The aqueous high-level waste (HLW) resulting from the reprocessing of civilian nuclear reactor fuel and other nuclear materials contains many long-lived fission products, including (sup 137)Cs(sup +) and (sup 90)Sr(sup 2+). These radionuclides present a serious health risk becausethey are a major source of radioactivity. In addition, they are significant contributors to the heat that is produced by HLW, which complicates the design of storage facilities. Both (sup 137)Cs(sup +) and (sup 90)Sr(sup 2+) have relatively small thermal cross sections and are therefore poor candidates for transmutation. Thus, HLW waste containing these species must be treated by conventional processing methods such as precipitation or ion exchange in order to achieve a reduction in volume and to lower the radioactivity of the waste. The liquid-liquid extraction of long-lived (sup 137)Cs(sup +) and (sup 90)Sr(sup 2+) radionuclides from aqueous tank waste into hydrophobic organic solvents, e.g., 1,2-dichloroethane, dichloromethane, toluene, xylene, with various ionophores, such as macrocyclic ligands, has been studied extensively. However, the high vapor pressure and toxicity associated with such solvents are problematic. As a result, hydrophobic room-temperature molten salts or ionic liquids have been investigated assubstitutes for the organic solvents normally used in this application (2-6). These ionic solvents have no detectable vapor pressure at room temperature. Although the toxicity of these solvents is unknown, they are believed to be less toxic than most of the molecular organic solvents listed above. One class of macrocycles that are promising candidates for the extraction of Cs(sup +) is the calix-arene-crown ethers, especially those based on calix(4)arenes. In a recent investigation, calix(4)arene-bis (t-octylbenzo-crown-6) (BOBCalixC6) dissolved in 1-C(sub n)-ethyl-3-methyl-imidazolium bis((trifluoromethyl) sulfonyl)imide (Tf9 sub 2)N-) ionic liquids was found to be an extremely effective extraction system for Cs(sup +). Likewise, dicyclohexano-18-crown-6 (DCH18C6) was found to be an effective extractant for Sr9 sup 2+) in this same ionic liquid. However,before this extraction system can be used to remove Cs(sup +) and Sr(sup2+) from aqueous tank waste on a practical scale, an economical method for recycling the expensive extraction solvent (macrocyclic ligands + ionic liquid) must be developed.
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