The numbers of rod cluster control assemblies (RCCAs) and rods per assembly are determined to secure necessary reactivity. RCCAs are inserted into some of the fuel assemblies (e. g., Fig. 3.30). In addition to securing the reactivity, the control rod arrangement is determined so that it flattens the pin power distribution in the fuel assembly when control rods are withdrawn, considering also mechanical integrity of RCCAs. After that arrangement is determined, a nuclear instrumentation guide tube is placed near the center of the fuel assembly and then fuel rods are arranged in the remaining locations. The 17 x 17 square lattice arrangement for a fuel assembly is shown in Figs. 3.30 and 3.34. This arrangement is composed of 24 control rod guide tubes (guide thimbles), one nuclear instrumentation guide tube (guide thimble), and the rest are fuel rods. Fuel assemblies of 14 x 14 and 15 x 15 types are also used in Japan.

(3) Fuel Assembly Height

A higher fuel assembly height can reduce the number of fuel assemblies and therefore improve the capacity factor of a plant. There is, however, a limitation in fuel assembly height. A higher assembly height causes an increased pressure drop in the thermal-hydraulic design and therefore requires a higher capacity primary coolant pump. It also gives rise to an increase in fuel assembly bowing in fuel mechanical design and therefore has an adverse effect on fuel loading operation into core. From the viewpoint of nuclear design, as discussed before, the core for which the ratio of the core height to the equivalent diameter is 1.0 gives a good neutron economy because of low neutron leakage. However, the fuel assembly active height was standardized as about 3.7 m for economy in fuel assembly fabrication.