Creep deformation of materials in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 68

Section 3.4). The reason for these differences is not clear. While the results
of Prasad et al . 122 indicated a stress exponent value close to 1 for pure zirco-
nium at low stress levels (1-3 MPa) revealing the operation of Coble creep,
the mechanism of creep at low stresses (0.2-14 MPa) at intermediate tem-
peratures is ascribed by Ruano et al. , to grain boundary sliding than to dif-
fusion mechanism. 123

3.8.2 Role of alloying elements in creep of Zr-alloys

Although addition of alloying elements is never based solely on the diffusiv-
ity criteria, the resulting creep rate of the alloy is the outcome of the diffusiv-
ity of the elements added and understanding the diffusion phenomenon in
these alloys will help in fi ne-tuning the concentration of the solute added. For
instance, though the strengthening effect of Nb in Zr increases with Nb con-
centration, the optimum level to obtain a low steady-state creep rate (<10 −8 /s)
as measured from the stress was found to be around 2.5 wt.%. 124 Nb with
low diffusion rate and high solubility limit can effectively enhance the creep
strength of Zr-2.5Nb alloy. 125 The creep strengthening effect by molybdenum
in zirconium is reported to be superior to that of niobium (for comparable
alloy fraction) in the temperature range 350-600°C. 126 Further, the modifi ca-
tion in the creep mechanism by addition of Nb is clearly seen (Fig. 3.16) to
transition from climb-controlled creep as in class-M to viscous glide creep as
in alloy class by correlating experimental results on Nb-added Zr-alloy sheet 127
and Zircaloy-4 cladding. 128 Addition of 1 wt.%Nb to Zircaloy-4 reduces the
creep rate by about 100 times and a region with n = 3 is introduced, absent in
Zircaloy-4 and which behaves like pure metal (class M).

3.8.3 Role of hydrogen in creep

￿ ￿ ￿ ￿ ￿ ￿

Since dry storage of spent fuel (SF) is gaining importance, it is necessary to
assure the fuel rod integrity during interim storage for relatively long times.
A clad with high burnup is likely to contain large amount of hydrogen (1000
ppm). The initial level of hydrogen is kept very low in order to reduce the
in-reactor hydride-related problems and hydrogen pickup during service is
controlled by employing new alloys. 129 Short term creep tests in Zircaloy-4
reveal that after a burnup of 64 MWd/kgU, hydrogen did not pose any dele-
terious effect and the material possessed suffi ciently good ductility. 130 But it
is interesting to note that hydrogen affects the creep rate in zirconium alloys
differently as atomic hydrogen and as hydride.
In Zircaloy-4, the creep rate was reported to depend on the condition of the
material - whether in cold-worked stress-relieved (CWSR) or annealed con-
dition; CWSR alloy shows a signifi cant strengthening on addition of hydrogen.
The reason for this behavior is attributed to hydrogen infl uencing the strain