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 dele­terious effect and the material possessed sufficiently 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 con­dition; CWSR alloy shows a significant strengthening on addition of hydrogen. The reason for this behavior is attributed to hydrogen influencing the strain hardening rate and static recovery of the material. Biaxial tests in Zircaloy-4 show that the presence of hydrides in the cladding will help to prevent the cold work microstructure from being annealed out of dislocations and thereby lower creep rates are maintained in the spent fuel cladding.131 The same alloy in annealed condition shows a decrease in creep rate when hydrogen is in solu­tion and an increase when part of the hydrogen is precipitated as hydrides. This behavior is attributed to the reduction in the stacking fault energy of Zr caused by diffusion of hydrogen to the core of the screw dislocations and an increase in their mobility. On the other hand when hydrogen is present in the form of hydrides, it increases the matrix strength and reduces the creep rate. The creep rate of Zircaloy-4 at a temperature of 693K and a stress of 150 MPa, a mar­ginal increase in creep rate is noted for a hydrogen content of 200 wt. ppm. The increase in the creep rate is believed to be brought out by the reduction in the modulus value when hydrogen is added.132 In a Zr-2.5wt.%Nb alloy, the creep rate at 723K is reported to increase by 2-2.5 times for a hydrogen content of 160 wt. ppm and the stress exponent reduces from 2.41 to 1.59, indicating the change in creep mechanism (Fig. 3.30).133

The results above indicate that hydrogen in dissolved state increases the creep rate and this is pertinent to SF which remains in this temperature range (~573K) and has sufficient hydrogen dissolved in it.