Waste Forms Technology and Performance Final Report (2011) / Chapter Skim
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6 Waste Forms and Disposal Environments
6 Waste Forms and Disposal Environments
Pages 153-174
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From page 153... ...
Consequently, the durability of a waste form in a disposal environment can be optimized by matching it with the appropriate physical and chemical conditions that foster long-term stability. The focus of this chapter is on waste form performance in disposal facilities.
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From page 154... ...
6.1 DISPOSAL OF WASTE FORMS Waste forms containing radioactive waste are intended for disposal in engineered facilities constructed in stable geologic formations. Geologic repositories are designed for the disposal of higher-hazard wastes3 such as SNF, HLW, and TRU waste.
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(b) Figure 6.1a.eps bitmap FIGURE 6.1 Examples of multiple-barrier disposal facilities for (a)
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Decades of research nationally and internationally have created a wealth of world-wide accumulated experience concerning the disposal of radioactive waste in various geologic environments (Table 6.1)
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157 WASTE FORMS AND DISPOSAL ENVIRONMENTS TABLE 6.1 Geologic Investigations of Potential Sites for Disposal of SNF/HLW SOURCE: NWTRB (2009)
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A waste form emplaced in a salt deposit well below Earth's surface will not contact flowing water unless a disruptive geologic event such as a meteorite impact or human intrusion takes place. Groundwater flow is thus not a factor in determining waste form performance in salt.
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. However, not all clay deposits have desirable characteristics for emplacement of radioactive waste forms.
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. Disposal of radioactive waste forms in shallow unconsolidated sediments may be suitable for relatively lowactivity materials when combined with appropriate supporting engineered barriers (see Figure 6.1b)
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These fractures provide pathways for the flow of groundwater through the tuff, which can result in the formation of a suite of lowertemperature minerals, such as zeolites, with high ion-exchange capabilities. Groundwater traveling through fractured rocks above the groundwater table contains dissolved oxygen and is therefore oxidizing.
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6.3 FACTORS AFFECTING WASTE FORMS IN DISPOSAL ENVIRONMENTS A disposal environment is defined as the time-dependent physical and chemical conditions in a facility designed for the disposal of radioactive waste. An initially undisturbed geologic environment can be substantially perturbed by the development of a disposal facility.
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From page 163... ...
Degradation can occur through a number of physical and chemical processes, including chemical corrosion through reactions of the waste form with groundwater or physical alteration through buildup of radiation-induced damage or in growth of daughter products. The following sections provide brief descriptions of some key processes and environmental factors that can affect waste form performance and radionuclide mobility in disposal environments.
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From page 164... ...
Consequently, a disposal facility in the vadose zone may experience significantly increased water flow rates and perhaps even fully saturated conditions in the future because of climate change. A disposal facility located below the water table would likely experience less variation as long as it remained saturated with water.
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. • Anionic species are highly mobile and can transport from their point of release from a waste form with the water front.
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The presence of cementitious materials in concretes used in disposal facilities promotes alkaline conditions. This can lead to a significant increase in the dissolution rate of silica-based waste forms, such as borosilicate glass, attributable to the increased solubility of silica at pH > 9.
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167 WASTE FORMS AND DISPOSAL ENVIRONMENTS FIGURE 6.2 Stability diagram for UO2 in various groundwaters at 25°C showing the pH-Eh stability fields for aqueous uranium species. The Eh-pH conditions for groundwaters in tuff (Yucca Mountain, Nevada)
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Terrestrial humic materials are often polycyclic aromatic moieties13 and may also reduce polyvalent metal ions, including actinides. Humic matter derived from aquatic sources tends to be more aliphatic14 in nature and is less likely to produce reducing conditions.
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. 6.3.7 Radioactive Decay and Radiolysis Radioactive decay and radiolysis may have profound effects on waste form performance in disposal facilities.
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This further illustrates the importance of matching waste forms to disposal environments.
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Several of the factors affecting waste forms in disposal environments delineated in the sections above are temperature dependent, as are the complex couplings between such factors. The boiling point of pure water in hydrologically saturated repository environments is about 220°C at a depth of 300 meters and is about 250°C at a depth of 500 meters.
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From page 172... ...
Waste forms in such facilities will perform optimally in a repository when they are matched with the appropriate physical and chemical conditions that foster long-term stability. An important implication of this fact is that the suitability of a waste form for disposal depends crucially on the characteristics of the disposal facility into which it will be emplaced.
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1988. "Spent Fuel," In Radioactive Waste Forms for the Future, W
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From page 174... ...
2009. Survey of National Programs for Managing High-Level Radioactive Waste and Spent Nuclear Fuel: A Report to Congress and the Secretary of Energy, Nuclear Waste Technical Review Board, Arlington, Va.
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