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ABSTRACT The behavior of the fission products, as they are released from fission events during nuclear reaction, plays an important role in nuclear fuel performance. Fission product release can occur through grain boundary (GB) at low burnups; therefore, this study simulates the mass transport of fission gases in a 2-D GB network to look into the effects of GB characteristics on this phenomenon, with emphasis on conditions that can lead to percolation. A finite element model was created based on the microstructure of a depleted UO2 sample characterized by Electron Backscattering Diffraction (EBSD). The GBs were categorized into high (D2), low (D1) and bulk diffusivity (Dbulk) based on their misorientation angles and Coincident Site Lattice (CSL) types. The simulation was run using different diffusivity ratios (D2/Dbulk) ranging from 1 to 10^8. The model was set up in three ways: constant temperature case, temperature gradient effects and window methods that mimic the environments in a Light Water Reactor (LWR). In general, the formation of percolation paths was observed at a ratio higher than 10^4 in the measured GB network, which had a 68% fraction of high diffusivity GBs. The presence of temperature gradient created an uneven concentration distribution and decreased the overall mass flux. Finally, radial temperature and fission gas concentration profiles were obtained for a fuel pellet in operation using an approximate 1-D model. The 100 µm long microstructurally explicit model was used to simulate, to the scale of a real UO2 pellet, the mass transport at different radial positions, with boundary conditions obtained from the profiles. Stronger percolation effects were observed at the intermediate and periphery position of the pellet. The results also showed that highest mass flux happens at the edge of a pellet at steady state to accommodate for the sharp concentration drop.
ContributorsLim, Harn Chyi (Author) / Peralta, Pedro (Thesis advisor) / Dey, Sandwip (Committee member) / Sieradzki, Karl (Committee member) / Arizona State University (Publisher)
Created2011
Description
Engineered nanoporous substrates made using materials such as silicon nitride or silica have been demonstrated to work as particle counters or as hosts for nano-lipid bilayer membrane formation. These mechanically fabricated porous structures have thicknesses of several hundred nanometers up to several micrometers to ensure mechanical stability of the membrane. However, it is desirable to have a three-dimensional structure to ensure increased mechanical stability. In this study, circular silica shells used from Coscinodiscus wailesii, a species of diatoms (unicellular marine algae) were immobilized on a silicon chip with a micrometer-sized aperture using a UV curable polyurethane adhesive. The current conducted by a single nanopore of 40 nm diameter and 50 nm length, during the translocation of a 27 nm polystyrene sphere was simulated using COMSOL multiphysics and tested experimentally. The current conducted by a single 40 nm diameter nanopore of the diatom shell during the translocation of a 27 nm polystyrene sphere was simulated using COMSOL Multiphysics (28.36 pA) and was compared to the experimental measurement (28.69 pA) and Coulter Counting theory (29.95 pA).In addition, a mobility of 1.11 x 10-8 m2s-1V-1 for the 27 nm polystyrene spheres was used to convert the simulated current from spatial dependence to time dependence.
To achieve a sensing diameter of 1-2 nanometers, the diatom shells were used as substrates to perform ion-channel reconstitution experiments. The immobilized diatom shell was functionalized using silane chemistry and lipid bilayer membranes were formed. Functionalization of the diatom shell surface improves bilayer formation probability from 1 out of 10 to 10 out of 10 as monitored by impedance spectroscopy. Self-insertion of outer membrane protein OmpF of E.Coli into the lipid membranes could be confirmed using single channel recordings, indicating that nano-BLMs had formed which allow for fully functional porin activity. The results indicate that biogenic silica nanoporous substrates can be simulated using a simplified two dimensional geometry to predict the current when a nanoparticle translocates through a single aperture. With their tiered three-dimensional structure, diatom shells can be used in to form nano-lipid bilayer membranes and can be used in ion-channel reconstitution experiments similar to synthetic nanoporous membranes.
To achieve a sensing diameter of 1-2 nanometers, the diatom shells were used as substrates to perform ion-channel reconstitution experiments. The immobilized diatom shell was functionalized using silane chemistry and lipid bilayer membranes were formed. Functionalization of the diatom shell surface improves bilayer formation probability from 1 out of 10 to 10 out of 10 as monitored by impedance spectroscopy. Self-insertion of outer membrane protein OmpF of E.Coli into the lipid membranes could be confirmed using single channel recordings, indicating that nano-BLMs had formed which allow for fully functional porin activity. The results indicate that biogenic silica nanoporous substrates can be simulated using a simplified two dimensional geometry to predict the current when a nanoparticle translocates through a single aperture. With their tiered three-dimensional structure, diatom shells can be used in to form nano-lipid bilayer membranes and can be used in ion-channel reconstitution experiments similar to synthetic nanoporous membranes.
ContributorsRamakrishnan, Shankar (Author) / Goryll, Michael (Thesis advisor) / Blain Christen, Jennifer (Committee member) / Dey, Sandwip (Committee member) / Thornton, Trevor (Committee member) / Arizona State University (Publisher)
Created2015