UC Berkeley

In an effort to increase the understanding of the mechanical properties of UO2 small scale mechanical testing techniques were developed at room and elevated temperature. The small scale mechanical testing techniques, such as micro cantilever testing and nanoindentation, focused on measuring the elastic and creep properties of the UO2 at room and elevated temperature. The elastic and creep properties of UO2 are important for pellet clad mechanical interactions during service in a reactor. During the lifetime of a reactor the fuel swells and the cladding creeps down onto the fuel which causes them to come into contact. Due to the elastic anisotropy of the UO2 this can lead to failures in the cladding and the release of radioactive material. In addition, the creep of the UO2 fuel is important during this contact as it a mechanism to relive stress in the cladding and fuel.

This work used small scale mechanical testing to measure the elastic and creep properties at room and elevated temperature. Microcantilever testing was performed ex-situ and in-situ in a scanning electron microscopy and transmission electron microscopy to measure the fracture stress and elastic modulus of the material over temperature. It was observed that when the microcantilevers had a ratio of (length/height) > 5 the elastic modulus values matched well with literature values. In addition, the microcantilevers measured higher values of fracture stress as compared with bulk sample as the microcantilevers had little to no porosity in the microcantilever. The measured values for the room temperature in-situ microcantilevers were in the 3-4 GPa range. While having little to no porosity the microcantilevers still exhibited a large spread in the results. Tests were performed on a single crystal with different loading orientations to measure the elastic anisotropy in the UO2. The results of the measurements show agreement between the experimental measured values and the theoretical values. The in-situ scanning electron microscope testing was also performed at elevated temperature in a reducing environment to measure the change in the elastic modulus values and fracture stress. The need for a reducing environment is because UO2 can further oxide to U3O8. The elastic modulus values measured at elevated temperature show good agreement with literature values. The in-situ transmission electron microscope testing showed that there was no dislocation motion at room temperature in the UO2. There was additional microcantilever testing to evaluate the effects of the microstructure on the calculated elastic modulus values of the UO2.

In addition to the microcantilever testing, nanoindentation was also performed on the UO2 fuel at room evaluated temperature on a variety of different samples. The two main studies performed with nanoindentation were to measure the effect of grain size on the hardness of the UO2 and to evaluate the effect of pre-straining the UO2 material at elevated temperature. The grains size study had 3 UO2 samples manufactured with spark plasma sintering with three different grain sizes (125 nm, 2 µm, and 10 µm) to evaluate the change in hardness of the samples. The 125 nm grain size sample had the highest hardness and maintained it hardness the best over the temperature range tested. The elastic modulus values measured with nanoindentation on all three samples agree with literature values. A sample was pre-strained prior to testing to evaluate the effects of an increased defect density in the sample. The pre-strained sample had a lower hardness and higher nanoindentation creep rates as compared with the un-strained results.

In summary this work demonstrates that small scale mechanical testing can be used to evaluate the mechanical properties of UO2 at room and elevated temperature and that these techniques have the ability to be applied to irradiated UO2 fuel.