Environmental Engineering Reference
In-Depth Information
10
-6
T
= 818K
B
10
-7
A
10
-8
C
10
-9
0.10
1.0
Effective stress, MPa
10.0
3.31
Abnormal creep in a Zr-2.5wt.%Nb alloy at low stresses.
3.8.4 The effect of thermal treatment and microstructure
on creep behavior
phase, in a Zr-2.5wt.%Nb alloy, has
resulted in a decrease in creep rate by 100 times over cold worked mate-
rial and rendered higher anisotropy at 450°C. This increase in creep rate
is attributed to segregation of Nb in grains that are favorably oriented for
easy slip.
133
The recent work on the thermal creep of Zr-2.5%Nb alloy by Kishore
et al
.
134
indicates that a microstructure containing a stable phase creeps faster
than one with a meta-stable phase and a phase redistribution is established
(Fig. 3.31). During creep deformation the stable
An intermediate cooling rate from
β
β
phase (with 80 wt.%Nb)
dissolves and re-precipitates as
phase (with ~35wt.%Nb), this resulting
strain due to phase change adds to the creep strain. Similar phase transfor-
mation is reported by Griffi ths wherein the Zr-2.5wt.%Nb alloy after 2-14
years of in-reactor service shows that the
β
phase has a distribution of com-
position, the Nb concentration varying from 37% to 75%.
135
However, the
effect of this phase change on the creep deformation is not well studied.
β
3.8.5 Irradiation creep
All the load bearing components in the core of the reactor, namely clad
tubes, guide tubes (GT), GT assemblies and BWR channels undergo irra-
diation creep, albeit at different rates. The clad tube is a crucial boundary
which has to withstand steep temperature and pressure gradient across its
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