SOIL-STRUCTURE INTERACTION EFFECTS ON NUCLEAR STRUCTURES FOUNDED ON ROCK SITES

Following the US NRC 50.54 letter, all operating plants in the United States must quantify the margin for beyond design basis events and take necessary actions to improve the margin if required. This effort is mostly completed by performing Seismic Probabilistic Risk Assessment (SPRA) requiring Soil-Structure Interaction (SSI) analysis as part of the overall analysis. Given the site profile data and the structural model developed as part of the Final Safety Analysis Report (FSAR) are generated many years ago, it is necessary to evaluate the SSI effects for structures founded on rock sites which were considered as fixed base condition during the original analysis. For this evaluation, the effect of SSI on a concrete nuclear containment structure in the Central Eastern United States (CEUS) is studied. The study shows the SSI effects are noticeable for rock profiles considered at frequencies above 5 Hz. However, below 10 Hz the effects are characterized by a small shift in frequency and amplification of the peak In-Structure Response Spectra (ISRS) over a narrow band of frequencies. These effects are more noticeable at higher levels in the structure indicating some degree of foundation rocking. The differences between the ISRS obtained from a rock profile and under fixed-base condition below 10 Hz, are not significant. This suggests that the use of fixed-base results for the subject structure is reasonable for the purpose of developing fragility curves and use in the SPRA evaluation.