A nuclear reactor core is a key component of a nuclear reactor. In reactor physics, the nuclear core is a bounded region, where a neutron multiplication occurs and where chain reactions take place. The reactor core contains especially the nuclear fuel (fuel assemblies), the moderator and the control rods. The core of the reactor contains all the nuclear fuel assemblies and generates most of the heat (fraction of the heat is genarated outside the reactor – e.g. gamma rays energy). The assemblies are exactly placed in the reactor according to a fuel loading pattern. (source)