Development of ultrasonic transducers for ranging and imaging in heavy liquid metal

Abstract

The paper is devoted to a description of development of high temperature, and neutron radiation resistant ultrasonic transducers which must operate continuously in a liquid Pb/Bi up to 450°C. Known ultrasonic transducers are not intended for such harsh conditions. The main problems are acoustic coupling of the piezoelement to the protector and wetting of the transducer by a heavy liquid metal. For the first time piezoelement was attached to the transducer body by gold-to-gold diffusion bonding process, acoustic coupling between the transducer and the liquid metal alloy was achieved by front face coating by a protective diamond like carbon (DLC) layer. Due to a high radiation only a limited number of materials may be used in the transducer design. The transducers with the bismuth titanate piezoelectric elements showed no noticeable changes of pulse responses and transfer coefficients. The housing of the transducers is made of stainless steel AISI 316, it is laser welded and the high temperature Ø1 mm 15 m long mineral cable is used. Key wordsHigh temperature ultrasonic transducer, heavy liquid metal INTRODUCTIONIn the field of nuclear technology the use of heavy liquid metal (HLM) -lead or lead-bismuth eutectic (LBE) -for reactor devices under development possess problems with the required inspection and maintenance due to the opaque nature of the medium that is employed as a coolant or target material. Use of HLM is foreseen for the accelerator driven systems (ADS) -where the sub-critical nuclear core is driven by primary neutrons that are generated in a LBE spallation target generated by a high energy proton beam. The MYRRHA device under design in the Belgian Nuclear Research Centre SCK•CEN at Mol in Belgium is an LBE cooled ADS [1].

A Review of Acoustic Impedance Matching Techniques for Piezoelectric Sensors and Transducers

“…Ultrasound Doppler velocimetry is another application where hot melts flows are monitored. High temperatures liquid metals inside modern reactors cooled by liquid metals such as Pb/Bi alloy [ 227 , 404 , 405 ] have wetting limitations and corrosive environment, posing difficulties for the ultrasonic measurements. Space exploration applications of piezoelectric transducers represent several extreme environments with combinations of high temperatures (460 °C), high pressure (9 MPa), and corrosiveness [ 406 , 407 ].…”

High temperature ultrasonic transducers for imaging and measurements in a liquid Pb/Bi eutectic alloy

“…It is reported the use of a metal foil for the coupling of ultrasonic energy between the AlN transducers and the object at elevated temperatures [31]. Dry coupling of a piezoelectric transducer with AISI-316 ultrasonic waveguide was used by us successfully and good experimental results were obtained [1,3]. The contacting surfaces of a piezoelement and a waveguide were flat polished to the optical quality.…”

“…The sensors are high temperature, γ and neutron radiation resistant, they can operate continuously in a liquid Pb/Bi alloy up to a temperature of 450°C [1][2][3]. Taking into account the design requirements it was necessary to solve the following main problems: to select suitable piezoelectric materials, to optimize the sensor design, ensure reliable acoustic coupling of a piezoelement to the protector (and the backing) in the temperature range 150-450°C, to get a durable and stable long-term acoustic coupling of an ultrasonic sensor to the liquid metal.…”

“…Therefore, ultrasonic methods are promising for on-line monitoring of molten metal properties. Possibility of ultrasonic ranging and imaging in liquid metals is very important for establishment of quasi-visibility inside modern reactors, cooled by Pb/Bi alloy [1][2][3]. Space exploration technology challenge requires new actuator and sensor designs.…”

Progress towards developing neutron tolerant magnetostrictive and piezoelectric transducers

“…Ultrasonic measurements have a long and successful history of use for materials characterization, including detection and characterization of degradation and damage, measurement of various physical parameters used for process control, such as temperature and fluid flow rate, and in non-destructive evaluation (NDE) [2]. Ultrasonic measurements have been demonstrated to successfully measure fission gas release [3], for under sodium viewing [4] for fuel porosity measurements [5,6], and for thermometry [7]. However, application of ultrasonic sensors in nuclear reactors has been limited to low neutron flux environments [1].…”