Zircalloy fuel cladding integrity during dry storage

Pursuant to Ch. 10, Sec. 72.72(h) of the code of Federal Regulations, the licensee of an interim spent fuel storage facility (ISFSI) must protect the spent fuel cladding against degradation and gross rupture. For the case where spent fuel is stored in casks under dry storage conditions, the only parameters that the cask designer has at his disposal to prevent such degradation is an inert internal cask environment and both the initial and decay temperatures of the spent fuel. The latter are governed by the type, quantity, specific power, and age of the spent fuel assemblies and by the passive heat dissipation properties of the storage cask. In view of this situation, the reviewers conducted their own investigation to determine whether the storage conditions specified in the TSAR assured compliance with the regulation. The reviewers interpreted the regulatory requirement to mean that not only should gross rupture be avoided during the design storage life of the ISFSI but also that the ability of the cladding to maintain its structural integrity during normal handling procedures at the end of storage life also be assured. After reviewing the current research relating to spent fuel cladding damage mechanisms, the reviewers concluded that a diffusion controlled cavity growth (DCCG) creep mechanism was the only damage mode applicable to dry storage of spent fuel that could lead to gross rupture of the cladding. Under the influence of the temperature and stress due to fuel rod internal pressure, the DCCG damage mechanism progresses by the nucleation and growth of cavities along the Zircalloy grain boundaries. This damage mechanism is insidious since it can progress without external evidence of damage, may not cause pin holes or cracks to relieve the internal rod pressure, and manifests itself by a sudden nonductile type of fracture