PNNL-13582 High-Level Waste Melter Study Report J.M. Perez, Jr. D.K. Peeler D.F. Bickford D.M. Strachan D.E. Day M.B. Triplett D.S. Kim J.D. Vienna S.L. Lambert R.S. Wittman S.L. Marra July 2001 Prepared for the U.S. Department of Energy under Contract DE-AC06-76RL01830 Pacific Northwest National Laboratory Richland, Washington 99352 Disclaimer This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor Battelle Memorial Institute, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. 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Box 62, Oak Ridge, TN 37831-0062 Ph: (865) 576-8401 Fax: (865) 576-5728 Email: [email protected] Available to the public from the National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Rd., Springfield, VA 22161 Ph: (800) 553-6847 Fax: (703) 605-6900 Email: [email protected] Online ordering: http://www.ntis.gov/ordering.htm PNNL-13582 High-Level Waste Melter Study Report J.M. Perez, Jr. D.K. Peeler D.F. Bickford D.M. Strachan D.E. Day M.B. Triplett D.S. Kim J.D. Vienna S.L. Lambert R.S. Wittman S.L. Marra July 2001 Prepared for the U.S. Department of Energy, under Contract DE-AC06-76RL01830 Abstract At the Hanford Site in Richland, Washington, the path to site cleanup involves vitrification of the majority of the wastes that currently reside in large underground tanks. A Joule-heated glass melter is the equipment of choice for vitrifying the high-level fraction of these wastes. Even though this technology has general national and international acceptance, opportunities may exist to improve or change the technology to reduce the enormous cost of accomplishing the mission of site cleanup. Consequently, the U.S. Department of Energy requested the staff of the Tanks Focus Area to review immobilization technologies, waste forms, and modifications to requirements for solidification of the high-level waste fraction at Hanford to determine what aspects could affect cost reductions with reasonable long-term risk. The results of this study are summarized in this report. iii Executive Summary At the Hanford Site in Richland, Washington, the path to site cleanup involves vitrification of the majority of the wastes that currently reside in large underground tanks. A Joule-heated glass melter is the equipment of choice for vitrifying the high-level fraction of these wastes. Even though this technology has general national and international acceptance, opportunities may exist to improve or change the technology to reduce the enormous cost of accomplishing the mission of site cleanup. Consequently, the U.S. Department of Energy (DOE) requested the staff of the Tanks Focus Area (TFA) to review immobilization technologies, waste forms, and modifications to requirements for solidification of the high-level waste (HLW) fraction at Hanford to determine what aspects could affect cost reductions with reasonable long-term risk. The results of this study are summarized in this report and are further distilled in this Executive Summary. It should be noted that there is a degree of uncertainty with the analyses that were performed in this study. Although we recognize the need for a propagation of error analysis, it was outside the scope of this task. Hence, we have taken the base case as certain even though there exists a significant uncertainty. Numbers and cases are treated accordingly. As a result, there are many more significant figures reported than could be supported had an error analysis been performed, e.g. 1833 canisters containing glass of a particular composition. The conclusions that are based on these numbers are indicative of where and the magnitude of the potential savings. The numbers that are show should be taken in that context. The vitrified product will be destined to a mined geologic repository. The current candidate repository is at Yucca Mountain, Nevada. Several requirements must be met to successfully emplace the waste glass in the repository. These are listed in the Waste Acceptance Systems Requirements. These requirements were reviewed to evaluate the bases for the requirements and the potential impact of changes or modifications. We conclude that modification or deletion of several of these requirements would have minor effects on waste loading or cost reductions but may simplify plant operations. In addition to product requirements, several processing requirements are impacted by the waste form or melter technology used. These requirements are imposed by the operating facility but can have a major impact on waste loading and production costs. Thus, processing requirements were considered in this review. It was concluded from this effort that the following requirements should be further evaluated to determine the extent to which they could be challenged such that waste loadings could be increased or costs could be reduced: • Waste form type (i.e., borosilicate glass) • Product Consistency requirement and strategy for compliance • Liquidus temperature limit. An evaluation of waste forms for Hanford high-level waste (HLW) was conducted. Waste compositions representative of those to be delivered to the HLW vitrification plant after retrieval, incidental blending, pretreatment, and separations were combined into groups of like-composition by cluster analyses. A review of literature and previous waste form-evaluation/down-selection activities were used to assess nearly 70 possible waste forms, some more thoroughly than others. The waste forms for which there is sufficient information and which can be fabricated in a HLW glass melter were selected for further consideration: • alkali-alumino-borosilicate (AABS) glasses • alkali-aluminosilicate (AAS) glasses iv • iron-phosphate (FeP) glasses • titanate based ceramics. Of these waste forms, the most complete information was available for Hanford’s reference waste form— AABS glasses. The loadings for each of the waste clusters in AABS glasses were estimated with existing glass property models and expert judgment for 21 different sets of property and composition constraints. The resulting glass-volume estimates were compared to those calculated for each individual waste composition before cluster analysis. This analysis showed roughly a 2.5% increase in the estimated glass volumes based on individual batch compositions. The impact of chromium leach factor, which is the fraction of chromium in the waste that is leached from the HLW, was also estimated; it had the largest influence on glass volume. Aside from the chromium leach factor, the volume of glass was most influenced by the limits set by liquidus temperature (TL) in the spinel primary phase field, by the Cr2O3 concentration, and by the melting temperature (TM) (because TL constraint increases with TM). The product consistency test (PCT) response constraint had little impact on estimated glass volume. The relaxation of constraints due only to glass-property-model validity and the allowance of multiple phase formation had a moderate impact on estimated glass volume. We conclude that melters that can tolerate crystals or an increase in TM would give the biggest glass-volume impacts of any technology/constraint studied. Available data suggested that the addition of boron to AAS glasses melting at low temperatures would be beneficial. It is possible that low boron or boron-free AAS glasses would give an advantage over AABS glasses at high temperatures. Waste forms obtained from melting Hanford HLW and enough SiO2 to meet PCT response constraints were found to give the highest waste loading (~80 mass%). The loading of waste clusters in FeP glasses was estimated with expert judgment and results from roughly 500 crucible melts. The waste loadings were estimated for both single-phase FeP glasses and partially crystallized FeP waste forms. The total glass volume for single-phase FeP glasses was slightly higher than that estimated from the reference case of AABS glasses. However, the glass volume for a few of the clusters was lower for FeP glasses. The glass volume estimated for partially crystallized FeP waste forms was lower than those estimated for AABS glasses. Waste-loadings in titanate ceramics that are produced in glass melter could not be reliably estimated. Significant technical issues must be resolved before implementing any waste form other than the reference-case AABS glass. This report gives some indication of the relative value that may be obtained by investing in these other waste forms or technologies in terms of waste-form volume savings. This report is structured around the five tasks that comprised the TFA study to evaluate HLW melters and the waste forms that could potentially be produced from them. The comparisons are made relative to the base case of an AABS glass. We start with a discussion of the changes to the glass volumes that would result from changes to the Waste Acceptance Systems Requirements Document (Section 3.0). We then discuss the waste types at Hanford and identify the components that limit waste loading for various waste form types.