Experimental investigation of the vibration response of a flexible tube due to simulated reactor core, cross and annular exit flows

Abstract

Bottom mounted instrumentation tubes in pressurized nuclear reactors have experienced wear in several plants due to excessive flow-induced vibrations. In some cases this wear has been sufficient to cause primary coolant leakage. The paper describes experiments conducted in order to identify the predominant flow excitation mechanism at a particular plant, and to develop a sleeve design to remedy the wear problem of the instrumentation tubes. An instrumented flow visualization model enabled simulation of a wide range of individual or combined reactor core flow, cross flow and thimble flow conditions. The instrumentation scheme adopted for these experiments used proximity displacement transducers and a force transducer to measure respectively tube motion and contact/impact forces at the wear region. Extensive testing of the original, in-plant configuration identified the normal core flow as the primary source of excitation. Shielding the In-Core-Instrumentation (ICI) thimble tube from the normal core flow curtailed vibration amplitudes; however, thimble flow excitation then became more pronounced. Various outlet nozzle configurations were investigated. An internal cavity combined with radial outlet slots became the optimum solution for the problem. The paper presents typical test data in the form of orbital tube motion, spectrum analysis and time history collages. The effectiveness of shielding (partial and complete) the instrumentation tube from the flow is demonstrated.