Flattening of the radial rating profile is achieved in the initial core by differential enrichment in several zones, and the use of burnable poisons in place of

Fig. 3.13 PWR axial rating shapes

some fuel rods. During a cycle the assembly power ratios change as burn-up proceeds, the peak positions tending to reduce further than the average and hence partially ‘burning out’ the peaks. On refuelling, the fuel assemblies are rearranged and some new assem­blies loaded in such a way as to minimise the rating

profile.

Flux fine structure effects within an assembly are smaller than in AGRs because of the presence of moderator throughout the fuel bundle and the absence of a separate moderator.