AbstractThe support structure of the International Thermonuclear Experimental Reactor (ITER) is suggested to have appropriate stiffness that accommodates both thermal distortion of the system and a maximum ground acceleration of 0.2 G (1 G=9.8 m s−1) due to earthquakes. For a site with earthquakes more severe than 0.2 G, the seismic isolation design is a possible candidate for keeping the seismic input low enough. The present design of ITER assumes that only the Tokamak pit portion is isolated, due to the whole building being very large and complex. In this study, dynamic analyses of the whole Tokamak building with a base-isolated Tokamak pit were carried out, and the effect of the isolation of the pit structure was evaluated. As a possible input level, a 0.4 G maximum ground acceleration was assumed. Particular attention was paid to the relative displacement between isolated and non-isolated portions, due to the smaller relative displacement being desirable for the design of pipes across the isolation gap.