Research Needs for High-Level Waste Stored in Tanks and Bins at U.S. Department of Energy Sites Environmental Management Science Program (2001) / Chapter Skim
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4. Retrieval and Pretreatment
4. Retrieval and Pretreatment
Pages 37-47
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From page 37... ...
Pretreatment consists of separating the HLW components from the bulk of the waste and preparing the waste stream for feed i ng to i mmobi I ization processes. Th is preparation is necessary to remove constituents that interfere with the HLW immobilization process and achieve the final objective of reducing the volume of solidified HLW produced and meeting quality criteria.
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From page 38... ...
. Pretreatment at SRS The alkaline salt solutions retrieved from different tanks are mixed in pretreatment tanks to"blend down"components that might cause problems in the vitrification step.The sludge is separated and pretreated by caustic leaching to solubilize aluminum from the HLW feed.The SRS is still in the process of selecting a method to separate cesium from the supernate, dissolved saltcake and sludge-leach solutions.The previous method (simultaneous large-scale in-tank precipitation with tetraphenylborate to remove cesium, and sorption with hydrous sodium titanate to remove strontium)
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From page 39... ...
Sludge leaching consists of R e t r i e v a I a n d P r e t r e a t m e FIGURE 4. 1 Simplified scheme of the pretreatment 39
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From page 40... ...
Following mixing to Dissolve soluble components e.g., saltcake containing alkali metal salts including cesium) and suspend insoluble solids (e.g., hydroxides of iron, chromium, nickel, transuranic elements, alkaline earth sulfates, and phosphates)
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From page 41... ...
in the order of 1 0,000 to 1 00,000 may be requ i red for some red ionucl ides to meet specifications for the immobilized LLW product; but solids removal by such large factors is not commonly achieved in practice in a single process cycle. Solids-liquid separations will likely be required at more than one point during the process.
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From page 42... ...
If not removed, these non-radioactive materials would increase the final volume of the HEW and/or lead to a hard-to-process slurry feed or an unacceptable final product. Removal of these undesirable species from the sludges is currently based on the "enhanced sludge leaching" method that is being used at the SRS and will likely be applied at the Hanford Site.
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From page 43... ...
Zirconium is the dominant element that has to be separated only for INEEL waste; however, in all cases, the leach solutions require a subsequent SLS step. Operations using precipitation and solids leaching can be affected negatively by several factors: · the presence of high salt concentrations and organic complexants that may solubilize strontium, and perhaps transuranic elements; · the solubility and speciation of actinides (plutonium, neptunium, americium, uranium)
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From page 44... ...
Liquid Decontamination After sludge leaching and SLS, the supernate and all leach solutions are carried through one or more processes designed to remove specific hazardous or radioactive constituents so that the liquid product can meet the acceptance criteriafor LLW immobilization and disposal. As indicated in Figure 4.1, these processes may include removal of radioactive materials such as cesium, strontium, transuranic elements, technetium, and other elements, as well as non-radioactive toxic metals and organics.
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From page 45... ...
In all cases, each separation process generates some quantity of secondary waste, such as ion-exchange sorbents, organic solvents, or various conditioning solutions. Dealing with these secondary waste streams is sometimes difficult because they can interfere with the separation processes and increase final immobilized waste volume.
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From page 46... ...
In addition, research on solvent extraction methods would be particularly valuable for use with acidic waste solutions for which solid sorbents are often less effective. Solvent extraction from acidic solutions is the standard for reactor fuel reprocessing, because fuel is soluble H ~ G H - L E V E E W A S T E 46
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From page 47... ...
However, investigators should not restrict consideration to alkaline methods only, even though that is the starting medium. There could be benefit from treating solutions from acid leaching of alkaline sludge or INEEL calcine, but the methods would be very different from fuel reprocessing.
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