In a nuclear power plant, the reactor vessel is a pressure vessel containing the coolant and reactor core. It is a device for containing and controlling a chemical reaction. The chemical process enables to convert raw material into final product under given pressure and temperature. During the reaction it becomes necessary to remove excess heat in the process to enable keep process under control. Vessels are built to withstand high pressure in the system.
Not all power reactors have a reactor vessel. Power reactors are generally classified by the type of coolant rather than the by the configuration of the reactor vessel used to contain the coolant. The classifications are:
Of the main classes of reactor with a pressure vessel, the PWR is unique in that the pressure vessel suffers significant neutron irradiation (called fluence) during operation, and may become brittle over time as a result. In particular, the larger pressure vessel of the BWR is better shielded from the neutron flux, so although more expensive to manufacture in the first place because of this extra size, it has an advantage in not needing annealing to extend its life.
Annealing of PWR reactor vessels to extend their working life is a complex and high-value technology being actively developed by both nuclear service providers (AREVA) and operators of PWRs.