This article was downloaded by: [McGill University Library] On: 17 September 2012, At: 04:28 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Solvent Extraction and Ion Exchange Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lsei20 A TBP/BTBP-based GANEX Separation Process. Part 1: Feasibility Emma Aneheim a b , Christian Ekberg a b , Anna Fermvik a b , Mark R. St. J. Foreman a b , Teodora Retegan a & Gunnar Skarnemark a a Nuclear Chemistry, Department of Chemical and Biological Engineering, Chalmers University of Technology, Gothenburg, Sweden b Industrial Materials Recycling, Department of Chemical and Biological Engineering, Chalmers University of Technology, Gothenburg, Sweden Version of record first published: 04 Jun 2010. To cite this article: Emma Aneheim, Christian Ekberg, Anna Fermvik, Mark R. St. J. Foreman, Teodora Retegan & Gunnar Skarnemark (2010): A TBP/BTBP-based GANEX Separation Process. Part 1: Feasibility, Solvent Extraction and Ion Exchange, 28:4, 437-458 To link to this article: http://dx.doi.org/10.1080/07366299.2010.480930 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms- and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan,
Transcript
This article was downloaded by: [McGill University Library]On: 17 September 2012, At: 04:28Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,UK
Solvent Extraction and IonExchangePublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/lsei20
A TBP/BTBP-based GANEXSeparation Process. Part 1:FeasibilityEmma Aneheim a b , Christian Ekberg a b , AnnaFermvik a b , Mark R. St. J. Foreman a b , TeodoraRetegan a & Gunnar Skarnemark aa Nuclear Chemistry, Department of Chemical andBiological Engineering, Chalmers University ofTechnology, Gothenburg, Swedenb Industrial Materials Recycling, Department ofChemical and Biological Engineering, ChalmersUniversity of Technology, Gothenburg, Sweden
Version of record first published: 04 Jun 2010.
To cite this article: Emma Aneheim, Christian Ekberg, Anna Fermvik, Mark R. St. J.Foreman, Teodora Retegan & Gunnar Skarnemark (2010): A TBP/BTBP-based GANEXSeparation Process. Part 1: Feasibility, Solvent Extraction and Ion Exchange, 28:4,437-458
To link to this article: http://dx.doi.org/10.1080/07366299.2010.480930
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes.Any substantial or systematic reproduction, redistribution, reselling, loan,
sub-licensing, systematic supply, or distribution in any form to anyone isexpressly forbidden.
The publisher does not give any warranty express or implied or make anyrepresentation that the contents will be complete or accurate or up todate. The accuracy of any instructions, formulae, and drug doses should beindependently verified with primary sources. The publisher shall not be liablefor any loss, actions, claims, proceedings, demand, or costs or damageswhatsoever or howsoever caused arising directly or indirectly in connectionwith or arising out of the use of this material.
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A TBP/BTBP-based GANEX Separation Process.Part 1: Feasibility
Emma Aneheim1,2, Christian Ekberg1,2, Anna Fermvik1,2,Mark R. St. J. Foreman1,2, Teodora Retegan1, and Gunnar Skarnemark1
1Nuclear Chemistry, Department of Chemical and Biological Engineering, ChalmersUniversity of Technology, Gothenburg, Sweden
2Industrial Materials Recycling, Department of Chemical and Biological Engineering,Chalmers University of Technology, Gothenburg, Sweden
Abstract: A GANEX (Group ActiNide EXtraction) separation system for transmuta-tion has been developed. In this separation process the actinides should be extracted asa group from the lanthanides and the fission and corrosion/activation products. Thiscan be achieved by combining BTBP (bis-triazine-bipyridine) with TBP (tri-butylphosphate) in cyclohexanone. From 4M nitric acid this organic system extracts theactinides (log(DAm) ¼ 2.19, log(DPu) ¼ 2.31, log(DU) ¼ 1.03, log(DNp) ¼ 0.53) andalso separates them from the lanthanides (log(DLa) ¼ -2.0, log(DCe) ¼ -1.72, log(DNd)¼ -1.05, log(DSm) ¼ -0.18, log(DEu) ¼ -0.02). One problem encountered is that someof the fission and corrosion products are also extracted. The new system however stilllooks feasible.
The waste from nuclear power plants all over the world has to be isolated fromman and his environment due to its high radiotoxicity. After being stored forover 100,000 years the radiotoxicity of the waste equals natural uranium.[1] Ithas been suggested that if the long-lived actinides could be separated from therest of the spent fuel, the storage time could be shortened to about 1000years.[2, 3] This shortening of the storage time, however, requires selective
Address correspondence to Emma Aneheim, Department of Chemical and BiologicalEngineering, Chalmers University of Technology, Kärnkemi, Kemivägen 4, Gothenburg41296, Sweden. E-mail: [email protected]
separation of parts of the waste. By separating the elements into fractions they canbe treated in different ways and hence both the amount and radiotoxicity of thewaste can be lowered. For example, some of the long-lived actinides can beirradiated with neutrons, and through nuclear reactions other nuclides that aremore short-lived or even stable can be formed; so-called transmutation.[4] It isespecially important to separate the long-lived actinides intended for transmuta-tion from the lanthanides. This is due to the fact that some of the lanthanides have ahigh neutron capture cross section resulting in serious scavenging of the neutronsavailable. Therefore the presence of lanthanides in transmutation fuelwould causean unwanted non-uniform heat distribution in the fuel during the irradiation.[5]
There are several different approaches towards a liquid-liquid extractionseparation method for transmutation. Earlier, the focus has primarily been on aseparation that will follow after a conventional PUREX process. One of theseconcepts is to separate both the actinides and lanthanides from the PUREXraffinate and then in a second step separate the two from each other. Muchresearch has been performed on this concept but the two processes that havebeen given the greatest interest are the TALSPEAK process[6] and theDIAMEX/SANEX process.[1,7] Today, however, the focus, at least inEurope, has been shifted towards a process that replaces PUREX by simulta-neously removing all the actinides from the dissolved spent fuel. This conceptis called the GANEX (Group ActiNide EXtraction) process.[8] This renders asingle process for separation of the actinides from both the lanthanides as wellas the rest of the fission and corrosion/activation products. This GANEXseparation can be done either directly from the dissolved spent fuel or fromdissolved fuel where the bulk part of the uranium already has been removed.This work focuses on the latter alternative.
Investigations of a ternary system containing cyclohexanone as diluentand the two extractants BTBP and TBP for the extraction of actinides havebeen performed. The BTBP molecules (Fig. 1) have a well documented abilityto separate trivalent and pentavalent actinides from trivalent lanthanides.[9,10]
TBP on the other hand (Fig. 2), has been known since the 1950s to extracturanium and plutonium.[11] By combining these two molecules the intentionwas to be able to extract not only the tetravalent and hexavalent actinides but
N N
N
N
N
R
R
N
N
N
R
R
Figure 1. BTBP (bis-triazin-bipyridine).
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also the trivalent and pentavalent ones from the dissolved spent fuel. At thesame time all the actinides should also be separated from the lanthanides aswell as the fission and corrosion/activation products. In this way the compli-cated process of redox control of plutonium and neptunium can be avoided.
Below follow short descriptions of the aims of the various experimentsrelated to these investigations.
Aim 1 – Reactivity
The aim of the experiments in this section was to determine if the extractingmolecules (BTBP and TBP) present in the organic system would react witheach other or not. A reaction between the two extractants is unwanted since itwould complicate the system and therefore make any future computer model-ing, which is necessary for real life process applications, much more difficult.It is already known that trimethyl phosphate can act as an electrophilicmethylation reagent[12] and therefore it is possible for TBP to react in thesame way. If this reaction occurs then the nitrogens on the BTBP coremolecule would be alkylated and the separate complex-formation ability ofboth TBP and BTBP might very well be lost. If a reaction between BTBP andTBP would occur it should be more or less the same for all types of BTBPsince the side chains on the BTBP would most likely not be involved.
If the molecules did not react with each other it was also important toestablish that the two molecules did not associate with each other in the extractedcomplex but extracted independently. This was tested by making several screen-ing extractions. These tests also served as an indication whether or not all theactinides could be extracted and, in the same time, be separated from thelanthanides as well as the fission and corrosion/activation products, as presumed.
Aim 2 – Extraction
The aim of the experiments in this section was to, under process like condi-tions, confirm the indications of extractions given in the previous section. Todo this the aqueous phase metal ion concentrations were targeted to be close to
OP
OO
O
Figure 2. TBP (tri-butyl phosphate).
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those found in real dissolved spent fuel, except for the radionuclides used(63Ni, 152Eu, 235U, 237Np, 238Pu, 241Am). These concentrations and also thecomposition of the dissolved fuel varies a lot between different sources [13–15]
depending on fuel type, burn-up, cooling time, the type of cladding used, andif the cladding is dissolved together with the fuel or not, etc. The amount ofcorrosion products present in a proposed GANEX process is also difficult toestimate since it would depend on many different factors.[16] Not only wouldthere already be an amount of activation products present in the dissolvedspent fuel but there would also be a continuous production of corrosionproducts in the extraction process due to dissolution, erosion, and wear ofthe equipment. They are nevertheless important to consider and extractions ofactinides, lanthanides, and fission products as well as corrosion/activationproducts were performed.
Aim 3 – Loading
To ensure that the actinide extraction will not be suppressed by the largeamounts of metals which will be present in process-scale applications, aloading extraction was performed. In this extraction the actinides and one ofthe lanthanides were extracted from a highly acidic aqueous phase loaded withfission products.
EXPERIMENTAL
In all of the following experiments the composition of the organic phase thatcombines BTBP with TBP is: 0.01 M of BTBP (C5- or CyMe4-) diluted incyclohexanone with an addition of 30 volume % of TBP. The cyclohexanonewas chosen as diluent due to the fact that is has shown to have fast kinetics forthe BTBP molecules[19] and that it also dissolves a reasonable amount of theBTBP. The composition of the organic phase with C5-BTBP (Fig. 3 Left) willfrom now on be referred to as “GANEX solvent 1” and the composition with
CyMe4-BTBP (Fig. 3 Right) will be referred to as “GANEX solvent 2.” C5-BTBP was used for the initial experiments due to the limited amounts avail-able of CyMe4-BTBP.
Reagents and Material
The BTBP molecules used were synthesized as described in reference 17[17]
and provided by Reading University, UK. All other inactive reagents usedwere commercially available and of analytical grade (see Tables 1 and 2)except for RhNO3 which was recycled from an old stock in house.
All radionuclides used in the experiments were added as tracers originat-ing from stock solutions. The preparation of the stock solution containing152Eu and 241Am (0.2 MBq/mL and 2 MBq/mL, respectively) has been
Table 1. List of inorganic chemicals used in the experiments
Substance Brand Purity (if available)
AgNO3 May & Baker pro analysi >99.9%Ba(NO3)2 Merck >99%Cd(NO3)2x4H2O Fischer Scientific certified reagentCe(NO3)3x6H2O Fluka purissCo(NO3)2x6H2O Fluka purum p.a. >99.0%Cr(NO3)3x9H2O Riedel de Haën AG. pure cryst.CsNO3 Merck extra pureFe(NO3)3x9H2O Merck pro analysi >99%HNO3 Sigma Aldrich 69% puriss p.a.KMnO4 Merck pro analysiLa(NO3)3x6H2O Riedel de Haën AG >99%MnSO4x1H2O Merck pro analyze >99%MoO3 Fluka purum p.a. 99.7%NaCO3 Fluka wasserfrei purum p.a.>99.0%NaNO3 Riedel de Haën AG p.a.Nd(NO3)3x6H2O Fluka purissPd(NO3)2x2H2O Fluka purum >97%RbNO3 Merck 99%Sb2O3 Fluka purum � 99.5%SeO2 Fluka purumSm(NO3)3x6H2O Fluka purum >98%SnO2 Fluka purumSr(NO3)2 Merck pro analysi, wasserfrei 99%Te Fluka puriss >99.999%Y(NO3)3 Fluka puriss 99%ZrO(NO3)2x5H20 ICN K&K Laboratories inc. -
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previously described.[18] Both the stock solution of 237Np (6 kBq/mL) and238Pu (10 MBq/mL) were prepared by dissolving the oxides (obtained fromAEA Technologies) in nitric acid. The radiochemical purity of 238Pu is>99.95% according to activity and >89.90% according to mass and the purityof 237Np is >99.95%. The 63Ni stock solution (0.3 MBq/mL) was obtained in0.1 M nitric acid from Amersham and then diluted. The stock solution of 235U(0.05 MBq/mL) was prepared by dissolving uranyl nitrate enriched to 84.88%(obtained from the Belgian atomic energy commission- today SCK-CEN) in6 M HNO3.
Extraction Experiments
The standard procedure for all extraction experiments is explained below andis valid unless otherwise stated.
The samples were done in duplicates or triplicates and both the organic andthe aqueous phases were pre equilibrated with the other phase, i.e. the organicphase was pre-equilibrated with the water phase and the water phase was preequilibratedwith cyclohexanone containing30%TBPwithout anyBTBPpresent.When radioactive tracers were used the water phases used for pre-equilibrationwere without metals. The actinides (235U, 237Np, 238Pu, 241Am), one lanthanide(152Eu) and one of the corrosion products (63Ni) were in all cases added to theaqueous phase in the form of radioactive tracers in small quantities (10–20 μL).
400–500 μL of organic phase was contacted with an equal amount ofaqueous phase in small (3.5 mL) glass vials. The contacting was performed byvigorous hand shaking in a thermal isolated canister for 10 minutes. This timeshould be sufficient to reach equilibrium since both CyMe4-BTBP andTBP have fast kinetics.[19,20] After letting the phases separate, an aliquotof 200–300 μL was removed from each phase for analysis. Excepted from thiswere all inactive analyzes made with ICP-OES. In this case the 200–300 μLaliquot was only removed from the aqueous phase and compared with a sampleremoved from the aqueous phase after pre-equilibration but before extraction.
Table 2. List of organic chemicals used in the experiments
Substance Brand Purity
cyclohexanone Acros Organics 99.8% extra pureTBP (tri-butyl phosphate) Fluka purum >97%ethanol BDH Pro Labo 95%dichloromethane Acros Organics pure >99%tri-ethyl amine Riedel de Haën AG purum >99%
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Analysis
The samples with 152Eu, 235U, and 241Am, were all analyzed using a HPGe(High Purity Germanium) detector (Ortec, GEM 15180-S) and Genie 2000 3.1(Canberra) software for evaluation.
The samples with 63Ni and 238Pu were analyzed using liquid scintillationcounting (Wallac 1414 WinSpectral).
The 237Np samples were analyzed using a NaI(Tl) detector (Intertechnique,CG 4000, Gamma Counter). In order to take the extraction of Pa impuritiespresent in the Np stock solution into account, the measurements were repeatedseveral times under up to four half lives of 233Pa (t1/2 ¼ 27d) until the values ofDNp were found stable. Due to the low activity of the stock solution thecounting time for the 237Np samples were set to 100 minutes, resulting in atotal number of counts >4000 for all aqueous phases and >11000 for all organicphases, thus resulting in a counting uncertainty of less than 2%.
The inactive metals were analyzed using ICP-OES (Thermo iCAP 6500Inductively Coupled Plasma Optical Emission Spectrometer) and comparingthe aqueous phase before and after extraction. With this method, the actualdetection limit is close to D¼0.01 (logD ¼ -2). Thus all values lower thanthat have been set to this value.
In all cases, uncertainties have been calculated using error propagationbased on measurement statistics.
PART 1: REACTIVITY – EXPERIMENTAL
Interaction Tests
All Thin Layer Chromatography (TLC)-tests were performed by comparing avery small volume (a spot) of the sample to reference samples on the samesilica plate. The BTBP-molecules were detected using a short wave UV-lampand to detect the TBP-molecule the whole plate was dipped in a colouringsolution (KMnO4 and NaCO3 in water) and heated. By comparing the Rf
values in the GANEX solvent to that of standard samples it can be seenwhether or not the BTBP and the TBP molecule have reacted with each other.
Several tests were made to determine the best composition of the mobilephase for the TLC-tests using different concentrations of ethanol and dichlor-omethane. A mobile phase consisting of 25% ethanol in dichloromethane waschosen. A drip of tri-ethyl amine was also added to 10 ml of the mobile phaseto diminish the blurriness of the detected spots by making the BTBP bindharder to the acid sites of the silica on the TLC-plates.[21]
The first test was performed as stated above and on the same day asthe GANEX solvent 1 was prepared. GANEX solvent 1 (diluted with dichlor-omethane) was compared with pure C5-BTBP dissolved in acetone and a third
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sample consisting of the first two ones on top of each other. The moleculeswere then detected and the Rf-values compared.
The GANEX solvent 1 was then left overnight in room temperature. Thenext day the TLC-experiment was repeated exactly as described above but withthe addition of a fourth sample consisting of TBP (diluted with dichloromethane)to ensure that the spot developed with the coloring solution actually was TBP.
Independent Extraction
The extractions were performed with four different organic phases: GANEXsolvent 2 (organic phase 1), 0.01 M CyMe4-BTBP in cyclohexanone (organicphase 2), 30% TBP in cyclohexanone (organic phase 3), and pure cyclohex-anone (organic phase 4). The aqueous media consisted of 1 M HNO3 and 3 MNaNO3 and the metals extracted were: Mo, Pd, Mn, Co, Ag, Cd, Sb, Sm, Eu,U, Np, Pu, and Am. Besides the pure aqueous phase used for the radioactivetracers, there were three additional aqueous phases prepared with the inactivemetals. To simplify the laboratory work Mo and Pd were prepared in separatesolutions by dissolving the oxide/salt in 1 M HNO3 and adding NaNO3 until atotal nitrate concentration of 4 M was reached. Due to incomplete dissolution,the two solutions were also filtered using a syringe filter (pore size 0.6 μm).The resulting metal concentrations were approximately 0.005 M for both Moand Pd. The third solution was prepared by dissolving the metal salts/oxidesof: Mn, Co, Ag, Cd, Sb, and Sm in 1 M nitric acid and adding NaNO3 up to atotal nitrate concentration of 4 M. To avoid loading issues the metal concen-trations were kept close to 0.001 M for Mn, Ag, Cd, Sb, and 0.0006 M for Coand Sm. Due to incomplete dissolution of the antimony oxide this solutionwas also filtered using a syringe filter (pore size 0.6 μm). All of theseextractions were then performed as stated earlier except that the Mo solutionand the mixed metal solution were contacted using a thermostatic (20+-1�C)shaking machine for a time of 1 hour instead of hand shaking. This shakingtime equals or exceeds 10 minutes of hand shaking and is hence enough toreach equilibrium.
PART 1: REACTIVITY – RESULTS AND DISCUSSION
Interaction Tests
The first TLC-test was performed on the same day as the GANEX solvent 1was prepared. From this test the conclusion could be drawn that the BTBP andthe TBP had not reacted with each other but was left as separate molecules.The GANEX solvent 1 was then aged (1 day) to see if any reactions that werekinetically slow would occur. The analysis showed no deviance in Rf value
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between the GANEX solvent 1 and the reference molecules, as can be seen inFig. 4, and hence no reaction had taken place.
The Effect of Nitric Acid
Nitric acid can be extracted into the organic phase by BTBP and also by TBP.TBP forms both 2:1[22] and 1:1 complexes with HNO3 and with 30% TBP inkerosene at 4 M nitric acid concentration the concentration of nitric acid in theorganic phase is app. 0.8 M.[23] Besides this, both TBP and cyclohexanonehave a mutual solubility of water. At 25�C the solubility of water in cyclohex-anone is 8.0 wt% and in TBP it is 4.67 %.[24] It is already known that C5-BTBP is degraded in the presence of nitric acid,[17] most likely due tooxidation on the α-carbons in the alkyl substituents, initiated by nitrous acidradicals present in the nitric acid. This is, however, not true for all BTBP typemolecules and for instance CyMe4-BTBP in octanol has shown a goodstability towards nitric acid.[25] This is because the CyMe4-BTBP side chainsdo not give the nitrous acid radicals any opportunity for hydrogen abstractionon the benzylic carbons. Due to this, the high presence of nitric acid in the
Figure 4. Sample 1 - C5-BTBP in acetone. Sample 2 - 0.1 M BTBP + 30% TBP incyclohexanone (diluted with dichloromethane). Sample 1+2¼ Sample 1 + Sample 2 onthe same place on the plate. Sample 3 - TBP (diluted with dichloromethane).
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organic phase is not considered to be a big problem even though an aqueousphase in a proposed GANEX process will consist of highly concentrated nitricacid (3–6 M). It is also already well known and documented that TBP is onlyto a very small degree degraded by nitric acid to dibutyl phosphate (DBP) andmonobutyl phosphate (MBP) and finally phosphoric acid.[26] The diluent is,however, an important factor when it comes to the stability of the system andtherefore the long term consequences of nitric acid contact with the GANEXsolvent is currently under investigation.
Independent Extraction
It has now been established that the BTBP and TBP do not react with eachother. The possibility remains, however, that the two molecules could form ajoint extractable complex. This is something that would not have been detectedin the TLC test. By performing separate and combined extraction experimentsthe occurrence of joint extractable complexes could be tested. When doing thisan indication is also given regarding whether or not the actinides actually willbe extracted together and separated from the lanthanides as well as the fissionand corrosion/activation products. In addition, this also gives information aboutwhich molecule extracts which metal under the conditions used.
The tests were done with four different organic phases using aqueousphases with high ionic strength but reasonably low acidity (1 M HNO3 + 3 MNaNO3). The reason for keeping the acidity low is because it was observedthat without TBP present, cyclohexanone, and nitric acid (>3 M) are almostcompletely miscible, so to be able to make a correct comparison between thephases this aqueous phase composition was chosen. The results from allextractions are presented in Table 3.
The first conclusion that can be drawn from Table 3 is that the BTBP andTBP clearly extract independently. Other interesting observations regardingwhich molecule that extracts which metal can also be made.
Considering the actinides it can be seen that Am is extracted by BTBP. Inthe same way both Np and Pu are almost entirely extracted by BTBP and not somuch by TBP as might be expected. This raises the question of the oxidationstate of the Pu since it was shown earlier that BTBP did not extract tetravalentor hexavalent actinides.[9] U on the other hand is extracted by TBP. Both Pu andU are, however, also extracted by the diluent, cyclohexanone. This extractionmay be undesired since it could cause problems in a subsequent stripping step.One solution to the problem could be carbonate stripping but this is an issue thatmust be further looked into. If one looks at the two lanthanides (Eu and Sm) itcan be seen that the extraction is caused by the BTBP.
All fission products that are extracted are almost entirely extracted by theBTBP except for Sb, which to some surprise also seems to be extracted by TBPand cyclohexanone. It can also be observed that among the fission products that
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Table3.
Log
(D)from
theextractio
nof
metals(in4M
nitrateconcentration(1
MHNO3+3M
NaN
O3))with
four
differentorganicphases
(org
1:0.01
MCyM
e 4-BTBP+30
%TBPin
cycloh
exanon
e.org2:
0.01
MCyM
e 4-BTBPin
cycloh
exanon
e.org3:
30%
TBPin
cycloh
exanon
e.org4:
cycloh
exanon
e)
organicph
ase1
organicph
ase2
organicph
ase3
organicph
ase4
log(D)
+/-
log(D)
+/-
log(D)
+/-
log(D)
+/-
Pu
2.55
0.00
05/0.000
52.59
0.00
04/0.000
41.21
0.00
01/0.000
11.04
8*10
-5/8*1
0-5
Am
2.58
0.00
6/0.00
72.18
0.00
6/0.00
6-1
.16
0.00
2/0.00
2-2
.09
0.00
5/0.00
5Eu
0.55
0.00
2/0.00
20.09
0.00
3/0.00
3-0
.94
0.00
4/0.00
4-1
.82
0.01
/0.01
U1.65
0.01
/0.01
0.91
0.00
5/0.00
51.67
0.02
/0.02
0.85
0.00
7/0.00
7Np
0.88
0.00
9/0.00
90.95
0.01
/0.01
-0.26
0.00
4/0.00
4-0
.42
0.00
3/0.00
3Ni
1.83
0.00
5/0.00
52.85
0.02
/0.02
-2.99
0.01
/0.01
-2.58
0.00
7/0.00
7Pd
1.33
0.01
/0.01
2.11
0.03
/0.03
-0.84
0.01
/0.01
-0.76
0.01
/0.01
Mo
1.21
0.00
3/0.00
31.46
0.00
4/0.00
4-0
.99
0.02
/0.02
-1.13
0.01
/0.01
Ag
1.78
0.01
/0.01
1.72
0.01
/0.01
-0.79
0.02
/0.02
-0.76
0.03
/0.03
Cd
**
-1.11
0.05
/0.06
-1.46
0.1/0.2
Co
4.06
0.6/>0.6
4.02
0.8/>0.8
-1.09
0.03
/0.03
-1.38
0.08
/0.1
Mn
3.49
0.1/0.1
3.47
0.1/0.1
-1.00
0.01
/0.01
-1.23
0.05
/0.06
Sb
3.05
0.4/>0.4
3.14
0.4/>0.4
0.06
0.01
/0.01
-0.08
0.01
/0.01
Sm
-0.38
0.01
/0.01
-0.73
0.07
/0.09
-0.52
0.00
8/0.00
8-1
.32
0.1/0.2
*Dvaluetoohigh
tomeasure
with
theequipm
ent.
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are extracted by BTBP, the majority display a lower D with organic phase 1 aswell as with organic phase 3 compared to organic phase 2 and 4. This suggeststhat the presence of TBP in the system lowers the distribution ratios. Thisdeviation in D is most likely explained by the large addition of TBP to thecyclohexanone. This is because when BTBP is the extractant then TBP just actsas part of the diluent, altering its properties. The fact that altering the nature ofthe diluent affects the D is something that is well established.[27]
PART 1: REACTIVITY – CONCLUSION
The BTBP and TBP molecules can coexist in a solution of cyclohexanonewithout reacting with each other. Neither do they associate with each other butextract independently and from a 1 M HNO3 + 3 M NaNO3 aqueous phase theactinides are readily extracted with GANEX solvent 2.
PART 2: EXTRACTION – EXPERIMENTAL
Extraction under Process Like Conditions
All of these extractions were performed using GANEX solvent 2 as theorganic phase. The aqueous phase used was 4 M nitric acid, to resembleprocess conditions (PUREX feed concentration is typically over 3 MHNO3).
[28] For the same reason the inactive metals were present in con-centrations comparable to those in dissolved spent fuel and the aqueousphases with inactive fission products were also based on 4 M nitric acid (seethe section: preparation of metal solutions). The extractions were performedas stated earlier except that all inactive fission and corrosion products(except for Pd) were contacted using a thermostatic (20+-1�C) shakingmachine for 1 hour. This shaking time was assumed to be sufficient toreach equilibrium for the fission- and corrosion products based on the factthat DNi could be compared to kinetic data for CyMe4-BTBP
[29] and thatextractions of various metals with C5-BTBP show no need for longershaking times for other metals compared to Ni.[30] The volumes shaken formetal solution 3 (see the section: preparation of metal solutions) was also900 μL and the aliquots removed were 800 μL.
Preparation of Metal Solutions
Eight different aqueous phases were prepared. Zr, Mo, and Pd were preparedin separate solutions by dissolving the metal salts/oxides in 4 M nitric acid.The three solutions were filtered using a syringe filter (pore size 0.6 μm).
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A separate solution was also made for Ru by diluting an ICP OES standardsolution (Ultra Scientific Analytical solutions, 1 analyte(s) at 1000 μg/mL)with 4 M nitric acid until the wanted concentration was reached. The rest ofthe fission products (FP) and corrosion/activation products were prepared infour different solutions depending on their abundance in the spent fuel.Solution 1 (FP with high abundance): Cs, Ba, La, Ce, and Nd. Solution 2(FP with medium abundance): Rb, Sr, Y, Te, and Sm. Solution 3 (FP withlow abundance): Rh, Ag, Cd, Sn, and Sb. Solution 4 (corrosion/activationproducts): Cr, Mn, Fe, and Co. All four solutions were prepared by dissol-ving the metal salts/oxides in 4 M nitric acid. An exception was Te, whichwas dissolved in a more concentrated nitric acid, later used for preparing the4 M nitric acid for solution 2. The dissolution of Te resulted in a finalconcentration that is much higher than what can be expected to be found inthe spent fuel. In solution 3 the tin oxide did not significantly dissolve andtherefore the solution was decanted. The concentrations of all the elementsin these eight solutions were then measured with ICP-OES after being preequilibrated. This was done by comparison with calibration curves con-structed with ICP-OES and ICP-MS standards (ICP-OES: Ultra ScientificAnalytical solutions, 1 analyte(s) at 1000 μg/mL and ICP-MS: High PurityStandards, 10 +- 0.05 μg/mL (MS)). The resulting concentrations can beseen in Table 4.
PART 2: EXTRACTION – RESULTS AND DISCUSSION
Extraction Test under Process Like Conditions
As can be seen in Fig. 5, all present actinides are readily extracted. Thelanthanides on the other hand are not extracted to any larger extent and alldistribution ratios are below one (Fig. 6). The extraction of the lanthanidesalso shows an obvious trend of being larger when moving to the right in theperiodic table. This behavior of the lanthanide extraction has been observedbefore for CyMe4-BTBP and the D reaches a maximum value at dysprosiumbefore it starts to decrease again.[25] This is a favorable result for processapplications since the first lanthanides (especially La, Ce, and Nd) are presentin the highest concentrations in the spent fuel. The D for the higher lantha-nides Tb, Dy, and Ho would most likely be over 1 when extracted withGANEX solvent 2 from 4 M nitric acid but the abundance of these elementsis however very low (below 10 ppm).[13–15]
The distribution ratios presented in Figs. 5 and 6 result in promisingseparation factors between actinides and lanthanides (Table 5) even forthe lanthanide that is extracted the most (Eu). When comparing theextraction of actinides with the extraction of the most abundant lantha-nide (Nd) the result looks even better (Table 5). Np is the actinide that
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has the lowest SF towards the lanthanides (SFNp/Eu ¼ 3.5) but this is stillsufficiently high.
In Fig.7 it can be seen that the elements among the fission products thatclearly have the highest distribution ratios (Ag and Cd) fortunately also have avery low abundance in the spent fuel. This unwanted extraction can, however,still become a problem in a future process if the metals cannot be scrubbedout. Two of the most abundant elements (Zr and Mo) are also extracted tosome degree. Even though the D for Zr is not very high this could still beproblematic due to the large amounts of the element available in spent fuel.Another troublesome element is Pd that also is extracted by the GANEXsolvent 2. Beside this, Pd has also been observed to quickly precipitate tometallic palladium when a ketone like cyclohexanone is present, which could
Table 4. Composition of metal solutions from OES-data
cause problems in a future process. This precipitation is, however, stoppedwhen BTBP is present in the organic phase forming complexes with the Pd.
As can be seen in Fig. 8, it is Ni, Co, and Mn among the corrosionproducts that are extracted to any great extent. This is a promising result sinceFe ought to be the most abundant corrosion product present in a reprocessingprocess. This is due to the high content in steel equipment and also in thepossible future use of steel cladding for fast reactors.[28]
It has also been observed that most of the results presented in the“Independent Extraction” section (part 1) not only apply to that specificaqueous phase but also to the process like conditions. Due to this, the
Figure 6. Extraction of lanthanides (152Eu, the rest as non radioactive metal salts)from 4 M nitric acid with 0.01 M CyMe4-BTBP and 30% TBP in cyclohexanone.
Figure 5. Extraction of actinides (235U, 237Np, 238Pu, 241Am) from 4 M nitric acidwith 0.01 M CyMe4-BTBP and 30% TBP in cyclohexanone.
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discussion on which element that is extracted by which molecule can also beapplied on most extractions with GANEX solvent 2 from 4 M nitric acid.
PART 2: EXTRACTION – CONCLUSION
Under process like conditions the actinides are readily extracted by GANEXsolvent 2. The actinides are separated from the lantahanides as well as frommost of the fission and corrosion/activation products.
Figure 7. Extraction of fission products from 4 M nitric acid with 0.01 M CyMe4-BTBP and 30% TBP in cyclohexanone. White bars ¼ concentrations well below 100ppm in dissolved fuel. Grey bars ¼ concentrations between 100–600 ppm in thedissolved fuel. Black bars ¼ concentrations above 1000 ppm in dissolved fuel. Thedashed line marks log(D)¼0 which is the dividing line between extraction and strip.
Table 5. Separation factors for the actinides (U, Np, Pu, Am) andtwo different lanthanides (Nd, Eu) after extraction from 4 M HNO3
with 0.01 M CyMe4-BTBP + 30% TBP in cyclohexanone
Elements Separation factor
Am / Eu 160.0Pu / Eu 210.0U / Eu 11.0Np / Eu 3.5Am / Nd 1700.0Pu / Nd 2300.0U / Nd 120.0Np / Nd 38.0
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PART 3: LOADING
In a real process case there will definitely be metal loading in the aqueousphase compared to the concentration of BTBP in the organic phase. To makesure that this is not an insurmountable problem, an extraction of U, Np, Pu,Am, and also Eu with GANEX solvent 2 from a heavily metal loadedaqueous phase was performed. The experiments were done to study howthe metal loaded water phase affected the D for the actinides and thelanthanides.
PART 3: LOADING – EXPERIMENTAL
The extraction experiments were performed as previously stated. One singlemetal solution with Rb, Sr, Y, Zr, Mo, Rh, Pd, Ag, Cd, Sb, Cs, Ba, La, Ce, Nd,Sm, and Te in approximately the same concentrations as seen in Table 4 wasprepared by dissolving the metal salts/oxides in 4 M HNO3.The total metalconcentration (after filtration through a glass microfiber syringe filter, particleretention: 1.0 μm) was over 9000 ppm. This metal solution was then used asaqueous phase in the extraction of 235U, 237Np, 238Pu, 241Am, and 152Eu withGANEX solvent 2.
PART 3: LOADING – RESULTS AND DISCUSSION
When comparing Fig. 9 to Figs. 5 and 6 it can be seen that the D for U is moreor less unaffected, as expected, due to the large amount of TBP present in the
Figure 8. Extraction of corrosion/activation products from 4 M nitric acid with 0.01 MCyMe4-BTBP and 30% TBP in cyclohexanone. Metal concentrations of 0.005 M.
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organic phase. The D for Np is however also unaffected even though it wasshown to be extracted by the BTBP and not TBP in the previous section. Thiscould be an effect of a change in oxidation state caused by the higher acidityor the other metals present in the aqueous phase. The D for both Am and Pu islowered but still sufficiently high, especially since the D for Eu also waslowered, rendering a total raise in SF for all actinides except Am (Table 6).The lowered D for Eu and hence also the other lanthanides under metalloading conditions is a good result considering that the higher lanthanides(Tb, Dy, and Ho) otherwise would have a D over 1.
In a future reprocessing process of spent transmutation fuel the contentof Pu will be much higher than in regular spent nuclear fuel. This is due tothe mixing in of Pu in the fuel for fast reactors.[31] This creates totallydifferent scenarios regarding the loading issue which are not considered inthis work.
Table 6. Separation Factors for the actinides (U, Np, Pu, Am)and one lanthanide (Eu) after extraction from metal loaded 4 MHNO3 (metal content >9000 ppm) with 0.01 M CyMe4-BTBP+ 30% TBP in cyclohexanone
Elements Separation factor
Am / Eu 101Pu / Eu 363U / Eu 91Np / Eu 28
Figure 9. Extraction of 235U, 237Np, 238Pu, 241Am, and 152Eu from a 4 M nitric acid -metal loaded aqueous phase with 0.01 M CyMe4-BTBP and 30% TBP in cyclohex-anone. Total metal concentration: >9000 ppm.
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PART 3: LOADING – CONCLUSION
The actinides can be extracted from a metal loaded water phase, however, withsomewhat decreased distribution ratios for Pu and Am. The separation factorsare, however, still high or even higher than without metal loading due to thedecrease in D for the lanthanides.
CONCLUSION
The two extractants BTBP and TBP can be used together in one solvent sinceit has been shown that they do not react with each other. The two moleculesalso extract independently and are able to extract the actinides and separatethem from the lanthanides as well as from most of the fission and corrosion/activation products. The results from the experiments clearly show, despite theneed for system optimization, that the combination of CyMe4-BTBP and TBPin cyclohexanone has a great potential and is a feasible candidate for a futureindustrial GANEX process.
ACKNOWLEDGMENTS
The authors thank M. Sc. Kristian Larsson who was very helpful and patientwith all ICP-OES measurements. Thanks also to the Swedish Nuclear Fueland Waste Management Company (SKB) and the European 7th FrameworkProgram ACSEPT (project number: 211267) for financial support.
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