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EXECUTIVE SUMMARY
The U.S. Department of Energy (DOE) is nearing a decision on how to process 30 million gallons of high-level radioactive waste salt solutions at the Savannah River Site in South Carolina to remove strontium, actinides, and cesium for immobilization in glass and eventual shipment to a geologic repository. The department is sponsoring research and development (R&D) 1 work on four alternative processes and plans to use the results to make a downselection decision in a June 2001 time frame. The DOE requested that the National Research Council help inform this decision by addressing the following charge:
- 1. evaluate the adequacy of the criteria that will be used by the department to select from among the candidate processes under consideration;
- 2. evaluate the progress and results of the research and development work that is being undertaken on these candidate processes; and
- 3. assess whether the technical uncertainties have been sufficiently resolved to proceed with downsizing the list of candidate processes.
The committee's interim report ( Appendix B) served as a response to the first point of this charge. In that report, the committee found that DOE's proposed criteria are an acceptable basis for selecting among the candidate processes under consideration, but that the criteria should not be implemented in a way that relies on a single numerical “total score.” Responses to the last two points are provided in this report.
A previous National Research Council report (NRC, 2000) found that there were potential barriers to implementation of all of the alternative processing options. A recommendation was made that Savannah River should proceed with a carefully planned and managed research and development program until enough information is available to make a defensible and transparent downselection decision. As a result of this report, DOE has developed and is vigorously pursuing an R&D program to resolve several outstanding issues related to the selection and implementation of these alternative processes. Consequently, technical,
1A list of acronyms and abbreviations may be found in Appendix E.Page 2
schedule, and cost risks have been more clearly defined and in some cases significantly reduced. The present committee has evaluated this R&D work and offers the following evaluations.
RESPONSE TO CHARGE 2: EVALUATE THE PROGRESS AND RESULTSOF THE RESEARCH AND DEVELOPMENT WORK THAT IS BEING UNDERTAKEN ON THESE CANDIDATE PROCESSES
The committee has provided a detailed evaluation of the R&D work in the second section of this report. On the basis of this review, the committee has identified the following unresolved issues for each alternative process:
Small Tank Tetraphenylborate Precipitation (STTP)
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The process by which tetraphenylborate (TPB) decomposes is not completely understood and is not predictable, either mechanistically or empirically.
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An unresolved issue is how cesium removal can be accomplished with waste batches where unexpected TPB decomposition occurs so rapidly that expected decontamination factors (DFs) are not achieved.
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Final selection of an antifoam agent has not been made.
Crystalline Silicotitanate (CST) Nonelutable Ion Exchange
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The mechanism of aluminosilicate precipitation on CST is not yet understood. This issue poses a potentially high technical risk.
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The reliance on a single supplier for CST poses a high schedule risk.
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Technical uncertainties—including column plugging, resistance to hydraulic transfer, irreversible desorption, and column system technologies—will continue to constitute a high risk for the use of this process for cesium removal. The method and resources required to resolve these risks are not clear.
Caustic Side Solvent Extraction (CSSX)
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Successful bench-scale demonstration of the complete CSSX process with actual tank waste is critical. These demonstrations are needed to clarify any residual risks.
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Actinide and Strontium Removal: Monosodium Titanate (MST)
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Two alternate precipitation processes are competitive with MST and should be studied further. These employ sodium nonatitanate, which behaves similarly to MST, and sodium permanganate. The selection of a process for actinide and strontium removal is mostly independent of the selection of a cesium removal process.
CHARGE 3: ASSESS WHETHER THE TECHNICAL UNCERTAINTIES HAVE BEEN SUFFICIENTLY RESOLVED TO PROCEED WITH DOWNSIZING THE LIST OF CANDIDATE PROCESSES
This report has focused exclusively on technical issues related to the candidate processes for radionuclide removal from high-level waste salt solutions at Savannah River. However, because the final downselection must be based on a number of issues in addition to science and technology, the committee makes no recommendation on which process(es) should be selected. Rather, the committee has attempted to identify residual technical risks that should be a component in the decision-making process for downselecting the list of candidate processes.
The committee believes, however, that technical uncertainties have been resolved sufficiently to proceed with downselecting the list of candidate processes. To this end, the committee offers the following advice:
Small Tank Tetraphenylborate Precipitation
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The STTP process has remaining technical uncertainties, but engineering solutions for most of these problems probably can be found. However, because of the unpredictability of the decomposition rate of the TPB, there remains the risk that one or more of the 67 high-level waste production batches will require additional or special treatment before it can be processed using this option.
Crystalline Silicotitanate Nonelutable Ion Exchange
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Of the three cesium separation processes under consideration, it is the committee's judgment that CST has the most technical uncertainties and the highest technical risks.
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Caustic Side Solvent Extraction
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Unless tests with actual waste encounter new problems, the CSSX option for cesium separation presents, at present, the fewest technical uncertainties of any of the three cesium separation alternatives.
Actinide and Strontium Removal: Monosodium Titanate
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All of the cesium separation processes depend upon a separate step to remove strontium, neptunium, and plutonium. Currently, that step uses MST. Because the success of this step is essential to all three of the processes for cesium separation, the committee believes that continued R&D on an alternate process to MST for removal of actinides and strontium is essential until MST processing can be demonstrated to meet the saltstone, Defense Waste Processing Facility (DWPF) throughput, and DWPF glass requirements.
