The procedure for designing a fast reactor core is schematically shown in Fig. 4.4. The core thermal power and the operation period per cycle are determined as the basic specifications of the core design. The fuel assembly maximum burnup (or discharged burnup), the maximum allowable linear heat rate and the breeding ratio are given from the design principles.

First, the total length of the fuel elements and the number of fuel assemblies are preliminarily determined according to the core thermal power, the linear heat rate of fuel and the core height. The fuel loading amount is obtained by determining diameter of the fuel element. Then, the control rods are arranged based on the functions. The core configuration is determined. The core char­acteristics are iteratively evaluated until the design principles are satisfied by adjusting the trade-off conditions as illustrated below.

• If the burnup is lower than the target, the fuel element is made thinner to reduce the fuel loading or the number of fuel exchange batches is increased.

• If the breeding ratio is lower than the target, the fuel element is made thicker or the core height is increased.

• If the void reactivity is too large, the core height is reduced.

When all the design targets and criteria are satisfied, the core design is fixed.