S. Sunder

The corrosion of nuclear fuel (UO2) in oxygenated solutions

Abstract The corrosion mechanism of UO2 (nuclear fuel) has been studied in 0.1 mol l−1 sodium perchlorate (pH = 9.5), with and without added sodium carbonate. The corrosion potential was followed for various exposure times. Subsequently, the electrode was either subjected to a cathodic stripping scan from the corrosion potential to −2.0 V to determine the presence and measure the thickness of surface films formed; or removed and examined by X-ray photo-electron spectroscopy to determine the composition of the electrode surface. In both perchlorate and perchlorate plus carbonate solutions two films were formed on the UO2 prior to the establishment of steady-state dissolution conditions. A layer of UO2.33 was formed over the first 10 h of exposure. The outer layers of this film slowly converted to hydrated UO3 (or uranyl carbonate) over the next ∼90 h. This conversion appeared to be concentrated at the grain boundaries. Corrosion rates were measured by extrapolating from the Tafel region for steady-state anodic dissolution to the corrosion potential. The corrosion process appears to be controlled by the kinetics of the anodic dissolution step.

Anodic oxidation of copper in alkaline solutions: Part IV. Nature of the passivating film

Abstract Potentiostatic and X-ray photoelectron spectroscopic (XPS) techniques have been used to study the passivation of copper electrodes in 1.0 mol dm −3 LiOH. For potentials 2 O is present, and an upper layer of Cu(OH) 2 is formed by nucleation and growth from solution. The dissolved Cu 2+ ions necessary for Cu(OH) 2 precipitation are produced by metal dissolution in the pores of the Cu 2 O layer. Under these conditions, the surface is only partially passivated, since metal dissolution can continue in the pores of the base layer. For potentials >−60 mV (vs. SCE), these pores, and eventually the whole surface, are covered by a layer of CuO identified by XPS. When this layer is formed, the extent of Cu(OH) 2 formation is drastically reduced. In addition the cupric ion dissolution rate is reduced, indicating a much higher degree of surface passivation.

The effect of ph on the corrosion of nuclear fuel (UO2) in oxygenated solutions

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.

Anodic oxidation of UO2: Part II. Electrochemical and X-ray photoelectron spectroscopic studies in alkaline solutions

Abstract The anodic oxidation of polycrystalline UO 2 has been studied in neutral Na 2 SO 4 solution (6 E E ≅+0.3 V), a thin film (t molecular layers) of UO 2+ x ( x ≤0.25) was formed. At slightly more positive potentials and longer times (1 t 3 O 7 grew on top of this UO 2+ x base layer. For potentials >+0.1 V, dissolution as U(VI) occurred, and for E =+0.3 V the surface film was transformed gradually to U 2 O 5 (≅1 h) and eventually to U 3 O 8 ( t t16 h). For E ≥+0.4 V a film of UO 3 · z H 2 O precipitated on the electrode surface in the absence of stirring. Also some dissolution of the U 2 O 5 film occurred due to a decrease in the local pH at the electrode surface. Both the precipitation and the film dissolution were prevented by stirring.

Oxidation of Candu UO 2 Fuel by the Alpha-Radiolysis Products of Water

The oxidation of CANDU fuel (UO/sub 2/) by the alpha-radiolysis products of water has been investigated using electrochemical and X-ray photoelectron spectroscopic experiments. Experiments with O/sub 2/ and H/sub 2/O, two of the expected products of radiolysis of water, indicate that the rate of oxidation of UO/sub 2/ by H/sub 2/O/sub 2/ is about 200 times faster than by dissolved oxygen. Oxidation by both H/sub 2/O/sub 2/ and O/sub 2/ shows pH dependence. Possible reaction paths for the oxidation of UO/sub 2/ by radiolysis products are discussed.

Anodic oxidation of UO2 IV. X-ray photoelectron spectroscopic and electrochemical studies of film growth in carbonate-containing solutions

Abstract The mechanism of oxidation of and film growth on UO2 in carbonate solutions has been studied using X-ray photoelectron spectroscopy and electrochemical techniques. It was found that the early stages of oxidation, leading to the formation of a film of composition UO2.33, are unaffected by the presence of carbonate in the solutions. The presence of carbonate in concentrations > 10-3 mol dm-3 prevents the formation of higher oxide films, which are formed at higher oxidation potentials in carbonate-free solutions. A reaction scheme is presented to explain the mechanism of film formation and oxidative dissolution of UO2.

ANODIC OXIDATION OF UO2-PART III. ELECTROCHEMICAL STUDIES IN CARBONATE SOLUTIONS

ABSTRACT The oxidative dissolution of UO 2 has been studied in aqueous carbonate solutions using steady-state potentiostatic and cyclic voltammetric techniques. The stoichiometry of the UO 2 electrode was determined by X-ray photoelectron spectroscopy. Dissolution occurs from a UO 2.33 surface layer at low anodic potentials and from a UO 2.5 layer at higher potentials. Steady-state dissolution currents, as a function of carbonate concentration and applied potential, show a variable reaction order with respect to carbonate concentrations, and a change from a two-electron to a one-electron rate-determining step as the potential increases.

Prediction of the Oxidative Dissolution Rates of Used Nuclear Fuel in a Geological Disposal Vault Due to the Alpha Radiolysis of Water

Effects of alpha radiolysis of water on the corrosion of nuclear fuel (UO 2 ) have been investigated in solutions at pH = 9.5, i.e., a value close to that expected in groundwaters at the depth of the disposal vault proposed in the Canadian nuclear fuel waste management program, CNFWMP. The corrosion potentials of UO 2 electrodes exposed to the products of alpha radiolysis of water were monitored as a function of alpha flux and exposure time in a specially designed thin-layer cell. The oxidative dissolution rates of UO 2 are calculated from the steady-state values of the corrosion potential using an electrochemical model. A procedure to predict the dissolution rate of used nuclear fuel in groundwater as a function of fuel cooling time is described, and illustrated by calculating the dissolution rates for the reference used fuel in the CNFWMP (Bruce CANDU reactor fuel, burnup 685 GJ/kg U). It is shown that the oxidative dissolution of used fuel in the CNFWMP will be important only for time periods ≤ 600 a at this burnup and assuming no decrease in pH.

Electrochemical studies of corrosion of SIMFUEL: Simulated used UO{sub 2} fuel

The corrosion of SIMFUEL, simulated high-burnup CANDU (CANadian Deuterium Uranium) fuel, was investigated in 0.1 mol{center_dot}dm{sup -3} NaClO{sub 4} solution (pH {approximately} 9.5) as a function of dissolved oxygen concentration using electrochemical techniques and X-ray photoelectron spectroscopy (XPS). Electrodes were constructed of SIMFUEL pellets with compositions close to those of a natural UO{sub 2} fuel that has undergone burnup of 3 at% and 6 at%. The XPS analysis of freshly polished SIMFUEL pellets showed that the extent of uranium oxidation in SIMFUEL was equivalent to that in stoichiometric UO{sub 2}. The SIMFUEL electrodes showed higher conductivity and electrochemical reactivity than the pure UO{sub 2} electrodes. A comparison of the open circuit corrosion potentials of the SIMFUEL electrodes with that of a pure UO{sub 2} electrode in oxygenated solutions showed a much faster increase in the potential for the SIMFUEL electrodes at short times. This suggests that the initial stages of UO{sub 2} oxidation, i.e., UO{sub 2} {yields} UO{sub 2+x} {yields} UO{sub 2.33}, are facilitated by fission product impurities in the UO{sub 2} matrix. However, the {open_quotes}steady-state{close_quotes} oxidation of uranium in SIMFUEL by the dissolved O{sub 2} was similar to that observed in pure UO{sub 2} under similar conditions.