Parametric Evaluation of Seismic Behavior of Freestanding Spent Fuel Dry Cask Storage Systems (NUREG/CR-6865, SAND2004-5794P)
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Manuscript Completed: December 2004
Date Published: February 2005
V.K. Luk and B.W. Spencer
Systems and Structures Department, 6864
Sandia National Laboratories
P.O. Box 5800, MS 0744
Albuquerque, New Mexico 87185-0744
Operated by Sandia Corporation
for the U.S. Department of Energy
Earth Mechanics, Inc.
Fountain Valley, California 92706
David Evans & Associates, Inc.
San Diego, California 92123
Syed K. Shaukat, NRC Project Manager
Division of Engineering Technology
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
NRC Job Code W6829
One of the high priority issues for the continuous operation of nuclear power plants is how to manage and store spent fuel. In recent years, dry storage of spent fuel above ground has become a de facto fuel "repository" solution worldwide. Arrays of dry cask storage systems have been installed at Independent Spent Fuel Storage Installations (ISFSI) at many nuclear power plant sites. Most of these storage systems are freestanding, leading to stability concerns in terms of potential excessive sliding displacements and tipping over in an earthquake event. Sandia National Laboratories has been contracted by the Office of Nuclear Regulatory Research of the U.S. Nuclear Regulatory Commission (NRC) to conduct a research project to develop a comprehensive methodology for evaluating the nonlinear seismic behavior of these storage systems. The main objective of this effort is to perform parametric analyses to characterize the sensitivity of the cask response to a number of important input parameters, which provides a guideline to the range of applicability of analysis results. The results from these parametric analyses have been compiled in nomograms to facilitate the safety review of licensing applications by the staff at the Office of the Nuclear Material Safety and Safeguards (NMSS). This report documents the details of analysis models and all parametric analysis findings.
In this research effort, the cask response is investigated using the finite element method with explicit time integration. The ABAQUS/Explicit code is used to analyze three-dimensional coupled models consisting of a freestanding cask, a concrete pad, and a soil/rock foundation interacting with frictional contact at interfaces. This modeling approach allows a realistic simulation of soil-structure interaction effects and the nonlinear cask behavior after the onset of cask rocking or rolling motion due to applied ground motions. The earthquake ground motions applied to the model are derived from actual recorded ground motions, fitted to conform to selected spectral shapes, and adjusted using a deconvolution procedure that enables the ground motion to be applied at the base of the foundation model.
Prior to performing parametric analyses, the coupled finite element models were developed for three site specific analyses including three-module rectangular Transnuclear West module/cask, and HI-STORM 100 casks at Hatch Nuclear Power Station and at Private Fuel Storage Facility. The lessons learned from the site-specific analyses help guide performing the much broader based parametric analyses.
The parametric analyses involve two cask system designs: the horizontal rectangular module with an aspect ratio of 0.58 defined as ½ the shorter width divided by the height of the center of gravity from the base and the vertical cylindrical cask with an aspect ratio of 0.56 defined as ½ the base diameter divided by the height of the center of gravity from the base. The seismic behavior of these cask designs was investigated with three different foundation types (soft soil, stiff soil, and rock) and three coefficients of friction (0.20, 0.55, and 0.80) at the cask/pad interface. Three spectral shapes (Regulatory Guide 1.60, NUREG/CR-0098, and NUREG/CR-6728) were selected, and for each of these spectral shapes, five different earthquake ground motion records were chosen. These ground motion records were linearly scaled to result in surface peak ground accelerations (PGA) ranging from 0.25 to 1.25 g. A total of 1165 analysis cases were performed in this investigation.
Nomograms of median cask responses ± one standard deviation of maximum cask top sliding displacements and angular rotations versus peak ground accelerations are plotted at a 5% damped 1 Hertz frequency (1 second period) of pseudo spectral acceleration (PSA) after compiling from the pool of parametric analysis results. These nomograms may provide a meaningful and practical tool to cask designers and reviewers in interpreting the seismic behavior of dry cask storage systems.