The core radial power distribution can be flattened by properly designing the control rod insertion positions (the control rod pattern) and the fuel loading pattern.

Figure 3.19 compares two examples of control rod patterns in BWRs. While control rods are shallowly inserted for control of the axial power distribution, they are generally deeply inserted for control of the radial power distribution. Neutron absorption by control rods suppresses the power of adjacent fuel assemblies and has an effect on the power of surrounding fuel assemblies through a change in the neutron flux distribution. Thus, the control rods control the radial power distribution of the whole core. In the previous design which lacked the axially two-zoned fuel concept, the same control rod is not inserted
for a long period of time to avoid slow burning of the adjacent fuel and all control rods are alternatively inserted to give uniform fuel burning in the core. The insertion positions of the control rods (control rod pattern) are changed during the operation cycle.

Such a change in control rod pattern during reactor operation is performed generally at a power level lower than the rated one to avoid a rapid variation in fuel rod power due to the control rod operation. This was a factor which interfered with improvement in the reactor capacity factor. A control cell core [15] has been developed as a core in which such a change in the control rod pattern is unnecessary and the reactor operation is simple. Figure 3.19 compares [16] the control rod pattern of the control cell core with that of the previous core. In the control cell core [17], there are several control cell regions in which low enrichment or high burnup low reactivity fuel assemblies are arranged near control rods. The control rods in the control cells are deeply inserted during reactor operation, while the other control rods are fully withdrawn. Although the control rods in the control cells are withdrawn to compensate for a reactivity loss with burnup, it does not cause a large power peaking because the neighboring fuel assemblies also have a low reactivity. The effect on fuel integrity is therefore small. Thus, the elimination of shallow insertion of control rods by adoption of the axially two-zoned core, the small excess reactivity by a proper addition of burnable poisons into fuel rods, and the reduction of necessary control rods by the control cells lead to a simple reactor operation without a change in control rod pattern.

In addition to the control of core radial power distribution by control rods, an exchange of fuel loading location (fuel shuffling) can be carried out to flatten the core radial power distribution, considering proper loading location of fresh fuel assemblies and the number and burnup of burned fuel assemblies.

BWRs are generally designed to have a scatter-loading pattern in which fuel assemblies are regularly dispersed in the core depending on the burned cycle. Another choice is a low leakage loading pattern in which high burnup fuel assemblies are loaded in the outermost region of the core to reduce the neutron leakage and increase the core reactivity. Figure 3.20 shows an example of the fuel loading pattern at the equilibrium cycle of the 1,100 MWe BWR core. In the 4-batch equilibrium core, fuel assemblies are kept in almost the same location until the third cycle after being loaded, and then moved to the outer­most region of the core or the control cell region. This scatter-loading pattern can produce a self-flattening effect of the power distribution with burnup and minimize the fuel shuffling for fattening of the power distribution.

In early BWR designs, generally, one type of fuel assembly was loaded in the initial core and the core radial power distribution was mainly controlled by control rods. In recent designs, an equilibrium core is simulated and different enrichments of fuel assemblies are employed [18]. The low leakage loading pattern mentioned above is usually used for improvement in economy. The control cell consists of four low enrichment fuel assemblies and other fuel assemblies with different enrichments are dispersed in other regions. In some of

Fig. 3.20 Example of BWR fuel loading pattern (1,100 MWe plant core). (a) Equilibrium core (b), initial core

the practical intial core designs, high enrichment fuel assemblies are loaded into the outermost region of the core.