Toward Development of Site-Specific Vertical Ground Motions for Resiliency of Nuclear Facilities
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Authors
Saxena, Swasti
Issue Date
2022
Type
Dissertation
Language
Keywords
finite element modeling , high-performance computing , physics-based simulations , regional-scale simulations , site response analysis , soil-structure interaction
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Abstract
Large-scale ground motion simulations have attracted a lot of research lately for various applications like supplementing ground motions for probabilistic seismic hazard analysis (PSHA), identifying new fault structures and improving general understanding of seismic wave propagation. The advent of superlative computational infrastructure has made it possible to perform intensive calculations to solve physics-based wave propagation equations. However, there is a struggle to represent the exact site conditions in the simulations due to (a) lack of comprehensive geotechnical in-situ data for a large area/domain, and (b) exponential increase in the HPC resources needed to compute high-frequency response from basin layers with very low shear wave velocity (Vsmin). Considering these limitations, soil-structure interaction analyses in a basin remain a challenge. Additionally, there is a lack in consensus on modeling vertical ground motions, which are currently determined based on the research carried out for horizontal ground motions.The objective of this research is to link ground motions from physics-based numerical simulations with a local finite element model of a nuclear building to evaluate soil-structure interaction of vertical ground motions. Regional simulations have a much larger domain than necessary to compute ground motions at a single station but they provide an opportunity to model earthquakes that have not been recorded or are anticipated in the future. This allows a more accurate characterization of hazard, especially when assessing the margin of safety of critical facilities like nuclear power plants. The simulated ground motions are then linked with a local FE model to model the kinematic interactions between seismic wavefield and existing/proposed structure. The free field motions from both codes are juxtaposed against the recorded ground motions and each other for an inter-code comparison. A three-step research methodology has been proposed to carry out the research. Firstly, the site of interest is identified to be Kashiwazaki-Kariwa Nuclear Power Plant (KKNPP) Japan, where ground motions have been recorded by a dense instrumentation in reactor buildings as well as free field geotechnical arrays. The site is thoroughly studied for subsurface conditions and spatial variability and geologic models are gathered to capture an accurate site response. In the second step, two aftershocks of the 2007 Niigata Chuetsu-Oki earthquake (NCEQ07) are simulated in SW4, a finite difference code developed by Lawrence Livermore National Laboratory that solves seismic wave propagation equations in the time domain up to 4th order accuracy. Three geologic models in increasing level of complexity are created to improve site response at KKNPP, which is represented by two different free field arrays in two separate studies. The simulated ground motions are validated qualitatively and quantitatively at site-specific and regional-scales. In the last step, ground motions are extracted from SW4 simulations at the base-level of an FE model created in LS-DYNA, which performs the SSI analysis. In conclusion, the application-based viability of each model is assessed and reported. The simulations discussed in this study have a maximum computable frequency of 3 Hz and 12 Hz which provide further insight into important considerations for high-frequency response under a linked approach. As the scientific community is gearing toward the establishment of simulated ground motion databases, one must be mindful of their limitations in application toward structural shaking and SSI analyses.
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Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 United States