Environmental Engineering Reference
In-Depth Information
120
100
100
80
80
60
60
40
40
20
20
0
0
110
22
210
22
Fast fluence (n/cm
2
)
310
22
410
22
0
1.27
Dissolution of SPPs with fl uence and the increase in oxide layer
thickness under BWR condition in Zircaloy-4 at 290°C.
41
A few factors under irradiation conditions are recognized as affecting the
corrosion of clad material: (1) The increase in thickness of the oxide fi lm
(water side) decreases the thermal conductivity of the fi lm and increases
the temperature of the zirconium matrix which results in a higher corrosion
rate;
41
(2) the changes that occur in the microchemistry of the Zircaloy-4
matrix due to irradiation are seen to accelerate weight loss (when com-
pared with unirradiated material);
39
,
42
(3) accumulation of lithium in the
oxide layer reduces the protective nature of the oxide at the metal oxide
interface and enhances the corrosion rate;
43
and (4) high concentrations of
zirconium-hydride are found in locations where the oxidation of Zircaloy
is high.
41
,
44
The vast research work conducted over the years has led to
some understanding of these problems: contrary to the expectation that
irradiation-induced defects can cause breakdown of the protective fi lm on
Zircaloy clads,
45
the examinations of irradiated clads did not show any evi-
dence of oxide damage;
46
,
47
though the exact mechanism of lithium induced
corrosion of zirconium is not clear, the general understanding of the lithium
effect is that Li gets incorporated in solid solution in the ZrO
2
and alters the
vacancy concentration and distribution in the oxide fi lm; since zirconium
alloy corrosion proceeds by oxygen diffusion through the fi lm, the increased
number of vacancies should increase the diffusion and, hence, the corrosion
rate; further, it has always been noted in irradiated Zircaloys that hydride
density is high in locations where the oxide thickness is high; this clearly indi-
cates that the cathodic hydriding and anodically favoured oxidation occur
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