NSUF 21-4275: Defect generation and phase stability in single crystal mixed uranium-thorium actinide oxides
Presently, new nuclear fuel designs are limited by a lack of a basic understanding of the microstructural mechanisms controlling property evolution. Generating a fundamental understanding is challenging when studying ceramic fuels fabricated in traditional commercial processes which result in heterogeneous microstructures with fine grains. Instead, an approach focusing on the study of single crystal fuel oxides and surrogates has demonstrated success in uncovering fundamental unit mechanisms of defect formation and agglomeration. Importantly, single crystals with these isolated defect populations (free of grain boundaries and large impurity concentrations) have proven the ideal platform upon which mechanisms of heat transport and thermal phonon scattering by defects may be studied. In this work, we will apply this framework to study mixed actinides of uranium-thorium dioxide which have been considered for advanced fuel applications. Traditional UO2 alloyed with thorium hopes to impart some of the increased thermal conductivity, higher melting point, chemical stability, and proposed proliferation resistance observed in ThO2. While fundamental data on unit phonon scattering mechanisms in either UO2 or ThO2 is still relatively scarce, no data presently exists for the (U,Th)O2 system with high (or equal) concentrations of both actinides. Importantly, a low-temperature miscibility gap has been computationally identified in these systems, but this miscibility gap is kinetically restricted in the few materials which have been synthesized in the relevant compositional range. Increased cation mobility under irradiation exposure may result in the exsolution of UO2 and ThO2 domains (on the nanoscale), which will dramatically affect the resulting thermal performance of the initially homogeneous solid solution microstructure.
To study both fundamental defect formation, the effects of defects on thermal transport, and the phase stability of mixed (U,Th)O2, we will conduct a series of ion beam irradiations at 600˚C using 2 MeV protons. Conditions and the targeted dose levels - 0.01, 0.1, and 1 dpa - are planned such that direct comparison may be made with single crystals of UO2 and ThO2 which have been irradiated in the past two years. Micro-scale thermal transport measurements using laser photothermal techniques available as part of ongoing programs at the Idaho National Laboratory will be used to directly measure the thermal conductivity of the defect-bearing crystal region following irradiation. Additionally, atom probe tomography will be utilized to interrogate whether nanoscale lamellar exsolution of uranium- and thorium-rich domains occurs under irradiation exposure. This study will provide direct and quantitative relationships between irradiation-induced microstructural changes and thermal transport behavior in (U,Th)O2. Such structure-property relations are key input parameters for mesoscale modeling of radiation effects in oxide fuels. The PIs are in possession of all necessary materials for this work, which will be completed within a nine month window upon award.
Additional Info
| Field | Value |
|---|---|
| Abstract | Presently, new nuclear fuel designs are limited by a lack of a basic understanding of the microstructural mechanisms controlling property evolution. Generating a fundamental understanding is challenging when studying ceramic fuels fabricated in traditional commercial processes which result in heterogeneous microstructures with fine grains. Instead, an approach focusing on the study of single crystal fuel oxides and surrogates has demonstrated success in uncovering fundamental unit mechanisms of defect formation and agglomeration. Importantly, single crystals with these isolated defect populations (free of grain boundaries and large impurity concentrations) have proven the ideal platform upon which mechanisms of heat transport and thermal phonon scattering by defects may be studied. In this work, we will apply this framework to study mixed actinides of uranium-thorium dioxide which have been considered for advanced fuel applications. Traditional UO2 alloyed with thorium hopes to impart some of the increased thermal conductivity, higher melting point, chemical stability, and proposed proliferation resistance observed in ThO2. While fundamental data on unit phonon scattering mechanisms in either UO2 or ThO2 is still relatively scarce, no data presently exists for the (U,Th)O2 system with high (or equal) concentrations of both actinides. Importantly, a low-temperature miscibility gap has been computationally identified in these systems, but this miscibility gap is kinetically restricted in the few materials which have been synthesized in the relevant compositional range. Increased cation mobility under irradiation exposure may result in the exsolution of UO2 and ThO2 domains (on the nanoscale), which will dramatically affect the resulting thermal performance of the initially homogeneous solid solution microstructure. To study both fundamental defect formation, the effects of defects on thermal transport, and the phase stability of mixed (U,Th)O2, we will conduct a series of ion beam irradiations at 600˚C using 2 MeV protons. Conditions and the targeted dose levels - 0.01, 0.1, and 1 dpa - are planned such that direct comparison may be made with single crystals of UO2 and ThO2 which have been irradiated in the past two years. Micro-scale thermal transport measurements using laser photothermal techniques available as part of ongoing programs at the Idaho National Laboratory will be used to directly measure the thermal conductivity of the defect-bearing crystal region following irradiation. Additionally, atom probe tomography will be utilized to interrogate whether nanoscale lamellar exsolution of uranium- and thorium-rich domains occurs under irradiation exposure. This study will provide direct and quantitative relationships between irradiation-induced microstructural changes and thermal transport behavior in (U,Th)O2. Such structure-property relations are key input parameters for mesoscale modeling of radiation effects in oxide fuels. The PIs are in possession of all necessary materials for this work, which will be completed within a nine month window upon award. |
| Award Announced Date | 2021-06-07T16:10:43.027 |
| Awarded Institution | None |
| Facility | None |
| Facility Tech Lead | Kevin Field, Yaqiao Wu |
| Irradiation Facility | Michigan Ion Beam Laboratory |
| PI | Cody Dennett |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | 4275 |