Designing plants to withstand earthquakes

The approach used in Japan to design nuclear plants capable of withstanding earthquakes is described. Earthquakes are classified into two types S1 and S2. In an S1 earthquake a nuclear plant must be capable of surviving essentially undamaged. In the more severe S2 earthquake, some damage may occur but there should be no release of radioactivity to the outside world. The starting point for the designer is the ground response spectrum of the earthquake which shows both the ground acceleration and the frequencies of the vibrations. From the ground response spectra synthetic seismic waves for S1 and S2 earthquakes are developed which can then be used to analyse a ''lumped-mass'' model of the reactor building to arrive at floor response spectra. These spectra are then used in further analyses of the design of reactor equipment, piping systems and instrument racks and supports. When a plant is constructed, results from tests with a vibration exciter are used to verify the floor response spectra and principle building resonances. Much of the equipment can be tested on vibrating tables. One large table with a maximum loading capacity of 1000 t is used to test large-scale models of containment vessels, pressure vessels and steam generators. Such tests have shown that the plants have considerable safety margins in their ability to withstand the design basis earthquakes. (UK)