Figure 3.7 shows a typical relation between H/U ratio and neutron infinite multiplication factor (kTO). The kTO increases as H/U ratio increases in the beginning because large numbers of neutrons are moderated to thermal neutrons by the moderation effect of water (specifically, its hydrogen atoms). However, a further increase in H/U ratio beyond the turning point contrarily leads to a decrease in kTO because the neutron absorption by hydrogen atoms becomes dominant. Therefore, kTO has a maximum value with respect to H/U ratio. The region on the left side of the maximum indicates an undermoderation region. If an H/U ratio which is a little smaller than the maximum is given at the cold temperature, the H/U ratio is decreased by the moderator density decrease and coolant voiding during reactor operation and then kTO decreases. The void coefficient and moderator temperature coefficient become negative. As shown in Fig. 3.8, an increase in H/U ratio leads to a less negative void reactivity coefficient and a large control rod worth because of neutron spectrum softening. In general, BWRs are designed to have an H/U ratio of 4 to 5 for the average void fraction of about 40 % at normal operation.

An increase in fuel enrichment for high burnup leads to a large amount of fissile materials in the fuel, and then the neutron absorption of hydrogen atoms in the moderator is lowered and the maximum value of kTO shifts to a higher H/U ratio as shown in Fig. 3.7. Figure 3.9 shows an example of the dependence of void reactivity coefficient and control rod worth on fuel enrichment. If the H/U ratio is set for low fuel enrichment at the reference core design, the void reactivity coefficient becomes more negative and the control rod worth becomes smaller as the fuel enrichment increases. It is, therefore, necessary to properly adjust the H/U ratio for high fuel enrichment.

As a way to increase H/U ratio, a more slender fuel rod can be made to reduce the fuel inventory, but that is not generally desirable from the viewpoint of fuel

economy. The position and amount of the non-boiling regions inside and outside the channel box are optimized instead as a feature of BWR core structures. The H/U ratio can be increased without changing the fuel inventory by the following measures.

(i) Increase the non-boiling region inside the channel box (water rod region).

(ii) Increase the non-boiling region outside the channel box (water gap region).

Figure 3.10 shows the effect [9] of such a non-boiling region increase on reactivity increase at the cold temperature under a constant fuel inventory; reactivity changes with the change from the normal to cold temperature condi­tion. Figures 3.11 and 3.12 show effects of the non-boiling region increase on void reactivity coefficient and infinite multiplication factor, respectively. To achieve measure (i), the central fuel rods in the fuel assemblies are replaced

with water rods and the fuel rod diameter is increased to maintain the same fuel inventory. The width of the channel box is reduced and the fuel rod pitch is adjusted to use measure (ii).

The suppression of the reactivity increase at the cold temperature and the improvement in the void reactivity depend more highly on the water gap region outside the channel box. It is because the neutrons can be effectively moderated before they are absorbed in the fuel since the water gap region is located relatively far from the fuel rods. On the other hand, the increase of the water gap region leads to a maldistribution of moderator and then it causes a large distortion of thermal neutron flux in the fuel assemblies. Therefore, the neutron absorption rate increases in the water gap region where the peak thermal neutron flux is seen, and the neutron infinite multiplication factor decreases. Fuel assembly design specifications suitable for target characteristics are deter­mined through control and adjustment of reactivity and reactivity coefficients by size and location of the non-boiling water region. Improvement of fuel assembly design for higher burnup can be achieved by increasing the water rod region with a proper H/U ratio to make reactivity high.