Properties of zirconium alloys and their applications in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 147

1.4

A high power adjacent UO 2 rod

1.2

1.0

0.8

2 w/o Gd rod

0.6

Representative
uncontrolled
cases at 40% void

0.4

3 w/o Gd rod

0.2

0

24
Nodal average exposure (GWd/T)

68 0 12 14 16 18 20 22

4.57 Relative power history of (U,Gd)O 2 and nearby high power UO 2
rods (Marlowe et al ., 1985).

understood, but perhaps related to the specifi cs of the boiling phenomena in a
BWR core, low heat fl ux rods, or regions of rods, have been shown to be par-
ticularly susceptible to nodular corrosion (Marlowe et al ., 1985 ). Therefore,
in gadolinia rods, Step 1b (in the incubation phase) is reached quickly. Figure
4.58 shows that when the nodular coverage reaches about 90%, copper depos-
its copiously on the crudded rod surface. The most likely deposition location
is between nodules, as it has been shown that microscopic heat fl ux increases
between nodules and, therefore, good conditions for wick boiling are estab-
lished (Wikmark & Cox, 2001). At this point, copper also begins to deposit in
lateral cracks in the thick nodules. Figure 4.59 is an elemental X-ray map of a
typical nodule, clearly showing copper (and zinc) in cracks in the nodule. At
this point the ZrO 2 /CuO layer can be more than 100 µm thick and heat trans-
fer through the nodule is inhibited. However, the conductivity of ZrO 2 /CuO
is still quite high (Wikmark & Cox, 2001, Table 12) and gross overheating
should not occur. The failure phase accelerates dramatically at step 2a, when
new cracks in the oxide 'sandwich' form and become steam-fi lled. This can be
facilitated by the deposited copper blocking normal ingress of coolant and
egress of steam from the cracks, and by expansion of the CuO 2 as the temper-
ature increases. Once steam-blanketed regions form, clad temperatures can
become very high, as steam has a very low conductivity compared to the crud
or oxides present. Step 2c follows, as the steam blanketing and the resulting
high temperatures dooms the cladding.

￿ ￿ ￿ ￿ ￿ ￿

Two-step steam testing (Cheng et al ., 1987) showed that only a small
percentage of material in failed fuel bundles is susceptible to nodular corro-
sion, thus explaining why most rods do not fail.