The reactors for the AGR stations, like those of the later magnox stations, are contained in concrete pres­sure vessels. The gas pressures and temperatures are higher and, with the exception of Hartlepool and Hey­sham /, the boilers are situated in an annulus round the core.

At Hartlepool and Heysham 1 the boilers are cylin­drical in shape and are located in the walls of the vessel. Horizontal ducts in the vessel walls connect the boilers to the main reactor vault. With this de­sign of station, the removal and replacement of boiler
units, although difficult, is easier than with other de­signs of AGRs.

Figure 2.80 shows the layout of reactor components within the concrete pressure vessel at Heysham 2, and the gas flow round the circuit.

In the AGR reactor design, the core graphite is cooled by gas at reactor gas inlet temperature which flows downwards through annuli round the fuel strin­gers. This gas then joins remaining gas and flows ver­tically upward through the fuel channels to transport the heat from the fuel to the boilers. To provide passages for the direction of the reactor coolant from the gas circulators into and out of the graphite core and to the boilers, large steel structures are necessary to separate the gas at reactor inlet pressure from the gas at reactor outlet pressure. These structures are called pressure cylinders and domes at Dungeness В, gas baffles at Hinkley Point B, Hunterston B, Hey­sham 2/Torness and hot boxes at Hartlepool and Heysham Ї,