This paper describes the shaking table testing performed on the full-scale prototype of a device developed in the framework of the EC Collaborative Project SILER (Seismic-Initiated events risk mitigation in Lead-cooled Reactors) to protect the seismic gap in Generation IV LFRs (Lead-cooled Fast Reactors) provided with base isolation. The need of a seismic gap is due to the relative movement between the seismically isolated building and the adjacent ones, during earthquakes. In fact, collisions between the reactor building and the pit wall during beyond design seismic events can be avoided only by the presence of sufficient clearance between the two structures. The joint cover shall be waterproof, fireproof and resistant to impacts. The specimen was a 2.5 x 3.45 m full-scale prototype subjected to three seismic tests, sine and combined sine-sweep tests at the UTTMAT-QUAL laboratory of the ENEA Casaccia Research Center. Together with traditional accelerometers, a 3D motion capture system was used to measure triaxial absolute displacements with easy and fast test set-up, high accuracy and the possibility to link the 3D motion time histories with CAD drawings of the structure and validate the FE models in real time experimental data assimilation. © 2015, International Society for Structural Health Monitoring of Intelligent Infrastructure, ISHMII. All rights reserved.