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--> Subcommittee on High-Level Waste in Tanks Introduction The Subcommittee on High-Level Waste in Tanks was formed in the second half of 1995 and has held two meetings. On the basis of the information obtained during those meetings and coupled with the experience of the members, this report is a preliminary assessment of the issues pertaining to technology needs for the disposition of radioactive waste in tanks. The approach was to comment on the technology issues and needs relative to the current approaches being taken for remediation of the tank wastes by DOE and its contractors. Prior to discussing technology needs, a limited amount of background information is provided based principally on Radioactive Tank Waste Remediation Focus Area (USDOE, 1995a). The high-level waste tank issue has to do with the safe management of over 100 million gallons of radioactive wastes in 332 underground storage tanks distributed among five sites. The five sites are Hanford, Savannah River, the Idaho National Engineering Laboratory (INEL), Oak Ridge National Laboratory (ORNL), and West Valley, New York. Of the 332 tanks across the complex, 177 are located at Hanford, which contain over 60 percent of the DOE tank waste (USDOE, 1995b). The waste in the 332 tanks is in the form of sludge, supernate, and saltcake. The wastes at the five sites differ in quantity, age, storage mode, originating process, chemical composition, and physical attribute. In most of the tanks, the wastes, originally formed as acidic solutions of radioactive nuclides, are now in a strong basic medium and hence have various solids associated with the caustic supernate. In addition, some sites have processed some of the wastes to concentrate the solutions and reduce the volumes, resulting in crystallized components of the waste also being present in tanks. Finally, some of the wastes were formed during processing of the original source of waste to remove selected components by processes that differ from
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--> those that formed the waste. In short, wastes in the tanks are a heterogeneous mixture of solutions, sludges, saltcakes, and other phases. Some of the tanks have leaked, and the ability of a significant but unknown number of tanks to confine the liquid phases has been lost. Further, some of the tanks have generated combustible gases leading to the potential of conflagrations or explosions. The current practice at all sites is to manage the safety issues on a high-priority basis. Processing for the final disposition of the contents of the tanks is being implemented at only two of the sites, the Savannah River Site/Defense Waste Processing facility (SRS/DWPF), and the West Valley Demonstration Project (WVDP)10. It is likely that at all of the sites, several types of actions using existing technologies will take place as discussed in the next section and highlighted in Figure 1. The report focuses on the identification of those technology needs that appear to be pertinent, generally, to the five sites currently holding such waste. These sites, i.e., Hanford, Savannah River, West Valley, Oak Ridge, and the Idaho Chemical Processing Plant (ICPP) are in various stages of developing processes and facilities for the conversion of waste in tanks to stable forms of high-level waste (HLW) and low-level waste (LLW)11 (Westinghouse Savannah River Company, 1994; Priebe and Valentine, 1980; Gephart and Lundgren, 1995; INEL, 1994). Technology pertinent to the management of the waste in tanks will include all processes, operations, and facilities used in the conversion of material in the tanks to the desired end products, including disposal of the tanks. The subcommittee has not been provided with, nor has it obtained a comprehensive catalog of the current activities of the various departments of the DOE that are pursuing the development of technologies related to the management of wastes in tanks. The subcommittee has obtained information on parts of some of the programs pertinent to waste in tanks, and members of the subcommittee have had access to information related to such activities. Nevertheless, this report should be considered preliminary. Finally, this report will not include details of technologies pertinent to the management of waste in tanks, but rather will address the general issues posed by these wastes and the planned management strategies. Future reports of the subcommittee will reach beyond the ''planned'' strategies and take a scenario approach to identifying technology needs. This approach is briefly discussed in the following section. 10 With respect to the Defense Waste Processing Facility, some safety aspects are not yet fully resolved in all the steps of the process. The result is that "hot" startup has been postponed until approximately March 1996. With respect to West Valley, following a leak in the melter, a review team identified the need for several improvements in the operation of the process equipment. Operations have been delayed until all improvements are implemented. 11 In some cases, these designations may not be entirely appropriate but are used here to designate waste that is highly radioactive and hazardous (HLW) and waste that is much less radioactive and generally much less hazardous (LLW). Since there is not now an accepted definition of such wastes based on their hazard, the definitions of HLW and LLW currently in use will be maintained.
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--> Figure 1: Relationship of Technology Development to Waste Remediation Scenario Approach It is the subcommittee's intention to emphasize what the scenario evaluations indicate as the overarching needs and issues that directly affect many components of technology development and indirectly affect all aspects of tank waste remediation. The subcommittee chooses a scenario strategy (i.e., how the subcommittee will conduct its investigations, not DOE strategy) as the best way to address overall technology issues and needs. The subcommittee will seek to distinguish between site-specific issues and issues applicable to multiple sites. Figure 1 is an attempt to diagram the relationship of technology development to the disposition of radioactive waste in tanks. The figure is very conceptual and is not an attempt to depict the details of what actually happens. Furthermore, some of the steps are yet to be defined, such as the peer-review process and the use of risk-benefit methods to prioritize the scenarios. For the case of high-level waste in tanks, the range of scenarios and end states might vary from the "no-action" scenario (perpetual surveillance and maintenance), and containment-in-place scenario (stabilize tank contents and tank forms), to scenarios that would lead to vitrified waste for disposal and a pristine site. Of course, all the in-between states involve different degrees of clean up. Except for the "in-place" scenarios, the sequence of events comprising the scenarios would consist of such activities as waste characterization, retrieval from the tanks, processing, immobilization, site closure,
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--> with differing technology-development needs. The result would be a clear display of the technologies involved, including those that are developed and available as well as those needing development. The task then becomes one of assessing the scenarios against the chosen set of performance measures leading to the identification of technology-development requirements. Candidate performance measures are safety of the workers and public, cost and schedule, public acceptance, environmental and economic impact, and technical feasibility. Prioritizing a scenario at this early stage will most likely be done against a much smaller set of performance measures such as safety and regulatory acceptability. Common Technology Needs Evaluation of the presentations made and DOE documents led to the conclusion that there are significant common technology-development needs related to the management of high-level waste in tanks. These needs are listed in Table 1 and are preliminarily ordered in decreasing order of importance. TABLE 1: Technology-Development Needs for Managing High-Level Waste in Tanks that are Common to All Sites12 Waste characterization. Final disposition of tanks containing residual waste. Retrieval of multiphase wastes from tanks with access limitations. Vitrification of wastes, including off gas treatment and recycle of volatile radionuclides. Removal of Cs from supernatant, dissolved solids, and secondary wastes. Disposition of used melters. Both Savannah River and West Valley have progressed in the development of processes and facilities for the treatment of wastes in tanks. Both sites plan to produce HLW glass and process the associated LLW wastes into grout during 1996. For these 12 The commonality of these needs to the Idaho site may be limited because of significant differences in the nature of the waste and the proposed management approach (i.e., acid dissolution).
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--> operations, the technology needs will focus on the final steps of the remediation process, namely the final disposition of the tanks and the residual waste in them. The subcommittee could not determine criteria from any site for the allowed residual waste in tanks or a general outline of plans for the final disposition of the tanks themselves after the remediation process. There could be significant technology-development needs when such decisions are made, because it is likely that complete removal of waste from the tanks is impractical and confinement of residue in the tanks for prolonged periods will be a requirement. Further, if complete removal of the tanks becomes a requirement, it is clear that considerable development and testing of techniques will be required. All of the sites have elected to produce a form of glass as the waste form for HLW. While the details of melter design, size, operation, expected life, and maintenance may be different among the sites, in all cases the melter off gas system will be an important part of the process. The subcommittee believes that technology to recycle the expected volatile components of the melter feed, with particular attention to some of the radioactive materials in that feed, will need to be developed and tested. Further, the disposition of massive highly radioactive melters that are no longer functional or have been subjected to unplanned incidents may require technologies that are not yet available and will need to be developed. Due to the differences in the compositions of the wastes, processes to effect required separations may be somewhat different among the sites. However, removal of multiphase waste from tanks in various states of disrepair will be an activity at all sites. While both Savannah River and West Valley apparently have produced satisfactory waste removal systems, such is not the case for Hanford and Oak Ridge. The waste at Hanford appears to be particularly recalcitrant in that aged sludge, saltcake, and supernate solution may be present in a single tank. The subcommittee concludes that techniques for the retrieval of tank contents, especially in light of the potential for many single-shell tanks to have leaks of radioactive liquids, are a common technology need. The common needs listed in Table 1 are directed primarily at the four DOE sites that have alkaline nitrate supernatant-salt-sludge wastes in tanks. While technology developed at or for one site may not be entirely applicable to the needs at another site, the subcommittee expects that there will be substantial commonalties that point to the need for an integrated technology-development program. The technology-development needs at the Idaho site will be significantly different because the wastes, the bulk of which are now present as solids, will be dissolved into acid solution as opposed to the alkaline wastes at other sites. There is significant European experience with processing and immobilizing acidic wastes that may be relevant, although the Idaho wastes are expected to have much higher sodium, aluminum, and fluoride content than those in Europe. It is recognized that there may be other potentially common technology-development needs and that numerous site-specific technology-development needs
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--> also exist. Technology-development activities related to all of these should continue to be integrated by an entity such as the Tank Focus Area. Specific Technology Needs The subcommittee has not had sufficient time to evaluate site-specific technology-development programs and believes that broader issues discussed herein should take precedence. Therefore, no specific delineation of such needs will be made until the subcommittee has had a better opportunity to examine the various programs in more detail. Major Concerns A number of concerns that profoundly affect the technology-development efforts depicted in Figure 1 are given in Table 2. These issues are thought to be inadequately addressed relative to the needs of the technology-development program. However, the information gathered thus far is not sufficient to state this as a conclusion. Some of the concerns (e.g., undefined end points, technology-development needs in a privatization scenario, and trade offs) in Table 2 define the boundary conditions for technology development and thus the nature and extent of technology development. The other concerns are more directly related to defining how much technology development is enough (or too much). If present approaches prevail, endpoints will be established by joint agreement among the cognizant DOE field office, the cognizant EPA, the host state, and possibly, Native American tribes. However, it is clearly within DOE's authority and ability to examine the history and trends in end points (i.e., waste-acceptance criteria) and establish moderately conservative interim end points that will provide a consistent focus for technology development.
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--> TABLE 2: Major Concerns Affecting Development of Technology for Managing High-Level Waste in Tanks Performing technology development when the end points (especially waste acceptance and tank-disposition criteria) are changing or unspecified, or when unrealistic requirements are imposed. Technology-development needs in a privatization scenario. The necessity and desirability for vitrifying low-level waste. Accommodating top-level tradeoffs (a) between the need for technology development and cost, schedule, and performance; and (b) between the performance requirements of major process operations such as characterization and pretreatment. Determination of the extent to which technologies developed and validated at a particular site are applicable to other sites and wastes. The need to perform technology-development work using actual wastes, i.e., the relevance of work performed using simulants. These concerns are important to the extent that (a) the subcommittee will focus on them as areas to be evaluated as it obtains data from DOE tank sites, and (b) DOE-EM should consider explicitly addressing them to provide the necessary framework for technology development. Conclusions and Recommendations Conclusion: Substantial technology-development needs remain to be addressed if high-level waste tanks are to be successfully remediated. Recommendation: The DOE should continue to support a balanced technology-development program integrated across all involved organizations, including EM-30, EM-40, and EM-50, and Energy Research.
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--> Conclusion: A relatively simple scenario-based approach to identifying and prioritizing technology-development needs is an effective and desirable method to focus technology-development efforts. Recommendation: DOE should develop and rank tank-remediation scenarios leading to a prioritized list of technology needs as described above in those cases where it is not already doing so. The scenarios should be structured to define only the major steps from characterization through disposal or storage and to highlight only the major choices to be made. It is important that the scenarios not be presented in so much detail as to obscure the basic steps and issues. Conclusion: There are a number of important technology needs related to managing high-level waste in tanks that are common to the four DOE sites that have alkaline nitrate supernatant-saltcake-sludge wastes. For the most part, the needs of the Idaho site are expected to be significantly different because the waste will be acidified. This indicates the need to integrate technology-development efforts through mechanisms such as focus area and cross-cutting programs, although the subcommittee has not yet had the opportunity to evaluate the effectiveness of the existing focus area. Recommendation: An integrated technology-development program such as a focus area continues to be desirable to cost effectively develop technologies and share the results to the extent they are mutually applicable. Conclusion: An effective technology-development program requires definition of key boundary conditions and the ability to establish how much technology development is required. A number of concerns in this regard have been identified that must be addressed to develop the technology necessary to manage high-level waste in tanks. These issues appear not to have been adressed definitively, and acceptable interim approaches have not been specified. Recommendation: The DOE should explicitly address these concerns. Ideally, this would take the form of firm answers. More realistically, a well-conceived and consistent interim approach (e.g., use of prudently conservative waste-acceptance criteria, development of contingency technologies) should be explicitly defined.
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--> References Gephart, R. E., and R. E. Lundgren. 1995. Hanford Tank Clean Up: A Guide to Understanding the Technical Issues. Prepared for Pacific Northwest Laboratory, PNL-10773. Idaho National Engineering Laboratory (INEL). 1994. ICPP Radioactive Liquid and Calcine Waste Technologies Evaluation Draft Technical Report. INEL-94/0119. Priebe, S. J., and J. H. Valentine. 1980. Description of a Pilot Plant to Produce a Pelleted Form from Simulated ICPP High-level Calcined Wastes, EINCO-1032. Idaho Falls, Idaho: Exxon Nuclear Idaho Company, Inc. U.S. Department of Energy (USDOE). 1995a. Radioactive Tank Waste Remediation Focus Area: Technology Summary. DOE Office of Environmental Management Technology Development. DOE/EM-0255. June 1995. U.S. Department of Energy (USDOE). 1995b. Integrated Data Base Report-1994; U.S. Spent Nuclear Fuel and Readioactive Waste Inventories, Projections, and Characteristics, page 47 et. seq., DOE/RW-0006, Rev. 11. Westinghouse Savannah River Company. 1994. High-Level Waste Processing Facilities, Fact Sheet. September 9, 1994.
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