Reactor core and fuel assemblies
Chain reactions that produce energy take place in the fuel assemblies. The high pressure water flows (150 atmospheres) between the fuel assemblies in order to remove heat from fission reactions. It also slow down the neutrons so that they can maintain the chain reaction. The core of a reactor producing 0.9 Gigawatts of electric power contains 157 fuel assemblies, 41,448 rods and 11,273,856 oxide pellets of enriched uranium.
Source COGEMA