Seismically induced uplift effects on nuclear power plants. Part 2: Demand on internal equipment

This work focuses on the dynamic response of nuclear power plant mechanical sub-systems (i.e., main cooling system, steam generators, emergency cooling injection tanks and piping) that are housed within the containment structure and are associated with power generation. More specifically, the numerical modeling procedure focuses on the internal R/C wall structural system used for supporting the mechanical equipment. Next, the complex grid of the mechanical components is modeled with shell finite elements. This internal equipment configuration is then excited by the ground motion numerically predicted in Part I οf this work by considering geometrically nonlinear soil-structure interaction effects. Following extensive parametric studies, the seismic demand imposed on the internal equipment is assessed on the basis of dynamic stress analysis of the critical components. Depending on frequency content of the incoming seismic motion, it is shown that abrupt uplift may take place. This is true even for moderate earthquake intensity, particularly when the containment structure rests on soft soils and the vertical component of ground motion is not negligible. This situation may produce peaks in the pipe elbow strains that could potentially affect serviceability, operation and under extreme conditions, the safety of the entire nuclear power plant.