The AGR concept was that uranium dioxide fuel with stainless steel dad would permit the channel gas out­let temperatures (about 650°C) necessary to give the steam conditions for conventional large turbine-gen­erators. The commercial fuel element design permits average channel powers of some 4.5 MW and hence some 300 to 400 channels are necessary to match the reactor power to the heat input required by a 660 MW turbine-generator.

The high gas outlet temperature precludes the use of in-pile refuelling grabs which had been trouble­some at the magnox stations and, furthermore, the change in channel power over its lifetime requires a variable gag to maintain a constant gas outlet tem­perature. The high channel powers and the pitch of the channels make individual channel access feasible and economic. Hence, each fuel channel has an in­sulated and watercooled standpipe above it passing through the pressure vessel top slab. Below the stand­pipe there is a guide tube which passes down through the top dome and into the top of the fuel channel in the core. The fuel stringer within the core consists of eight fuel elements (seven at Dungeness B) with top and bottom fitments and is lifted by a tie bar which passes up the middle of the stack and is con­nected to the plug unit. The plug unit sits on top of the fuel stringer and extends up the guide tube/ standpipe to the pile cap level. The plug unit together with its fuel stringer is called a fuel assembly. This has an overall length of some 23 m and is handled as a single unit. The refuelling grab engages with the top of the plug unit and never enters the pressure vessel.