Abstract: Safe and reliable energy supply is the key foundation for the development of unmanned underwater vehicle (UUV). Unlike land and space missions, underwater missions are more demanding on energy, which requires higher power level, longer operating time, faster response time, and better concealment. Heat pipe cooled reactor has the advantages of large power capacity, simple structure, easy control of reactivity, and rapid thermal response. The solid-state reactor core does not require coolant and relies entirely on heat pipes to dissipate heat, thereby providing good inherent safety, controllability and concealment. Therefore, heat pipe cooled reactor is considered as one of the most promising options for UUV energy supply. In this study, a conceptual design of Nuclear Silent Thermal-Electrical Reactor (NUSTER-100) was proposed to meet the energy requirement of China’s heavy ocean UUV research and development. 109 sodium heat pipes were arranged in the reactor core for passive cooling, and the thermoelectric generators (TEGs) were employed in the reactor to convert fission heat to electric power. A set of heat pipe cooled reactor system-wild mathematical and physical models were established, including core neutron physics model, core channel heat transfer model, heat pipe model, thermoelectric conversion model and cold junction heat transfer mode. Based on the efficient and robust numerical algorithm and modular modeling ideas, heat pipe advanced reactor transient analysis code HEART was developed with independent intellectual property rights, and the key modules of HEART were validated and verified by heat pipe experiment and thermoelectric power experiment. The steady-state, cold-start transient and reactivity insertion transient conditions of NUSTER-100 were calculated and analyzed by HEART, and the full power operation characteristics of NUSTER-100 were obtained. Steady-state performance of the NUSTER-100 indicates that the solid-state core has good temperature flattening ability. The surface temperature of the heat pipe is less than 1 300 K, and the average temperature drop of the TEG module in central channel is 724 K, which can produce an electrical power of 1 207.8 W. Based on the cold-start transient thermal-hydraulic analysis, a three-stage heat pipe cooled reactor start-up scheme with high safety was proposed, and the stability and safety of heat pipe reactor under reactivity insertion transient were evaluated. The results show that the three-stage heat pipe cooled reactor start-up scheme can improve the reactor start-up efficiency. This study can provide support for the development of UUV and heat pipe cooled reactor technology in China.