Waste Forms Technology and Performance Final Report (2011) / Chapter Skim
Currently Skimming:
7 Waste Form Performance in Disposal Systems
7 Waste Form Performance in Disposal Systems
Pages 175-196
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 175... ...
calls for the identification and description of "state of-the-art tests and models of waste forms used to predict their performance for time periods appropriate to their disposal system." This chapter provides a discussion of the modeling portion of this charge, including waste form performance in disposal systems and models for evaluating waste form performance. Waste form testing is discussed in Chapter 5.
|
From page 176... ...
Radioactive Waste Generation and Management Interdependencies, which states:1 Since the steps of radioactive waste management occur at different times, there are, in practice, many situations where decisions must be made before all radioactive waste management activities are established. As far as reasonably practicable, the effects of future radioactive waste manage ment activities, particularly disposal [emphasis added]
|
From page 177... ...
• Evaluate the radiological safety of disposal systems in the larger context of costs, schedules, alternative options, and optimization of overall waste management policies. This chapter focuses primarily on the first type of PA modeling: i.e., assessment and demonstration of compliance with regulatory guidelines.
|
From page 178... ...
Repository programs typically employ a hierarchy of PA models to assess long-term safety, barrier design, and regulatory compliance of disposal systems containing radioactive waste. This hierarchy, referred to here as the PA Pyramid, is illustrated in Figure 7.2.
|
From page 179... ...
. 7.3 MODELS FOR WASTE FORM PERFORMANCE IN DISPOSAL SYSTEMS As noted in Chapter 6, the dominant potential pathway for radionuclide release from a disposal facility to the biosphere is via groundwater transport.
|
From page 180... ...
. The application of mass-transfer models to disposal system performance assessment is now routine for all types of radioactive waste (e.g., Andra, 2005; JNC, 2000; Nagra, 2002; SKB, 2006)
|
From page 181... ...
. Such a buffer has several important safety functions, including promoting diffusive transport of all radionuclides released from the dissolution of waste forms.
|
From page 182... ...
. Performance analyses of LLW disposal systems also typically apply these solubility limits as controls on radionu 5 The timeframe for regulatory compliance and the half-lives of key radionuclides present in the waste are also considerations; the dissolution rate of a waste form may limit radionuclide releases from disposal systems for an initial period before the onset of solubility limits imposed by precipitation of radionuclide-bearing solids.
|
From page 183... ...
Application of PA models for disposal systems can place diverse factors such as waste-form dissolution rate, waste loading, and solubility limits of the solid phases containing radionuclides into a common system-level context for evaluation and optimization. Furthermore, such PA models can also provide guidance to future decisions on whether there is a safety-based reason for further development of advanced waste forms.
|
From page 184... ...
, and fractional dissolution rates. For the reference fractional dissolution rate of 10–5 parts per year, the releases of selenium-79, technetium-99, and neptunium-237 from the disposal system would be constrained by their respective solubility limits, TABLE 7.1 Sensitivity of Calculated Flux Ratios Using Equation 7.1 for Radioelements with a Key Long-lived Radionuclide Present in a Reference HLW Borosilicate Glass Waste Loading Solubility, Fractional CSAT Radioelement/ (kg of radionuclide/ Dissolution Rate Flux (kg/m3)
|
From page 185... ...
It would require a speculative fractional dissolution rate on the order of 10–9 parts per year (i.e., the waste form would take 1 billion years to completely dissolve) for a waste form to control, and thereby lower, the release rates of these key radionuclides from the disposal system.
|
From page 186... ...
! 7.3.2 Intermediate Level Models Significant insights on the performance of waste forms in disposal systems can be gained from the application of relatively simple performance assessment models described in the previous section.
|
From page 187... ...
Such models can be used to obtain best-estimates of waste form performance in particular disposal environments; to select suitable combinations of waste forms and engineered-barrier configurations; and to evaluate system performance using metrics other than dose, which can aid in optimizing facility designs. More complex models for waste form durability need to account for waste form material properties (Chapter 3)
|
From page 188... ...
To understand the underlying mechanism it was necessary to conduct a series of experimental studies supplemented with detailed microscopic characterizations of the evolving glass surface layers. Similar studies will be necessary for any candidate waste forms considered by DOE-EM because a mechanistic understanding of the controls on waste form dissolution provides the basis for understanding waste form performance at long time scales.
|
From page 189... ...
• redox conditions • speciation in solution • radiolysis • interactions with corroded canisters and near-field geology • formation and mobility of colloids These individual factors often interact and are coupled in a repository environment. The dissolution of waste forms containing radioactive waste can be complex, particularly following the closure of a disposal facility when thermal, radiological, mechanical, hydrological, and chemical perturbations to the disposal system are highest.
|
From page 190... ...
The challenge for using coupled models to evaluate waste form performance in disposal systems is the identification of key processes in the near-field environment, including: • Rate-limiting steps. These could include the dissolution mechanism of the waste form; the formation and decomposition of radiolytically produced species in solution; initial surface sorption/desorption reac tions; or the nucleation and precipitation of secondary phases.
|
From page 191... ...
7.4 DISCUSSION The PA of waste forms containing radioactive waste can only be meaningfully accomplished within the context of disposal system PA, in which health-risk consequences are the appropriate basis for evaluations. As shown in Figure 7.2, there is typically a hierarchy of PA models employed in assessing any waste form/disposal system, with each level of PA models having the appropriate fitness for purpose, for example, design optimization, identification of risk-informed R&D needs, or regulatory compliance.
|
From page 192... ...
PA can provide safety-based insights to guide future decisions on further development of waste forms. The most notable application relates to estimating thresholds at which extremely low waste-form dissolution rates would control (and simplify the calculation of)
|
From page 193... ...
2008. Yucca Mountain Repository License Application: Safety Analysis Report, DOE/ RW-0573, Rev 0, Office of Civilian Radioactive Waste Management, Las Vegas, N.V.
|
From page 194... ...
2009. Survey of National Programs for Managing High-level Radioactive Waste and Spent Nuclear Fuel, NWTRB, Arlington, Va.
|
From page 195... ...
2006. Geological Challenges in Radioactive Waste Isola tion: Fourth Worldwide Review, LBNL-59808, Lawrence Berkeley National Laboratory, Berkeley, Calif.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.