Changing a fuel stringer on an AGR has a much greater effect on core reactivity than changing a chan­nel of fuel on a magnox reactor, partly because it represents a much larger fraction of the total core fuel inventory and partly because of the much greater reactivity difference between start of life and discharge. The effect on an AGR core is about 60-90 mN per stringer, compared with about 0.5 mN per channel in a magnox reactor.

As the irradiated stringer is discharged the adja­cent fuel channels run hotter, as in a magnox reactor, so the regulating rods will be driven in by the auto control system to maintain average channel gas outlet temperatures constant. As the new stringer is loaded, the regulating rods will be driven in further to main­tain criticality due to the greater reactivity worth of the new fuel. Over the two days following refuelling the regulating rods will be driven out slightly to match the build-up of Xe-135 to its equilibrium concentra­tion іплЬе new fuel. Also some trimming will be re­quired to optimise the reactor output with the new fuel, not only the new fuel stringer itself but also the nearby fuel stringers because of the effect of the new stringer on them (see Section 8 of this chapter). The strategy by which the temperature and power distribution is optimised is carefully chosen to mini­mise temperature cycling of the fuel.

Consideration of certain faults which may arise during refuelling on an AGR currently require that this operation be carried out at reduced power, there­fore the concern for exceeding temperature limits on fuel in the vicinity of the refuelling is greatly re­duced. Because of this, and the fact that moderator temperature is maintained largely constant by the re­entrant gas flow, and the greater reactivity invest­ment in regulating rods (supplemented by trim rods at Dungeness B), the kinetics of reactor parameters during refuelling is much less onerous on an AGR than on a magnox reactor from the control engineer’s point of view. His attention is concentrated more on the safety of the fuel stringer being handled, parti­cularly in the removal of the irradiated stringer, and on the effects on the boilers of the gas at reactor gas inlet temperature which streams from the empty channel.