Properties of zirconium alloys and their applications in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 146

Table 4.10 Typical elemental analysis of crud
composition in a CILC-susceptible plant

Standard crud CILC crud

Major phase Fe 2 O 3 CuO

Iron 87% 21.1%

Copper 2.0 52.8

Zinc 4.4 11.1

Nickel 3.3 2.5

Manganese 2.2 3.3

Chromium 1.1 2.5

Cobalt 0.3 0.6

Source: A.N.T. International (2011) and Baily et al .
(1985).

90% of the failed rods contained (U,Gd)O 2 fuel (i.e. they were 'gadolinia
rods'). However, most fuel reloads and fuel bundles were not affected, even
in susceptible plants. Poolside and hot cell examinations revealed unusual
crud scale deposits, with high copper concentrations, rather than the typical
fl uffy, Fe 2 O 3 crud. Table 4.10 gives a breakdown of crud elemental composi-
tion in a CILC-susceptible plant (Baily et al ., 1985 ).
It was reported (Marlowe et al ., 1985) that three factors interacted to
cause CILC fuel failures: reactor water chemistry, fuel duty and Zircaloy
resistance to nodular corrosion. Marlowe et al . ( 1985 ) and Wikmark and
Cox (2001) provide details and analysis. An interpretive summary is given
here as an illustration of a crud-induced failure process. Failure in the gado-
linia rods proceeds by the following steps:

1 Incubation phase (low to moderate power)

(a) extensive nodular corrosion occurs early in fi rst cycle
(b) oxide nodules grow on some fuel rods to produce 90-100% coverage
(c) copper, in reactor water, deposits between oxide nodules
(d) copper deposition continues, crud grows within oxide nodules to
form a thick sandwich structure (ZrO 2 / crud /ZrO 2 )

￿ ￿ ￿ ￿ ￿ ￿

2 Failure phase (moderate to high power)

(a) cracks form within sandwich structure producing local, steam-insu-
lated regions
(b) insulating effect accelerates cladding corrosion and hydriding
(c) cladding penetrations occur locally by formation of auto-catalytic
corrosion pits or by cracking of hydrided Zircaloy in spalled regions.

Gadolinia rods are at low power during the fi rst cycle and never become the
highest power rod in a bundle (see Fig. 4.57). For reasons that are still not fully