Determination of Pu in Spent Fuel Assemblies by X-Ray Fluorescence

One of the more difficult non-destructive assay problems for gamma-ray spectroscopy is the determination of the Pu in spent nuclear fuel. In spent nuclear fuel the high gamma flux from fission products makes direct observation of Pu gamma rays impractical. However, high-Z elements, such as uranium and plutonium, have large photoelectric cross sections for absorption of the spent fuel gamma rays. As a result the gamma-ray flux in the spent fuel will generate fluorescent X rays from uranium and plutonium. This leads to a substantial generation rate of uranium and plutonium K X rays in the 95 -117 keV energy range. The nuclear fuel cladding for many reactor types is thin enough so that a significant fraction of the X-ray flux escapes the fuel assembly, and can be measured with gamma-ray detectors. The relative ratio of Pu to U X-ray peaks in the gamma-ray spectrum can be used to determine the Pu/U elemental ratio in the spent fuel. The Pu content of the assembly can be estimated by multiplying this ratio by the better-known mass of uranium. This paper describes the details of the calculation and applies it using fluorescent X-rays observed in the gamma-ray spectra of spent fuel assemblies of the BN-350 fast breeder. This technique provides a new method of estimating the Pu content of spent fuel assemblies.