ChemInform Abstract: The Effect of pH on the Corrosion of Nuclear Fuel (UO2) in Oxygenated Solutions,

Abstract

Abstract The oxidative dissolution of UO2 has been studied in NaClO4 and Na2SO4 solutions as a function of pH over a range of 0.8 ⩽ pH ⩽ 12 using a combination of electrochemical and X-ray photoelectron spectroscopic techniques. The relative stability and solubility of solid uranium oxides and uranium speciation in aqueous solutions were examined using thermodynamic calculations. In neutral to alkaline solutions (pH ≥5), dissolution is preceded by the growth of a thin film of UO2 33 on the UO2 surface. This film achieves a steady-state thickness (∼6 nm) in 5 to 10 h, and the thickness increases with an increase in pH. Over the next 10 to 100 h, further oxidation to a hydrated form of UO3 occurs, after which steady-state dissolution conditions are achieved. Formation of UO3 · xH2O is a precursor to dissolution and its presence appears to be confined mainly to the grain boundaries. In acidic solutions UO2.33 formation does not occur. Oxidation proceeds directly to the UVI state (possibly to UO3 · xH2O) and the dissolution rate, measured by extrapolation of steady-state currents to the corrosion potential, is approximately 50 times greater at pH = 2.5 than it is at pH = 9.7. The electrochemically measured corrosion rate value of ∼45 × 10−8 g cm−2 d−1 at pH = 2.5 compares well with the value of ∼ 25 × 10−8 g cm−2d−1 measured chemically by Thomas and Till. The dependence of the steady-state corrosion potentials on pH suggests possible rate control by the anodic dissolution reaction for pH > 10. For pH values between 2 and 10, the corrosion potential varies only slightly with pH, and it is unclear whether the anodic or cathodic reaction is rate-controlling in this region.