ABSTRACT
All reactors have to be able to produce more neutrons than they consume for the fission chain reaction to continue over time. There are five basic types of reactors in relatively widespread use in the world. These are Pressurized water reactors (PWR), Boiling water reactors (BWR), Heavy water reactors, Gas-cooled reactors, and Liquid metal fast breeder reactors. These reactors use different combinations of coolants and different combinations of uranium and plutonium to generate the electrical power the world needs. The number of power reactors in the world today has increased steadily since nuclear power was first introduced in the 1950s. Aside from the nuclear reactor and the containment building, most nuclear reactors are remarkably similar in design to large coal-fired power plants. For a number of reasons, BWRs and PWRs have taken a slightly different evolutionary path when it has come to the design of their control rods.
