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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].

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The association reactions between trivalent ions of lanthanum, cerium, neodymium, gadolinium, erbium, ytterbium, and nitrate or sulphate ligands has been investigated from ultrasound velocity dispersion measurement data. It is assumed that ultrasound velocity dispersion in aqueous solutions of lanthanide nitrates and sulphates is caused by an association process. The ultrasound velocity was measured by the ultrasonic laser interferometer within the frequency range of 3–200 MHz. The rates of formation of inner sphere complexes [LnNO3]2+aq and [LnSO4]+aq were calculated and the interaction between individual ions Ln(III) and water molecules is discussed across the series. A comparison of relaxation parameters for the nitrate solutions with corresponding sulphates was made. It has been shown that a magnitude of an ultrasound dispersion may be the quantity of complexation. Ultrasound velocity dispersion for nitrate solutions is much smaller than that of more diluted lanthanide sulphate solutions. The lanthanide nitrates form predominantly outer sphere complexes with some inner sphere substitution. In order to produce an appreciable amount of inner complexes in nitrate solutions for a similar dispersion and relaxation, a tenfold higher concentration of the nitrates must be used.