Transactions, SMiRT-22 San Francisco, California, August 18-23, 2013 Division 5 THE NRC ESSI SIMULATOR PROGRAM, CURRENT STATUS N. Tafazzoli 1 , F. Pisan` o 1 , J. A. Abbel M. 1 , B. Kamrani 1 , C.-G. Jeong 1 , B. Aldridge 1 , R. Roche 2 , A. Kammerer 2 , and B. Jeremi´ c 1,3 , 1 University of California, Davis, CA, U.S.A. 2 U.S. Nuclear Regulatory Commission, Washington, DC, U.S.A. 3 Lawrence Berkeley National Laboratory, Berkeley, CA, U.S.A. (jeremic@ucdavis.edu; bjeremic@lbl.gov) ABSTRACT In this paper we overview some current modeling and simulation features of the NRC ESSI Simulator Program, a parallel, nonlinear, time domain finite element program developed to solve dynamic problems for the soil/rock – structure interaction of Nuclear Power Plant (NPP) system. The NRC ESSI Program is part of the NRC ESSI Simulator System, a software, hardware and educational system for high fidelity modeling and simulation of dynamic response of NPPs. INTRODUCTION We have been developing the ESSI Simulator System for last three years. The ESSI Simulator systems consists of the program, computer and notes. Focus in this paper will be on ESSI Simulator Program. The motivation for developing the ESSI Simulator system is based on a need to develop tools for high fidelity modeling and simulation of nonlinear behavior of earthquake soil/rock structure interaction. Sim- plifying modeling assumptions (for example linear elasticity for all components and material of a SSI NPP system) can lead to introduction of modeling uncertainty in results. This increase in uncertainty of results (on top of uncertainty in material modeling, seismic input motions, etc.) contributes to diminishing accuracy of numerical predictions, which are to be used in assessment of safety and economy of an NPP system. The ESSI Simulator Program features a number of advanced modeling and simulation models, methods and algorithms. In this paper we will briefly overview just two features. Firstly, we will overview the so called Pisan` o material model, a full 3D incremental elastic-plastic mate- rial model, that can be calibrated from (and that can be replicated accurately) modulus reduction (G/G max and damping curves. While modulus reduction and damping curves are inherently one dimensional, 3D