Environmental Engineering Reference
In-Depth Information
shadow corrosion (ESSC)) can apparently be mollifi ed by control of both
Zircaloy microstructure and by reactor water chemistry. No harmful effects
have been reported or been related to the use of Pt in noble water chem-
istry treatments in BWRs, but further studies are underway. BWR channel
bowing has been attributed to manifestations of shadow corrosion, and has
produced major in-reactor problems to be discussed later in this chapter,
but further study is required to understand the details (Mahmood et al .,
2007 ; Blavius et al ., 2008 ; M ü nch et al ., 2008). There does not appear to be
an immediate remedy for shadow corrosion - for instance an oxide prefi lm
on Zircaloy does not prevent shadows. Coatings on the shadower (such as
Inconel springs) would appear to be promising, but may not be practicable.
4.5.3 Classical crud-induced localized corrosion (CILC)
The major sources of crud formation on fuel rod surfaces in BWRs (as in
PWRs) are the metallic impurities in the coolant that result from the cor-
rosion of reactor coolant system materials. Other sources are leakage of
solids, liquids and gases into the system and impurities from the surface of
components placed in the core (fuel assemblies, etc.).
Crud types identifi ed on BWR fuel surfaces have generally been:
￿
tightly-adherent, dense Fe
2 O 3 - generally a thin layer
￿
loosely-adherent Fe
2 O 3 - various levels of 'looseness'
a combination of the two types - tight and loose layers
￿
￿
tightly-adherent, dense Fe
2 O 3 + Cu or CuO
￿
tightly-adherent, dense Fe
2 O 3 + ZnO (ZnFe 2 O 4 ) .
The effect of the thin, tightly adherent Fe 2 O 3 layers on clad surface tem-
peratures is minimal. The loosely adherent layers are generally very porous,
fi lled with water and, as a result, have high thermal conductivity and only a
small effect on clad surface temperatures.
Zn, injected to reduce activity transport, tends to form the spinel with
Fe, as listed above, but is not known to have been the cause of fuel failures.
A particularly detrimental crud type is Fe 2 O 3 infi ltrated with Cu. This has
been associated with CILC cladding failures. The topic has been thoroughly
reviewed in a ZIRAT report (Wikmark & Cox, 2001) and by Marlowe et al .
(1985). An overview of CILC mechanisms is presented here.
In 1979 and the early 1980s, localized fuel cladding corrosion failures
occurred in some plants that had copper alloy (Admiralty brass) condenser
tubes and fi lter-demineralizer condensate clean-up systems. Such plants have
higher levels of soluble copper in the water than those with stainless steel
or titanium condenser tubes or with deep-bed resin clean-up systems. A few
rods per bundle failed in susceptible bundles at burnups >15 GWd/MT. Over
￿ ￿ ￿ ￿ ￿ ￿
Search WWH ::




Custom Search