Figure 10.6 shows the RPV and its internals. The vessel consists of a cylindrical shell with a hemispherical lower head. Inlet and outlet nozzles are set into the upper part of the vessel with a flange above these. A flanged hemispherical head

Control rod drive mechanism

Upper support plate

Internals support ledge

Core barrel Support column

Upper core plate Outlet nozzle

Baffle radial support Baffle

Core support columns Instrumentation thimble guides Radial support

Core support

closure assembly is connected to the vessel flange by a system of threaded studs and nuts. The integrity of the pressure vessel is vital since its failure cannot be tolerated because it will almost certainly lead to a large release of radioactivity. Older RPVs were constructed from plates, which were shaped and welded together but modern RPVs are made from cylindrical forgings. This obviates the need for vertical welds, which will be in the highest stress regions. This is particularly important in the cylindrical part of the vessel adjacent to the core since this will
be subject to the highest radiation levels. RPV materials are subject to embrittlement as a result of prolonged exposure to radiation and the weld material is more susceptible than the forgings themselves. For this reason some plants use a very large single forging for the core region.

The reactor vessel’s internal structures support the core, maintain fuel assembly alignment, limit fuel assembly movement and maintain proper alignment of the control rod guide tubes between fuel assembly and control rod drive. They also direct the flow of reactor coolant, provide gamma and neutron shielding, provide guides for in-core instrumentation between the reactor vessel bottom head and the fuel assemblies and support the surveillance specimen capsules. The internals are divided into the upper core support structure, which is removed for refuelling, and the lower core support structure, which can be removed for vessel inspection. The lower core support structure, which directly supports the core, consists of:

• the core barrel, a cylinder surrounding the core

• the thermal neutron shield

• the lower core plate and the lower core support, supporting the whole weight of the core

• the baffle assembly, inside the core barrel, which limits the by-pass flow of the core

• the secondary core support

The upper core support structure, which is located above the core, consists of:

• the upper support plate, directly above the fuel assemblies

• the control rod guide tubes and the thermocouple columns and their supports

The internals are supported on a ledge formed in the reactor vessel wall just below the vessel main flange. The neutron shield is provided to give additional shielding of the reactor vessel material in regions where the neutron flux might otherwise cause significant reduction in fracture toughness. Reactor vessel material samples are sited adjacent to the shielding pads to permit checks to be made on this property. Cooling water enters the vessel via the inlet nozzles and flows, down the annulus (downcomer) between the core barrel and the RPV, to the lower head. It then passes through the core removing heat. Coolant then flows from the upper regions through the outlet nozzles and into the steam generators.