With the rapid development of nuclear power plant constructions

it has become an unavoidable issue to choose a soil site as the plant site. Soil-pile-structure interaction (SPSI) effect has an important influence on the seismic safety evaluation of a nuclear island structure (NIS). A 3D integrated simulation method is developed to evaluate the seismic responses of a pile-mat-founded AP1000 nuclear-island building system subjected to multidirectional earthquake motions. Considering the regional tectonic setting and historical seismicity around the plant site

a set of three-component recordings for scenarios near-field moderate-strong

moderate-far field strong

and far-field large earthquakes are selected and justified for determining the bedrock shakings used in this study. The whole nonlinearity including both the primary and secondary nonlinearities of soil and only the primary nonlinearity of soil are considered by one-step method and two-step method for performing the 3D response analysis of the SPSI system

respectively. The quantitative influence of soil secondary nonlinearity (SSN) on the NIS seismic responses (i.e.

SSN effect) under earthquake scenarios can be obtained by comparing the results obtained from the above two methods. A finding is that the influences of SSN on the horizontal seismic responses of NIS are obviously greater than those of the vertical ones of NIS

and the SSN effect makes the SPSI system more flexible. The SSN effect is strongly related to the characteristics of bedrock shaking scenarios and is the largest under the near-field moderate-strong earthquake scenarios. Given SSN effect can significantly increase the NIS seismic responses

the SSN effect should not be ignored in the NIS seismic design.