The Phase state at high temperatures in the MOX-SiO system

高温下におけるMOX-SiO系における相状態

中道 晋哉  ; 加藤 正人   ; 砂押 剛雄*; 内田 哲平; 森本 恭一 ; 鹿志村 元明; 木原 義之

Nakamichi, Shinya; Kato, Masato; Sunaoshi, Takeo*; Uchida, Teppei; Morimoto, Kyoichi; Kashimura, Motoaki; Kihara, Yoshiyuki

「常陽」での短期照射試験後、燃料中心部の限定された領域で燃料製造時に不純物として混入するSiOの凝集が認められた。SiOを混合したMOXについて酸素分圧を変えて1700C, 2000C, 2400Cの温度で熱処理を行った。低酸素分圧下で熱処理した試料において、MOXの粒界にPu, Am及びSiの化合物の形成が観察され、特に2400Cで熱処理した場合において、その傾向が顕著となった。

Japan Atomic Energy Agency researchers have developed mixed oxide (MOX) fuels containing minor actinides (MA). These fuels were irradiated for ten minutes in the FBR Joyo in some short-term irradiation tests. The Si-condensed phases were observed at the center of the pellets in the post irradiation examination. Si impurities came to be mixed into the raw materials in the ball milling process, because Si rubber was used as the lining of the milling pot. Content of Si in the pellets was within the specification of the fuel. It is important to investigate the Si state in MOX at high temperatures like the reactor operating temperature of the fuel to evaluate irradiation behavior. In the present work, MOX specimens with mixed SiO impurity were prepared. The ratio of MOX to SiO was controlled at a mol fraction of 3 to 1. The specimens were first heated at 1973K in atmospheres of three different oxygen partial pressures to adjust the O/M ratio. Then these specimens were sealed in a tungsten capsule, and heated at 2273K or 2673K. Compounds consisting of Pu and Si were observed at grain boundaries of the MOX matrix in specimens after heat treatment. These compounds were not observed in grain interior and MOX matrix was not affected significantly by Si impurity. These compounds tended to form in specimens with low O/M ratio and in specimens heated at higher temperatures.