The most restrictive degradation mechanism for light water reactor spent fuel during a 40-yr dry storage period in an inert atmosphere is creep rupture of the Zircaloy cladding. To develop the best possible predictive methodology, the deformation and fracture theories were used to develop deformation and fracture maps. These maps were validated, where possible, with experimental data. Allowable storage temperatures were calculated using these maps. A cumulative damage model was used to account for the declining temperature history. The allowable initial storage temperature depends on the cladding stress for different fuel types, the decay heat history of the spent fuel, and the heat dissipation capability of the storage installation.