The mechanism causing a fatigue failure of an in-vessel mechanical component has been identified through fundamental thermal-hydraulic analysis, and verified by experiments at several scales. It was found that pressure oscillations, caused by acoustic standing wave interaction with flow-instabilities, similar to vortex shedding, created the oscillatory driving force that initiated the failure. It was necessary to determine the dominant scaling laws in order to design scale models, which would result in time-dependent behavior that was representative of full size systems. The scaling laws showed, and it was confirmed experimentally, that room temperature air could be used in place of steam in small-scale testing, to provide representative behavior of the interactive acoustics and the flow instabilities. Tests of the scale models showed that minor changes in geometry and edge sharpness caused significant changes in the pressure oscillations at a given flow rate. It was also found that the piping configuration attached to a vessel may significantly alter acoustic pressure oscillations within the vessel. The study of interactive acoustic and flow instability phenomena in small scale models makes it possible to study the effect of parameter changes on the magnitude of thermal-hydraulic forces, which can play a role in fatigue failures.

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