Soil Structure and Fluid Interaction Assessment of New Modular Reactor: Part 1 — Numerical Simulation of Fluid Motion due to Seismic Waves

In recent years, the nuclear industry has proposed design of affordable small modular reactors (SMR), which will be installed below grade. A complex soil-structure-fluid interaction is expected to occur during a seismic event at such installation sites. A thorough understanding of this interaction is needed for the purpose of designing damping or isolation systems as well as to determine the adequacy and safety of these devices. A fully dynamically coupled analysis of the surrounding soil, reactor structure, and contained fluid within the reactor would provide the most accurate estimate of the forces acting on the SMR, but such an exercise is difficult to accomplish due to large discrepancies in length and time scales of each subsystem. It also would be computationally intensive to explicitly model all the detail physical features that affect system response in a single analysis framework. A sequential one-way explicit coupling between parts of the system, such as soil-structure or fluid-structure interaction in response to seismic ground motion, would provide some reasonable engineering information useful to designers and regulators. A two part study was conducted to understand the soil-structure and fluid-structure interaction in response to a seismic event for an SMR. The present paper describes the latter (fluid-structure interaction), where the containment fluid behavior during a seismic event is studied. A simplified two-dimensional computational fluid dynamics (CFD) model, representing a mockup structure based on the mPower reactor is developed in the study. It is used to simulate the sloshing motion of the fluid during a seismic event. A general volume of flow (VOF) approach is employed to simulate the sloshing motion and track the air-water interface. Ground acceleration calculated from a separate mechanical analysis is adopted in the study to specify the body forces experienced by the fluid. CFD simulations are performed for two different cases that correspond to two different input seismic waveforms. Simulated results highlight the movement of air-water interface due to sloshing within the containment building. The total horizontal and vertical forces on the structure, resulting from the sloshing motion were calculated. A Fourier analysis of the calculated fluid forces shows the dominant frequencies of the force, due to fluid sloshing, are different from that of the seismic acceleration. Similar dominant frequencies of the forces are predicted using two different input seismic waveforms. The magnitudes of the forces varied, depending on the magnitude of the seismic waveform input.