The materials used for the FA components are Zr alloys, Inconel (precip­itation hardened Inconel X-750, Inconel 718 and solution treated Inconel 625) and stainless steel (SS 304L) (Cox et al., 2006). A low cobalt content is desired in the stainless steel and nickel base alloys to keep the activity trans­port by the coolant and the radiological exposure of the workers (man-rem) low. Since these alloys have high thermal neutron capture cross sections

(a)

4.6 (a) VVER-440 operating FA; (b) VVER-1000 FA (Cox et al., 2006).

that lead to reactivity losses, most of the FA components within the core are made of low cross section Zr alloys. These Zr-Sn based alloys are heat treated to reach optimum mechanical and corrosion resistance properties. Spring materials need to be made of materials with low stress relaxation rates, such as Inconel X-750 or Inconel 718. These Ni base alloys are gen­erally heat treated to reach an optimum precipitation hardening. Table 4.2 presents an overview of alloys used in LWR and their typical compositions.

Table 4.3 provides data for different Zr alloys used by different fuel vendors (Cox et al. , 2006). It is noteworthy that there are so many differ­ent Zr-alloys for PWR applications. Originally, Zircaloy-4 (Zr-4) was used in PWRs, but increased corrosion rates at extended burnups resulted in the

need to develop more corrosion-resistant alloys. However for BWRs, whilst the material originally selected, namely Zircaloy-2 (Zry-2), appeared to have adequate corrosion performance, recent burnup experience has shown that alloys with better corrosion and hydriding resistance are needed. Improved versions of Zry-2 and advanced alloys are being developed. Initially pure Zr-sponge liner was used as a PCI remedy for BWR applications. It was later found that the pure Zr sponge material results in a tendency for secondary degradation of failed fuel and therefore all fuel vendors added some alloying elements to increase the resistance towards secondary degradation of failed rods. The most potent alloying element to obtain this increased resistance is Fe by improving corrosion resistance of the liner material. However, Fe also has a tendency to decrease PCI performance. In RBMK and VVER reactors, E110 has always been used as fuel cladding material while E125 is being used for some of the structural components in the fuel assemblies.