[en] The structure of pool-type fast breeder reactor (FBR) is quite simplified in order that the calculation model may both be reflecting the structural dynamic characteristics and convenient to compute it. A simple spring-mass system could be approximatively in effect replacing a complex structure. The vessel deformation and appointed dynamic fluid pressure suppositions are taken. The motion equation of the system under horizontal seismic ground motion can be obtained by means of virtual displacement principle. Finally, the seismic response of the structure is computed