1st Kashiwazaki International Symposium on Seismic Safety of Nuclear Installations Niigata Institute of Technology Dr. Annie Kammerer Office of Nuclear Regulatory Research November 2010 Technical standards for seismic isolation in the US and NRC activities
Research Team • Robert Budnitz (LBNL) • Annie Kammerer & Richard Rivera-Lugo (NRC) • Andrew Whittaker & Yin-Nan Huang (U of Buffalo) • Bozidar Stojadinovic & Michael Mieler (UC Berkeley) • Boris Jeremic (UC Davis) External Research Review Group • Robert Kennedy, Chair • Nilesh Chokshi (NRC) • Antonio Godoy (IAEA) • Jim Johnson (James J. Johnson and Associates) • Don Moore (Southern Nuclear) • Jon Stewart (UCLA) Acknowledgements
• Regulatory and research history & activities • Key (new) concepts from the NRC Seismic Isolation Research Program • Design elements and analysis needs • Construction and operability needs Overview
• Pendulum and rubber bearing isolators are widely used in the US and are considered “mature” and effective technology. No US NPPs are isolated. • Seismic isolation viewed as approach to bring new designs to high seismicity areas and areas with very high frequency ground motions • Holding pre-application meetings with vendors developing NPPs with seismically isolated systems • Reviewing and communicating with code committees & international community, particularly • ASCE 4 (some differences will exist) • JNES and JAEG guidance Regulatory History & Activities
• In 2008 NRC began new research in seismic isolation and currently developing NUREG • Continued NRC research to address key items • Vertical and beyond-design-basis loading • Numerical model development for NRC’s in-house analytical tool (NRC ESSI Simulator) • Incorporation of seismic isolation systems in Seismic PSA and Risk-Informed decision making • Developing performance-based criteria for regulation of NPPs using seismic isolation systems • Testing of isolator systems to confirm analysis tools, models, and assumptions (not yet included) • Sensitivity study on isolator mechanical properties Regulatory Activities
• A number of items/issues of interest have been identified in the research program and are being incorporated into the draft NUREG. The draft NUREG will be provided to other NRC offices for input and concurrence prior to publication. No agency positions are yet set. • Possible Regulatory Guide to follow (still to be determined) • Possible contribution to IAEA Safety Report Regulatory Activities
IMPORTANT NOTE: NRC work on seismic isolation is still in the research phase-not yet in a regulatory implementation phase. The following slides present research results and are NOT agency policy. Regulatory Activities
• Analyzability is a design requirement. Both (1) failure of an isolator due to excessive deformation and (2) pounding of the superstructure against the moat are un-analyzable conditions. Therefore, it will be recommended that: • Moats are sized to allow for 3 times the deformation anticipated in the design basis earthquake, • Isolation systems are designed to have an adequately low probability of failure at 3 times to design basis earthquake and, • The isolator system must include provision for a “soft landing” if the superstructure comes in contact with the moat. Key Findings from NRC Research
• A “soft landing” is a loss in momentum that remains analyzable. It must be demonstrated that the loads do not exceed the in-structure design response spectrum. A HCLPF (high confidence of low probability of failure) of 1.67 should be demonstrated. • Passive systems to assure “soft landing” are preferred and should be recommended. Active systems could be used for beyond design basis design elements. • Isolation systems must be addressed within the defense-in-depth concept. Key Findings from NRC Research
• Soil-structure-isolator-systems must be analyzed using three-dimensional time-domain fully non- linear analysis of the complete system. Appropriate long-period content and duration of input earthquake records must be demonstrated. • To understand risk, it is important to analyze isolators within the foundation and structural system, therefore it is recommended that the NRC guidance is based on the “isolator system”, which includes the slabs above and below the isolators, in addition to the isolators themselves. Key Elements from NRC Research
• Isolation devices differ in their size and design. While test results may exist for similar devices, they may have been obtained from tests on scaled models or on full-scale, but smaller, devices. It will be recommended that testing on full size prototypes representing actual design is performed before application submission. • Isolator system is non-redundant, but isolators are. Therefore is treated like a critical non-redundant system (like the pressure vessel). Must assure low risk through appropriate design details and redundancy of isolators. Key Findings from NRC Research
• Seismic isolation systems should be analyzed for a variety of situations in which multiple isolators fail simultaneously and in which isolators fail at the perimeter of the system. • Vertical load is an important input parameter for determining isolator properties. The slabs below and above the individual isolators are not perfectly rigid elements, and rocking motions can occur. Therefore it is recommended that load cells be installed in the isolator system to confirm the spatial variability of loading on the isolators. Key Elements from NRC Research
• Guidance will focus on technologies with track record in US (i.e., rubber bearings and pendulum isolators). • It is recommended that “low damping” bearings as currently designed are not used in the US because (1) secondary systems are required and (2) it is hard to demonstrate acceptable risk in beyond design basis events. • Vertical isolation systems could be allowable. • Fragilities and reliabilities should be carefully assessed and incorporated into risk assessment. Key Elements from NRC Research
• Evaluation of the probability of exceeding the isolator displacement capacity in design-basis and in beyond-design-basis events to demonstrate the required High Confidence of Low Probability of Failure (HCLPF) is achieved • Investigation of the effect of additional damping devices on the response of the isolated superstructure • Umbilicals and other connections outside the isolated system are critical. Safety related umbilical must be analyzed for beyond-design-basis events. Significant review will revolve around this area. Additional Design and Analysis Issues
• Effect of seismic excitation in the vertical direction, including vertical isolation, propagation of vertical excitation through the structure and its effect on internal systems and components through the evaluation of design-basis and beyond-design-basis floor spectra • Likelihood and possible consequences of rocking and/or vertical uplift of the isolated superstructure on the base isolation devices and on the internal systems and components • Aging and radiological degradation must be accounted for in the analysis, design, and inspection plan of NPPs Additional Design and Analysis Issues
• Wind, fire, flood and aircraft impact loads on the base isolation devices • Effect of statistically significant differences among the base isolation device mechanical properties and the possibility of asymmetric excitation of the isolated superstructure due to such differences • Development, verification and validation of computer simulation models of base isolation devices under multi-directional excitation • Interaction between the base isolation layer and the foundation and soil underneath it, particularly at NPP sites not classified as rock Additional Design and Analysis Issues
• Manufacture and construction QA/QC and testing. Many isolators are constructed outside the US (making oversight challenging) and are not set up for the levels of QA/QC required by the NRC. • Testing of every isolator before construction • Aging and degradation must be considered, particularly in those devices made using rubber. There is a lack of accelerated ageing test procedures. • Ability to change out isolators as needed must be assured Construction and Operability Issues
• Acceptable access to isolators must be maintained. Must ensure inspectability; and inspection plan must assure that isolators continue to perform. Inspection plan must account for aging, degradation, change in vertical confinement due to creep in geologic materials and in the upper and lower rafts. Construction and Operability Issues
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