Overview of ageing and degradation issues in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 375

(a)

(b)

Usual texture
of basal poles
in tubing

1.25 (a) Effect of neutron radiation exposure on a Zr-single crystal
leading to decreased c-axis (vertical) and increased a-axis (horizontal);
(b) preferred orientation (texture) in a typical Zircaloy cladding tube.

or textures such that the c-axis of the grains are mainly oriented at 30° from
the radial (thickness) direction towards the hoop (transverse) direction as
shown in Fig. 1.25b. This results in small, positive strains along the axial and
hoop directions equal to the contractile strain along the radial or thickness
direction such as that the total sum is zero:

ε z + ε θ + ε r = 0. [1.24]

￿ ￿ ￿ ￿ ￿ ￿

Since all the radiation-induced strains are relatively small, no observable
changes occur in the diameter (~9.5 mm) and thickness (~0.56 mm) while
measurable changes occur along the axial direction of the ~3.9 m long clad-
ding tubes. This lengthening of the cladding tubes due to radiation exposure
is known as radiation growth with no change in volume; this is in contrast
to void swelling where dimensional changes are accompanied by increased
volume. Radiation growth of Zircaloy cladding leads to rod bow and, in
cases where a gradient in neutron fl ux and/or texture along the cladding
tubes exist, then the entire assembly can bow; more details may be found in
the later chapters on Zr-alloys.
Deformation due to creep occurs in the presence of external stress during
irradiation. At relatively low temperatures, where thermal creep is negligible,