Environmental Engineering Reference
In-Depth Information
et al
., 2010a). If the fuel cladding outward creep rate exceeds the fuel
swelling rate (due to fi ssion product production during irradiation), the
pellet-cladding gap may increase. This phenomenon is denoted cladding
liftoff. Since this gap constitutes a signifi cant heat fl ux barrier, such a gap
increase may result in increased fuel pellet temperature. This higher tem-
perature will in turn increase the gaseous fi ssion product release rate, fur-
ther increasing the fuel rod overpressure and leading to an even higher
outward cladding creep rate. Such a thermal feedback condition may lead
to fuel failure.
A larger fuel rod free volume, lower FGR rate and increased clad creep
strength increases the margins towards liftoff (i.e. a larger rod internal pres-
sure can be accepted without getting liftoff) (Strasser
et al
., 2010a ). Free
volume refers to the void volume bounded by the inner surfaces of the clad-
ding and end plugs and the outer pellet surface minus the volume of ple-
num springs and other internal hardware. Note that closed pellet porosity is
within the pellet volume, while open porosity, dishes, chips and other surface
irregularities with fi nite, open volume are in the free volume.
5.2.4 Degradation of failed fuel rods
Degradation of failed fuel rod is a situation where the leakage path(s)
through the damaged cladding increases to the point where the fuel itself is
dispersed into the primary system (Strasser
et al
., 2008). This may occur if
the rods degrade to such a point that the water contacts the fuel pellet, par-
ticularly if the contact also involves active fl ow of the water over exposed
fuel pellets, one example being a large axial cladding crack. Steam will not
be able to cause fuel washout while water can by oxidising the fuel grain
boundaries thereby causing disintegration f the fuel grains. Normally, util-
ities are much more concerned about fuel washout than high iodine and
noble gas release. This is because it may take up to ten years to clean the
core from the tramp uranium resulting from the fuel dissolution, while the
high iodine and noble gas activities released from the failed rod will be elimi-
nated when the failed rod is extracted from the core.
Degradation has historically been more of an issue in BWRs than in
PWRs (Strasser
et al
., 2008). Failed rods in PWRs may degrade, but the
amount of dispersed fuel is lower than in a BWR. The rationale may be
that the coolant chemistry in a PWR is more reducing than in BWRs.
During the period 1992-93, six plants in the United States and Europe
were forced into unscheduled outages because of concerns about failed
Zr-sponge liner fuel (IAEA, no. 388, 1998). This is a liner produced from
Zr sponge material to which no alloying elements have been added; its
major impurities are oxygen (about 600-900 wt.ppm) and iron (about
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