Evaluation of the Hydride Reorientation Behavior for Zircaloy-2 Liner Cladding with High Hydrogen Content during Interim Dry Storage Conditions

Abstract

One concern during the interim dry storage of spent fuel is hydride reorientation and its effect on cladding integrity. Previous studies with liner cladding show that a low-alloyed zirconium liner will act as a sink for hydrogen and, given sufficient time, hydrogen in solid solution will migrate toward the liner, leaving less hydrogen available for hydride reorientation in the interior. Most previous studies on liner cladding have, however, been performed with hydrogen levels below 350 wppm. The objective of this study was to evaluate what happens at higher hydrogen levels where the liner might be saturated with hydrogen. An irradiated Zircaloy-2 cladding with an average hydrogen concentration up to at least 790 wppm was chosen for the studies. The tests were performed under conservative conditions with an internal pressure of 125 MPa and an upper temperature limit of 400°C. The cooling rate varied from 60°C/h to 0.6°C/h. To dissolve additional hydrogen from the rim some tests were preceded by thermal cycling before the reorientation treatment. Posttest light optical microscopy showed that a cooling rate of 60°C/h resulted in major hydride reorientation in which radial and circumferential hydrides formed a continuous network across the cladding wall. When the cooling rate was lowered to 6°C/h most of the dissolved hydrogen migrated to the liner, providing a hydride-free interior. During pretest thermal cycling additional hydrogen dissolved and formed long radial hydrides at the intermediate cooling rate. However, when the cooling rate was decreased further to 0.6°C/h, in principle all hydrogen migrated to the liner. At the pellet-pellet interface, the formation of a dense hydride rim in the entire liner indicated saturation of hydrogen. However, even at the highest average hydrogen levels, the liner was not fully saturated with hydrogen at all axial levels after three thermal cycles and the lowest cooling rate.