Conference: Proposed for presentation at the 9th United States National Congress on Computational... more Conference: Proposed for presentation at the 9th United States National Congress on Computational Mechanics (USNCCM IX) held July 23-26, 2007 in San Francisco, CA.
ABSTRACT A discrete element method (DEM) representation of coupled solid mechanics, fracturing an... more ABSTRACT A discrete element method (DEM) representation of coupled solid mechanics, fracturing and heat conduction processes is developed and applied to explicitly simulate the random initiation and subsequent propagation of interacting thermal cracks in a ceramic nuclear fuel pellet during initial rise to power and during power cycles. The DEM model clearly predicts realistic early-life crack patterns including both radial and circumferential cracks. Simulation results clearly demonstrate the formation of radial cracks during the initial power rise and formation of circumferential cracks as the power is ramped down. In these simulations, additional early-life power cycles do not lead to the formation of new thermal cracks. They do, however clearly indicate changes in the apertures of thermal cracks during later power cycles due to thermal expansion and shrinkage. The number of radial cracks increases with increasing power, which is consistent with the experimental observations.
Experimental observations relative to both in-reactor irradiation and post-irradiation annealing ... more Experimental observations relative to both in-reactor irradiation and post-irradiation annealing of oxide nuclear fuel indicate that substantial fission gas release can occur on a small time scale during temperature transients (burst release). The rapid kinetics of the process cannot be interpreted as purely diffusion-controlled. Micrographs demonstrate the presence of patterns of grain-face separations (micro-cracks) in transient tested fuel, thus indicating micro-cracking as the basic mechanism of burst release. In this work, a new model for transient fission gas behaviour in oxide fuel is developed. The treatment extends a previously developed model for diffusion-controlled fission gas release to introduce the effect of micro-cracking, which is interpreted as a reduction of the grain-face gas inventory and storing capacity during transients. The process is characterized through an empirical temperature-dependent function, based on the experimentally observed characteristics of ga...
ABSTRACT Multiscale analysis aims to improve predictions at the macro-scale by utilizing high-fid... more ABSTRACT Multiscale analysis aims to improve predictions at the macro-scale by utilizing high-fidelity models running at a lower length-scale. The lower length-scale models provide a detailed view of the material behavior that is used to determine the average material response to be used at the macro-scale. This approach is especially useful in the nuclear field, since irradiation experiments are difficult and expensive to conduct. The lower length-scale models complement the experiments, reducing the total number of experiments that are needed. Multiscale modeling is a critical part of the BISON-MARMOT fuel performance codes being developed at Idaho National Laboratory (Williamson et al. (2012), Tonks et al. (2010)). One critical aspect of multiscale modeling is the ability to extract the relevant information from the lower length-scale simulations. One approach, the asymptotic expansion homogenization (AEH) technique (Guedes et al. (1990), Ghosh et al. (1995), Hassani and Hinton (1998), Laschet and Apel (2010), and Oliviera et al. (2011)), has proven to be an effective method for determining homogenized material parameters. The AEH technique prescribes a system of equations to solve at the micro-scale that are used to compute homogenized material constants for use at the macro-scale. In this work, we employ AEH to explore the effect of evolving microstructural thermal conductivity on nuclear fuel performance. This builds on the work of Tonks et al. (2010) and Williamson et al. (2012), where a direct calculation was used to obtain average thermal conductivity. We show that the AEH approach fits cleanly into the BISON and MARMOT codes and provides a natural, multidimensional homogenization capability.
Conference: Proposed for presentation at the 9th United States National Congress on Computational... more Conference: Proposed for presentation at the 9th United States National Congress on Computational Mechanics (USNCCM IX) held July 23-26, 2007 in San Francisco, CA.
Conference: Proposed for presentation at the 9th United States National Congress on Computational... more Conference: Proposed for presentation at the 9th United States National Congress on Computational Mechanics (USNCCM IX) held July 23-26, 2007 in San Francisco, CA.
ABSTRACT A discrete element method (DEM) representation of coupled solid mechanics, fracturing an... more ABSTRACT A discrete element method (DEM) representation of coupled solid mechanics, fracturing and heat conduction processes is developed and applied to explicitly simulate the random initiation and subsequent propagation of interacting thermal cracks in a ceramic nuclear fuel pellet during initial rise to power and during power cycles. The DEM model clearly predicts realistic early-life crack patterns including both radial and circumferential cracks. Simulation results clearly demonstrate the formation of radial cracks during the initial power rise and formation of circumferential cracks as the power is ramped down. In these simulations, additional early-life power cycles do not lead to the formation of new thermal cracks. They do, however clearly indicate changes in the apertures of thermal cracks during later power cycles due to thermal expansion and shrinkage. The number of radial cracks increases with increasing power, which is consistent with the experimental observations.
Experimental observations relative to both in-reactor irradiation and post-irradiation annealing ... more Experimental observations relative to both in-reactor irradiation and post-irradiation annealing of oxide nuclear fuel indicate that substantial fission gas release can occur on a small time scale during temperature transients (burst release). The rapid kinetics of the process cannot be interpreted as purely diffusion-controlled. Micrographs demonstrate the presence of patterns of grain-face separations (micro-cracks) in transient tested fuel, thus indicating micro-cracking as the basic mechanism of burst release. In this work, a new model for transient fission gas behaviour in oxide fuel is developed. The treatment extends a previously developed model for diffusion-controlled fission gas release to introduce the effect of micro-cracking, which is interpreted as a reduction of the grain-face gas inventory and storing capacity during transients. The process is characterized through an empirical temperature-dependent function, based on the experimentally observed characteristics of ga...
ABSTRACT Multiscale analysis aims to improve predictions at the macro-scale by utilizing high-fid... more ABSTRACT Multiscale analysis aims to improve predictions at the macro-scale by utilizing high-fidelity models running at a lower length-scale. The lower length-scale models provide a detailed view of the material behavior that is used to determine the average material response to be used at the macro-scale. This approach is especially useful in the nuclear field, since irradiation experiments are difficult and expensive to conduct. The lower length-scale models complement the experiments, reducing the total number of experiments that are needed. Multiscale modeling is a critical part of the BISON-MARMOT fuel performance codes being developed at Idaho National Laboratory (Williamson et al. (2012), Tonks et al. (2010)). One critical aspect of multiscale modeling is the ability to extract the relevant information from the lower length-scale simulations. One approach, the asymptotic expansion homogenization (AEH) technique (Guedes et al. (1990), Ghosh et al. (1995), Hassani and Hinton (1998), Laschet and Apel (2010), and Oliviera et al. (2011)), has proven to be an effective method for determining homogenized material parameters. The AEH technique prescribes a system of equations to solve at the micro-scale that are used to compute homogenized material constants for use at the macro-scale. In this work, we employ AEH to explore the effect of evolving microstructural thermal conductivity on nuclear fuel performance. This builds on the work of Tonks et al. (2010) and Williamson et al. (2012), where a direct calculation was used to obtain average thermal conductivity. We show that the AEH approach fits cleanly into the BISON and MARMOT codes and provides a natural, multidimensional homogenization capability.
Conference: Proposed for presentation at the 9th United States National Congress on Computational... more Conference: Proposed for presentation at the 9th United States National Congress on Computational Mechanics (USNCCM IX) held July 23-26, 2007 in San Francisco, CA.
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