NSUF 16-618: Nano-Indentation Investigations on Nano-grained UO2 with and without High-Energy Ion Irradiation
Grain boundary affects a series of key material properties, including radiation tolerance and mechanical performance. On one hand, grain boundaries in nuclear fuel materials help relieve radiation-induced damage and postpone fission gas release. On the other hand, the grain boundaries pin dislocations and meanwhile originate inter-granular cracks. Therefore, grain size influences some key factors that determine fuel performance. Nano-grained UO2 is supposed to provide exceptional radiation tolerance and survive high burnup compared with conventional UO2 fuels. Meanwhile, the nanoscale grain size also alters the mechanical properties. These mechanical properties, especially the fracture toughness, are crucial for fuel performance. Nano-grained UO2 also has different responses to irradiation. Hence, a comprehensive comparison of mechanical properties before and after irradiation between nano-grained and micro-grained UO2 samples is essential for confirming the feasibility of using nano-grained UO2 as a fuel material. In addition, the fracture behavior of UO2 with various grain sizes and irradiation doses is an important phenomenon for fuel performance codes. As an indispensable effort within the Nuclear Energy Advanced Modeling and Simulation (NEAMS) program, the development of MARMOT requires experimental references on the fuel cracking of UO2 fuels as a function of grain size and irradiation dose. Thus, the success of this proposed study will effectively support the validation of MARMOT and the evaluation of nanao-grained UO2 as a fuel material. To realize that, four UO2 specimens with various grain sizes (50 nm, 500 nm, 5 micron, and 50 micron) were sintered from powders. The samples were vibration polished to 0.02 micron and then irradiated by 80 MeV Xe ions at 200°C to 7.9×1016 ions/cm2 at the Argonne Tandem Linac Accelerator System (ATLAS). The selected irradiation conditions simulate the behavior of a major fission fragment. The ultra-high energy ions create a large zone (~4 micron) of flat damage profile and free of Xe implantation. This region is sufficient for accurate low-load nano-indentation. For irradiated UO2, only low-load mode (depth<400 nm) will be used to limit the measurement within the flat damage profile region. For those control UO2, both low- and high-load modes will be adopted for comparison and then connecting the existing results from macroscopic measurement. Also, as the indenter is larger than the grains, the mechanical properties of the nano-grained specimens measured by nano-indentation are isotropic. However, for those micro-grained specimens, the grain is much larger than the indenter. Thus, the measured values are sensitive to the lattice orientation. To quantify this anisotropic phenomenon, the orientation of the indented grains will be determined by electron backscattering diffraction (EBSD). The elastic moduli and nano-hardness will be obtained by analyzing the load-displacement curves. More importantly, the fracture toughness will be deduced from measuring the length of cracks. Thus, the mechanical properties of UO2 samples with various grain sizes and irradiation conditions can be obtained for validating nano-grained UO2 as a fuel material and supporting the development of MARMOT. Nano-indentation takes eight(8) days, 1 day per specimen; SEM-EBSD takes two(2) days. The total time is within the RTE proposal limitation.
Additional Info
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| Abstract | Grain boundary affects a series of key material properties, including radiation tolerance and mechanical performance. On one hand, grain boundaries in nuclear fuel materials help relieve radiation-induced damage and postpone fission gas release. On the other hand, the grain boundaries pin dislocations and meanwhile originate inter-granular cracks. Therefore, grain size influences some key factors that determine fuel performance. Nano-grained UO2 is supposed to provide exceptional radiation tolerance and survive high burnup compared with conventional UO2 fuels. Meanwhile, the nanoscale grain size also alters the mechanical properties. These mechanical properties, especially the fracture toughness, are crucial for fuel performance. Nano-grained UO2 also has different responses to irradiation. Hence, a comprehensive comparison of mechanical properties before and after irradiation between nano-grained and micro-grained UO2 samples is essential for confirming the feasibility of using nano-grained UO2 as a fuel material. In addition, the fracture behavior of UO2 with various grain sizes and irradiation doses is an important phenomenon for fuel performance codes. As an indispensable effort within the Nuclear Energy Advanced Modeling and Simulation (NEAMS) program, the development of MARMOT requires experimental references on the fuel cracking of UO2 fuels as a function of grain size and irradiation dose. Thus, the success of this proposed study will effectively support the validation of MARMOT and the evaluation of nanao-grained UO2 as a fuel material. To realize that, four UO2 specimens with various grain sizes (50 nm, 500 nm, 5 micron, and 50 micron) were sintered from powders. The samples were vibration polished to 0.02 micron and then irradiated by 80 MeV Xe ions at 200°C to 7.9×1016 ions/cm2 at the Argonne Tandem Linac Accelerator System (ATLAS). The selected irradiation conditions simulate the behavior of a major fission fragment. The ultra-high energy ions create a large zone (~4 micron) of flat damage profile and free of Xe implantation. This region is sufficient for accurate low-load nano-indentation. For irradiated UO2, only low-load mode (depth<400 nm) will be used to limit the measurement within the flat damage profile region. For those control UO2, both low- and high-load modes will be adopted for comparison and then connecting the existing results from macroscopic measurement. Also, as the indenter is larger than the grains, the mechanical properties of the nano-grained specimens measured by nano-indentation are isotropic. However, for those micro-grained specimens, the grain is much larger than the indenter. Thus, the measured values are sensitive to the lattice orientation. To quantify this anisotropic phenomenon, the orientation of the indented grains will be determined by electron backscattering diffraction (EBSD). The elastic moduli and nano-hardness will be obtained by analyzing the load-displacement curves. More importantly, the fracture toughness will be deduced from measuring the length of cracks. Thus, the mechanical properties of UO2 samples with various grain sizes and irradiation conditions can be obtained for validating nano-grained UO2 as a fuel material and supporting the development of MARMOT. Nano-indentation takes eight(8) days, 1 day per specimen; SEM-EBSD takes two(2) days. The total time is within the RTE proposal limitation. |
| Award Announced Date | 2015-12-16T00:00:00 |
| Awarded Institution | None |
| Facility | None |
| Facility Tech Lead | Yaqiao Wu |
| Irradiation Facility | None |
| PI | Yinbin Miao |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | 618 |