Environmental Engineering Reference
In-Depth Information
1
H
2
O+Zr
→
ZrO
2
2
p
H
2
/p
H
2
O
> (p
H
2
/p
H
2
O
)
critical
t
oxide thickness
<
t
critical oxide thickness
ZrH
1.66
3
p
H
2
/p
H
2
O
→ ∞
Transversal break formation
4
5.6
Schematic showing the events resulting in transversal break
formation. The numbers in the fi gure relate to the sequence of events
that may lead to a transversal break as described in the text (Strasser
et al
., 2008).
from hydrogen ingress thus causing secondary hydriding (3 in Fig. 5.6) (e.g.
Olander
et al
., 1997). If the hydride precipitates along the whole fuel clad
circumference, the fuel rod may fracture transversally due to the hydride
embrittlement effect (4 in Fig. 5.6 ).
Transversal break in PWRs/VVERs
- are caused by a mechanistic develop-
ment similar to that of BWRs (Strasser
et al
., 2008). However, the second-
ary hydride defects tend to form in the upper part of a PWR
/
VVER rod.
The processes involved in developing a transversal break in a PWR rod are
shown in Fig. 5.7 .
1 .
Axial cracks in BWRs
- Formation of long axial cracks has three prereq-
uisites, (Strasser
et al
., 2008 ):
1a. A sharp primary defect such as a PCI crack or cracks in hydride
blisters formed due to a primary defect. However, in this case the
hydride blister is very local and does not exist along the whole fuel
clad circumference, as seen in formation of transversal breaks.
1b. A fuel cladding hydrogen content larger than the hydrogen solid
solubility.
1c. A stress intensity (
K
I
) at the crack tip above the critical value for
crack extension.
K
I
will increase with clad tensile stress level which
in turn depends on:
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