Characterization of Uranium Metal Alloy Fuel Forms for Advanced Nuclear Reactor Applications

An engineering investigation of the fabrication process for low smear density uranium alloy nuclear fuel forms was completed and the resulting alloys were characterized. The metal fuel alloys investigated in this project were uranium metal, uranium – 5 wt% zirconium (U-5Zr), and uranium – 10 wt% zirconium (U-10Zr). The physical fuel forms of interest included 1) dense, solid rods that could be used be used in low smear density nuclear fuel applications, 2) extruded tubes designed to begin fuel service in constant contact with cladding yet having a low smear density, and 3) low density pellets designed to simulate low density fuel structures. Extrusion was shown to be an industrially viable option for the production of uranium and U-10Zr rods and tubes. Extrusion experiments were performed in the uranium alpha phase in the temperatures range of 550 to 650°C. The reduction ratio was varied between 4 and 15. The extrusion constant, K, was empirically estimated from the data collected and ranged from 45 to 160 ksi over the course of this study. Larger extrusion ratios as well as higher temperatures resulted in lower extrusion constants. The microstructures of the extruded products were studied using scanning electron microscopy and neutron diffraction. The specimens exhibited elongated grains in the extrusion direction. The U-10Zr also displayed aligned and elongated α zirconium grains. Neutron diffraction of the U-10Zr product revealed α uranium texture in the (100) and (110) direction as well as δ-UZr2 texture in the (0001) direction. The δ-UZr2 texture was found to persist through high temperature heat treatment, but the α uranium texture did not. To examine low density porous alloys, U, U-5Zr, and U-10Zr powders were fabricated with a range of densities from ~45 to ~89 %TD. The resulting alloy microstructures were characterized via scanning electron microscopy. The structures of the pellets were typically homogeneous. The microstructures created were similar to the microstructures of fuel that has undergone irradiation swelling. The porosity-dependent thermal diffusivity of the alloys was measured in the temperatures range of 20 to 300ºC. As sample porosity increased, the diffusivity was observed to decrease linearly over the entire sample set. Diffusivity values for uranium ranged from 10.7 mm^2/s at 88.6 %TD and 300ºC to 2.54 mm^2/s at 46.6 %TD and 30ºC. The empirical relationship between the thermal conductivity and porosity corresponded well with the predicted values from the effective medium theory structural model for modelling the thermal conductivity of two phase materials

Characterization of Uranium Metal Alloy Fuel Forms for Advanced Nuclear Reactor Applications

An engineering investigation of the fabrication process for low smear density uranium alloy nuclear fuel forms was completed and the resulting alloys were characterized. The metal fuel alloys investigated in this project were uranium metal, uranium – 5 wt% zirconium (U-5Zr), and uranium – 10 wt% zirconium (U-10Zr). The physical fuel forms of interest included 1) dense, solid rods that could be used be used in low smear density nuclear fuel applications, 2) extruded tubes designed to begin fuel service in constant contact with cladding yet having a low smear density, and 3) low density pellets designed to simulate low density fuel structures. Extrusion was shown to be an industrially viable option for the production of uranium and U-10Zr rods and tubes. Extrusion experiments were performed in the uranium alpha phase in the temperatures range of 550 to 650°C. The reduction ratio was varied between 4 and 15. The extrusion constant, K, was empirically estimated from the data collected and ranged from 45 to 160 ksi over the course of this study. Larger extrusion ratios as well as higher temperatures resulted in lower extrusion constants. The microstructures of the extruded products were studied using scanning electron microscopy and neutron diffraction. The specimens exhibited elongated grains in the extrusion direction. The U-10Zr also displayed aligned and elongated α zirconium grains. Neutron diffraction of the U-10Zr product revealed α uranium texture in the (100) and (110) direction as well as δ-UZr2 texture in the (0001) direction. The δ-UZr2 texture was found to persist through high temperature heat treatment, but the α uranium texture did not. To examine low density porous alloys, U, U-5Zr, and U-10Zr powders were fabricated with a range of densities from ~45 to ~89 %TD. The resulting alloy microstructures were characterized via scanning electron microscopy. The structures of the pellets were typically homogeneous. The microstructures created were similar to the microstructures of fuel that has undergone irradiation swelling. The porosity-dependent thermal diffusivity of the alloys was measured in the temperatures range of 20 to 300ºC. As sample porosity increased, the diffusivity was observed to decrease linearly over the entire sample set. Diffusivity values for uranium ranged from 10.7 mm^2/s at 88.6 %TD and 300ºC to 2.54 mm^2/s at 46.6 %TD and 30ºC. The empirical relationship between the thermal conductivity and porosity corresponded well with the predicted values from the effective medium theory structural model for modelling the thermal conductivity of two phase materials

A GIS-based method for archival and visualization of microstructural data from drill core samples.

Core samples obtained from scientific drilling could provide large volumes of direct microstructural and compositional data, but generating results via the traditional treatment of such data is often time-consuming and inefficient. Unifying microstructural data within a spatially referenced Geographic Information System (GIS) environment provides an opportunity to readily locate, visualize, correlate, and explore the available microstructural data. Using 26 core billet samples from the San Andreas Fault Observatory at Depth (SAFOD), this study developed procedures for: 1. A GIS-based approach for spatially referenced visualization and storage of microstructural data from drill core billet samples; and 2. Producing 3D models of sample billets and thin section positions within each billet, which serve as a digital record after irreversible material loss and fragmentation of physical billets. This approach permits spatial registration of 2D thin section ‘base maps’ within the core sample billets, where each billet is represented by 3D solid surface (produced via SFM photogrammetry) and internal structure models (acquired with micro-CT scans) created prior to sectioning. The spatial positions of the base maps were established within locally defined coordinate systems in each core billet’s solid surface model. The GIS database structure provided interactive linkage to the results of various analyses performed throughout the map at a wide range of scales (e.g. SEM and CL images as well as text and numerical data) within each thin section. The viability of the proposed framework was demonstrated via display of integrated microstructural data, creation of vector point information associated with features of interest in CL imagery, and development of a model for extraction and unsupervised classification of a multi-generation calcite vein network from the CL imagery. The results indicate that a GIS can facilitate the spatial treatment of 2D and 3D data even at centimeter to nanometer scales, building upon existing work which is predominantly limited to the 2D space of single thin sections. Conversely, the research effort also revealed several challenges, particularly involving intensive 3D representations and complex matrix transformations required to create geographically translated forms of the within-billet coordinate systems, which are suggested for consideration in future studies

Advanced damage modelling of free machining steels

The current available damage models do not accurately predict effective plastic strain to failure in low triaxiality stress states. A damage model was developed for low triaxiality that is appropriate to hot rolling of steel. This work focuses on nucleation and growth of damage as well as the effect of the strain and stress path. The latter is especially important for the rolling of bar and other complex cross-section products. A study of damage mechanisms and methods to model them has been undertaken. It is pointed out that the many models are only useful under certain conditions but can be used when the expected damage mechanisms are active. Several test types were evaluated to assess their ability to simulate stress state in rolling. A program has been written to evaluate the stress state for plane and axisymmetric tests, which allows one to choose the most appropriate test-piece geometry. A test has been designed and implemented. Thermal and mechanical data was gathered, which has been used to relate the stress triaxiality to damage growth and identify appropriate damage growth models. The size and spacing distributions of inclusions in free cutting steels have been measured. The different distributions have an effect on the ductility of the different steels. This effect has been found to change at different strain rates and temperatures. By better accounting for the effect of inclusions on damage growth under a range of test conditions, the damage model can be significantly improved. Free cutting steels that contained different additions of heavy metals were tested. The ductility and damage mechanisms were compared in each of the steels. The effect of the precipitation of the different heavy metals at the inclusion to matrix boundary was highlighted. The same damage mechanisms were observed in each steel but the ability to accommodate damage varied between the steels. Ex-situ synchrotron x-ray micro-tomography was used to better measure and quantify the distribution of inclusions and damage evolution in a free cutting steel. Localised damage coalescence away from the centre of the uniaxial tensile test-piece was attributed to the effect of inclusion clustering. This research was used to develop a realistic damage model, which can predict damage growth and coalescence for a range of forming parameters and different stress-state conditions related to hot rolling applications. The micro-mechanics based model includes the effects of inclusion distribution on damage. The model is calibrated using twenty six temperature based material constants