Fission Site is a term used to describe a facility where nuclear fission takes place. The two crossword solutions for this term are CORE, which has 4 letters, and REACTOR, which has 7 letters. A CORE is the central part of a nuclear REACTOR, where the fission process occurs. A REACTOR is the entire facility where the fission process takes place, including the CORE and other components such as control rods and cooling systems.