In this section, the common characteristics of BWR and PWR fuel assemblies as well as features particular to each type will be described. CANDU and VVER fuel designs are not treated in detail in this chapter. Useful information about these latter types was given by Boczar et al. (2003) and in Nuclear Engineering International (2010). See also Chapter 11 for more details on CANDU fuel design.

The CANDU fuel design is very different from that of LWR fuel. In CANDU reactor terminology, bundle is used instead of assembly, and ‘fuel element’ means ‘fuel rod’. The CANDU fuel bundle is short and has few different components. It is composed of 37 or 43 elements (rods) of length 48 cm held together by bundle end plates in a circular arrangement (three rings plus centre rod). Each element consists of uranium-dioxide pellets encased in a thin-walled zircaloy tube. CANDU fuel is designed for on-power refuelling.

VVER (Vodo-Vodyanoi Energetichesky Reaktor = Water-Water Energetic Reactor) is the Russian type of pressurised water reactor. Its fuel assemblies differ in many respects from Western PWR assembly designs. Immediately noticeable are the hexagonal outer shape of the assembly and the corresponding triangular matrix of the fuel rods filling the space within a hexagonal shroud in the VVER — 440 type (VVER-1000 assemblies are without a shroud). The fuel rods are smaller in diameter than in PWRs, and the pellets have a centre hole. In contrast to PWRs, a VVER fuel assembly does not include space (tubes) for insertion of control rods.

Typical BWR and PWR fuel assemblies are shown in Fig. 9.1, Fig. 9.2 and Fig. 9.3, and typical dimensions are listed in Table 9.1 , The assemblies are approximately 3.9-4.8 m long and have a square cross section. Their bearing structure consists of end fittings linked by tie rods (BWR) or guide tubes (PWR). The fuel rods are inserted into this structure in a square lattice (14 x 14 to 18 * 18 for PWRs; 8 x 8 to 10 x 10 for BWRs) and kept from touching each other by spacer grids distributed along the length of the fuel rods. The BWR assembly has a shroud forming a flow channel to direct the flow of coolant along the fuel rods. Control rods can be inserted into the guide tubes of a PWR assembly, while the smaller BWR assemblies are arranged four and four in cells with space for insertion of a cruciform control blade unit between them.

The top and bottom end fittings are called ‘nozzle plates’ (PWR) or ‘tie plates’ (BWR). They provide lateral support to the ends of the fuel rods and control of the coolant flow through the assembly. The top nozzle or tie plate also prevents an

Cross wing

Bottom support

Screw

Transition piece

9.1 I llustration of a BWR assembly (SVEA-96 Optima3) and its main components (courtesy of Westinghouse Electric Company LLC).

upward movement of the fuel rods. It can be removed on site for retrieval of rods, inspection and repair. The BWR assembly has a handle attached to the top tie plate for lifting the assembly into and out of the core.

PWR assemblies are installed between the lower and upper core plate. Alignment holes at two diagonally opposite corners of both end fittings match pins in the upper and lower core plate. In this way, the fuel assembly is secured laterally in the reactor core. Axially, a PWR assembly is fixed by four strong,

9.2 Atrium 10xp BWR assembly (courtesy of AREVA pic).

9.3 PWR fuel assembly (courtesy of Westinghouse Electric Company LLC).

Table 9.1 Typical overall dimensions of PWR and BWR fuel assemblies Feature PWR BWR 14 x 14 15 x 15 16 x 16 17 x 17 18 x 18 9×9 10 x 10 Assembly length, (mm) 3900-4060 4060-4200 4060-4800 4060-4800 4830 4470 4420-4480 Assembly square width, (mm) 197-206 214-215 197-230 214 230 139 139 Rod length, (mm) 3730-3870 3860-3920 3880-4490 3850-4490 4390-4430 4075-4090 3890-4150 Number of fuel rods 176-179 204-208 236 264 300 72 91-96 Average heat rating, (W/cm) 204-220 203-238 176-211 171-200 166-167 158-160 124-158

multi-leaf springs, which are attached to the top end fitting plate by hold-down bolts. Other designs use four helical springs. The springs, when pressed down by the upper core plate, provide a counter force against the lifting force exerted by the upwards coolant flow.

The bottom nozzle or tie plate has several functions. It rests on the lower core plate carrying the load of the assembly, secures it laterally and directs the coolant flow to the assembly. It consists of a perforated plate preventing a downward movement of the fuel rods from the fuel assembly. Another important function is keeping foreign objects (for example metal particles, chips, turnings; collectively called debris) from entering the assembly and causing damage to the fuel rods (debris fretting is the main cause of fuel failure). To this end, the holes in the plate must be small enough to preclude or minimise the passing of debris while still allowing sufficient coolant flow. Fuel vendors have extended considerable effort to improve the debris filtering capability and developed various solutions, for example a bent flow path such that long, thin objects (wires) will not be able to pass through (Gotoh et al., 1999).

PWR assemblies contain 16-24 guide tubes made of a zirconium alloy. They provide the structural connection between the nozzle plates, serve as attachment points for the spacer grids and guide the control rods into and out of the assembly. The guide tubes are designed to ensure a fast drop of the control rods without damage to the latter or the assembly. The control rods fall easily through the upper part with a larger diameter and holes in the guide tube wall such that the water can be displaced without too much resistance. The lower part has a smaller diameter and no holes, thus serving as a dashpot softening the impact.

The centre position of the PWR assembly lattice is taken by a tube for insertion of in-core instrumentation.

A BWR fuel assembly is encased in a thin-walled square tube forming a flow channel. This feature is necessary to avoid reactor instability and cross flow of coolant and steam with the risk of locally inadequate cooling. So-called ‘flow trippers’ redirect the coolant onto the fuel rods for improved thermal performance. The tube is kept centred by leaf springs attached to the lower tie plate and is fixed to the upper tie plate by a fastener with a two-leaf positioning spring. The assembly casing also guides the control blade cross that can be inserted between four assemblies.

In light water reactors, the water has a double function as coolant and moderator. However, in a BWR, the moderator capability is diminished as more and more of the fuel channel towards the top is filled with steam. Water rods are therefore included in BWR assemblies to enhance neutron moderation especially in the upper part with most steam. In some designs, the water rods are cross shaped dividing the assembly into four sub-assemblies. The water rods may also function as tie rods connecting the lower and upper tie plate, and they may be used to attach the spacer grids.

The thin fuel rods, about 4 m long, would vibrate, bend and touch each other without additional lateral support between the end fittings. Spacer grids are therefore inserted about 50 cm apart and attached to the tie rods or guide tubes.

The grids usually consist of square cells formed by a lattice of metal straps. They support the fuel rods at several contact points, which are fixed, rigid dimples and springs providing lateral and axial forces to keep the rod in place while allowing for fuel rod thermal expansion and irradiation-induced growth.

The grids are mostly fabricated from zirconium alloy materials (low neutron absorption), but in some designs Inconel is used for the grids at both ends. Another variant is the bi-metallic grid where the dimples and springs are made of Inconel while a zirconium alloy is used for the straps. The middle grids may contain mixing vanes to increase the turbulence of the flow and coolant mixing within an assembly for improved thermal performance.

The fuel rods in all types of water moderated reactors (CANDU, VVER, BWR, PWR) consist of a cylindrical, zirconium alloy tube (cladding) filled with cylindrical fuel pellets, and two end plugs welded onto the cladding. The rod is filled with helium for good heat transfer through the gap between the pellets and the cladding. The cladding outside diameter is largest for CANDU elements (about 13 mm) and smallest for VVER reactors (9.1 mm), while BWR and PWR dimensions are in between. The rod diameters of the latter two types have decreased over the years as the assembly designs evolved from 7 x 7 to modern 10 x 10 lattices for BWRs and from 14 x 14 to 17 x 17 for PWRs. Typical values are 9.5 mm (PWR, 17 x 17) and 9.62-10.28 mm (BWR, 10 x 10). More details are given in Section 9.4 on fuel rod design and fabrication.