Since the BWR core has a low void fraction in the lower part and a high void fraction in the upper part due to steam being directly generated in the core, this leads to an axial distribution of void fraction in the core. The axial void distribution causes a difference in the moderation effect between the core lower and upper parts and the lower part, with the large moderation effect, has a relatively high multiplication factor compared with the upper part. This, therefore, gives rise to power peaking in the lower part. The mitigation of the axial power peaking is an important challenge to improve the plant capacity

Power

Distribution

(Bottom) Core Height (Top)

Fig. 3.16 Improvement in flattening of axial power distribution

1.5 —

И 0.5

Fig. 3.17 Improvement in axial power distribution at EOC

factor in considering an increase of operating easiness as well as maintaining the core thermal margin.

As shown in Fig. 3.16, control rods were shallowly inserted from the core bottom and gadolinia was added to the lower part of the fuel rods to suppress the distortion of axial power distribution in early BWR designs. However, this strategy caused high power peaking in the lower part of the core, as shown in Fig. 3.17, because the core excess reactivity decreases with burnup and control

rods are withdrawn to compensate for the decrement near the end of the operating cycle.

As a solution of this problem, the uranium enrichment in the core upper part can be increased a little more than that in the lower part to compensate for the decrease of the infinite multiplication factor due to the void in the core upper part. This strategy balances the infinite multiplication factor between the core upper and lower parts, and is practically employed to flatten the core axial power distribution; it is referred to as the axially two-zoned fuel concept [10, 11]. Figure 3.18 shows an example of an axially two-zoned fuel core design [10]. The enrichment of upper pellets of some fuel rods is higher by about 0.2-0.5 wt% than that of lower ones and the cross-sectional average enrichment of the fuel assembly upper part is higher by about 0.2 wt% to give a balance of infinite multiplication factors between the upper and lower parts. While control rods are withdrawn and the effect of burnable poisons is decreas­ing with burnup, the effect of the axially two-zoned fuel on the flat axial power distribution can be maintained with burnup even at the end of the operating

Fig. 3.19 Comparison of control rod pattern between (a) Previous core and (b) Control cell core

cycle [14] as shown in Fig. 3.17. The axially two-zoned BWR core consider­ably improves the plant capacity factor by decreasing the maximum linear heat generation rate by 20 % compared with the previous core and by simplifying the control rod operation [12—14].

Thus, different axial enrichments, independent of the core reactivity control, are applied to control of the axial power distribution in the axially two-zoned BWR core. This makes it easy to optimize the axial power distribution and makes it possible to use burnable poisons in core reactivity control separately from control of the axial power distribution.