Seismic Isolation

Over the last decade, particularly since implementation of the certified design regulatory approaches outlined in 10 CFR 52, “Licenses, Certifications, and Approvals for Nuclear Power Plants,” interest has been increasing in the use of seismic isolation (SI) technology to support seismic safety in nuclear facilities. In 2009, the United States (U.S.) Nuclear Regulatory Commission (NRC) initiated research activities to develop new guidance targeted at isolated facilities because SI is being considered for nuclear power plants in the U.S. One product of that research, which was developed around a risk-informed regulatory approach, was a draft NRC nuclear regulatory commission (NUREG) report that investigated and discussed considerations for use of SI in otherwise traditionally-founded large light water reactors (LWRs). A coordinated effort led to new provisions for SI of LWRs in the forthcoming standard ASCE/SEI 4-16, “Seismic Analysis of Safety-Related Nuclear Structures.” The risk-informed design philosophy that underpinned development of the technical basis for both of these documents led to a set of proposed performance objectives and acceptance criteria that was developed to serve as the foundation for future NRC guidance on the use of SI and related technology.
Although the guidance provided in the draft SI NUREG report and ASCE/SEI 4-16 provides a sound basis for further development of nuclear power plant designs incorporating SI, these initial documents were focused on surface-founded or near-surface-founded LWRs and were, necessarily, limited in scope. For example, there is limited information in either the draft NUREG report or ASCE/SEI 4-16 related to nonlinear analysis of soil-structure systems for deeply-embedded reactors, the isolation of components, and the use of vertical isolation systems. Also missing from the draft SI NUREG report are special considerations for licensing of isolated facilities using the certified design approach in 10 CFR 52 and a detailed discussion of seismic probabilistic risk assessments (SPRAs) for isolated facilities.
To identify and address limitations in the initial guidance, Idaho National Laboratory (INL) has initiated several projects focused on further developing the technical and licensing underpinnings for facilities using SI technology. These efforts include a 2014 workshop focused on SI, development of new structural analysis tools and methodologies, and development of this report to identify and describe regulatory gaps and challenges related to licensing of advanced reactors using SI. Nearly all of the gaps and challenges identified in the report fall outside the scope of current efforts (including those at INL). This report provides information for developing a roadmap for future activities related to SI of advanced reactors. Although design optimization and commercial aspects related to the use of SI have been identified in Coleman and Sabharwall (2014) and elsewhere as possible issues or areas of opportunity, only topics that may impact efficient and successful licensing are addressed in this report.
Because efforts to date related to regulatory guidance development (e.g., the draft SI NUREG report) have principally considered designs similar to those currently being licensed, the existing literature (some of which is discussed in this report) is reflective of traditional LWR designs. All of the regulatory gaps and challenges that apply to LWRs also apply to advanced reactors; and often the LWR case provides a simplified example as compared to the range of cases found in advanced reactors. Advanced reactor designs often lead to additional gaps and challenges not faced in LWR design. Although this report discusses advanced reactors broadly, the exact set of challenges and potential solutions for any particular reactor design is technology-specific.
SI offers potential significant economic benefits for advanced reactors because the isolation system can be used to reduce the site-dependent seismic demands below pre-qualified levels in the certified design. Site-independent SSCs such as reactor vessels and steam generators would be analyzed, designed and qualified just once. Their protective isolation systems would be site-specific.
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Advanced reactors will often be designed and constructed very differently from LWRs, regardless of whether or not they employ SI and damping devices. The key technical advances in civil and structural engineering needed to deploy advanced reactors are: (1) development of performance-based seismic design and assessment procedures for non-LWR reactors; (2) development and deployment of analysis methodologies suitable for computing the response of deeply-embedded power reactors, including nonlinear time domain, soil-structure-interaction analysis; and (3) development, prototyping, and deployment of two-dimensional and three-dimensional isolation systems suitable for components ranging in size and complexity from diesel generators to reactor vessels. Advances in Items 1 and 2 are needed regardless of whether seismic protective measures (i.e., such as those noted in Item 3) are deployed. Not covered in this report are other important technical advances in civil and structural engineering needed for economical deployment of advanced reactors such as development and deployment of modular construction strategies used to minimize “one-off” field work, schedule delays, and construction cost.
The identified gaps and challenges are addressed throughout this report and summarized in Section 7. High-impact/high-value topics that should be addressed in the short term, and that must be satisfactorily completed before substantial progress on other tasks can be made, include the following topics listed below. The number in parentheses refers to the subsection number in this report where the topic is discussed. These topics are also highlighted in Table 4.
• Verification and validation of models and tools for nonlinear soil structure interaction of embedded facilities (2.1.3)
• Guidance for ground motion selection and modification for design and SPRA (2.1.5)
• Procedures for SPRA of nonlinear soil-isolator-structure systems (2.3.1, 2.3.2)
• Component isolation systems (2.3.3)
• Certified design process for deeply-embedded advanced reactors (3.1, 3.2)

Due to their large displacement capability and stable energy dissipation associated with a compact shape and new highly performing materials, the use of concave sliding isolators have been continuously increasing for application in buildings and bridges. In this paper the results of dynamic tests on full scale devices are presented. Their response was studied in a wide velocity range, for bi-directional patterns under different compressive loads. In this range of loading characteristics, which is typical of design for earthquake excitation, the behavior of these isolators appears significantly affected by the multi-directionality of the motion, and more specifically by the degradation of the coefficient of friction due to heating phenomena at the sliding surface. An analytical model, applicable to the prediction of bi-directional sliding behavior of friction-based isolators has been experimentally validated. Results of this study suggest that these phenomena should be considered in the design of structures equipped with these popular anti-seismic devices.

The mechanical properties of elastomers can change significantly due to air temperature variations. In particular, prolonged exposure to subzero temperatures can result in rubber crystallization, with a considerable increase in the shear stiffness of the material. As a result, the seismic response of structures with elastomeric isolators can be strongly influenced by air temperature. Current seismic codes, indeed, require an upper and lower bound analysis, using suitable modification factors, to account for the changes in the cyclic behavior of elastomeric isolators due to air temperature variations. In this study, the sensitivity of the cyclic behavior of elastomeric isolators to air temperature variations is investigated based on the experimental results of an extensive test program on six different elastomeric compounds for seismic isolators, characterized by a shear modulus ranging from 0.5 to 1.2 MPa at 100% shear strain and 20 • C. The cyclic tests have been performed on small-size specimens, subjected to shear strain amplitudes and frequency of loading typical for elastomeric seismic isolators, at seven different air temperatures, ranging from 40 to −20 • C. The effects of rubber crystallization due to prolonged exposure to low-temperatures have been also investigated. A finite element model for the evaluation of the temperature contour map inside a full-size elastomeric isolator exposed to low air temperatures has been also developed. In the paper, the experimental outcomes are compared with the modification factors provided by the current seismic codes to account for the temperature effects on the mechanical properties of elastomeric isolators.