Rupture and Internal Pressurization Creep of Nb-Modified Zircalloy-4 Tubing: Application to Dry Storage Feasibility

Burst rupture properties of Zircaloy-4 and Nb-Modified Zircaloy-4 are investigated at temperatures ranging from 638K to 843K by internal pressurization of closed-end tubing samples. Various pressurization levels were imposed and rupture times were noted. The data enabled an evaluation of Larson-Miller parameter and the present experimental results on Zircaloy-4 were in excellent agreement with those reported in the literature. The effect of 1% Nb addition to Zircaloy-4 was studied since these materials are now known to resist long-time corrosion in water reactors and are thus considered for high burn-up applications. Negligible differences were noted in the rupture characteristics between the standard Zircaloy-4 and Nb-added Zircaloy-4. In addition, a limited amount of testing was performed to characterize the hoop creep behavior of these materials using a creep tester with hoop strain monitored in-situ by a Laser telemetric extensometer. Both the alloys followed an exponential stress variation of the creep-rate at high (>10⁻³E) stresses. At very low stresses, viscous creep (n = 1) was noted identifiable with Coble creep corresponding to small grain sized materials. These results have significance on the feasibility of surface storage of spent fuel where the creep deformation of the cladding could be a failure mode because of the residual heat and fission products following exposure to neutron irradiation in commercial nuclear plants. Blind extrapolation of short-term creep results to low stresses encountered during dry storage lead to nonconservative estimates of the creep-rates and creep-strains due to contributions of viscous creep to the total strain.