[1] Setting of core geometry

The fast reactor core consists of the core fuel assemblies, the control rod assemblies, and the surrounding blanket fuel assemblies and reflectors (cf. Fig. 4.3). The core fuel is MOX. The blanket fuel is depleted UO2. The core fuel region consists of two types of core fuel assemblies with different plutonium enrichments. The outer core has higher plutonium enrichment to flatten the power distribution. The core fuel assembly consists of the fuel

elements containing upper and lower axial blanket fuels as well as the core fuel. The blanket fuel assemblies surround the core region. They contribute to breeding by efficiently capturing leaking neutrons from the core region and also by reducing neutron leakage to the outside. The blanket region is surrounded by the reflectors in order to further reduce the neutron exposure of the structures.

In the core design, the total length of core fuel elements for target thermal power is calculated first from the linear heat rate. The total number of core fuel elements is calculated from the core height. The number of core fuel elements in a core fuel assembly is determined by selecting the diameter of the core fuel element. Then, the number of core fuel assemblies is determined. The relation between the core thermal power and the number of core fuel assemblies is

(4.5)

Na : Number of core fuel assemblies Q : Core thermal power (MW) q : Average linear heat rate (MW/m)

Hc : Core height (m)

Nc : Number of core fuel elements in a core fuel assembly

The amount of fuel loading is determined from the diameter of the core fuel element. The relationship among the core thermal power, the operation period per cycle, the fuel loading amount, and the number of batches is given by Eq. (4.6).

W = Q x D x Nb/BU (4.6)

W : Fuel loading amount (t)

Q : Core thermal power (MW)

D : Operation period per cycle (d)

Nb : Number of batches (reciprocal of the fraction of refueled assemblies at a refueling)

BU: Average discharge burnup (MWd/t).

The core thermal power and the burnup are set as the design conditions. After selecting the operation period, the variables are the fuel loading amount and the number of batches. The fuel loading amount (diameter of the fuel element) strongly relates to the breeding ratio.

The breeding ratio is one of the important indices for the core characteris­tics of a fast reactor. On the other hand, the core size is limited from the viewpoint of safety because a larger core has a more positive coolant void reactivity. The breeding ratio increases with the fuel volume fraction by making the fuel element thicker and reducing the fuel element pitch. The breeding ratio also increases by making the ratio of core height and diameter closer to unity due to smaller leakage of neutrons. However, those changes lead to more positive void reactivity. Therefore, the following procedures are carried out.

(a) The core height and the thickness of blanket region etc. are preliminarily determined. Then, the possible ranges of the fuel element diameter and the fuel element pitch etc. for achieving the target breeding ratio are identified through parametric surveys.

(b) The possible ranges of the core height for achieving the target breeding ratio and the coolant void reactivity below the limit are identified through a parametric survey. Generally, the ratio of the core height and diameter is set small to reduce the void reactivity and coolant pressure drop. Designs so far have adopted ratios of 0.3-0.5 [9].

(c) The possible range of the thickness of the blanket region for achieving the target breeding ratio is identified through a parametric survey.

(d) The number of batches to achieve the target fuel burn-up with the designed operation period is considered.

As described above, the core characteristics are iteratively evaluated by changing the design parameters and finally the design specifications for achiev­ing the design targets are determined.

The core configuration is generally made as symmetric as possible from the viewpoint of flattening the power distribution [9]. The total number of the core fuel assemblies and control rod assemblies is set as a multiple of 6 plus 1. The core fuel region including the control rod assemblies is divided into two regions: inner core and outer core.

The heights of the upper and lower axial blanket fuels are determined so as to achieve the designed breeding ratio. The number of radial blanket layers is determined in the same manner.

The number of control rods is determined so as to ensure the necessary reactivity worth. In the designs so far, 7-10 % of the core fuel region was occupied by the control rod assemblies [9]. The control rod assemblies are symmetrically arranged for flattening the power distribution, and the influences of the inserted rods and fully withdrawn rods at normal operation on the power distribution are also considered.