The AGR fuel assembly designs for Dungeness B, Hinkley Point Bt Hunterston В, Hartlepool/Heysham l and Heysham 2/Torness reactors are developments of the basic type proven in the Windscale advanced gas cooled reactor (WAGR) prototype operation since 1963.

They comprise a plug unit and fuel stringer joined together by a central tie bar or coupling to form a

long flexible assembly.

The upper plug unit incorporates a closure unit, gag actuator mechanism, gag and shields and is described in detail in Section 6,3 of this chapter.

As originally proposed, the lower section of the first civil fuel assembly design comprised a number of 36-pin fuel elements of the floating pin type, develop­ments of prototype WAGR 9 and 18 pin configura­tions. However, early developments of this design led to the adoption of a fixed pin element arrangement in which the fuel was attached to the support grid and located radially at central and upper positions by braces mounted from the graphite sleeves which form the coolant channel (see Fig 2.72).

Both improved cluster performance and vibrational stability were achieved by fixing the fuel to the grid and reducing the inter-element gap. Increases in fuel length and reductions in inter-sleeve leakage and ab­sorber content were also possible.

Improvements in fuel pin endurance were also found to be possible when hollow fuel pellets were used in place of the earlier solid design, by assisting in the problem of accommodating fission product gases in providing increased pin voidage and eliminating the hot central core region associated with solid fuel. The use of fuel cans manufactured from 25cold-worked tubing followed by assembly annealing was also found to be beneficial.

Requirements of on load refuelling have necessi­tated the incorporation of charge/discharge and in­pile stabilising features and the use of element anti­gapping devices.

More recently a Stage 2 fuel element has been developed adopting a single thicker graphite sleeve and integrally spot-welded grid and braces, which with additional strength is more impact resistant providing prospects of full power on-load refuelling (see Fig 2.73).

The need for operational control on the radioactive contamination of boilers and ancillary reactor parts has also involved the development and installation of inertial collectors in stringers in order to filter gas — borne particular matter from the coolant.