This is an example of a Pressurized Water Reactor used for generating electricity. In the core the uranium undergoes fission so that a lot of heat is released. The control rods shown regulate the rate of the reaction, and therefore the heat yield, by absorbing some of the moving neutrons. In the Pressurised Water Reactor the core is surrounded by ordinary water and is enclosed in a very thick steel pressure vessel. The water, under high pressure, serves as both coolant and moderator. It is circulated to a heat exchanger (steam generator) where water in a separate circuit is turned into steam. This information and picture were taken from the Uranium Information Center.