The fuel cladding is a tube whose length, diameter and wall thickness depend on reactor type (Table 9.2) and assembly design. It has to withstand corrosive high temperature water on the outside, stresses and strains imposed by expanding pellets in contact with the tube on the inside, and corrosive fission products released from the pellets. (Modelling of these phenomena is discussed in Chapter 14.) The tube is manufactured in several steps, Fig. 9.9, starting from an ingot produced by 2-3 times vacuum melting of reactor grade zirconium, recycled scrap and the alloying elements for obtaining the desired product. Several drawing and heating steps are required before the hollow billets are reduced to the size of the cladding, guide tube or water rod. The beta-quenching to form second phase particles and the final heat treatment (stress relief annealing, partial or full recrystallisation) are important steps, which influence corrosion, creep and yield properties of the material.

In the past, CANDU reactors and BWRs particularly suffered from so-called pellet-clad interaction (PCI) failures (Cox, 1990). The mechanism is often a stress

corrosion cracking (SCC) process induced by localised mechanical stresses and the fission product iodine. The problem was solved by introducing graphite — coated cladding in CANDU reactors and cladding with an inner liner in BWRs. The latter consists of a layer of material (e. g. zirconium with 400 ppm Fe) with better SCC resistance and higher ductility than Zry-2. This SCC barrier may be protected by an additional, thinner layer against rapid corrosion and deterioration from the inside should the cladding be breached and water enter the fuel rod. The liner is co-extruded with the substrate and metallurgically bonded to it.

The same technique is used to obtain an outer, corrosion resistant layer on PWR Zry-4 cladding. The Zry-4 tube is metallurgically bonded with an extra-low tin (Sn) outer layer about 100 pm thick. In this corrosion-resistant layer, the Sn level is below and the Fe and Cr levels are above the range specified by ASTM for Zry — 4. Such DUPLEX cladding combines high corrosion resistance with the good mechanical properties of Zircaloy-4 (Garzarolli et al., 2000).