Soil-Structure Interaction Assessment of New Modular Reactors, 20-R8270
Inclusive Dates: 11/14/11 – Current
Background — To meet the growing demand for inexpensive power, the nuclear industry is developing several new, advanced nuclear reactor designs with scalable modules. Each module will produce relatively small amounts of electricity compared to current nuclear power plants. To meet the power demand and infrastructure constraints, several modules of these reactors can be installed at a given site, as needed. Because the construction is relatively simple and small in size, the lead time to start power generation is shorter. Each of these new reactors consists of an integrated reactor module and a reactor containment vessel. These containment vessels are located below the ground surface and are either fully or partially submerged in water. In addition, these containment vessels are attached to the support structures via seismic damping systems. These new reactor designs pose a complex soil structure-fluid interaction problem from earthquake-induced ground motion. Understanding this soil structure-fluid interaction phenomenon is essential to designers and regulators to ensure adequacy of the seismic damping/isolation system for safe operation of the modular reactors during seismic events.
Approach — This project uses a simplified, sequentially coupled analysis methodology for assessing the response of a containment structure housing a small modular reactor during a seismic event using the geomechanical code FLAC and the computational fluid dynamics (CFD) package ANSYS-FLUENT. The FLAC code analyzes the amplification of the earthquake motion as it propagates upward through the geological medium and to the containment structure. The time-dependent forces or velocities at the containment structure wall boundary from the FLAC analysis are used as the perturbation to initiate fluid sloshing simulated in the ANSYS-FLUENT package. The force generated by the sloshing process from the ANSYS FLUENT will be recorded and can be used, in turn, as an internal boundary condition in the FLAC analysis at a later time step.
Accomplishments — A site response analysis was conducted considering a hypothetical site to study the amplification of seismic waves while propagating from the bedrock to the ground surface and resulting soil-containment structure-fluid interaction with a modular reactor. Figure 1(a) shows the model used for soil-structure interaction analysis under seismic excitation. Ground acceleration at the base of the reactor from a strong-motion earthquake in California is shown in Figure 2(a). A simplified CFD model in two-dimensional space was developed to understand fluid sloshing in response to ground motion. As the wall viscous effect is expected to be negligible compared to the fluid sloshing impact, grid clustering was not done in the near-wall region. The water contained within the structure was set to have a sloshing motion in response to the applied acceleration signal. The sloshing motion continued after the input acceleration signal stopped. The vertical force on the reactor due to sloshing is shown in Figure 2(a) (negative sign because the forces are acting downwards). A representative water surface profile to highlight the sloshing motion in the reactor is shown in Figure 2(b). It shows the free water surface at an angle with the horizontal plane, indicating fluid motion and deformation due to external disturbance.