System used to initiate, maintain and control a fission chain reaction (chain reaction). The main part is a core with fissile nuclear fuel. In general, a reactor is equipped with a moderator, a shield and control devices. Reactors are built for research or power generation purposes. Reactors where the chain reaction is maintained by thermal neutrons (see neutrons, thermal) are called thermal reactors; if the chain reaction is maintained by fast neutrons, one refers to fast reactors. The first reactor was put into operation on 2nd December 1942 by a group of researchers headed by Enrico Fermi. (See pressurized water reactor, boiling water reactor).