Properties of zirconium alloys and their applications in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 160

constant volume deformation. Also, in addition to the natural anisotropy of
the zirconium crystal lattice, another factor is the concept that anisotropic
diffusion is enhanced by stress.

Basic mechanism

Many mechanisms of irradiation creep have been proposed. No single mech-
anism has been accepted as the dominating mechanism, and it seems very
likely that several processes contribute simultaneously. The most compre-
hensive review of creep mechanisms is quite old (Matthews & Finnis, 1988)
with newer reviews given by Holt (2008) and Adamson et al . (2009). The
two most prominent mechanisms are stress induced preferential absorption
(SIPA) and climb and glide. SIPA assumes a bias of the motion of vacancies
and SIAs to dislocations depending on the orientation of the Burgers vec-
tors with respect to the applied shear stress. There are several variations of
SIPA, including the elasto-diffusion modifi cation which invokes the effect
of stress on the diffusion anisotropy itself. Elasto-diffusion appears to have
the strongest effect on creep within the SIPA 'family'.
The most straightforward irradiation creep mechanism is the climb and
glide mechanism, by which deformation-producing dislocations are aided in
bypassing obstacles to their motion by irradiation-produced point defects.
As long as an individual dislocation attracts a net fl ux of either vacan-
cies or SIAs, it can 'climb around' a barrier and under the infl uence of an
applied stress, glide to the next barrier, thereby producing strain and even-
tually causing a slip step at the material surface. The weak dependence of
creep rate on dislocation density suggests that glide may not be the main
strain-generating process.
A further contributor to the strain measured in a creep experiment is
irradiation growth. Although not strictly in the 'creep' category because it
occurs in the absence of an applied stress, it is inevitably measured as part
of the overall strain in all in-reactor experiments, except for most bent beam
stress relaxation tests. Irradiation growth results from mechanisms similar
to irradiation creep in that it is dependent on the anisotropic properties of
the zirconium crystal lattice. It was discussed in the previous section.

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Parameters

Creep experiments in a neutron irradiation environment have been con-
ducted since the early 1960s and continue today. Since creep without irra-
diation tends to have a relatively high initial rate, and since irradiation
damage builds up with time, the creep curve (strain vs time or fl uence) is
usually divided into primary and secondary stages, as shown in Fig. 4.68.
Whether or not a true 'steady state' creep rate is obtained in-reactor is
problematic to prove, but in any case it is assumed for the analysis of the