Nuclear Reactor Pressure Vessel
The reactors in a nuclear power station are simply sources of heat that are used to boil water and so produce high pressure steam.  Each reactor core is contained within a pressure vessel through which a coolant (either gas or liquid) passes under pressure to transfer the heat produced in the core to the boilers which make the steam.  This steam is then used to turn the turbines which produce electricity.  Reactor pressure vessels are manufactured from either steel of reinforced concrete.

Steel reactor pressure vessels, normally between 50 and 125 mm thick, are the most common.  Steel can be manufactured to tight specifications to give the required chemical composition and mechanical properties.  It can also be fully inspected to ensure there are no significant defects present.  The vessels are constructed from either large plates, which are hot formed into shape, or forgings.  A number of plates or forgings are welded together to form the reactor pressure vessel.

An alternative material to steel for pressure vessel manufacture is pre-stressed concrete.  It is made by mixing water, Portland cement and aggregate together before pouring into a mould.  Like rock, concrete has a high resistance to fire, corrosion and abrasion.  It can be cast into any shape and size, sets within a short time and is relatively cheap.  Because concrete is stronger in compression than tension, steel reinforcing wires are embedded in concrete vessels to provide tensile strength.  To ensure the vessel is gas tight the concrete is cast around a steel line.

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