Recently, a base isolation system used to be introduced for the seismic safety of nuclear power plants. The isolation system should keep its function over the design level earthquake with a high confidence like any other equipment in nuclear power plants. The seismic response of isolators by the extended design level earthquakes should be controlled not to exceed the ultimate limit state of it. In this study, lead-rubber bearings (LRBs) were tested. The small scale test specimen of a LRB had 550 mm diameter and the full scale was 1,500 mm diameter. The displacement controlled horizontal displacements were applied to the specimens with a constant vertical load in a test. The small scale model was tested under the various vertical load conditions to define the failure mode when the compressive force is excessive. In this test, the shear fracture by a large horizontal displacement with a relatively low axial load around the design axial load and the buckling fracture by a high axial load with a small effective area resisting the axial force were compared. The full scale model was tested to understand various characteristics such as the dependency of strain rate, the function of excitation displacement and the bidirectional behavior on the two-dimensional horizontal plane. As an experimental result, the behaviors of isolators under the ultimate limit state were investigated and the considerations for the prototype test of isolation devices were discussed.
