Influence of thermal expansion bend and tubesheet geometry on guided wave inspection of steam generator tubes of a fast breeder reactor

Periodic assessment of steam generator tubes of a sodium-cooled nuclear reactor is very crucial for smooth operation of steam generators. To examine the integrity, an in-bore magnetostrictive transducer capable of launching and receiving longitudinal ultrasonic guided waves (L(0,2) mode) from the inner diameter side of a steam generator tube developed in-house is used. Preliminary tests conducted on defective steam generator tubes with thermal expansion bends (three successive bends) of the mockup steam generator test facility yield a good sensitivity of 20% wall thickness deep flaw (0.46-mm deep and 1-mm wide half-circumferential groove) and the location accuracy of 10 mm. In order to remove high noise, wavelet-based denoising using discrete wavelet transform is used which improves the signal-to-noise ratio by 5–10 dB. In addition, cross-correlation technique is also used to denoise and unambiguously identify the defect echoes amid noise and multiple reflections between the defects. Furthermore, influence of the thermal expansion bend and tubesheet–spigot structure on L(0,2) mode is studied using the finite element analysis. It is observed that in the thermal expansion (multiple) bend, axisymmetric L(0,2) mode becomes non-axisymmetric (maximum and minimum amplitudes at extrados and intrados, respectively) and undergoes mode conversion to a weak flexural mode F(1,3). The results are validated experimentally. In the tubesheet–spigot structure, L(0,2) mode is found to have ∼10% reflection from spigot–tubesheet transitions, and it is seen to mode convert to bulk waves in the tubesheet. In conclusion, thicker tubesheets are found to be better from the perspective of inspection.