The place inside a nuclear reactor where a controlled fission reaction takes place in the nuclear fuel and generates heat that is further used. The core typically contains fuel (e.g., enriched uranium in fuel assemblies), a moderator (in the form of a liquid or as solid graphite blocks), coolant (liquid or gas flowing around the fuel), support structures that hold the fuel in position, and areas for the insertion and withdrawal of control rods that regulate the intensity of the reaction, or areas for the insertion of emergency rods used to terminate the fission reaction immediately.