Design and Assembly of MiniFuel Targets for Characterization of High Burnup UO₂ Specimens Irradiated in the High Flux Isotope Reactor

Safety concerns have been established regarding the performance of high-burnup fuel during a loss of coolant accident, and the observed performance is coupled to the microstructural formations across the UO₂ fuel pellet. These microstructural features are a result of the radial temperature and burnup profile across the fuel pellet; an understanding of the conditions driving the formation of these features may prove critical for minimizing fuel fragmentation, relocation, and dispersal consequences. To study the effects of temperature on the microstructure of UO₂ fuel, a MiniFuel experiment was designed in the High Flux Isotope Reactor on small UO₂ disk specimens fabricated at Oak Ridge National Laboratory. The High Flux Isotope Reactor affords the opportunity to accelerate burnup in order to rapidly answer key questions associated to fuel performance. The density and grain size of the fabricated UO₂ disk specimens represent commercial UO₂ fuel pellets. The irradiation conditions selected for this experiment include a fuel average temperature ranging between 600 to 1,000°C in 100°C increments and a sample burnup ranging from 50 to 70 MWd/kgU. The goals of this experiment are (1) to investigate the burnup and temperature thresholds for dark zone formation UO₂, (2) to explore fission rate effects on microstructure formations, and (3) to assess other fuel performance considerations such as fission gas release and unconstrained swelling under various irradiation conditions. Seven MiniFuel targets were successfully assembled and inserted into the reactor in the Removable Beryllium and Vertical Experiment Facility locations. This report summarizes the experiment design and test matrix, the fuel fabrication and pre-characterization, the target assembly, as well as the successful insertion of targets into the reactor.