The reactor core is the central part of a nuclear reactor where nuclear fission occurs. It consists of four basic systems and components: the fuel (including fuel rods and the fuel assembly structure), the coolant, the moderator and the control rods, as well as additional structures such as reactor pressure vessel internals, core support plates, and the lower and upper internal structure in light water reactors. This Safety Guide provides recommendations on meeting the safety requirements established in SSR-2/1 (Rev. 1) applied to the design of the reactor core for nuclear power plants. The publication addresses the safety aspects of the core design and includes neutronic, thermohydraulic, thermomechanical, and structural mechanical aspects. Other aspects considered are those relating to reactor core control, shutdown and monitoring, and core management.