F. King

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...

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...

Localized Dissolution of Millscale-Covered Pipeline Steel Surfaces

Abstract Electrochemical impedance spectroscopy, corrosion potential measurements, and surface analysis by scanning electron microscopy/energy-dispersive x-ray spectroscopy (SEM/EDX) and Raman spectroscopy were used to investigate the localized dissolution of millscale-covered pipeline steel surfaces. The porous millscale originally present on the pipe surface exerts an influence on the corrosion of the pipeline and may contribute to the eventual onset of stress corrosion cracking (SCC). Three regions in the corrosion potential-time plot were observed after exposure to an aqueous environment, corresponding to the initial attempts at breakdown of the millscale, coupling of the dissolution of the underlying steel to reductive dissolution of the millscale, and active corrosion of the steel at the base of pores in the film supported by water reduction either on the metal or on the millscale surface. The corrosion rate increases as the dissolved carbon dioxide (CO2) concentration increases. Changes in the solu...

Modelling long term corrosion behaviour of copper canisters in KBS-3 repository

Abstract The well known KBS-3 repository design involves the disposal of spent fuel in copper canisters in a deep geological repository sealed with clay based buffer and backfill materials. A one-dimensional reactive transport model has been developed to predict the evolution of the general corrosion behaviour of the copper canisters by the initially trapped O2 and by sulphide ions. Various sources of sulphide are considered, including the microbial reduction of sulphate, the dissolution of pyrite impurities and the ground water itself. The model has been used to simulate the evolution of the canister corrosion behaviour for various scenarios, including both the Olkiluoto and Forsmark proposed repository locations, the vertical and horizontal KBS-3 design variants, increased ground water sulphide or chloride concentrations, different microbial scenarios, different rates of repository saturation, and with and without the dissolution of pyrite. Following a brief description of the model, the results of these and other simulations are described.

Nominally Anaerobic Corrosion of Carbon Steel in Near-Neutral pH Saline Environments

Abstract Gas transmission pipeline corrosion commences when coatings disbond, exposing the steel to groundwater. When this occurs, a number of anaerobic and aerobic corrosion scenarios can be envisaged. The initial nominally anaerobic corrosion period has been investigated by applying a combination of electrochemical methods (i.e., corrosion potential, linear polarization resistance, and electrochemical impedance spectroscopy [EIS] measurements) and surface analytical techniques (scanning electron microscopy, energy-dispersive x-ray spectroscopy, and Raman spectroscopy). An evolution in film properties was observed and attributed to the entry of adventitious oxygen into faults within the preformed film. This leads to an increase in overall corrosion and a change in properties of the film as detected by EIS and Raman analysis. This article describes the mechanism involved in this transition, and provides a basis for a more extensive study of the corrosion process encountered on switching between anaerobic ...

Modelling the Consumption of Oxygen by Container Corrosion and Reaction with Fe(II)

A model is described that predicts the rate of O 2 consumption in a sealed nuclear fuel waste disposal vault as a result of container corrosion, reaction with biotite and the oxidation of organics and other oxidizable impurities in the clay. The most important reactions leading to the consumption of O 2 for Cu containers in a conceptual Canadian disposal vault are container corrosion, the oxidation of dissolved Cu(I) and the oxidation of organics and other impurities in the clay. Consumption of O 2 by the oxidation of dissolved Fe(II) from biotite is significant in backfill materials containing crushed granite and in the rock itself. The O 2 initially trapped in the disposal vault is predicted to be consumed in between 50 and 670 a.

The Electrochemistry of Copper in Aqueous Sulphide Solutions

Using a variety of electrochemical and surface analytical techniques, the mechanism and kinetics of Cu corrosion in anoxic, aqueous, sulphide-containing environments are being investigated. Under these conditions ([S]total = 10 −4 to 3 ×10 −3 mol/L), the anodicgrowth ofa film (XRD identifies Cu 2 S/Cu 1.8 S as major/minor phases, respectively) is supported by the cathodic reduction of water thereby destabilizing the copper surface. For more oxidizing conditions, the subsequent growth of a partially passivating film is observed. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry at rotating disc electrodes show film growth occurringwith negligible dissolution and under partial SH - transport control. Current-potentialrelationships as a function of [S]total give Tafel slopes of ∼ (40 mV) −1 suggesting reaction occurs via a 2step process: Aninitial rapid adsorption of SH - leading to an equilibrium surface concentration, followed by a rate determining electron transfer to form a sulphide film. It is proposed thatfilm growth propagates via transport of Cu I through the film to the solution interface. The primary goal of this research is the development of a mathematical model which can be used to assess the performance of copper nuclear waste canisters in granitic repositories.

Simulations of long-term anaerobic corrosion of carbon steel containers in Canadian deep geological repository

Abstract Carbon steel is a candidate container material for the disposal of used fuel in a deep geological repository in sedimentary host rock in Canada. Prediction of the long term anaerobic corrosion behaviour of the container is important, not only because it partly determines the container lifetime but also because the products of the corrosion reaction, Fe(II) and H2, can impact the properties of the clay based sealing materials and host rock. A mechanistically based numerical model is described that predicts both the long term corrosion rate and the effects on the repository environment, including the periodic build-up and release of H2, the precipitation of Fe3O4 within the bentonite buffer, and the alteration of montmorillonite to a non-swelling clay. The results of a reference simulation for a repository in shale host rock are described.

Modelling Procedures for Predicting the Lifetimes of Nuclear Waste Containers

Approaches to modelling the corrosion of nuclear waste containers are reviewed. The required containment of many thousands of years makes this a daunting task. The process has been simplified for a disposal vault in which redox conditions evolve from initially oxidizing to eventually non-oxidizing. The corrosion behaviour can be divided into two periods: an early hot, oxidizing period when localized corrosion damage is to be expected; and a later cool, non-oxidizing period when localized processes would be stifled, or repassivated, and general corrosion will predominate. At present, deterministic models to predict localized corrosion damage during the early period are unavailable or, at best, preliminary. Generally, the approach to predicting localized penetration of the container has been stochastic in nature and extreme value statistical analyses have been used to predict the expected penetration of carbon steel or copper containers by pitting. Experiments to determine the rate of crevice propagation in titanium are discussed and a model developed to predict failure of titanium waste containers by either crevice corrosion or hydrogen-induced cracking described. General corrosion occurring in the second, less oxidizing, period is more amenable to modelling by deterministic methods. Models based on electrochemical descriptions of the interfacial kinetics are described for carbon steel and copper containers, two materials expected to corrode actively under waste vault conditions. To date, no adequate model exists to predict the slow general corrosion of passivated materials.

The Stress Corrosion Cracking of Copper:Nuclear Waste Containers

The extent of stress corrosion cracking (SCC) of copper nuclear waste containers is being predicted on the basis of a “limited propagation” argument. In this argument, it is accepted that crack initiation may occur, but it is argued that the environmental conditions and material properties required for a through-wall crack to propagate will not be present. In this paper, the effect of one environmental parameter, the supply of oxidant (J ox ), on the crack growth rate is examined. Experiments have been conducted on two grades of Cu in NaNO 2 environments using two loading techniques. The supply of oxidant has been varied either electrochemically in bulk solution using different applied current densities or by embedding the loaded test specimens in compacted buffer material containing O 2 as the oxidant. Measured and theoretical crack growth rates as a function of J ox are compared with the predicted oxidant flux to the containers in a disposal vault and an estimate of the maximum crack depth on a container obtained.