Reactor pressure vessel

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

A reactor pressure vessel in a nuclear power plant is the pressure vessel containing the nuclear reactor coolant, core shroud, and the reactor core.

Classification of nuclear power reactors

Not all power reactors have a reactor pressure vessel. Power reactors are generally classified by the type of coolant rather than by the configuration of the reactor vessel used to contain the coolant. The classifications are:

Light water reactor - Includes the pressurized water reactor and the boiling water reactor. Most nuclear power reactors are of this type.
Graphite moderated reactor - Includes the Chernobyl reactor (RBMK), which has a highly unusual reactor configuration compared to the vast majority of nuclear power plants in Russia and around the world.
Gas cooled thermal reactor - Includes the advanced gas-cooled reactor, the gas cooled fast breeder reactor, and the high temperature gas cooled reactor. An example of a gas cooled reactor is the British Magnox.
Heavy water reactor - utilizes heavy water, or water with a higher than normal proportion of the hydrogen isotope deuterium, in some manner. However, D₂O (heavy water) 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 - utilizes a liquid metal, such as sodium or a lead-bismuth alloy to cool the reactor core.
Molten salt reactor - salts, typically fluorides of the alkali metals and of the alkali earth metals, are used as the coolant. Operation is similar to metal-cooled reactors with high temperatures and low pressures, reducing pressure exerted on the reactor vessel versus water/steam-cooled designs.

Of the main classes of reactor with a pressure vessel, the pressurized water reactor 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 boiling water reactor 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 pressurized water 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 pressurized water reactors.

Components of a pressurized water reactor pressure vessel

All pressurized water reactor pressure vessels share some features regardless of the particular design.

Reactor vessel body

The reactor vessel body is the largest component and is designed to contain the fuel assembly, coolant, and fittings to support coolant flow and support structures. It is usually cylindrical in shape and is open at the top to allow the fuel to be loaded.

Reactor vessel head

This structure is attached to the top of the reactor vessel body. It contains penetrations to allow the control rod driving mechanism to attach to the control rods in the fuel assembly. The coolant level measurement probe also enters the vessel through the reactor vessel head.

Fuel assembly

The fuel assembly of nuclear fuel usually consisting of uranium or uranium/plutonium mixes. It is usually a rectangular block of gridded fuel rods.

Neutron reflector or absorber

Protecting the inside of the vessel from fast neutron escaping from the fuel assembly is a cylindrical shield wrapped around the fuel assembly. Reflectors send the neutrons back into the fuel assembly to better utilize the fuel. The main purpose though is to protect the vessel from fast neutron induced damage that can make the vessel brittle and reduce its useful life.