Design of an Independent Spent Fuel Storage Installation (ISFSI) pad requires examining seismic stability and determining the static and dynamic loads of casks in accordance with the provision of 10 CFR 72.212 (Ref. 1). Since spent fuel storage casks are free standing structures on a concrete pad, cask rocking and sliding are expected to occur during an earthquake. NUREG/CR-6865 (Ref. 2) requires that the dynamic analyses consider the effect of soil-and-structure interactions between supporting soil and concrete pad as well as between casks and concrete pad. When casks exhibit rocking or sliding, nonlinear analyses are performed to handle the contact, and explicit time integration analyses with very short period of time step are often used to analyze the dynamic contact behavior of casks.

Explicit time integration analyses are more prone to contain high-frequency response than implicit time stepping analyses because of the use of very small time step and the limitation of damping model. A cask analysis case is presented in this paper to illustrate high frequency responses associated with the use of Rayleigh mass proportional damping, which is a commonly used damping model for explicit time integration analyses. A modified mass proportional damping expression is proposed to improve the responses of soil and casks.

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