Along with the reactivity control, control of the power distribution in core is one of the important challenges in core design. Technologies for power distribution control have been developed and applied to the pin power distribution in a fuel assembly, the core axial power distribution, and the core radial power distribution.

[1] Power distribution in a fuel assembly

The relative power density of each fuel rod to the average power density of fuel rods in the fuel assembly is referred to as the local power peaking. Areas inside and outside of the channel box of the fuel assembly are completely separated in BWRs. When a void occurs inside the channel box, it does not occur in the water gap region outside the channel box. This, therefore, leads to high power of the outermost fuel rods neighboring the water gap which increases the neutron moderation effect; in other words, a high local power peaking is caused. The water rods, which are arranged in the central region to optimize the H/U ratio, increase the power of the central fuel rods by increasing the moderation effect, therefore contributing to a flat power distribution.

To flatten the pin power distribution and suppress the local power peaking, different enrichments of fuel rods are properly arranged as shown in Fig. 3.5; low enrichment fuel rods are near the channel fox and high enrichment fuel rods are in the central region.

The pin power distribution in a fuel assembly varies with burnup. Generally higher power fuel rods at initial burnup lead to a larger power decrease with burnup, and therefore the high local power peaking at the initial burnup is mitigated with burnup. The enrichment zoning of fuel rods and the arrangement and concentrations of burnable poison rods are determined so as to maintain a flat power distribution during burnup until the fuels are discharged. Since the gadolinia-added fuel rods suppress pin powers at initial burnup, the burning speed of fissile materials in the fuel rods is slower than that of neighboring fuel rods. The gadolinia-added fuel rods are designed to have a relatively low enrichment compared with the neighboring fuel rods in order to avoid a high local power peaking caused by the remaining fissile materials after burning out of gadolinia. Since the variation in gadolinia concentration with burnup depends on the thermal flux near the gadolinia-added fuel rods, the concentra­tion is properly determined to optimize the fuel burnup and gadolinia burning, corresponding to the location of the gadolinia-added fuel rods.