# Research on the Dynamic Characteristics Analysis and Power Control Method of Heat Pipe Reactors
## 1 Introduction  
According to the International Atomic Energy Agency (IAEA), micro nuclear reactors are classified as a type of small modular reactor (SMR) that possess an electrical power output of less than 10 MW [1]. One of the modern reactors in SMR systems is the HPR, which has a solid core and utilizes heat pipes to transfer heat from the core. Thanks to the solid-state property of the reactor and the use of heat pipes, the HPR possesses several advantageous features, including inherent safety, compactness, simple system architecture, and strong transportability. Furthermore, HPRs exhibit broad application prospects in low-power, unmanned applications, such as remote islands and underwater vehicles [2]. Since the 1960s, space exploration has made significant contributions to the advancement of space reactors, including HPRs, liquid metal reactors, and gas-cooled reactors. In 1968, a space power system was conceptualized by [PERSON] and colleagues, which utilized a heat pipe to establish a connection between the core and the thermionic diodes [3]. The proposed design concept effectively mitigated the adverse effects of high neutron flux and core expansion on the performance of thermionic diodes through utilizing heat pipes to transfer heat and isolate them from the harsh reactor environment. [PERSON] et al. conducted a study in the same year on a space reactor power system that utilized heat pipesfor heat transfer, out-of-pile thermionic diodes for electricity generation, and dual central absorber rods for reactor power control. This resulted in a reduction in the size and weight of the reactor system compared to the conventional space reactor that employs thermionic diodes within the reactor at the same power capacity [4]. A study conducted by the Los Alamos National Laboratory (LANL) in the United States involved the comparison of three distinct designs of heat pipe cooled reactors. The electrical power output of these reactors ranged from 1 to 500 kW, with core temperatures up to 1200-1700 K [5]. According to the calculation results and analysis, it was determined that none of the three HPRs surpassed the safety thresholds for any of the operation parameters. The authors of the study noted that augmenting the power of the HPR would necessitate an increase in the quantity of heat pipes, which would result in the amplification of system weight, size, and intricacy. Since the 21 st century, the development of materials, high-temperature heat pipes, and other key technologies have once again promoted the development of HPRs. In 2000, based on LANL's heat-pipe power system (HPS) project, [PERSON] et al. proposed a heat-pipe cooled nuclear reactor power system [6] for a Mars exploration mission and gradually developed it into the heat-pipe-operated mars exploration reactor (HOMER) [7] in subsequent studies. After that, many institutions put forward different HPR designs for the different application scenarios and requirements: for example, SAIRS [8] and HP-STMCs [9] proposed by NMU, MSR-A [10] proposed by MIT, Kilopower [11] proposed by LANL, eVinci [12] proposed by Westinghouse, etc. In 2012, LANL and NASA's Glenn Research Center (GRC) conducted the first nuclear test in the history of the HPR, in which the heat pipe exported heat from the core and served as a heat source for a Stirling engine to generate kilowatts of electrical power [11]. LANL and the GRC then continued to refine the Kilopower project, culminating in the development of the KRUSTY (kilowatt reactor using Stirling technology) [13; 14; 15]. After almost six decades of development, the HPR has established a comprehensive design plan and technical route and has achieved significant advancements in numerous pivotal technologies. The HPR also faces several obstacles, including the manufacturing process of heat pipes that can withstand high temperatures, the use of monolith and cladding materials that are resistant to high temperatures, effective thermoelectric conversion technology, and independent and reliable control methods.