Fluid-Structure Interaction Simulations in Applications for Nuclear Engineering

Abstract

Nuclear reactors pump coolant through their core and heat exchangers at massive mass flow rates to sustain energy production. These demanding requirements push engineers to extreme levels of optimization to safely sustain the transfer of energy. High flow rates introduce the possibility of flow-induced vibrations. Reactor core and heat-exchanger/ steam generator designs go through many stages of experimentation to ensure that problematic flow-induced vibrations do not arise. Advances in computational capabilities introduce the possibility of creating predictive simulations that accelerate the iterative design process and replace expensive physical experiments. Simulation methods for fluid-structure interactions are rapidly developing and undergoing extensive verification and validation. Computational fluid dynamics code Nek5000 and computational structural mechanics code Diablo have been coupled to create a highly scalable, high-fidelity fluid-structure interaction code. A fully coupled model of crossflow through a tube bundle has been simulated using the Nek5000-Diablo code for validation purposes. Simulations at three velocities were performed to test the method’s capabilities of capturing the onset of large amplitude vibrations that occur at a critical velocity for the tube bundle. The simulation results compared favorably to the experiment on which it was based and gave further insight into the mechanisms behind the vibrations. A 7-pin bundle of wire-wrapped fuel pins was simulated using Nek5000 and the forces exerted on the pins captured. A scheme was developed to synthesize force histories of indefinite length replicating the Nek5000 force signals, forming a modified one-directional coupling procedure. Multiple structure simulations were performed, observing the effects of pin-to-pin, and pin-to shroud contact scenarios on the resulting vibrations. The shroud was found to effectively limit vibrations to short wave-lengths on the order of 1/6^th helical pitch even when extensive gaps between pins formed in the reactor. Both the one-way and two-way coupling methods are successful in capturing the fluid and structure behaviors and provide a convenient method of analysis for these geometries.