(1) Fuel Assembly Size

Large-size fuel assemblies can reduce the number of fuel assemblies to be loaded in core and to be handled in refueling, and therefore they result in an improved capacity factor of a plant. However, a larger size leads to an increase in effective multiplication factor and even one fuel assembly may result in criticality. It is necessary to ensure that one fuel assembly to be handled outside the core should be subcritical in unborated water. For 5 % fuel enrichment, the maximum fuel assembly size to satisfy this condition is a cross-sectional width of about 22 cm.

(2) Fuel Rod Pitch

Fuel rod pitch can be determined by fuel rod diameter and moderator-to — fuel volume ratio Vm/Vf (H/U atomic ratio). Since control rods as well as fuel rods are placed in fuel assemblies, the number and arrangement of fuel rods are determined according to the following items. Control rods are usually withdrawn in normal operation. Therefore, the control rod volume

in fuel assembly is also regarded as the moderator volume in evaluating the moderator-to-fuel volume ratio.