Environmental Engineering Reference
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during creep deformation and re-precipitates as meta-stable
phase (~35
wt%Nb), this phase change adding to the creep strain. Moreover, transients
during sudden load drops or changes in temperature need to be considered
in proper predictions.
85
The importance and signifi cance of studying the tran-
sitions in creep mechanisms along with microstructural characterization fol-
lowing deformation were clearly outlined by Gollapudi
et al
.
86
and the reader
is referred to Chapter 3 on Creep for further details.
β
Role of hydrogen on creep
In Zircaloy-4, the creep rate was reported to depend on the condition
of the material - whether in CWSR or annealed condition; CWSR alloy
showed a signifi cant strengthening upon addition of hydrogen. The reason
for this behaviour is attributed to hydrogen infl uencing strain-hardening
rate and static recovery of the material. Biaxial tests in Zircaloy-4 showed
that the presence of hydrides in the cladding will help to prevent the cold
work microstructure from being annealed out of dislocations and thereby
maintain lower creep rates in the spent fuel cladding.
87
The same alloy
in annealed condition showed an increase in creep rate when the hydro-
gen is in solution and a decrease when part of hydrogen is precipitated
as hydrides. This behaviour is attributed to the reduction in the stacking
fault energy due to diffusion of hydrogen to the core of the screw disloca-
tions and their increased mobility. On the other hand, when the hydrogen
is present in the form of hydrides, it increases the matrix strength and
reduces the creep rate. Other researchers noted that the increase in creep
due to hydrogen content of around 200 wt ppm may be due to the reduc-
tion in the modulus value when hydrogen was added.
88
In a Zr-2.5 wt%Nb
alloy, the creep rate at 723 K was reported to increase by 2-2.5 times for
a hydrogen content of 160 wt ppm and the stress exponent reduced from
2.41 to 1.59, indicating a change in the creep mechanism.
89
Since dry storage of spent fuel is gaining importance, it is necessary to assure
clad integrity during interim storage. The high burnup rods are likely to con-
tain large amounts of hydrogen (1000 ppm) and with a hoop stress of 100-120
MPa the clad should not creep to failure. In order to reduce the hydride prob-
lem the initial level of hydrogen (and other impurities) is kept low and pickup
during service is controlled by using new corrosion resistant alloys.
Fuel failure data (PWRs and BWRs)
From the vast reactor operating experience it is noted that the cause of
fuel failures in terms of the number of units with leakers has decreased
over the years (Fig. 1.33) and the US nuclear industry has been tending
toward 100% no-leakers. As of July 2010, more than 90% of units in the
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