Properties of zirconium alloys and their applications in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 150

channel wall, again leading toward control blade interference. Fuel rods,
water rods or boxes, guide tubes and tie rods can lengthen, possibly leading
to bowing problems. (For reference, a recrystallized ( RX or RXA ) Zircaloy
water rod or guide tube could lengthen due to irradiation growth more
than 2 cm during service; a CWSR component could lengthen more than
6 cm.) Even RX spacer/grids could widen enough due to irradiation growth
(if texture or heat treatment was not optimized) to cause uncomfortable
interference with the channel. In addition, corrosion leading to hydrogen
absorption in Zircaloy can contribute to component dimensional instability
due, at least in part, to the fact that the volume of zirconium hydride is about
16% larger than zirconium.
The above discussion leads to the concept that understanding the mecha-
nisms of dimensional instability in the aggressive environment of the nuclear
core is important for more than just academic reasons. Reliability of materi-
als and structure performance can depend on such understanding.
Comprehensive reviews of dimensional stability have been given in the
ZIRAT Special Topical Reports (Adamson & Rudling, 2002; Adamson et al .,
2009; Cox et al ., 2005).The sources of dimensional changes of reactor com-
ponents (in addition to changes caused by conventional thermal expansion
and contraction) are: irradiation growth, irradiation creep, thermal creep,
stress relaxation (which is a combination of thermal and irradiation creep),
and hydrogen and hydride formation.
Irradiation effects are primarily related to the fl ow of irradiation-produced
point defects to sinks such as grain boundaries, deformation-produced dis-
locations, irradiation-produced dislocation loops, and alloying and impurity
element complexes. In zirconium alloys, crystallographic and diffusional
anisotropy are key elements in producing dimensional changes.
In the past, hydrogen effects have been considered to be additive to and
independent of irradiation. Although this independency has yet to be defi n-
itively proven, it is certain that corrosion-produced hydrogen does cause
signifi cant dimensional changes simply due to the 16-17% difference in
density between zirconium hydride and zirconium. A length change in the
order of 0.20% can be induced by 1000 ppm hydrogen in an unirradiated
material (Fig. 4.60) (King et al ., 2002 ; Seibold et al ., 2000 ). That the presence
of hydrides contributes to the mechanisms of irradiation creep and growth
is highly suspected but yet to be determined in detail.
Fuel rod diametral changes are caused by stress dependent creep pro-
cesses. Fuel rod length changes are caused by several phenomena:

￿ ￿ ￿ ￿ ￿ ￿

￿ Stress free axial elongation due to irradiation growth.

￿ Anisotropic creep (before pellet/cladding contact) due to external

reactor system pressure. Because of the tubing texture, axial elongation
generally results from creep down of the cladding diameter; however for