ABSTRACT
Since December 2, 1942, when the first man-made nuclear reactor produced a selfsustaining chain reaction, several hundred different types of reactor systems have been constructed. Despite the many possible differences in design, there are a number of general features which all reactors have in common. The heart of every reactor is an active core in which the fission chain reaction is sustained. The active core contains (1) fissile fuel which through its fissioning is the main source of neutrons, (2) moderator material if the fission neutrons are to slow down, (3) coolant if the heat generated by the fissions is to be removed from the core, and (4) structural material which maintains the physical integrity of the core. Surrounding the active core is usually either a reflector whose purpose is to scatter neutrons back towards the core or a blanket region which captures neutrons leaking from the core to produce useful isotopes such as 60Co or 239Pu. The reactor core and reflector/blanket are, in turn, surrounded by a shield to minimize radiation reaching personnel and equipment near the reactor. Finally, all reactors must have a means of control to allow the chain reaction to be started up, maintained at some desired level, and safely shut down.
