region of a reactor in which a chain reaction can take place, that part of a nuclear reactor which contains fissile material, in which a chain reaction can take place, the central portion of a nuclear reactor, which contains the fuel assemblies, moderator, neutron poisons, control rods, and support structures, the reactor core is where fission takes place., piirkond reaktoris, kus toimub tuumalõhustumise ahelreaktsioon