Radial Characterization of As-Irradiated and Post-LOCA Tested High Burnup UO₂ Fuel Using SEM and S/TEM-EDS

High burnup UO2 fuel under steady-state irradiation experiences microstructural changes including subgrain formation and fission gas bubbles production. The fuel is susceptible to fragmentation, pulverization and eventual dispersal under loss-of-coolant accident (LOCA) conditions. Analysis is needed to quantify inter- and intragranular bubble density to determine a correlation of bubble radial distribution and aid in understanding the driving mechanisms controlling restructuring and high burnup structure formation. Two fuel samples were analyzed in the as-irradiated and simulated LOCA test condition with an average sample burnup of 69 and 77 MWd/kgU. Particle measurements from the scanning electron microscopy (SEM) data revealed an overall decrease in pore number density and increase in average pore area from the fuel periphery to centerline in all samples. However, an increase in pore number density is observed approximately at the fuel mid-radius (0.60 r/r0) before decreasing again towards the fuel center. An initial image processing technique was performed to identify inter- and intragranular pores and displayed primarily intergranular pores at the fuel periphery which decrease rapidly towards the center. Scanning/transmission electron microscopy (S/TEM) with energy-dispersive X-ray spectroscopy (EDS) data exhibited a similar trend observed with the pore SEM data. Further analysis is ongoing to identify inter- and intragranular bubbles using the S/TEM-EDS data and the proposed image processing technique.