Efficient removal and salt-phase fixation of hazardous uranium oxide contaminants from 316 L stainless steel using nitrate-containing molten salt.

Hazardous uranium oxide contamination on stainless-steel components presents a persistent challenge in nuclear-facility decommissioning because effective contaminant removal must be balanced against secondary uranium release and substrate damage. Here, a nitrate-containing NaOH-K2CO3-NaNO3 molten-salt system was developed for the decontamination of UO3- and U3O8-contaminated 316 L stainless steel. Response surface methodology identified optimized conditions yielding decontamination efficiencies of 96.62 ± 0.35% for UO3 at 396 ℃ for 27 min and 93.38 ± 0.57% for U3O8 at 376 ℃ for 30 min. Surface and recovered-salt characterization showed effective removal of the uranium oxide contaminants, transfer of uranium into the salt phase, and subsequent formation of Na2U2O7, supporting a release-transfer-salt-phase fixation pathway. Nitrate/nitrite-related salt chemistry was further evidenced by ion chromatography. Compared with the chloride-containing reference system, the nitrate-containing formulation generally provided better electrochemical stability and mechanical-property retention under suitable dosage conditions. Density functional theory calculations indicated weaker intrinsic interaction of NaNO3 than NaCl with a simplified α-Cr2O3(0001) passive-film model, providing qualitative atomistic support for the experimentally observed differences. These results demonstrate a promising molten-salt approach for efficient uranium oxide decontamination, salt-phase fixation, and mitigation of substrate damage.