Seismic analysis of a PWR 900 Reactor: Study of reactor-building with soil-structure interaction and evaluation of floor spectra

The study deals with the building; computation of natural modes and frequencies, seismic response, floor spectra at various levels. The building includes the concrete containment building and the internal building (internals). The concrete containment building is axi-symmetric, internals are roughly axi-symmetric except buckles, steam generators casemates and some holes in floors. The calculation has been performed on a 3-D thin shell model of internals with equipments as masses. The results show that the first two modes have the same frequencies (5.87 Hz), and each of them represents 90% of the total mass of internals. A seismic analysis with this model shows that only the first two modes play some role (this model is sufficiently simple to include some equipments as thin shells and beams). Then an equivalent axi-symmetric model has been built. We get with this model the same mode and the same frequency as in 3D model. On this model a parametric study of the soil-structure interaction effect has been performed using 'Bechtel' springs: - evaluation of the modes and frequencies versus the soil stiffness. - evaluation of seismic response of the building for 2 different spectra (EDF and DSN) and different soils. - evaluation of floor spectra at different levels of the reactor building in order to study seismic responses of equipments and eventually internals. The paper shows how a complex structure can be simply modelised in order to perform a parametric study of the seismic response and to come back to a more refined description of some parts of building or equipments. (orig./HP)