Reactor pressure vessel

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

Not all power reactors have a reactor 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 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 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 - Utilize 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 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 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.

Components of a PWR Reactor Pressure Vessel

PWR Nuclear Reactor Pressure Vessels share some features regardless of 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. The fuel assembly 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 which can make the vessel brittle and reduce its useful life.

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