Extraction of Some Elements by Mixture of DIDPA-TBP and Its Application to Actinoid Partitioning Process

Effect of TBP on the extraction of Pu(IV), Zr(IV), Nb(V), Nd(III) and Am (III) was studied with diisodecylphosphoric acid(DIDPA) as an extractant. Hydrolyzable elements contained in the high-level liquid waste of fuel reprocessing,i.e. Pu(IV), Zr(IV) and Nb(V), could be extracted with mixtures of DIDPA and TBP of different compositions. Addition of TBP to DIDPA causes an increase and a decrease of respectively distribution rate and ratio of Zr(IV). These elements extracted were completely stripped with the aid of oxalic acid. An effect of TBP on separation of transplutonides (III) from lanthanoids(III) in the DIDPA and DTPA extraction system was also studied.

Based on the results, a process flow sheet utilizing the extractant of DIDPA and TBP mixture was contrived for partitioning actinoids in the high-level liquid waste.     

Extraction of Lanthanoids (III) with Di-isodecyl Phosphoric Acid from Nitric Acid Solution

The extraction of Nd(III) with di-isodecylphosphoric acid was studied and compared with that by di(2-ethylhexyl) phosphoric acid. DIDPA possesses higher D_f values in a strong acid medium but poorer separation factors against lanthanoids(III) than that of DEHPA. Mono (2-ethylhexyl) phosphoric acid increased extraction of Nd(III) with DIDPA in a similar manner as in the case of DEHPA, by a factor of D_f of 10³.

The extraction of Sr(II) was also investigated with DIDPA, which showed a maximum of D_f at pH 5.0. Results showed that DIDPA is a promising extractant for the partitioning of the high level liquid waste.     

Influence of Monoisodecyl Phosphoric Acid on Extraction of Neptunium with Diisodecyl Phosphoric Acid

The influence of monoisodecy1 phosphoric acid (MIDPA) on the separation of Np with diisodecy1 phosphoric acid (DIDPA) was studied from the aspects of the extraction rate and the back-extraction with H₂C₂O₄ solution. This study is important for the development of the partitioning method for high-level waste because MIDPA is one of the radiolysis products of DIDPA The increase in the MIDPA concentration was found to accelerate the extraction of Np initially in the pentavalent oxidation state. When the MIDPA concentration was 0.1 M, the extraction rate was ten times faster in the absence of H₂O₂ and twice faster in the presence of 0.5 M H₂O₂. It was also found that the extraction mechanism in the aspect of Np redox reactions did not change even in the presence of MIDPA

On the other hand, Np became less back-extracted with increasing MIDPA concentration. However, the tolerable concentration of MIDPA, determined by the required distribution ratio for the Np back-extraction, is higher than the estimated concentration of MIDPA produced radioiytically in the partitioning process.     

萃取分离法处理高放废液的进展

评述了近几年用萃取分离法从高放废液中去除超铀锕系元素的进展情况，着重介绍世界上已有的应用前景较好的ＴＲＵＥＸ流程（美）、ＤＩＡＭＥＸ流程（法）、ＤＩＤＰＡ流程（日）、ＣＴＨ流程（瑞典）和ＴＲＰＯ流程（中国）。     

Radiation Damage of Organic Extractant in Partitioning of High-Level Liquid Waste, (II)

Radiation dose absorbed in the organic extractant was estimated for the counter- current extraction with mixer-settler in the partitioning of the high-level liquid waste of fuel reprocessing. The radiation dose due to the absorption of α and β-particles was calculated to be 10, 5.0 and 0.42 Wh/l for 150 days, 1 year and 10 years after fuel discharge from the reactor, respectively. About 70–90% of the total dose was due to the absorption of β-particles. These values were discussed in comparison with the published data for the fuel reprocessing. On the basis of the calculated data, the radiolytic effects upon the extraction of strontium, lanthanoids and transplutonium elements in partitioning were estimated be sufficiently small.     

Separation of Precipitate-forming Elements during High Level Waste Treatment Using Acidic Zirconium Salt of Dibutyl Phosphoric Acid

Iron extraction and the limits of iron dibutyl phosphate precipitation were investigated for use in TPE/RE recovery and partitioning with the use of the acidic Zr salt of dibutyl phosphoric acid (ZS HDBP, Zr:HDBP = 1:9), dissolved in 30% TBP with Isopar-L. The presence of Mo and the solvent loading with RE affected the Fe extraction in the opposite ways. Slow kinetics and process irreversibility were found for the Fe extraction with the ZS HDBP solution. To increase acceptable Fe concentration in a HLW, reduction of Fe(III) by ascorbic acid (AA) was studied to be carried out continuously in the feed flow, just before entering the head contactor of the partitioning extraction cycle. The Mo extraction is of the same order as that of TPE and RE; so its selective stripping prior to the TPE/RE separation is required. This can be done using the DTPA or H₂O₂ solution in diluted nitric acid. The effectiveness of the process was verified by laboratory scale trials on the centrifugal contactor rig using simulated HLW. The Zr-to-HDBP ratio of 1:6 was found to be useful to decrease the Fe and Mo extractability for their better backwashing with complexants.     

Enhancement of the Mutual Separation of Lanthanide Elements in the Solvent Extraction Based on the CMPO-TBP Mixed Solvent by Using a DTPA-Nitrate Solution

The mutual separation behavior of rare earth elements was studied in the system based on the neutral extractant CMPO (n-octyl(phenyl)-N,N-diisobutylcarbamoyl methylphosphine oxide) and aminopolyacetic acid DTPA (diethylenetriamine pentaacetic acid). The extractability of lanthanides decreased monotonously with increasing of atomic number in the system of 0.2 M CMPO-1.0 M TBP-n-dodecane (TRUEX solvent) and 0.05 M DTPA-NaNCO₃ or hydroxylamine nitrate (HAN) solution. The separation factor of La/Lu was 6,600 using 0.05 M DTPA-3M NaNO₃ solution (initial pH=2.0). The mutual separation of lanthanides in the CMPO/DTPA system was greatly improved when compared with the general TRUEX system. The separation factor was not affected by the initial pH of the solution (1.8–2.2), the salting out reagent (NaNO₃ and HAN), or its concentration (2, 3 M). The experimentally obtained separation factors of lanthanide elements were reproduced by a simple model based on the distribution ratios in the TRUEX extraction system and the stability constants of DTPA-metal complex. The mutual separation between light lanthanides is attributed to the selectivity of DTPA, while CMPO mainly influences the separation of heavy lanthanides.     

Solvent Extraction of Americium(VI) by Tri-n-Butyl Phosphate

Solvent extraction of Am(VI) by tri-n-butyl phosphate (TBP) from nitric acid solutions was investigated to develop a novel method for partitioning americium from high level liquid waste generated through spent nuclear fuel reprocessing. Am(VI) was prepared using ammonium peroxodisulfate and silver nitrate. The distribution coefficients of Am(VI) were determined for extraction systems of various concentrations of nitric acid and TBP. Sufficiently stable Am(VI) could be extracted and the extraction reaction of Am(VI) was found to be the same as for other hexavalent actinides. The apparent equilibrium constant varied with the concentration of peroxodisulfate used for the valence control, which was ascribed to the competitive reaction of the extraction of Am(VI) and the complex formation of Am(VI) with sulfate ion produced by the decomposition of peroxodisulfate. A distribution coefficient of Am(VI) above 1 was obtained with undiluted TBP and the separation factor between Am(VI) and Nd(III) was 87±9. TBP extraction of Am(VI), after implementing valence control, was proved to be an effective method for the partitioning of americium from fission products such as rare earth elements.     

Separation of Americium(III) and Europium(III) by thermal-swing extraction using thermosensitive polymer gel

Extraction separation of Am(III) and Eu(III) was examined by the thermal-swing extraction technique using a thermosensitive gel, poly-N-isopropylacrylamide (NIPA) crosslinked with a TPEN derivative, N,N,N′,N′-tetrakis(4-propenyl-oxy-2-pyridylmethyl)ethylenediamine (TPPEN). The separation of Am(III) from Eu(III) was observed in the swelling state of gel (5 °C) and the separation factor of Am(III) was evaluated as about 18 at pH 5.2. The distribution ratio of Am(III) was reduced to about 1/8 by the volume phase transition from the swelling state (5 °C) to the shrinking one (40 °C). These results suggest that Am(III) can be separated and recovered effectively from Eu(III) in nitrate aqueous solution by the thermal-swing extraction technique using TPPEN–NIPA gel. This technique is applicable to the MA partitioning process, which is one of the important chemical processes in the P&T technology.     

Continuous extraction and separation of Am(III) and Cm(III) using a highly practical diamide amine extractant

A highly practical diamide-type extractant, which is an alkyl diamide amine with 2-ethylhexyl alkyl chains (ADAAM(EH)), was investigated for the mutual separation of Am(III) and Cm(III). ADAAM(EH) is a multidentate ligand with one soft N-donor atom and two hard O-donor atoms as part of its central frame. This tridentate arrangement of donor atoms provides selective binding to Am(III) compared to that with Cm(III) in highly acidic media (1.5 M HNO₃), resulting in separation factors of up to 5.5. A continuous liquid–liquid extraction and stripping test was conducted using a multistage countercurrent mixer-settler extractor with ADAAM(EH) in n-dodecane. In this test, the separation of Am(III) and Cm(III) was achieved with very high yield.     

Extraction of Pentavalent Neptunium with Di-Isodecyl Phosphoric Acid

As a fundamental study for separating neptunium from high-level radioactive waste, the mechanism and the rate of extraction of pentavalent neptunium with DIDPA were investigated. From the analysis of oxidation state of neptunium in organic phase, it was proved that disproportionation reaction of Np(V) was concerned in the extraction.

The reaction order of the extraction with respect to neptunium concentration was larger than unity. The extraction rate was much reduced by the decrease of DIDPA concentration. The dependence of the rate on nitric acid concentration and on the temperature was also examined.     

Measurement of CO2 Laser Pulse Narrowing in CF2HCl

The oxidation of Np(V) by nitrous acid was investigated spectroscopically in 0.2 M CMPO—1.4 M TBP—n-dodecane and the oxidation rate constant of Np(V) was estimated assuming the first-order reaction. By using the obtained oxidation rate constant as well as taking account of the Np(V) oxidation in aqueous phase and the distribution ratios of Np(V) and Np(VI) cited from literature, extraction kinetics of Np from 4 M nitric acid solution was calculated, and was compared with the extraction kinetics measured experimentally.     

Magnetic Properties of Uranium Carbides after Neutron Irradiation

Fundamental investigations on valence control and solvent extraction of americium were carried out to develop a method for americium separation from reprocessing solution. In order to adjust americium valency from III to IV and VI, (NH₄)₁₀P₂W ₁₇O₆₁ synthesized was used as complexant stabilizing Am(IV). Oxidation behavior of americium was investigated as a function of (NH₄)₁₀P₂W ₁₇O₆₁ americium ratio. Using 0.1M (NH₄)₂S₂O₈ and 0.01M AgNO₃ as oxidation reagent, Am(IV) was obtained quantitatively at the ratio of 15. On decreasing the ratio to 0.6, 92% of americium was adjusted to Am(VI). The concentration of (NH₄)₂S₂O₈ could be reduced to 1/15 compared to the previously reported method in which no complexant was used. Americium(IV) was also prepared by reacting O₃ and AgNO₃ but no Am(VI) was obtained even at low (NH₄)₁₀P₂W ₁₇O₆₁ to americium ratio.

Americium(VI) could be extracted by tri-n-butyl phosphate stably without influence of (NH₄)₁₀P₂W ₁₇O₆₁. The distribution coefficient of Am(VI) was 4 between 100% tri-n-butyl phosphate and 1 M nitric acid, and separation factor from Nd(III) was 50.

With regard to the americium separation method which implemented valence control followed by extraction, adding (NH₄)₁₀P₂W ₁₇O₆₁ led to minimization of waste volume and improvement of extraction efficiency.     

Extraction of cerium(III) with N,N′-dimethyl-N,N′-dioctyl-diglycolamide in [Bmim][PF6] compared in 40% octanol/kerosene

The extraction of Ce(III) nitrate has been investigated by using N, N′-dimethyl-N, N′-dioctyl-diglycolamide (DMDODGA = L) as extractant with 1-Butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF₆]) as diluent in comparison with 40% octanol/kerosene. DMDODGA in two different diluents shows good extraction ability for Ce(III), but the trends are quite different depending on the concentrations of the ligand in solvent phase and nitric acid in aqueous phase. The slope analysis method is used to obtain the stoichiometric ratio of the extracted complexes. 1:3 complex of Ce(III) to DMDODGA may be formed in both systems, while 1:2 complex may be also formed in DMDODGA-40% octanol/kerosene system. The extraction of Ce(III) is by the cation exchange mechanism in DMDODGA-[Bmim][PF₆] system. Unlike the expected positively charged 1:3 complex of Ce(III) formed in the DMDODGA-40% octanol/kerosene system, the 1:2 complex is assumed to be a neutral complex with two DMDODGA molecules bonding to Ce(III). In addition, the stronger interaction between the C = O groups of L and Ce(III) in the DMDODGA-[Bmim][PF₆] system than in the DMDODGA-40% octanol/kerosene system is confirmed by IR spectroscopy.     

Supposed existence of Np4+ in a genuine dissolver solution from the results of extraction simulation by PARC-L code

With a genuine spent fuel solution (a dissolver solution), a laboratory-scale reprocessing experiment of an extraction–separation process was performed using mixer-settlers as extractors. In the experiment, n-butyraldehyde was utilized as a reducing reagent of Np^(VI)O₂²⁺ to Np^(V)O₂⁺ for the purpose to distinguish Np^(VI)O₂²⁺ from Np⁴⁺. From the Np concentration in the aqueous phase, Np would be extracted from the dissolver solution together with U and Pu. The scrutiny of Np behavior was performed utilizing 66 cases of calculation results by a Japan Atomic Energy Agency open extraction simulation code, the Program for Advanced Extraction with Radiation Effect Calculation–Lightened version. From the scrutiny, the authors found that the calculation result with 60% of Np⁴⁺ in the dissolver solution represented the best experimental extraction–separation behavior of Np. Therefore, it was supposed that the dissolver solution contained sufficient proportion of Np⁴⁺ to affect the extraction–separation behavior of Np.     

Strontium Isotope Effect in Liquid-Liquid Extraction of Strontium Chloride Using a Crown Ether

Isotope effects in a liquid-liquid extraction of strontium with dicyclohexano-18-crown-6 (DC18C6) were investigated. Unit mass enrichment factors were observed to increase with concentrations of strontium salt in an aqueous phase. Isotope distinguishing ability of DC18C6 to strontium isotopes was calculated as an intrinsic separation factor to be K _c =1.00051±0.00004. An odd mass number isotope, ⁸⁷Sr, was recognized to behave differently from even mass number isotopes, ⁸⁴Sr, ⁸⁶Sr and ⁸⁸Sr. The enrichment factor induced by a nuclear property (odd or even mass number) other than the mass difference was ε_O/E = ?8.0 × 10^?4 which was observed with 3.2M Sr aqueous solution. Isotope shift of energy state for 5S-orbital of strontium produces the isotope shift in vibrational energy between the strontium ion and the DC18C6.     

Extraction of Am(III) at the Interface of Organic-Aqueous Two-Layer Flow in a Microchannel

Extraction of Am(III) was performed at the interface of organic-aqueous two-layer flow in a micro-channel having an asymmetric cross section. A solution of 3 mol/dm³ nitric acid containing ²⁴³Am(III) and octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphineoxide diluted with n-dodecane were introduced into the microchannel as the aqueous phase and organic phase, respectively. The two phases formed a stable two-layer flow with an interface parallel to the sidewall of the microchannel, and they were separated from each other at the divergence point of the microchannel. The extraction reaction of Am(III) proceeded at the interface of the two phases, and reached the equilibrium state while the two phases passed through the microchannel.     

Development of a simplified separation process of trivalent minor actinides from fission products using novel R-BTP/SiO2-P adsorbents

From a viewpoint of direct separation of trivalent minor actinides (MA: Am, Cm etc.) from fission products (FP) including rare earths (RE) in high level radioactive liquid waste, the authors have developed a simplified separation process using a single column packed with novel extraction adsorbents. Attention was paid to a new type of nitrogen-donor ligand, R-BTP (2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridine, R: alkyl group) as an extractant because it has higher extraction selectivity for Am(III) than RE(III). Since the R-BTP ligands show different properties such as adsorbability and stability when they have different alkyl groups, several R-BTP extraction adsorbents were prepared by impregnating the R-BTP ligands with different alkyl groups (isohexyl-, isoheptyl- and cyheptyl-BTP) into a porous silica/polymer composite support (SiO₂-P particles). This work investigated: (1) fundamental properties of the synthesized R-BTP/SiO₂-P adsorbents, (2) adsorption and desorption properties of Am and FP in nitric acid solution and water using the adsorbents in a batch experiment, (3) radiolytic and chemical stabilities of the adsorbents, and (4) the possibility for developing a simplified separation process of MA using the most promising adsorbent (isohexyl-BTP/SiO₂-P) under temperature control between 25 and 50°C.     

Solvent Extraction of Pentavalent Neptunium with n-Octyl(phenyl)-N,N-Diisobutyl- carbamoylmethylphosphine Oxide

Solvent extraction of Np(V) from HNO₃ solution was experimentally studied with n-octyl(phenyl)-N, N-diisobutylcarbamoylmethylphosphine oxide (CMPO) as an extractant mixed with TBP and n-dodecane. In the low HNO₃ concentration, the Np(V) is weakly extractable, but effectively extracted in the high HNO₃ concentration (>4M) due to the disproportionation of Np(V) to Np(IV)and Np(VI). The distribution ratio of Np was increased by use of H₂O₂as a reductant of Np(V). More than 95% of Np was extracted with 0.8 M H₂O² in 2.2 M HNO₃ solution. It was shown that most of the Np extracted is in tetravalent state as expected from redox mechanism. The Np(IV) extracted in the organic phase was effectively stripped to the aqueous phase with H₂C₂O₄.     

Separation of Uranium from Fission Products

A simple procedure has been developed for separating U from F.P. and neutron-irradiated Th. The separation is performed with tri-n-butyl phosphate in a system of dodecane-mixture of sulfuric acid and aluminium nitrate.

Uranium dioxide was irradiated with 20 MeV bremsstrahlung, which produced both ²³⁷U and F.P. The target was dissolved in dilute nitric acid and U was extracted into the organic phase of the above mentioned system. Finally, U in the organic phase was back-extracted into an aqueous phase. The γ-ray spectrum and decay curve of the separated U fraction show no radioactive nuclides other than U isotopes and its decay products.

This method can also be applied to the preliminary separation of ²³⁸U from neutron- irradiated Th.

The distribution ratios (Kd) for U, Th and some other elements in the extraction system are also given.