Seismic Isolation of Nuclear Structures by a Non-Oscillating System SERB Type

The paper presents the advantage of non-oscillating systems — SERB type — for the seismic isolation of buildings compared with other solutions for seismic protection of the buildings which are now employed worldwide. The non-oscillating isolation devices — SERB type — have a capsulated structure and are capable to overtake large compression and tension loads on vertical direction with very small deflection and cut off the seismic action in horizontal direction. The stiffness of the devices is zero for the translations on any direction in horizontal plane upto a pre-set displacement which usually, is between 0.6–0.8 of the maximum seismic soil displacement. After having reached that value, a non-linear increase of the device stiffness is occurring on the horizontal direction in order to provide a non-shock limitation of some random seismic displacements higher than the maximum displacement accommodated by the device. The motion inside the device is controlled all time by the dynamic friction and restricted by nonlinear stiffness and damping material after a pre-set displacement. On vertical direction, the stiffness of the devices is big in order to avoid the additional stresses on the structural elements of the isolated suprastructure, due to the differential settlement of the devices on the suprastructure permanent loads that could appear. The system provides very small deflection of the device on vertical direction with high damping. On request, SERB isolators can be so fabricated to have the desired stiffness on vertical direction. Standard SERB devices 960×960×230 are capable to overtake a permanent compression load of 3000KN and tensile load of 1500KN, respectively, over which dynamic loads of similar values may overlap. The maximum seismic acceleration transferred to the isolated structure on horizontal direction is ranging between 0.03g and 0.05g depending on the friction coefficient. The maximum relative displacement between the isolated suprastructure and the infrastructure embedded into the ground, is about 3 times smaller than the maximum seismic displacement showed by the oscillating systems employed today and smaller then the maximum seismic soil displacement. In series SERB device 960×960×230 allows for a maximum seismic displacement of 230mm. An application of the non-oscillating SERB devices 960×960×230 as the solution to seismically isolate the heavy water detritiation system building at Cernavoda NPP, is presented.