To meet the growing demand of affordable power, several designs of Small Modular Reactors (SMRs), which will be installed below-grade, have been proposed by the nuclear industry. The containment vessels of these reactors will be under water. During a seismic event, these reactors will experience a complex soil (ground)-structure (SMR)-fluid (water) interaction that can affect the integrity of the system. Each of these reactors uses a seismic damping or isolation system to protect its important to safety structures, systems, and components from a design-basis earthquake. Designers of these damping/isolation systems need to have a thorough understanding of the complex soil-structure-fluid interactions to ensure the adequacy of the isolation system. In addition, regulators need to understand these interactions to evaluate the safety of such installations and systems.
This study was initiated to understand the complexities in modeling facility responses that may accompany a design-basis earthquake. The ability to model these complexities is important to designers and regulators. It was recognized that a three-way coupled approach that can satisfactorily model the unique dynamic characteristics of soil surrounding the reactor, the reactor structure, and fluid contained within the reactor is not available. As a first step in understanding the complex interaction phenomena, a sequential coupling approach was adopted in this study. It was assumed that the feedback loop (such as structural deformation affecting ground motion and sloshing) has limited influence because of the high inertia of the massive structure. The general-purpose geological continuum package FLAC was used to simulate the propagation of earthquake-generated ground motion. The fluid analysis was conducted using the commercial computational fluid dynamics (CFD) package ANSYS-FLUENT. This paper briefly discusses the modeling techniques used in soil-structure and structure-fluid interaction analyses. Using a strong motion earthquake record, the ground acceleration at the base of the SMR was calculated and used as input to the CFD analysis of fluid motion inside the structure.