As shown in Fig. 3.5, the set of a fuel assembly, water gap outside channel box, and a quarter of cruciform control rod form a unit lattice and unit lattices are regularly arranged radially in the BWR core.

As main parameters of the unit lattice, there are the size and shape of the channel boxes wrapping the fuel assemblies, the number and diameter of fuel rods, the fuel rod pitch and pellet size, the size and location of the water rods, specifications of control rods, and so on. The amount of fissionable materials in each fuel rod, namely, the enrichment and density of U235 in the case of uranium fuel and the amount of gadolinia in case of burnable poison-mixed fuel are the basic nuclear design parameters.

Figure 3.6 [3] shows the procedure flow in the fuel assembly design. An optimal ratio of fuel to moderator is established from the viewpoint of the reactor nuclear performance, and the fractions of coolant and fuel regions are selected. The fuel assembly design proceeds with the fuel rod pitch and water rod size maintaining consistency with the design requirements to the number and diameter of fuel rods from the viewpoints of fuel inventory and fuel integrity. In the actual work, the final design of fuel assemblies is determined by repeating this sequence.

Unit Lattice

Fig. 3.5 Fuel lattice of BWR fuel assembly

[ Discharge Burnup j

Fuel Assembly Design

Fig. 3.6 Fuel assembly design flow

In determining the fuel assembly design, it is especially important to consider the relation between the moderator-to-fuel ratio in the unit lattice, which has a large effect on the nuclear performance, and the parameters mentioned above.