Environmental Engineering Reference
In-Depth Information
between IASCC and other environmental cracking phenomena (e.g. SCC)
are that in the former (1) the microstructure is modifi ed by fast neutrons
with time and (2) the chemistry of the environment is altered by the ionizing
radiation. However, the overall stability of water increases with increasing
temperature and the yields of molecular decomposition products (H
2
, O
2
and H
2
O
2
) correspondingly reduced.
59
Austenitic stainless steel is the major
material that has been the subject of IASCC investigation as compared to
other grades. In the case of in-core structures, radiolysis increases the elec-
trochemical potential in that region where the SCC susceptibility is high.
Among the various radiolytic products, H
2
O
2
is the most concentrated spe-
cies present in irradiated, aerated water which gives rise to high corrosion
potential for stainless steels. However, the critical potential to mitigate SCC
of irradiated materials has not yet been established.
60
,
61
Slow strain-rate tests have been carried out on type 304 stainless steel
with prior thermal sensitization of the grain boundaries (to produce grain
boundaries with chromium depletion) that show that the electrochemical
potential of stainless steel increased signifi cantly on irradiation in oxygen-
ated water but decreased slightly in the hydrogen treated water. Though
the mechanism is not fully understood, it is now realized that neither Cr
depletion near grain boundaries
62
nor RIS (of S or P) at the boundar-
ies
63
alone plays the detrimental role in IASCC. Further, the low stacking
fault energy (SFE) of the matrix leads to localized deformation through
dislocation channelling and irradiation has been found to accelerate the
IASCC process.
64
Studies done under simulated BWR environments to
examine the susceptibility of four steels with varying SFE under irradia-
tion showed that the one with the highest SFE exhibited good resistance
to cracking whilst that with the lowest SFE was seen to be susceptible to
cracking at all of the doses studied
65
(Fig. 1.32). In 316 SS, the initiation
and propagation of IASCC in a water environment depends on the dis-
solved hydrogen and the stress required decreases with (a) increase in
dissolved hydrogen and (b) decrease in the rate of straining.
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Corrosion
problems are equally important in storage and disposal of nuclear wastes
where long term safety and reversibility act as guidelines in designing the
basic layout of a geological repository. Unlike conventional engineering
structures, the ageing and degrading clad tubes should not only serve trou-
ble free all through their service life but also maintain their integrity in
repository conditions.
67
1.4 Degradation mechanisms of specific nuclear
reactor structures
Following the introduction to various degradation phenomena, fundamen-
tals of radiation damage and radiation effects, we now turn our attention to
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