中性子標的容器の高出力対応化の現状

Present status of design improvements for high power operation of J-PARC neutron target vessel

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

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

現在運転中の中性子標的容器は、熱応力の制限から利用運転に供する最大出力を700kWとしているが、次の標的容器では、J-PARCの目標である1MW運転を可能とするため、熱応力を低減する新たな冷却水流路と構造の設計を行い、現在製作中である。新しい設計では、発熱密度の高い標的前半部で水銀容器と保護容器を分離し、保護容器の冷却水流路の配置を最適化することで、容器の熱膨脹差で生ずる熱応力を大幅に低減している。また、全体の部材数を減らすように製作設計を行い、各部材はワイヤー放電加工を用いてブロック状の材料から成型することで溶接箇所を大幅に減らした。一部の溶接は電子ビーム溶接を新たに導入し、溶接に伴う変形・残留応力を低減した。さらに、製作工程で放射線検査、超音波検査を積極的に導入し、溶接部の健全性を確認している。圧力波によるキャビテーション損傷に関しては、150kW200kWの出力で使用した使用済み中性子標的容器から試験片を切り取って損傷深さを計測し、気泡の無い条件における損傷予測の精度を向上させるデータが得られた。

The maximum beam power for user operation of the present neutron target vessel is limited to 700 kW due to the allowable thermal stress. However, to allow the rated 1-MW operation, the next neutron target vessel changed the target structure and the water channel arrangement to reduce the thermal stress for achieving 1MW beam power. In the new target vessel design, the mercury vessel and water shroud were separated in the front half part of the target vessel where the heat generation density is high, and the water channel arrangement was optimized to reduce the thermal stress caused by the difference of thermal expansion between those two vessels. The fabrication design was changed to reduce the total parts number to be the minimum, and the number of welding lines was drastically reduced by fabricating each part from the block of stainless steel with wire electrode discharge machining. The welding deformation and the residual stress were also reduced by applying electron beam welding to part of the welds. Furthermore, the radiation tests and ultra-sonic tests were applied in the fabrication processes to check the soundness of welds. As for the cavitation damage caused by the pressure wave, the damage depth of the specimen cut out from the used neutron target vessel which was operated at the beam power of 150 kW to 200 kW was measured, and damage data to improve the damage evaluation under the non-bubble condition was obtained.