Cesium distribution ratios

The significant selectivity for cesium over rubidium (,S Cs/Rb = 39), compared with that obtained for the calix[4]arene-crown-6 derivatives (8 for MC8 and less than 5 for MC14), has to be pointed out. Replacement of 0.1 M of sodium hydroxide by potassium hydroxide strongly decreases the cesium distribution ratios for calix[4] arenes-crown-6 (DCs < 1). On the contrary, they are slightly increased for tBuC[4], leading to a cesium-over-rubidium selectivity exceeding 150. This selectivity value... 

Distribution ratios and transport were carried out on real HAW arising from dissolution of a mixed oxide of uranium and plutonium (MOX) fuel (burnup 34,650 MW d/tU), where uranium and plutonium have been previously extracted by TBP.86 The experiments were performed in the CARMEN hot cell of CEA Fontenay aux Roses with two dialkoxy-calix[4]arene-crown-6 derivatives (diisopropoxy and dini-trophenyl-octyloxy). High cesium distribution ratios were obtained (higher than 50) by contacting the HAW solution with diisopropoxy calix[4]arene-crown-6 (0.1 M in NPHE). Moreover, the high selectivity observed with the simulated waste was confirmed for most of the elements and radionuclides (actinides or fission products Eu, Sb, Ce, Mo, Zr, and Nd). The residual concentration or activity of elements, other than cesium, was less than 1% in the stripping solution, except for iron (2%) and ruthenium (8%) the extraction of these two cations, probably under a complexed... 

Some of the modifiers in this study (1-5) were also evaluated in the isoparaffinic diluent Isopar L for cesium extraction performance from a Savannah River waste simulant. The effect the modifiers have on the cesium distribution ratio in aliphatic diluents is more pronounced than it is in polar diluents, such as 1,2-DCE, previously used. The cesium distribution results are shown in Table 4.17 alongside those obtained from 1 M NaN03 in 1,2-DCE. 

DCE diluent. However, the differences between the stronger fluorinated modifiers and the weaker nonfluorinated modifiers are much more pronounced. This could be due to the fact that the stronger modifiers may be sufficiently acidic to be partly deprotonated when contacted with the alkaline simulant. The alkoxide form of the alcohol would serve as the counteranion to the extracted cesium, eliminating the need for an anion from the aqueous phase (e.g., nitrate) to be coextracted, and, in turn, making it energetically easier to transfer the cesium cation to the solvent phase, hence increasing the cesium distribution ratio. 

The cesium distribution ratio afforded by alcohol modifiers, such as 1, dissolved in hydrocarbon diluents, such as Isopar L or dodecane, increased dramatically as the concentration of the modifier increased from below 0.10 M to 1 M, then leveled off. 

Cesium Distribution Ratios and Corresponding Values for 1,2-DCE Solutions of BC6 at 0.005 M as a Function of Modifier (1) Concentration... 

As the concentration of the modifier is increased, the net polarity of the solvent and accordingly both the cesium distribution ratio and the magnitude of the hypsochro-mic shift of the dye increases. The relationship between DCs and Ej as a function of modifier 1 concentration is nonlinear. 

Although solvent samples have been observed for approximately one year without any solids formation, work was completed to define a new solvent composition that was thermodynamically stable with respect to solids formation and to expand the operating temperature with respect to third-phase formation.109 Chemical and physical data as a function of solvent component concentrations were collected. The data included BC6 solubility cesium distribution ratio under extraction, scrub, and strip conditions flowsheet robustness temperature range of third-phase formation dispersion numbers for the solvent against waste simulant, scrub and strip acids, and sodium hydroxide wash solutions solvent density viscosity and surface and interfacial tension. These data were mapped against a set of predefined performance criteria. The composition of 0.007 M BC6, 0.75 M l-(2,2,3,3-tetrafluoropropoxy)-3-(4-.sw-butylphenoxy)-2-propanol, and 0.003 M TOA in the diluent Isopar L provided the best match between the measured properties and the performance criteria. 

The developed cesium and potassium extraction model, based on extraction data obtained from simple aqueous media, was tested to ensure the validity of the prediction for the cesium extraction from actual waste.112 The predicted values agreed with the measured values for the simulants predicted values also agreed, with some exceptions, with measured values for the tank wastes. Discrepancies were attributed, in part, to the uncertainty in the cation/anion balance in the actual-waste composition, but likely more so to the uncertainty of the potassium concentration in the waste, given the demonstrated large competing effect of this metal on cesium extraction. It was demonstrated that the upper limit for the potassium concentration in the feed should not exceed 0.05 M, in order to maintain suitable cesium distribution ratios. 

The influence of the two active components with one another was studied. It was shown that increasing the DtBuCH18C6 concentration in the mixture results in increasing Sr distribution ratios, but slightly decreasing cesium distribution ratios (see Figure 6.6). 

Flgu re 1. Fit of cesium distribution ratios for nitrate media... 

Real waste batch tests conducted at the Savannah River Site provided the concentrations of major components (sodium, potassium, cesium, hydroxide, and nitrate) of the waste. From these results, simulants were prepared using either nitrite or chloride. These initial concentrations were entered into a data file, and the program SXFIT predicted the cesium distribution ratio based on all the species and formation constants presented above. Table 10 presents the comparison between the measured distribution ratios and those that were predicted. The O/A volume ratios were all 113. All experiments were carried out at 25°C. 

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