluid flow and convergence behavior within the wick structure on the heat transfer characteristics of the heat pipe were simulated using a combination of a liquid film model and a volume of fluid (VOF) model. Sodium high temperature heat pipe experiments were used to verify the accuracy of the numerical simulations with a maximum error within 10%. The effects of operating angle and wick structure on the heat pipe are investigated, and this study lays the foundation for the design and analysis of the heat transfer characteristics of high-temperature heat pipes.

A Small Modular Reactor (SMR) requires advanced safety features capable of providing long-term passive cooling and maintaining the integrity of the final heat sink. Many designs of a water-cooled SMR employ a water reservoir as the final heat sink, but this water can potentially dry out during an accident due to decay heat. Therefore, in this study, a modular passive-cooling tower equipped with a wickless heat pipe is proposed as the passive safety system to delay the complete depletion of water through air convection. Since the modular passive-cooling tower transfers heat from the final heat sink to the ambient air by air natural circulation, the analysis of natural circulation and the convective heat transfer is crucial to assess its feasibility of the modular passive-cooling tower. An in-house developed code, a nuclear reactor thermal-hydraulic system code, and a CFD program were used in the assessment and their results were compared to each other. Additionally, the system code was used to optimize the system design, which has design to achieve the best heat removal capability. Consequently, this study evaluated structural effects on the overall heat transfer performance and confirmed that the modular passive-cooling tower has significant potential in delaying the depletion of the water in the final heat sink, contributing to the long-term passive cooling capability of the SMR.

This study examines the free piston Stirling engine (FPSE) as a promising candidate for supporting compact, long-lasting, and high-efficiency microreactors suited to remote operation. A 20 kW FPSE was designed and analyzed using Sage software, producing about 19.86 kW at 26.48% thermal efficiency. Key losses, including friction in the regenerator’s wire mesh, conduction and shuttle heat losses, and regenerator performance, were optimized to meet design constraints. Integrating a linear alternator created a free piston Stirling generator (FPSG) that converts mechanical to electrical energy at around 18.99 kW_e and 97.62% conversion efficiency. EMWorks2D, an extension of SOLIDWORKS, helped visualize and refine the permanent magnet configuration, improving the magnetic field path and validating the Sage model. This theoretical investigation supports the viability of FPSEs in microreactor applications. Further improvements could include employing heat flux calculations rather than standard heat exchangers, replacing wire mesh with robust foil in the regenerator, and performing 3D modeling simulations to maximize the engine’s potential.

This study investigates the intermittent boiling phenomenon in sodium heat pipes with wick structures during startup. The experiments focus on the effects of heating power, filling amounts, and capillary wick support structures on temperature oscillations and flow instabilities. A novel stainless steel porous thin-walled tube was designed as a wick support to enhance structural stability. Key findings reveal that intermittent boiling primarily occurs during the continuous flow region expansion phase, characterized by periodic temperature fluctuations. The amplitude and period of oscillations are non-monotonically influenced by heating power, peaking at 800 W with a maximum amplitude of 155.2°C and a cycle of 220 s. Reducing the liquid filling volume decreases oscillation intensity but risks localized dry-out at high heat fluxes. Comparative tests between porous thin-walled tubes and conventional 50-mesh screen supports demonstrate that the former reduces the stable power threshold by 42% by mitigating vapor-liquid interfacial shear stress in the adiabatic section. The study establishes optimal parameters for balancing heat transfer efficiency and operational stability, providing critical insights for the design of high-temperature alkali metal heat pipes in nuclear reactor applications.