Properties of zirconium alloys and their applications in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 149

Experience of CILC failures drove the BWR industry to develop
nodule-resistant Zircaloy microstructures, to refi ne the 500°C-type steam
test as a tool to predict in-reactor corrosion resistance and to tighten-up
water chemistry specifi cations, particularly for copper.
It should be noted that there is no evidence that the presence of copper in
the water enhances nodule nucleation. In fact at least one careful study indi-
cates that copper either has no effect or improves nodular corrosion resistance
(Ito et al ., 1994 ; Shimada et al ., 1997). Since nodular corrosion by itself has
never been shown to affect fuel performance (assuming oxide spallation does
not occur), elimination of either copper in the water or of nodular corrosion
is claimed, with considerable justifi cation, to eliminate CILC-type failures. In
fact, the industry has been free of classical CILC failures for the past 15 years.
Recent fuel failures in the United States may be caused by a crud-induced
process, but the characteristics are different from CILC. Both, however, induce
fuel rod failures by temperature-driven corrosion processes.

4.6 Dimensional stability of zirconium alloys
One of the most unique aspects of material behaviour in a nuclear power
plant is the effect of radiation (mainly neutrons) on the dimensional stabil-
ity of the reactor components. In fast breeder reactors the Fe and Ni-based
alloys creep and swell, that is, they change dimensions in response to a stress
and change their volume in response to radiation damage. In LWRs, zirco-
nium alloy structural components creep, do not swell, but do change their
dimensions through the approximately constant volume process called
irradiation growth. Radiation effects are not unexpected since during the
lifetime of a typical component every atom is displaced from its normal
lattice position at least 20 times (20 dpa). With the possible exception of
elastic properties like Young's Modulus, the properties needed for reliable
fuel assembly performance are affected by irradiation. A summary of such
effects is given by Adamson (2000).
Practical effects of dimensional instabilities are well known and it is rare
that a technical conference in the reactor performance fi eld does not include
discussions on the topic. Because of the difference in pressure inside and
outside the fuel rod, cladding creeps down on the fuel early in life, and then
creeps out again later in life as the fuel begins to swell. A major issue is to
have creep strength suffi cient to resist outward movement of the cladding
if fi ssion gas pressure becomes high at high burnups. PWR guide tubes can
creep downward or laterally due to forces imposed by fuel assembly hold
down forces or cross fl ow hydraulic forces - both leading to assembly bow
which can interfere with smooth control rod motion. BWR channels can
creep out or budge in response to differential water pressures across the

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