Development of observation techniques in reactor vessel of experimental fast reactor Joyo

高速実験炉「常陽」における炉内観察技術開発

高松 操 ; 今泉 和幸 ; 長井 秋則; 関根 隆 ; 前田 幸基

Takamatsu, Misao; Imaizumi, Kazuyuki; Nagai, Akinori; Sekine, Takashi; Maeda, Yukimoto

高速実験炉「常陽」では、炉内干渉物対策の一環として、ビデオカメラを用いた炉心構成要素頂部の観察,ファイバースコープを用いた炉心上部機構下面の観察を実施した。炉心構成要素頂部の観察は、回転プラグの炉内検査孔上にデジタルビデオカメラを設置した簡易なシステムにより実施した。炉心構成要素頂部に異物や損傷がないことを確認するとともに、当該システムが約1mmの分解能を有することを確認した。炉心上部機構下面の観察においては、炉心上部機構下面部と炉心構成要素頂部の間隙約70mmにファイバースコープを挿入する観察装置を新規に開発し、炉心上部機構の下面を観察した。本装置により、厚さ約0.8mmの整流格子を判別し、炉心上部機構下面の状況を把握することができた。これらの観察により、Na冷却型高速炉の炉容器内観察の高度化に資する有用な知見を得ることができた。

During the investigation of an incident that occurred with the experimental fast reactor Joyo, in-vessel observations using a standard Video Camera (VC) and a Radiation-Resistant Fiberscope (RRF) took place at (1) the top of the Sub-Assemblies (S/As) and the In-Vessel Storage rack (IVS), (2) the bottom face of the Upper Core Structure (UCS) under the condition with the level of sodium at -50 mm below the top of the S/As. A simple 6 m overhead view of each S/A, through the fuel handling or inspection holes etc, was photographed using a VC fixed to the rotating-plug with the acrylic panel for making observations of the top of S/As and IVS. About 650 photographs were required to create a composite photograph of the top of the entire S/As and IVS, and a resolution was estimated to be approximately 1 mm. In order to observe the bottom face of the UCS, a remote handling device equipped with RRFs (approximately 13 m long) was specifically developed for Joyo with a tip that could be bent into an L-shape and inserted into the 70 mm gap between the top of the S/As and the bottom of the UCS. A total of about 35,000 photographs were needed for the full investigation. Regarding the resolution, the sodium flow regulating grid of 0.8 mm in thickness could be discriminated, and the base of thermocouple sleeves 6 mm in diameter located 450 mm above the top of S/As were also clearly observed. In both types of observations, it was confirmed that lighting adjustments play a critical role. Particularly in narrow space observations, scattered lighting with automatic dimming controlled light source was available for achieving close observations of the in-vessel structures. In addition to the successful result of the incident investigation, these experiments provided valuable insights for use in further improving and verifying in-vessel observation techniques in sodium cooled fast reactors.