Reactor vessel

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The reactor vessel used in the first commercial nuclear power plant, the Shippingport Atomic Power Station. Photo from 1956.

In a nuclear power plant, the reactor vessel is a pressure vessel containing the coolant and reactor core.

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:

Light water reactor - Includes the PWR, BWR. Vast majority of nuclear power reactors are of this type.
Graphite moderated reactor - Includes the Chernobyl Reactor RBMK that has a highly unusual reactor configuration compared to the vast majority of nuclear powerplants in Russia or around the world.
Gas cooled thermal reactor - Includes the AGR,the Gas Cooled Fast Breeder Reactor or GCFR, and the HTGR. An example of a Gas Cooled Reactor is the British Magnox.
Heavy water reactor - Utilize deuterium in some manner, however it should be noted that D20 is more expensive and may be used as a main component, but not necessarily as a coolant in this case. An example of a Heavy-Water reactor is Canada's CANDU reactor.
Liquid metal cooled reactor - Utilize a liquid metal, such as sodium or a lead-bismuth alloy to cool the reactor core.
Molten Salt Reactors - Special organic coolants are used in this unique design, such as the MSBR.

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.