ROADRUNNER: a MiniFuel Experiment to Collect Irradiation Performance Data on Uranium Nitride Specimens using the High Flux Isotope Reactor

High-density uranium nitride (UN) is a promising fuel candidate for advanced nuclear reactor designs, including Liquid Metal Fast Reactors (LMFRs), Small Modular Reactors (SMRs), micro-reactors, and space-based fission reactors. However, limited data exist across this broad operational reactor space, as well as on how fabrication impurities and density variations affect UN’s irradiation performance. To address these knowledge gaps, the ROADRUNNER (Research On ADvancing the peRformance of UraNium Nitrides in Extreme enviRonments) irradiation campaign was launched as a collaborative effort among the University of Texas at San Antonio (UTSA), Westinghouse Electric Company, Oak Ridge National Laboratory (ORNL), and Los Alamos National Laboratory (LANL) under the Nuclear Science User Facilities (NSUF) program. This campaign aims to support UN fuel qualification by systematically assessing the effects of density and impurity variations across different irradiation temperatures and burnup levels. The experiment is being conducted in the High Flux Isotope Reactor (HFIR) using ORNL’s MiniFuel irradiation platform, which enables separate-effects testing by decoupling fuel temperature from the fission rate through gamma heating. The test matrix consists of six MiniFuel targets, spanning three burnup levels (37.5, 60, and 75 MWd/kgU) and three irradiation temperatures (600°C, 900°C, and 1200°C). Neutronics and thermal analyses guided the experimental design, predicting six, nine, and twelve HFIR cycles for the respective burnup targets. UN pellets, fabricated at LANL with controlled density and carbon impurity levels, were further refined at UTSA to the required thickness. Comprehensive pre-irradiation characterization included dimensional and mass measurements, density analysis, carbon and oxygen content assessment, microstructural examination, X-ray computed tomography, Raman spectroscopy, and laser flash analysis. All six targets were successfully assembled and inserted into HFIR, with lowburnup targets expected to complete irradiation by the end of the year. Post-irradiation examination (PIE), to be conducted at ORNL’s Irradiated Fuels Examination Laboratory, will include fission gas release measurements, visual inspection, gamma spectrometry, swelling analysis, optical microscopy, and scanning electron microscopy with energy-dispersive spectroscopy. The work presented will include the experimental test matrix and design, pre-characterization, assembly process, current irradiation status, and planned PIE activities.