Design of fuel assembly

(1) Features of fast reactor fuel assembly

A fast reactor fuel assembly consists of triangularly arranged fuel elements and a containing wrapper tube (cf. Fig. 4.3). Positions of the fuel elements are kept by the wire spacers or the grid spacers, so that the coolant channels are ensured. The upper shielding is contained in the upper part of the wrapper tube. The top of the wrapper tube is connected to the handling head which has a gripper function for handling the fuel assembly. The lower shielding is also contained in the lower part of the wrapper tube. The bottom of the wrapper tube is connected to the entrance nozzle through which the coolant enters. The wrapper tube, along with the entrance nozzle, forms the individ­ual coolant flow passage and hence enables flow distribution among the fuel assemblies. The wrapper tube also protects the fuel element bundle and would act as one of the barriers against propagation of fuel damage during accident conditions.

Upper, lower and intermediate spacer pads are provided on the outer surface of the wrapper tube in order to keep the spacing between the neighboring assemblies and to take loads during operation including seismic load.

(2) Major design principles of fast reactor fuel assembly

For the design of a fuel assembly, the assembly size, i. e. the number of fuel elements, needs to be determined first. The following factors influence the assembly size.

(a) Reactivity worth per assembly

(b) Decay heat of an assembly

(c) Weight of an assembly

(d) Fuel cycle cost

(e) Refueling time

(f)Degrees of freedom for flow allocation

(g) Degrees of freedom for control rod arrangement

(h) Stable support of a fuel element bundle

Among them, (a), (b) and (c) increase with assembly size. High reactivity worth per assembly increases the degree of risk associated with a reactivity accident during fuel handling and criticality in case of submergence caused by an accident during transportation of the assemblies. High assembly decay heat or large assembly weight leads to excess loads on the cooling facility during fuel handling and on the refueling machines.

On the other hand, (d) and (e) decrease with increasing assembly size. Generally, as the number of fabricated assemblies is larger, the fuel fabri­cation cost is higher. Thus, larger assembly size reduces the fuel cycle cost including the fabrication cost. Also, larger assembly size means a smaller number of assemblies which shortens the time needed for refueling and hence improves the plant availability.

As for (f) and (g), the degrees of freedom decrease with increasing assembly size, i. e. decreasing the number of assemblies. As for (h), espe­cially if the wire spacer is adopted, larger assembly size worsens the support because the degrees of freedom of the fuel elements’ displacement increase with the number of the fuel elements, i. e. the assembly size.

From the above and some other considerations, an adequate assembly size is determined and then the assembly is designed based on two design principles.

• The assembly has and keeps sufficient mechanical and structural strength at normal operation and anticipated operational occurrences during the duration of service

• The assembly has and keeps sufficient mechanical and structural strength against normal loads during transporting and handling.

(3) Major evaluation items in fuel assembly design

(1) Stress evaluations

According to the design principles, sufficient strength of each compo­nent of the fuel assembly against various loads is confirmed and kept by evaluating the stresses associated with the loads at normal operation and anticipated operational occurrences by using the finite element method or other approaches.

(2) Duct-to-duct interaction (DDI)

DDI is evaluated by confirming that the neighboring wrapper tubes do not contact each other (i. e. the refueling function is not obstructed) due to expansion of the wrapper tube such as by thermal expansion, swell­ing and creep. In the DDI evaluation, bending of the wrapper tube due to thermal deformation by the temperature distribution and swelling by neutron irradiation must be considered as well as the expansion.