The term PART OF A REACTOR can be broken down into two crossword solutions, ROD and CORE. A ROD is a cylindrical piece of material used in nuclear reactors to control the rate of fission. It is usually made of boron steel or a similar material that absorbs neutrons. The CORE of a reactor is the central part of the reactor where the nuclear reaction takes place. It is typically made up of fuel rods, moderator rods, and control rods. Together, these two parts form the heart of a nuclear reactor.