[en] Risk calculations should focus on providing best estimate results, and associated insights, for evaluation and decision-making. Specifically, seismic probabilistic risk assessments (SPRAs) are intended to provide best estimates of the various combinations of structural and equipment failures that can lead to a seismic induced core damage event. However, in some instances the current SPRA approach has large uncertainties, and potentially masks other important events (for instance, it was not the seismic motions that caused the Fukushima core melt events, but the tsunami ingress into the facility). SPR's are performed by convolving the seismic hazard (this is the estimate of all likely damaging earthquakes at the site of interest) with the seismic fragility (the conditional probability of failure of a structure, system, or component given the occurrence of earthquake ground motion). In this calculation, there are three main pieces to seismic risk quantification, 1) seismic hazard and nuclear power plants (NPPs) response to the hazard, 2) fragility or capacity of structures, systems and components (SSC), and 3) systems analysis. Two areas where NLSSI effects may be important in SPRA calculations are, 1) when calculating in-structure response at the area of interest, and 2) calculation of seismic fragilities (current fragility calculations assume a lognormal distribution for probability of failure of components). Some important effects when using NLSSI in the SPRA calculation process include, 1) gapping and sliding, 2) inclined seismic waves coupled with gapping and sliding of foundations atop soil, 3) inclined seismic waves coupled with gapping and sliding of deeply embedded structures, 4) soil dilatancy, 5) soil liquefaction, 6) surface waves, 7) buoyancy, 8) concrete cracking and 9) seismic isolation The focus of the research task presented here-in is on implementation of NLSSI into the SPRA calculation process when calculating in-structure response at the area of interest. The specific nonlinear soil behavior included in the NLSSI calculation presented in this report is gapping and sliding. Other NLSSI effects are not included in the calculation. The results presented in this report document initial model runs in the linear and nonlinear analysis process. Final comparisons between traditional and advanced SPRA will be presented in the September 30th deliverable.

[en] Design Basis Ground Motion is generally defined on rock outcrop because, in general safety related nuclear structures are founded on rock. For sites at which the bedrock is very deep, these safety related structures are founded on soil. Consequently, the behaviour of the foundation of structures resting on soil is very much different than on bedrock. Hence, seismic ground response analysis is required to develop a site-specific response spectrum for the design of important superstructures in the region. One-dimensional ground response analysis is a commonly used method to estimate the ground responses under earthquake excitation in both equivalent linear and non-linear domains for both low to high strains. To understand the above phenomena, tests on F-55 Ottawa sand have been performed in a large scale geotechnical laminar box (GLB) at the Buffalo State University, New York. In this study, these tests have been simulated using numerical procedures involving equivalent linear and nonlinear time history analysis (using hyperbolic stress strain curve) using the author’s in-house code which is limited to low to medium strains. The developed code, when tested for high strains, results in response accelerations that are lower than the experimental observations. The limitation of the model is that it predicts higher damping under larger strains. To match the damping under large strains, a modification is introduced in the developed nonlinear model which takes into account the above problem. With this modification, the predicted accelerations are in line with the experimental observation. Further, a study illustrating the performance of hyperbolic and multilinear backbone curves are studied based on the generation of the amplitude of high frequency harmonics in the response of nonlinear soil at high degree of nonlinearity. It is observed that the amplitude of odd harmonics is dependent on the number of parallel springs (or number of points) chosen for generating the multilinear stress strain curve and it is least (minimum) if a continuous (hyperbolic) backbone curve is chosen for conducting the nonlinear analysis of soil column. In addition, the results of the dynamic model tests are compared with the results from a plane strain finite difference program in terms of acceleration time history at the top and bottom accelerometer locations and it is found out that the numerical predictions are in reasonable agreement with the experimental observation using hyperbolic nonlinear stress-strain soil model. (author)

[en] Seismic isolation is a mature technology that has been implemented in many civil structures, including buildings, bridges, liquid natural gas tanks, and off shore oil platforms, both in the United States and other countries, to mitigate the damaging effects of earthquakes. Seismic isolation has also been used in nuclear structures in France and South Africa, but not yet in the United States: neither in the Department of Energy facilities nor in commercial nuclear power plants (NPPs). This is primarily due to a lack of guidelines, codes and standards for the analysis, design and construction specific to seismically isolated nuclear structures. However, seismic isolation of nuclear structures has seen increased research interest in the recent years and the recently published national consensus standard, America Society of Civil Engineers (ASCE) Standard 4-16 'Seismic analysis of safety related nuclear structures', now incorporates language and commentary (Chapter 12) for seismically isolating surface or near-surface-mounted nuclear facilities, including NPPs. Seismic isolation substantially reduces horizontal seismic loads (demands) on structures, systems, and components. Reduction in demand results in four potential benefits: (1) economic: reduction in capital cost, (2) increased safety: reduction in the mean annual frequency of unacceptable performance, (3) insurance: protection against increases in the known seismic hazard after construction by minimizing the effort to re-qualify and re-certify structures, systems and components, and (4) recertification: the opportunity to certify an existing NPP design for a region of higher seismic hazard. Item (2) above, an increase in safety (reduction of seismic risk), was explored in Huang et al., wherein it was demonstrated that the implementation of seismic isolation reduced seismic risk in nuclear power plant structures. Studies that assess item (1) have not been performed although the use of isolation may lead to large reductions in the capital cost of safety-related nuclear facilities. Funding provided by Nuclear Safety Research and Development in the Department of Energy (DOE) allowed the authors to develop a framework for assessing the economic benefits and reductions in seismic risk afforded by the use of seismic (base) isolation. The framework includes probabilistic risk assessment and estimation of overnight capital costs (OCC's) of a sample generic DOE nuclear facility (GDNF). The project is documented in detail in Bolisetti et al. and Yu et al., and key results are presented here. The GDNF considered in this study is based on a facility constructed at the Idaho National Laboratory. The GDNF is a two-story reinforced concrete building with an embedded basemat and is founded on soft rock at the boundary of Site Class B and Site Class C (B/C boundary) as characterized by ASCE/SEI Standard 7. The GDNF is assumed to handle radiological materials, and its structure, systems and components (SSC's) are designed to effectively confine these materials in the event of an internal failure. The GDNF is populated with components that are generic to safety-related nuclear structures. A single system, which is intended to confine a loss of materials-at-risk (MAR) to the interior of the structure, is chosen for this study. Accordingly, a hypothetical event tree and fault tree are assumed for risk assessment. The isolation system for the GDNF is designed per Chapter 12 of ASCE/SEI 4-16 but is not optimized. The isolation system consists of 38 lead rubber bearings installed in one horizontal plane below the basemat. Details of the GDNF, including the SSC's, event tree and fault tree, and isolation system, are presented in the project report and Yu et al.. This study includes seismic risk assessment and cost estimation for four cases: (1) the GDNF is located at a site of low to moderate seismic hazard and is constructed on a conventional (non-isolated) foundation; (2) the GDNF is located at the same site as case 1, but is seismically isolated; (3) the GDNF is located at a site of high seismic hazard and is constructed on a conventional foundation; and (4) the GDNF is located at the same site of high seismic hazard as case 3, but is seismically isolated. In this study, the DOE site at Idaho National Laboratory (INL) is chosen as the low to moderate seismic hazard site for cases 1 and 2, and the Los Alamos National Laboratory (LANL) is chosen as the high seismic hazard site for cases 3 and 4

[en] The Nuclear Regulatory Commission (NRC) regulation 10 CFR Part 50 Appendix S requires consideration of soil-structure interaction (SSI) in nuclear power plant (NPP) analysis and design. Soil-structure interaction analysis for NPPs is routinely carried out using guidance provided in the ASCE Standard 4-98 titled “Seismic Analysis of Safety-Related Nuclear Structures and Commentary”. This Standard, which is currently under revision, provides guidance on linear seismic soil-structure-interaction (SSI) analysis of nuclear facilities using deterministic and probabilistic methods. A new appendix has been added to the forthcoming edition of ASCE Standard 4 to provide guidance for time-domain, nonlinear SSI (NLSSI) analysis. Nonlinear SSI analysis will be needed to simulate material nonlinearity in soil and/or structure, static and dynamic soil pressure effects on deeply embedded structures, local soil failure at the foundation-soil interface, nonlinear coupling of soil and pore fluid, uplift or sliding of the foundation, nonlinear effects of gaps between the surrounding soil and the embedded structure and seismic isolation systems, none of which can be addressed explicitly at present. Appendix B of ASCE Standard 4 provides general guidance for NLSSI analysis but will not provide a methodology for performing the analysis. This paper provides a description of an NLSSI methodology developed for application to nuclear facilities, including NPPs. This methodology is described as series of sequential steps to produce reasonable results using any time-domain numerical code. These steps require some numerical capabilities, such as nonlinear soil constitutive models, which are also described in the paper.

[en] Highlights: • Seismic risk reduced significantly by the implementation of base isolation. • Isolation reduces capital cost at sites of moderate and high seismic hazard. • Innovative seismic risk assessment procedure can be implemented to infuse risk calculations into the design process. - Abstract: The implementation of seismic base isolation can substantially reduce horizontal seismic demands on structures, systems, and components (SSCs) in a nuclear facility, potentially providing significant benefits in terms of increased safety (smaller seismic risk) and reduced capital construction cost. Although increased safety of SSCs has been demonstrated previously, the possible reduction in their capital cost has not been explored. To quantify the reduction in risk enabled by isolation, nonlinear response-history analysis of a conventionally founded and a base-isolated model of a generic nuclear facility (GNF) is performed at the sites of the Idaho National Laboratory and the Los Alamos National Laboratory: sites of moderate and high seismic hazard, respectively. Seismic probabilistic risk assessment is performed to compute the mean annual frequency of unacceptable performance. The seismic risk is reduced by 7 to 8 orders of magnitude by the implementation of isolation. The costs of addressing seismic loadings are estimated for the GNF in both the conventionally founded and base-isolated GNF. The possible reductions in the required seismic ruggedness and in the cost of SSCs in the isolated GNF are quantified at both sites. A reduction in cost enabled by isolation is possible at nearly all sites of nuclear facilities in the United States, with the greatest benefit at sites of high seismic hazard, such as LANL. Two risk-calculation procedures are used in the assessment: a simplified method based on Boolean mathematics and a rigorous method based on Monte Carlo analysis. The simplified procedure, which is suitable for implementation with preliminary design calculations, produces accurate estimates of risk unless the mean annual frequencies of unacceptable performance are very small, measured here as smaller than 10⁻¹⁰. The sensitivity of the calculated risk in the conventionally founded GNF, to the choice of anchor period for the seismic hazard curve, is investigated and found to be insignificant over the range considered: 0 to 0.10 s.

[en] Multi-hazard Analysis for STOchastic time-DOmaiN phenomena (MASTODON) is a finite element application that aims at analyzing the response of 3-D soil-structure systems to natural and man-made hazards such as earthquakes, floods and fire. MASTODON currently focuses on the simulation of seismic events and has the capability to perform extensive ‘source-to-site’ simulations including earthquake fault rupture, nonlinear wave propagation and nonlinear soil-structure interaction (NLSSI) analysis. MASTODON is being developed to be a dynamic probabilistic risk assessment framework that enables analysts to not only perform deterministic analyses, but also easily perform probabilistic or stochastic simulations for the purpose of risk assessment.

[en] Highlights: • Performed equivalent linear and nonlinear site response analyses using industry-standard numerical programs. • Considered a wide range of sites and input ground motions. • Noted the practical issues encountered while using these programs. • Examined differences between the responses calculated from different programs. • Results of biaxial and uniaxial analyses are compared. - Abstract: Site response analysis is a precursor to soil-structure interaction analysis, which is an essential component in the seismic analysis of safety-related nuclear structures. Output from site response analysis provides input to soil-structure interaction analysis. Current practice in calculating site response for safety-related nuclear applications mainly involves the equivalent linear method in the frequency-domain. Nonlinear time-domain methods are used by some for the assessment of buildings, bridges and petrochemical facilities. Several commercial programs have been developed for site response analysis but none of them have been formally validated for large strains and high frequencies, which are crucial for the performance assessment of safety-related nuclear structures. This study sheds light on the applicability of some industry-standard equivalent linear (SHAKE) and nonlinear (DEEPSOIL and LS-DYNA) programs across a broad range of frequencies, earthquake shaking intensities, and sites ranging from stiff sand to hard rock, all with a focus on application to safety-related nuclear structures. Results show that the equivalent linear method is unable to reproduce the high frequency acceleration response, resulting in almost constant spectral accelerations in the short period range. Analysis using LS-DYNA occasionally results in some unrealistic high frequency acceleration ‘noise’, which can be removed by smoothing the piece-wise linear backbone curve. Analysis using DEEPSOIL results in abrupt variations in the peak strains of consecutive soil layers. These variations are found to be a consequence of the underlying hysteresis rules. There are differences between the site response predictions from equivalent linear and nonlinear programs, especially for large strains and higher frequencies, which are important for nuclear applications. The acceleration predictions from nonlinear programs are reasonably close for most cases, but the peak strain predictions can be significantly different despite using identical backbone curves. Variability in the predictions of different site response analysis programs is significant for large strains and at higher frequencies, underlining the need for the validation of these programs. Biaxial horizontal site response analyses are also performed for the stiff soil site using LS-DYNA. Results from these analyses show that the inclusion of the orthogonal component of the ground motion in site response analysis can significantly influence the acceleration response