When the initial enrichments have been chosen, fur­ther cost benetits can be secured it the loading pattern itself is arranged as a chequerboard array of ‘high’ and ‘low’ enrichment channels in a large inner-region, together with a third higher enrichment loaded into some of the outer-region channels (the others con­taining vacancy stringers) in order to provide some enrichment flattening of the radial power shape (Fig 3.49). Following replacement of the majority of the vacancies, refuelling is then arranged of the inner — region low’ enrichment channels, punctuated only by the occasional visit to the outer zone to keep the power shape under control. The intent will be to load the centre again with one enrichment, the outer re­gion with another, with possibly an intermediate en­richment region between the two in the interests of radial power shaping (Fig 3.50). Remembering that the inner region originally consisted of a chequerboard array of high and low enrichments, it is clear that the greatest reactivity benefit will be obtained by re-

fuelling the low enrichment (low reactivity) channels firsts leaving the higher enrichment channels to do more work until they too become refuelled later in the cycle. In this way, core reactivity is sustained and the desired irradiation of the initial charge is achieved with- the last stringer of initial fuel being discharged at (or near) the target discharge limit. The overall effect of adopting this strategy is that the CAI at discharge is maximised for all the initial fuel with the added bonus of a significantly reduced re­fuelling rate.

The feed fuel enrichments need to be higher than those of the initial charge in order to compensate for the higher average irradiation level of the fuel at equilibrium and the correspondingly lower average reactivity level. However, the increased average age of the fuel at equilibrium means that the higher rated fuel elements, situated in more central channel posi­tions ‘burn up’ more rapidly than the other elements within the channel, and consequently reactivity and therefore rating declines more rapidly, producing a degree of ‘automatic’ axial power flattening. During the approach to equilibrium, however, the fuel on average is considerably younger and axial power flat­tening is accommodated by introducing higher enrich­ment elements into the outer positions of the fuel stack (positions 1, 7 and 8 usually) during the initial
loading. Such fuel assemblies represent the inner — region low’ channels within the mixed enrichment scheme, whilst the adjacent channels containing fuel uniformly enriched to the same level as the outer elements of the low enrichment channels will therefore constitute the inner ‘high’ positions.