Properties of zirconium alloys and their applications in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 93

Table 4.4 Radiation damage: <a> loops in Zircaloy

Nature Vacancy(+), interstitial

Size 8-20 nm (80-100 )

Density 8 × 10 14 m −2

Saturation fl uence 1 × 10 25 n/m 2 ( E >1 MeV)

Thermal stability To about 400 ° C (673K)

Effect Strength, ductility, dimensional stability

Source: A.N.T. International (2011).

4.7 <a> type dislocation loops in neutron irradiated Zircaloy-2 (after
post-irradiation annealing at 723K for 1 h). (Source: Adamson, 2000.)

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These <a> loops form early in the irradiation and the number density
reaches a saturation value at a fuel burnup below 5 GWd/MT (1 × 10 21 n/
cm 2 , E > 1 MeV). The size of the loops increases with irradiation temper-
ature, and the loops become unstable (start to disappear) at about 673K
(400˚C). As will be discussed later they have a strong effect on mechanical
properties and dimensional stability.
The <c> type of loop lies on the basal plane and has its Burgers vector, or
at least a strong component of it, in the c-direction of the HCP cell. As indi-
cated in Table 4.5, and unlike the <a> loop, it is strictly a vacancy-type loop,
is relatively large (100 nm) and does not form until considerable irradiation
effects have occurred. In Zircaloy, <c> loops are fi rst observed by TEM at
a burnup of around 15 GWd/MT (~3 × 10 25 n/m 2 , E > 1 MeV) and increase
in density for the remainder of the fuel lifetime. They are thermally stable