Nuclear reactor scaffold.
1/ fuel rods; contains the U-235 and is where fission takes place.
2/ a moderator, heavy water slows free neutrons making fission more efficient. A chain reaction or sustained reaction is set up.
3/ control rods made of cadmium control the rate of reaction, inserting the rods slows the reaction, removing the rods speeds the reaction. During the sustained reaction the number of neutrons remains constant.
4/ the cooling fluid ..liquid sodium…collects the heat produced by fission and transfers it to water. This creates steam turning a turbine.
NUCLEAR ENERGY–> heat energy–> mechanical energy
4/ the core..containing the above is enclosed in a case made of lead and concrete preventing the escape of neutrons and radiation.