Seismic performance assessment of base isolated nuclear power plants

Abstract

Retrofit of structures is an inevitable task especially when buildings are not designed for seismic actions or their design was based on older design codes. Many retrofit strategies have been proposed and practiced. Use of dampers, base isolators, and active and semi-active energy dissipation devices are among the most common retrofitting methods. For base isolating, passive base isolator has been widely employed by engineers for conventional structures and bridges. However, very few applications of these base isolators for Nuclear Power Plants (NPPs) can be found in the literature. A new base isolation methodology based on intensity, which considered different earthquake parameters such distance of earthquake center to site was proposed by FEMA 58 in 2012 and this methodology has not been addressed in previous studies. Thus, this research investigated the effects of FEMA 58 base isolation method on the results of an analysis. This new analysis method investigated the distance effect of earthquake center to site. In this research, application of three types of base isolators for seismic retrofit of first generation NPP was investigated. Three levels of return periods comprising 105 years, 2.5*104 years and 104 years were introduced to investigate the highest level of performance for NPP based on FEMA 58. The study applied both experimental and numerical analysis. For the experimental part, two scaled NPPs were constructed in laboratory with a scale factor of 1:36 and a total weight of approximately 1 ton. The NPPs were tested with the pushover method for two conditions: fixed and isolated base. Numerical studies were performed to investigate the effects of 11 earthquakes on the obtained results from the finite element models. Results indicated that regardless of the employed base isolators, the isolated NPP had a higher natural period and displacement compared to the fixed-base NPP. However, the isolated NPP showed significantly lower acceleration, stress, base shear, and overturning moment when compared with the fixed-base NPP. It was also observed that when frictional pendulum base isolator was used to retrofit, the highest energy dissipation and lowest base shear as well as overturning moment; and stress were achieved. Monitoring the strain distribution between base-isolated and fixed-base NPP revealed that the base isolators had reduced the strain in the containment of the NPPs. With regard to the results based near the fault and far field earthquake characteristics, it is concluded that the base isolators are more effective under or near the fault earthquake.