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2/27/2007 4:57 PM
Blogs (Monday PM session)
1) Role of Grain Boundary Engineering on Creep behavior of F-M alloy T91.
F-M alloys such as T91 are proposed for applications in high temperature nuclear reactor core materials, where creep strength becomes an issue. Grain boundary engineering has been adopted as a mitigation technique to improve creep properties. GBE results in an increase in the number of subgrains w/o changing any other microstructural parameter. Subgrain boundaries induce long-range internal stresses which reduce the effective stress and thus lead to reduced creep rates. Internal stresses due to subgrain boundaries depend on the effective obstacle distance (distance between the lath and subgrain boundaries). Introduction of more subgrains in the modified condition reduces the effective obstacle distance. Calculation of internal stress this way definitely makes the problem simple and actually the barrier strength term on which internal stress depends is dynamic and depends on both the shape and size of the obstacle.
2) An Analytical Model for Amoeba Effect in UO2 Fuel Pellets.
Significant temperature gradients are present across the fuel particles when a nuclear reactor is producing power. Fuel particles can migrate to high temperature regions leading to complete failure of the coating due to damage to the barrier retaining the fission product. This is termed as the Amoeba effect. For mathematical simplicity a cylindrical shaped Kernel is assumed to treat the flow behavior of CO gas molecules through the buffer layer packed with pyrolytic carbon atoms. This effect is evaluated in terms of an interactive transport phenomenon between solid-state diffusion of oxygen ions and flow of CO gas. Concentration gradient of oxygen ion also causes the Amoeba effect.
3) Effect of Microstructural Heterogeneity on Stress Variability in ZrN Pellets.
Manual Parra Garcia
ZrN fuel pellets are important for next generation nuclear energy systems. Effect of porosity and local density of these pellets was studied by evaluating the microstructure. 2-D models have been formulated to simulate the effects of sintering on overall mechanical response. 2-D models account for grain, pellet size, porosity and density, as well as orientation of individual grains. Difference in sintering temperature also had its effects. Reasons for which ZrN was chosen were also evaluated. Effective modulus was calculated for samples of different density. 2-D models of high density ZrN pellets were constructed using FEM.
4) In-situ Neutron Diffraction Measurements of Intergranular Strains in a Zirc-Alloy.
Advantages of neutron experiments for strain measurements were introduced. Neutron diffraction measurement for internal strains is based on Braggs law signals from grains. Axial and transverse strain measurements were taken. Microscopic insights of macroscopic values were obtained. Fundamental understanding of elastic-plastic deformation of Zr alloy under applied stress was studied. Localized stress-distribution around a crack tip in the plastic zone was also discussed and evaluated. An increase in elastic lattice strains near the crack tip was observed with increase in applied stress.
5) The Mechanism of Zr and Hf in Reducing Radiation Induced Segregation in 316 Stainless Stress.
Role of radiation-induced segregation in causing IASCC was discussed. Theory of RIS was introduced and how over-size solute additions can lead to a reduction in RIS was also discussed. TEM results were compared to that with the MIK model and it was observed that the effect on segregation was more due to the binding energy as compared to the size effects. Modeling results were based on Ab-Initio calculations. Radiations were performed using 3.2MeV energy protons at high temperature.
6) Corrosion Behavior of Alloy 617 in Impure He Environment
Reactive impurities in coolant He in very high temperature reactor can cause corrosion (oxidation, carburization and decarburization) depending on their ratios. A new system built at University of Michigan evaluated this behavior at high temperature for long periods of time by varying the ratio of various impurities. Weight-gain measurements, microstructural characterization (EDS, SEM) were used to evaluate the results. A power law was found to be applicable for weight gain results, though a linear relation was observed for a low carbon monoxide/ carbon dioxide ratio for shorter time periods. This was discussed on the basis of both oxidation and decarburization.
7) Strain Hardening and Plastic Instability in Structural Alloys for Advanced Nuclear Systems.
Role of testing temperature and cold deformation was studied for materials exposed to high temperature radiation and high doses. Role of twinning in deformation processes was studied. High work hardening is related to the stacking fault energy of the material. Bilinear response of hardening and uniform elongation was studied. Role of SFE in promoting cross-slip was also studied. Different deformation mechanisms were found to be applicable to different stages. EBSD was used to examine the competiting mechanism between the mechanical twinning and dislocatio-based planar slip.
8) Synchrotron X-rays for Microstrutural Investigations of Reactor Materials.
X-rays provide a powerful tool for analyses particularly in conditions where long-term material degradation under complex loading systems is important. Properties of yttria dispersoids in ODS steels proposed for next generation reactors were studied. Magnetic properties of FeCr alloys was studied and also results were compared with that from modeling. The significance of the use of magnetic interatomic potential was found to be useful.
10)Stable Nanoclusters in MA/ODS Ferritic Alloys
ODS alloys are proposed for components in next generation reactors due to their excellent mechanical properties (creep, strength). Their microstructural analysis was done with electrode atom probe and spectrum imaging in TEM together with first principle calculations, which were used to validate the results. These alloys are studied as the dispersed components trap helium and defects. They also depict high stablity in exterme conditions.
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