Fuels

 Effect of Processing Conditions on Grain Boundary Character Distribution and Mobility in Oxide Nuclear Fuels

The initial microstructure of an oxide fuel pellet can play a key role in the fuel performance. At low burnups, the transport of fission products has a strong dependence on oxygen content, grain size distribution, porosity and grain boundary (GB) characteristics (crystallography, geometry and topology), all of which, in turn depend on processing conditions. These microstructural features can also affect the pellet densification, thermal conductivity and microstructure evolution inside the reactor. Understanding these effects can, in turn, provide insight into microstructure evolution of fuels in-pile. Data on the aforementioned microstructural features can be collected using Electron Microscopy tools (Secondary Electron Images, Electron Backscatter Diffraction Scanning, Electron Diffraction  Spectroscopy) to obtain both 2-D and 3-D data. Data from serial sectioning (maps and crystallographic data), using either mechanical polishing or Focused Ion Beam (see figures 1 and 2), can be used  to obtain 3-D reconstructions (Dream 3DÔ, AvizoÔ) of the microstructure (see figure 3) that can provide information on: porosity shape, pore size distribution, pore connectivity, grain size and shape, grain orientation, GB misorientation angle distribution, Coincidence Site Latice,crystallographic texture, GB normals; as well as correlations between this microstructural features. The data can be used to determine how different manufacturing parameters can affect the evolution of the microstructure and, as initial inputs for microstructurally-explicit mesoscale simulations and atomistic models of fuel behavior.

Figure 1. Inverse pole map of a d-UO2 sample

Figure 2. Serial sectioning images for 3-D reconstruction

Figure 3. Reconstructed model of the microstructure of a d-UO2+x sample.

Heat and Mass Transport

Harn_Fig1 Harn_Fig2 Harn_Fig3
Multi-physics simulation studies transport mechanisms in UO2 as a nuclear fuel element. Understanding fission product behavior is important to the development of nuclear fuels as they can affect fuel performance and fuel life. It is known that microstructure, GB property and connectivity are factors that are important to the study. Mesoscale models in both 2-D and 3-D are developed to tackle this problem using Finite Element Analysis. In particular, the transport of fission gas is temperature dependent, and heat conduction of the fuels is affected by the distribution of these fission gases. This multi-physics relationship is coupled in the simulation, and the mechanisms of both fission gas transport and the thermal performance can be studied.

High-Temperature Mechanical Testing

This project is focused on improving the mechanical properties of uranium dioxide (UO2) at high temperatures. In this research, we are investigating the use of chemical doping to improve the mechanical properties of UO2 in a temperature range that is applicable to standard nuclear reactors. Our UO2 samples contain <1.0 wt% of various oxides such as alumina, yttria, titania, and vanadia. Currently, there is a lack of information on the effect of dopants on the mechanical performance of UO2. The mechanical behavior of UO2 at high temperatures is important for understanding how it will behave in a reactor. In a reactor core, nuclear fuel is prone to swelling and cracking that can lead to poor performance and shortened operating lifetimes. Improving the mechanical properties of nuclear fuel will mitigate these problems and allow power plants to operate longer without having to re-fuel. We perform high temperature Vickers indentation experiments using the assembly we have installed in the MTL. Vickers indentation provides information on mechanical properties such as hardness and fracture toughness. With this information, we can quantitatively analyze the mechanical character of each sample doped with a different oxide