Mitigation of steam-oxidation in uranium nitride fuel pellets by carbon addition

Uranium nitride (UN) is an accident-tolerant fuel candidate for light-water reactors because of its high uranium density and thermal conductivity. During fabrication, oxygen- and carbon-related impurities can be introduced, yet their influence on the steam oxidation of UN has not been well quantified. To address this gap, we fabricated UN pellets with controlled additions of UO2 and carbon, and investigated their behavior under flowing steam at 350°C. Pristine UN pellets showed partial surface oxidation after 30 min, followed by complete disintegration after 90 min. UO2-added pellets were less stable and disintegrated within 45 min. In contrast, carbon addition markedly improved the steam oxidation tolerance, as evidenced by intact pellet surfaces with reduced spallation. UN pellets containing 0.25–1.40 wt% carbon retained their shapes and exhibited weight gains below 0.2% even after 240 min. Overall, our experimental results underscore the need to carefully control fabrication-related impurities, as minimizing oxygen contamination and moderately incorporating carbon are effective measures for improving the steam oxidation resistance of UN pellets.