There are essentially, four different types of commercial light water cooled reac­tors, whose main characteristics are provided in Table 4.1 (Cox et al, 2006).

4.1.1 Pressurized water reactors (PWRs) and boiling water reactors (BWRs)

There is a wide variety of fuel assembly (FA) types for BWRs and PWRs. The fuel rod array for BWRs was initially 7 x 7 but there has been a trend over the years to increase the number of FA rods and today most designs

* Variation from lower to upper part of the core and from plant to plant. Source: A. N.T International (2011) and Cox et al. (2006).

are either of 9 x 9 or 10 x 10 square configuration design (Cox et al., 2006). The driving force for this trend was to reduce the linear heat generation rate (LHGR), which resulted in a number of fuel performance benefits such as lower fission gas release (FGR) and increased pellet clad interaction (PCI) margins. However, to increase utility competitiveness, the LHGRs of 9 x 9 and 10 x 10 FA have successively been increased, and peak LHGRs are today almost comparable to those of the older 7 x 7 and 8 x 8 designs.

Also for PWRs there has been a trend to greater subdivision of fuel rods, for example from the Westinghouse 15 x 15 to 17 x 17 design (Cox et al, 2006). However, since PWRs do not have the same flexibility with core internals and control rods as BWRs, to accomplish this requires modifica­tion of the reactor internals. There is, however, one exception, namely DC

Cook 1, which is switching to 17 x 17 through changing the reactor internals. Figure 4.2 shows the current PWR fuel rod array designs.

In most PWRs, the assemblies are positioned in the core by bottom and top fittings, and the lateral clearances are restricted by the assembly-to-assembly contacts at the spacer-grid levels (Cox et al, 2006). Furthermore, the control rods consist of rod cluster control assemblies (RCCAs), the poison part of which moves into guide thimbles (GTs). These guide thimbles are an inte­gral part of the assembly structure.

In all BWRs, the assemblies are enclosed in ‘fuel channels’ surrounding the assemblies and between which the blades of the control rods moves.

Irrespective of the many possible different shapes, sizes and configura­tions, the common FA design requirements are (Cox et al, 2006):

• Maintain proper positioning of the fuel rods under normal operating conditions and in design basis accidents (DBAs) (e. g. seismic effects, LOCA, RIA).

• Permit handling capability before and after irradiation.

Figures 4.3 and 4.4 show a typical BWR and PWR FA, respectively. Also, the different FA components are shown and the material selections for these

Rod cluster control

Top nozzle

304 L stainless steel

springs

inconel 718

Control rod

304 L stainless steel

clad

Bottom view

4.4 Typical PWR FA (Cox et al., 2006).

components are provided. The selection of the different structural materi­als is based on their nuclear and mechanical properties as well as their cost, in order to ensure acceptable performance during normal operation and accidents.

4.5

Draft of the RBMK-1500 fuel assembly. (Cox et al., 2006).