Multimodal Serial Sectioning and Synchrotron Micro-Computed Tomography Characterization of High-Burnup U-Mo Fuel

Uranium-molybdenum (U-Mo) based fuels are candidates for converting high-performance research and test reactors from high enrichment uranium to low enrichment uranium as a non-proliferation effort. U-Mo fuels undergo irradiation-induced grain refinement at approximately 3-5x10²¹ fissions/cm³ (37-62% burnup), resulting in a porous microstructure. As the irradiation continues, the pores increase in size and can create interconnected networks. This increase in porosity and interconnected pore networks dramatically increases swelling of the material, creates avenues for gas escape, lowers the thermal conductivity, and decreases the mechanical stability. To properly assess the distribution, morphology, and interconnectedness of the pore network and potential fuel phase relationship of the pores, it is necessary to measure the microstructure using a three-dimensional (3D) characterization technique. Two different 3D characterization techniques are typically used with their perspective pros and cons; X-ray micro-computed tomography (X-ray µ-CT) and scanning electron microscopy (SEM) serial sectioning. X-ray µ-CT is a non-destructive technique that uses X-ray attenuation to reconstruct images of the interior of the sample. This technique can assess the microstructure with spatial accuracy and high contrast but has a lower resolution and smaller sample size capabilities than SEM serial sectioning, and compositional information needs to be inferred from expected constituents. On the other hand, SEM serial sectioning is a destructive technique preventing any further testing on the sample. SEM serial sectioning typically produces a higher resolution image, revealing smaller pores that would be overlooked in X-ray µ-CT and can investigate the composition via simultaneous energy-dispersive x-ray spectroscopy (EDS). However, the technique does not have identical X, Y, and Z resolution making the geometric measurements of the porosity more difficult. Damage from the serial sectioning can also cause curtains on the image surface, which distort the features of the sample. To properly assess the capabilities and limitations of each technique on the characterization of U-Mo porosity and the surrounding features; this study applied synchrotron high energy X-ray µ-CT at the Advanced Photon Source at Argonne National Laboratory, and plasma focused ion beam SEM serial sectioning at Irradiated Materials Characterization Laboratory at Idaho National Laboratory on the same samples irradiated to 5.4x10²¹ fissions/cm³ (68% burnup). One sample was obtained from the UMo-Zr barrier interaction region to characterize Mo depletion, interfacial pore interaction, pore morphology, and Zr barrier penetration, and a second sample was removed from the bulk fuel to assess the pore morphology without interfacial effects. By analyzing these two samples using multimodal techniques, a detailed assessment of the capabilities and limitations of the techniques will be created and provide a complimentary model on how each technique can be applied to reveal the true microstructure of the fuel.