An update on the copper corrosion program for the long-term management of used nuclear fuel in Canada

In 2012, the Nuclear Waste Management Organization developed a comprehensive proof test plan (PTP) to evaluate the feasibility and safety of their copper-coated used fuel container and their novel bentonite buffer box emplacement concept to be employed in its proposed deep geological repository for the long-term management of used nuclear fuel. Research within the PTP includes several programs to evaluate the possible extent of damage that may be caused by various copper corrosion mechanisms: oxic-, radiolytic-, anoxic-, and sulfide-induced, with particular attention being paid to the possibility of localization of any of these processes. Programs remain on track to support and refine a maximum copper corrosion allowance of <1.27 mm over a one million year emplacement in a deep geological repository.     

The anaerobic corrosion of candidate disposal canister materials in compacted bentonite exposed to natural granitic porewater containing native microbial populations

The materials corrosion test (MaCoTe) is a long-term, multinational in situ corrosion experiment setup at the Grimsel Test Site, Switzerland. The experiment has been operating since 2014 with a focus on the corrosion behaviour of container materials for the disposal of high-level waste and spent nuclear fuel under conditions representing a granitic deep geological repository. The experiment consists of eight modules containing metal coupons and bentonite. Two of the modules, each with a different bentonite density, have been retrieved after 394 days of exposure and have been analysed using a range of techniques aimed at studying the corrosion behaviour of the metals and the mineralogical evolution of the bentonite. Weight loss measurements show that carbon steel had a relatively low average corrosion rate (~2 µm year⁻¹). Much lower average corrosion rates were measured for the various types of copper (0.13–0.32 µm year⁻¹). No detectable corrosion was measured on stainless steel coupons. To date, no significant differences were observed in the corrosion behaviour and rate of the test metals in bentonite with different dry densities.     

Corrosion of copper-coated used nuclear fuel containers due to oxygen trapped in a Canadian deep geological repository

The Nuclear Waste Management Organization is evaluating the safety and feasibility of the permanent disposal of used nuclear fuel in a deep geological repository. Their current design concept utilises copper-coated steel used fuel containers to isolate the waste from the environment. Immediately following repository closure, a finite quantity of O₂ will be trapped inside the repository and could cause some amount of oxic corrosion to the outer copper layer of the containers. On a per container basis, 13?mol of O₂ will be trapped in the repository rooms at the time of closure, based on reference design dimensions. This corresponds to a maximum depth of copper corrosion of 81?μm, assuming a uniform distribution. This work also considers the sensitivity of this oxic corrosion allowance to various hypothetical design changes to the repository that may occur before or during construction.     

The anoxic corrosion behaviour of carbon steel in anoxic alkaline environments simulating a Swiss L/ILW repository environment

The Swiss waste management programme foresees that low- and intermediate-level radioactive waste will be disposed of in a deep geological repository constructed in Opalinus Clay. Gas generation is expected in the repository due to the decomposition of organic materials and the corrosion of metals, with carbon steel being the primary source. The corrosion behaviour of mild steel under anoxic conditions has been studied over the course of several years to better understand the long-term hydrogen evolution profile under anticipated repository conditions. Steel, either bare or encased within mortar, was tested in water vapour or immersed in electrolytes representative of aged cement waters at 50°C. The corrosion rate was measured indirectly through the hydrogen analysis using a solid-state probe. The hydrogen evolution behaviour of grout was also monitored to more accurately determine the hydrogen generating from the corrosion of the embedded steel. For steel in water vapour or in alkaline environments, embedded in cementitious material or immersed in simulated aged cement pore water, corrosion rates were invariably <1 nm/year after several years of analysis.     

The design of copper-coating overpack for the high-level radioactive waste disposal concept in Japan

For the geological disposal system in Japan, a vitrified waste will be contained in a metal overpack, which, in turn, will be surrounded by a thick bentonite buffer. The overpack is aiming to prevent the contact of groundwater to vitrified waste during the high radioactivity and heat generation period of the first 1,000 years at least after emplacement. Within the Japanese program, consideration for overpack candidate materials has included carbon steel, copper–steel composite, and titanium–steel composite. Within the extensive safety assessment conducted in 2000, steel was selected. This selection was partly based on the manufacturability of carbon steel, as well as its well-understood corrosion behavior. However, the understanding of copper corrosion and welding/manufacturing technologies have greatly progressed over the past two decades. In this study, we focus on the copper-coated container developed by the Nuclear Waste Management Organization, because this technology is seemingly effective to maintain very long-term containment of hundreds of thousands of years, with a low cost of manufacturing within the Canadian program. We are investigating the applicability of the copper-coating technologies in terms of the corrosion allowance and mechanical design.     

Sulphide-induced stress corrosion cracking and hydrogen absorption of copper in deoxygenated water at 90°C

Stress corrosion cracking (SCC) of oxygen-free phosphorous-alloyed copper was investigated in sulphide- and chloride-containing deoxygenated water at 90°C with sulphide concentrations of 0.001 and 0.00001 M. Several intergranular defects were found in the specimen exposed to the high sulphide environment. Similar defects were not found in the low sulphide environment, where only slight corrosion on grain boundaries and slip lines occurred. Hydrogen content measurements show an increase in hydrogen uptake of the plastically deformed specimens, which is dependent on the sulphide concentration and on plastic deformation of copper. However, the highest hydrogen content was measured in friction stir welds, welded in air without shielding gas, and tested in the high sulphide environment. The embedded oxide particles in the weld metal act as local hydrogen trapping sites and selectively react with the sulphide solution. A relatively thick air-formed oxide film covers the copper canisters when deposited, which transforms into a sulphide film in the repository conditions. Thus, some of the coupon specimens were pre-oxidised. The conversion of the pre-existing Cu₂O film into Cu₂S film occurs quickly and the transformation is almost 100% efficient. The structure and properties of the Cu₂S films, susceptibility of copper to sulphide-induced SCC and hydrogen uptake of copper in reducing, anoxic repository conditions are discussed.     

Corrosion behaviour of carbon steel,titanium and titanium alloy in simulated underground water in Beishan area preselected for nuclear waste repository in China

The groundwater will completely infiltrate to the surface of nuclear waste container after the closure of its deep geological disposal. Therefore, the corrosion behaviour of Q235 carbon steel, titanium and titanium alloy, which were the candidates as the container materials for high-level nuclear waste disposal in simulated groundwater solution of Beishan to be served as the preselected high-level nuclear waste disposal area in China at different temperatures, was studied through electrochemical methods including open circuit potential, electrochemical impedance spectroscopy and potentiodynamic polarisation curve measurements. The results show that the corrosion rate of titanium and titanium alloy is lower than that of carbon steel at all temperatures, and they are more promising as container materials. Another phenomenon is that higher temperatures facilitate the protection performance of corrosion products compared with lower temperatures.     

Speciation of copper in high chloride concentrations,in the context of corrosion of copper canisters

Canisters with a cast iron insert for mechanical strength and a 50-mm thick copper shell as corrosion protection are planned to be used for disposal of spent nuclear fuel in Sweden and Finland. Chloride can be considered “beneficial”, as it promotes active dissolution of copper rather than passivation (which might result in pitting), but a high concentration of chloride in solution would increase the driving force for corrosion through the formation of soluble copper chloro complexes. Thermodynamic calculations are performed in this study with the PHREEQC software and three of its accompanying databases, and a comparison with experimental data is performed to select the database to be used when evaluating repository performance. The activity coefficient models are given special attention. For the assessment of chloride-assisted corrosion of a KBS-3 canister, chloride concentrations pessimistically up to 5 mol/kg are used (in Finland and Sweden, the groundwater and bentonite porewater chloride concentrations are not expected to exceed 1 mol/kg). The resulting copper solubilities are then considered in different mass transport cases.     

Predicting the Lifetimes of Nuclear Waste Containers

As for many aspects of the disposal of nuclear waste, the greatest challenge we have in the study of container materials is the prediction of the long-term performance over periods of tens to hundreds of thousands of years. Various methods have been used for predicting the lifetime of containers for the disposal of high-level waste or spent fuel in deep geological repositories. Both mechanical and corrosion-related failure mechanisms need to be considered, although until recently the interactions of mechanical and corrosion degradation modes have not been considered in detail. Failure from mechanical degradation modes has tended to be treated through suitable container design. In comparison, the inevitable loss of container integrity due to corrosion has been treated by developing specific corrosion models. The most important aspect, however, is to be able to justify the long-term predictions by demonstrating a mechanistic understanding of the various degradation modes.     

Effect of hydrogen sulphide on The corrosion behaviour of titanium

In this paper the effect of hydrogen sulphide on the electrochemical and corrosion behaviour of VT 1–0 commercial titanium has been studied in acidic solutions containing polarization and analytical methods. Neither in the presence of sulphate nor chloride ions hydrogen sulphide affected electrochemical corrosion parameters in less acidic solutions but in 1 M sulphuric and hydrochloric acid where the increase of the corrosion rate can be attributed first of all to the decrease of the hydrogen overvoltage. The interpretation of the results is based on considerations including the role of titanium hydrides formed during the processes.     

Materials Options and Corrosion-Related Considerations in the Design of Spent Fuel and High-Level Waste Disposal Canisters for a Deep Geological Repository in Opalinus Clay

Opalinus Clay has been selected as a host rock for a future high-level waste deep geological repository in Switzerland, whereas concepts for spent fuel and high-level waste disposal canisters are currently under development. The development of disposal canisters is done with respect to the expected evolution of the near-field conditions in the repository and requirements set by long-term safety, the handling concept, and manufacturing. A wide range of issues such as manufacturing, materials options, corrosion, closing weld, postweld heat treatment, structural performance, and inspection are considered. A detailed design study has shown that a carbon steel concept may be suitable for a geological repository in Opalinus Clay. Additionally, there are good prospects for using carbon steel design concepts as a substrate for copper coatings, which would largely eliminate gas production as an issue in long-term safety and would avoid the need of postweld heat treatment of steel. In parallel, several possible materials and design options are explored and their feasibility is considered through risk-assessment criteria.     

Modeling microbial sulfate reduction and the consequences for corrosion of copper canisters

The copper sulfide model (CSM) is a one-dimensional reactive transport code for predicting the evolution of the corrosion behavior of a copper canister in a deep geological repository. Here, the CSM has been extended to simulate the microbial reduction of sulfate in the repository and the consequences for corrosion of the canister. Organotrophic and chemotrophic sulfate reduction are represented by Monod kinetics, along with the dissolution of solid organic matter and gypsum as sources of nutrient and an electron acceptor, respectively. Siderite dissolution in the buffer and tunnel backfill materials acts as a source of Fe(II), which can then precipitate the microbially produced sulfide as mackinawite. Results are presented for a simulation representing the expected evolution of the corrosion behavior and repository environment and for a series of sensitivity analyses designed to identify the most important processes in the overall reaction scheme.     

Material corrosion issues for nuclear waste disposition in Yucca Mountain

For more than two decades, an extensive scientific effort has been underway to determine whether Yucca Mountain, Nevada, is a suitable site for a deep underground repository for spent nuclear fuel and high-level radioactive waste. Even though the geologic site is stable, additional engineered barriers are planned, including waste packages, drip shields, and tunnel inverts that will be within the emplacement tunnels. Research is under way into the best materials for corrosion prevention in those engineered barriers to ensure their long-term mechanical integrity.     

Carburization and metal dusting like attack of 9Cr-1Mo steel resulting from sulphide corrosion

The influence of two sulphur-bearing refinery environments: (1) liquid hydrocarbons with steam present, and (2) liquid hydrocarbons with hydrogen present on the corrosion behaviour of commercial 9Cr-1Mo steel has been investigated. It has been stated that the progress of sulphide corrosion is followed by carburization of this steel due to the release of carbon from the sulphur-attacked metal carbides and the diffusion of carbon ahead of the advancing corrosion front. The rapid progress of sulphide corrosion with the presence of hydrogen results in a steep carbon concentration profile and metal dusting of the carburized layer. When metal dusting starts, rapid acceleration of scale growth and localised metal thinning takes place.     

硼酸缓冲溶液中pH值和Cl~-浓度对Cu腐蚀行为的影响

在硼酸缓冲溶液中,采用动电位极化、电化学阻抗谱(EIS)和半导体电容分析方法分别研究了Cu电极的极化行为及其表面人工Cu_2O钝化膜的化学稳定性.结果表明,低pH值、高Cl~-浓度均造成Cu_2O钝化膜的破坏和溶解.高Cl~-浓度时,Cu_2O钝化膜的半导体性质由p型转变为n型,使Cl~-更容易进入钝化膜与Cu~+络合,并破坏钝化膜从而加速腐蚀.高pH值、低Cl~-浓度有利于Cu_2O钝化膜稳定.     

The kinetics of copper corrosion in nitric acid

The strategy for the permanent disposal of high-level nuclear waste in Canada involves sealing it in a copper-coated steel container and burying it in a deep geologic repository. During the early emplacement period, the container could be exposed to warm humid air, which could result in the condensation of nitric acid, produced by the radiolysis of the humid air, on the copper surface. Previous studies have suggested that both nitrate and oxygen reduction will drive copper corrosion, with the nitrate reduction kinetics being dependent on the concentration of soluble copper(I) produced by the anodic dissolution of copper in the reaction with oxygen. This study focused on determining the kinetics of nitrate and oxygen reduction and elucidating the synergistic relationship between the two processes. This was investigated using corrosion potential and polarization measurements in conjunction with scanning electron microscopy and X-ray photoelectron spectroscopy. Oxygen reduction was shown to be the dominant cathodic reaction with the oxidation of copper(I) to copper(II) by nitrate, promoting the catalytic cycle involving the reaction of copper(II) with copper to reproduce copper(I).     

Autoklaven-Untersuchungen der Spannungsrißkorrosion von Fe-Cr-Ni-Legierungen in NaCl/Co2/H2S-Medien

Autoclave investigation of stress corrosion cracking behaviour of Fe-Cr-Ni alloys in NaCl/CO₂/H₂S-environment In oil and gas production, the corrosion problems increase as the depth of the reservoirs increases. The oil and gas products contain chloride-rich waters and mixtures of H₂S and CO₂ at high pressures and temperatures. Materials that can be used under these conditions are only high strength high alloy steels and nickel base alloys. These materials must be assessed for corrosion resistance under these conditions. The environment contain chloride ions and hydrogen sulphide, which are known to be critical components for SCC. With the aid of autoclave experiments, the fields of corrosion resistance for the materials no. 1.4462, 1.4563 and 2.4618 were determined as a function of temperature and hydrogen sulphide pressure. The base environment was a 5 Molar sodium chloride solution at 20 bar carbon dioxide. While the corrosion resistance of the duplex steel, material no. 1.4462, decreases markedly as the strength of the material and the hydrogen sulphide pressure increase, the two austenitic materials are completely resistant up to 300 °C and hydrogen sulphide pressure of 15 bar. Only at 300 °C and high partial pressures of hydrogen sulphide the material no. 1.4563 did fail, when stressed to stress levels higher than the YS. The crack path was predominantly transgranular with minute fractions of intergranular cracking. The microstructure appears to have no effect. All results indicate that a mixed mechanism of hydrogen- and chloride induced SCC is operting, while a corrosion enhancement due to interaction of both critical components takes place.     

HCO3-和SO42-对Cu点蚀行为的影响

在不同浓度配比的HCO₃^-和SO₄^2-混合溶液中,利用循环极化电化学测试方法和SEM,对Cu工作电极的循环极化行为和点蚀表面形貌进行了系统的研究.结果表明,在高电位范围的循环极化实验中,Cu的点蚀行为可分为活性溶解型点蚀和钝化膜破裂型点蚀;随SO₄^2-浓度的升高Cu点蚀的敏感性增大.由于HCO₃^-与SO₄^2-的协同作用,随HCO₂^-浓度升高点蚀敏感性呈先增大后减小的规律.在钝化膜破裂型点蚀中,SO₄^2-提高Cu点蚀的诱发能力;HCO₃^-降低Cu点蚀的诱发能力.2种离子对点蚀自修复能力的影响无明显规律.     

Anticipated Degradation Modes of Metallic Engineered Barriers for High-Level Nuclear Waste Repositories

Metallic engineered barriers must provide a period of absolute containment to high-level radioactive waste in geological repositories. Candidate materials include copper alloys, carbon steels, stainless steels, nickel alloys, and titanium alloys. The national programs of nuclear waste management have to identify and assess the anticipated degradation modes of the selected materials in the corresponding repository environment, which evolves in time. Commonly assessed degradation modes include general corrosion, localized corrosion, stress-corrosion cracking, hydrogen-assisted cracking, and microbiologically influenced corrosion. Laboratory testing and modeling in metallurgical and environmental conditions of similar and higher aggressiveness than those expected in service conditions are used to evaluate the corrosion resistance of the materials. This review focuses on the anticipated degradation modes of the selected or reference materials as corrosion-resistant barriers in nuclear repositories. These degradation modes depend not only on the selected alloy but also on the near-field environment. The evolution of the near-field environment varies for saturated and unsaturated repositories considering backfilled and unbackfilled conditions. In saturated repositories, localized corrosion and stress-corrosion cracking may occur in the initial aerobic stage, while general corrosion and hydrogen-assisted cracking are the main degradation modes in the anaerobic stage. Unsaturated repositories would provide an oxidizing environment during the entire repository lifetime. Microbiologically influenced corrosion may be avoided or minimized by selecting an appropriate backfill material. Radiation effects are negligible provided that a thick-walled container or an inner shielding container is used.     

Corrosion and Cavitation Erosion Behaviours of Cast Nickel Aluminium Bronze in 3.5% NaCl Solution with Different Sulphide Concentrations

The effect of sulphide(Na_2S) concentration(SC) on the corrosion and cavitation erosion behaviours of a cast nickel aluminium bronze(NAB) in 3.5% NaCl solution is investigated in this study.The results show that when the SC exceeds 50 ppm,the hydrogen evolution reaction dominates the cathodic process,and a limiting current region appears in the anodic branch of the polarisation curve due to the formation of a copper sulphide film,which is a diffusion-controlled process.After longterm immersion,the increased mass loss rate of NAB with the sulphide additions of 20 and 50 ppm is attributed to the less protective films,which contains a mixture of copper oxides and sulphides.Moreover,NAB undergoes severe localised corrosion(selective phase corrosion,SPC) at the β' phases and eutectoid microstructure α+κ_Ⅲ.By comparison,NAB undergoes general corrosion and a copper sulphide film is formed in 100 and 200 ppm sulphide solutions.Cavitation erosion greatly increases the corrosion rate of NAB in all solutions and causes a negative potential shift in 3.5% NaCl solution due to the film destruction.However,a positive potential shift occurs in the solutions with SC higher than 50 ppm due to the accelerated mass transfer of the cathodic process.The cavitation erosion mass loss rate of NAB increases with the increase of SC.The occurrence of severe SPC decreases the phase boundary cohesion and causes brittle fracture under the cavitation impact.The corrosion-enhanced erosion is the most predominant factor for the cavitation erosion damage when the SC exceeds 50 ppm.