David W. Shoesmith

Fuel corrosion processes under waste disposal conditions

The release of the majority of radionuclides from spent nuclear fuel under permanent disposal conditions will be controlled by the rate of dissolution of the UO2 fuel matrix. In this manuscript the mechanism of the coupled anodic (fuel dissolution) and cathodic (oxidant reduction) reactions which constitute the overall fuel corrosion process is reviewed, and the many published observations on fuel corrosion under disposal conditions discussed. The primary emphasis is on summarizing the overall mechanistic behaviour and establishing the primary factors likely to control fuel corrosion. Included are discussions on the influence of various oxidants including radiolytic ones, pH, temperature, groundwater composition, and the formation of corrosion product deposits. The relevance of the data recorded on unirradiated UO2 to the interpretation of spent fuel behaviour is included. Based on the review, the data used to develop fuel corrosion models under the conditions anticipated in Yucca Mountain (NV, USA) are evaluated.

Electrochemical techniques in corrosion science and engineering

Introduction, R.G. Kelly Electrochemical Thermodynamics and Kinetics of Relevance to Corrosion, R.G. Kelly Passivity and Localized Corrosion, R.G. Kelly The Polarization Resistance Method for Determination of Instantaneous Corrosion Rates, J.R. Scully The Influence of Mass Transport on Electrochemical Processes, J.R. Scully Current and Potential Distributions in Corrosion, J.R. Scully Development of Corrosion Models Based on Electrochemical Measurements, D.W. Shoesmith Electrochemical Studies of Surface Treatments for Metals and Alloys, R.G. Buchheit Experimental Procedures, R.G. Kelly and J.R. Scully Index

A Mixed-Potential Model to Predict Fuel (Uranium Dioxide) Corrosion within a Failed Nuclear Waste Container

Abstract A mixed-potential model is described to predict the corrosion behavior of used nuclear fuel inside a steel-lined failed Canadian nuclear waste container under anticipated waste vault (repository) conditions. The model accounts for the effects of the alpha radiolysis of water, the precipitation of corrosion products on both the fuel and the carbon steel (CS), and redox reactions between species produced by either radiolysis or corrosion at the fuel surface and by corrosion on the CS liner. The model is based on a series of ten one-dimensional reaction-diffusion equations, each describing the mass-transport, precipitation/dissolution, adsorption/desorption, and redox processes of the ten chemical species included in the model. These equations are solved using finite-difference techniques. A three-layer spatial grid is used, with the two outer layers (of time-varying thickness) representing porous precipitated corrosion products on the uranium dioxide (UO2) and CS surfaces. The middle layer represen...

Effects of Temperature and Potential on the Passive Corrosion Properties of Alloys C22 and C276

Potentiostatic polarization experiments were performed as a function of potential (200 to 700 mV Ag/AgCl ) and temperature (25-85°C) on the Ni-Cr-Mo alloys C22 and C276. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF SIMS) were used to determine the chemical composition and thickness of the films formed. The currents recorded as a function of applied potential were due to dissolution, with only minor increases in film thicknesses observed. Measured currents for C22 were lower than for C276 and decayed over the entire period of measurement at each temperature. Those on C276 more closely approached steady state. The temperature dependence of the currents on C22 was significantly lower than that on C276. Surface analyses, performed on specimens anodically treated at one potential but after a sequence of temperatures up to 85°C, confirmed that the passive films on both alloys consisted of a Mo, Cr, and Ni oxide, with Cr present as Cr 3 + and Mo present in several oxidation states. The passive films on C22 showed a distinct layered structure, consisting of an inner layer rich in Cr and Ni, and an outer layer enhanced in Mo. By contrast, the oxide films on C276 did not show such a clear separation into layers, and the relative Cr content was much lower. The increase in oxide thickness with increasing anodic potential and the low temperature dependence of the passive current observed for C22 are consistent with an oxide dissolution rate which is low compared to the rate of creation of oxygen vacancies leading to film growth. The absence of a dependence of film thickness on potential and the higher temperature dependence of the passive currents on C276 are consistent with control of the overall anodic process by ion-transfer at the oxide/solution interface.

The Electrochemical Response of Preoxidized Copper in Aqueous Sulfide Solutions

The conversion of a Cu 2 O film on copper to Cu 2 S in aqueous sulfide solutions has been followed using a combination of electrochemical techniques and in situ Raman spectroscopy. Oxide films were electrochemically grown in alkaline solutions and their composition and morphology determined using Raman spectroscopy and scanning electron microscopy. Although corrosion potential measurements indicate that the aqueous sulfide solution rapidly penetrates the porous Cu 2 O layer, the oxide-to-sulfide reaction appears to proceed chemically at the oxide/solution interface rather than via the galvanic coupling of Cu oxidation to Cu 2 S and Cu 2 O reduction to Cu. In situ Raman spectroscopy confirms that the sulfide formed is Cu 2 S, and cathodic stripping voltammetry shows that the reaction is initially rapid and then proceeds at a constant rate until the conversion is complete. Comparison of the amounts of oxide initially present and sulfide eventually formed demonstrates that the conversion is 100% efficient. These studies are part of a larger project to determine the important corrosion processes on copper high-level nuclear waste containers exposed to anoxic aqueous sulfide containing groundwaters.

Sulfide Film Formation on Copper under Electrochemical and Natural Corrosion Conditions

Abstract The mechanism and kinetics of Cu corrosion in anoxic aqueous chloride solutions containing sulfide (10−3 mol/L) have been investigated electrochemically and under natural corrosion conditi...

Assessing the Corrosion Performance of High-Level Nuclear Waste Containers

Abstract The assessment of the corrosion performance of containers (waste packages) for the disposal of high-level nuclear waste has been underway for more than 20 years. A review of the rationale behind disposal site selection, container design, materials selection for fabrication, and the bases for the development of corrosion models is discussed.

Corrosion Behavior of Uranium Dioxide in Alpha Radiolytically Decomposed Water

The response of the corrosion potential of a UO 2 electrode to oxidants (H 2 O 2 ,O 2 ) produced by the alpha radiolysis of water has been measured in a thin-layer electrochemical cell. This cell allows the electrode to be brought within ∼25 μm of a gold-plated alpha source, thereby ensuring the uniform distribution of radiolytic oxidants in the aqueous solution filling the gap. The kinetics of water radiolysis, coupled with the surface-catalyzed decomposition of H 2 O 2 and diffusive transport of radiolytic species out of the electrode-source gap, was modeled using a finite difference description of the cell and the commercial numerical integration software, Facsimile. Using this approach, the mechanism of UO 2 corrosion in the presence of alpha radiolysis was shown to be dominated by reaction of the fuel with radiolytically produced H 2 O 2 . To explain the results, two surface-catalyzed H 2 O 2 decomposition processes were invoked: one with H 2 to produce H 2 O, and a second process leading to the production of O 2 and H 2 O. For sufficiently high alpha source strengths, the fuel behavior becomes redox-buffered, i.e., independent of alpha source strength, due to the presence of a corrosion product deposit.

Kinetics of Corrosion Film Growth on Copper in Neutral Chloride Solutions Containing Small Concentrations of Sulfide

The corrosion behavior of oxygen-free copper in an anoxic 0.1 M NaCl + 5 × 10 ―4 M Na 2 S solution was studied using electrochemical impedance spectroscopy, scanning electron microscopy equipped with a focused ion beam, X-ray photoelectron spectroscopy and micro X-ray diffraction. The corrosion film grew as a coherent, compact crystalline layer with a nonuniform thickness. The growth kinetics obeyed a parabolic law consistent with control by a diffusion process. Once a coherent layer was established, film growth kinetics appeared to be controlled by Cu + ion diffusion through the film. At shorter times before a coherent film was established, SH- diffusion in solution exhibited a considerable influence on the growth rate.

A Review of Corrosion of Titanium Grade 7 and Other Titanium Alloys in Nuclear Waste Repository Environments

Abstract Titanium alloy degradation modes are reviewed in relation to their performance in repository environments. General corrosion, localized corrosion, stress corrosion cracking, hydrogen-induced cracking, microbiologically influenced corrosion, and radiation-assisted corrosion of Ti alloys are considered. With respect to the Ti Grade 7 drip shields selected for emplacement in the repository at Yucca Mountain, general corrosion, hydrogen-induced cracking, and radiation-assisted corrosion either do not initiate or propagate sufficiently fast over a 10,000-year time frame in likely repository environments to cause penetration of the drip shield. Stress corrosion cracking (in the absence of disruptive events) is of no consequence to barrier performance; and localized corrosion and microbiologically influenced corrosion are not expected to occur. To facilitate the discussion, Ti Grades 2, 5, 7, 9, 11, 12, 16, 17, 18, and 24 are included in this review.