Metallographic examination of irradiated core fuel subassembly (fab.No. PPJX13) for "JOYO" MK-I

Enokido, Yuji*; not registered; Unno, Ichiro

The morphological observation and the instrumental analysis of fuel elements of core fuel subassembly (Fab. No. PPJX13) irradiated in "JOYO" MK-I have been performed. The burnup of the subassembly (40, 100MWD/MTM) was the highest in breeder core (MK-I) of the reactor. It was expected that the subassembly should exhibit almost all irradiation characteristics throughout the irradiation duration of MK-I. For the examination at Metallography Cell of Analysis and Evaluation Section (AES) in Fuel Monitoring Facility (FMF), samples from two pins selected in the subassembly were prepared. One was the pin with the highest cladding temperature history, and another was a sibling of the center pin. The samples were prepared in the cell, in order to carry out the morphological observation of fuel and cladding by optical microscope and scanning electron microscope, and the instrumental analyses by electron microprobe analyser and ion micro-probe mass analyser. The experimental results are summarized as follows: (1)Central void of about 200 m was formed. From the void toward the outer surface of the fuel, fuel restructuring was observed as an elongated grain zone, a porous equiaxed grain zone and a densified zone. The structure resembles to the typical high burnup fuel structure. (2)Fission products Cs, Te, I, Ag, Pd etc. accumulated in the fuel-cladding gap, but the corrosion caused by fuel-cladding chemical interaction was 12m in maximum. (3)Extraodinal precipitations and grain bundary attack in cladding were not observed. (4)At the outer surface of the cladding, fine particles were found. Reduction of Ni and Mn content in the cladding within 4m from the surface at high cladding temperature position resulted in the enrichment of Cr, Fe and Mo. On the contrary, Ni and Mn deposited on the surface of cladding at low temperature region. (5)The maximum local burnup of the core fuel was 5.08 at.%. (6)The behaviour of fission products in fuels of "JOYO" MK-I was ...