Metallographic examination of irradiated core fuel subassembly; (Fab. No.PPJX12) for "JOYO" MK-I

Ukai, Shigeharu  ; not registered; Unno, Ichiro

Core Fuel Subassembly (Fab. No. PPJX12) was irradiated at the first row of reactor core all through the "JOYO" MK-I period, and estimated to be the highest average burnup of 39,000 MWD/MTM next to PPJX13 of 41,000 MWD/MTM in the "JOYO" MK-I Subassembly. This sabassembly had been intentionally kept in the water pool with insufficient removal of Sodium after irradiation. The observation of outer surface of cladding by scanning electron microscope (SEM), morphological observation of fuel and cladding by Optical microscope, and instrumental analyses by electron microprobe analyser (EPMA) and ion microprobe analyser (IMA) were performed in order to confirm the integrity of cladding kept in pool with insufficient removal of sodium, and to supplement the data base of high burnup region on "JOYO" MK-I fuel. The experimental results are summarized as follows: (1)The condition of outer surface of cladding observed by SEM was similar to that of cladding kept in pool after sufficient removal of sodium, and intergranular attack and corrosion of outer surface of cladding were not observed. (2)Central void and Columner grain were not formed. Fuel structure was observed as fission gas bubble region, densified region and unrestructured region. Gap width between fuel and cladding at oxial middplane of fuel column was reduced to 14m in radius. These results supplement the data base of high burup region on "JOYO" MK-I fuel. (3)Redistribution of Uranium and Plutonium was not observed. Volatile fission product such as Cesium accumulated in the fuel-cladding gap, but the corrosion depth of cladding caused by fuel-cladding chemical interaction was 10m in maximum. (4)Burnup measured by IMA showed good agreement with that calculated using "JYHIST" code.