Boris F. Myasoedov

Maxim D. Samsonov

Trofim I. Trofimov

Yurri M. Kulyako

Comptes Rendus Chimie

Full paper / MémoireDissolution and extraction of actinide oxidesin supercritical carbon dioxide containing the complexof tri-n-butylphosphate with nitric acidTrofim I. Trofimov *, Maxim D. Samsonov, Yurri M. Kulyako, Boris F. MyasoedovLaboratory of Radiochemistry, V.I.Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences,Kosygin str., 19, Moscow, RussiaReceived 1 December 2003; accepted after revision 16 April 2004Available online 13 October 2004AbstractDissolution of individual actinide oxides (Th, U, Pu, Np), or their mechanical mixtures, as well as of solid solutions U–Pu,U–Np, U–Am and U-Pu-Eu oxides in supercritical fluid carbon dioxide (SF-CO2) containing the complex of tri-n-butylphosphate (TBP) with nitric acid (TBP–HNO3) has been investigated. The effect of the calcination temperature of solid solutionsof dioxides on the separation of actinides during supercritical fluid extraction (SFE) has been studied as well. It was shown forthe first time that milligram amounts of uranium dioxide could be quantitatively dissolved in (SF-CO2) containing theTBP–HNO3 complex and efficiently separated from Pu, Np, and Th during SFE of mechanical mixture of these oxides. On thecontrary, both U and Pu are quantitatively dissolved in SF-CO2–TBP–HNO3 during SFE from solid solutions of U–Pu dioxide.An increase of the calcination temperature of the mixed U(IV)–Pu(IV) dioxide from 850 to 1200 °C has no influence on therelative extraction yield of these actinides during SFE. To cite this article: T. Trofimov et al., C. R. Chimie 7 (2004).© 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.RésuméDissolution d’oxides d’actinides et extraction d’éléments dans le dioxide de carbone supercritique contenant lecomplexe tri-n-butylphosphate–acide nitrique. La dissolution d’oxydes de Th, U, Pu et Np, de leurs mélanges et de solutionssolides U–Pu, U–Np, U–Am et U–Pu–Eu dans le dioxyde de carbone supercritique (CO2-SC) contenant le complexe tri-n-butylphosphate–acide nitrique (TBP–HNO3) a été étudiée, et notamment l’effet de la température de calcination des solutions solides.On montre que quelques milligrammes de UO2 peuvent être dissous dans le système CO2-SC–TBP–HNO3 et être séparés de Pu, Npet Th en traitant un mélange d’oxydes. En revanche, U et Pu sont dissous dans la phase CO2-SC–TBP–HNO3 durant le traitementdes solutions solides U(IV)–Pu(IV). Une augmentation de la température de calcination de 850 à 1200 °C de ces solutions solidesn’a pas d’effet sur le rendement d’extraction des actinides. Pour citer cet article : T. Trofimov et al., C. R. Chimie 7 (2004).© 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.Keywords: Supercritical carbon dioxide; ActinidesMots clés : Dioxyde carbone supercritique ; Actinides* Corresponding author.E-mail address: trofimov@geokhi.ru (T.I. Trofimov).C. R. Chimie 7 (2004) 1209–1213http://france.elsevier.com/direct/CRAS2C/1631-0748/$ - see front matter © 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.doi:10.1016/j.crci.2004.04.0101. IntroductionExtraction and purification of U and Pu in spentnuclear fuel reprocessing (SNF) is a key chemicalprocess in the nuclear industry. The most widely usedcommercial process to achieve this objective is thePlutonium Uranium Reduction EXtraction process(PUREX), which involves the dissolution of SNF innitric acid followed by solvent extraction of U(VI) andPu(IV) using tri-n-butyl phosphate (TBP) as extractant[1,2]. The PUREX process, though highly efficient,has the inherited drawbacks of liquid–liquid extrac-tion, including generation of large volumes of aqueousand organic high-level radioactive wastes. So a searchfor new alternative techniques for reprocessing of SNF,which will allow minimizing waste generation, is amajor task, challenging the future of the nuclear indus-try.The possibility of using (SF-CO2) as a solvent forreprocessing of SNF was suggested recently [3]. Ofparticular interest is the idea of direct dissolution ofSNF in SF-CO2 using a suitable complexing agent thatshould form a SF-CO2 soluble complex with the maincomponents of SNF, basically with uranium dioxide.SF-CO2 is considered as a green solvent because it isnon-toxic and environmentally benign. It is also cheap,readily available in relatively pure form, and has mod-erate critical constants (Tc = 31.3 °C, Pc = 72.8 atm,and qc = 0.47 g cm–3) [4]. A major advantage of usingsupercritical CO2 for reprocessing of SNF is the elimi-nation of the use of acid and organic solvent required inthe conventional PUREX process.The hexavalent uranyl ion UO22+ is known to formSF-CO2 soluble complexes with a number of complex-ing agents, including TBP and b-diketones [5]. It hasbeen demonstrated that UO3 can be dissolved in SF-CO2 using thenoyltrifluoroacetone (HTTA) and TBP[6,7]. Recent studies have also demonstrated the pos-sibility of lanthanide extraction from their oxides usingSF-CO2 containing the TBP–HNO3 complex [8].However, the extraction efficiency of all elements in-volved did not exceed 40%.We have reported earlier [9] about the quantitativedissolution of milligram amounts of UO3 in SF-CO2containing HTTA and TBP, which was performed us-ing ultrasonication. Also, it was reported that uraniumcan be quantitatively extracted and efficiently sepa-rated from Pu, Np and Th in the process of SFE of themechanical mixture of their dioxides using SF-CO2modified with TBP–HNO3 complex [10].The study of dissolution of solid solutions of ac-tinide dioxides in UO2 using the system SF-CO2–TBP–HNO3 is the purpose of this work. Preliminaryresults obtained are presented below in comparisonwith previous results.2. ExperimentalCommercial oxides were used in the above-mentioned research about the dissolution of individualactinide oxides in SF-CO2 containing the TBP–HNO3complex [9,10]. In the present work, we used the syn-thesized samples of solid solutions of actinide dioxidesand Eu2O3 in UO2. Their preparation was carried out inaccordance with the scheme presented in Fig. 1. Ac-cording to [1,11] Np and Pu dioxides are formed in theprocess of air calcination of many Np and Pu com-pounds, such as hydroxide, acetate, nitrate and oxalate.Though Np and Pu may have any oxidation state inthese compounds, the product of their calcinations isalways NpO2 and PuO2. Since UO2 was the basis of allsamples of the solid solutions of actinide dioxidesprepared, it should be obtained in the form of pureUO2. For this purpose, drying, decomposition, andcalcinations of mixed actinide oxalates up to the solidsolution of their oxides were conducted under argon–hydrogen atmosphere. These operations were per-formed in glass-graphite crucibles, increasing the tem-perature from 20 to 850 °C for 5 h. Further calcinationFig. 1. Preparation of the solid solutions of mixed actinide and Euoxides.1210 T.I. Trofimov et al. / C. R. Chimie 7 (2004) 1209–1213was continued for 8 h at 850 °C. In the case of U–Pu–Eu, about a half of the mixed oxide sample preparedwas additionally calcined at 1200 °C for 8 h in order tosimulate samples that may be obtained from real SNF,since the temperature in the fuel elements in thenuclear reactor can reach 1200 °C. A schematic dia-gram of the experimental set-up for SFE of actinideoxides, the preparation technique of the TBP–HNO3complex and experimental procedures were describedearlier [9,10]. Types and composition of the solid solu-tions of the corresponding actinides and Eu are pre-sented in Table 1.3. Result and discussionThe results of X-ray structure analysis (Table 2) ofthe prepared U–Pu oxides carried out by the method ofthe X-ray powder diffraction pattern have shown that,in all cases, UO2 was the matrix of the mixed oxides.Unit cell parameters for the samples #1, #2, and #3 areequal to 5.4703 Å, 5.4232 Å, and 5.4421 Å, respec-tively. Reference data for UO2 are taken from thedatabase PDF-2 (JCPDS-ICDD, 1999). According toit, a(Å) = 5.4704 – 0.0935 x, for UO2+X at 25 °C. Anincrease of Pu content in sample #2 in comparison with#1 results in a considerable decrease in the unit-cellparameter of UO2. At the same time, additional calci-nation of the sample #3 at 1200 °C results in its in-crease. Potentially, these changes in the unit-cell pa-rameter value may influence the processes ofdissolution of actinide dioxides in the course of SFE.We present in Tables 3 and 4 the results obtainedearlier [9,10] of SFE of actinides from their individualand mixed oxides using SF-CO2 modified with theTBP–HNO3 complex. The new results on SFE of U,Np, Pu, Am and Eu from the solid solutions of theiroxide are presented in Table 5. As seen from the datapresented (Tables 3 and 4), uranium can be quantita-tively extracted from all the oxides investigated. It wasshown by spectrophotometry that uranium is present inthe oxidation state +6 only in the trap solution (Fig. 2).Therefore, the interaction of TBP–HNO3 complexwith UO2 results in the oxidation of the tetravalenturanium to the hexavalent one. This leads to the forma-tion of highly soluble UO2(NO3)2·2 TBP complex inthe SF-CO2 phase. Oxidation of Np and Pu up to theoxidation state +6 under the conditions investigateddoes not occur. So, the extraction efficiencies of Pu,Np, and Th from both individual oxides and theirmixtures with uranium dioxide are extremely poor.At the same time, it can be seen from the datapresented in Table 5 that during SFE of the solidsolution of U–Np, U–Am, U–Pu, U–Pu–Eu oxides,Table 1Samples of the solid solutions of actinide and Eu oxidesComposition of the solidsolutionsWeight %AmO2 10.7UO2 89.3NpO2 5.0UO2 95.0PuO2 5.0UO2 95.0PuO2 25.0Eu2O3 5.0AmO2 0.05UO2 69.95Table 2Interplanar spacing d(Å) and relative intensities (Int) for solid solutions of U–Pu oxides prepared and reference data for UO2Reference data for UO2Measured data for the samples preparedU–Pu(5%) (Sample 1) U–Pu(25%)–Eu (Sample 2) U–Pu(25%)–Eu (Sample 3)Tcal = 850 °C Tcal = 850 °C Tcal = 1200 °Cd (Å) Int. (%) d (Å) Int. (%) d (Å) Int. (%) d (Å) Int. (%)3.156 100.0 3.154 100.0 3.135 100.0 3.147 100.02.733 34.7 2.729 44.4 2.716 50.0 2.725 50.01.932 40.9 1.934 56.4 1.920 70.0 1.927 70.01.648 35.0 1.649 63.6 1.636 70.0 1.642 70.01.578 7.6 1.579 18.4 1.570 10.0 1.572 20.01.366 5.0 1.368 14.2 1.359 10.0 1.361 10.01.254 11.5 1.255 29.9 1.245 30.0 1.249 30.01.222 8.5 1.224 23.4 1.214 20.0 1.217 20.01211T.I. Trofimov et al. / C. R. Chimie 7 (2004) 1209–1213practically quantitative extraction of Np, Pu, Am andEu occurs. It turned out that the Np and Pu passing intothe trap solution exists in the oxidation state +4 (seeFig. 2). Other oxidation states of Np and Pu in absorp-tion spectrum of this solution are not seen on thespectra. It means that during the contact of the TBP–HNO3 complex with the solid phase of U-Pu or U-Npoxides U(IV) easily oxidizes to U(VI), resulting inbreaking the structure of the actinide solid solutionsand in initiating the formation of the SF-CO2-solublecomplexes of Pu(IV) and Np(IV) with the TBP–HNO3complex. The results obtained show that trivalent eu-ropium and americium pass quantitatively into the trapsolution as well. Increase in the Pu content in thematrix of UO2 from 5 to 25% has practically no effectTable 5Dissolution of solid solutions of mixed radionuclide oxides in SF-CO2 with the TBP–HNO3 complex Vcomplex = 3 ml (0.01 mol), T = 65 °C,P = 200 atmCalcinationtemperature (°C)Solid solution ofmixed oxidesWt % Actinide content inoxide (mg)Extractionefficiency (%)850 AmO2 10.7 0.55 92.0UO2 89.3 4.56 98.2850 NpO2 5.0 0.25 84.0UO2 95.0 4.70 93.6850 PuO2 5.0 0,67 89.0UO2 95.0 14,60 93,3850 PuO2 25.0 2.15 86.1UO2 69.95 6.14 94.4Eu2O3 5.0 0.43 93.6AmO2 0.05 0.0037 86.0PuO2 25.0 3.91 89.61200 UO2 69.95 11.17 93.1Eu2O3 5.0 0.78 91.0AmO2 0.05 0.0066 88.0Table 3SFE of actinides from their individual oxides using SF-CO2 with theTBP–HNO3 complex. T = 65 °C, P = 250 atmOxide Actinide added(mg)Molar ratioAn:complexExtractionefficiency (%)UO2 61,5 1:50 > 99334.4 1:10 90 ± 5367.9 a 1:10 65 ± 3UO3 175.1 1:20 92 ± 5U3O8 177.3 1:20 85 ± 3PuO2 8.1 1:250 < 0.150.1 1:50 < 0.1NpO2 5.6 1:250 < 0.155.0 1:50 < 0.1ThO2 52.9 1:50 < 0.1a Non-milled uranium fuel pellet.Table 4Separation of U from Pu, Np, and Th using SF-CO2 with the TBP–HNO3 complex T = 65 °C, P = 250 atm, molar ratio An:complex ~1:20Mixtureof oxidesActinideadded (mg)Extractionefficiency (%)Separationfactor (SU/An)UO2 150.5 87 ± 4 ~1200PuO2 37.4 < 0.1UO2 120.6 91 ± 5 ~1400NpO2 11.5 < 0.1UO2 133.5 89 ± 4 ~1100ThO2 58.5 < 0.1Fig. 2. Absorption spectra of the trap solutions obtained from SFE ofUO2 (1) and of solid solutions of mixed U–Np (2) and U-Pu (3)dioxides.1212 T.I. Trofimov et al. / C. R. Chimie 7 (2004) 1209–1213on the relative yield of U and Pu on SFE for solidsolution or their mixed dioxides under the conditionsinvestigated. An increase of the calcination tempera-ture of the mixed U–Pu–Eu oxide from 850 to 1200 °Chas no influence on the relative yield of radionuclidesinvolved.4. ConclusionUranium can be quantitatively extracted from UO2,UO3 and U3O8 with SF-CO2 containing the TBP–HNO3 complex. On dissolution of UO2 in the system,the oxidation of U(IV) to U(VI) proceeds withthe formation of the SF-CO2-soluble complexUO2(NO3)2·2 TBP. Uranium can be efficiently sepa-rated from Pu, Np and Th in the course of SFE frommechanical mixtures of their oxides. Contrary to themechanical mixtures of actinide oxides, solid solutionsof mixed actinide oxides are quantitatively dissolved inSF-CO2 containing the TBP–HNO3 complex. Pu andNp trapped during SFE of solid solutions of mixedoxides turn out in the trap solution in the oxidationstate +4, whereas U is in the oxidation state +6. Anincrease in the Pu content in the matrix of UO2 from5 to 25% has practically no effect on the relative yieldsof U and Pu on the SFE of the solid solution of theirmixed dioxide under the conditions investigated. Anincrease in the calcination temperature of the mixedU–Pu–Eu oxide from 850 to 1200 °C has no influenceon the relative yield of radionuclides involved duringSFE as well. Application of SFE technique may bevery promising for SNF reprocessingAcknowledgementsThis work was supported by Russian Foundation forBasic Research, grants # 99-03-32819 and 00-15-97391.References[1] J.J. Katz, G.T. Seaborg, L.R. Morss, The Chemistry ofActinide Elements, 2nd Ed, Vol. 1, Chapman and Hall, Lon-don, 1986 p. 525.[2] I. Denniss, A. Jeapes, in: P.D. Wilson (Ed.), The Nuclear FuelCycle: From Ore to Waste, Oxford University Press, Oxford,UK, 1996, p. 123 Ch. 7.[3] N. Smart, C. Phelps, C. Wai, Chem. Br. 34 (1998) 34.[4] S.B. Hawthorn, Anal. Chem. 62 (1990) 633A.[5] Y. Lin, C. Wai, Anal. Chem. 66 (1994) 1971.[6] C. Phelps, N.G. Smart, C.M. Wai, J. Chem. Educ. 73 (1996)1163.[7] C. Phelps, Recycling Uranium: Hello SFE, GoodbyePUREX? PhD Thesis, University of Idaho, Department ofChemistry, 1997.[8] O. Tomioka, Y. Enokida, I. Yamamoto, J. Nucl. Sci. Technol.35 (1998) 515.[9] T. Trofimov, M. Samsonov, S. Lee, B. Myasoedov, C. Wai,Mendeleev Commun 4 (2001) 129.[10] M.D. Samsonov, T.I. Trofimov, S.E. Vinokurov, S.C. Lee,B.F. Myasoedov, C.M. Wai, Dissolution of actinide oxides insupercritical fluid carbon dioxide, containing various organicligands, J. Nucl. Sci. Technol. (Suppl. 3) (2002) 263.[11] L. Brewer, Chem. Rev. 52 (1953) 1 BB.1213T.I. Trofimov et al. / C. R. Chimie 7 (2004) 1209–1213

Dissolution and extraction of actinide oxides in supercritical carbon dioxide containing the complex of tri-<i>n</i>-butylphosphate with nitric acid

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Dissolution and extraction of actinide oxides in supercritical carbon dioxide containing the complex of tri-<i>n</i>-butylphosphate with nitric acid

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