The methodology currently applied in France and Europe to develop new highly selective hydrophilic or lipophilic compounds for partitioning LLRN is more or less the same as elsewhere in the world:

(i) It starts with the design of a new structure of complexing molecule that could suit the chemical properties of the target LLRN. Radiochemists can search, through comprehensive literature reviews, for existing natural or tailored molecules known for their ability to complex or extract mimicking nonradioactive elements. However, although significant progress has been made in recent years on the use of computational tools to develop macrocyclic ligands for selective metal binding (Hay et al., 2004, 2005), radiochemists can hardly rely on computational modelling approaches, such as those adopted by pharmacologists to design effective drugs, to develop potentially interesting ligands to selectively extract minor actinides from PUREX raffinates, probably because the intrinsic complexity of the chemical systems implemented in their partitioning processes deters chemists from identifying and understanding the physicochemical phenomena underlying solvent extraction, both qualitatively and quantitatively.

Whatever the objectives when describing a chemical system (its energy, the behaviour of its molecular orbital electrons, the main source of its chemical reactivity, its mechanics, or the geometry of its studied complexes), this system can be conceptualized and modelled in different referentials. Nevertheless, these referentials consist of parameters and basic data only achievable after establishing chemical models, which in turn have to be confronted with experimental physi­cal data. Unfortunately, these experimental data are not easy to obtain, since preparing and analyzing radioactive samples in glove boxes is often difficult and limited to specific chemical environments.

Furthermore, the conditions used are usually far from those encoun­tered when reprocessing spent nuclear fuel at industrial scale, where concentrated solutions are implemented. In concentrated solutions, the chemical reactivity of the radionuclides is actually considerably modified as compared to that in diluted solutions. This difficulty, which is currently overcome by using approximate interpolated and/or extrapolated functions, makes the quantitative prediction of the behaviour of a solute in a complex medium almost impossible without experimental data. However, recent coupling of two mathematical approaches proved to be successful in predicting the thermodynamic properties of concentrated solutions: Molecular Dynamics on the one hand, which allows microscopic parameters to be calculated (such as the diameter of the hydrated cations in infinitely dilute solutions or the parameters describing the changes in the size of the hydrated cations with the concentration), and the Binding Mean Spherical Approximation theory on the other hand, which allows osmotic coef­ficients and thus activity coefficients (yb which accounts for the solute chemical reactivity in concentrated solutions) to be estimated via a microscopic representation of the solutions (Ruas et al., 2006).

Statistical approaches aiming at establishing empirical mathemati­cal relationships between sets of ligand structures and their chemical properties (such as complexation or extraction of target LLRN) are sometimes used to help design optimized ligand structures (Ionova et al., 2001). For instance, a quantitative structure-activity relationship can correlate chemical structures with well defined chemical reactivity parameters (Drew et al., 2004a, Varnek et al., 2007). On a more fun­damental level, quantum chemistry and molecular dynamics calcula­tions, such as density functional theory, have been used concomitantly with structural and spectroscopic characterization to enrich basic knowledge of actinide complex formation and/or extraction (Boehme and Wipff, 1999a, b, Karmazin et al., 2002, Baaden et al., 2003, Coupez et al., 2003). Beyond the difficulties of characterizing the radioactive systems investigated, this modelling approach seeks to define inde­pendent physical criteria that can be experimentally observed at different scales (molecular, microscopic as well as macroscopic) and correlated with modelling calculations (Guillaumont et al., 2006, Foreman et al., 2006, Petit et al., 2007, Gaunt et al., 2008).

(ii) Small quantities (sometimes a few hundred milligrams) of the desired compound are synthesized and chemically characterized to validate its structure and determine its purity utilizing suitable analytical tools such as nuclear magnetic resonance (NMR), mass spectrometry or elementary analysis.

(iii) The complexing and/or extracting properties of the synthesized com­pound are then assessed by general screening tests in micro-tubes, first on surrogate solutions, then on solutions spiked with the target LLRN and containing some of (or all) the competing elements present in the genuine nuclear waste.

(iv) If the ligand presents promising complexing and/or extracting proper­ties toward the target LLRN, the chemical reactions involved may be studied more deeply from a thermodynamic and kinetic standpoint, but this requires the synthesis of larger quantities of compounds. These investigations are carried out both at molecular and supra — molecular scales implementing analytical methods adapted to the nuclear environment to probe radioactive complexes. Analytical tech­niques used to investigate complexes at molecular scale include NMR (Wietzke et al., 1998, Lefranqois et al., 1999, Dozol and Berthon, 2007), UV-Visible spectrophotometry (Miguirditchian et al., 2006), X-ray crystallography (Wietzke et al., 1998, Berthet et al., 2005, Baaden et al., 2003, Coupez et al., 2003, Foreman et al., 2006, Gaunt et al., 2008), X-ray absorption spectroscopy (Hudson et al., 1995, Denning et al., 2002, Den Auwer et al., 2004, Gannaz et al., 2006), time-resolved laser — induced fluorescence spectroscopy (Colette et al., 2004, Pathak et al., 2009), microcalorimetry (Miguirditchian et al., 2005), gas chromatog­raphy and electrospray ionization mass spectrometry (Lamouroux et al., 2006, Leclerc et al., 2008, Antonio et al., 2008). Examples of analytical techniques used to investigate complexes at supra-molecu — lar scale are vapour pressure osmometry, small-angle neutron scatter­ing, and small-angle X-ray scattering (Hudson et al., 1995, Erlinger et al., 1999, Berthon et al., 2007).

(v) As the compound will sooner or later be degraded whilst being imple­mented to partition the target LLRN, through acidic hydrolysis and a/у radiolyses of the spent nuclear fuel dissolution solution, under­standing the ligand degradation pathways will help improve its chemi­cal resistance (by modifying its structure or changing the formulations of the separation system) and minimize the effect of its degradation products on the process efficiency (by developing specific solvent washing methods).

(vi) In parallel, systematic parametric tests to acquire partitioning data in test tubes (such as the solvent loading capacity or the variation of the distribution ratios of the target LLRN with a given parameter, as for instance the extractant concentration, acidity, pH, or ionic strength) will allow the formulations of the different aqueous and organic solu­tions to be optimized and the mass action laws to be determined. Thermodynamic models will consequently be developed to interpo­late the extraction isotherms.

(vii) In some cases a counter-current separation flowsheet is calculated utilizing process simulation codes, taking into account both the phase transfer kinetics and the hydraulic characteristics of the contactors (e. g., mixing efficiency, droplet size). The implementation (after scaling up the synthesis of the compound) of this counter-current flowsheet in laboratory scale contactors (mixer-settlers, centrifuges, small-scale pulsed or rotating columns), first on surrogate feeds to test the hydro­dynamic behaviour of the partitioning system, and then on genuine nuclear waste feeds (arising from the dissolution of spent nuclear fuel in concentrated nitric acid) validates or invalidates the process separa­tion performance. Analysis of the counter-current pilot test and com­parison of the observed experimental results with modelling calculations improve the accuracy of the process simulation codes, which can subsequently be employed to extrapolate the design of the workshops in the industrial reprocessing plant.

This methodology is rather long and complicated: it requires a range of skills and involves many researchers as the work progresses. The design of a highly selective hydrophilic or lipophilic compound is therefore intrinsi­cally combined with the development of the partitioning process in which the compound is used alone or in a synergistic mixture. The optimizations of both the selective compound itself and of the related partitioning process are therefore almost inseparable.

In fact, only a very few molecules are developed up to the counter-current demonstration test and most often the compounds investigated do not reach the third step, thus implying continuous iteration between steps (i) and (iii). It is commonly accepted, although difficult to satisfy, that highly efficient and selective compounds must fulfil many criteria, among which are:

• simplicity of preparation: excessive synthesis costs to scale up the pro­duction of a highly selective complexant or extractant might be a disin­centive for an industrial application;

• high complexation or extraction efficiency and a high selectivity toward the target element(s) to ensure high decontamination factors;

• reversibility of the target element complexation or extraction to ensure a high recovery yield;

• high resistance to chemical and radiochemical attack, generating only manageable degradation compounds;

• no formation of precipitate, the occurrence of which could trap some radionuclides and thus decrease their recovery yields, not to mention a criticality event;

• minimum secondary waste generation (burnable compounds consisting of carbon, hydrogen, oxygen, and nitrogen atoms are expected to release only gases);

• fast mass transfer kinetics to ease the implementation of the partitioning process in short time contactors;

• no occurrence of stable emulsions, which block counter-current process implementation.

With regard to the extractant loading capacity (that is to say the amount of metallic cation it can take up from the aqueous feed without inducing third phase formation or organic phase splitting), the major factor to con­sider is the balance between its hydrophilic character and its lipophilicity. The hydrophilic character of a ligand is enhanced by the polar nature of its chemical functions bearing the electron-donor atom(s), whereas the lipophilicity of a ligand is favoured by the length of the hydrophobic carbon chains grafted onto its skeleton. However, due to the presence of these two differing parts in its structure (the polar head interacting with the metallic cation(s) at the water/oil interface and the long hydrocarbon tail(s) facilitat­ing its dissolution and that of its metallic complex(es) in organic diluents), the compound usually presents surface active properties which may cause aggregation in the organic phase. Although aggregation could improve the extraction efficiency through the formation of reverse micelles (Chiarizia et al., 1999, Yaita et al., 2004, Jensen et al., 2002, 2007, Testard et al., 2008), it may also lead to the formation of a stable emulsion in certain experimen­tal conditions (Nave et al., 2004) and consequently to hydrodynamic prob­lems when running the process.

With regard to the selectivity and the complexation/extraction efficiency of a given ligand, considered as a base in Pearson’s ‘Hard and soft acids and bases’ theory (Pearson, 1963), the main factors governing the thermo­dynamics of its chemical reactivity are (i) the nature (‘soft’ or ‘hard’) and (ii) the number of its electron-donor atom(s), which interact with the elec­tron deficient metallic cation (considered as an acid in Pearson’s theory), as well as (iii) its structure (i. e., the three-dimensional geometric orientation of its chemical function(s) bearing the electron-donor atom(s)). The better the fit between the ligand donating function(s) and the metallic cation free orbitals, the stronger the coordination interactions. Multiple denticity, chelation, cyclization, and pre-organization of the chemical functions are complementary but nevertheless significant parameters that induce energy- stabilizing effects on the thermodynamics of the chemical reactivity and selectivity of a given ligand. For instance, the higher the degree of coordina­tion of a neutral extractant to the targeted metallic cation, the stronger the metal-ligand bonding interactions, because of the increased entropy varia­tion term (AS > 0) due to amplification of the system disorder, mainly resulting from dehydration of the metallic cation (Musikas, 1986, Nash, 1993).

Carbon dioxide is widely used for supercritical fluid extraction studies because of its moderate critical constants (Pc = 73 atm and Tc = 31 °C), chemical inertness, and low cost (Phelps et al. 1996). Above the critical point, CO2 becomes a fluid that has both gas-like and liquid-like properties as illustrated in Fig. 14.1. Supercritical fluid CO2 has mechanical properties

14.1 Phase diagram of CO2 and some properties of sc-CO2.

like a gas and yet has solvation strength like a liquid. Therefore, it is capable of penetrating into small pores of solid materials and dissolving organic compounds from the solid matrix. After extraction, the fluid phase can be vented as CO2 gas by reduction of pressure causing precipitation of the extracted solutes. In principle, no liquid waste is produced by this extraction technique according to the idealized operation.

One important factor which determines the efficiency of supercritical fluid extraction is the solubility of the target compound in the fluid phase (Darr and Poliakoff, 1999). Because CO2 is a linear molecule with no dipole moment, sc-CO2 is a good solvent for dissolving non-polar organic com­pounds, but is ineffective for dissolving highly polar compounds or ions. Metal ions are not soluble in sc-CO2. Searching for methods of dissolving metal ions in sc-CO2 was the focus of the initial research started in the author’s laboratory at the University of Idaho two decades ago. In 1991, Laintz et al. (1991) noticed that complexes formed by transition metal ions with a fluorinated dithiocarbamate chelating agent bis(trifluoroethyl) dithiocarbamate (FDDC) exhibited unusually high solubilities (by 2-3 orders of magnitude) relative to their non-fluorinated analogs. Based on this information, the authors demonstrated that copper ions (Cu2+) in aqueous solutions can be effectively extracted into sc-CO2 with the aid of FDDC (Laintz et al. 1992). The idea of using fluorinated chelating agents for dissolution of metal ions in sc-CO2 was actually inspired by the fact that perfluorinated alkanes were considered at that time as blood substitutes because of their high solubilities for oxygen and carbon dioxide. The reverse is apparently true for dissolution of fluorinated compounds in sc-CO2.

Today, fluorine-containing compounds are generally referred to as “CO2- philic” because of their high solubilities in sc-CO2. The successful demon­stration of Cu2+ extraction immediately led to the investigation of extracting lanthanides and uranium in sc-CO2 using fluorinated ligands because of its potential relevance to nuclear waste management. Several reports appeared after 1992 demonstrating that trivalent lanthanide ions and uranyl ions dis­solved in water or spiked in solid materials could be effectively extracted by sc-CO2 using fluorinated P-diketonates as extractants (Lin et al. 1993, Lin and Wai, 1994, Lin et al. 1994). Among a number of fluorinated P-diketonates tested for sc-CO2 extractions, thenoyltrifluoroacetone (Htta) was more often used than the others because it is a solid at room tempera­ture and is easier to handle experimentally (Wai and Wang 1997). After these initial reports, dissolution of uranium oxides directly in sc-CO2 with fluorinated P-diketones was investigated (Wai and Waller, 2000). Direct dissolution of solid UO3 in sc-CO2 with Htta occurs according to the fol­lowing equation:

UOs(s) + 2 Htta ^ UO2(tta)2-H2O 14.1

The dissolution of UO3 proceeds with a much higher efficiency if tri-n- butylphosphate (TBP) is present with Htta in sc-CO2. The enhance effi­ciency is attributed to the fact that TBP is a stronger Lewis base which can replace water in UO2(tta)2-H2O to form a more soluble adduct complex UO2(tta)2-TBP as shown in equation (14.2).

UO3(s) + 2 Htta + TBP ^ UO2(tta)2-TBP + H2O 14.2

The solubility of UO2(tta)2-TBP in sc-CO2 is reasonably high to be poten­tially useful for reprocessing consideration. For example, at a fixed tempera­ture of 40 °C its solubility in sc-CO2 is 7.5 x 10-3 mol/L at 200 atm and increases to about 1.5 x 10-2 mol/L at 300 atm. Reaction (14.2) occurs effi­ciently for UO3 because uranium is in the +6 oxidation state, which leads to the formation of the uranyl-tta-TBP adduct complex with no need of an oxidation step. However, this reaction does not occur with UO2 because uranium is in the +4 oxidation state, which does not form a stable complex with tta and TBP. A CO2-soluble oxidizing agent such as H2O2 has been shown to promote the dissolution of UO2 in sc-CO2 with Htta and TBP but the efficiency is limited (Trofimov et al. 2001). Research in extraction of actinides in sc-CO2 using fluorinated P-diketones and organophosphorus reagents may still find applications in nuclear waste treatments. For example, a report in 2003 showed that plutonium and americium in soil could be effectively removed by sc-CO2 augmented with Htta and TBP (Fox and Mincher 2003). The solubilities of uranium, plutonium, neptunium, and americium P-diketonates and their adducts with organophosphorus reagents in sc-CO2 were also reported (Murzin et al. 2002).

A significant development in supercritical fluid extraction of uranium was made in 1995 by Lin et al. (1995) who reported that uranyl ions in nitric acid solutions could be extracted into sc-CO2 with the aid of TBP, a well — known ligand for uranium extraction in the conventional PUREX (Plutonium Uranium Extraction) process. TBP happens to be highly soluble in sc-CO2. An earlier report showed that about 10% by mole of TBP could be dissolved in sc-CO2 under normal extraction conditions (Page et al. 1993). Actually, TBP becomes miscible with sc-CO2 above a certain pressure at a given temperature according to a later report (Joung et al. 1999). The efficiency of extracting uranium by the sc-CO2/TBP process depends on the nitric acid concentration and follows closely the same trend as the tradi­tional solvent extraction process using dodecane and TBP as shown in Fig. 14.2 (Lin et al. 1995). The extracted uranium species in sc-CO2 was identified as UO2(NO3)2(TBP)2 similar to the uranyl complex extracted from nitric

14.3 Solubility of UO2(NO3)2(TBP)2 in sc-CO2. From Carrott et al. 1998. Reproduced by permission of The Royal Society of Chemistry.

acid into dodecane with TBP (Carrott et al. 1998). To evaluate the solubility of the uranyl complex in sc-CO2, a high-pressure fiber-optic cell was designed to measure the absorption spectrum of the complex in situ in the fluid phase (Carrott and Wai 1998). The solubility of UO2(NO3)2(TBP)2 in sc-CO2 mea­sured by the spectroscopic method is surprisingly high. At 40 °C and 200 atm, the solubility of the uranyl complex in the fluid phase reaches 0.45 moles per liter, a concentration comparable to that used in the conventional PUREX process (Wai 2001). When the solubility of the uranyl complex in sc-CO2 is plotted against the density of the fluid phase, a linear relationship is observed as shown in Fig. 14.3. The results given in Fig. 14.3 demonstrate the solvation strength of the supercritical fluid is tunable with respect to density. Consequently, the solubility of UO2(NO3)2(TBP)2 in sc-CO2 can be predicted based on the relationship log S = a log ф + b, where S is the solu­bility in g/L, ф is the density also in g/L, and a and b are constants. The tunable solvation property is unique for sc-CO2 and suggests a possibility of selective dissolution or separation of metal complexes in sc-CO2 by manipulation of density (i. e., temperature and pressure) of the fluid phase. Conventional liquid solvents are basically not compressible and the solva­tion strength cannot be altered at a given temperature by varying pressure except perhaps at extremely high pressures. Other studies of uranium extraction using sc-CO2 and TBP were reported by Meguro et al. (1998a, 1998b). In another study, Iso et al. (2000) showed that the distribution of Pu(IV) between 3 M nitric acid and sc-CO2 containing TBP depends on pressure at a given temperature (60 °C) and follows the equation log D = a log ф + b, where D is the distribution coefficient, ф is the density of the

fluid and a and b are constants. The D values as well as the constants a and b for U(VI) in the same system are different from those of Pu(IV) suggest­ing some degree of separation of Pu(IV) and U(VI) can be achieved by adjusting the pressure of the sc-CO2 extraction process. These studies provide a basis for using sc-CO2 to replace the organic solvents used in the PUREX process.

Based on the information available in 1998, Smart et al. proposed a supercritical fluid-PUREX process for reprocessing spent nuclear fuel as illustrated in Fig. 14.4 (Smart et al. 1998). In this process, sc-CO2 is employed to replace the organic solvent conventionally used in the PUREX process. The advantages of the supercritical fluid-PUREX process include conceiv­ably higher mass transport properties, faster extraction rates, and tunable distribution coefficients of uranium and other actinides. However, since nitric acid is used to dissolve spent fuel in the supercritical fluid-PUREX process, it would still produce acidic liquid waste like the PUREX process. After the proposed application by Smart et al. (1998), research in finding a dry process for dissolution of uranium dioxide in sc-CO2 intensified (Wai 2006a).

As the energetic beta and alpha particles traverse through a crystalline medium, atoms are displaced from their stable lattice positions, forming Frenkel pairs (ion pairs, vacancy and interstitials). For example, the autora­diolysis in metallic 239Pu (T1/2 = 2.41 x 104 y) causes significant structural changes. When the 239Pu atom decays, an a-particle with an kinetic energy of 5 MeV and the recoil nucleus of 235U atom with an energy of 86 keV are created. In metallic Pu, the a-particle deposits its kinetic energy in approxi­mately 10 pm, while for the heavier recoil atom it is only 12 nm. For each disintegration event, approximately 2600 ion pairs are formed, and over the course of 20 years, every atom in a piece of Pu metal changes its position. This property becomes critical when trying to predict the long-term behav­ior of Pu materials (Keogh, 2005). However, while metallic structures can undergo a significant degree of self-annealing (heating to a melting point and cooling over a long period of time), molecular complexes can suffer irreversible structural damage. A typical example would be autoradiolysis causing amorphization of crystalline compounds of relatively short-lived americium 241Am (T1/2 = 432.7 y). As confirmed by the single crystal X-ray diffraction analysis, the intensity of the diffraction decreases with time. Visible damage of crystal lattice can be observed by opacity of originally clear crystal and increased solubility. Similar structural disordering in mate­rials was observed after irradiation with high-energy electron beam. Electrons, being much smaller than alphas or heavy ions, penetrate deeper through the bulk of the irradiated crystal, and cause amorphization of the crystalline lattice. Uranophane and other uranium solid matrices (Douglas et al., 2002; Utsunomia et al., 2003), important for the development of radia­tion resistant waste forms have been studied using accelerated beams. Thermochemical investigation of stability of microporous and mesoporous materials such as frameworks in silica zeolites or selected metal-phosphates (Petrovic et al., 1993; Hu et al., 1995) is another example of material studies
for high-level nuclear waste immobilization (Navrotsky et al., 2009; Weber et al., 2009).

A material very resistant to radiation damage is glass, because it is a noncrystalline “solid” liquid. Though at an atomic level the same phenom­ena occur in glass, it is not appropriate to speak of dislocations in glass: the random structure of the glass allows it to accommodate foreign species throughout the sample (Sales, Boatner, 1984; Oelkers, Montel, 2008). Glasses have been intensely studied as the matrix for high active waste consisting of fission products and actinides (Ewing, Wang, 2002).

The goal of Raman and vis-NIR spectroscopic measurements of the dis­solved ATM-109 fuel solutions was to assess the utility of optical process monitoring methods employing Raman and vis-NIR for the direct measure­ment of dissolved fuel feed, 30% TBP/n-dodecane extraction, and raffinate phases under the conditions of complex composition of the dissolved com­mercial fuel containing multiple light-absorbing species potentially interfer­ing with detection and quantification of the target analytes.

Direct Raman measurements of the aqueous nitric acid feed and raffinate solutions performed on commercial fuel ATM-109 are shown in Fig. 4.10 (left). This figure also contains a spectrum of the Simple Feed fuel simulant (containing 1.3 M UO2(NO3)2/0.8 M HNO3) for comparison of the Raman response between simulants and actual fuel samples. The spectral features responsible for the UO22+ and NO3- bands (870 and 1047 cm-1, respectively) in the fuel feed and raffinate samples are in excellent agreement with those contained within the Simple Feed simulant. The TBP/n-dodecane extract phase of ATM-109 fuel was also measured by Raman spectroscopy. Figure

4.10 (right) compares the Raman spectra of TBP/n-dodecane extracts of ATM-109 feed and Simple Feed simulant. No shift of the UO22+ and NO3- bands Raman bands (858 and 1029 cm-1, respectively) between the extrac­tion phases loaded using actual commercial fuel and simulant solutions was observed. Other bands observed in the extraction phase Raman spectra were assigned to the solvent (TBP and n-dodecane).

Spectrophotometric measurements of the aqueous feed solutions of the ATM-109 commercial fuel samples were performed using the vis-NIR spec­tral region. Plutonium in both Pu(IV) and Pu(VI) oxidation states was observed in the commercial fuel feed, with varying concentrations depend­ing on the HNO3 concentration, as is apparent in Fig. 4.11. Neptunium as Np(V) was evident in the ATM-109 fuel (Fig. 4.11).

Figure 4.12 (top) shows the vis-NIR spectra of the organic extraction phase from ATM-109 in 5.1 M HNO3. The spectral bands observed in

Fig. 4.12 are those diagnostic for Pu(IV). For comparison, a fuel simulant containing a feed composition of 1.33 M UO2(NO3)2 in 0.8 M HNO3 with a Pu(IV) concentration of 2 mM was contacted with the 30% TBP/n-dodecane PUREX solvent followed by spectroscopic measurement by vis-NIR spec­troscopy, with the resulting spectra shown in Fig. 4.12 (bottom). There is excellent agreement in comparing the Pu(IV) bands between the actual commercial fuel extract (Fig. 4.12, top) and fuel simulant extract (Fig. 4.12, bottom).

The Raman and vis-NIR spectra of the ATM-109 feeds were subjected to chemometric analysis and standard Beers Law spectral analysis to deter­mine the concentrations of U, Pu, and Np present in solution. The resulting concentrations are contained in Table 4.1. For comparison, the ICP-MS results are also displayed, along with both the ORIGEN code calculations for these fuel samples and the computed ratios of analytical results for ORIGEN/ICP and Spectroscopic/ICP data. From this table, it is evident that the spectroscopic method is in excellent agreement with the standard ICP-MS analysis.

The behavior of Tc in the COEXTM process is basically the same as in PUREX flowsheet presented in Fig. 6.2. More than 95% of Tc from the dissolution solution is recovered in raffinate of the complementary extrac­tion, so in an effluent presenting a relative low activity (the bulk of the other fission products is eliminated during the first extraction-scrubbing steps). The other 5% of Tc is recovered in the raffinate of the second U/Pu puri­fication cycle (“mini-cycle”).

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

At the ‘salt waste treatment’ stage, the salt-loaded zeolite is mixed with fresh zeolite at approximately 500 °C to absorb the free salt, and the mixture is then heated up to approximately 925 °C to convert the salt-loaded zeolite into a more stable sodalite structure, Na8(AlSiO4)6Cl2, with borosilicate glass powder as a bonding material to microencapsulate the grains of soda — lite. The glass-bonded sodalite obtained is referred to as ceramic waste form (CWF). The long-term stability of CWF is expected from the natural ana­logue of sodalite mineral, which contains chloride salt and iodine in its cage structure, in a stable form. Actually, sufficiently stable characteristics to meet Yucca Mountain requirements were reported for CWF based on the extensive studies carried out by ANL and INL from the early 1990s (Ebert, 2005).

The first nitrogen based systems investigated in France and in Europe to selectively extract An(III) from nitric acid solutions consisted of synergistic binary mixtures of tridentate polyazine ligands and organophilic acids. They played the role of cation exchangers by extracting the cationic complexes formed between the An(III) and the tridentate polyazine ligands, through proton exchange. As illustrated in Fig. 11.11 and more deeply detailed in

11.11 Examples of nitrogen-donor ligands used in synergistic mixtures with cation exchangers.

the comprehensive reviews of Kolarik (2008) and Ekberg et al. (2008), the tridentate polyazine ligands studied belonged to the families of the:

• terpyridines (Drew et al., 2000a);

• monopyridyl-1,2,4-triazines (Hudson et al., 2003a) and dipyridyl-1,2,4- triazines (Boubals et al., 2002, Hudson et al., 2003a, b, Drew et al., 2006);

• dipyridyl-1,3,5-triazines (Drew et al., 2000b) and tripyridyl-1,3,5-tri — azines (Chan et al., 1996, Cordier et al., 1998, Hagstrom et al., 1999);

• dioxa-/benzoxa-/binzimida-/zol-pyridines (Kolarik and Mullich, 1997, Andersson et al., 2003, Weigl et al., 2003, Drew et al., 2004b);

• amino-dipyridyl-1,3,5-triazines (ADPTZ).

These tridentate nitrogen-donor ligands all provided moderate selectivity of complexation toward An(III), with selectivity factors versus Ln(III), SFAn/Ln, ranging from 5 to 20 at most, as demonstrated by Miguirditchian et al. (2005, 2006), who investigated the thermodynamics of M(III): ADPTZ complex formation (M = An or Ln) and pointed out the more exothermic complexation reactions of An(III) than Ln(III), thus highlighting the assumed greater covalent character of the N-An(III) bonds, which is sup­ported by quantum chemistry calculations. The X-ray crystal structures of metal : terpyridine solvates resolved by Berthet et al. (2002a) also revealed that the U-N(central pyridine) distances are shorter than the U-N(distal pyridines) distances, while the reverse is true for lanthanide compounds. It has been suggested that these differences reflect the presence of a n back — bonding interaction between U and terpyridine.

Unfortunately, the use of carboxylic acids (e. g., octanoic or alpha-bromo — capric acids) and even dinonylnaphthalene-sulphonic acid (HDDNS), as sources of lipophilic anions in synergistic mixtures, was of little help in discriminating between An(III) and Ln(III) from aqueous feeds whose acidity exceeded 0.1 mol. L-1, as is the case for DIAMEX An(III)-product solutions. Therefore, the only two reported counter-current tests imple­mented in laboratory-scale mixer-settlers used surrogate spiked feeds of low acidity. The first one was carried out by Vitart et al. in 1986 and made use of the synergistic mixture composed of TPTZ (Fig. 11.11, R = — H) and HDNNS, dissolved in carbon tetrachloride. The second one was carried out in 2000 with the binary mixture composed of 2-(3,5,5-trimethylhexanoyl- amino)-4,6-di(pyridin-2-yl)-1,3,5-triazine (TMHADPTZ, Fig. 11.11) and octanoic acid (Madic et al., 2002). Although the experimental profiles were in good agreement with the calculated flowsheet, the latter system has not been further developed because of its high sensitivity to pH variations and the necessity to buffer the feed acidity (pH > 3).

Within the NEWPART collaborative project (Madic et al., 2000), the discovery of the bis-(1,2,4-triazinyl)-pyridines (BTP) synthesized by Zdenek Kolarik appeared as a real breakthrough in the design of nitrogen-donor extractants selective toward An(III). Surprisingly, BTP ligands firstly pre­cipitated when combined with organophilic carboxylic acids (Kolarik et al., 1999a). Even more unexpectedly, these tridentate compounds, consisting of two dialkylated 1,2,4-triazines attached sideways to a central pyridine (Fig. 11.12), proved to extract Am(III) from acidic aqueous solutions ([HNO3] > 1 mol. L-1) when dissolved in polar diluents, such as mixtures of HTP and alcohols. Furthermore, their selectivity was shown to be amazingly high as compared to any of the aforementioned nitrogen-donor ligands: SFAm/Eu > 100 (Kolarik et al., 1999b).

Various BTP ligands have been further synthesized in the framework of NEWPART and PARTNEW collaborative projects (Madic et al., 2000, 2004). The parametric and thermodynamic studies carried out on these molecules revealed that the nature (e. g., normal or branched alkyl chains, aromatic rings) and the length, as well as the position of the substituting groups, not only influence the complexing and extracting properties of BTP compounds (An(III)/Ln(III) selectivity and kinetics of extraction), but also their chemical stability (Iveson et al., 2001, Colette et al., 2002, Hill et al., 2002, Hudson et al., 2003a, Weigl et al., 2006).

Several reasons could explain the peculiarity of BTP’s extracting behaviour:

• The unsymmetrical positions of the nitrogen atoms within the lateral triazine rings, which give these compounds low and great affinities respectively for protons and An(III) (Petit et al., 2006). As opposed to bis-(1,2,4-triazinyl)-pyridines, symmetrical bis(1,3,5-triazinyl)-pyridines present bad extraction properties (Drew et al., 2004c).

• The M :L3 (metal : ligand) stoichiometry of the extracted complexes pointed out by different structural studies (Fig. 11.13, Drew et al., 2001a, b, Berthet et al. 2002b, Colette et al., 2003), which results in the fully saturated inner coordination spheres of the extracted trivalent f cations, wrapped by three triply bonded BTP ligands, hence leaving no spare space for extra water or nitrate coordinating molecules (Colette et al., 2004, Deneke et al., 2005, 2007).

• The chemical reactivity of the carbon atoms located on the a-positions of the alkyl groups grafted onto the lateral triazines. Nitrous acid and oxygen tend to oxidize non fully substituted carbon atoms, leading to hydrophilic alcohol and ketone derivates, which degenerate the intrinsic extraction properties of BTP ligands, as experienced during the demon­stration hot tests carried out with 2,6-bis-(5,6-di-n-propyl-1,2,4-triazine — 3-yl)-pyridine (nPr-BTP, dissolved at 0.04 mol. L-1 in a mixture of HTP and n-octanol (70/30 vol.%)) on genuine DIAMEX An(III)-product solutions, both at the CEA Marcoule (France) and the ITU (Karlsruhe, Germany) in 1999.

FL .N. J-L — Y. Ns .R Y Y N T Y rAn"n N’fA 2,6-Bis-(5,6-dialkyl-1,2,4-Triazin-3-yl)-Pyridines (BTP) JL Ns. YY Y n n Y/ n’nYU Bis-Annulated-Triazine-Pyridine (BATP): CyMe4-BTP Bis-Annulated-Triazine-Pyridine (ВАТР): фСуМе4-ВТР ) Vn n- N=( Yn r4 ,n _>r /—N N Y R R YY ) Yn n4 ,,N 6,6’-bis(5,6-dialkyl-[1,2,4]-triazin-3-yl)-[2,2’]-bipyridines (BTBP) Bis-Annulated-Triazine-Bis-Pyridine (BATBP): CyMe4-BTBP 77.72 Polypyridine-triazine extractants developed for An(lll)/Ln(lll) separation.

• The longer and bulkier are the alkyl chains grafted onto the lateral triazines of the BTP compounds, the slower are their kinetics of mass transfer, probably due to adsorption difficulties at the interface (BTP molecules are not assumed to aggregate). Therefore, the formulation of BTP based solvents most often requires the addition of a mass transfer catalyst (such as a diamide) to aid phase transfer and thus enhance kinetics of extraction and stripping (Hill et al., 2002). However, this additive tends to decrease the selectivity of the BTP toward An(III).

Physical chemical processes utilize chemicals or chemical properties of surfaces to chemically reduce or physically extract the radionuclides based on chemical charge or size. Processes that have been tried include the use of specialized ion exchange resins, chemical oxidation and adsorption columns. Mechanisms and materials employed include the following.

Adsorption

This is the most widely used among the available processes. It is primarily used for the removal of soluble organics with activated carbon serving as the adsorbent. Removal of organics on the adsorbent surfaces is mostly predicted by the Freundlich Isotherm:

y = kclln 15.1

where y = adsorbent capacity, mass pollutant/mass carbon, c = concentration of the pollutant in waste (mass/volume), and k and n = empirical constants.

Other non-linear models are also available that are applicable under par­ticular conditions.

To effectively manage the recycle and disposal of the fission byproducts, the chemistry of the most challenging elements must be addressed in two dif­ferent stages, during the recycling process (generally solution chemistry) and wastes produced from the recycling process (generally solid-state chemistry). Some nuclear and radiological features have been discussed in the previous section. In this section, the emphasis will be on discussing the essential features of the solution phase and solid-state chemistry of these elements, starting with the lightest, proceeding to the heaviest and empha­sizing the aqueous chemistry that is of the greatest significance. The discus­sions of the chemistry start from the lightest elements to the heaviest.