Conference Agenda

Heat pipe reactors have broad application prospects in deep space power, military bases, marine power and so on. For system level analysis of heat pipe cooled microreactors(HP MicroRx), the coupling between neutronics analysis and thermal-hydraulics analysis was usually solved iteratively. This study aims at establishing a multi-physics coupled simulation framework for performing transient safety analysis of HP MicroRx. This simulation framework is based on the system analysis software RETA. RETA is a multi-physics solver framework for advanced nuclear reactors based on C++ and object-oriented design methods. This work includes the following subtasks: 1) developing the heat pipe modeling capability with a fully-implicit solution method; 2) deforming the geometry of the reactor core so that it’s convenient for the finite volume method(FVM); 3) using the Preconditioned Jacobian-Free Newton-Krylov(PJFNK) method to uniformly solve the heat pipe reactor. In the modeling of heat pipes, a one-dimensional compressible flow model was used to model the vapor core, a two-dimensional axisymmetric heat conduction model was used to model the heat pipe wall and wick region, and the heat pipe wick and vapor core were coupled through a conjugate heat transfer interface. The KRUSTY reactor was selected and analyzed using the above simulation framework. Simulation results match the experimental data produced by Los Almos National Lab well. To conclude, this work provides an accurate and reliable tool for safety analysis of heat pipe microreactors.

High-parameter pressurized water reactors (HP-PWRs) with plate-type fuel operate at a higher power density than conventional pressurized water reactors (PWRs). This is accompanied by higher void fractions and the potential presence of slug flow, which can significantly affect reactor neutronic behavior. However, most neutronics and thermohydraulics analyses for PWRs rely on subchannel codes, and the impact of subchannel homogenization remains uncertain for HP-PWRs. This study models slug flow and bubbly flow in both the XY and XZ planes to investigate the effects of void distribution resolution on neutronic behavior using RMC. For slug flow, subchannel homogenization results in a noticeable overestimation of keff in the XY plane. The maximum relative power deviation (MRPD) between the homogenized and reference schemes reaches 3.70% in the XY plane and 5.38% in the XZ plane. MRPD increases with increasing overall void fraction and gas slug void fraction, as well as decreasing gas slug width and length, while it shows limited sensitivity to variations in small bubble radius in slug flow. For bubbly flow, although void distribution resolution has only a marginal influence on keff, its impact on power distribution is non-negligible—especially as the bubble radius increases, the void distribution becomes more non-uniform, and the overall void fraction rises. The MRPD between the homogenized and reference schemes exceeds 2%. These findings highlight the potential inaccuracies introduced by subchannel homogenization in high-void, non-uniform flow environments. Fine-resolution void modeling is essential for accurate N/TH coupling in HP-PWRs, particularly in reactor safety analysis.

To analyze the startup process and load tracking operation characteristics of a heat pipe reactor with Brayton cycle for thermoelectric conversion, this study employs a hierarchical component model to establish simulation software for the heat pipe reactor system. The study analyzes the impact of key parameters in the Brayton cycle on power generation efficiency. By utilizing the control method of the PID model, the simulation of the heat pipe reactor startup is conducted through the control of Brayton’s rotor speed. The control of the rotor speed under normal operating conditions is achieved by controlling the filling amount of the secondary loop working fluid, thereby exploring the inherent safety characteristics and load tracking operation characteristics of the heat pipe reactor under controlled and uncontrolled rotor speed conditions. The results show that pressure ratio, degree of superheat, and the temperature at the inlet and outlet of the unit significantly affect power generation efficiency; the PID control model can simulate the startup process of the heat pipe reactor, and the rotor speed can be well controlled; compared to uncontrolled rotor speed, the controllable rotor speed results in smaller changes in power and thermal parameters such as fuel temperature during the system’s load increase and decrease processes, which is more conducive to the safety of the reactor core.

High-temperature heat pipes are promising devices for advanced microreactor technologies in terrestrial and space applications. Understanding their performance and safety characteristics is critical to the successful deployment of heat pipe microreactor systems. One key safety consideration influencing the design and operational limitations of heat pipes is the occurrence of dryout in the liquid-wick region. This study investigates the effects of temporary dryout conditions induced by pulsed heat inputs to the evaporator that exceed the capillary limitation. Using water as the working fluid, experiments were conducted to examine the transient response of the heat pipe’s external wall temperatures, internal liquid and vapor temperatures, and vapor pressure under pulsed heat input conditions. Pulse lengths were varied to control the duration and severity of the pulsed dryout conditions and study rewetting and the long-term effects on heat transfer performance. Spatial temperature profiles during transients were obtained using an optical fiber temperature sensor in the vapor core. Thermal resistance and hysteresis were evaluated under steady state conditions before and after pulses to assess their impact on overall heat pipe performance. This study provides valuable insights into the internal two-phase flow behavior during dryout and rewetting of the wick. The experimental data set can be used to benchmark numerical codes and validate computational models. Future work will investigate the effect of pulsed dryout conditions with alternative wick designs, varying filling ratios, and liquid metal high-temperature heat pipes to optimize their design and enhance resilience.

Advanced reactor designs will use sodium heat pipes as the primary means of heat transfer from the core block to the heat exchanger system. Such devices provide an efficient and reliable method for transferring heat over a small temperature gradient and at near-atmospheric pressures. However, robust experimental data is needed to better characterize these devices and provide validation metrics for the Sockeye simulation code. To meet these needs, several heat pipes will be manufactured and tested at high powers (~10 kW) to explore manufacturing repeatability, test operating limits, and measure the properties of the working fluid. This work summarizes the heat pipe design and optimization process used to determine the dimensions of the heat pipes that will be manufactured. Analytical expressions from a variety of sources were used to calculate a theoretical ideal design to meet multiple experimental goals. The wick geometry and properties were tuned to potentially encounter four power limits over the range of operation supported by the experimental facilities. To employ the analytical expressions, a simple yet novel averaging scheme was proposed to account for an annular gap surrounding the wick structure. This averaging scheme was applied to limiting experiments in the literature to evaluate its accuracy. Finally, numerical and analytical methods were applied to evaluate the heat pipe operating conditions to ensure the experimental facilities will be able to test the power limits.

High-temperature heat pipes are critical components in the core of solid-state reactor heat pipe cooled reactors, serving as the exclusive and essential means of heat transfer from the core to the energy conversion system. The Geyser Boiling Phenomena (GBP) of high-temperature heat pipes has a significant impact on the safety and stability of solid reactors. This investigation encompasses the design, fabrication, and testing of sodium heat pipes with varying filling ratios, ranging from 33.3% to 100.1%. An extensive array of experimental studies has been carried out to evaluate the heat transfer properties of these sodium heat pipes under diverse conditions, including different heat transfer rates and inclination angles. The results indicate that the design parameters and operational settings, such as filling ratio, heat transfer rate, and inclination angle, significantly affect the GBP of high-temperature heat pipes. This research combines experimental data with relevant theoretical analysis to establish a semi-empirical relationship for predicting the temperature fluctuation period caused by the GBP of high-temperature heat pipes. Furthermore, based on the improved network thermal resistance model, a GBP analysis model is proposed, providing valuable reference for the design and engineering application of heat pipe-cooled reactors.