Creep deformation of materials in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 66

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 fi ssion products and it is essen-
tial 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 signifi cantly 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 densifi es 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 touch-
ing 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 over-
come 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 infl uencing the
cladding performance in the repository - is estimated to reach a tempera-
ture of ~325°C, although the average temperature of the cladding is esti-
mated to be less than 240°C. 115 At this temperature and with a hoop stress of
around 100 MPa due to fi ssion gases the clad material can undergo thermal
creep. The creep in clad tubes becomes all the more important with dry stor-
age becoming common. 116 , 117

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3.8.1 Thermal creep of zircaloys

The creep behavior of unirradiated material is taken as a benchmark to pos-
tulate its performance in the reactor. Though these out-of-pile tests may not
be representative of their in-reactor behavior, they have been successfully
used to grade various materials during alloy development programs and to