b'Probing Microstructure-inducedAdvancing fundamental understandings of the role of radiation-induced Swelling and Thermal Propertydamage on the thermal transport and mechanical properties of nuclear fuels supports the efficient, reliable, and safe operation of existing and future nuclear Changes in Ion-irradiated Oxidepower plants.Fuels using Laser-generatedL aser-based characterization methods have emerged as promising non-Surface Acoustic Waves contact, non-destructive tools to measure the evolution of microstructure-induced material property changes. Moreover, laser methods offer the promise of in situ characterization while the material is being irradiated. Although recent advances have been made in laser-based property evaluation of irradiated metallic material, little effort has been dedicated to corresponding measurements in non-metallic materials that include semiconductors and ceramics. This research TOTAL APPROVED AMOUNT:improved the understanding of radiation-induced changes to material microstructure $210,000 over 3 years and on the thermal and elastic properties of ceramic nuclear fuels. Low-dose ion irradiation was used to seed specific defect types in the microstructure of thorium PROJECT NUMBER:oxide (ThO2) fuel samples to simulate and systematically study the effects of fission 19P43-012 fragment damage and lattice swelling on fuel material performance. Transient PRINCIPAL INVESTIGATOR:grating spectroscopy (TGS) was used to simultaneously measure thermal diffusivity Amey Khanolkar and elastic properties at the micrometer length-scale in pristine (unirradiated) and ion-irradiated thorium oxide. TGS uses two crossed laser beams derived from a CO-INVESTIGATORS: pulsed nanosecond laser to create a spatially sinusoidal temperature profile (thermal David Hurley, INL grating) in the sample. Rapid thermal expansion of the sample in the peaks of Zilong Hua, INL this thermal grating launches counter-propagating surface acoustic waves with COLLABORATORS: wavelength defined by the thermal grating spacing. The dynamics of the thermal Air Force Research Laboratory and acoustic response are monitored via the time-dependent diffraction of a Texas A&M University continuous wave laser with high beam quality.The Ohio State University Results from this study demonstrate the sensitivity of the laser-generated surface acoustic waves in detecting radiation-induced elastic and thermal property changes in ceramic energy materials and provide insight into the role of early-stage lattice defects (predominantly point defects) on phonon-mediated thermal transport and elastic properties. This research provides foundational work to validate the influence of atomic-level lattice defects in fuel performance codes. Future studies can use the TGS technique to measure the phonon mean free path at low temperatures. The multi-modal nature of the laser-based TGS technique coupled with the experimental methodology being developed in this project can accelerate new nuclear fuel materials qualification for advanced reactor concepts by reducing the number of iterative measurement steps. In addition to studying radiation effects of thermal and elastic properties of ceramic fuels, the experimental methodology can be applied to intra-granular measurements in complex metal alloys, as well as investigating spatially-confined temperature- and pressure-dependent microstructural phenomena in material systems of interest to the broader condensed matter physics and materials science communities beyond the realm of nuclear fuels.60'