Environmental Engineering Reference
In-Depth Information
σ
σ
Pellet
Pellet thermal
expansion
(a)
(b)
5.3
Schematics showing the fuel rod condition (a) before the ramp and
(b) during the ramp (Strasser
et al
., 2010a).
fi ssion products and susceptible material may result in SCC of the fuel clad-
ding, as shown in Fig. 5.4 .
PCI failures may occur in PWRs/VVERs and BWRs (Strasser
et al
., 2010a ).
The failure mechanism is much more prevalent in BWRs, since reactor power
is controlled in part by control rod movements that subject the fuel to rapid
power level changes. (The reactor power in both BWRs and PWRs is also reg-
ulated by fl ow control.) In PWRs and VVERs, reactor power is not normally
controlled by insertion and extraction of the control rods in the core; rather,
reactor power is controlled by the boron concentration that is continuously
decreased during operation to compensate for the decrease in reactivity. This
type of reactor power control is much smoother than in the BWR case and,
consequently, PCI failures are less common in PWRs. However, during reac-
tor power increases, and specifi cally during a class II transient (anticipated
operational occurrences, AOO), PCI failures may occur in a PWR.
To prevent PCI failures, it is necessary to remove at least one of the fun-
damental conditions (tensile stress, sensitive material, aggressive environ-
ment) which cause SCC. There are two principal types of remedy (Strasser
et al
., 2010a ):
1.
One is to develop reactor operation restrictions that will ensure cladding
stresses are always below the PCI threshold stress during power increases.
This is the main measure in avoiding PCI defects and the only measure
used in PWRs. Operating rules (also called management recommenda-
tions, or pellet-cladding interaction operating management restrictions
(PCIOMRs)) to limit local power increases and 'condition' fuel for
power ramping were implemented in BWRs during the late 1970s to mit-
igate the PCI issue. The rules are usually a function of exposure and were
developed by the different fuel vendors, so they differ between various
fuel types. To establish and validate these rules, extensive power ramp
tests were performed by the fuel vendors in experimental reactors.
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