Development of ultrasonic waveguide sensor for under-sodium inspection in a sodium-cooled fast reactor

The reactor core and in-vessel structures of a sodium-cooled fast reactor (SFR) cannot be examined visually due to the opaque sodium. The examination of the in-vessel structures is possible using ultrasonics to penetrate the sodium. A plate-type ultrasonic waveguide sensor using a leaky Lamb wave (A0 mode) has been developed for under-sodium visual inspection of the reactor core and in-vessel structures. In the plate waveguide sensor, the A0 leaky Lamb wave is utilized for the single mode generation and the effective radiation capability in a fluid. The liquid wedge is applied for the generation of the A0 mode in the low frequency range. The long pulse tone-burst excitation should be applied to minimize the dispersion effect in 10 m long distance propagation of the A0 Lamb wave. And a novel technique which is capable of steering a radiation beam of a waveguide sensor without a mechanical movement of the waveguide sensor has been suggested. The control of the beam angle can be achieved by a frequency tuning method of the excitation pulse in the dispersive low frequency range of the A0 Lamb wave. A 10 m long ultrasonic waveguide sensor module which consists of a plate waveguide, a liquid wedge, an ultrasonic sensor, and an acoustical shielding protection tube has been designed and manufactured. The possibility of applying the ultrasonic waveguide sensor module to an under-sodium visual inspection has been investigated. The experimental tests such as the long distance propagation test of A0 Lamb wave, the beam profile measurements, and C-scanning experiments in water have been carried out for the performance of the ultrasonic waveguide sensor. The feasibility of the ultrasonic waveguide sensor technique has been successfully demonstrated.