Materials used in the reactor undergo irradiation-assisted creep as well as thermal creep (which predominates if stress and temperature are high enough). The in-pile creep deformation of a material is the net contribu­tion by both of these processes and it is difficult to distinguished between them. Thermal creep rate of unirradiated material is different from that of

irradiated material and both are different from that for a material undergo­ing irradiation.

Zirconium base alloys, with slightly differing chemical compositions, are used for various components inside a reactor. The clad tubes in BWR and PWR are made of Zircaloy-2 or Zircaloy-4 for most of the operating reac­tors while new alloys are being proposed for the forthcoming reactors which have to withstand higher burnups (Table 3.2). The Zr-Nb alloy was intro­duced for spacer grids in place of stainless steel (from 1987 in the WWER-

Alloy Nominal chemical composition (wt.%) Component Type of reactor Sn Fe Cr Ni 0 Others Zr Zircaloy-2 1.5 0.15 0.1 0.05 0.1 Bal Fuel clad, channel, BWR (UNS grade R60802) calandria tube Zircaloy-4 1.5 0.2 0.1 0.1 Bal Guide tube, PWR (UNS grade R60804) instrument tube, calandria tube ZIRLO 1 0.1 0.1 1 Nb Bal Fuel clad, spacers PWR M5, E110 0.1 1 Nb Bal Fuel clad, spacers PWRWWER E635 1.2 0.35 0.1 1 Nb Bal Fuel clad WWER HANA 0.4 0.2 0.1 0.1 1.5 Nb Bal Fuel clad Zr-2.5wt.%Nb 0.1 2.5 Nb Bal Pressure tube (UNS Grade R60904) EXCEL alloy 3.5 0.8 Mo Bal 0.8 Nb

440 and the mid-1990s in the WWER-1000). With the recent developments in WWER fuels, Zr-1%Nb/Sn/Fe alloys, with higher resistance to irradiation induced growth, creep and corrosion, are being used for guide tubes and for fuel rod cladding with extended residence time (5-6 years).114

Fuel cladding is a key barrier in containing fission products and it is essen­tial that this barrier is strong and remains intact over a prolonged period — both in service and during repository storage. Fuel failure occurs when this barrier is degraded and breached. The fuel rod failure rate in LWRs has been significantly reduced since 1987. This achievement, besides design improvements, is due to the introduction of many improved variants of Zr base alloys over the years — the latter ones improved in properties over the earlier ones. The clad tubes in reactors undergo creep extension due to many service conditions. At low burnup, the pellet densifies and the external water pressure causes the clad tube to creep-down. On power ramp, the pellet expands and applies excess strain on the clad. This leads to the pellet touch­ing the clad thus leading to PCI failure or hydride related cracking (which are described in detail in later chapters). The sheath should have good creep rupture properties to withstand this additional strain. A non-symmetric axial growth or creep of the fuel assembly (and guide thimble) can lead to bowing of the assembly. There is another deformation which adds to the creep strain. An analysis performed at Ringhals revealed that the bowing of the rods in this reactor had been due to a large creep deformation caused by excessive compressive forces of the hold down spring on the fuel assemblies and a decrease in lateral stiffness. This problem, though, can be partly over­come by introducing advanced materials with a low growth rate and higher creep resistance (e. g. M5 or ZIRLO) for cladding and guide thimble which improves the dimensional stability of the assemblies albeit irradiation creep remains a matter of concern for these materials.

At the repository the Zircaloy clads of the fuel rods face a challenging environment. The clad temperature — a crucial parameter in influencing the cladding performance in the repository — is estimated to reach a tempera­ture of ~325°C, although the average temperature of the cladding is esti­mated to be less than 240°C.115 At this temperature and with a hoop stress of around 100 MPa due to fission gases the clad material can undergo thermal creep. The creep in clad tubes becomes all the more important with dry stor­age becoming common.116,117