No 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 onedimensional reactive transport model has been developed to predict the evolution of the general corrosion behaviour of the copper canisters by the initially trapped O 2 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.

The mechanism and kinetics of Cu corrosion in anoxic aqueous chloride solutions containing sulfi de (10 -3 mol/L) have been investigated electrochemically and under natural corrosion conditions. Under these conditions Cu is thermodynamically unstable in anoxic water, and the anodic growth of a chalcocite (Cu 2 S)/digenite (Cu 1.8 S) fi lm is supported by the cathodic reduction of water. Electrochemical experiments at rotating disc electrodes and impedance spectroscopy show that the fi lm growth occurs under SHtransport control as stagnant conditions are approached. At this concentration, fi lm growth can follow two distinct pathways. The initially formed fi lm grows rapidly via an ion (or associated defect) transport process. If this fi lm remains coherent, subsequent fi lm growth/corrosion is extremely slow. If the development of interfacial stresses leads to fi lm fracture, then growth continues and a much thicker nodular deposit is formed. The primary goal of this research is to develop a mixed potential model, which can be used to assess the performance of copper nuclear waste containers in granitic nuclear waste repositories.

No abstract

Microbiologically influenced corrosion (MIC) is one of a num-ber of threats to the long-term integrity of nuclear waste containers. As such, the potential for, and extent of, MIC must be assessed and suitable models developed for predicting the long-term behavior of the container. There are two broad approaches to assessing the threat posed by MIC; first, to determine whether the environment will support microbial activity and, if so, where and when it will occur, and second, to estimate the maximum amount of damage that could occur if microbial activity in the repository is possible. A decisiontree approach is used to present evidence for both of these approaches and to decide whether MIC is a significant threat to the integrity of the container. Examples are provided from various international nuclear waste management programs. It is concluded that microbial effects will not compromise the safety of the overall disposal system because they will not lead to either early container failures or to a large number of simultaneous failures, both factors that can lead to an increase in the peak dose.

No abstract

A wide range of alloys have been considered as candidate container materials for the storage and disposal of nuclear waste. The goal of the majority of national nuclear waste management programs is the ultimate disposal of the waste, although, depending upon the strategy being followed, disposal may come only after an extended period of storage. The management strategy depends on the nature of the waste, with intermediate level waste (ILW) generally being stored for a longer period before disposal than is the case for higher activity wastes, such as high-level waste (HLW) from reprocessing activities or spent fuel (SF). This review describes the corrosion issues associated with the storage and disposal of both ILW and HLW/SF. Various factors enter into the decision of which material to select for the container, of which the corrosion behavior in the expected service environment is only one. The corrosion behavior of the container material(s) is closely tied to the nature of the environment to which the containers will be exposed and how that environment changes with time. A general discussion of the corrosion behavior of the materials selected or proposed as container materials is provided, and the specific corrosion issues associated with each class of material highlighted. The classes of material considered for the storage and/or disposal of ILW and HLW/SF include copper, carbon steel and cast iron, stainless steels, titanium alloys, and nickel-based alloys.

A model has been developed to predict the impact of microbiological processes on the long-term corrosion behaviour of copper containers in a deep geologic repository. The model accounts for a range of aerobic and anaerobic microbial processes. Various factors expected to limit the extent of microbial activity in the repository, such as the lack of water, evolving redox conditions, and the nutrient-poor environment, are taken into account in the model. Amongst other effects, the model predicts that microbial activity will not occur close to the container in the presence of highly compacted bentonite buffer material.