Properties of zirconium alloys and their applications in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 152

4.6.1 Irradiation growth

Irradiation growth occurs simultaneously with irradiation creep if there
is an applied stress. The two processes are considered to be independent
and additive, even though they compete for the same irradiation-produced
defects mechanistically. Earlier ZIRAT reviews providing more detail can
be found in the ZIRAT7 STR (Adamson & Rudling, 2002) and the Fuel
Material Technology Report, Vol. 2 (Rudling et al ., 2007 ).
Irradiation growth is a change in the dimensions of a zirconium alloy
reactor component even though the applied stress is nominally zero. It is an
approximately constant volume process, so if there is, for example, an increase
in the length of a component, the width and/or thickness must decrease to
maintain constant volume. Understanding of the detailed mechanism is still
evolving; however a clear correlation of growth to microstructure evolu-
tion exists, and many empirical observations have revealed key mechanistic
aspects. The inherent anisotropy of the Zr crystallographic structure plays
a strong role in the mechanism, as materials with isotropic crystallographic
structure (like stainless steel, copper, Inconel, etc.) do not undergo irradia-
tion growth. It should not be confused with irradiation swelling, which does
not conserve volume and does not occur in zirconium alloys under normal
reactor operating conditions.
Irradiation growth is strongly affected by fl uence, CW, texture, irradiation
temperature and material chemistry (alloying and impurity elements).

Growth characteristics and rate

Figure 4.61 gives schematic growth curves for Zircaloy illustrating several
points. Note that L-textured (longitudinal, or in the original rolling direction)
material grows, while T-textured (transverse to the rolling direction) mate-
rial shrinks; when taking into account shrinkage of a component in the third
direction (N, normal to the rolling direction), this behaviour results in approx-
imately constant volume. The long direction (L) of a component is the most
important: for instance the length of a fuel rod, channel box or GT. Note that
cold worked (CW or SRA) material grows at a high and almost linear rate,
while recrystallized (RXA) material grows in a 3-stage process, with the fi nal
high rate being called 'breakaway' growth. The various stages can be directly
correlated to the irradiation-produced microstructure described earlier. For
RXA Zircaloy, at low fl uences where only <a> component loops exist, growth
is small (~0.1%) and saturates. When <c> component loops begin to appear
the growth rate increases and becomes nearly linear with fl uence in the range
6-10 × 10 25 n/m 2 , E >1 MeV. For L-texture material growth can reach 1% at 20
× 10 25 n/m 2 . In initially cold worked (CW) or stress relieved material (SRA),
<c> component dislocations occur as part of the deformation-induced struc-
ture and more are formed during irradiation (Holt et al ., 1996 ). The growth

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