Waste Forms Technology and Performance: Final Report (2011)

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Findings and Recommendations

The task statement for this study (Box 2.1 in Chapter 2) calls on the National Academies to provide “Findings and recommendations … to assist DOE in making decisions for improving current methods for processing radioactive wastes and for selecting and fabricating waste forms for disposal.” Findings and recommendations are provided in this chapter. Support for these findings and recommendations can be found in Chapters 2-9.

The task statement specifically enjoins the committee that carried out this study (Appendix A) from making “recommendations on applications of particular production methods or waste forms to specific EM waste streams.” Although the committee has not made recommendations on specific applications, it has identified potential opportunities for applying waste forms and production methods to DOE-EM waste streams. The committee has focused on waste forms and production methods for high-level radioactive waste (HLW) streams because they represent the highest-cost and highest-risk waste streams in the DOE-EM cleanup program (see Chapter 2). The committee recognizes that DOE-EM decisions to adopt any of these committee-identified opportunities involve policy, regulatory, and technical considerations, the former two of which are well outside the scope of this study.

Findings to address the five study charges shown in Box 2.1 in Chapter 2 are given below and are followed by two overarching findings and one overarching recommendation.

FINDING ON STUDY CHARGE 1

The role of waste forms in disposal systems is discussed in Chapters 6 and 7. The primary role of a waste form is to immobilize radioactive and hazardous constituents in a stable, solid matrix for disposal. The waste form and other engineered barriers in the disposal system, if present, work in concert to isolate the waste. The near-field environment¹ of the disposal system establishes the physical and chemical bounds within which the waste form performs its sequestering function.

The capacity of a waste form for immobilizing radioactive and hazardous constituents depends on intrinsic properties of the material, as discussed in Chapter 3. Some materials have the capacity to chemically incorporate radioactive and hazardous constituents at atomic scales. Other materials have the capacity to encapsulate constituents by physically surrounding and isolating them.

Durability is a measure of the physical and chemical resistance of a waste form material to alteration and the associated release of contained radioactive and hazardous constituents. The durability of a waste form material depends on its intrinsic properties as well as the physical and chemical conditions in the disposal facility into which it is emplaced. Waste forms perform optimally in a disposal environment 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.

FINDING ON STUDY CHARGE 2

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¹ The near-field environment is generally taken to include the engineered barriers in a disposal facility (e.g., waste canisters) as well as the host geologic media in contact with or near these barriers whose properties have been affected by the presence of the facility.

Regulatory and legal requirements are described in Chapter 8. There are well-established regulatory requirements for assessing the long-term performance of disposal systems to meet human health-protection standards; for example, DOE Order G 430.5 for disposal of low-level radioactive waste; Title 40 Part 191 of the Code of Federal Regulations for disposal of defense transuranic waste in the Waste Isolation Pilot Plant in New Mexico; and Title 10 Part 63 of the Code of Federal Regulations for disposal of spent nuclear fuel and HLW at Yucca Mountain, Nevada. Not all of these requirements have a technical basis, and none establishes specific requirements for waste form performance.

There are also established technical criteria for waste acceptance in current and planned disposal facilities; for example, the Waste Acceptance System Requirements Document (WASRD) for HLW and spent nuclear fuel managed by DOE’s Office of Civilian Radioactive Waste Management.² Some of these criteria establish requirements for specific characteristics of the waste form in terms of physical or chemical characteristics, but they do not establish requirements for waste form performance.

DOE has signed agreements with two states (Washington and South Carolina) that specify the types of waste forms that will be used for immobilizing the low-activity waste (LAW) fraction of HLW at those sites: Saltstone for LAW immobilization at the Savannah River Site and borosilicate glass, or another waste form that is “as good as glass” (see Sidebar 8.1 in Chapter 8), for immobilizing LAW that will be produced in the Waste Treatment Plant at the Hanford Site. DOE has also selected waste forms for immobilizing sodium-bearing waste and HLW calcine at the Idaho Site.

The lack of waste form performance requirements gives DOE flexibility in selecting waste forms for immobilization and disposal of waste in con-

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² This office was being subsumed into DOE’s Office of Nuclear Energy when the present report was being finalized.

sultation with its regulators and other Agreement stakeholders. Moreover, the ability of DOE to modify its Agreements (again in consultation with its regulators and stakeholders) is evident from the numerous past modifications to reflect scope and schedule changes. The established flexibility in such Agreements provides DOE-EM with the opportunity to pursue optimization of its overall waste management system, including the consideration of new waste forms and processing methods to reduce costs and risks and increase efficiencies. Of course, such alterations have to be supported by scientifically sound analyses.

The Resource Conservation and Recovery Act (RCRA) requirements for disposal of hazardous waste, which DOE has agreed to follow under Order 5400.1, could reduce DOE-EM’s flexibility to pursue optimization of its overall waste management system, especially for disposal of Hanford HLW/LAW and Idaho HLW. Vitrified HLW from Savannah River and West Valley currently qualify for disposal because they meet the Environmental Protection Agency’s (EPA’s) Best Demonstrated Available Technology (BDAT) requirements. However, it is not clear whether immobilized Hanford HLW/LAW and Idaho HLW would also satisfy RCRA requirements under a BDAT rationale. DOE-EM will need to consult with its regulators (EPA and states hosting the disposal facilities for these waste streams) to clarify this issue.

Scientific and technical requirements for waste form performance are described in Chapters 5 and 8. Borosilicate glass was selected for immobilization of defense HLW in the 1980s based on the industrial simplicity of the process, extensive experience in Europe, adequate waste loading, acceptable processing rates processing costs, durability, and a number of other factors. It was judged that borosilicate glass would provide acceptable performance in any of the several geologically diverse repository host rocks (salt, basalt, granite, tuff, and clay) then under consideration (see Section 8.3.3 in Chapter 8).

Advances in science and technology can inform future waste form selection decisions that could reduce costs, expedite schedules, reduce risks, and improve stakeholder acceptance. The absence of specific waste form performance requirements means that a risk-informed, adaptive repository program should readily accommodate new waste forms through the itera-

tive process of modifying the repository design and updating performance assessment, as discussed in Chapter 7.

Reliance on solubility controls on the release of radionuclides, independent of the waste form, could also aid in evaluating strategies for the future development of advanced waste forms. As an example, a radionuclide released from a glass might arrive at a low concentration because of the low solubility product of secondary phases. This is often the case for actinides. In this case, it does not matter what the waste form is (assuming that it meets other waste acceptance criteria) because the concentrations in solution are controlled by secondary phases. In the case where the calculated releases from a disposal system meet safety criteria because of radioelement solubility limits, then the motivation for developing advanced waste forms would be based more on factors such as waste loading and ease of processing rather than durability.

FINDING ON STUDY CHARGE 3

Waste form tests have several purposes, as discussed in Chapter 5. Tests can be used to identify ranges of processing variables that result in acceptable waste forms (production consistency testing). Tests, combined with experimental studies, can also be used to determine mechanisms of release of radioactive and hazardous constituents from waste form materials over short (days to months) time scales. Once release mechanisms are determined, tests can be used to measure waste form release rates over short time scales. The release mechanisms and rates can be used in modeling studies to estimate long-term (10³-10⁶ year) waste form performance in specific disposal environments.

A suite of waste form tests have been developed; these are described in Chapter 5. These tests are material-specific, and no single test can be used

to elucidate waste form durability in a given material. Tests to determine release behavior and measure release rates have been developed and qualified for borosilicate glass, glass-ceramic, and some crystalline ceramic materials. However, these tests have not been qualified for some other classes of waste form materials, including non-silicate glasses, hydroceramics, and geopolymers. Additional work will be needed to determine the suitability of existing tests for these materials if DOE-EM intends to use them in its cleanup program.

Models of waste form and disposal system performance are described in Chapter 7. Models can be useful for predicting waste form performance in disposal systems when they are based on an adequate scientific understanding of waste form–near-field interactions and reactive transport in those systems. Most critically, valid estimates of waste form performance cannot be made in the absence of knowledge about the near-field environment of the disposal system.

Many of the current models that are being used in the United States to model waste form behavior in disposal systems are based on ad hoc simplifications specific to the proposed repository at Yucca Mountain, Nevada. Other national programs have developed a substantial capability for modeling the long-term behavior of some types of waste forms based on fundamental principles; for example, the GLAMOR program in Europe is a cooperative effort of several researchers, including researchers from the United States, to elucidate the mechanisms controlling long-term durability of vitrified high-level waste.

U.S. regulations have adopted risk-based health standards for assessing the long-term safety of geological disposal using performance assessment (PA) models. PA modeling of waste forms containing radioactive waste can only be meaningfully accomplished within the context of PA modeling of the entire waste disposal system, in which health-risk consequences are the appropriate basis for evaluations. There could be significant benefits in providing more realistic and risk-informed safety analyses by improving these models to capture the full complexity of waste form–near-field interactions, including the durability of waste forms as well as waste form interactions with other engineered and natural barriers in the near-field environment.

Additional R&D on waste form–near-field interactions and reactive transport would likely improve quantitative modeling capabilities for esti-

mating long-term waste form performance in different disposal environments. Having such an improved modeling capability could allow DOE-EM to take credit for waste form performance in future disposal system performance assessments. In addition, study of relevant natural analogue materials, where available, could also provide additional lines of evidence and arguments to increase confidence in waste form performance over 10³-10⁶ year time scales.

FINDING ON STUDY CHARGE 4

Waste form production methods are described in the committee’s interim report (see Appendix C) and in Chapter 4 of this report. The committee identified three opportunities for more efficient production of waste forms in its interim report:

• Fluidized bed steam reforming for conditioning waste feed streams and processing HLW and associated waste streams.
• Cold crucible induction melters as substitutes for Joule-heated melters for processing HLW and LAW.
• Hot isostatic pressing for processing waste streams that are difficult or inefficient to process by other methods.

These identified opportunities are just examples; there are probably many other good ideas that have not yet been investigated.

Chapter 4 of this report provides a more complete discussion of processing technologies and their potential applicability to DOE-EM waste streams. Chapter 9 describes some recent advances in computational science and recently emerging tools in computational fluid dynamics that have applicability in the DOE-EM cleanup program.

FINDING ON STUDY CHARGE 5

As discussed in Chapter 3, there are a wide range of waste form materials that could potentially be used in the DOE-EM cleanup program: single-phase (homogeneous) glasses, glass-ceramic materials, crystalline ceramics, metals, cements, geopolymers, hydroceramics, and ceramicretes. The baseline technology for immobilization of HLW in the cleanup program is single-phase borosilicate glass. Other waste form materials are potentially suitable for HLW immobilization:

• Other types of glass (e.g., iron phosphate glass) might be useful for immobilizing waste streams with constituents that are sparingly soluble or chemically incompatible with borosilicate glasses (e.g., phosphate and sulfate).
• Crystalline ceramic waste forms produced by fluidized bed steam reforming have good radionuclide retention properties and waste loadings comparable to, or greater than, borosilicate glass. This waste form material is also potentially useful for immobilizing LAW.

Examples of other opportunities are identified in Chapter 9 of this report for immobilizing actinides and/or fission products in

• Glass-ceramic materials
• Crystalline ceramics (e.g., pyrochlore, murataite, garnet, and apatite)
• Metal-organic frameworks
• Mesoporous materials

Additional research and development work will be required to apply these materials in the DOE-EM cleanup program.

No single waste form is suitable for all EM waste streams or suitable for all disposal environments. Consequently, DOE-EM would benefit from having a “toolbox” of waste forms available for different waste streams and disposal environments. However, compatibility of the waste form with its intended disposal environment is not the only important consideration when making a selection decision, as explained in the following overarching finding.

OVERARCHING FINDINGS AND RECOMMENDATION

DOE-EM asked the National Academies to examine “requirements for waste form technology and performance in the context of the disposal system in which the waste form will be emplaced” (see Box 2.1 in Chapter 2). The phrase “in the context of the disposal system in which the waste form will be emplaced” explicitly recognizes that waste form requirements do not exist in isolation of the overall DOE-EM waste management system (Figure 7.1). Consequently, decisions on waste form development and selection are best made in a systems context. Additionally, because the ultimate goal of disposal is to protect public health, such development and selection decisions are best made (to the extent practical) on public health risk considerations.

To illustrate this point, consider the selection of a waste form for immobilizing HLW containing technetium-99. As noted in Chapter 6, technetium-99 is soluble in groundwater under oxidizing conditions and can therefore be mobile in the environment. Consequently, an important consideration in selecting a waste form for immobilizing HLW is its capacity to sequester technetium-99, for example by chemical incorporation (Chapter 3), to reduce the mobility of this radionuclide after disposal. However, there are other systems considerations that are equally important in this selection decision, for example:

• Is the process for making the waste form compatible with the waste stream? One might select a durable waste form such as borosilicate glass for immobilizing a HLW stream. However, the process for making glass (vitrification) can drive technetium and other volatile radionuclides into off-gas streams, which creates secondary waste that can be difficult to manage.
• Is the waste form suitable for its intended disposal environment? As noted in Chapter 6, the long-term durability of a waste form depends on the physical and chemical conditions in the disposal environment in which it is emplaced. Borosilicate glass waste forms are durable in many, but certainly not all, disposal environments. Disposal of borosilicate glass in an environment that is under-saturated in silica, for instance, could result in accelerated degradation and release of technetium-99.
• Will the waste form function with other barriers in the disposal facility to protect public health? As discussed in Chapter 6, the

This example illustrates the importance of understanding the interactions among the various elements of the waste management system when making waste form selection decisions. Critical factors can be overlooked, and suboptimal decisions can be made, when waste form selections are considered in isolation of other system components.

As the committee observed in its interim report (see Appendix C), the DOE-EM cleanup program is successfully processing waste and producing waste forms at several sites (see also Chapter 2 of this report). For example, DOE-EM has completed HLW immobilization at the West Valley site, but residual liquid and sludge heels remain in the tanks. DOE-EM is also retrieving HLW from tanks at the Savannah River Site, separating it into high-activity waste (HAW) and LAW streams, and processing these waste streams into HLW glass for disposal in a future geologic repository and LAW Saltstone for near-surface onsite disposal. DOE-EM is also building facilities to process and immobilize HLW at the Hanford Site in Washington.

As the cleanup program continues DOE-EM will have opportunities to incorporate emerging developments in science and technology on waste forms and waste form production technologies into its baseline approaches. As noted in Chapters 3, 4, and 9, waste form-relevant science and technology are advancing rapidly along several fronts—for example, materials science research and development, chemical and materials processing in industry, waste management in advanced nuclear fuel cycle programs, and management of special nuclear materials in national security applications. These advances could lead to the development of

• Waste form materials designed for higher waste loadings or for improved performance in specific disposal environments.
• Waste processing technologies that can handle large volumes of highly radioactive wastes, operate at high throughputs, and/or produce high-quality waste forms.
• Advanced analytical and computational techniques that can be used to understand and quantitatively model interactions between waste forms and near-field environments of disposal facilities.

The committee’s interim report (see Appendix C) and this final report provide only snapshots of these advances.

Computational techniques for materials discovery and design have longer-term applications in the DOE-EM cleanup program. Computational simulations can be used to investigate new waste form compositions or structure types and to focus experimental efforts on critical chemical systems and conditions.

Incorporating new science and technology need not (and should not) halt the progress that is currently being made in the cleanup program. In fact, if done wisely, the incorporation of new science and technology can improve the cleanup program by increasing efficiencies, reducing lifecycle costs and risks, and advancing scientific understanding of and stakeholder confidence in waste form behavior in different disposal environments. In short, scientific advances, both now and in the future, offer the potential for more effective solutions to DOE-EM’s waste management challenges.

To take full advantage of future scientific and technological advances, DOE-EM will need to identify, develop where needed, and incorporate where appropriate state-of-the-art science and technology on waste forms, waste form production processes, and waste form performance. This will require:

• Active engagement with governmental, academic, and industrial organizations that are researching, developing, and implementing these technologies.
• Development and/or expansion of intellectual capital, both within DOE-EM and in external contractor staff, to identify and transfer this knowledge and technology into the cleanup program.
• Appropriate resources to support these capabilities.

Such engagement can take a variety of forms: For example, DOE-EM could collaborate or partner with the DOE Office of Science and Office of Nuclear Energy to identify and, where appropriate, fill knowledge gaps on waste forms, waste form production, and waste form performance.³ International organizations and large-scale chemical processing industries are also potentially rich sources of information. DOE-EM is already engaging with other organizations for some of its technology development needs: Examples include the development of fluidized bed steam reforming and cold crucible induction melter technologies, which are discussed in Chapter 4. With carefully targeted investments, the costs of establishing and maintaining such collaborations need not be high.

As discussed in Chapter 8, DOE-EM is operating its cleanup program under various and sometimes conflicting regulatory requirements and legal agreements with states and the U.S. Environmental Protection Agency. Modifications of existing requirements or agreements might be necessary before DOE-EM can implement the technologies identified in this report. However, it is outside of the committee’s task to consider how the use of the technologies identified in this report might impact those requirements and agreements.

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³ The Office of Science, for example, sponsors research needs workshops that are relevant to EM needs (see http://www.er.doe.gov/bes/reports/files/brn_workshops.pdf and http://www.er.doe.gov/bes/reports/list.html). The Office of Nuclear Energy sponsors a fuel cycle R&D program. See http://www.ne.doe.gov/fuelcycle/neFuelCycle.html.