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Shinji KOSUGI, Tetsuo IMAOKA, Kenji KANAZAWA, Takafumi HIRAKI, Seiji NAGATA, Takashi NAKAYAMA, Kunihiko SATO, Masakazu JIMBO and Yoshito UMEKI Application of a seismic base-isolation system using lead rubber bearings (LRBs) has been developed for the purpose of enhancing the seismic safety of nuclear power plants (NPPs). To grasp the mechanical properties of the seismic isolator over the design limit, the break tests for a 1600-mm diameter LRB have been conducted using full-scale specimens in order to avoid the difficulties considering size effect. As a result, linearity limit state and break limit state of the full-scale LRBs were found to be well-evaluated as expected; i.e., the breaking shear strain under axial compression shall exceed the commercial reference value of 400%. Further, statistical models of linearity limit and break limit of LRBs are evaluated on the basis of the real-sized break tests. By using these statistical models of limit state of the LRBs, a residual risk for seismic base-isolated NPPs against earthquake events beyond design basis can be investigated more accurately.

This paper provides a part of the series “Development of an Evaluation Method for Seismic Isolation Systems of Nuclear Power Facilities.” This part shows the fundamental properties of full-scale lead rubber bearings with 1600 mm diameter based on break tests. The following results are mainly obtained. One: The deformations and the acting loads of the full-scale specimens were accurately obtained up to break by the measurement system for the break tests. Two: The fundamental properties for the full-scale lead rubber bearings with a large-diameter lead plug were obtained by the basic property tests. The load-displacement relations were stable and similar basic properties were obtained among the specimens. Three: The result of shear break tests showed that the hardening property of the specimens had a certain harmony with the hardening stiffness model which was used in the seismic response analysis to investigate the safety margin for severe earthquakes beyond design basis earthquakes of nuclear power facilities. The effect of axial pressure on hardening property was not specifically observed. The evaluated linear strain limit was larger than 250% for every specimen. Four: The softening property of the specimens was obtained from the tensile break tests. The axial stress of tensile yield was approx. 1.4 MPa and the axial stress did not show any negative gradient at least up to approx. 10% axial strain after the tensile yield even with offset shear strains. Five: The tensile force acting on the bolts which secure the specimen to the testing machine was lower than the estimated tensile force at shear break, which indicates tensile force was conservatively calculated to maintain safety in the design for foundation of lead rubber bearings.