Actinide, Lanthanide, and Fission Product Speciation and

Chapter 14

Actinide, Lanthanide, and Fission Product Speciation and Electrochemistry in Ionic Melts

Downloaded by PENNSYLVANIA STATE UNIV on August 9, 2012 | http://pubs.acs.org Publication Date: June 9, 2006 | doi: 10.1021/bk-2006-0933.ch014

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A. I. Bhatt , H. Kinoshita , A. L. Koster , I. May *, C. Sharrad , H. M. Steele , V. A. Volkovich , O. D. Fox , C. J. Jones , B. G. Lewin , J. M. Charnock , and C. Hennig 1

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Centre for Radiochemistry Research, Department of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom Department of Rare Metals, U r a l State Technical University-UPI, Ekaterinburg, Russia Nexiasolutions, B N F L , Sellafield, Seascale, Cumbria C A 2 0 1PG, United Kingdom C L R C Daresbury Laboratory, Warrington, Cheshire W A 4 4AD, United Kingdom E S R F , R O B L - C R G , B P 220, Grenoble, Cedex, France *Corresponding author: [email protected] 2

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We report the results of recent research that we have undertaken to increase our understanding of key actinide and fission product speciation in a range of ionic melts. These results will be used to develop novel electrochemical methods of separation of uranium and plutonium from irradiated nuclear fuel. Our studies in high temperature alkali metal melts (including L i C l and the eutectics L i C l - K C l and CsCl-NaCl) have focussed on in-situ speciation of U , Tc and Ru using both Electronic Absorption Spectroscopy (EAS) and X-ray Absorption Spectroscopy (XAS). The X A S studies have included Extended X-Ray Absorption Fine Structure (EXAFS) and X-Ray Absorption Near Edge Structure (XANES) measurements. We report what could be unusual uranium speciation in high temperature melts and evaluate the © 2006 American Chemical Society

In Separations for the Nuclear Fuel Cycle in the 21st Century; Lumetta, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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likelihood of Ru or Tc volatilisation during plant operation. Our studies in lower temperature melts, commonly known as ionic liquids, have focussed on salts containing tertiary alkyl group 15 cations and the bis(trifluoromethylsulphonyl)imide anion, melts which we know to have exceptionally wide electrochemical windows. We report Ln, Th, U and Np speciation ( X A S , E A S and vibrational spectroscopy) and electrochemistry in these melts.

Introduction The development of novel methods for the safe processing of irradiated nuclear fuel is a key challenge that the nuclear industry must face. Current research is focussed on advancements to P U R E X process technology, including actinide partitioning and subsequent transmutation, and the development of novel alternative fuel treatment technologies to an industrial scale. These technologies include supercritical fluids and pyrochemical processing.

Pyrochemical Processing High temperature molten salt processing of irradiated nuclear fuel is not a novel concept and there are currently two operating plants at the Research Institute for Atomic Reactors (RIAR) in Russia and at the Argonne National Laboratory (ANL) in the US. The two facilities both operate electrochemical processes for irradiated nuclear fuel treatment using molten chloride melts - a N a C l - K C l eutectic at the RIAR and a L i C l - K C l eutectic at the A N L . Advanced molten salt technologies are currently being developed by both institutes and by other nuclear research laboratories in Japan, Europe and the US. (1-9)

The Application of Low Temperature Ionic Liquids in Nuclear Waste Management Low Temperature Ionic liquids (LTILs) are relatively novel solvent systems which are currently receiving a lot of interest for a whole range of chemical processes. (10-11) These ionic melts are generally classed as salts with melting points less than 100° C and tend to comprise of inorganic anions and organic


221 cations. With regards to specific nuclear applications there have been studies into the use of water immiscable ionic liquids for solvent extraction processes, with particular reference to { U 0 } , S r and C s extraction. (12-14) The replacement of alkali metal chloride melts with lower temperature ionic liquids in an electrochemical process is also attractive, with the obvious decrease in operating temperature and the potential for greatly reduced corrosion issues. Previously studies focussed on chloroaluminate melts, (15-16) although the hygroscopic nature of such melts would probably inhibits large scale operation. More recent studies have focussed tertiary alkyl ammonium or pyrrolidinium cation and bis(trifluoromethylsulfonyl)imide anion (TFSI, [N(S0 CF ) ]") based salts which are much less moisture sensitive and appear to have electrochemical windows wide enough to undertake the electrodeposition of uranium and plutonium. (17-21) 2+

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Current BNFL/Nexiasolutions Research Programme For the past decade B N F L (British Nuclear Fuels Ltd) have undertaken a research and development programme into the application of ionic melt technologies for the electrochemical separation of U and Pu from irradiated nuclear fiiel. This work continues through the new organisation Nexiasolutions which supercedes the Research and Technology group of B N F L . Since 2000 the Centre for Radiochemistry Research (CRR) at the University of Manchester in the U K has undertaken basic chemical research as part of this programme. This paper summarises some of the key results from the past four years collaboration in which both high temperature melts and low temperature ionic liquid studies have been undertaken.

High Temperature Molten Salt Studies

Experimental A detailed account of the experimental method used to undertake high temperature molten salt spectroscopic measurements is given elsewhere. (22-25) X A S measurements were recorded at the Daresbury Laboratory in absorbance mode on station 9.3 (U L -edge and Ru K-edge) and in fluorescence mode (Tc K-edge and Re L -edge) on station 16.5. E A S with other supporting analysis was undertaken at the C R R laboratories. ni

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222 Uranium Speciation

Previous Research Probing actinide speciation in high temperature melts is extremely challenging due to both the harsh chemical environment, limiting the spectroscopic techniques available, and the radiological hazards. Radiological limitations restrict most studies to low specific activity radionuclides such as U . E A S has been our standard method for probing speciation and is an excellent method for determining oxidation state, but not so useful in determining coordination environment. (22) Recent studies into actinide speciation in ionic melts have given an indication that simple monomeric complexes may not dominate actinide speciation in all cases. A Raman spectroscopy study has indicated that ThCl :CsCl melts contain oligomeric Th species with bridging chlorides. (26) E A S has provided evidence for { N p 0 } : { U 0 } cationxation complexes in a CsCl melt (27) and a peroxide bridged { U 0 } dimer in a NaCl:CsCl melt. (28) Novel actinide complexes may also be present in alkali metal chloride melts of relevance to the pyrochemical processing of irradiated nuclear fuel. X A S spectroscopy is an element specific technique which may be applied to probing chemical speciation in a wide range of diverse chemical matrices. It has previously found a wide range of applications for the determination of metal speciation, including actinide cation speciation. (29) A recent X A S study probing the U chemical environment in in a L i C l - K C l melt at 550 °C has been undertaken. (30) In this study E X A F S data was collected to an E of 17.7 keV and information obtained on the first coordination sphere which was fitted with 7.3 chloride ligands at 2.82 A . This group found no evidence of oligomerisation in the melt system chosen with simple monomeric species appearing to dominate speciation.


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In situ XAS Measurements The furnace set-up that we have previously used to record in situ E A S measurements (22) has recently adapted to allow us to undertake in situ X A S ( X A N E S and E X A F S ) measurements. (23, 25) Currently, we are only able to obtain data in absorbance mode and our salt systems are restricted to low absorbing alkali metal halides (i.e. L i C l , L i C l - K C l and LiCl-BeCl ). Uranium could be introduced into the melt by either direct dissolution of UC1 , UC1 or U 0 C 1 or by in situ chlorination (by HC1 or C l ) of U 0 or U 0 . 2

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223 Preliminary results obtained from probing the U L i edge reveal that we can use the X A N E S spectrum to distinguish between uranium in the +III, +IV, +V and +VI oxidation states. (23) E X A F S data fitting is in good agreement for classical 6 coordinate uranium species (e.g. [ U C l ] \ [U C1 ] ", [U 0 C1 ] " and [U 0 C1 ] "). A n example E X A F S data fit is shown in Figure 1. However, there is also often evidence for longer range interactions which we believe represent U-alkali metal and U - U interactions indicating that oligomeric species may be present in these high temperature melts. n

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Fission Product Speciation in Molten Salts Most nuclear power plants around the world use U 0 based fuel and we are thus interested in the electrochemical processing of irradiated oxide fuel types. One method for the dissolution of oxide fuel in a high temperature melt is chlorination (either by HC1 or C l ) . However, this has the potential to bring additional fission products into the molten solution, including T c and R u and R u . Our specific interest in the chemistry of these two elements lies in the fact that both merit specific consideration during conventional P U R E X process operations, with [Tc0 ]" coextraction through solvent extraction operations (3132) and care required to avoid the formation of volatile R u 0 during any high temperature waste treatment process. (33) We have previously reported a detailed study into the speciation of Tc and Re in a range of high temperature melts which have shown that Re is not always a good non-radioactive analogue for Tc in these systems. (24) The reaction between Re metal, R e 0 , R e 0 and T c 0 with HC1 and C l were studied in L i C l K C l , NaCl-CsCl and N a C l - K C l melts using in situ E A S . Quenched melts were also probed by X A S . This work has shown that [ReCl ] " is the dominant species formed for Re for all experimental conditions with T c 0 alo reacting with HC1 in a range of melts to form only [TcCl ] ". However, the reaction between T c 0 and C l leads to the formation of volatile chlorides and/or oxychlorides with [Tc0 ]" remaining behind in the melt. In the design of any new molten salt fuel processing plant care should thus be taken to avoid any Tc volatility issues. 2

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We have now turned our attention to Ru, studying Ru and R u 0 dissolution by HC1 and C l in N a C l - K C l , L i C l - K C l , NaCl-CsCl and L i C l . In situ E A S is potentially very informative but we have not yet conclusively been able to assign the broad absorbance bands that are observed. However, in situ E X A F S measurement in L i C l and L i C l - K C l have yielded more information and the data fit is consistent with the formation of [Ru Cl ] " as the major species irrespective of initial Ru starting material, ionic melt or chlorination method. Typical E X A F S data is given in Figure 2. We cannot, however, rule out the possibility that the major Ru oxidation state in these melts is R u and more detailed experimental studies are currently being undertaken. 2

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Figure 1. U L edge EXAFS spectrum andfourier transform ofU0 exposed to HCl in LiCl/KCl eutectic at 700 °C (experimental - solid line; theory - dashed line). Data can be fitted with the following model: 2x0 - 1.75 A; 4xCl - 2.62 A. %U = 1.56 %, k = 3.8, k = 9.0, R = 28.5 nr

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Figure 2.Ru K-edge EXAFS spectrum andfourier transform of Ru0 chlorination with HCl in LiCl-KCl eutectic at 500°C (experimental - solid line; theory - dashed line). Data can be fitted with the following model: 6xCl - 2.39 A. %Ru = 0.60%, 2


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Low Temperature Ionic Liquid Studies Experimental


More detailed experimental methods are described elsewhere. (34) X A S measurements were undertaken on station 16.5 at the C L R C Daresbury Laboratory and at the R O B L - C R G beamline at the ESRF. Synthetic chemistry, spectroscopic characterisation, X-ray crystallography and electrochemical measurements were all undertaken at the C R R laboratories.

Electrochemical window measurements Our initial aim was focused on evaluating novel LTIL systems with wide enough electrochemical windows to undertake the electrochemical deposition of U and Pu metal. Initial studies focused on the Group 15 quaternary alkyl salts of general formula [Me X][TFSI], where X = N , P and As. Their electrochemical windows were measured both in the molten state (at 160 °C) and as supporting electrolytes in M e C N . (34) A l l had electrochemical windows wide enough to potentially undertake U and Pu metal deposition, as observed by through the observation of the Li /Li° couple. (34) We have since extended this study to ionic liquids with lower symmetry cations, including [ BuNMe ][TFSI], which is a liquid at room temperature. A l l the ionic liquids were synthesized by metathesis reactions between Group 15 based alkyl cation halides and LifTFSI]. For all the room temperature measurements the ferrocenium/ferrocene couple was used as an internal standard. (35). 4

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F-element Speciation Due to the presence of electron withdrawing C F groups it would be expected that TFSI would be a weakly coordinating anion (Figure 3). This may explain why, until recently, the only X-ray structural study of coordinated TFSI is in the Cu complex, [Cu(CO)(TFSI)], where the ligand coordinates through the central nitrogen. (36) There have, however, been more recent reports indicating that TFSI is a flexible ligand, with several possible coordination modes. (37-38) We have recently structurally characterized [La(TFSI) (H 0) ] as a benchmark trivalent f-element complex. The three TFSI ligands coordinate to the La centre through bidentate sulfonyl oxygens (Figure 4). We have also been able to prepare a range of additional solid state TFSI complexes with other f-element cations, including Eu , S m , T h , U and { U 0 } , all characterised by a range of analytical and spectroscopic techniques. 3

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Figure 3. The TFSI anion

Figure 4 The structure

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228 Studying in situ speciation of f-element cations in TFSI based LTILs have been limited to E A S and X A S measurements. We have undertaken extensive X A N E S and E X A F S measurements on a range of uranium complexes in TFSI based melts. E A S has been used to confirm oxidation state purity in solution. The results confirm that TFSI is a weak ligand, for example it unable to displace coordinated chloride anions from UC1 . Interestingly, a recent E X A F S study into { U 0 } solvent extraction into a TFSI based LTIL with nitrate and C M P O gave no evidence for the coordinated TFSI. [12] However, we have found evidence for TFSI remaining coordinated to U with [U0 (TFSI) ] appearing to remain intact when dissolved into [BuNMe ][TFSI], as might be expected. 4

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We attempted to prepare a Np -TFSI complex by reacting N p hydroxide with 4 molar equivalents of HTFSI, removing H 0 in vacuo. However, on dissolution of the resultant solid in [BuNMe ][TFSI] oxidation to { N p 0 } was indicated by E A S (39) despite our best attempts to eliminate oxygen and moisture from the system. This tends to suggest that TFSI is not effective at stabilizing the lower oxidation state. Two major bands are observed around the region for the major { N p 0 } f-f transition suggesting the presence of more than one species in solution. 2

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F-element Electrochemistry Electrochemical studies have been undertaken in several of ionic liquids with a series off-element cations, including La ", E u , Sm , T h , U , { U 0 } and { N p 0 } . In most cases reduction of L n (or Ln ) to Ln° or An to A n was observed although often plating and stripping behaviour was not observed, probably due to the presence of moisture in the solvent system leading to the immediate formation of oxide on reduction to the metallic state. In addition, electrochemical features attributed to higher oxidation state processes (e.g. U , N p ) were also observed at more positive potentials. Typical cyclic voltammagrams showing the T h couples for [ BuNMe ][TFSI] is shown in Figure 5. 1

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Conclusions Chemical research is required to support the development of novel ionic melt technologies for the electrochemical separation of U and Pu from irradiated nuclear fuel. Higher temperature melt studies have focused on in situ speciation of uranium and key fission products with X A S spectroscopy yielding information on U , Tc and Ru speciation which compliments E A S measurements. Currently we are looking into the application of N M R and Raman spectroscopy to probe f-element speciation in these melt systems further.



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Our ionic liquid research has focused on group 15 quaternary alkyl bistriflimide salts which have electrochemical windows wide enough to potentially see the electrodeposition of U and Pu. Electrochemical and solution speciation studies indicate that it may be possible to electrochemical separate out actinides using this technology. Further studies are required to develop novel ionic liquid solvents that can be used for bulk electrodeposition of U and Pu.

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Actinide, Lanthanide, and Fission Product Speciation and

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