Properties of zirconium alloys and their applications in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 164

suggest a moderate dependency ( ρ 0.2 ), whereas for irradiation growth it is

about 0.8 , summarized by Holt ( 2008 ).

Since the HCP crystal structure is anisotropic, many properties are
expected to be anisotropic, and so it proves to be. The often-preferred slip
(glide) direction is <a> (prism slip), as is the preferred diffusion direction of
SIAs, both occurring parallel to the basal plane. The prevalence of basal slip
increases in irradiated material, but even so prism slip predominates at low
stress, so the climb and glide mechanism predicts higher creep strain in the
<a> direction (in Zircaloy components this is called the longitudinal direc-
tion), as do conventional and modifi ed SIPA mechanisms. Although texture
(as defi ned by basal pole fi gures) is nearly the same for RXA and SRA
materials, the anisotropy as expressed by anisotropy coeffi cients are differ-
ent for the two. Forfuel rods, for example, creep in the rod results in a rod
length decrease for SRA material and an increase for RXA material (AGP,
2009). Analyses confi rm that for uniaxial stress conditions, creep of Zircaloy
components in the longitudinal direction is greater than in the transverse
direction (Fidleris, 1988). Zr-2.5Nb pressure tubes have a different texture to
most Zircaloy components, so the anisotropy is different to that for typical
Zircaloy components, but the basic principles are the same (Holt, 2008).

ρ

4.7 Future trends and research needs

￿ In PWRs it is found the Zircaloy-4 no longer meets corrosion and hydriding

needs; therefore virtually all current PWR cladding use a zirconium alloy
containing niobium. Table 4.11 lists materials currently being explored for
use as cladding and structural materials, with the most widely used to date
in the West, in addition to Zircaloy-4, being M5 and ZIRLO.

For BWRs, Zircaloy-2 cladding, with various heat treatments to optimize ￿

the second phase precipitate size and distribution, remains the standard
for BWR components. However, new channel materials are currently
being explored to meet the challenges of channel bowing (described in
the next chapter and in Garzarolli et al ., 2011a ). Table 4.12 lists some of
those materials and heat treatments currently under development and
early usage.
Other trends and needs are given in the appropriate section of the next

￿ ￿ ￿ ￿ ￿ ￿

￿

chapter.

4.8 Sources of further information
Major sources are many, including:

￿ ZIRAT Annual Reports and ZIRAT Special Topic Reports, A.N.T.

International, M ö lnlycke, Sweden ( www.antinternational.com ).