PNNL

Abstract

On-line structural health corrosion monitoring in advanced lead fast reactor heat exchangers and molten salt reactor heat exchangers is desirable for detecting tube degradation prior to leaks that may allow mixing of heat exchanger fluids or release of radiological contamination beyond the design containment boundary. This program, On-Line Lead/Water Heat Exchanger Sensor/System Feasibility – PNNL 76092, demonstrates feasibility for an ultrasonic torsional wave mode sensor attached to the outside of a long (30.5-m) heat exchanger tube in the stagnant flow area where the tube joins a heat exchanger plenum and where it is possible to protect a sensor and cable from high-force flows. The sensor must be cable connected to a monitoring instrument near the heat exchanger. The sensor and cable management approach for periodic in-service inspection will be impractical to implement on existing heat exchangers; rather permanently mounted sensors must be installed in conjunction with heat exchanger fabrication. Previous work has shown low-temperature lead zirconate titanate (PZT) piezoceramic sensors are able to detect anomalies of interest in 3.0-m long tubes. These sensors have hereby been extended to a 30.5-m long tube more representative of commercial power heat exchanger designs. The program will continue to investigate higher temperature piezoelectric ceramics and long-term performance of high temperature adhesives and sealants and mechanical pressure coupling for 350-500 oC lead reactor environments and higher temperature (700 oC) molten salt environments. • Based on the demonstrated ability of the sensor to detect flaws of interest and survive elevated temperature tests at 350 oC for more than 8 days and at 450 oC for more than 9 days, the project has demonstrated feasibility for an online heat exchanger tube monitoring system. • Generally, the feasibility for detecting flaws of interest [50% through-wall (TW), 6.4-mm diameter, flat-bottom hole and 50% TW, 90o arc notch] in both a 3.0-m (10.0-ft) and 30.5-m (100.0-ft) long tubes with low temperature sensors is encouraging. Clear detection of all flaws of interest with sensors having only 15 pairs of high temperature Pz46 [bismuth titanate (BiTi) with listed maximum working temperature of 550 oC] piezoelectric elements on the 3.0-m long tube further supports the claim that flaws of interest are detectable. • The acoustic coupling failure of the high temperature adhesive was disappointing after one temperature cycle but redirected efforts to examine mechanical pressure acoustic coupling as an alternative were successful. Based on the adhesive’s successful electrical conductivity tests and the continued macro-adhesion of the Pz46 material to the stainless-steel, an assumption was that micro-cracks and micro-debonds formed in the adhesive due to the differential thermal expansion between the Pz46, stainless-steel, and adhesive. This rendered the adhesive to be opaque to ultrasound in the frequency range being of interest. The reason for adhesive failure was not extensively explored; rather – efforts shifted to mechanical pressure acoustic coupling. • The adapted half-cylinder high temperature test was highly successful with the pressure-based coupling of a Pz46-aluminum-foil design. The tests ran for over 8 days at 350 oC in argon with a temperature cycle, over 5 continuous days at 450 oC in argon and then for over 9 continuous days at 450 oC in air. Although the test fixture seemed to mechanically relax several times and required manual tightening / intervention, the nature of these interventions suggests that if an appropriate compliant pressure is applied, the sensor would continue to run for extended periods.