Metal Container Materials for Nuclear Waste
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MRS BULLETIN/DECEMBER 1994
waste from the accessible environment. The disposal container may be made from a single material or two or more materials. Several countries—France, the United Kingdom, Japan, and Germany—plan to reprocess waste into a vitrified form. The vitrified waste will be furnished in a pour canister (most likely made from a stainless steel), and the pour canister will be overpacked into a disposal container. Canada, Germany, Finland, and Sweden plan to encanister spent fuel rods into a disposal container for emplacement in a repository. The U.S. program will separately encanister both vitrified waste and spent fuel into disposal containers. Because the isotopic content and, therefore, the thermal output from the two kinds of waste are much different in the U.S., different strategies and
Figure 1. Schematic potential-pH (Pourbaix) diagram for an electrochemically active metal in water. The diagram shows stability regions for the metal, its soluble ioqic corrosion products, and its insoluble oxides. Water is stable between the A and B lines.
hence different criteria for container materials selection are anticipated. Some countries will store waste above ground to await repository construction and to allow additional radionuclide decay. Once the aged waste is emplaced in the repository, surface temperature on the container is reduced, as well as radiolysis effects generated by gamma radiation from the waste. Most waste disposal programs are characterizing repository sites where waste will be emplaced below the water table. When the repository is sealed, hydrostatic pressure will build around the waste package. Groundwater can then contact the container surface at temperatures determined by the total thermal output. Temperatures can exceed 100°C, since the prevailing pressure will exceed atmospheric pressure. Waste disposal programs examining salt beds consider the lithostatic pressure from the salt as it creeps around the buried waste package, and the formation of brine pockets in the salt and their migration toward the container. The impact of a rock collapse onto the container must also be considered. The U.S. program differs from other national programs because the repository site (Yucca Mountain) would dispose the waste above the water table. Water temperatures will not exceed the atmospheric boiling point; in fact, the waste heat may be used advantageously to keep the repository dry for a long period of time and to delay aqueous corrosion. Environments An understanding of the many different kinds of corrosion and the environments in which they occur is fundamental to making long-term predictions. Except for copper under certain circumstances, all the candidate metals are electrochemically active in water. Stability diagrams, such as the schematic for an electrochemically active metal, M, shown in Figure 1, illustrate regions where the metal, metal oxides, and ionic species are stable over domains of electrochemical potential and pH.1 The schematic metal in Figure 1 is multivalent, typical for
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