One of the most important developments for nuclear separations in the past two decades has been the emergence and rapid implementation of technologies for fission-product separation from aqueous streams based on molecular recognition. Simple neutral crown ethers provided an initial starting point for developing extractants selective for cesium and strontium,25’26’39-43’61’63’7374 but calixarenes have now supplanted crown ethers for cesium extraction.61,64 As its name implies, molecular recognition is associated with high selectivity, making the job of extraction simpler and more efficient, but just as important is the ability to strip with water or very dilute aqueous solutions. Downstream operations for production of waste forms or perhaps a radiation source stand to be greatly simplified with less secondary waste if the radionuclide is concentrated into a pure solution in water. A major challenge with using simple crown ethers for process devel­opment has been weak extraction strength, leading researchers at first to use large counteranions like phosphomolybdate or hexachloroantimonate, which was demonstrated for cesium extraction from medium-activity waste.74’75 Alternatively, by mixing crown ethers with cation exchangers like

HCCD43,61,62 or lipophilic sulfonic, carboxylic, or phosphoric acids,41,73,76 syn­ergistic extraction of Cs and Sr can be effected. However, these systems have the same disadvantage of stripping with strong acid. Furthermore, their use would make sense only for Sr, since no synergist for dicarbollide is needed for good Cs selectivity. The problem of extraction strength was adequately solved by judicious selections of crown ether, diluent, and some­times modifier; in addition, rather high concentrations of crown ether are needed, on the order of 100-fold more capacity than required for stoichio­metric loading. A careful study of the effect of diluent showed that Sr(NO3)2 could be adequately extracted from nitric acid by cK-dicyclohexano-18- crown-6 (ds,-DCH18C6),77 giving rise to the successful SREX process, which employs the more lipophilic crown ether bis(im-butylcyclohexano)- 18-crown-6 (DtBuCH18C6) in either 1-octanol78 or TBP-modified Isopar L.79 Stripping is effected with dilute nitric acid. Among a number of coun­tercurrent demonstrations of SREX, for example, workers at INL achieved 99.997% removal of the 90Sr in acidic waste.80 In the same time period, Russian researchers employed fluorinated alcohols as diluents, making pos­sible an effective process with DCH18C6.81-83 More recently, Chinese researchers, who call the process CESE, have also employed DCH18C6, achieving a >99.96% removal of the 90Sr in high-level liquid waste effluent from a TRPO process raffinate.84 DtBuC18C6 may be added to the TRUEX solvent to achieve a combined extraction of 90Sr and actinides.85

Cesium extraction with crown ethers proved even more difficult, hin­dered by weak extraction strength for CsNO3 and modest selectivity over Na and K. Benzo-substituted crown ethers of various ring sizes exhibit useful cesium selectivity to varying degrees, the best selectivity being achievable with dibenzo-21-crown-7 (DB21C7) or its more lipophilic analog bis(im-butylbenzo)-21-crown-7.76,86-88 Russian workers proceeded with process development using DB21C781 or a derivative bearing phosphoryl substituents on the benzo groups, used in a polyfluorinated alcohol diluent (Fluoropol-732).88 The latter crown ether was also mixed with DCH18C6 to achieve a simultaneous recovery of Cs and Sr. Workers in the United States noted that extraction strength was highest with dibenzo-18-crown-6 bearing 1-hydroxy-2-ethylhexyl substituents on each benzo group.87 This crown ether was mixed with DtBuC18C6 in Isopar L modified with TBP and lauryl nitrile to obtain the CSEX-SREX combined process for simul­taneous removal of Cs and Sr, demonstrated in a countercurrent test with

INL waste simulant.63,85,89

In the mid-1990s, the advent of the family of calix[4]arene-crown-6 com — pounds90,91 dramatically changed the technological possibilities for cesium extraction, as recently reviewed.61,64 These compounds feature cesium binding strength92 and selectivity93 both on the order of 100-fold higher than DB21C7, and stripping can again be effected with water or dilute aqueous solutions. A useful way to understand the structure of the calix[4]arene — crown-6 compounds is to consider them as crown ethers with aromatic rings preorganized to lie above and below the crown cavity. The resulting pocket is extraordinarily complementary for the Cs+ ion. French investigators showed the feasibility of developing useful processes on the acid — or alkaline — side based on a TBP-modified aliphatic diluent.94 In the United States, acid-side conditions were worked out with alcohol modifiers in Isopar L, which allowed much lower calixarene concentrations to be employed.95 It was subsequently shown that a simultaneous extraction of Sr and Cs could be obtained with calix[4]arene-bis(iert-octylbenzocrown-6) (BOBCalixC6) mixed with DtBuCH18C6 in Isopar L modified with 1-(2,2,3,3-tetrafluoropropoxy)-3-(4-s, ec-butylphenoxy)-2-propanol (Cs-7SB modifier).96 Further development on this system, called the FPEX process, is continuing, as BOBCalixC6 suffers from low solubility and mild nitration by nitric acid.95 While the removal of Cs and Sr in reprocessing may be questionable,30 alkaline-side process development has proceeded quickly to industrial implementation by more urgent needs for processing alkaline tank waste in the United States.32,33 The current implementation of the process, known as the Caustic-Side Solvent Extraction process (CSSX), employs a solvent system consisting of 0.007 M BOBCalixC6, 0.75 M Cs-7SB modifier, and 0.003 M tri-n-octylamine in Isopar L.9798 The loaded solvent is scrubbed and stripped with 0.050 M and 0.001 M HNO3, respectively, to produce a practically pure stream of CsNO3 in mildly acidic water. Countercurrent demonstrations using centrifugal contactors with simulated99 and real100 SRS tank waste showed that decontamination factors of >40 000 and concentra­tion factors of 15 can be readily achieved, and plans call for implementation in the SRS Salt Waste Processing Facility (SWPF) in 2014.3233 The Modular CSSX Unit (MCU) at the SRS started operation in 2008 as an interim pilot facility intended for pretreatment of several million gallons of low-curie SRS salt waste.101 It achieves 137Cs decontamination and concentration factors of >100 and 12, respectively, and the strip product is being vitrified in the DWPF. A proposed next-generation CSSX flowsheet using the more soluble extract­ant calix[4]arene-bis(2-ethylhexylbenzo-crown-6) (BEHBCalixC6)102 con­centrates 137Cs into a dilute boric acid strip solution, which is itself a glass component for the downstream borosilicate vitrification process.103