Low-Temperature Rupture Behavior of Zircaloy-Clad Pressurized Water Reactor Spent Fuel Rods Under Dry Storage Conditions

Creep rupture studies on five well-characterized Zircaloy-clad pressurized water reactor spent fuel rods, which were pressurized to a hoop stress of ∼145 MPa, were conducted for up to 2101 h at 323°C. The conditions were chosen for limited annealing of in-reactor irradiation hardening. No cladding breaches occurred, although significant hydride agglomeration and reorientation took place in rods that cooled under stress. Observations are interpreted in terms of a conservatively modified Larson-Miller curve to provide a lower bound on permissible maximum dry-storage temperatures, assuming creep rupture as the life-limiting mechanism. If hydride reorientation can be ruled out during dry storage, 305°C is a conservative lower bound, based on the creep-rupture mechanism, for the maximum storage temperature of rods with irradiation-hardened cladding to ensure a 100-yr cladding lifetime in an inert atmosphere. An oxidizing atmosphere reduced the lower bound on the maximum permissible storage temperature by ∼5°C. While this lower bound is based on whole-rod data, other types of data on spent fuel behavior in dry storage might support a higher limit. This isothermal temperature limit does not take credit for the decreasing rod temperature during dry storage. High-temperature tests based on creep rupture as the limiting mechanism indicate that storage at temperatures between 400 and 440°C may be feasible for rods that are annealed.