Nondestructive assay of mixed uranium--plutonium oxides by gamma-ray spectrometry

Gamma-ray spectroscopy measurements have been made on mixed uranium and plutonium oxides in sealed containers to determine the uranium and plutonium enrichment and isotopics. Experimental results obtained using two different methods were in good agreement with the known contents. The first method is applicable to thick samples of freshly reprocessed mixed oxide and determines isotopic abundances from measured absolute gamma-ray intensities. Measurement times depend on plutonium enrichment, but for mixed oxide enriched to 12% in plutonium, the fissionable content can be determined to better than 0.5% in 2 h. The second approach utilizes intensity ratios of selected pairs of gamma-rays to determine plutonium enrichment and uranium and plutonium isotopes. This method requires at least 12 h to determine the plutonium enrichment to an accuracy of 0.5%. However, it cannot be applied until the /sup 238/U daughter activities in the mixed oxide reach equilibrium, which requires at least 5 months after separation. Preliminary conclusions drawn from these two noninvasive and nondestructive measurement techniques, and recommendations for future experiments are discussed.