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Detection of Hydride Blisters in Zirconium Pressure Tubes using Ultrasonic Mode Conversion and Velocity Ratio Method  

Cheong, Yong-Moo (Nuclear Materials Technology Team, Korea Atomic Energy Research Institute)
Lee, Dong-Hoon (Nuclear Materials Technology Team, Korea Atomic Energy Research Institute)
Kim, Young-Suk (Nuclear Materials Technology Team, Korea Atomic Energy Research Institute)
Publication Information
Journal of the Korean Society for Nondestructive Testing / v.23, no.4, 2003 , pp. 334-341 More about this Journal
Abstract
When the pressure tubes(f are in contact with the calandria tube(CT) in the pressurized heavy water reactor(PHWR), the temperature difference between inner and outer wall of W results in a thermal diffusion of hydrogen (deuterium) and hydride blisters are formed on the outer surface of PT. Because the hydride blisters and zirconium matrix are acoustically continuous, it is not easy to distinguish the blisters from the matrix with conventional ultrasonic method. An ultrasonic velocity ratio method was developed to detect small hydride blisters on the zirconium pressure tube. Hydride blisters were grown in the PT specimen using a steady state thermal diffusion device. The flight times of longitudinal echo and reflected shear echo from the outer surface were measured accurately. The velocity ratio of the longitudinal wave to the shear wave was calculated and displayed using contour plot. Compared to the conventional flight time method of longitudinal wave, the velocity ratio method shows superior sensitivity to detect smaller blisters as well as better images for the blister shapes. Detectable limit of the outer shape of the hydride blisters was conservatively estimated as $500{\mu}m$, with the same specifications of ultrasonic transducer used in the actual PHWR pressure tube inspection.
Keywords
ultrasonic mode conversion; ultrasonic velocity ratio; hydride blister; zirconium alloy;
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