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Demonstration of the LUCA Process for the Separation Of Radiochim. Acta 98, 193–201 (2010) / DOI 10.1524/ract.2010.1708 © by Oldenbourg Wissenschaftsverlag, München Demonstration of the LUCA process for the separation of americium(III) from curium(III), californium(III), and lanthanides(III) in acidic solution using a synergistic mixture of bis(chlorophenyl)dithiophosphinic acid and tris(2-ethylhexyl)phosphate By G. Modolo1,∗,P.Kluxen1 and A. Geist2 1 Forschungszentrum Jülich, Institute for Energy Research, Safety Research and Reactor Technology, 52425 Jülich, Germany 2 Forschungszentrum Karlsruhe GmbH, Institut für Nukleare Entsorgung, Postfach 3640, 76021 Karlsruhe, Germany (Received December 11, 2008; accepted in revised form November 21, 2009) Minor actinides / Lanthanides / Americium / Curium / from spent fuel using the well-established PUREX process, Partitioning / Solvent extraction / Centrifugal contactors / which can also be adapted for partitioning Np [2]. There is LUCA process no potential for recovering Am or Cm using the PUREX process because the tri-n-butylphosphate (TBP) extractant shows a very low affinity for trivalent actinides. During the Summary. The LUCA process was developed at last decade a large amount of research has been conducted in Forschungszentrum Jülich for the selective separation of several countries on the separation and recovery of Am and Am(III) from an acidic solution containing the trivalent ac- Cm from the high-level liquid waste (HLLW) fraction of the tinides Am(III), Cm(III), and Cf(III) as well as lanthanides. PUREX process. A comprehensive survey on actinide sep- A mixture of 0.4mol/L bis(chlorophenyl)dithiophosphinic aration science and technology is given by Nash et al. [3]. acid and 0.15 mol/L tris(2-ethylhexyl)phosphate dissolved in 20% isooctane/80% tert-butyl benzene was used as the ex- Most partitioning strategies rely on the following separation tractant. The process was carried out in centrifugal contactors processes. using an optimized flowsheet involving 7 stages for extraction, 1. Separation of uranium and/or plutonium from spent fuel 9 stages for scrubbing and 8 stages for back-extraction. Very dissolution liquors (e.g. PUREX, UREX). encouraging results were obtained. A high feed decontamina- tion factor was obtained for Am(III) (> 1000), and recovery 2. Co-extraction of the trivalent actinides and lanthanides in the product after stripping was higher than 99.8%. The (e.g. TRUEX, DIAMEX, TRPO, TODGA). Am(III) product was contaminated with 0.47% Cm(III). More 3. Separation of trivalent actinides from lanthanides (e.g. than 99.9% Cf(III), Eu(III) and > 99.5% Cm(III) inventories TALSPEAK, SANEX). were directed to the raffinate and the contamination with The last step is important because it is essential to sep- Am(III) (< 0.08%) was low. The experimental results were in good agreement with the predictions of a computer code. arate americium and curium from trivalent lanthanides to avoid the strong absorption of thermalized neutrons by the lanthanides, particularly if the trivalent actinides are to be re- 1. Introduction cycled as fuel (or targets for transmutation) in a current gen- eration reactor. Due to similarities in the chemical properties Plutonium and the minor actinides (MA) Np, Am and Cm and behaviour of trivalent actinides and lanthanides extrac- are mainly responsible for the long-term radiotoxicity of the tants or complexing agents containing soft donor atoms such waste generated from nuclear power production. If these ra- as N, S, Cl, etc. are required for reliable group separa- dionuclides are removed from the waste (partitioning) and tions [4–8]. After the An(III)/Ln(III) separation process, the converted by neutron fission (transmutation) into shorter- product fraction contains approx. 0.35 to 0.45 g/LAmand lived or stable elements, the remaining waste loses most Cm (e.g. after a BTBP process [9]). of its long-term radiotoxicity. Thus, partitioning and trans- In principal, both elements could be transmuted together mutation are considered attractive options for reducing the in a fast reactor or ADS system. However, because of the burden on geological disposals. This is important both in the high heat decay and neutron emission of curium, any dry case of a nuclear phase-out, as well in the case of the con- or wet fabrication process will require remote handling and tinuous use of nuclear power as part of a sustainable energy continuous cooling in hot cells behind thick concrete shield- supply [1]. Today, Pu and uranium are industrially separated ing. The development of a simple, compact and robust fab- rication process appears to be a great challenge [10]. One *Author for correspondence (E-mail: g.modolo@fz-juelich.de). option involves the interim storage of curium for about Bereitgestellt von | Forschungszentrum Jülich Angemeldet Heruntergeladen am | 24.04.18 15:28 194 G. Modolo, P. Kluxen and A. Geist 100 years, after which the relatively short-lived curium iso- aration factor of 1.6 is low, this process requires a large topes (242Cm, 243Cm, and 244Cm) decay to form plutonium number of stages. Nevertheless, a flowsheet comprising 24 isotopes which can then be easily separated from americium. extraction, 24 scrubbing and 8 stripping stages was success- Therefore, an effective method for separating Am from Cm fully tested in 2002 using surrogate solutions without sig- prior to re-fabrication is a major prerequisite for the discus- nificant difficulties. The performance of this test was good, sion of further fuel cycle scenarios [1]. as was predicted by calculations: more than 99.9% of each The separation of adjacent trivalent actinides represents actinide was recovered. 0.6% Am was found within the Cm an even more challenging task than the An(III)/Ln(III) sep- product solution, 0.7% Cm within Am product solution, and aration. It is known that the separation of americium from only 0.02% Am and 0.01% Cm remained in the stripped curium is a very difficult operation, due to the very similar solvent. properties of these elements [3]. The development of solid Recently, Myasoedov et al. [22] reported on Am(III)/ ion-exchange materials, which are capable of capturing and Cm(III) separation by counter-current chromatography reversibly releasing the metal ions back into the contacting (CCC) using the DIAMEX solvent. The application of CCC, solution, represents a big step forward in separating elem- a multistage extraction technique, makes it possible to sepa- ents with similar properties. These separations depend more rate the elements within 100 min: the Cm fraction contains on differences in the complexing power of the eluants to- 99.5% Cm(III) and 0.6% Am(III) inventories and the Am wards the metal ions than on the selectivity of the resin. Ac- fraction contains 99.4% Am(III) and 0.5% Cm. However, cording to this method, Am/Cm separation is carried out by CCC can only be applied to analytical and radiochemical selective elution of Am(III) and Cm(III), and the quality of separations on a laboratory scale, whereas solvent extrac- separation is a function of the nature of the eluant. Promis- tion processes are predominantly proposed for use on an ing results for transplutonium separations were obtained on industrial scale. a DOWEX 50 cation exchanger using α-hydroxy-isobutyric The synergistic mixture (Fig. 1) composed of bis(chloro- acid as an eluant [11]. The α-hydroxy-isobutyric acid pro- phenyl)dithiophosphinic acid [(ClPh)2PSSH] and tris(2- vides average separation factors for adjacent lanthanides or ethylhexyl)phosphate (TEHP) showed a very high affinity trivalent actinides of about 1.3–1.5. for actinides(III) over lanthanides(III). Am(III)/Eu(III) sep- Numerous other techniques, including high-pressure ion aration factors were over 2000. Surprisingly high Am(III)/ exchange, extraction chromatography, and solvent extrac- Cm(III) separation factors of 6–10 were also reported by tion using e.g. di(2-ethylhexyl)phosphoric acid (HDEHP) Modolo et al. [23]. Using 1H-NMR, the aggregation of have also been used for Am(III)/Cm(III) separation and (ClPh)2PSSH has been determined, and its effect on the ex- purification [12–15]. However, the Am/Cm separation fac- traction has been considered. Treatment of distribution data tors were low and do not exceed 3, necessitating a large by slope analysis suggests that the extracted An(III) and number of stages in order to obtain a pure product. The Ln(III) complexes have the composition of ML3(Syn)xorg, TALSPEAK process can be adapted for liquid membrane where HL = (ClPh)2PSSH and Syn = neutral synergist separations [16]. The authors report on the use of a “sup- TEHP. Furthermore, the thermodynamic parameters ∆H 0, ported liquid membrane” impregnated with HDEHP for the ∆S0,and∆G0 of the extraction have been determined in the separation of Am(III) and Cm(III) using DTPA, citric acid, temperature range between 10 and 45 ◦C. and the potassium salt of a heteropolyacid, potassium phos- Based on the extraordinary extraction properties of the photungstate (K10P2W17O61), as aqueous complexants. The above synergistic mixture, the LUCA process [24]wasin- optimum separation factors reported are SFAm(III)/Cm(III) ≈ 5.0. vented. LUCA is the acronym for Lanthaniden Und Curium The best separation of transplutonium elements has been Americum Trennung. The present paper relates to the devel- obtained using methods based on the various oxidation opment and demonstration of a continuous LUCA process states of the separated elements. Contrary to Cm, Am can for the selective recovery of Am(III) from an aqueous ni- be oxidized in aqueous solutions to oxidation states higher tric acid solution (≈ 0.1mol/LHNO3) containing trivalent than III, i.e. IV, V and VI. Nevertheless, these Am oxida- actinides (i.e. Am(III), Cm(III) and Cf(III)) and trivalent lan- tion states are thermodynamically unstable in acidic aqueous thanides.
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