The Kyoto University Critical Assembly (KUCA) has three types of cores: two solid-moderated and solid-reflected cores (A and B cores), and one light-water-moderated and light-water-reflected core (C core). The three cores are operated at room temperature and very low power less than 1 W, as a normal operation mode, and referred as zero-power reactors. Also, they have reactor components, including the low-enriched uranium fuel with the uranium-235 enrichment less than 20 wt%, moderators, reflectors, three control rods, three safety rods, six neutron detectors (three fission chambers and three uncompensated ionization chambers). Of three cores, in this chapter, essential parts of the C core are mainly introduced to make an easy understanding of the reactor core itself, including core characteristics, utilization purposes, functions of shutdown systems, and core components.