Non-destructive inspection technique for weld part of mercury target vessel

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 triple-walled structure, which is assembled with thin plates (minimum thickness of 3 mm) by welding. A structural analysis of the new vessel evaluated that allowable defect size not to influence structural integrity was 0.4 mm at a highest stress region. Radiographic testing using the sample with triple-walled structure could not detect the defect within allowable detection size. On the other hand, a non-contact immersion ultrasonic method, which is insensitive to distorted surface of the vessel after welding, was able to detect the defect of 0.2 mm by using a high frequency sensor. We inspected all welding lines (total length of ca. 17 m) of newly fabricated vessel, detecting only six defects and evaluated that their sizes never influence on the structural strength. The present development of the non-destructive inspection technique has improved the reliability of the vessel.