Tracking uranium oxide surface reactions by open-circuit voltage fluctuations: Oxidative dissolution of a La-doped uranium oxide particle film

Mixed-valence uranium oxide particles are immobilized on an electrode surface. • Electrochemical response to oxidation in Cl-SO4-Na type groundwater is investigated. • Power spectral density of time series data reveal a fractal dimension (Df) = 1.6. • Voltage fluctuations are ascribed to uranium dissolution and reprecipitation. An electrode comprising uranium oxide particles doped with lanthanum at the outermost atomic layers was utilized as a surrogate surface for spent nuclear fuel. XPS analysis confirmed the formation of a mixed-valence (La,U)O 2+ x surface with an average uranium oxidation state of +4.78 (U(IV):U(V):U(VI) ≈ 4:3:2). The electrochemical response of a modified electrode to oxidative dissolution by H 2 O 2 (10 -4 mol dm -3 ) in either 3.5 wt. % NaCl solution or Cl-SO 4 -Na type groundwater were studied by open circuit voltage (OCV) measurements and power spectral density analysis. These methods aimed to characterize the electrode surface fingerprint in contact with each electrolyte environment. In the groundwater medium, the OCV time series exhibited a fractal dimension ( D f ) of 1.6. Inductively coupled plasma mass spectrometry (ICP-MS) revealed substantially reduced uranium and lanthanum release in groundwater (U: 160 μg/L; La: 89 μg/L) compared with NaCl solution (U: 749 μg/L; La: 1027 μg/L). The increased complexity of voltage fluctuations observed in groundwater, as evidenced by fractal time series analysis, is attributed to protective mineralization of the particle surfaces by Na and Ca and U ions. Download: Download high-res image (212KB) Download: Download full-size image