10.32. Design flexibility has been the most significant trend as operating experience has accumulated during the post decade and assemblies have been available from several vendors. As the use of lumped burnable poisons has increased, more flexible designs have been developed that do not require separate rods in control cluster positions. For example, absorber material can be fabricated with the fuel, either as a coating or integral with the fuel pellet [6].

10.33. Gadolinia bearing rods, which have been used in BWR assem­blies for some time, are now being used in PWR assemblies. Such use of gadolinia as a solid solution with the U02 pellets permits a variation of absorber concentration as well as a selection of the number and location of absorber rods within the assembly. The capability to shape the power distribution is thereby enhanced. Similar advantages may be obtained by using a thin coating of ZrB2 as a burnable poison on the surface of selected fuel pellets. Boron is somewhat easier than gadolinium to model neutron — ically, although it has the disadvantage of not being combinable with ura­nium oxide. On the other hand, for gadolinium rods, the buildup during exposure of the high neutron capture isotope, gadolinium-157, results in a slightly higher residual poison penalty than for boron-coated rods.

10.34. Increasing the water/fuel ratio in PWR lattice design improves uranium utilization and reduces fuel costs. This could be done by reducing the rod diameter. The resulting slight decrease in the negative moderator temperature coefficient can be compensated for by decreasing the soluble boron and, in turn, increasing the solid absorber in the assembly. Uranium utilization is also improved by using some natural uranium oxide pellets at each end of the fuel rod to serve as an axial blanket. However, some increase in axial power peaking results. Thus, the use of partially enriched axial blankets is another option.

10.35. Hardware improvements are also receiving continued attention. For example, the use of zircaloy-4 instead of Inconel in spacer grids im­proves neutron economy. Assembly components, such as the upper tie plate, have been redesigned to ease the replacement of damaged rods between burnup cycles, and if desirable, to reconstitute (rebuild with some different rods) the assembly. Another significant PWR improvement is the use of debris filters in the assembly bottom nozzles to prevent foreign particles from entering the core. In the past, such foreign material would be trapped in the rod spacer grids and cause vibration-induced wear (fret­ting) of the rods.