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核破砕中性子源の水銀ターゲット容器の溶接部に対する超音波検査技術

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

涌井 隆 ; 若井 栄一 ; 直江 崇  ; 粉川 広行 ; 羽賀 勝洋 ; 高田 弘  

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

J-PARCの核破砕中性子源の水銀ターゲット容器(幅1.3m、長さ2.5m)は、水銀容器と二重壁構造を持つ保護容器からなる薄肉(最小厚さ3mm)の多重容器構造で、溶接組立による複雑な構造を持つので、溶接部の検査が重要である。溶接検査の精度を向上することを目的として、新たなフェーズドアレイ超音波法の適用性を検討した。長さ約1mmの欠陥を有する厚さ3mmの薄肉試料を用いた試験を行い、計測誤差は7%程度であることを測定した。また、長さ0.5mmの欠陥を有する薄肉試料のシミュレーションにおいて、センサの周波数の増加につれて、超音波エコー画像における欠陥の輪郭が明確になり、測定精度の向上が見込まれる。

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.

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