The moderator-to-fuel ratio in the unit lattice relates to the size and shape of fuel rods and water rods, and the void fraction, and is derived from the

structural feature of the BWR core where the coolant in the flow path of the channel box boils, but the water in the water rod and the assembly gap outside the channel box do not boil. The H/U ratio, which is defined as the ratio of hydrogen atoms in moderator to uranium atoms ( U + U) in fuel, rather than the moderator-to-fuel volume ratio (Vm/Vf) is used as the moderator-to-fuel ratio in the unit lattice. The evaluation of the effect of lattice shape and void fraction based on the H/U ratio is suitable for a consistent investigation into the effect of each parameter.

The H/U ratio is given by

H/U ratio = (No. of hydrogen atoms in unit lattice)/

(No. of fuel atoms in unit lattice)

No of hydrogen atoms in unit lattice = Щ/ x (1 — a) H nH x a} x (Cross-sectional area of boiling region inside channel box) + n°H x Cross-sectional area of non-boiling region inside channel box (water rod region)) H n/x

(Cross-sectional area of non-boiling region outside channel box (Water gap region))

No. of fuel atoms in unit lattice = nu x Nrod x n(Dp/2)2

Here a is the void fraction inside the channel box, n/ is the atomic density of hydrogen in the saturated water, nsH is the atomic density of hydrogen in the steam, and nu is the atomic density of uranium in the pellet. It is necessary to remark that the hydrogen atoms in the water gap and water rods (non-boiling region) are considered in the H/U ratio and moreover the H/U ratio depends on the void fraction inside the channel box. The H/U ratios of typical BWR fuel assemblies are shown in Table 3.8.