Chromatographic Separation of Actinides and Fission Products - Core

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 39 (2013) 110 – 119

Asian Nuclear Prospects 2012 (ANUP2012)

Chromatographic Separation of Actinides and Fission Products Wei Yuezhoua,b , Liu Ruiqina, Wu Yana, Zu Jianhuaa, Wang Xinpenga, Chen Zia a

School of Nuclear Science and Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Rd., Shanghai 200240,China b Cyclotron and Radioisotope Center, Tohoku University, Aoba6-3, Sendai 980-8578, Japan

Abstract Several novel adsorbents which are immobilized in a porous silica/polymer composite support (SiO2-P) were prepared to separate MA (Am, Cm) and some fission product elements (FPs) such as Tc, Pd, Cs and Sr from high level liquid waste (HLLW) by chromatographic techniques systematically. Adsorption and separation behavior of various elements contained in HLLW was studied experimentally. Small scale separation tests using simulated and genuine waste solutions were carried out to verify the feasibility of the proposed process. The obtained experiment results indicated that the proposed process is essentially feasible.

© 20xx The Authors. Published by Elsevier Ltd. © 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and/or peer-review under responsibility Instituteand ofNew Nuclear and New Energy Technology, Selection and peer-review under responsibility of Institute of of Nuclear Energy Technology, Tsinghua University Tsinghua University Keywords: HLLW; minor actinides; fission products; waste minimization, valuable elements utilization; adsorbent; chromatographic separation

1.

Introduction

The long-term radiotoxicity in the high level liquid waste (HLLW) generated by the spent fuel reprocessing system such as the PUREX process is governed by the content of several long-lived radionuclides, among which the most significant nuclides are the so-called minor actinides (MA: Am, Cm and Np) and 99Tc. Through the transmutation technology such nuclides will fission to short-lived and Corresponding author. Tel.: +86-21-34205684; fax: +86-21-34205182 E-mail address: [email protected].

1876-6102 © 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license.

Selection and peer-review under responsibility of Institute of Nuclear and New Energy Technology, Tsinghua University doi:10.1016/j.egypro.2013.07.197

Wei Yuezhou et al. / Energy Procedia 39 (2013) 110 – 119

finally stable nuclides in an advanced nuclear reactor or transmutation system. Then, the remaining radionuclides in the waste may decay within a few centuries to the level of natural uranium instead of more than 106 years [1]. Furthermore, the separation and recovery of some fission product elements (FPs) will give a beneficial impact on the HLLW management and valuable resource utilization. Representative valuable FPs are the platinum group metals (PGM = Pd, Ru and Rh) whose most nuclides are stable or relatively short lived. Such PGM may be potentially used as industrial catalyst or electronic material. In addition, 137Cs and 99Sr with half-life less than 30 years will be possibly utilized as radiation or heat emitting sources. Although a number of partitioning processes have been proposed and studied to separate the MA and some FPs from nuclear wastes, most of these processes essentially utilize liquid-liquid extraction technology by using a mixture of organic extractant and hydrocarbon diluents [2-4]. A large amount of the secondary waste, which is difficult for treatment and disposal of, will be generated by the extraction process. In addition, the liquid-liquid extraction process is basically only advantageous for a large-scale process. Compared to U and Pu, the MA and FPs are significantly less abundant in the spent fuel, so that the scale of an efficient partitioning process for nuclear wastes should be reasonably small and result in less waste amount. To separate the minor actinides and some specific FPs such as Tc, Cs, Sr and the PGM from nuclear waste solutions, we have been studying an advanced aqueous partitioning process, which uses extraction chromatography and anion exchange, i.e. selective adsorption as separation method. For this process, we prepared several novel chelating adsorbents and anion exchangers which are immobilized in a porous silica/polymer composite particles (SiO2-P). These new type of resins is characterized by fast diffusion kinetics, improved chemical stability and low pressure drop in a packed column. Adsorption and separation behavior of various elements contained in HLLW was studied experimentally. Small scale separation tests using simulated and genuine waste solutions were carried out to verify the feasibility of the proposed process. 2.

Outline of the proposed separation process

We have proposed an advanced partitioning process based on column separation technique using silicabased novel adsorbents and the flowsheet is shown in Figure 1. The process consists of three chromatographic columns for the target elements separation from HLLW, respectively. The first column is packed with the adsorbent containing a mixture of calix-crown (Calix[4]arene-R14) and crown ether (DtBuCH18C6) for the removal of Cs and Sr. These super-molecular compounds have highly selectivity for Cs and Sr in concentrated nitric acid solution through ionic size fitting effect. In our previous studies, it was found that Cs and Sr could be effectively separated from simulated HLLW containing 2-4M HNO3 by using silca-based super-molecular adsorbents [5]. The second column is packed with an R-BTP(2,6bis(5,6-dialkyl-1,2,4-triazine-3-yl)pyridine) adsorbent such as isoHexyl-BTP/SiO2-P. In this column, MA (Am and Cm) can be separated from various FP elements, because R-BTP shows high adsorption selectivity for MA over most FPs including trivalent lanthanides. In addition, since an R-BTP adsorbent exhibits significantly strong adsorption for Pd in a broad range of nitric acid concentration [6], Pd is expected to be selectively recovered from the adsorbent and separated from the MA. The effluent from the second column is then applied to the third column packed with an anion exchanger named AR-01 to achieve effective separation of Tc and possibly Ru/Rh from other FPs in HLLW based on their different adsorption and desorption behavior.

111

112


HLLW Washing (3M HNO3) Cs-Sr elution (H2O) Calix-crown/ Crown ether

FPs elution (1-3M HNO3) MA elution (H2O or DTPA) Pd elution (thiourea)

Washing (1-3M HNO3) Ru-Rh elution (H2O) Tc elution (U(IV), etc.)

Cs/Sr adsorbent column

Other Elements

Cs/Sr

R-BTP adsorbent column

FPs Am/Cm Pd

AR-01 resin column

Other FPs Ru/Rh Tc

Figure 1 Schematic flowsheet of the partitioning process for HLLW by chromatographic separation.

3.

Materials and methods

We prepared several novel chelating adsorbents and anion exchangers which are immobilized in a porous silica/polymer composite particles (SiO2-P) developed in our previous work [7]. SiO2-P contains a macroreticular styrene-divinylbenzene copolymer which is immobilized in porous silica particles with pore size of 0.6 μm, pore fraction of 0.69 and mean diameter of 60 μm. Figure 2 shows the molecular structures and photos of these adsorbents. A silica-based anion exchanger, AR-01, was synthesized for the separation study of Tc and Ru/Rh from HLLW. In AR-01, a macro-reticular organic resin was embedded into the pores of the silica particles. It contains N-methyl-benzimidazole (weak-base group) and N,N’methyl-benzimidazolium (strong-base group) as functional groups. The total exchange capacity is 3.4 eq/kg-resin and the strong-base capacity is 2.0 eq/kg-resin. For the separation of MA(Am, Cm), novel silica-based R-BTP adsorbents were prepared by impregnating an R-BTP ligand into the SiO2-P support. In this work, 2,6-bis(5,6-dibutyl-1,2,4-triazine-3-yl)pyridine (nButyl-BTP) and 2,6-bis(5,6-bis(4-methyl pentyl)-1,2,4-triazin-3-yl)pyridine (isoHexyl-BRP) were selected as the ligands, respectively. To separate Cs(I) and Sr(II) from HLLW, we prepared two novel silica-based adsorbents by impregnating the macrocyclic compounds, 1,3-[(2,4-diethyl-heptyl-ethoxy) oxy]-2,4-crown-6-calix[4] arene (Calix[4]arene-R14), and 4,4 ,(5 )-di-(tert-butyl-cyclohexano)-18-crown-6 (DtBuCH 18C6) into the SiO2P support. These new type of adsorbents is characterized by fast adsorption kinetics, improved chemical stability and low pressure drop in a packed column. Adsorption characteristics of metal ions onto the adsorbents were evaluated with distribution coefficient (Kd) which was measured by a batch adsorption experiment. The leakage behavior

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measurement of R-BTP ligand from the adsorbent into aqueous solution was carried out in a batch contact system. Separation behavior of the specific elements from simulated and genuine HLLW was investigated by means of column experiment using an adsorbent packed glass column. The details of experimental procedures were described in our previous articles [7, 8]. CH3 N

R

N

N R

(a) AR-01 anion exchanger

O

O

R

N

R

N

N

CH3

O

N N

(b) R-BTP O

O O

O O O

N

O O

O

O

O

O O

(c) Calix-arene R14

(e) SEM image of SiO2-P support

(d) DtBuCH18C6

25 μm (f) SEM image of R-BTP/SiO2-P

Figure 2 Molecular structures and SEM image of silica-based novel adsorbents.

4.

Results and discussion

4.1 Separation of minor actinides Mutual separation of the trivalent actinides (Am and Cm) and lanthanides is one of the most difficult tasks in nuclear fuel cycle due to their similarly chemical properties. In recent years, Kolarik et al. discovered that a new type of nitrogen-donor extractant, 2,6-bistriazinyl pyridine (BTP) shows high selectivity for Am(III) over Ln(III) [9,10]. Since 2000 we have synthesized some R-BTP compounds with different alkyl groups (C=1-8) and investigated their separation properties for MA/Ln and chemical/radiation stability in nitric acid medium [11-13]. We found that the adsorption behavior and chemical stability of R-BTP depends strongly on the length and structure of alkyl group. Among the RBTP family, isoHexyl-BRP/ SiO2-P adsorbent showed significantly strong adsorbability for Am(III) over Ln(III) in quite high concentration of HNO3 (up to 4M). The separation factor for Am/Eu-Gd was around 100 at 2-3M HNO3. Furthermore, the leakage of isoHexyl-BTP from the solid phase into nitric acid solution was negligibly slight. These results indicated that it would be applicable for the MA direct separation from a strongly acidic waste effluent. Figure 3 illustrates the distribution coefficients (Kd) of Am(III), Ln(III) and other typical FP ions contained in HLLW onto isoHexyl-BTP/SiO2-P adsorbent as a function of HNO3 concentration[6]. As can be seen, the adsorbent showed almost no adsorption towards the light rare earth elements such as Y(III), Ce(III), Nd(III) as well as Cs(I), Sr(II), Mo(VI), Zr(IV) and Rh(III) at the experimental HNO3 concentrations(0.1-4M) and their Kd values were less than 5 cm3/g. The adsorption of Am(III) increased

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dramatically with increasing HNO3 concentration from 1 M and the Kd values reached near 104 cm3/g at 3M HNO3. The medium-heavy rare earths, i.e. Eu(III) and Dy(III) as well as Ru(III) exhibited weak adsorption onto the adsorbent. The separation factor between Am(III) and Ce(III)-Eu(III) is as high as 100-1000 in 2-3 M HNO3, which would be sufficiently high for effective separation. Notably, since Am(III) showed almost no adsorption in very dilute HNO3, the adsorbed Am is expected to be eluted out with decreasing HNO3 concentration in chromatographic separation process.

Kd (cm3/g)

5

10 4 10 3 10 2 10 1 10 0 10 -1 10 -2 10

Pd Sr Y Zr Mo Ru Rh Pd Cs Ce Eu Dy Am

Am

0

1

2

3

4

5

Concentration of HNO3 (mol/L) Figure 3 Effect of nitric acid concentration on elements adsorption onto isoHexyl-BTP/SiO2-P adsorbent.

To examine the mutual separation of trivalent MA from lanthanides, a hot test using a genuine MALn effluent was carried out using a nButyl-BTP/SiO2-P packed Pyrex-glass column with 10 mm inner diameter and 300 mm in length. Prior to the separation of MA/Ln, the HLLW sample was supplied to a CMPO/SiO2-P packed column for the elemental group partitioning. Then a genuine MA-Ln effluent containing Am, Cm and almost all of the FP lanthanides was obtained [14]. NaNO3 was added into the MA-Ln effluent to the concentration of 1 M, to enhance the adsorption of MA. The pH value in the effluent was around 1. Figure 4 shows the results of separation experiment for the genuine MA containing effluent, illustrating the elution profiles of the typical nuclides, 241Am, 244Cm, 237Np, 141Pr and 145Nd. As can be seen, the Ln represented by Nd and Pr showed almost no adsorption. These elements flowed out with the sample solution and the 0.3 M NaNO3 washing solution. Np was also not adsorbed and behaved similarly to the Ln. On the other hand, Am and Cm were strongly adsorbed by the nButyl-BTP/SiO2-P adsorbent, and then were effectively eluted off by using pure water as an eluent. A complete separation between Am-Cm and the Ln was achieved. R-BTP is a neutral compound and the adsorption reaction of trivalent metal ions such as Am(III) and Cm(III) from nitrate solution onto the adsorbent is presumed as: M3+ + 3NO3- + x(R-BTP)res = [M(NO3)3 (R-BTP)x]res

(1)

Since the stoichiometric ratio of M : NO3- is 1 : 3, the adsorption affinity of trivalent MA strongly depends on the concentration of nitrate ion (NO3-) surrounding the extractant in the adsorbent as shown in Figure 3. Therefore the adsorbed MA could be eluted easily from the adsorbent by supplying water to the column, corresponding to the decrease of nitrate concentration in the adsorbent bed. Chemical stability of R-BTP/SiO2-P adsorbents is a serious concern for the HLLW partitioning process. We have examined and reported some properties related to the stability [15]. Table 1 shows the

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leakage behavior of isoHexyl-BTP/SiO2-P with contact in 3M HNO3 till 120 days. The concentration of isoHexyl-BTP leaked into the aqueous solution from the adsorbent was calculated from the TOC (total organic carbon) measurement. So it is considered to result from the leaching and/or decomposition of isoHexyl-BTP contained in the adsorbent. As seen, the leakage amount of isoHexyl-BTP increased only slightly with contact time till 120 days, revealing that the adsorbent is relatively stable in 3M HNO3 during a long term. In addition, our previous evaluation results indicated that its adsorbability for RE (Eu, Gd and Dy) were steadily maintained at an equilibrium state in 3M HNO3 solution for 21 days [16], reflecting its good resistance against nitric acid. On the other hand, since HLLW contains strong - and -emitters, the resistance of the R-BTP adsorbents against radiation needs to be evaluated in detail. A

B

C

30

D

Concentration / g g

25

0.6

-1

-1

0.7

145

0.5

20

Nd

15

0.4 0.3

241

141

Pr

Am

10

0.2 244

5

Cm

0.1

Concentration / g g

0.8

237

Np

0 0

50

100 150 Effluent weight / g

200

0

Figure 4 Results of separation test for the genuine MA effluent by nButyl-BTP/SiO2-P packed column (A: dead volume, B: sample solution, C: 0.3 M NaNO3, D: H2O).

Table 1. Leakage behavior of isohexyl-BTP from isohexyl-BTP/SiO2-P contacted with 3 M HNO3. Contact time (d)

Concentration of leaked isohexyl-BTP (ppm)

Percentage of leaked isohexyl-BTP (%)

30

138.0

0.8

60

251.1

1.5

90

282.9

1.7

120

328.0

2.0

4.2 Separation of palladium Pd and other PGM namely Ru and Rh are produced as man-made precious metals after nuclear fissions. At a burn up of 33 GWd/t, one metric ton of spent fuel contains more than 1 kg Pd. This would be a considerable addition to the yield from natural sources [17]. Fission Pd is comprised of isotopes (wt%): Pd-104 (17), Pd-105 (29), Pd-106 (21), Pd-107 (17), Pd-108 (12), and Pd-110 (4). Among them,

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Pd-107 is the only one radioactive isotope with a half-life of 6.5×106 years. Since the maximum energy of -ray emitted by Pd-107 is 35 KeV and the radiation intensity at the surface of fission Pd is only merely 520 Bq·cm-2. Thus, if recovered, its week radioactivity can be tolerated for many industrial applications. On the other hand, In the process of vitreous solidification for HLLW, PGM are harmful, and so far, Rokasho reprocessing plant in Japan has been compelled to stop running because they formed separate phases, causing deterioration in the stability of the glass, which is very adverse to the storage of vitreous solidification. Based on these facts, to separate and recover PGM, especially Pd from HLLW is desirable. As shown in Figure 4, Pd(II) presented significantly strong adsorption even at very low acid concentration and the Kd values were much higher than Am(III) and other FPs. The adsorption behavior is characterized by the strong adsorption in a broad range of HNO3 concentration. Pd(II) is well kwon as a “soft” metal ion which has strong ability of electron acceptance. Therefore the unique adsorption of Pd(II) is presumed to result from the complexation reaction between Pd(II) ions and the R-BTP adsorbent which contains the nitrogen atoms bearing lone-pair electrons. Because the adsorbed Pd(II) could not be eluted from the adsorbent by changing HNO3 concentration, a complexing agent, thiourea (Tu: CH4N2S) was attempted to desorb the Pd(II). Figure 5 shows the desorption behavior of Pd(II) with 0.01 and 0.1M thiourea (Tu: CH4N2S) solution at 25 and 50 ºC- [18]. The most fraction of Pd can be desorbed with 0.01M CH4N2S at 50 °C. Tu is well known to have a strong complexation affinity with Pd(II). The desorption of Pd(II) is considered to result from the ligand substitution reaction between the Pd(II) complex in the adsorbent and Tu as the following possible reactions: +

[Pd(NO3)2 (R-BTP)x]ads + 4Tu + 2H = x(R-BTP)·2HNO3 + Pd(Tu)4

2+

(2)

The experimental results indicate that Pd can be selectively removed from the isohexyl-BTP adsorbent with thiourea solution as eluent and separated with Am and other FP elements.

Desorption (%)

100 80 60 0.1 M CH4N2S, 50ºC 0.1 M CH4N2S, 25ºC

40

0.01 M CH4N2S, 50ºC 0.1 M CH4N2S, 25ºC 0.01 M CH4N2S, 25ºC

20 0

0

2 4 6 Desorption time (h)

8

Figure 5 Desorption of Pd(II) with 0.01 and 0.1 M thiourea (CH4N2S) solution at 25 and 50 ºC.

4.3 Separation of technetium In our previous study, it was found that in acidic solution Tc(VII) strongly adsorbed onto AR-01 anion exchanger as the oxoanionic form of TcO4- [19]. However, since the adsorption affinity of Tc(VII) increases with the decrease of HNO3 concentration, it could not be eluted from the anion exchanger by


using a diluted HNO3. Conversely, a concentrated HNO3 is generally attempted to elute the adsorbed Tc(VII). However, concentrated HNO3 with strong oxidation ability will result in damage to the resin, so that the concentration of HNO3 as eluent could not be very high and the elution efficiency was always low. In this work, we examined the reductive elution of Tc(VII) from AR-01 anion exchanger by using U(IV) as a reducing agent by both batch and column elution experiments. Figure 6 illustrates the batch elution experiment results of Tc(VII) adsorbed in AR-01 by using different concentrations of U(IV) in HNO3-N2H4 solution. It was found that the elution rate of Tc(VII) increased significantly with the increase of U(IV) concentration. Furthermore, the elution rate was accelerated by increasing the nitric acid concentration in the U(IV) solution. It can be concluded that Tc(VII) was quantitatively reduced to lower oxidation states such as Tc(IV) and/or Tc(II), which exhibit no or very weak adsorption on anion exchanger [19].

80

Desorption ratio of

99

TC / %

100

60 40 20 0

0

10

U(IV) conc. :

20 30 40 Desorption time / min 10

20

50

50

60 100

99

Feed : [ Tc]=1mM, [HNO ]=6M 3

Eluent : [U(IV)]=1~100mM, [HNO ]=1M, [N H ]=1~100mM 3

2

4

Figure 6 Batch desorption results of Tc(VII) adsorbed in AR-01 using U(IV) as reductive eluent.

Figure 7 shows the column adsorption and desorption experiment results by using 100mM Tc(VII)3M HNO3 as feed solution and U(IV)-6M HNO3 as reductive eluent. As seen, Tc(VII) was completely adsorbed by AR-01 anion exchange resin. Then, it was effectively eluted from the resin with supplying the U(IV) solution into the column and the recovery rate was near 90% under the experiment conditions. As seen in Figure 7, most amount of U was separated from the eluted Tc except a small portion of U(VI) which was converted from U(IV). In another column experiment, it was found that near 100% of Tc could be recovered by using 80mMU(IV)-6M HNO3 as eluent and the separation performance between Tc and U was also improved significantly [20]. The redox reactions between Tc(VII) and U(IV) in nitric acid medium are reportedly very complicated [21]. The following one is presumed to be a representative reaction elution process. The reaction mechanism needs to be investigated in detail in future work. TcO4- + U4+ + 2H+ + 2NO3- = TcO(NO3)2 + UO22+ + H2O (3)

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Wei Yuezhou et al. / Energy Procedia 39 (2013) 110 – 119 250

0.5

Tc concentration / mM

Dead volume Feed

6M-HNO

3

100mM-U(IV) / 3M-HNO

3

H O 2

200

0.4 U(IV) 0.3

Tc Tc recoverly : 89.0%

150

0.2

100

0.1

50

0

U(IV) concentration / mM

118

0

Figure 7 Column adsorption and desorption experiment results of Tc(VII) using U(IV) as reductive eluent.

5.

Conclusions

To separate the long-lived minor actinides and specific fission products (FPs: Tc, Pd, Cs, Sr, etc.) from HLLW, we have investigated an advanced partitioning process based on column separation technique using some porous silica-based adsorbents and obtained the following results. Am(III) presented significantly high adsorbability and selectivity onto R-BTP/SiO2-P adsorbents over most FPs including Ln(III). The separation factors of Am (III)/Ce(III)-Eu(III) were as high as 100-1000 in 2-3 M HNO3. The hot test results using a genuine MA-Ln effluent reveal that a complete separation between MA and Ln was achieved. The R-BTP adsorbents were fairly stable in 3M HNO3, but its stability against radiation needs to be evaluated in detail. Pd(II) was strongly adsorbed by R-BTP/SiO2-P adsorbents in a broad range of HNO3 concentration due to complexation reaction. Pd could be selectively desorbed from the adsorbent with thiourea solution and is expected to separate with Am and other FPs. Tc(VII) exhibited strong adsorption onto AR-01anion exchanger and could be eluted off by using U(IV)-HNO3 solution as a reductive eluent. A novel partitioning process consisting of three separation columns for the target elements separation from HLLW was proposed and the experimental results indicated that the process is essentially feasible. Acknowledgements A part of this study is the results of “Development of a Simplified MA Separation Process Using Novel R-BTP Adsorbents” carried out under the Strategic Promotion Program for Basic Nuclear Research by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). A part of this work was supported by the National Natural Science Foundation of China (91026019, 91126006 ), which are gratefully acknowledged. References [1] Magill J, Berthou V, Haas D. Impact limits of partitioning and transmutation scenarios on the radiotoxicity of actinides in radioactive waste. Nucl. Energy, 2003;42:263–277 . [2] Madic C. Overview of the hydrometallurgical and pyrometallurgical processes studied world-wide for the partitioning of high active nuclear wastes. Proc. JAERI-Conf. 2002-004, Tokai, Japan, Oct. 31-Nov. 2, 2001, 27-37. [3] Kolarik Z. Current european research on the separation of actinides from high-level radioactive wastes. J Nucl Fuel Cycle


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