Reanalysis and evaluation of the seismic capacity of the CANDU 700 MW NPP CERNAVODA

The dynamic response of structures of the CANDU 700 MW NPP due to seismic loadings was conventionally analyzed in the time domain using modal substructure procedures. The frequency-independent parameters were tuned to the main frequencies of the soil-structure system. This is a common procedure widely used in the preliminary design of power plant structures and provides conservative results. However, parallel to the rapid progress being made in upgrading the capability of data processing systems, methods and software tools have become available which work in the frequency domain using complex (soil-structure) mathematical models or models in which the soil is represented by frequency-dependent impedances. In order to demonstrate the reserves existing in the design of the CANDU 700 reactor building, frequency-domain calculations were additionally prepared. The analyses were based on appropriate mathematical 3D-models of the coupled vibrating structures of the reactor building and as the soil represented by frequency-dependent impedances. The results obtained by using the time and frequency domain methods were compared and the safety margins of the CANDU design discussed.