PNNL

Abstract

Uranium-molybdenum (U-Mo) monolithic fuels represent a viable option for converting civilian research and test reactors operating with high enriched uranium (HEU) to low enriched uranium (LEU), effectively reducing the threat of nuclear proliferation world-wide. This study demonstrates the utility of a comprehensive microanalytical characterization toolbox, which includes Energy-Dispersive X-ray Spectroscopy (EDS), Secondary Ion Mass Spectrometry (SIMS) and the correlative analysis of their image data streams using image fusion. The elemental and isotopic microstructural components of a sample from the Advanced Test Reactor (ATR) Full-sized plate In-center flux trap Position (AFIP)-7 were analyzed and compared to previous optical and electron microscopy results. The imaging results reveal the AFIP-7 sample to be elementally heterogeneous with Mo banding along the transverse cross-section of the fuel foil. Additionally, observation of the U signal intensity alone suggests that the AFIP-7 sample is isotopically homogeneous with little to no variation in the 235U/238U isotopic ratio. However, quantitative analysis revealed an average 235U atom percent value of 17.856 ± 2.351%. This value is outside of the process uniformity specification of ± 0.2% and is indicative of a non-uniform U isotopic distribution. The image fusion results from the AFIP-7 sample are characteristic of SIMS isotopic maps, but with the spatial resolution of EDS images and, therefore, can be used to increase the effective spatial resolution of the SIMS imaging results. Development of this capability is useful for understanding the connections between the properties of LEU fuel alternatives and the ability to predict their performance under irradiation conditions.