b'Thermal diffusivityLaser assisted atom probe tomography provides direct evidence of measurement in nano- defects affecting thermal properties in ceramic fuels at nanoscale. crystalline oxides usingF actors affecting thermal diffusivity in ceramics include stoichiometric variation caused by nuclear fission and structural defects such as fission gas laser assisted atombubbles, defect clusters, solute segregations, and grain boundary interfaces. probe tomography Features like defects are nanosized, and measuring properties like thermal diffusivity at nanoscale is challenging with conventional lased based techniques. In this work, we developed a thermal pulse model for laser assisted atom probe tomography to indirectly measure thermal diffusivity from a nanosized tip (100 nanometers). Atom probe tomography analysis was performed on pristine and proton irradiated thorium oxide to study their field evaporation behavior. Systematic variation in laser energy confirmed thermal assisted field evaporation from thorium oxide, which has PROJECT NUMBER:been topic of debate in the past. The peak shape in mass spectra changed for pristine 22P1071-028FP and irradiated thorium oxide as function of laser energy, showing clear presence of TOTAL APPROVED AMOUNT:field evaporation behavior. The peak shape difference was fitted with an analytical $125,000 over 1 year model that considers laser-tip interaction and material properties. Thermal pulse model showed large variation in thermal diffusivity for a pristine sample, whereas PRINCIPAL INVESTIGATOR:material with defects was fitted with constant thermal diffusivity for different Mukesh Bachhav temperature of the tip. These results were independently verified with literature CO-INVESTIGATORS: data using conventional methods at bulk scale. These results are the first-of-a-kind Kaustubh Bawane, INL to determine thermal properties from irradiated fuels at nanoscale-based laser-tip Krzysztof Gofryk, INLinteractions in atom probe tomography and provide a framework on how defects Rahul Kancharla Reddy, INL affect thermal properties of ceramic materials. Tsvetoslav Pavlov, INL(a) Reciprocal lattice points elongated in one direction to rodsrel-rodimages of atom probe needles showing the presence of interstitial Frank loops (circled in red) in proton irradiated (0.1 dpa, 600C) thorium oxide (ThO 2 ) single crystals. Thermal response of ThO2+ ions in atom probe mass spectrum subjected to different laser energy from (b) pristine and (c) proton irradiated 0.1 dpa at 600C ThO 2 . 142'