STUDY OF FISSION-PRODUCT SEGREGATION IN USED CANDU FUEL BY X-RAY PHOTOELECTRON-SPECTROSCOPY (XPS) .2.

A thorough investigation of the grain-boundary chemistry of used CANDU fuel from one intact element has been conducted by X-ray photoelectro n spectroscopy (XPS). Selected findings from more extensive XPS measur ements on other used CANDU fuels exposed to storage conditions are inc luded for comparison. Cesium, rubidium, tellurium and barium have been commonly observed, often reaching high degrees of surface enrichment, although their relative abundances can vary widely with a complex dep endence on the fuel irradiation history. Lower concentrations of cadmi um, molybdenum, strontium and iodine have also been occasionally detec ted. Except for iodine, chemical-shift data are indicative of oxidized species, possibly uranates. Segregation at monolayer-level coverages has been demonstrated by sequential XPS analysis and argon-ion sputter ing. Calculations based on an idealized thin-film model are consistent with the depth profiles. The interpretation of these results is discu ssed in the context of previous studies, especially on LWR fuels.