Limit state domain of high damping rubber bearings in seismic isolated nuclear power plants

This work is framed into the research activity performed at Politecnico di Milano within the field of seismic isolation of NPP (Nuclear Power Plant) buildings. One of the most challenging objectives for advanced NPPs is to identify cost effective engineering solutions to increase the current level of safety. Literature presents many factors affecting NPPs safety, among which an important role is played by the plant protection against external events, naturally or man-induced. The significant number of investigations recently performed worldwide has shown that earthquakes are among the most impacting external events in defining the annual damage frequency of nuclear reactor core. Moreover, it has been noticed that the risk induced by seismic events can be considered comparable with the risk caused by internally initiated events, because of the specific attitude of earthquakes to initiate multiple failure events. In the framework of a seismic Probabilistic Safety Assessment (PSA), fragility evaluation of safety related components is, therefore, a fundamental issue for a correct evaluation of risk. The seismic fragilities of single equipments shall be combined with the seismic hazard, i.e. the frequency of occurrence of a given intensity of the earthquake motion, to evaluate the probability of different core damage states. The NPPs are designed for Serviceability Limit State (SLS) earthquake and for Ultimate Limit State (ULS) earthquake. The first is of medium intensity with higher return probability, after which the plant shall be operative with minor maintenance. The latter is of higher intensity with a lower return probability, after which a huge crisis shall be prevented by keeping all radioactive components contained and cooled. Recent seismic events in Japan induced significant damages to NPPs (July 2007 - Kashiwakazi NPP; March 2011 – Fukushima NPP) and focused world public opinion on the risk due to major earthquakes. In particular, Fukushima NPP, put into operation in the 70’s-80’s, suffered accelerations widely bigger than designed at the time (the registered acceleration were between 3,3 and 6,4 m/s²; design ones were between 2,4 and 3,2 m/s²). Recent studies performed at Politecnico the Milano (2006-2011) adopted the IRIS reactor (International Reactor Innovative and Secure) as reference case to develop innovative solutions in reducing the seismic hazard. IRIS is an integral, modular, medium size (335MWe) Pressurized Water Reactor (PWR), whose preliminary design was developed through an international partnership including over twenty organizations from nine countries, which provides a viable bridge to Generation IV reactors. Its features include safety equipments which can be activated without human intervention or electricity, also known as passive systems, and base isolation system at foundation level. Results of these studies identified base isolation system as one of the most effective technical solution to mitigate seismic risk. In particular: - Isolation system leads to much lower horizontal peak accelerations. The frequency energy content of the absolute accelerations at different locations is significant only in low frequency range (0.3÷2 Hz), and common NPPs components usually exhibit natural frequencies higher than 5÷10 Hz. - The Soil-Structure Interaction (SSI) becomes less important for horizontal vibration modes. Safety assessments and design procedures become less dependent on the actual soil dynamic stiffness and damping. - Horizontal accelerations are almost the same throughout the building. This facilitates the design of safety related components, since the seismic demand is the same regardless of the floor level, allowing a simpler and less conservative design, which leads to more standardized equipments. - the main contributor to seismic risk becomes the isolation system itself. Reliable limit state domains shall be defined both for first-damage and complete failure conditions, in order to perform PSA. In previous works published by Politecnico di Milano, a complete FE HDRB model, consisting of alternate high damping rubber and steel layers, was set and extensive numerical tests were performed to identify the most suitable element type, mesh refinement and analysis parameters, in order to account for highly nonlinear geometric and mechanical material properties. At the same time, a refined FE model of a single rubber layer was developed to investigate in detail different ANSYS® hyperelastic material models and tune them against experimental results. In the present study, the development of a new FE model of a HDRB in ANSYS® has been pursued, mainly aimed at the definition of a reliable limit state domain under seismic excitation. Along with the numerical domain, an analytical approach was developed to support designers with a computational efficient tool. A theoretical framework of mechanical continuum is firstly given and, after a comprehensive study of the global and local behavior of numerical and analytical models, a limit state domain for delamination mode is assessed in terms of both stresses and global actions. Finally, the influence of the compressibility on stresses and on limit state domains is approached.