Access to the core is via standpipes on the reactor pressure envelope. When not in use the standpipes contain shield plugs which provide a pressure seal and preserve the biological shielding. Standpipes are also necessary for control rod mechanisms. In mag­nox reactors the ratio of fuel channels to control rods is approximately 16:1. Because of the difficul­ties experienced at Calder Hall where the same stand­pipes were used for both control rods and refuelling, the early magnox stations had separate refuelling standpipes. At Hunterston A, the refuelling stand­pipes were placed below the core for bottom refuel­ling but this led to a costly increase in the height of the reactor complex. To minimise the number, each refuelling standpipe must serve a number of channels with the greatest number being 64 at Berkeley. Be­fore refuelling, a chute with a parallel motion dog­leg must be inserted. The FM can rotate this chute and move the dog-leg to align with the desired chan­nel (Fig 2.32). Because the fuel elements are too long to negotiate the dog-leg angles, the hoist has to be stopped and the leg moved in or out as appropriate whenever the grab is engaged.

ing requires reliable first time engagement. There are two independent sets of three jaws. Should one set rest on the fuel element sprung arm spider, the second set operates correctly. The operating plunger motion is controlled by the interrupter which gives alternately long and short strokes. The grab is suspended on a wire rope formed around a stainless steel spiral in­corporating an insulated conductor which serves the locking solenoid and provides a ‘jaws engaged’ signal.