Overview of ageing and degradation issues in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 376

radiation may induce creep due to the increased concentration of point
defects during irradiation which enhances diffusion which is thus known
as radiation-induced creep. At high temperatures where thermal creep can
take place, radiation enhances creep due to increased defect concentration
and is referred to as radiation-enhanced creep. In an extremely simplifi ed
way one may express the vacancy concentration as due to thermal and radi-
ation so that the creep-rate equation (Equation [1.8]) becomes

β

2 t h i Q / RT Q / RT irr

n

=

r r

AD , Dv

β α β α

v ( ( CC + C ) e − Q

RT

, C e d C C irr d

−

ε σ + = e p . pa

σ = e ∝

D Dv

C

v

C

v v

6

[1.25a ]

In general, however, the radiation component of the creep rate is seen to be
temperature insensitive and proportional to the fl ux and stress:

ε B φ

[1.25b ]

σ

,

ir r

and with very little primary creep so that the strain due to radiation creep
is given by

ε Bt φσ φσ

[1.25c ]

.

ir r

In-reactor results are often sensitive to the neutron spectrum and are
very scattered to unequivocally describe the stress and time dependences.
However, irradiation creep is not just the thermal creep imposed with high
defect density; in the former the interstitial and vacancy loops that form
during irradiation play a major role in the creep mechanism while in the
latter the creep rate increases with temperature. Two mechanisms are pro-
posed to explain the irradiation creep phenomenon: (a) in a stress-induced
preferential absorption (SIPA), 35 extra planes of atoms accumulate on
crystal planes so as to produce creep strain in the direction of the applied
stress, whereas (b) stress-induced preferential nucleation (SIPN) assumes
that nucleation of loops is preferred on planes with a high resolved normal
stress. 36 , 37 Both of these mechanisms assume that the growth or formation
of loops occurs at a favourable orientation with respect to applied stress
and causes macroscopic strain. Irradiation generated point defects (vacan-
cies and self interstitials) migrate to different sinks like dislocations and
grain boundaries, in order to reduce the energy of the system, and do so in
a preferential manner due to the anisotropy of the zirconium crystal lattice.
Because of the diffusional anisotropy, interstitial atoms tend to migrate to
dislocations lying on prism planes and to grain boundaries oriented paral-
lel to prism planes while vacancies drift preferentially to dislocations lying

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