Nuclear power station. Pile cap at a nuclear power station. The pile cap covers the reactor core. Each of the coloured squares is a standpipe providing access to fuel channels within the core. The core contains rods of uranium, atoms of which split, releasing neutrons and generating heat. Control rods are inserted into the reactor through the pile cap to control the amount of heat generated. A control rod insertion machine (yellow & black) stands nearby. The rods absorb neutrons, thus preventing from them splitting further uranium atoms. Lowering the control rods completely can shut down the reactor.