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Cesium migration effects on irradiation behavior of fast reactor MOX fuel pins

高速炉MOX燃料ピンの照射挙動におけるCs移動の影響

丹野 敬嗣  ; 岡 弘   ; 生澤 佳久   ; 上羽 智之 ; 大塚 智史   ; 皆藤 威二 ; 前田 誠一郎 

Tanno, Takashi; Oka, Hiroshi; Ikusawa, Yoshihisa; Uwaba, Tomoyuki; Otsuka, Satoshi; Kaito, Takeji; Maeda, Seiichiro

Csは揮発性FPであり、高速炉燃料ピン内で高温の炉心部から低温のUO$$_{2}$$ブランケットに軸方向移動する。UO$$_{2}$$ブランケットに蓄積したCsは低密度のCs-U-O化合物を形成し、特に高燃焼度では厳しいFCMIを生じることが懸念される。本研究ではCsの軸方向移動とその影響を理解するため、EBR-IIで照射された燃料ピンについて、外径測定とガンマスキャンによるCs軸方向分布測定を実施するともに、計算コードを用いて燃焼によるCs生成量を求め、Csの軸方向分布を定量化した。これらの結果、ブランケット部Cs蓄積位置でFCMIが生じていること、Cs蓄積量に応じて燃料ピンの外径変化が増大していることが分かった。

Cesium (Cs), which is a volatile fission product (FP), migrates axially from hot core region to cold top and/or bottom ones in fast reactor (FR) fuel pins. The Cs accumulated in cold region such as axial UO$$_{2}$$ blanket fuel can form Cs-U-O compounds having lower density than that of the fuel pellet, causing fuel cladding mechanical interaction (FCMI). The severe FCMI would arouse concern about the integrity of fuel pins. This work aims to understand Cs axial migration behavior and its effect on FR fuel pins. Two MOX fuel pins irradiated in EBR-II were evaluated the Cs migration behavior. Pin diameter measurement and gamma-scanning were carried out, and calculations with the ORIGEN-2 code was also done to estimate FP inventory with burnup in the pins. It was found from the comparison between the calculated pellet swelling by the Cs-U-O compounds and the measured pin diameter increase that the localized pin diameter increases at the MOX fuel-blanket interfaces were due to the FCMI caused by the pellet swelling associated with the formation of the Cs-U-O compounds.

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