In line with the designs of the more modern Magnox reactors, all AGRs are constructed with a pre-stressed concrete pressure vessel (PCPV).

The reactor core is contained within a single cavity in the centre of the PCPV. It consists of a 16-sided stack of graphite bricks on a square lattice, interconnected with graphite keys to provide stability. The graphite acts as the core moderator (see Section 12.4). The bricks have central holes through them into which fuel assemblies are loaded and control rods can be inserted. Different AGRs have different numbers of fuel channels. Hinkley Point B and Hunterston B have 308 channels per reactor, Hartlepool and Heysham 1 have 324, Heysham 2 and Torness have 332 but Dungeness B has 408.

There are graphite reflectors (to minimise neutron escape) around the core. Outboard of the reflectors there are shields, usually of steel and graphite although Hinkley Point and Hunterston use calcium hydroxide, to reduce neutron radiation levels such that access to the outer parts of the reactor cavity is possible when it is shut down. The shields also limit radiation damage to reactor components made of steel, which cannot be removed, and to the PCPV.

The boilers on all AGRs, excepting the Hartlepool and Heysham 1 sister stations are also contained within the PCPV cavity. Part way through construction of the earlier stations, accelerated corrosion tests on some steels used for boiler tubing showed that they might be life-limiting. Removal of the boilers through the PCPV is not impractical but would be very difficult and time consuming. Because of this, the Hartlepool and Heysham 1 reactors were designed so that the boilers could be replaced if corrosion became an issue. This design is such that the boilers are set into deep, circular pits, called pods, within the walls of the PCPV. The closures above the pods were removable to allow access to and replacement of the boilers. However, in practice, the boilers have not been corroding as quickly as the accelerated tests predicted and the Heysham 2 and Torness boiler designs reverted to the Hinkley Point/Hunterston arrangement. Furthermore, safety concerns expressed by the UK regulator over the possibility of failure of the closures above the boiler pods in the Heysham/Hartlepool arrangement led to modifications, which would make it very difficult to remove a boiler.

Figure 12.3 shows a cross section of a Hinkley Point B type design with integral boilers. In the pod boiler design used at Hartlepool and Heysham 1, the boilers are set into vertical cylindrical ducts (the pods) in the concrete of the PCPV.

High pressure CO2 is used to cool all AGRs. Most AGRs operate at 4 MPa (40 bar) pressure, Dungeness B being the exception with a pressure of 3 MPa. The gas is pumped around the reactor by large circulators, contained in penetrations through the PCPV at the bottom of the reactor. From the gas circulator outlet, the gas is discharged into a lower plenum below the core where the flow divides with approximately half going directly up the core over the fuel with the residual (called re-entrant flow) being directed up an annulus outside the core, returning downwards through passages in and between the graphite bricks and thence to the fuel channel inlets. This gas cools the neutron shield for the boilers (the boiler shield wall) and the graphite core.

At one stage, as the AGR design was developed, it was thought that oxidation by the CO2 would limit the operation of the plant to below design output. As a consequence, a study was initiated to consider the possible use of helium as an alternative coolant. However, it was concluded that the helium in an AGR would still require the addition of a gas such as CO2 in order to produce an oxide layer on steel surfaces to reduce the friction between moving parts. The idea was therefore abandoned. A conceptual design was also proposed for a helium-cooled

fast reactor using essentially AGR PCPV technology, but was abandoned due to the size of the core and potential difficulties with the helium leak rate.

Gas emerging from the top of the core passes down the boilers and is pulled into the circulators. The typical design gas outlet temperature from the core is 650 oC and the outlet from the boilers is 300 °C.

The boilers are a once-through design consisting of re-heater, super-heater, economiser and evaporator sections. There are penetrations through the PCPV for each boiler section through which the steam passes. Boilers in most AGRs were of serpentine-wound sections in mild steel, low-alloy austenitic or stainless steel depending on operating temperature. In the podded Heysham/Hartlepool design, helically wound boilers were used.

There is a steel gas baffle between the neutron shielding and the boilers. On all AGRs except the Hartlepool and Heysham 1 pair, the baffle consists of a vertical cylindrical wall topped by a welded dome, which is provided with holes that align with the channels in the core. The dome is in the plenum above the top neutron shield and below the underside of the top cap of the PCPV. The baffle is effectively the boundary between the hot and cold gas. At Hartlepool and Heysham 1, with their pod boiler design, the gas baffle is a curved plate, which spans the upper plenum above the top neutron shield and is fixed, around its circumference, into the PCPV concrete.

The weight of the core is taken on a plate (the core support plate) beneath the lower core reflector. This plate is held in place by the diagrid, which is an open lattice steel structure to allow open coolant flow to the core channels. The weight of the diagrid is taken by supporting struts, which are anchored into the PCPV.

The PCPV is lined with insulation covered by steel plates, which are bolted to the concrete. The liner serves as a gas-tight containment membrane. There are a number of penetrations through the PCPV, which serve the boilers, the circulators and the fuel and control rod channels. Below the liner are cooling water pipes, which maintain the concrete temperatures at acceptable levels.