Influence of ISFSI Design Parameters on the Seismic Response of Dry Storage Casks

Many nuclear utilities have considered using upright cask systems for the dry storage of spent nuclear fuel. These casks are in most cases free standing and rest on a reinforced concrete pad in a variety of arrays. Stability requirements to prevent incipient tipping and sliding of the casks are often based on the cask not exceeding specific limits on either the ZPA of the site ground spectrum or the acceleration at the cask/pad interface (top of pad). Implicit in the use of either the ZPA or the acceleration at the top of the pad, is the assumption that the acceleration at the top of the pad is the same as the acceleration at the center of gravity of the cask, and, therefore, no amplification occurs between the top of the pad and the cask's center of gravity. In contrast to this assumption, the author's experience in the evaluation of Independent Spent Fuel Storage Installation (ISFSI) sites has shown that the cask/pad/soil system can significantly amplify the acceleration response at the cask center of gravity to levels well above the acceleration at the top of the pad. This paper presents the results of an investigation to determine the influence of three parameters on cask response: pad flexibility (i.e., pad thickness), soil properties and cask layout. A total of 16 soil-structure interaction (SSI) analyses were performed with various combinations of these parameters using the SASSI program. The results show that the most important parameter affecting cask response is the out-of-plane flexibility of the pad, and that this parameter can significantly amplify cask acceleration response at the cask center of gravity. Graphs and tables showing the influence of each parameter on response are presented. These results should be helpful to engineers making preliminary or confirmatory seismic response evaluations of ISFSI sites and design parameters. However, it is important to point out that these results only apply to the prediction of the onset of sliding or tipping. Once tipping or sliding has occurred these results no longer apply, and one must perform either an uncoupled linear/non-linear analysis or a coupled non-linear analysis, much like that in NUREG/CR-6865 [6].