Ultrasonic inspection technique for weld in mercury target vessel for spallation neutron source

Wakui, Takashi  ; Wakai, Eiichi  ; Naoe, Takashi   ; Kogawa, Hiroyuki  ; Haga, Katsuhiro  ; Takada, Hiroshi   

A mercury target vessel has been used for the spallation neutron source at J-PARC. It has a complicated multi-layered structure composed of a mercury target and surrounding double-walled water shroud, which is assembled with thin plates (minimum thickness of 3 mm) by welding. Thus, welding inspection during the manufacturing process is important. We investigated the applicability of new ultrasonic inspection, named as phased array ultrasonic method, to improve the accuracy of welding inspection for the mercury target vessel. It was measured that the new ultrasonic method could detect a 1-mm-long defect formed in a 3-mm-thick specimen with an error of about 7%. Moreover, simulations of the phased array ultrasonic method demonstrated that the outline of a 0.5-mm-long defect in a thin specimen got clearer in an ultrasonic echo image with increasing frequency of the sensor, suggesting a possibility of further improvement.