The term 'region of a nuclear reactor' can be summarized by the word 'CORE, ' which is a four-letter word. The CORE of a nuclear reactor is the central part of the reactor, where the nuclear reaction takes place. It is typically made up of fuel rods containing uranium or plutonium, and is surrounded by a moderator, such as water or graphite, to slow down the neutrons released during the reaction. The CORE also contains control rods, which are used to regulate the rate of the reaction.