The Department of Energy's Office of Environmental Management (DOE-EM) is responsible for cleaning up radioactive waste and environmental contamination resulting from five decades of nuclear weapons production and testing that are stored at over 100 sites across the United States. A major focus of this program involves the retrieval and processing of stored waste to reduce its volume and incorporate it into suitable waste forms to facilitate safe handling and disposal. Waste forms immobilize radioactive and hazardous constituents of wastes in a stable, solid matrix. This report was produced to assist DOE in making decisions for improving current methods for processing radioactive wastes and for selecting and fabricating waste forms for disposal.

The ultimate goal of DOE-EM is to protect human health; therefore, the report concludes that waste form development and selection decisions are best made in a risk-informed, systems context that considers: how the waste form will be produced; in what disposal environment it will be emplaced; and how the waste form will function with other barriers in the multi-barrier disposal system to protect public health. Because the scheduled cleanup program will not be completed for several decades, there is time for DOE-EM to use the many recent advances in waste form science and technology to guide future waste form selection decisions.

Identifying, developing, and utilizing state-of-the-art science and technology on waste forms will require that DOE: actively engage with governmental, academic, and industrial organizations that are researching, developing, and implementing these technologies; develop and/or expand intellectual capital, both within DOE-EM and in external contractor staff, to identify and transfer this knowledge and technology into the cleanup program; have access to appropriate resources.

Key Messages

• Two essential characteristics of waste forms govern their performance in disposal systems: (1) capacity for immobilizing radioactive or hazardous constituents; and (2) durability.
• On regulatory and legal factors: U.S. laws, regulations, and other government directives and Agreements under which DOE-EM operates are not all based on technical factors, and none establish specific requirements for waste form performance in disposal systems. Performance requirements have been established for disposal systems as a whole to meet human health-protection standards; however, waste forms are just one of several engineered barriers in such systems and do not have any subsystem performance requirements. The lack of waste form-specific performance requirements gives DOE-EM flexibility in selecting waste forms for immobilization and disposal of waste in consultation with regulators and other Agreement stakeholders.
• On scientific and technical factors: Scientific and technical considerations have underpinned some waste form selection decisions in the past. Looking forward, DOE-EM has substantial opportunities to use advances in waste form science and technology since these original decisions were made to guide future waste form selection decisions.
• On tests: Waste form tests are used for three purposes: (1) to ensure waste form production consistency; (2) to elucidate waste form release mechanisms; and (3) to measure waste form release rates under a range of conditions. Information on release mechanisms and rates can be used to model waste form behavior in near-field environments over time scales of interest for disposal (10³ - 10⁶ years). Tests have been developed and qualified for some waste form materials. There is a need to demonstrate the application of current tests to new waste forms if they are to be used in the DOE-EM cleanup program.
• On models: Models of waste form performance are used to estimate the long-term (10³ - 10⁶ years) behavior of waste forms in the near-field environment of disposal systems. There is a need to improve these models to capture the full complexity of waste form-near-field interactions.
• Opportunities exist to adapt more efficient waste form production methods to DOE-EM waste streams to reduce costs, expedite schedules, and reduce risks.
• Opportunities exist to develop new waste forms for immobilizing DOE-EM waste streams to reduce costs, expedite schedules, and reduce risks. For examples actinides and/or fission products could be immobilized in glass-ceramic materials; crystalline ceramics (e.g., pyrochlore, murataite, garnet, and apatite); metal-organic frameworks; and mesoporous materials.
• 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 findings.
• OVERARCHING FINDING 1: Waste forms are a central component of the DOE-EM waste management system whose ultimate goal is to protect public health. Consequently, waste form development and selection decisions are best made in a risk-informed systems context by considering, for example: how the waste form will be produced; what disposal environment it will be emplaced in; and how the waste form will function with other barriers in the multi-barrier disposal system to protect public health.
• OVERARCHING FINDING 2: Because the currently scheduled DOE-EM cleanup program will not be completed for several decades, there is time to advance and apply scientific understanding of waste form properties and behavior. Materials, processing technologies, and computational methods are under constant development; these developments could lead to improvements in current DOE-EM cleanup operations as well as new and innovative applications in future cleanup and nuclear fuel cycle programs.
• OVERARCHING RECOMMENDATION: DOE-EM should enhance its capabilities for identifying, developing where appropriate, and utilizing 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.