Abstract: Compared with nuclear thermal propulsion (NTP) and dual-mode propulsion (BN-TEP), nuclear electric propulsion (NEP) has the advantages of high specific impulse (low thrust) and mature technology. It can greatly reduce the mass of propellant required for long-term exploration missions. And it is suitable for deep space and manned exploration missions. In the field of NEP, China has completed the development of the principle prototype of 100 kWe magnetic plasma power thruster (MPDT) and its ignition experiment. The maximum ignition power is 114 kWe with the thrust of 3 N, maximum specific impulse of 5 360 s, and the efficiency of 69%. According to the application requirements of NEP technology, a scheme of 100 kWe class reactor power system used on NEP was studied. And the design concept of 100 kWe NEP reactor power system was put forward. After a preliminary design and optimization, the overall design parameters and schematic diagrams of the system were presented. And a detailed design of thermoelectric conversion system was carried out. Besides physics, shielding, thermal-hydraulics and structure of the system were analyzed and demonstrated. The results show that the system scheme is reasonable and feasible. It can satisfy the requirements of physics, thermal-hydraulics, safety and service life. NEP reactor power has the characteristics of high working temperature, high conversion efficiency, high reliability, small size and compact structure. It is composed of reactor core, shadow shielding, heat transfer system, heat discharge system, thermoelectric conversion system, etc. The reactor adopts liquid metal lithium as its coolant. The thermoelectric conversion system uses closed Brayton cycle generator. And the heat discharge system adopts heat pipe radiator. The service life of the reactor is 10 years and the thermal power is 532 kWt. During normal operation, the inlet/outlet temperature of the reactor is 1 288 K/1 331 K, the inlet temperature of the turbine is 1 150 K and the inlet temperature of the compressor is 410 K, the coolant flow is 2 kg/s. The reactor has a fast neutron spectrum. The critical safety in case of falling accident (k_eff<0.98 under any falling accident condition) must be considered. In the design, the structural materials and safety rod in the reactor use spectral shift absorber (SSA) materials (molybdenum rhenium alloy and gadolinium oxide) which have small absorption cross section of fast neutron and large resonance absorption cross section of medium energy neutron. The use and reasonably arrangement of these materials successfully solve the critical safety problem of falling accident.