Plutonium and Fission Product Redistribution in Mixed-Oxide Fuels During Irradiation

Plutonium and fission product migration were studied in a low burnup vibrationally compacted UO₂-2 wt% PuO₂ specimen utilizing autoradiography, micro sampling, microgamma scanning, and shielded electron microprobe analytical techniques. The results show that a significant change in radial plutonium concentration can occur rapidly in high performance mixed-oxide fuels by a vapor transport mechanism that is coincident with the fuel restructuring phenomenon. This results from the condensation of uranium-rich vapor in the cooler peripheral regions of the fuel rod. Cesium, ruthenium, and rhodium migrate significantly in mixed-oxide fuels, whereas zirconium, niobium, cerium, and praseodymium do not redistribute significantly.