Seismic Back Calculation of an Auxiliary Building of the Nuclear Power Plant Muehleberg, Switzerland

The auxiliary storage building of the Muehleberg nuclear power plant houses all the equipment for checking, intermediate storage and the treatment of all solid, liquid and gas products deriving from the operation of the plant, which may have been exposed to radiation. Due to the systems installed in the building it has to resist an earthquake of intensity level OBE (Operating Basis Earthquake). A corresponding verification was not carried out when the plant was constructed. To fulfil the requirements of the regulatory commission (HSK) the seismic safety of the storage facility had to undergo a detailed investigation. The following studies were made within the framework of this investigation: preparation of a concept; specifying the most important aspects and boundary conditions; calculation of the frequency-dependent impedance function of the ground with 3-D soil and foundation computational models for different depths of embedment; execution of the soil-structure interaction analysis with 3-D lumped mass models; preparation of a refined 3-D finite element model of the structure; verification of the model by checking the masses; execution of the modal analysis to determine the dynamic behaviour; execution of the static analysis with the operational loads; execution of the seismic analysis using the response spectrum method; superposition of stresses from the static and seismic analyses; execution of the geotechnical stability analyses; execution of the local stress and bearing capacity verifications; determination of the floor response spectra. The results are in agreement with observations from real earthquakes, which is a good indicator of the reliability of the methods employed and the computational models