Simulated High Level Waste Solution Research Articles

Extraction of Cesium, Strontium, and Stable Simulated HLW Components with Substituted Crown Ethers in New Fluorinated Diluents

ABSTRACT The extraction of cesium, strontium, and a number of stable components of simulated high-level waste solutions from nitric acid media with solutions of crown ether derivatives in new fluorinated diluents was studied. Based on the data on the solubility of the extractants into the aqueous phase and the physicochemical properties of the diluents used, the most promising extraction systems were chosen: crown ethers 4,4‘(5’)-di-tert-butyldibenzo-18-crown-6 (L1) and 4,4‘(5’)-di-tert-butyldicyclohexyl-18-crown-6 (L2) in bis(2,2,3,3-tetrafluoropropyl) carbonate (BK-1) and bis(2,2,3,3-tetrafluoropropoxy)methane (FN-1). For these systems, the extraction of a number of stable components and their subsequent stripping with water and solutions of chelating agents were studied. Lead was found to be co-extracted to the greatest extent. To a lesser extent, barium and calcium cations are co-extracted with strontium, and rubidium and potassium cations with cesium. Lead, barium, and silver are the most difficult to strip; however, the problem of lead stripping can be solved by using DTPA or citric acid solutions neutralized with ammonia to weakly alkaline (pH 8) media.

Exploring novel functionality for efficient extraction of UO22+ and Th4+in ionic liquid: Mechanism, speciation, selectivity, stability and stripping

Novel ligand functionality, 5-bromo-N1, N3-bis(4,6-dimethylpyridin-2-yl)isophthalamide, was explored for highly efficient and selective separation of UO22+ and Th4+ in pyridinium based ionic liquid. The comprehensive investigation revealed that, both in ionic liquid as well as in molecular diluents the extraction predominantly proceeded via ‘solvation’ mechanism. However, the species involved in extraction were UO2(NO3)2, L and Th(NO3)4, L in ionic liquid, while UO2(NO3)2, 2 L and Th(NO3)4, 3 L in molecular diluent. The highly viscous ionic liquid resulted the mass transfer slower during extraction. In ionic liquid based system, the extraction processes were endothermic, spontaneous and entropy driven. Ionic liquid based solvent systems exhibited high radiation stability, which was attributed to the formation of resonance-stabilized aromatic cation as well as the restricted movement of radicals in viscous ionic liquid. The aqueous solution of sodium carbonate was found to be effective for the stripping of UO22+, while oxalic acid solution was found to be effective for Th4+. The solvent systems were used for processing of simulated high level waste solutions of a Research Reactor (RR) and Fast Breeder Reactor (FBR) origin.

Comparison in the aggregation behaviour of amide extractant systems by dynamic light scattering and ATR-FTIR spectroscopy

The aggregation behavior of N,N,N’,N’-tetraoctyldiglycolamide (TODGA) in n-dodecane was studied in the presence of two different phase modifiers, N,N-di-hexyloctanamide (DHOA) and 1-octanol using dynamic light scattering (DLS) spectroscopy. The results were compared with the aggregation behavior of a modifier free extractant system N,N-di-octyl-2-hydroxyacetamide (DOHyA) in n-dodecane. Aggregation studies were done as a function of concentration of nitric acid and Nd (III) ions in the aqueous phase. The size distribution of reverse micellar aggregates revealed different aggregation mechanisms in the presence and absence of phase modifiers. For TODGA-modifier systems, the sizes of reverse micellar aggregates were bigger than DOHyA and among the two phase modifiers studied, use of 1-octanol resulted in the formation of bigger aggregates compared to DHOA. The observations from DLS measurements were co-related with the results from ATR-FTIR spectroscopy for all the three extractant systems. The sizes of reverse micellar aggregates increased with increase in extraction of nitric acid and/or Nd (III) ions and no third phase was visible in all the three systems even under very high loading conditions. The aggregation studies under co-current extraction conditions from fast reactor simulated high level waste solution (FR-SHLLW) showed the formation of smaller reverse micellar aggregates and third phase or crud formation after 3–4 successive contacts for all the three systems.

Quaternary ammonium based task specific ionic liquid for the efficient and selective extraction of neptunium

Abstract: Liquid–liquid extraction of neptunium from aqueous acidic solution using quaternary ammonium based task specific ionic liquid (TSIL) was investigated. The extraction of Np was predominated by the ‘cation exchange’ mechanism via [NpO2·Hpth]+ species for NpO2 2+, while NpO2 + was extracted in ionic liquid as [NpO2·H·Hpth]+. The extraction process was thermodynamically spontaneous while kinetically slower. Na2CO3 as strippant showed quantitative back extraction of neptunium ions from TSIL. TSIL showed excellent radiolytic stability upto 500 kGy gamma exposure. Finally, the TSIL was employed for the processing of simulated high level waste solutions revealing high selectivity of TSIL towards neptunium.

Radium and Thorium Sorption by Monosodium Titanate (MST) and Modified MST (mMST)

To confirm performance assessment inputs for the Saltstone Facility at the Savannah River Site (SRS), a series of experiments were performed to examine the removal of Ra and Th by monosodium titanate (MST) and modified MST (mMST). The solubility of Th in the simulated high-level waste solutions was found to be extremely low; therefore, its removal by MST could not be measured. Radium was rapidly removed from the high-level waste simulant by both MST and mMST.

Comparative study on the radiolytic stability of TBP, DHOA, Cyanex 923 and Cyanex 272 in ionic liquid and molecular diluent for the extraction of thorium

The radiolytic stability of TBP, DHOA, Cyanex 923 and Cyanex 272 in ionic liquid vis a vis molecular diluents have been evaluated for the extraction of thorium. The decrease in DTh value was found to follow the trend of irradiated (Xylene + ligand) > irradiated (ionic liquid + ligand). Ionic liquid was found to act as a sink to protect the ligand functionality from radiation damage. On gamma exposure, the other metal ions were also found to be co-extracted into organic phase from simulated high level waste solution of fast breeder reactor origin consequently reducing the selectivity. The reduction in selectivity on gamma exposure was found to be more in xylene than C8mimNTf2 based systems.

Electrodriven selective transport of Cs+ using chlorinated cobalt dicarbollide in polymer inclusion membrane: a novel approach for cesium removal from simulated nuclear waste solution.

The work describes a novel and cleaner approach of electrodriven selective transport of Cs from simulated nuclear waste solutions through cellulose tri acetate (CTA)/poly vinyl chloride (PVC) based polymer inclusion membrane. The electrodriven cation transport together with the use of highly Cs+ selective hexachlorinated derivative of cobalt bis dicarbollide, allows to achieve selective separation of Cs+ from high concentration of Na+ and other fission products in nuclear waste solutions. The transport selectivity has been studied using radiotracer technique as well as atomic emission spectroscopic technique. Transport studies using CTA based membrane have been carried out from neutral solution as well as 0.4 M HNO3, while that with PVC based membrane has been carried out from 3 M HNO3. High decontamination factor for Cs+ over Na+ has been obtained in all the cases. Experiment with simulated high level waste solution shows selective transport of Cs+ from most of other fission products also. Significantly fast Cs+ transport rate along with high selectivity is an interesting feature observed in this membrane. The current efficiency for Cs+ transport has been found to be ∼100%. The promising results show the possibility of using this kind of electrodriven membrane transport methods for nuclear waste treatment.

Studies on evaluation of modified TRUEX solvent for the partitioning of minor actinides

An attempt to modify the TRUEX solvent has been initiated by replacing TBP with long-chain alcohols such as isodecanol. Solvent extraction experiments have been carried out determining distribution ratios for Am(III) as a function of concentrations of nitric acid and isodecanol. Acid uptake constant for the combined organic phase has also been arrived at which strongly suggests the interaction of CMPO and isodecanol. The suitability of the organic phase was tested for simulated high-level waste solutions.

Process development for separation of cesium from acidic nuclear waste solution using 1,3-dioctyloxycalix[4]arene-crown-6 + isodecyl alcohol/n-dodecane solvent

A new process for selective separation of cesium from acidic nuclear waste solution has been described using 1,3-dioctyloxycalix[4]arene-crown-6 (CC6)/n-dodecane modified with isodecyl alcohol. The process solvent was optimized as 0.03M CC6+30% isodecyl alcohol/n-dodecane for efficient recovery of cesium. The solvent has shown strong extraction ability for cesium between 1 and 6.0M nitric acid. Stoichiometry of the extracted complex determined by slope analysis method reveals 1:1:1 molar ratio for Cs+, CC6 and HNO3. Extraction of cesium was found to be independent of nitrate ion concentration, possibly due to formation of solvated free ions CsCC6+·HNO3 and NO3− in the organic medium. Extracted cesium from organic phase was readily stripped with deionized water. Radiolysis of the process solvent up to an absorbed dose of 0.6MGy produces essentially no deterioration in its performance. Counter-current mixer–settler experiments with four extraction, two scrubbing and four stripping stages gave about 99% recovery of cesium with very high selectivity from simulated high level waste solution. The results demonstrated the effective use of CC6+isodecyl alcohol/n-dodecane for separation of cesium from acidic high level waste solution.

Solvent systems containing diglycolamide-functionalised calix[4]arenes in room temperature ionic liquid for metal ion extraction: studies with simulated high-level wastes

The extraction of actinide ions such as Am(III), Pu(IV), Np(IV), U(VI) and Pu(VI) was studied from simulated high-level waste (SHLW) solutions using several diglycolamide-functionalised calix[4]arene (Calix-DGA) ligands dissolved in the room temperature ionic liquid (RTIL) [C8mim][NTf2]. The Calix-DGA ligands were three lower-rim functionalised ligands with varying alkyl substituents (L-I, L-II and L-III), an upper-rim functionalised ligand (L-IV) and an upper- and lower-rim functionalised ligand (L-V). The SHLW solutions used contained 0.5 M HNO3 and the extraction trend of the actinides was found out to be Am(III) > Pu(IV)∼Np(IV)≫ and the extraction efficiency of the Calix-DGA ligands for Am(III) being L-V>L-I>L-II>L-IV>L-III. Reasonably, good extraction of Am(III) was obtained with 5.0 × 10− 4 M ligand solutions suggesting that higher concentrations of the ligand can be used for a higher loading of the metal ions. The extraction of other metal ions representing fission products, structural materials and process chemicals was studied by Inductively Coupled Plasma –Atomic Emission Spectroscopy analysis. The extraction mechanism follows the cation-exchange mechanism often observed with RTIL-based extraction systems. Enthalpy changes were measured from van't Hoff plots and were found to be significantly different than those observed with tracer studies reported previously.

Amberlite XAD-16 Functionalized with 2-Acetyl Amide Group for the Solid Phase Extraction and Recovery of Palladium from High Level Waste

Acetyl amide grafted XAD-16 (ACAM-XAD16) resin has been synthesized by acetylating Amberlite XAD-16 resin followed by coupling it with N,N′-diisobutyl-2-chloroacetamide. Various instrumental techniques such as FT-IR, TGA, and elemental analysis were used to characterize ACAM-XAD16 resin. The resin has been evaluated for the solid phase extraction of palladium and other metal ions present in high level waste (HLW). The sorption kinetics of Pd onto the resin was found to be fast, with equilibrium being attained within 45–60 min. The experimental data fit well with the pseudo-second-order kinetics model of sorption kinetics. With the increase in aqueous phase acidity, Pd sorption increases initially, reaches a maximum around 3.0 M and then decreases. The sorption process involves the complex formation between Pd(II) and the resin which contains the carbonyl groups from the acetyl and the amidic moieties. The loaded palladium can be effectively eluted using acidic thiourea solutions and HEDTA solutions. The isothermal adsorption data fit well with both the Langmuir and the Freundlich models with maximum sorption capacity equal to 9 mg/g, which corroborates well with the equilibrium sorption capacity. Sorption studies carried out with simulated high level waste solution have shown very high uptake of palladium and small coextraction of lanthanides compared to the other elements.

Amberlite XAD-16 functionalized with 2-acetyl pyridine group for the solid phase extraction and recovery of palladium from high level waste solution

Amberlite XAD-16 has been functionalized with 2-acteyl pyridine by coupling it with 2-chloro pyridine after acetylation. The resulting resin (AP-XAD 16) has been characterized by various techniques like FT-IR, TGA, elemental analysis, etc. and has been evaluated for the solid phase extraction of palladium and other metal ions present in high level waste (HLW) solution. Sorption equilibrium was attained within 30–45min. The pseudo-second order kinetics model yielded the best fit for the experimental data of sorption kinetics. Pd sorption was found to decrease gradually with increase in aqueous phase acidity. The sorption process involves the complex formation between Pd(II) and the resin which contains the carbonyl group and soft ‘N’ atom of pyridine moiety with lone pair of electrons. The loaded palladium can be effectively eluted using acidic thiourea solutions. The adsorption isotherm data fitted well with Langmuir as well as Freundlich models. The maximum sorption capacity of the resin was found to be 8mg/g which is in agreement with the equilibrium adsorption capacity. Sorption studies carried out with simulated high level waste solution have shown very high uptake of palladium as compared to the other elements, thus showing the potential for possible use of the resin for efficient separation and recovery of palladium from HLW solution.

Removal of Cs from simulated high-level waste solutions by extraction using chlorinated cobaltdicarbollide in a mixture of nitrobenzene and xylene

Efficacy of chlorinated cobaltdicarbollide in a modified diluent, 20% nitrobenzene in xylene was tested for the extraction and recovery of Cs from simulated high-level waste (HLW) solutions generated from PHWR-fuel reprocessing. Concentration of the reagent, composition of the diluent, numbers of contacts, the nature of stripping agents are some of the parameters optimized for the complete removal of Cs from such waste solutions. The above solvent extraction procedure can be applied to genuine HLW solutions for effective reduction of the dose due to Cs so that HLW can be handled in fume hoods for its characterization.

Gross alpha-activity estimation in simulated high level waste after actinides partitioning by solid state nuclear track detection

The estimation of low level alpha activity is difficult in waste samples containing large concentration of salts and beta–gamma activity. In the present study, the feasibility of gross alpha-activity measurement for simulated high level waste (SHLW) in solution medium by alpha-track registration technique has been attempted. The results showed that it is possible to use this technique for gross alpha-activity estimation of ~200 Bq/mL in solution medium with a precision and accuracy of ~30%. The importance of measuring 200 Bq/mL alpha activity in SHLW solutions is that this value corresponds to about 4,000 Bq/g activity in the solid medium which is the safe disposable limit. The advantage of this method over other methods is that it is not sensitive to beta–gamma emitters and salts and is very simple and inexpensive.

Comparative evaluation of two substituted diglycolamide extractants for ‘actinide partitioning’

Extraction behaviour of actinides, lanthanides, fission products and structural elements has been studied with the two diglycolamide extractants, namely N,N,N 0 ,N 0 - tetra-2-ethylhexyl diglycolamide (T2EHDGA) and N,N,N 0 ,N 0 - tetraoctyl diglycolamide (TODGA). The acid extraction studies suggested that T2EHDGA (KH: 1.8) is less basic as compared to its linear homologue, TODGA (KH: 4.1). The distribution ratio of Am(III) by 0.1 M diglycolamides fol- lowed the order: TODGA ( T2EHDGA. The number of ligand molecules present in the stoichiometry of the extracted species of Am(III) was found to be three and four for T2EHDGA and TODGA, respectively. Thermodynam- ics studies suggested that the extraction of Am(III) by both the extractants is exothermic in nature. The radiolytic sta- bility of TODGA and T2EHDGA solutions in n-dodecane has been investigated. Due to lower distribution ratio of Am by T2EHDGA, 0.2 M of its solution has been used as compared to 0.1 M solution of TODGA. The distribution behaviour of various metal ions, viz. Am, Nd, Fe, Mo, Cr, Sr and Cs has been studied from nitric acid as well as from simulated high level waste solution.

Use of Calix[4]-bis-2,3-naphthocrown-6 for Separation of Cesium from Pressurized Heavy Water Reactor Simulated High Level Waste Solutions (PHWR-SHLW)

Distribution studies on Cs(I) were carried out from pressurized heavy water reactor (PHWR) simulated high level waste (SHLW) solution using calix[4]-bis-2,3-naphtho-crown-6 as the ligand. A mixture of 1:1 nitrobenzene and toluene was evaluated as a suitable diluent. The distribution ratio of Cs(I) increased with the aqueous feed acidity upto 3 M HNO3 and decreased thereafter due to extraction of hydronium ions. The maximum DCs value at ∼3 M HNO3 suggested the possible application of the system for the recovery of radio-cesium from high level waste solution. The addition of 0.4% (v/v) Alamine 336 (a tertiary amine) facilitated the quantitative stripping of Cs(I) with distilled water. Quantitative extraction of Cs(I) from SHLW containing 0.32 g/L of Cs was observed in five contacts at O/A = 1/2 with 2.5 × 10−3 M calix[4]-bis-2,3-naphtho-crown-6. Similarly, quantitative stripping of Cs(I) from the loaded organic phase was achieved in two contacts with distilled water at a volume ratio (O/A) of 2. Selectivity studies carried out using several radiotracers such as 143Ce, 140La, 140Ba, 137Cs, 103Ru, 99Mo, 99mTc, 97Zr, 91Sr, etc. indicated excellent selectivity for Cs. The reagent exhibited excellent chemical stability up to a period of six months.

Performance of some extractants used for ‘actinide partitioning’ in a comparative hollow fibre supported liquid membrane transport study using simulated high level nuclear waste

The performance of different extractants used for ‘actinide partitioning’, viz. TRUEX, DIAMEX and TODGA is compared in hollow fibre supported liquid membrane (HFSLM) technique for the separation of trivalent lanthanides/actinides from simulated high level waste solutions. In the case of TODGA solvent (0.1 M TODGA + 0.5 M DHOA), quantitative transport of Nd(III) and Am(III) was achieved in 30 min from 500 mL of feed solution containing 0.6 g/L Nd at 3 M HNO 3 as well as from pressurized heavy water reactor-simulated high level waste (PHWR-SHLW). In the case of TRUEX solvent (comprising of 0.2 M CMPO as the extractant + 1.2 M TBP as the modifier in normal paraffin hydrocarbon), only 50% transport was observed which was attributed to the significant amount of acid transport, which in turn affected the dissociation of metal–ligand complex at the membrane–aqueous strip interface. However, quantitative transport of metal ions was even possible for TRUEX solvent after neutralization of feed acidity. On the other hand, DIAMEX solvent (1 M DMDBTDMA) resulted in about 85% transport under identical conditions which was explained on the basis of relatively lower D Am value under HLW conditions.

N,N,N′,N′‐Tetraoctyl Diglycolamide (TODGA): A Promising Extractant for Actinide‐Partitioning from High‐Level Waste (HLW)

N,N,N′,N′‐Tetraoctyl diglycolamide (TODGA) has been evaluated as an extractant for the partitioning of minor actinides from simulated high level nuclear waste solutions. Acid uptake studies suggested that TODGA is more basic (KH: 4.1) as compared to TRUEX and DIAMEX solvents viz. CMPO (KH: 2.0) and DMDBTDMA (KH: 0.32), respectively. TODGA molecules display a tendency toward aggregation in n‐dodecane at lower acidities. Effect of diluent on the distribution behavior of Am(III) was studied employing diluents of different dielectric constants. N,N‐dialkyl amides with different alkyl groups viz. dibutyl decanamide (DBDA), di(2‐ethylhexyl) acetamide (D2EHAA), di(2‐ethylhexyl) propionamide (D2EHPRA), di(2‐ethylhexyl) isobutyramide (D2EHIBA), dihexyl octanamide (DHOA) and dihexyl decanamide (DHDA) were evaluated as phase modifiers. Distribution behavior of various metal ions viz. Am(III), Pu(IV), U(VI), Eu(III), Fe(III), Sr(II) and Cs(I) was studied from pure nitric acid solution as well as from simulated high level waste solution.

Studies on the Separation of Minor Actinides from High-Level Wastes by Extraction Chromatography Using Novel Silica-Based Extraction Resins

To develop an advanced partitioning process by extraction chromatography using a minimal organic solvent and compact equipment to separate minor actinides such as Am and Cm from nitrate acidic high-level waste (HLW) solution, several novel silica-based extraction resins have been prepared by impregnating organic extractants into the styrene-divinylbenzene copolymer, which is immobilized in porous silica particles (SiO2-P). The extractants include octyl(phenyl)-N, N-diisobutylcarbamoylmethylphosphine oxide (CMPO), di(2-ethylhexyl)-phosphoric acid (HDEHP), and bis(2,4,4-trimethylpentyl)dithiophosphinic acid (Cyanex 301). Compared to conventional polymer-matrix resins, these new types of extraction resin are characterized by rapid kinetics and significantly low pressure loss in a packed column.The results of separation experiments revealed that trivalent actinides and lanthanides can be separated from other fission products, such as Cs, Sr, and Ru in simulated HLW solution containing concentrated nitric acid by extraction chromatography using a CMPO/SiO2-P resin-packed column. Satisfactory separation between Am(III) and a macro amount of lanthanides from simulated HLW solution with pH 4 was achieved by using a newly purified Cyanex 301/SiO2-P resin. However, the Am(III) separation was very sensitive to the purity of Cyanex 301, and the improvement of its stability is an important task for practical utilization.

Separation and Recovery of Uranium, Neptunium, and Plutonium from High Level Waste Using Tributyl Phosphate: Countercurrent Studies with Simulated Waste Solution

ABSTRACT The present work deals with countercurrent extraction studies on the partitioning of uranium, neptunium, and plutonium using 30% tributyl phosphate (TBP) from simulated high level waste solution generated during reprocessing of spent uranium fuel from pressurized heavy water reactors. The oxidation states of neptunium and plutonium were adjusted either by 0.01 M potassium dichromate or 0.01 M dioxovanadium ion. Neptunium and plutonium, extracted in the TBP phase, were stripped together using a mixture containing 0.05 M ascorbic acid and 0.25 M hydrogen peroxide in 2.0 M nitric acid solution. Although dioxovanadium ion is more effective for proper adjustment of the oxidation states of plutonium and neptunium, subsequent recovery of these actinides from loaded TBP is better if potassium dichromate is used for the valency adjustment. Results of the stagewise analysis of extraction and stripping of actinides using mixer-settlers are presented.