Profiles of activity ratios of fission products, cesium-134, cesium-137 and ruthenium-106 in low-enriched PuO2-UO2 fuel assemblies.

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Distributions of fission products of low-enriched PuO2-UO2 fuel assemblies irradiated in a D2O-moderated power reactor were non-destructively measured by r-ray spectrometry with a Ge(Li) detector. Proportionality was observed between the 134Cs to 137Cs activity ratio and the activity of 137Cs, which was a measure of the burnup of a fuel element. The proportionality constant depends on the specifications of the fuel element, such as size, density and enrichment as well as on the irradiation history. The burnup ratio between two fuel assemblies could be determined from the values of the activity ratio between 134Cs and 137Cs or from the activity of 137Cs. The results proved to be in fair agreement with the indications obtained from reactor instrumentation. The distributions of the 106Ru to 137Cs activity ratio were almost flat, both along the axis and across the fuel assembly. This flat distribution is a distinctive characteristic of irradiated PuO2-UO2 fuel rods and could be utilized as an indication of whether or not Pu was originally contained in a fresh fuel rod.
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