The design of in-structure components in Port Plug (PP) requires the accurate reaction of the seismic events on the particular points of interest to properly define the vibration behavior of PPs. The most common way to represent the seismic event used is by Floor Response Spectral (FRS). To define the seismic FRS at the particular locations of the ITER diagnostics components, IO has been developed a suitable methodology by a “direct spectra-to-spectra method”. The developed methodology in the frequency domain, has four main steps: first, conversion of the response spectral into artificial equivalent Power Spectral Density (PSD); second, performing random vibration analysis using the artificial equivalent PSD; third, extraction of the output PSD at specific points; fourth, inverse conversion of the generated PSD into response spectra. This methodology to find seismic FRS at the particular points of in-structure in Upper Port 18 (UP18) is presented in this work. A comparison result for different Upper Port Plug (UPP) models is also discussed.