Overview of ageing and degradation issues in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 389

tubes and for fuel rod cladding with extended residence time (5-6 years). 76
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 touching the clad and results
in PCI failure or hydride related cracking. The sheath should have good
creep rupture property not to fracture. Further, the creep of the fuel assem-
bly and guide thimble can lead to bowing of the assembly. 77 An analysis
performed at Ringhals concluded that the bowing in this reactor had been
caused by a large creep deformation due to excessive compressive forces
of the hold down spring on the fuel assemblies, with a decrease in lateral
stiffness. The proposal to introduce advanced cladding and guide thimble
materials with a low growth rate and higher creep resistance to improve the
dimensional stability of assemblies is also being considered (M5 by Areva
and ZIRLO of Westinghouse); 78 further details can be found in Part II on
Zr-alloys.
The creep behaviour of unirradiated material is taken as a benchmark to
postulate its performance in reactor. Though these out-of-pile tests may not
be representative of their in-reactor behaviour, they have been used success-
fully to grade various materials during alloy development programmes and
to gain a basic understanding of the material behaviour. It has been recog-
nized that hoop strain in a clad tube (in-pile or during spent fuel storage)
is a vital parameter in the breach of fuel clad and evaluation of their creep
and burst behaviours is very important to assess the integrity of the tube.
Biaxial creep 79 and burst 80 characteristics are usually studied using internally
pressurized tubing over a range of pressures and temperatures. One may
also use ring-creep 81 tests to characterize hoop creep behaviour under uni-
axial hoop loading; this might be advantageous in cases where only a limited
amount of material is available and also in evaluating radiation effects that
require relatively small size samples. It becomes relatively more complex
for Zircaloys that exhibit distinct textures leading to anisotropic deforma-
tion and creep 82 that need to be accounted for, along with possible radiation
effects, in predicting the dimensional changes in reactor. As demonstrated by
Murty and co-workers, the relatively weak hoop direction for CWSR mate-
rial became stronger following recrystallization annealing, illustrating the
profound effect of heat treatment on the creep anisotropy of Zircaloys. 83
The observation that under equi-biaxial stress state the secondary slip sys-
tems (basal and pyramidal) are also favoured along with the easier prism
slip, concur with the observation that the irradiated recrystallized Zircaloy
exhibited a creep locus similar to that of isotropic material (texture reduced
as all slip systems were favoured). The recent work on the thermal creep of
Zr-2.5%Nb alloy by Kishore et al . 84 indicates that a microstructure contain-
ing a stable phase creeps faster than the one with a meta-stable phase and a
phase redistribution is established. The stable

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β phase (80 wt%Nb) dissolves