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Effects of Beyond Design Basis Earthquakes on NPPs:

Time Domain Nonlinear Modeling and Simulations

Using the NRC ESSI Simulator

B. Jeremić*, R. Budnitz** and A. Kammerer***

* University of California, Davis, CA, USA and Lawrence Berkeley National Laboratory, Berkeley, CA, USA

** Lawrence Berkeley National Laboratory, Berkeley, CA, USA

***U.S. Nuclear Regulatory Commission, Rockville, MD, USA

Earthquake Soil Structure Interaction (ESSI) Effects: Earthquake motions feature 3D, inclined seismic waves that propagate through nonlinear rock/soil, foundation and Nuclear Power Plant (NPP) Structures, Systems and Components (SSC). Earthquake energy is dissipated at a number of locations at different times during a seismic event. Location and timing of such seismic energy dissipation controls the amount of damage an NPP SSCs will sustain. Most of the seismic energy travels (occurs) in the form of surface waves (Rayleigh, Love, etc.) while embedded structures objects experience significant body waves (compressional (P), and shear (S)) as well.

Beyond design basis earthquakes give rise to development of (significant) nonlinear response in rock, soil, foundation and NPP SSCs. Such nonlinear response can result in (a) (significant) energy dissipation (frictional and viscous damping) and in (b) altering dynamic characteristics of the seismic motion. As noted earlier, during ESSI, seismic energy dissipation happens at different locations within soil/rock, foundation and structure and at different times. Such energy dissipation, during ESSI, can have beneficial effects on NPP SSC response, particularly if significant amounts of energy is dissipated before it reaches safety related SSCs. However, in some cases ESSI can have detrimental effects on overall dynamic response of the NPP SSCs. This is particularly true if altered seismic motions excite different parts of the NPP SSC, changing seismic energy frequency content and distribution through SSCs.

High Fidelity modeling and simulation of all of the above phenomena (inclined 3D seismic motions (translations and rotations) consisting of both body and surface waves, nonlinear effects, including elastic-plastic behavior of rock, soil, concrete and steel, gaping and frictional contact, coupling and buoyant effects (pore fluid in soil/rock skeleton) is needed in order to reduce modeling uncertainty and empower regulators to gain better understanding of the NPP seismic response.

Presented here is the NRC ESSI Simulator System, developed for high fidelity modeling and simulation (including all of the above phenomena) of Earthquake Soil Structure Interaction behavior of NPPs.

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Modeling and Simulation System. The NRC ESSI (Earthquake-Soil-Structure Interaction) Simulator is a software, hardware and documentation system for high fidelity, high performance, time domain, nonlinear, 3D, finite element modeling and simulation of earthquake-soil/rock-structure interaction of NPPs. The NRC ESSI Simulator System consists of three components, namely the NRC ESSI Simulator Program, the NRC ESSI Simulator Computer, and the NRC ESSI Simulator Notes.

The NRC ESSI Program is a 3D finite element program specifically developed for high fidelity modeling and simulation of Earthquake Soil/Rock Structure Interaction problems for NPPs on ESSI-Computer. The ESSI-Program features a number of elastic, elastic-plastic and elastic-damage-plastic model for soil. rock, concrete and steel. Included are also a variety of solid (bricks), structural (trusses, beams and shells), and special (gaping and frictional contact, isolator, etc.), finite elements, full coupling of pore fluid with soil/rock/concrete skeleton, buoyancy loading, as well as analytic input of seismic motions (including body and surface waves). Significant verification and validation program ensures high fidelity of modeling and increase confidence in simulation results.

The NRC ESSI Computer is a distributed memory parallel computer, based in part on a Beowulf concept and more recent developments of clusters of clusters. Compute nodes are Shared Memory Parallel (SMP) computers, that are connected, using high speed networks, into a Distributed Memory Parallel (DMP) computer.

The NRC ESSI Notes represent detailed hypertext documentation system (theory, formulation, examples) for modeling and simulations of ESSI problems as well as the Application Programming Interface for the NRC ESSI Program.

The NRC ESSI Simulator system is licensed through an open source license, and can be accessed through http://nrc-essi-simulator.info/.

Acknowledgment: The NRC ESSI Simulator system is developed in collaboration and with funding from U.S.-NRC. Such support is much appreciated.