In a BWR, water boils directly in the reactor to make steam that drives a turbine, eliminating the need for steam generators. In a PWR, the uranium fuel supply consists of small ceramic pellets sealed inside fuel rods. The rods are bundled in lots to form fuel assemblies and are mounted inside the reactor vessel - a large metal vessel shaped somewhat like a cold capsule. The vessel walls are made of carbon steel from 5 to 9 inches thick for added strength to contain heated water under pressure. Water used as reactor coolant contains boron in solution to help control the reaction.
Control rods absorb neutrons within the fuel assemblies.
The rods can be withdrawn from or lowered into the fuel assemblies to control or shut down the fission process. Water circulated through the reactor is never allowed to boil, even though it is heated hundreds of degrees higher than the usual boiling point for water. That's because the water is kept under pressure, in the same way a car radiator keeps the coolant from boiling over by keeping it under pressure. This heated water is pumped to another vessel called the steam generator. There, the water passes through thousands of small metal tubes, while cooler water from a second system circulates around the outside of the tubes.