Extraction Chromatographic Separation of Trivalent Minor Actinides

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Chapter 11

Extraction Chromatographic Separation of Trivalent Minor Actinides Using iHex-BTP/SiO2-P Resin N. Surugaya,* Y. Sano, M. Yamamoto, A. Kurosawa, and T. Hiyama Japan Atomic Energy Agency, 4-33 Muramatsu, Tokai, Ibaraki 319-1194, Japan *[email protected]

The separation of trivalent minor actinides (Am(III) and Cm(III)) from lanthanides is a challenging issue because of difficulties associated with their mutual similarity in chemical behavior. However, this separation is important for the future nuclear fuel cycle for environmental friendship. Extraction chromatographic separation of the long-lived trivalent minor actinides has been performed to study the potential application of 2,6-bis(5,6-di-iso-hexyl-1,2,4-triazin-3-yl)-pyridine (iHex-BTP) impregnated into a porous silica support coated with styrene-divinylbenzene copolymer (SiO2-P). The adsorption and elution characteristics of Am(III) and Cm(III) in nitric acid medium have been investigated to separate them from lanthanides using a column packed with iHex-BTP/SiO2-P resin. Depending on the concentration of nitric acid solution as well as aqueous flow rate, a certain condition allowed us to selectively recover the fractions containing Am(III) and Cm(III) from a sample solution derived from the PUREX raffinate.

Introduction Trivalent minor actinides like Am(III) and Cm(III) are present in nuclear wastes generated from spent nuclear fuel reprocessing activities. Their separation is important because the minor actinides with long half-life and high radiological toxicity are highly abundant in irradiated nuclear fuel (1, 2). Nowadays group partitioning used to reprocess spent nuclear fuels through a number of © 2010 American Chemical Society In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


stages as shown in Figure 1 is under investigation for future nuclear cycle in Japan. Uranium in dissolved spent nuclear fuels is coarsely separated by crystallization. Uranium, plutonium and neptunium are co-recovered by the extraction with tri-butyl phosphate (TBP). The minor actinides are separated from lanthanides and fission products by extraction chromatography (3, 4) using extractants like octylphenyl-N,N-diisobutylcarbomoyl phosphine oxide (CMPO), bis triazinyl pyridine (BTP), N,N,N′,N′-tetraoctyl diglycolamide (TODGA), and di-(2-ethylhexyl) phosphoric acid (HDEHP). Americium and curium are mutually separated using ion exchange. Currently, extraction chromatography is being investigated for the separation of minor actinides from lanthanides by combination utilization of several ligands (Figure 1). Extraction chromatography offers several advantages over conventional liquid-liquid extraction: less organic material is required, no third phase formation occurs, phase modifiers are not necessary, the equipment is simple and compact, and a rapid operation is possible. Therefore, extraction chromatography may be a suitable option for the separation of minor actinides from lanthanides as a part of advanced nuclear fuel cycle. The mutual separation of minor actinides and lanthanides is generally very difficult due to the similarity in their chemical behavior. Some potential extractants have been proposed to facilitate the separation of Am and Cm from lanthanides. In 1999 Kolarik et al. firstly reported that BTP is one of the strongest candidates (5, 6). The BTP containing nitrogen atom, which is a soft donor, has a greater extraction selectivity for 5f elements compared to 4f elements through the covalent coordination (7–11). Recent studies have been performed using 2,6-bis-(5,6-dialkyl-1,2,4-triazine-3-yl)-pyridine (R-BTP) derivatives like 2,6-bis-(5,6-dibutyl-1,2,4-triazine-3-yl)-pyridine (nBu-BTP) (12) and its isomer i-Bu-BTP (13). In this study, we investigated the performance of 2,6-bis(5,6-di-i-hexyl-1,2,4-triazin-3-yl)-pyridine (iHex-BTP), whose longer alkyl side-chains might facilitate the selective separation of minor actinides (Figure 2) and decrease its solubility in nitric acid. A series of preliminary experiments using iHex-BTP in liquid-liquid extraction systems actually suggested that Am and Cm have high distribution ratios and separation factors from lanthanides in 1-3 M nitric acid media. In this paper, iHex-BTP was impregnated into a porous silica support coated with styrene-divinylbenzene copolymer (SiO2-P) for the extraction chromatographic separation. The resulting iHex-BTP/SiO2-P resin has been used for a series of separation tests for Am and Cm existing in a real sample of highly radioactive liquid waste generated by the current PUREX process. This paper describes the experimental results and the adsorption characteristics of the iHex-BTP/SiO2-P resin based on the extraction chromatography using the sample solutions containing 1 and 3M nitric acid media.

132 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 1. An advanced separation process for nuclear fuel cycle planed for the future in Japan (Extraction chromatography is under investigation as an option for separation of An(III) from Ln using several ligands).

Figure 2. 2,6-bis(5,6-di-iso-hexyl-1,2,4-triazin-3-yl)-pyridine (iHex-BTP).


Experimental


Materials The SiO2-P resin was prepared according to the published procedures (2, 3) using spherical silica particles with a diameter of 40-60 µm, a mean pore size of 600 nm, and a pore fraction of 0.69. The iHex-BTP extractant obtained from the Institute of Research and Innovation was impregnated into the SiO2-P resin based on the reported procedure. The extractant was placed in a glass flask and diluted with dichloromethane as a diluent. Subsequently, the dried SiO2-P particles were added to the solution. The mixture was then shaken mechanically for 2h at room temperature. The diluent was evaporated under reduced pressure and the residue was finally dried up in vacuo. The weight ratio of iHex-BTP to SiO2-P resin was approximately 1:2. The iHex-BTP/SiO2-P resin was transferred into a φ0.8 cm × h4 cm Bio-Rad poly-prep column with a bed volume of 2 mL. All commercially available chemicals like nitric acid were of analytical grade and were used without further purification. The water used for buffer preparation was deionized, sub-boiled, distilled, and further purified using the Milli-Q academic purification system. Radiotracers, 241Am and 244Cm, for analytical calibration were obtained from laboratory stocks. Stock standard solutions were prepared from reagent grade chemicals obtained from Wako Chemical Co., Inc. for other metal ions. The nuclear waste sample was derived from a PUREX raffinate, which was generated at the Tokai Reprocessing Plant of the Japan Atomic Energy Agency. The reprocessed spent fuels originated from the advanced thermal reactor FUGEN (Burn-up = 17.2 GWd/t, cooling time ≈ 2800 d). Major fission product (FP) elements selected in the sample included La, Ce, Nd, Sm, Eu, Gd, Sr, Y, Zr, Mo, Ru, Pd, and Ba, which were dissolved in 3 M nitric acid medium. The concentrations of Am and Cm were 35 and 0.56 mg/L, respectively. The highly radioactive sample was treated in shielded analytical facilities. Separation Tests The column packed with iHex-BTP/SiO2-P resin was first conditioned using a nitric acid solution (1 or 3 M) at a volume that was 30 times greater than the bed volume. Then, a sample solution of 1 mL was introduced into the packed column using a custom made pipetting device. The nitric acid concentration was about 3 M in the original sample. Sample loading was carried out using the original sample or its diluted solution, which was adjusted to 1 M by adding 0.01 M nitric acid. Taking advantage of the high affinity and selectivity of the iHex-BTP stationary phase for trivalent minor actinide species in nitric acid media, 3 or 1 M nitric acid (8 mL) was passed through the column to wash off the lanthanides and other fission products. Water (15 mL) was subsequently added to elute Am and Cm. All the fractions were collected as 0.5 mL in the course of the experiment. The chromatographic separations were performed at room temperature by gravity flow with no pressure added. The flow rate was about 0.1-0.4 mL/min. Each collected fraction was diluted with 1 M nitric acid (500-5000-fold dilution) for subsequent measurements. An Elan 6000 inductively coupled plasma mass 134 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

spectrometer (Perkin Elmer) was used to analyze 241Am and 244Cm. An SPS 7700 inductively coupled plasma atomic emission spectrometer (Seiko Instrument) was used to analyze other elements. Each instrument was installed in a containment glove-box for handling radioactive sample solutions.

Results and Discussion


Extraction Chromatographic Behavior and Acid Dependence of Extraction Chromatographic Separation The chromatographic separation profiles of Am, Cm, and other selected FP elements in the sample are shown in Figure 3a. The nitric acid concentration was 3 M in the sample and washing solution, and the flow rate was 0.3-0.4 mL/min. The relative elemental concentrations determined in each fraction with respect to the initial sample, C/C0, were plotted as a function of effluent volume. The plots show that almost all elements, except Am, Cm and a part of Sm and Eu, were eluted with 3 M nitric acid during the column wash step, while Am and Cm were eluted immediately with addition of water. The chromatographic behavior of Am and Cm was quite similar. This experiment also reveals that Sm and Eu appeared both in nitric acid and water fractions, indicating that Am and Cm could be separated from the FP elements, except a part of Sm and Eu. The distribution coefficient Kd of each element was calculated by:

where C0 denotes the initial concentration of element in the sample. Csi is the concentration of element in ith eluted fraction (1 or 3 M nitric acid); V is the volume of aqueous phase and W is the weight of dried resin. Kd values calculated for selected representative elements are listed in Table 1. The separation factor between minor actinides (Am, Cm) and the adsorptive lanthanides (Sm, Eu) which was calculated as the ratio of their Kd values, was about 10; this is not high enough to separate them completely. It is also noted that Pd adsorbed strongly on the resin and was not eluted in water. A similar experiment was carried out for the sample solution containing 1 M nitric acid and 1 M nitric acid was used as washing solution at a flow rate of about 0.1 mL/min (Figure 3b). This flow rate was slower than in the previous experiment, because the flow rate control was unreliable in gravity experiments. In this experiment, Am and Cm were eluted with addition of water and the water fractions did not contain Sm and Eu. In addition, Pd was not detected in the original sample in this experiment. This fortuitous observation may indicate the potential of Pd precipitation under these experimental conditions and/or under sample storage circumstances in the hot cell. Moreover, Ru concentration was not determined because of analytical problems. Despite missing some experimental data, Kd values for the analyzed elements are summarized in Table 1. The separation factor between Am/Cm and other elements, including Sm and Eu, was found to be about 101 to 102. As the result, a large number of the coexisting matrix 135 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


elements showed no or little effect on the separation, and they were separated from Am and Cm fractions. Excellently selective separation and recovery of Am and Cm was therefore achieved by adjusting the nitric acid concentration to 1 M using extraction chromatography with iHex-BTP/SiO2-P resin.

Figure 3. Extraction chromatographic profiles of selected elements with iHex-BTP/SiO2-P resin. Sample: PUREX raffinate, temperature: 25°C, nitric acid concentration: (a) 3 M, and (b) 1M, flow rate: (a) 0.3-0.4 mL/min. and (b) 0.1 mL/min. 136 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Table 1. Distribution coefficients, Kd, of selected elements obtained from extraction chromatographic separation with iHex-BTP/SiO2-P resin at different nitric acid concentrations using a PUREX raffinate samplea


Element

a

Distribution coefficient, Kd at 3 M

at 1 M

Sr

44

83

Y

48

71

Zr

65

71

Mo

71

85

Ru

81

-

Pd

250

-

Ba

57

70

La

53

68

Ce

62

71

Nd

70

70

Sm

161

47

Eu

164

62

Gd

101

72

Am

989

1374

Cm

1608

592

Temperature: 25°C.

Recovery Yield Table 2 shows recovery yields for the elements in the extraction chromatographic separation with the iHex-BTP/SiO2-P resin using the sample solutions containing 3 and 1 M nitric acid, respectively. The recovery yields of Am and Cm were 50-60% which are much less than 100%, suggesting possible hydrolysis of Am and Cm nitrates in elution with water. The use of a very dilute nitric acid solution (0.1-0.01 M) as eluent may be able to prevent the hydrolysis. Moreover, elution of Am, Cm, and other elements using nitric acid or water by gravity flow is subject to slow kinetics, which causes these elements to remain attached to the resin. In addition, radiolytic degradation of iHex-BTP/SiO2-P resin may be one of the reasons for incomplete recovery.


Table 2. Recovery yields of selected elements in extraction chromatographic separation with iHex-BTP/SiO2-P resin using a PUREX raffinate samplea


Element

a

Recovery yield, % at 3 M HNO3 sample

at 1 M HNO3 sample

Sr

115

67

Y

115

76

Zr

76

77

Mo

71

60

Ru

62

-

Pd

26

-

Ba

92

77

La

92

77

Ce

79

58

Nd

73

77

Sm

82

105

Eu

78

94

Gd

53

82

Am

60

50

Cm

58

60

Temperature: 25°C.

Conclusions We demonstrated that extraction chromatography using a column packed with iHex-BTP/SiO2-P resin is a suitable technique for the selective separation and recovery of trivalent minor actinides like Am(III) and Cm(III) from lanthanides existing in highly radioactive nuclear waste. Although more comprehensive experiments are required, the optimal nitric acid concentration in this study was found to be 1 M in the sample solution to achieve high separation factors for Am and Cm from lanthanides. Thus, the nitric acid concentration should be adjusted to 1 M prior to separation. We observed that Am and Cm elution recovery using water was incomplete. Overall, approximately 60% of Am and Cm was recovered in both tests using the sample solutions with 1 and 3 M nitric acid, respectively. Nevertheless, the separation procedures and experimental results described above are useful for further optimizing separation and recovery conditions using the iHex-BTP/SiO2-P resin for extraction chromatography. An unexpected finding was the observation of strong affinity of Pd to the iHex-BTP/SiO2-P resin. Some complexing agents may be useful to remove the adsorbed Pd for repeatedly using the resin packed in the column. 138 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

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Extraction Chromatographic Separation of Trivalent Minor Actinides

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