Corrosion Issues in High-level Nuclear Waste Containers

Open Access
Author:
Sharifiasl, Samin
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 21, 2013
Committee Members:
  • Digby D Macdonald, Dissertation Advisor
  • Digby D Macdonald, Committee Chair
  • James Hansell Adair, Committee Member
  • Michael Anthony Hickner, Committee Member
  • Mirna Urquidi Macdonald, Committee Member
  • Gary Lynn Messing, Committee Member
Keywords:
  • Passivity
  • Corrosion
  • High-level nuclear waste
  • Sulfide
  • Thermodynamics
  • Kinetics
Abstract:
In this dissertation different aspects of corrosion and electrochemistry of copper, candidate canister material in Scandinavian high-level nuclear waste disposal program, including the thermodynamics and kinetics of the reactions that are predicted to occur in the practical system have been studied. A comprehensive thermodynamic study of copper in contact with granitic groundwater of the type and composition that is expected in the Forsmark repository in Sweden has been performed. Our primary objective was to ascertain whether copper would exist in the thermodynamically immune state in the repository, in which case corrosion could not occur and the issue of corrosion in the assessment of the storage technology would be moot. In spite of the fact that metallic copper has been found to exist for geological times in granitic geological formations, copper is well-known to be activated from the immune state to corrode by specific species that may exist in the environment. The principal activator of copper is known to be sulfur in its various forms, including sulfide (H2S, HS-, S2-), polysulfide (H2Sx, HSx-, Sx2-), poly sulfur thiosulfate (SxO32-), and polythionates (SxO62-). A comprehensive study of this aspect of copper chemistry has never been reported, and yet an understanding of this issue is vital for assessing whether copper is a suitable material for fabricating canisters for the disposal of HLNW. Our study identifies and explores those species that activate copper; these species include sulfur-containing entities as well as other, non-sulfur species that may be present in the repository. The effects of temperature, solution pH, and hydrogen pressure on the kinetics of the hydrogen electrode reaction (HER) on copper in borate buffer solution have been studied by means of steady-state polarization measurements, including electrochemical impedance spectroscopy (EIS). In order to obtain electrokinetic parameters, such as the exchange current density and the cathodic Tafel slope, two stages of optimization have been performed. From the optimization process, the activation energy (Eac) of the HER on copper was obtained as ≈32 kJ mol-1. Moreover, the mechanism of the hydrogen evolution reaction (HER) on copper in mildly alkaline media has been studied by means of EIS over the frequency range of 0.01 Hz ≤ f ≤ 5 kHz. The impedance spectra were modeled using a mechanism based upon the Volmer-Heyrovsky-Tafel steps for hydrogen evolution and by considering the reactions involved in hydrogen atom and hydroxyl group adsorption on the copper surface. A single set of kinetic parameters, including the rate constants and transfer coefficient, have been derived for each pH by optimization of the mechanistic model on the experimental impedance (EIS) data. It is postulated that the HER proceeds through the Volmer-Heyrovsky-Tafel mechanism with the Volmer reaction being the rate-determining step. The kinetics of growth of the passive sulfide film on copper in deaerated aqueous sodium chloride solution as a function of applied potential, sulfide species concentrations and temperature were explored by means of potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The electronic and compositional properties of the passive layer were investigated with Mott-Schottky analysis and X-ray photoelectron spectroscopy (XPS). It is found that metal vacancies are predominant defects in the barrier layer which is in agreement with the p-type character of the film observed experimentally. A point defect model (PDM) for formation and dissolution of the passive sulfide film on copper is proposed. Finally, the behavior of the system interpreted in terms of reaction mechanisms and kinetic parameters extracted from the experimental impedance data by mathematical optimization using a genetic algorithm approach. The diffusion coefficient of cation vacancies is obtained directly from optimization of the proposed model onto the EIS experimental data and was found to be essentially independent from the applied potential within the passive range. The diffusivity of the predominance defects (cation vacancies) found to increase with increase in solution temperature from approx. 10-13 (cm2 s-1) at 25 oC to 10-11 (cm2 S-1) at 75oC.