The seismic analysis of nuclear power plant structures and components is considered to evaluate the convenience to update the brazilian design routine through the use of frequency domain techniques provided by codes such as SASSI. For this purpose one uses a FEM model of a reactor building which has been already analyzed by current techniques in time domain, and comparisons are made with results obtained in the frequency domain, with special attention to the following points: - analysis of the main structures using foundation impedance functions variable with the excitation frequency; - generation of coupled floor response spectra and their application to the analysis of secondary systems; - consideration of complementary criteria based on a minimum seismic spectral density function to generate artificial ground motion accelerograms compatible with a prescribed design response spectrum. As a first step, an exploratory work on the usage of SASSI program is made running a couple of simple examples. In the continuation of the work, a series of runs is made using the above-mentioned structural model under currently prescribed ground motions in the design of brazilian NPP structures. The results are compared with those from previous analyses using different methodologies and some interesting conclusions are brought up.