The overarching goal of the proposed work is to develop fundamental chemistry-structure-property relationships for UO2 grains and grain boundaries that serve as a basis for optimizing chemistry and engineering microstructure for damage tolerant fuels. Small scale testing will be employed in combination with in situ laser heating and ion irradiation with a goal of understanding and isolating the mechanical properties of individual microstructural features, e.g. the fracture toughness along specific crystallographic directions in the bulk and along grain boundaries, under conditions intended to model those experienced by fuels, T>1200 oC. The work builds on prior collaborations between the PI and Khalid Hattar’s group at Sandia National Laboratories where we have previously developed capabilities to perform in situ ion irradiation induced creep at high temperatures as well as in situ transmission electron microscopy based ultrahigh temperature, T>2000 oC, in situ mechanical testing. Experiments performed as a function of dopant chemistry, dose, and temperature will be used to develop fundamental structure-property relationships that will inform continuum scale mechanical models of fuels.

Fabrication of samples used for this work is supported by the DOE-NE Advanced Fuels Campaign; within the goals of this broader effort our work is applying advanced characterization to improve the capability for development of science-based fuel development. Chemical additives (e.g. dopants) can be used to control the microstructure of UO2 fuels, which is an effective means to affect the distribution of fission gas product bubbles and related failures. Chemical additives in oxides may strongly influence mechanical properties and atomic diffusion at high temperatures. The effects of dopants on high temperature mechanical properties and high temperature properties under irradiation remain poorly defined. As a result, little data is available to inform or validate models necessary to predict damage evolution in doped fuels at high temperature under irradiation. This work will develop and demonstrate advanced characterization techniques that reasonably simulate fuel operating temperature, irradiation damage in terms of dpa and gas implantation, and irradiation flux. The effort will specifically focus on Cr2O3-doped UO2, which is a common additive in commercial fuels. The work scope addresses validation needs related to meso-scale microstructure evolution of UO2 in separate effects experimentation and is therefore coupled to Appendix D. It is envisioned that the single grain boundary and microstructural fracture toughness UO2 grain boundaries can provide validation for calculated data in BISON.