Research Terms
Mechanical Engineering Aerospace Engineering
This uranium dioxide composite fuel pellet, which boasts improved thermal conductivity, will extend the life of UO2 fuel within nuclear reactors to save money and enhance safety. Radioactive crystalline powder uranium dioxide (UO2) is a naturally occurring substance and the most commonly used material in nuclear reactors' fuel rods. UO2 has a high melting point, good high-temperature stability, chemical compatibility with cladding and coolant, and resistance to radiation. Its main disadvantage is its low thermal conductivity, which can cause large temperature gradients in UO2 fuel pellets. These temperature gradients produce thermal stresses that can lead to extensive fuel pellet cracking, an increased rate of fission gas release, more fission gas bubbles and fuel pellet swelling - issues that limit UO2 fuel's life within reactors. Using conventional pellet processing techniques, it is not possible to obtain high density UO2/SiC composite pellets. Researchers at the University of Florida have employed unique spark plasma sintering to develop an inexpensive high-density urania/SiC composite with enhanced thermal conductivity. This cooler-running nuclear fuel pellet will increase reactor output, save money and enhance safety.
Uranium dioxide composite fuel pellet with high density and thermal conductivity for nuclear reactors
University of Florida researchers have developed a high-density nuclear fuel pellet with increased conductivity and a technology that rapidly sinters powder to create the pellet. The researchers' spark plasma sintering (SPS) technique, essential for manufacturing the fuel pellet, uses significantly lower processing temperatures and much shorter processing times to combine a highly conductive material with uranium dioxide (UO2). The result is a dense pellet with excellent thermal conductivity. Examples of thermally conductive starting materials include silicon carbide (SiC) and diamond.
This compact stress waveguide enables the delivery of long-duration impact pulses or transfer stress signals in a shorter footprint. Additionally, the waveguide is applicable for designing high-frequency fatigue testing equipment, where large displacements are needed at one end while applying long-duration stress at the other. It can also test equipment for obtaining the stress-strain response of materials in the low-to-intermediate strain-rate range. Strain rate refers to the deformation a material accumulates over a specific time interval. Knowledge of the strain-rate dependent stress-strain response of materials is a fundamental requirement to assess the suitability of materials in many impact applications.
Quasistatic or low-strain deformation tests are often needed to assess the suitability of materials in many engineering applications. However, numerous applications, such as crashworthiness of vehicles, impact, ballistics, and high-speed machining, require the high strain-rate response of materials. The split Hopkinson pressure bar (SHPB) is conventional equipment for testing materials in the intermediate to the high strain-rate range but not suitable to test materials in the low-to-intermediate strain-rate ranges. It features longer rods to accommodate longer stress wave durations, translating into lower strain rates, thus fulfilling this drawback. Unfortunately, manufacturing and housing the apparatus is space and cost-prohibitive, requiring several tens of meters of length in floor space and tens of thousands of dollars. Similarly, drilling and piling equipment, and fatigue testing machines are also currently designed with long rods to deliver impact or high-frequency stress pulses.
Researchers at the University of Florida have developed the Millipede Bar, a compact stress waveguide with a folded-bar design, enabling longer-duration stress waves to transfer undisturbed or obtain a stress-strain response in materials at lower strain rates.
Compact stress waveguide for the propagation of long-duration impact pulses and for testing the stress-strain response of materials in the low-to-intermediate strain-rate range
The Millipede waveguide is a folded-bar design, allowing a longitudinal stress wave to “flow” through a 180-degree bend junction if the stress wave duration is sufficiently high. The folded-bar design acts as a compact stress waveguide for use in various drilling tools and testing equipment. For example, it is applicable in piling or drilling equipment to reduce the length of the tool without losing the ability to deliver a long-duration impact stress pulse. Similarly, it can be used in Hopkinson Bar for testing materials in the low to intermediate strain-rate ranges. Traditional split Hopkinson pressure bars use 1-dimensional stress wave propagation principles to determine the stress-strain response of materials in the intermediate range of strain rates (10^2/s-10^4/s). Achieving longer-duration stress waves and lower strain rate requires longer rods. The Millipede Hopkinson Bar features a folded-bar design, including any number of folds to accommodate longer-duration stress waves. By enabling the generation of long stress waves, the apparatus makes achieving strain rates in the 10^1/s-10^2/s range possible. Additionally, each folded bar maintains the length-to-diameter ratio of at least 20 to obtain 1-dimensional stress wave propagation characteristics.
