ROADRUNNER uranium nitride MiniFuel: Experimental design, fabrication and pre-irradiation baseline characterization for accelerated burnup testing

Abstract

Uranium Nitride (UN) is considered as a possible driver fuel for a variety of advanced reactor designs; however, its qualification remains limited by the lack of consistent, high-quality irradiation performance data. To address this gap, the ROADRUNNER (Research On ADvancing the peRformance of UraNium Nitrides in Extreme enviRonments) irradiation campaign leveraging the MiniFuel platform at the High Flux Isotope Reactor (HFIR). This campaign enables accelerated irradiation testing under controlled iso-thermal and iso-neutronic configurations. Systematically varied UN specimens were fabricated to isolate the effects of fuel density and impurity content (carbon) on UN performance behavior under irradiation. The campaign includes 36 UN minidiscs irradiated at nominal temperatures of 873 K, 1173 K, and 1473 K, targeting burnups of 4, 6 and 8% FIMA, with density ranging from 86 to 96% of the theoretical UN density. The UN minidisc also vary the carbon impurity levels ranging from 961-5240 ppm and grain size of 2.5 to 24 μm. Neutronic and thermal simulations have been conducted to define the specific irradiation conditions for each minidisc. Pre-irradiation characterization—including impurity composition, phase analysis (X-ray diffraction ), local crystallography (Raman), minidisc density and microstructure, and dimensional inspection via X-ray Computed Tomography (XCT) and confocal profilometry—establishes a detailed baseline to support post irradiation performance. Pre-irradiation data were used to generate preliminary estimates of fission gas release (FGR) and swelling using existing empirical models. Fuel performance evaluations of the UN minidiscs revealed gaps in current datasets, with the most significant divergences observed at high burnup and elevated temperatures. The proposed UN irradiation and post-irradiation examination campaign intends to generate data to fill these gaps and support the development of physics-based fuel performance models.