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ALTERNATIVES FOR HIGH-LEVEL WASTE SALT PROCESSING AT THE SAVANNAH RIVER SITE 3 Strontium and Actinide Removal The removal of strontium (Sr) and actinides (especially plutonium [Pu] and neptunium [Np]) is an important step in the salt processing flowsheet at the Savannah River Site (SRS). As presently envisaged, strontium and actinides will be removed from the salt solutions in all four of the cesium processing options discussed in this report —small tank tetraphenylborate (TPB), solvent extraction, ion exchange, and direct grout. Because of its position near the beginning of the processing flowsheet (Figure 1.2), the strontium and actinide removal step is referred to as “front-end” processing. As noted in Chapter 8, however, this processing step does not necessarily need to be performed prior to removal of cesium. In fact, there may be advantages to performing this step later in the processing sequence. This chapter provides a review of this front-end process and a discussion of the remaining technical uncertainties. Most of the information presented in this chapter was collected at the committee's two information-gathering meetings and from written responses to committee questions outside of those meetings (Westinghouse Savannah River Company, 1999a,c; Jones, 1999a,b; Jones, 2000a,b). Unless otherwise noted, the information used in the discussions in this chapter are taken from these references. BASELINE APPROACH The “baseline” approach for this processing incorporates the use of monosodium titanate (MST), NaTi2O5H, an amorphous solid consisting of porous, irregular-shaped particles, to remove actinides and strontium from the high-level waste salt solutions. Although the process details differ for some of the cesium removal options, in its simplest form, the process combines MST with the high-level waste salt solutions in a reaction vessel, mixing (typically for 24 to 48 hours) to promote the sorption of strontium and actinides with the MST solids, and then filtering to separate the MST solids from the “decontaminated” salt solutions. The MST solids are then washed to re-
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ALTERNATIVES FOR HIGH-LEVEL WASTE SALT PROCESSING AT THE SAVANNAH RIVER SITE move residual salt solutions and are transferred to the Defense Waste Processing Facility (DWPF) for additional processing and eventual immobilization in borosilicate glass. The MST step is performed in a separate reaction vessel for the ion exchange, solvent extraction, and direct grouting options. For the small-tank TPB option, the MST step is carried out concurrently with the cesium removal step in a single reaction vessel. To operate successfully, the baseline process must meet the following requirements: After treatment with MST, the “decontaminated” salt solutions must meet the strontium and actinide limits shown in Table 3.1 to be acceptable for disposal in the onsite saltstone facility. The process must deliver sufficient feed of MST solids to the DWPF to provide for continuous operation of the glass melter. To this end, MST sorption kinetics for strontium and actinides, MST solids filtering, and MST solids washing must be rapid enough to support the required process cycle times. The MST solids feed to the DWPF must have a composition that is compatible with the DWPF glass. In particular, the concentration of titanium (Ti), must be less than the limits established for the DWPF, or else the feed will have to be diluted, resulting in the production of additional glass canisters. TABLE 3.1 Saltstone Waste Acceptance Criteria for Decontaminated Salt Solutions Radionuclide Limit (nanocuries per gram of saltstone) Strontium-90 40 Plutonium-241 200 Neptunium-237 0.03 Total Alpha 20 SOURCE: Jones (2000b). During its information-gathering sessions, the committee received written information and briefings on all of these issues, several of which are reviewed below. STRONTIUM AND ACTINIDE REMOVAL The mechanisms for strontium and actinide removal by MST are not well understood. Presumably, the removal mechanism involves an ion-exchange reaction of the sodium ions in the MST, primarily with cations in
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ALTERNATIVES FOR HIGH-LEVEL WASTE SALT PROCESSING AT THE SAVANNAH RIVER SITE higher oxidation states (e.g., strontium, plutonium, neptunium, and uranium), but also, to a lesser extent, with monovalent cesium and potassium cations. The mechanism also may involve the sorption of these cations into the MST structure. Experimental work has demonstrated that under alkaline conditions, MST has a higher removal capacity for strontium and actinides than for other cationic species, particularly cesium and potassium, that are present in the salt solutions. Indeed, MST has been demonstrated empirically to remove strontium and actinides from the SRS high-level waste salt solutions with high efficiency relative to cesium and potassium. Although MST can be demonstrated to remove strontium and actinides from the tank waste, it is not clear that removal rates are sufficiently high to provide the needed throughput to the DWPF, especially at low MST concentrations. This is particularly true for salt solutions with high ionic strengths (i.e., high Na+ concentrations), or salt solutions with high plutonium-238 concentrations. In the 1983 in-tank precipitation (ITP) demonstration and the 1995 startup operations (see Chapter 4), MST was used successfully to remove strontium and actinides from salt solutions in Tank 48. However, these solutions apparently contained low concentrations of strontium and actinides and therefore met the saltstone limits (Table 3.1) without MST processing. The MST concentrations used in these operations were 0.6 gram of MST per liter of salt solution in the 1983 demonstration and 1.1 grams of MST per liter of salt solution in the 1995 startup operations. As discussed elsewhere in this chapter, these MST concentrations may be too high to meet DWPF glass limits. Work on MST kinetics following 1995 startup operations has employed both real and simulated salt solutions using lower concentrations of MST (0.2 to 0.4 gram MST per liter of salt solution) compared to the 1983 demo and 1995 startup. Perhaps the most significant finding from this work is that removal rates differ for the various cationic species of interest. At 0.2 gram of MST per liter of salt solution, removal rates for actinides are lower than for strontium. Indeed, SRS representatives reported to the committee that neptunium removal rates appear to be insufficient to meet the saltstone limits (Table 3.1) after 24 hours of contact. Moreover, plutonium removal rates appear to change significantly after about 10 hours of contact with MST, suggesting that this actinide exists in more than one oxidation state in the salt solutions. Uranium removal is not a major concern, owing to its low concentrations in the salt solutions, although it does consume some of the sorption capacity of the MST and, therefore, can have an effect on strontium and actinide removal rates. There are several possible ways to increase actinide removal rates to meet the saltstone limits and DWPF throughput requirements. For example, MST concentrations can be increased. SRS representatives reported to the committee that doubling the MST concentration (to 0.4 gram of MST per liter of salt solution), for example, is sufficient to ensure neptunium and plutonium removal for the “average” actinide concentrations encountered in the tank waste. To obtain the required levels of Pu removal, however, it may be necessary to dilute the salt solutions from tanks having high actinide concentra-
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ALTERNATIVES FOR HIGH-LEVEL WASTE SALT PROCESSING AT THE SAVANNAH RIVER SITE tions with salt solutions from tanks having lower actinide concentrations, a process known as blending. This would require an extra processing step, potentially involving the transfer and mixing of waste from several tanks prior to MST processing. Additionally, reaction vessel size can be increased to provide for longer contact times. Experimental work suggests that the required levels of actinide removal can be obtained by increasing the planned reaction vessel size by 25 percent (from the current baseline design of 100,000 gallons [380,000 liters] to 125,000 gallons [470,000 liters]) and diluting the salt solutions from 6.4 molar Na+ to 5.6 molar Na+ through the addition of water. MST SOLIDS REMOVAL Once MST solids have been added to the salt solutions to sorb strontium and actinides, these solids (along with any sludge solids in the waste) must be separated from the liquids and transferred to the DWPF. The process for removal of MST solids from the tank waste has been described by SRS as having a potentially significant impact on the success of the strontium and actinide removal step, especially for the ion exchange, solvent extraction, and direct grout options. The baseline technology for MST solids removal is crossflow filtration, in which the MST-sludge slurry is streamed tangentially across the face of a microporous filter. The MST and sludge solids are retained in the slurry stream, whereas the decontaminated salt solutions pass through the filter. Tangential flow helps reduce filter clogging. According to SRS, crossflow filtering provided an acceptable rate of solids removal in both the 1983 ITP demo and the 1995 ITP startup operations. Both of these operations, however, involved the co-filtering of MST, sludge, and TPB solids. SRS characterized the filter performance in the 1983 demonstration as “good” and the filter performance of the 1995 startup operations as “excellent.” The improved filter performance in 1995 startup was attributed to enhancements in the filter design and a narrower size distribution of MST solids, which helped prevent filter clogging. Filtration performance for slurries containing only MST and sludge solids (i.e., without TPB solids) exhibits a 2- to 3-fold decrease compared to MST-TPB-sludge filtering. SRS suggested that a possible reason for this difference was that the larger particle sizes of the TPB solids reduces filter clogging. There are several ways to increase MST and sludge removal rates to achieve needed throughputs should the ion exchange, solvent extraction, or direct grout options be selected. For example, filter size can be increased, flocculents and filter aids [e.g., poly(ethylene oxide), copolymers, or bentonite] can be used to enhance filter performance, or alternate separation technologies (e.g., involving density separations) can be employed. Several of these alternatives are being examined at SRS.
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ALTERNATIVES FOR HIGH-LEVEL WASTE SALT PROCESSING AT THE SAVANNAH RIVER SITE GLASS COMPATIBILITY MST exhibits good phase stability in the alkaline high-level waste salt solutions at SRS. Consequently, almost all of the MST solids that are added to the processing vessel would be in the waste feed to the DWPF, and very little dissolved MST will end up in the decontaminated salt solutions that are sent to the saltstone facility. The issue of MST compatibility in the DWPF glass focuses on the addition of Ti, which can reduce glass durability and change its processing properties. The current limit for TiO2 in feed to the DWPF is 1 weight percent. SRS estimates that at the currently planned MST treatment levels of 0.2 to 0.4 grams of MST per liter of salt solution, the concentration of TiO2 in the DWPF would range from 1 to 2 weight percent (Dimenna et al., 1999, p. 49). If the crystalline silicotitanate ion exchange option were selected, however, even more TiO2 would be sent to the DWPF—between 2.5 and 4 weight percent (Dimenna et al., 1999, p. 134). In other words, all of the current processing options appear to exceed current DWPF glass limits for TiO2 (Hobbs, 2000). SRS is aware of and has begun to study this problem (Edwards, Harbour, and Workman, 1999a). In briefings to the committee, SRS representatives presented data suggesting that the increased TiO2 loading would not cause significant changes in glass durability (Edwards, Harbour, and Workman, 1999b). Site personnel acknowledged, however, that a complete variability study would need to be done to ascertain the effects of increased TiO2 loading on glass durability, homogeneity, liquidus temperature, and viscosity, and that DWPF glass formulation models would need to be updated to reflect this information before any of the processing options could be implemented. There appeared to be some difference of opinion among SRS staff on the cost and time for completing this study. MST AVAILABILITY If SRS selects MST for strontium and actinide removal, it would have to procure this material in large quantities from a commercial vendor. Two suppliers have indicated interest in production of MST, and one has made sufficient amounts to validate their process. There is no history of problems resulting in the production of substandard batches. To the committee's knowledge, however, SRS has not yet established detailed material specifications for MST, so the potential for future manufacturing problems remain uncertain. ALTERNATE PROCESSES The committee's interim report noted that SRS was not considering alternative materials to MST for removal of strontium and actinides (National
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ALTERNATIVES FOR HIGH-LEVEL WASTE SALT PROCESSING AT THE SAVANNAH RIVER SITE Research Council, 1999b, p. 7). Although SRS personnel reported that they were confident that MST performance would be sufficient to meet the processing needs for the small tank precipitation process, the committee learned at its November 1999 meeting that alternate processes were being reviewed in case MST fails to meet expectations. Among alternate processes to MST that are being considered are the following: sorption by sodium nonatitanate (Na4TinO2n+1 or Na4TinO2n+2), or SNT; ferric hydroxide flocculation; permanganate reduction; and sodium uranate formation. Some testing is planned to determine the ability of these alternate processes to achieve required decontamination levels. It also was suggested that it might be possible to coat the crystalline silicotitanate (CST) particles that would be used in the ion exchange process with an actinide-absorbing agent. However, no further details of this possibility were provided to this committee. Among the reasons for choosing MST is its capacity for actinides, which is adequate at reasonable loadings. Importantly, the loading at full capacity is insufficient to have a criticality incident. The Allied Signal (now Honeywell) SNT is an appealing alternative from the standpoint of reliable sources because it is produced in large amounts for a worldwide market. If SNT is an acceptable alternative, a range of possibilities can be considered—for example, SNT could be used in an ion exchange mode, either alone or in some combination with crystalline silicotitanate (see Chapter 5), which would mean that filtration might be limited to sludge alone. Filtration is a consideration since, as discussed previously, MST has a 2-3 fold increase in filtration efficiency if the filtration is carried out in the presence of the TPB precipitate. R&D ACTIVITIES TO RESOLVE UNCERTAINTIES In its briefings to the committee (Westinghouse Savannah River Company, 1999a, 1999c) and in written responses to subsequent committee questions (Jones, 2000a), SRS has provided the committee with an outline of its planned R&D activities to resolve the remaining uncertainties with the strontium and actinide removal step. The R&D plans were being completed as this report went to review. The committee did not have the opportunity to obtain or perform a detailed evaluation of these plans. The planned R&D work falls into the following broad categories: MST filtration studies to understand the role of TPB in enhancing filtration of TPB-MST-sludge slurries, bench-scale screening to identify flocculents that can be used to increase filtering rates, and larger-scale filtering tests of these flocculents using real waste. Alternate methods to remove MST solids from the decontaminated salt solutions involving the testing of a small number of removal technologies identified in a recent Tank Focus Area report (U.S. Department of Energy, 1999).
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ALTERNATIVES FOR HIGH-LEVEL WASTE SALT PROCESSING AT THE SAVANNAH RIVER SITE Studies to obtain a more detailed understanding of removal rates of strontium and actinides using MST, and investigation of the ways to increase these rates by varying MST concentrations and processing conditions. Evaluation of the efficacy of other materials (sodium nonatitanate, ferric hydroxide, permanganate, sodium diuranate) for removing sodium and actinides from high-level waste salt solutions. Evaluation of the effects of plutonium oxidation states on MST sorption rates. Determination of concentrations of soluble actinides in the salt solutions through direct measurements of tank samples to aid design of MST processing conditions. ANALYSIS In its interim report (National Research Council, 1999b) the committee reached the following conclusion with respect to the strontium and actinide removal step: (a) there appear to be some remaining technical questions that will need to be resolved before this process can be successfully implemented at SRS; and (b) Westinghouse Savannah River Company appears to be pursuing these questions vigorously. The committee also concluded that the information it received subsequent to the completion of its interim report does not justify a change in its position; that is, the committee continues to believe that the MST process holds sufficient promise to justify its continued development. Nevertheless, in the view of the committee, two major issues remain to be resolved before SRS can successfully implement this process: SRS must demonstrate that strontium and actinide removal can be accomplished within saltstone limits and at the throughput rates required by the DWPF; and SRS must demonstrate that the MST concentrations used to remove the strontium and actinides will not exceed compatibility limits for DWPF glass. That SRS now appears to be considering alternatives to the MST process is perhaps a sign that there is some uncertainty concerning whether MST can be shown to work. The committee concurs with this concern. The consolidation of alternatives represents prudent engineering practice in the face of technical uncertainties. The committee believes that further R&D is needed to confirm that the MST process can operate successfully within the technical requirements. The plans for R&D work that were presented to the committee in outline form seem appropriate for resolving many of the technical uncertainties with this processing option. As noted above, however, the details of these plans were still being developed as the committee finalized this report; consequently, the
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ALTERNATIVES FOR HIGH-LEVEL WASTE SALT PROCESSING AT THE SAVANNAH RIVER SITE committee was unable to evaluate the R&D plans in detail. The committee cannot provide an endorsement of the R&D in the absence of sufficient information that allows evaluation of the details of these plans. RECOMMENDATIONS In light of the foregoing discussion, the committee offers the following recommendations to promote the resolution of the technical uncertainties with respect to the MST processing step. SRS should resolve the technical uncertainties with the MST processing step as soon as possible by implementing an R&D program focused on the technical uncertainties noted above, and others that are consistent with a systems engineering approach to salt processing. This R& D should provide a demonstration that MST functions satisfactorily with each of the cesium separation process options. Simultaneously, a well-focused R&D program should be conducted to examine alternatives to MST. This program should have a level of effort commensurate with the risk of process failure and should continue until MST processing can be demonstrated to meet the saltstone, DWPF throughput, and DWPF glass requirements. As part of its efforts to resolve technical uncertainties, SRS should establish requirements for and reliable sources for the manufacture of MST.
Representative terms from entire chapter: