Within virtually all advanced power reactor designs, the thinnest barriers between primary and secondary fluids are the heat exchanger tubes. Consistent with light water reactor experience, these thin-wall barriers are the most susceptible regions for corrosion and leakage. Moreover, traditional inspections and repairs using robotics will be expensive or impractical. On-line structural health corrosion monitoring is therefore desirable to detect tube degradation prior to leaks that may allow mixing of heat exchanger fluids. Guided wave ultrasound from heat exchanger tube ends or tube inside diameters using conventional low-temperature piezoelectric sensors have been shown to be sensitive to pits and cracks less than 50% through-wall in long pipes and tubes. Mode and frequency selection are important for practical sensor design decisions and ultimate performance. The L(0,1) wave mode is easily excited by L-mode transducers but they have surface-normal wave motion that will be sensitive and more attenuative because of fluid interaction with the tube surface. Torsional shear wave or T(0,1) mode motion produced by shear horizontal (SH) piezoelectric crystals have material motion parallel to the tube surface and, therefore, are virtually unaffected by fluid interaction. For T(0,1) to be nondispersive in steel, the frequency-thickness product (MHz-mm) must be less than approximately 1.5. Thus, for heat exchanger tubes with a wall thickness of nominally 0.8–1.5 mm, nondispersive frequencies should be &lt; 1–1.5 MHz. Final frequency choice should be guided by appreciating the need to be nondispersive and noting that higher frequencies can have better spatial resolution, but lower frequencies will be less attenuative. 
SH mode transducers must be coupled by a material that can support shear stress; a thin layer of honey works well for temporary sensors; however, an adhesive or a brazed metallurgical bond is required for permanent sensors. Sensor placement has historically mostly been at tube ends or within the tube because this is the only accessible location after the heat exchanger is fabricated. This region is typically a high flow area that would experience significant forces on both the sensor and wiring during operation, plus the sensor would occlude fluid flow through the tube thereby compromising the heat-exchange function of that tube. If sensors are incorporated into the fabrication process, however, the same kinds of SH-0 waves can be generated from the stagnant flow area of the tube outer diameter (OD) without compromising the heat-exchange tube function. Benefits to the advanced reactor system include: 
• Minimizing the expense of a reactor shut down for periodic inspection—only shutdown for cause. 
• Reduce risk of unexpected tube rupture by detecting flaws before they grow to through-wall leaks. 
• Cost reduction by minimizing need to design for periodic manual/robotic inspectability. 
This project attempts to demonstrate feasibility to develop a practical sensor that can be incorporated into Hydromine lead fast reactor heat exchanger design. This heat exchanger configuration is particularly amenable to the high temperature sensors because the 100-ft long spiral heat-exchanger tubes have their ends exposed to an argon gas blanket at 350 oC– not immersed in lead. The feasibility assessment consists of demonstrating that flaws of interest (50% flat bottom holes and notches) can be detected from one or both tube ends and that the adhesively applied sensors can survive at design temperatures and within the argon blanket atmosphere.

Feasibility of Lead Fast Reactor Heat Exchanger Tube On-Line Monitoring

Paper Proceedings: https://asmedigitalcollection.asme.org/qnde/QNDE2021/volume/85529

ASME 2021 48th Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE2021)

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