Environmental Engineering Reference
In-Depth Information
06
.1
0
1
2
N
2
f
σ
σ
π
f
N
f
n
2 10
4
D
[1.26 ]
Δ ε
u
N
35
+
=
f
10
,
.
E
E
ε
where f is the frequency of cycling,
the strain-rate, D the ductility, n the
ε
work-hardening parameter,
f are true tensile and fracture stresses
and the remaining factors are as defi ned earlier.
Fatigue crack growth follows the same kinetics as described in section
1.2.4 but with K IC replaced by K I irr in Equation [1.14], similar to the case of
environmental effects where K IC is replaced by K ISCC .
σ
u and
σ
1.3.5 Corrosion-related problems
Corrosion is a major concern for reactor structures because in their con-
struction many different materials are used which corrode at different rates
by electrochemical effect and the corrosion (pitting, cracking, etc.) is accel-
erated by neutron radiation. More importantly, the corrosion products from
steam generators, piping and other components are transported through the
core and deposit on the fuel rods leading to formation of crud, in turn lead-
ing to increased fuel temperature and fuel failure. Corrosion can also lead
to deposition of radioactive corrosion products on out-of-pile surfaces of
the primary loop (e.g. heat exchanger) which becomes a safety concern for
maintenance personnel. Further, the fl ow of the medium replenishes the
concentration and pH at the corroding site which aggravates the corrosion
(fl ow-assisted corrosion).
An important aspect in the degradation of the Zircaloy clads, the sole bar-
rier between the hot fuel and coolant, is the oxidation and corrosion problem.
The various oxidation processes in Zircaloys have led to major degradation
phenomena that are described in detail in subsequent chapters. Stable, adher-
ent oxide fi lms form which act as a protective coating and offer resistance
to environmental cracking as in the case of stainless steels. However, in the
case of Zircaloys long exposures lead to the fi lm fl aking - that results in wall
thinning - which in some cases may result in through-wall failures. More and
specifi c details can be found in the chapters on Zr-alloys in Part II.
The addition of transition metals (Fe, Cr, Ni) to zirconium was aimed
at reducing the severe oxide growth stresses and localized cracking of the
grain boundaries that exposes more grains to the corrosive environment.
Formation of second phase precipitates (SPPs) containing the transition
elements help by aiding uniform oxidation of the grains and preventing
localized cracking and spalling as the oxide grows. 37 , 39 It has been noted in
Zircaloy-4 that the fraction of SPPs decrease and correspondingly the oxide
thickness increases with fl uence ( Fig. 1.27 ). 40
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