Environmental Engineering Reference
In-Depth Information
p
H
2
/p
H
2
O
→ ∞
p
H
2
/p
H
2
O
>(p
H
2
/p
H
2
O
)
critical
ZrH
1.66
H
2
O+Zr
→
ZrO
2
5.7
Schematic description of the events resulting in transversal break
formation (Strasser
et al
., 2008).
1c1. The initial pellet-cladding gap prior to the power ramp, which
depends on:
1c1a. Burnup since the gap is decreasing with increased bur-
nup due to fuel swelling and fuel clad creep-down. This
is the reason that axial cracks do not form in low burnup
fuel since the fuel pellet-clad gap is so large.
1c1b. The corrosion properties of the cladding inner surface
(Edsinger, 2000). The pellet-cladding gap decreases if the
corrosion properties of the cladding inner surface are
poor, resulting in formation of a thick porous oxide layer
in the failed rod. The decrease in gap is related to the zir-
conium oxide having a larger specifi c volume than that
of the zirconium metal. It also turns out that, if the corro-
sion resistance of the cladding inner surface is poor, then
formed oxide is less dense due to the many cracks and
pores which will decrease the pellet-cladding gap further.
The fi rst type of Zr-liner materials used in the nuclear
industry were non-alloyed with very poor corrosion prop-
erties. Once it was realized that the corrosion properties
of the Zr liner have a large impact on the tendency to
form axial cracks in failed fuel, all fuel vendors did alloy
their liners to improve the corrosion resistance. However,
it is important to ensure that the alloying additions will
not degrade the PCI performance of the fuel cladding.
1c2. The magnitude of the rod power increase.
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