The coolant gas temperatures at the AGR stations during normal operation range from about 290°C at reactor inlet to 650°C at reactor gas outlet. As with the magnox station vessels, insulation and a cooling system are attached to the liner to keep the concrete temperatures and thermal gradients at, or below, the design values.

The insulation design is different at each station. There are two basic types which have been used; metallic foil at Dungeness В, Hartlepool and Heysham

1, and ceramic fibre at Hinkley Point В and Heysham

2.

The insulation on the central region of the floor and lower parts of the walls of the Dungeness В vessel is similar in design to that at Oldbury, i. e., corrugated rings, foils, spreader plates and coverplates on the floor, and foil and wire mesh on the walls. The design of the insulation was changed for the upper part of the sidewall and roof of the vessel to enable a multi­stud fixing of the coverplate to be used. This was necessary because of the high turbulence in that part of the vessel and the cyclic loadings which it imposed on the coverplate. The insulation is made up of layers of elements, each element consisting of a stainless steel dimpled foil covered by plain foils. Sealing be­tween adjacent elements is achieved by thin metallic strips. The elements are covered by a hot-face skin to limit the penetration of gas into the insulation, and an acoustic skin to reduce the effects of noise on the elements. The coverplates and insulation are supported by studs, usually four per coverplate, which are welded to the liner and attached to the coverplate with hoops. The hoops are to provide flexibility in the retention system to accommodate the differential thermal expansion between the coverplate and the liner to which it is attached (Fig 2.82).

The insulation for Hartlepool and Heysham 1 is similar to the element-type insulation at Dungeness B. The insulation and coverplates are supported by either studs or strips of metal welded to the liner. Four studs or strips per coverplate are required to give adequate support (Fig 2.83).

A ceramic fibre insulant material, ‘Triton Kaowool’, was selected for Hinkley Point В and the insulation design has been repeated at Heysham 2. (At Heysham 2 the terminology has been changed and the insula­tion is now called a ‘thermal shield’.) The insulation consists of layers of fibre blanket, the number and thickness of the layers varying according to the local gas conditions and the thermal performance require­ments. In between the layers of blanket are stainless steel foils. On the vertical surfaces there are ‘Z sec-

lions’, or shelves, to give support to the blankets. The blankets are contained at the hot face by a sys­tem of overlapping foils and 0.9 mm thick secondary coverplates. The whole of the insulation is retained by primary coverplates which also are used to com­press the blankets. Each coverplate is retained by a stud welded to the vessel liner. Studs positioned at the corners of the coverplates provide additional blanket support and act as anti-rotation devices for the inter — blanket foils, secondary coverplates, etc. Secondary retention of the coverplates is provided by a system of hoops which interconnect adjacent coverplates (Fig 2.84).

Interblanket support in the standpipe region of the roof is by retention plates held in position by washers welded to studs. The coverplates are welded to exten­sions of the fuelling standpipe liners, and secondary retention devices are fitted between coverplates.