Seismic isolation of nuclear reactor vessels considering soil-structure interaction

The research presented in this dissertation investigates the influence of soil-structure
interaction on seismic isolation of nuclear reactor vessels using numerical simulations. This
research is motivated by the nuclear industry searching for viable solutions to standardize
the design of reactor vessels. Seismic isolation of reactor vessels is a potential solution as it
enables deployment of standardized reactor vessels irrespective of site seismic hazard
thereby saving time and cost by allowing large-scale factory fabrication of standard
modules and by eliminating the need for repeated approval of reactor vessel design. Seismic
isolation is also a technology that has matured from successful implementation in buildings
and bridges allowing easier transition to nuclear applications. Currently, the
implementation of component-level seismic isolation in nuclear industry is challenging due
to gaps in research and lack of specific guidelines.

In this research, the effectiveness and potential limitations of using conventional friction
pendulum bearings for component-level isolation are investigated based on conceptual
numerical models of seismically isolated reactor vessels at different nuclear power plant
sites subject to a variety of ground motions. The numerical modeling and analysis
approach presented in this research are checked using experimental data and results from
multiple numerical codes to ensure reliability of the obtained analysis results.

Within the scope of this study, it is found that slender vessels are particularly vulnerable
to rotational acceleration at the isolation interface. Rotational acceleration at the isolation
interface is caused by rotation at the foundation level of the containment building housing
the isolated reactor vessel and by excitation of higher horizontal translational modes of the
seismically isolated system. Rotation of the building foundation increases with decrease in
shear wave velocity of the soil surrounding the building foundation. When the containment
building is embedded below the soil surface, the effect of embedment on peak horizontal
acceleration of the isolated vessel depends on the amount of increase in shear wave velocity
at the foundation level of the building. When embedment does not result in any change in
shear wave velocity, it is found to have negligible impact on the acceleration response of the
isolated vessel.

  The optimum location to support a vessel for seismic isolation is found to be on a plane
passing through its center of mass. It minimizes horizontal acceleration in the isolated
vessel as well as the tendency of isolator to uplift. Isolator uplift and exceedence of
displacement capacity of the isolator during extreme events are possible drawbacks in using
seismic isolation technology since they produce impact forces due to uplift and
re-engagement of the isolator or due to collision between the isolated system and the moat
wall. If such cases are avoided, seismic isolation of reactor vessel could provide more than
50% reduction in peak acceleration of vessel except for low-intensity motions that do not
engage the isolator.