Before discussing the more detailed physics aspects it would be prudent at this stage to look at the aims of the fuel cycle itself, since there are several of them and not all are compatible. Quite simply, the overall objective must be to ensure that both the capital and the continued running costs of the reactor are kept as low as possible. At the same time, the reactor must provide safe and dependable operation at maxi­mum power throughout its working life. It is worth emphasising that the properly optimised fuel cycle, which encompasses choice of enrichment as well as the formulation of a detailed refuelling strategy, needs to take into account fuel handling machinery and reactor safety as well as the more obvious needs ot reactor optimisation and control.

Throughout reactor life, from first power raising and subsequent refuelling of the initial core (І. Є., the approach to equilibrium) and through the various replacement charges thereafter, it is of prime impor­tance that acceptable axial and radial power distri­butions should be maintained. That is to say the shape of the power distribution should be as flat as possible, both along the fuel stack and across the core in all directions, so that maximum power can be obtained from as much of the fuel as possible. This is assisted in practice by using two or three different fuel enrichments, as will be explained later, but choice of enrichment level must also reflect the need to pro­vide sufficient reactivity at the fuel irradiation dis­charge limit chosen and must ensure adequate shut­down margins throughout reactor life. Therefore the design and operation of a suitable control rod system, w’hich is capable of providing adequate control and shutdown capacity, forms an integral part of the over­all task. Under normal circumstances the fuel enrich­ments for an AGR system are chosen to match a particular discharge irradiation limit, the higher the enrichment, the higher the achievable irradiation.

The complete fuel cycle study examines the highly complex interplay between these issues so that, within the framework of the various constraints on reactor operation, balanced judgements can be made in re­spect of choice of enrichments and overall refuelling strategy (i. e., refuelling rate and sequencing) which will ultimately enable a uniform distribution of re­activity to be maintained throughout life.