Environmental Engineering Reference
In-Depth Information
(a)
(b)
100
μ
m
100
μ
m
4.27
Hydride orientation in Zircaloy-4 (SRA) cladding: (a) circumferential,
(b) radial (Chu
et al
., 2005).
can form in the radial direction (Fig. 4.27b). Because in high power rods a
temperature gradient encourages hydrogen to diffuse to the colder outer
clad surface, rims of hydrides can form, illustrated in Fig. 4.28a.
Hydrides effects are listed here, giving appropriate fi gures and references.
The effect of hydrides is strongly dependent on testing temperature.
Material at 300°C (573K) (reactor operating temperature regime) retains
much more ductility than at 20°C. Figures 4.28b and 4.29 indicate the
ductile-to-brittle transition for unirradiated material is less than 200°C,
for circumferentially oriented hydrides. Figures 4.30 and 4.31 indicate
that at 332°C the primary reduction in ductility comes from the irradia-
tion effect, while at room temperature the effect on ductility of irradia-
tion and hydrides is additive for uniformly distributed hydrides below
about 1000 ppm. It is apparent that below 100°C ductility is very low.
The distribution of hydrides is important. Dense layers of hydrides (for
instance at fuel cladding surfaces) retain little ductility at any tempera-
ture, and are susceptible to crack formation. Whether or not the crack
will be arrested by the relatively ductile zirconium matrix depends on
the layer thickness, as shown in Fig. 4.32.
The strength of irradiated or unirradiated Zircaloy is insensitive to
hydrogen content. See Fig. 4.33 .
Existence of radial hydrides can substantially reduce ductility, particu-
larly at room temperature. Figure 4.34 shows the failure strains for the
range of hydride orientations given in Fig. 4.35. When radial hydrides
exist as in Fig. 4.35c failure strain is low. Figure 4.36 indicates that a
high percentage of radial hydrides reduces the failure strain at room
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