Assessment of Base-Isolated Nuclear Structures for Design and Beyond-Design Basis Earthquake Shaking

Keywords: Nuclear power plants (NPP).  Base isolation.  Seismic isolation systems.  ASCE 43-05.  Damage.  Design level earthquakes.  Moats.  Gravity loads.  Seismic loads.  Lateral displacement.  Nonlinear response-history analyses.  Ground motion.  Lead rubber dampers.  Friction pendulum bearings.

Abstract: This report presents the technical basis for proposed changes to the 2010 edition of ASCE Standard 4, Seismic Analysis of Safety-related Nuclear Structures. Three performance statements aiming at achieving the objectives of ASCE 43-05, Seismic Design Criteria for Structures, Systems and Components in Nuclear Facilities, are assessed in the study: 1) individual isolators shall suffer no damage for design level earthquake shaking; 2) the probability of the isolated nuclear structure impacting surround structure (moat) for 100% (150%) design level earthquake shaking is 1% (10%) or less; and 3) individual isolators sustain gravity and earthquake-induced loads at 90th percentile lateral displacements consistent with 150% design level earthquake shaking. Nonlinear response-history analysis is performed in support of performance statements 2 and 3, accounting for the variability in both earthquake ground motions and seismic isolator properties. Lead rubber, low damping rubber, and Frictional Pendulum base isolators are considered. Representative rock and soft soil sites in the Eastern, Central, and Western United States are addressed. Eleven sets of ground motions are recommended for response-history analysis of base isolated nuclear structures. The median displacement response of a best-estimate model subjected to spectrum compatible design level ground motions should be increased by a factor of 3 to achieve the performance objectives of ASCE 43-05.