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

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

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 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.