Overview of studies conducted at the JAEA on evaporation to dryness accidents of high-level liquid waste in reprocessing plant

原子力機構で実施した蒸発乾固事故研究の概要

吉田 尚生  ; 田代 信介 ; 天野 祐希 ; 吉田 一雄 ; 山根 祐一  ; 阿部 仁 

Yoshida, Naoki; Tashiro, Shinsuke; Amano, Yuki; Yoshida, Kazuo; Yamane, Yuichi; Abe, Hitoshi

福島第一原子力発電所事故を踏まえ、核燃料サイクル施設に対しても重大事故が定義され、対応策の有効性評価が求められている。再処理施設における重大事故の1つとして、高レベル濃縮廃液貯槽の蒸発乾固事故が定義されている。この事故は、貯槽内の冷却機能が喪失した場合に、内在する放射性物質の崩壊熱により廃液温度が上昇し、事故対策に失敗した場合には廃液の蒸発および乾固に至ることで、放射性物質の放出が引き起こされるというものである。この事故事象中で、ルテニウム(Ru)は四酸化ルテニウム(RuO)等の揮発性化合物を形成することで、他の元素よりも多い割合で貯槽外へ放出されると想定されている。本発表では、主に気体状Ruの放出と移行挙動に関して、原子力機構で実施してきた研究を紹介する。

After the Fukushima-Daiichi Nuclear Power Plant accident, Japan's licensing standards of the nuclear fuel cycle facilities were further strengthened and countermeasures against "severe accidents" were newly required as regulatory items. The severe accidents in the licensing standards were defined as accidents that occur under conditions beyond design-basis conditions. The "Evaporation to Dryness due to the Loss of Cooling Functions (EDLCF)" of high-level liquid waste (HLLW) was defined as one of the severe accidents for the reprocessing plant. In the EDLCF, it is assumed that radioactive materials (RM) are released to the gas phase due to evaporation and dryness of HLLW which induced by decay heat derived from fission products. The release behaviour of the RM is affected by the condition of the HLLW and the migration behaviour of RM is affected by the condition of gas phase. We have been conducted experimental studies for these behaviour of the RM. In recent years, behaviour of ruthenium (Ru) has attracted much attention in the field of EDLCF study. Existing studies have shown that Ru was released at a rate greater than other elements because it formed volatile compounds such as ruthenium tetroxide (RuO). For the above reasons, we have been conducting studies focusing on the behaviour of Ru among the radioactive materials that could be released in the accident. In the presentation, we will report a summary of our studies about the release and migration behaviour of gaseous Ru.