Environmental Engineering Reference
In-Depth Information
60
PWR fuel rod with
constant treat flux
q = 75 W/cm 2
To
50
=
340°C
40
Second
transition
30
Classical
transition
20
Isothermal in-PWR 340°C
10
5 μ m
Isothermal out-of-pile 340°C
0
0
200
400
600
800
Exposure time (d)
4.41 Schematic of PWR corrosion kinetics (Adamson et al .,
2006/2007). (Source: Reprinted, with permission, from Garzarolli et al .
(1996), copyright ASTM International, 100 Barr Harbor Drive, West
Conshohocken, PA 19428.)
thickness at high burnup may reach or exceed 100 µm, while some of the
newer alloys have less than half of that. As noted in Table 4.8, only uniform
corrosion occurs in PWRs under non-boiling, hydrogenated conditions.
The study of uniform corrosion in the laboratory at temperatures rele-
vant to reactor operation suffers from three problems: (1) irradiation effects
are absent, (2) corrosion rates are very low at 280-360°C (553-633K) and
(3) very long times (hundreds of days) are required to differentiate between
material variables. The most widely used test appears to be 360°C (633K)
pressurized water for very long time periods. An example is given in Fig.
4.42 where a series of Zircaloy-type alloys with different Sn contents are
compared. Garde et al . (1994), show that in-reactor results after high burnup
give the same ranking as the autoclave laboratory tests. Also, adding Li to
the water is thought to improve comparisons for PWRs (Sabol et al ., 1994 ).
Both Zircaloy-2 and -4 form a protective uniform oxide in typical BWRs and
PWRs. In BWRs the various types of oxidation kinetic are shown in Fig. 4.43.
￿ ￿ ￿ ￿ ￿ ￿
Uniform corrosion continues at a very low rate out to high burnups.
During reactor exposure the microstructure of the Zircaloys used in BWRs
is continually evolving due to irradiation damage and SPP dissolution. In
the range 30-50 MWd/kgU (6-10 × 10 21 n/cm 2 , E > 1 MeV), changes in the
microstructure induce an acceleration of uniform corrosion, as described
earlier. First, patches of white oxide appear in the otherwise black or grey
uniform background, as illustrated in Fig. 4.44. These patches remain very
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