b'HBu UO2 Transmission Electron MicroscopyPrincipal Investigator: Chad M. ParishTeam Members/Collaborators: Nathan Capps, Lauryn Reyes, Casey McKinneyThis project providesL oss-of-coolant accident (LOCA)Specifically, HBu fuel from North tests showed significantAnna 1 and North Anna 2 (NA1 and fine fragmentation in highNA2) LWRs have been examined. quantitativeburnup (HBu) fuel such that someSamples were prepared by focused experimental data thatof the fuel was reduced to a sand- ion beam (FIB) for S/TEM analysis will allow validationlike consistency. This HBu fueland S/TEM was used to image and benchmarking offragmentation raised concerns ofgrains, cavities, FGBs, and FMPs.predictive models forthe fuel fragments relocating into improving safety andthe ballooned region of the clad- S/TEM methods were used to quan-efficiency of LWR fuel. ding and possible dispersal fromtify the microstructures present in the cladding through the rupturethe as-irradiated and post-LOCA opening, collectively known assamples as a function of radial fuel fragmentation relocation andlocation. For different radial loca-dispersal. In this work, as-irradiatedtions (where r/r0=1.0 is the fuel-clad and LOCA-tested HBu fuel samplesinterface and r/ro=0.0 is the center are examined in scanning/transmis- of the pellet) we examined grain sion electron microscopy (S/TEM) tostructures, large pores (~microns) correlate microstructural features,that were prior-Xe bubbles, the primarily fission gas bubblesFMPs, and the nanometric FGBs. We (FGBs), to fragmentation behavior. are also developing and refining an analytical method to estimate the Project Description: pressure of the Xe in the nanometric The goal of this project is to enableFGBs, because the pressurization understanding and predicting howof the FGBs is a vital parameter light-water reactor (LWR) fuel willfor modeling and predicting fuel behave under HBu and LOCA condi- behavior and experimental confir-tions. Here, we are obtaining quan- mation of the predicted pressures is titative microstructural data fromvital to validate current models.as-irradiated and post-LOCA samples of HBu fuel. This microstructuralAccomplishmentsdata is essential input to modellingThe most important results from efforts to understand and predictthe present work are comparisons the performance of HBu fuel duringof as-irradiated and post-LOCA both normal and accident condi- samples from approximately the tions. The HBu micro- and nano- same radial regions in NA1 samples, structure is so complex (large grains,and secondarily, comparison to the small grains, high- and low-angleas-irradiated NA2 samples. In all grain boundaries, FGBs, five-metalthe samples, a combination of large particles [FMPs], etc.) that model- and fine UO 2grains are observed, ling the behavior under irradiationintermixed with large (~microns) or LOCA conditions requires theregions of free space that were large quantitative insight to the structureprior-xenon bubbles before the obtained by these experiments.samples were thinned to electron 116 2023|AFC ACCOMPLISHMENTS'