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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.

In the Niigataken-Chuetu earthquake occurred on October 23, 2004, the JMA seismic intensity scale 6 was recorded at many seismic stations. Especially at Kawaguchi, Kawaguchi-machi seismic station, JMA seismic intensity scale 7 was recorded for the first time since introduction of seismic intensity meters. Strong ground motion records with building damage data around the seismic stations make us possible to investigate correlation between damage to buildings and strong ground motion characteristics. We carried out damage investigation around the seismic stations where high JMA seismic intensity scale were recorded. We did not find heavily damaged buildings except for around the Kawaguchi-machi, JMA Ojiya and K-NET Ojiya seismic station in spite of high JMA seismic intensity scales and PGAs(peak ground accelerations). We investigated correlation between damage to buildings and strong ground motion characteristics using observed strong ground motion records. We found that the very short period, less than 0.5 sec., dominated many records whose JMA seismic intensity scales and PGAs are high but which did not bring about heavy damage to buildings. On the contrary, the 1-2 sec. period region dominated in the Kawaguchi-machi record which brought about heavy damage to buildings.

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This paper is a part of the series “Development of an Evaluation Method for Seismic Isolation Systems of Nuclear Power Facilities.” This part presents a break test plan and development of a test machine for a full-scale lead rubber bearing (LRB). Application of seismic base-isolation systems using LRBs of 1600 mm in diameter to reactor building has been considered for the purpose of enhancing seismic safety. It is important to obtain the ultimate properties of isolators in order to estimate the seismic safety margin of seismic base-isolated structures against a beyond design-basis earthquake events. Recent studies reveal that the scaled effect appears on the ultimate properties of seismic rubber bearings. However, because of the limitation of the loading capabilities of loading machines, the ultimate property of such a large scale LRBs has not been confirmed. In this study, the break tests for LRBs of 1600 mm in diameter is planned on the basis of estimation that refers to previous studies on break tests for small-scale LRBs and natural rubber bearings. The world-largest class test machine is designed and constructed to conduct static break tests for the full-scale LRBs. Furthermore, the performance of the test machine is evaluated from test results including those for break tests.