Combining a nuclear reactor with thermoelectric converters is expected to be one of the few promising options to supply a propulsion power for deep space explorers. In the present study, the criticality of low enriched uranium reactor core is surveyed. The fuel materials of uranium oxide, nitride and metal were examined. Zirconium and yttrium hydrides, beryllium, zirconium beryllide and graphite were considered as the moderator materials. The reflector materials of beryllium, beryllium oxide and graphite were taken into consideration. The investigation of criticality of the low enriched uranium core was performed by changing the ratio of the fuel, moderator and structure in the core, and the thickness of the reflector. As a result from the viewpoint of the smaller critical mass of the low enriched uranium fueled reactor, it is found to be better to use the thermal spectrum system than the fast system, and the metal hydride moderators than beryllium or graphite. Promising reactor core concepts, which attain criticality with the reactor mass of less than 1,000kg, are obtained when zirconium hydride, yttrium hydride or beryllium is used for moderator. For example, the combination of uranium nitride fuel, yttrium hydride moderator and beryllium reflector achieves the reactor mass of as low as 500kg.