Solvent Extraction Research and Development, Japan, Vol. 22, No 1, (2015)

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1 Solvent Extraction Research and Development, Japan, Vol. 22, o 1, (2015) Extraction Separation of U and Pu by,-di(2-ethylhexyl)-2,2-dimethylpropanamide (DEHDMPA) and,-di(2-ethylhexyl)butanamide (DEHBA) Using Mixer-Settlers in the Presence of Degradation Products of DEHDMPA and DEHBA Yasutoshi BA,* Shinobu HTKU, Yasuhiro TSUBATA, ao TSUTSUI and Tatsuro MATSUMURA Japan Atomic Energy Agency, Research Group for Partitioning and Transmutation Cycle, uclear Science and Engineering Center, Shirane 2-4, Tokai, , Japan (Received June 15, 2014; Accepted July 29, 2014) A continuous counter-current experiment using mixer-setters was performed to evaluate the separation performance of,-dialkylamides for U and Pu in the presence of their degradation products. The experiment consisted of two cycles: the first cycle employed,-di(2-ethylhexyl)-2,2-dimethylpropanamide (DEHDMPA) for selective extraction of U(VI) and the second cycle employed,-di(2-ethylhexyl)butanamide (DEHBA) for co-extraction of U(VI) and Pu(IV). DEHDMPA extracted most of the U(VI) from the feed, and the ratio of U(VI) recovered from the U fraction was 99.57%. DEHDMPA hardly extracted Pu(IV), and the decontamination factor for U with respect to Pu in the U fraction was The raffinate from the first cycle was used as the feed for the second cycle, and almost all the U(VI) and Pu(IV) were co-extracted by DEHBA. The degradation products of the extractants had no detrimental effect on the two-phase separation and operation of the mixer-settlers. 1. Introduction Solvent extraction has been widely used for nuclear fuel reprocessing and disposal of high-level nuclear waste via hydrometallurgical methods [1-4]. An important task in spent nuclear fuel reprocessing and disposal is the extraction of U and Pu, and extensive studies have been carried out on this extraction. While several methods have been proposed for the extraction, the PUREX process is the only method employed in commercially operating reprocessing plants. Tri-n-butyl phosphate (TBP) is used in the PUREX process to extract U and Pu from spent nuclear fuels dissolved in nitric acid. TBP has high distribution ratios for U(VI) and Pu(IV), and has a high loading capacity for U(VI). Although these specific features make TBP an appropriate extractant for reprocessing, it has a few drawbacks. For example, TBP contains phosphorus, which cannot be decomposed to a gas by incineration, and this will increase the volume of secondary waste. Furthermore, the degradation products of TBP, such as dibutyl phosphate and monobutyl phosphate, can cause the formation of a third phase and/or precipitation [5,6].,-Dialkylamides (monoamides) have been proposed as alternative extractants to overcome the drawbacks of TBP. Since monoamides consist of C, H,, and elements, they can be decomposed into gases by incineration. Some of the monoamides have high distribution ratios for U(VI) and Pu(IV), and have a high loading capacity for U(VI). Furthermore, monoamides can be diluted by hydrocarbons such as n-dodecane, which has already been used in the PUREX process, and can be easily synthesized. Many

2 investigations on monoamides as extractants for reprocessing have been carried out, and their extraction properties for U, Pu, and Th have been revealed in batch experiments [7-9]. Continuous counter-current experiments have also been carried out, and the applicability of monoamides as extractants for recovering U and Pu has been demonstrated [10,11]. Furthermore, as shown below, studies on the radiolysis of monoamides and effects of radiolytically degraded monoamides on solvent extraction have been carried out. Gasparini and Grossi compared the radiolytic stability of,-dihexyloctanamide (DHA, Figure 1(a)) and TBP in the presence of nitric acid by using a 60 Co irradiator, and showed that the stability of DHA is at least similar to that of TBP [12]. Mowafy and Aly conducted batch experiments for U(VI), Th(IV), and Zr(IV) extraction with gamma-ray-irradiated DHA,,-dihexyl-2-ethylhexylhexanamide (DHEHA, Figure 1(b)), and TBP, and compared the extraction behavior of DHA or DHEHA and TBP [13]. Furthermore, Mowafy reported the effect of the absorbed gamma doses on the percentage extraction of Th(IV), Zr(IV), Eu(III), and Am(III) in 3.5 mol/dm 3 (M) nitric acid with a few monoamides [14]. Suzuki et al. irradiated a few monoamides diluted in dodecane with a 60 Co source, and reported that the ratio of degraded monoamides was ca. 15% at an absorbed dose of ca. 500 kgy [9]. Ban et al. investigated radiolysis and extraction properties of branched,-dialkylamides for U(VI) [15]. They irradiated,-di(2-ethylhexyl)-2,2-dimethylpropanamide (DEHDMPA, Figure 1(c)),,-di(2-ethylhexyl)butanamide (DEHBA, Figure 1(d)), and,-di(2-ethylhexyl)isobutanamide (DEHiBA, Figure 1(e)) diluted to ca. 2 M in n-dodecane pre-equilibrated with 5 M nitric acid by gamma-rays from a 137 Cs source. They observed a gradual decrease in the distribution ratio of U(VI) as the integral dose increased, and reported that the degradation ratios of DEHDMPA, DEHBA, and DEHiBA were 22 27% at an integral dose of ca kgy. They carried out batch experiments for extracting U(VI) and fission products in nitric acid by the irradiated monoamides, and showed that the degradation products of the monoamides had hardly any detrimental effects on the selective extraction of U(VI) from simulated fission products except Pd. Furthermore, they detected several degradation products such as (a) (b) (c) (d) (e) Figure 1. Molecular structure of (a),-dihexyloctanamide (DHA), (b),-dihexyl-2-ethylhexan- amide (DHEHA), (c),-di(2-ethylhexyl)-2,2-dimethylpropanamide, (DEHDMPA), (d),-di(2-ethyl- hexyl)butanamide (DEHBA), and (e),-di(2-ethylhexyl)isobutanamide (DEHiBA)

3 (a) (b) (c) (d) (e) Figure 2. Molecular structures of the degradation products of DEHDMPA and DEHBA: (a),-di(2-ethylhexyl)amine (DEHA), (b),-di(2-ethylhexyl)formamide (DEHFA), (c) -(2-ethylhexyl)-2-ethylhexylamide (EHEHA), (d) -(2-ethylhexyl)-2,2-dimethylpropanamide (EHDMPA), and (e) -(2-ethylhexyl)butanamide (EHBA).,-di(2-ethylhexyl)amine (DEHA, Figure 2(a)),,-di(2-ethylhexyl)formamide (DEHFA, Figure 2(b)), -(2-ethylhexyl)-2-ethylhexylamide (EHEHA, Figure 2(c)), -(2-ethylhexyl)-2,2-dimethylpropanamide (EHDMPA, Figure 2(d)), and -(2-ethylhexyl)butanamide (EHBA, Figure 2(e)). The abovementioned studies on the radiolysis of monoamides have mainly evaluated the extraction behavior of metal ions, such as U, Pu, and other fission products by using single-stage batch experiments with radiolytically degraded monoamides. Thus, the effect of the degradation products of monoamides on U and Pu extraction in a multistage system is still unclear. In the present study, continuous counter-current experiments with DEHDMPA and DEHBA using mixer-settlers were performed in the presence of monoamide degradation products observed in a previous study [15], and the separation performance of U and Pu was evaluated. 2. Experimental 2.1 Reagents DEHDMPA, DEHBA, and DEHA were purchased from Wako Pure Chemical Industries, Ltd. (Wako). DEHFA, EHEHA, EHDMPA, and EHBA were purchased from Hitachi Chemical Co., Ltd. A stock solution of the fission products was prepared by mixing a nitric acid solution of Sr( 3 ) 2, Ba( 3 ) 2, Zr( 3 ) 2 2H 2, a 2 Mo 4 2H 2, Ru()( 3 ) 3, Rh( 3 ) 3, Pd metal powder, and d( 3 ) 3 6H 2. The reagents used to prepare the stock solution were purchased from Wako (Sr, Zr, Mo, and d), Kojundo Chemical Laboratory Co., Ltd. (Ba and Pd), and.e. Chemcat Co. (Ru and Rh). These metal elements were chosen as representatives of alkaline earth elements (Sr and Ba), transition elements (Zr and Mo), platinum group elements (Ru, Rh, and Pd), and lanthanides (d). The reagents used in the present study were used without further purification. 2.2 Apparatus Mixer-settlers purchased from Tsunakawa Engineering Co. were used in the continuous counter-current experiments. Each unit of the mixer-settlers consisted of 16 stages, and the volumes of the mixing and settling parts in each stage were 6 cm 3 and 17 cm 3, respectively. A continuous counter-current experiment consisting of two cycles was carried out using the mixer-settlers. 2.3 The first cycle experiment Experimental conditions for the first cycle are shown in Figure 3. The objective of the first cycle was

4 the selective extraction of U, which is intended to produce a U fraction as raw material for uranium oxide fuel. Two mixer-settlers, labeled bank 1 and bank 2, were used in this experiment. Bank 1 consisted of a 4 stage U extraction step and a 12 stage scrub step, and bank 2 consisted of a 16 stage U back-extraction step. DEHDMPA diluted in n-dodecane to 1.4 M was fed to the 1st stage as the organic phase. This organic phase contained DEHA, DEHFA, EHEHA, and EHDMPA as the degradation products of DEHDMPA, and the concentration of each degradation product was 20 mm. A former study reported that the dose absorbed by the solvent was ca. 5 kgy/cycle in a first extraction cycle during the reprocessing of a fast breeder reactor fuel [16]. Another study showed that the degradation ratios of DEHDMPA and DEHBA are at most 0.2 % for an integrated dose of 5 kgy [15]. Thus the total concentration of the degradation products in the organic phase was estimated to be a few millimoles per liter. In order to clearly observe the effects of the degradation products on U and Pu extraction, the concentration of each degradation product employed in the present experiment exceeded the estimated value. The stock solution of the fission products was mixed with nitric acid solutions of U(VI) and Pu(IV), and used as the feed solution after adjusting the concentrations of U, Pu, and nitric acid to 0.85 M, 0.81 mm, and 3.5 M respectively. The feed solution, 3.5 M nitric acid, and 0.01 M nitric acid were fed to the 4th, 16th, and 32nd stages, respectively, as the aqueous phases. The U extraction step was used to extract U(VI) from the feed with DEHDMPA, and the extracted U(VI) was fed to the 17th stage. The U back-extraction step, used for back-extracting U(VI), was performed by contacting the organic phase from bank 1 with 0.01 M nitric acid, and the aqueous phase from the 17th stage was collected as the U fraction. The scrub step was used to back-extract some of the fission products (Sr, Ba, Zr, Mo, Ru, Rh, Pd, and d) and Pu with 3.5 M nitric acid fed to the 16th stage, and the aqueous phase from the 1st stage was collected in a vessel as Raffinate M DEHDMPA containing degradation products of DEHDMPA 135 cm 3 /h Feed 3.5 M H 3 containing U, Pu, and FPs 37 cm 3 /h 3.5 M H 3 53 cm 3 /h Bank 1 U extraction 4 stages Scrub 12 stages (1) (4) (16) Raffinate M H 3 91 cm 3 /h Vessel Bank 2 U fraction U back-extraction 16 stages (17) (32) Used solvent 1 Figure 3. Experimental conditions for the first cycle. Solid and dashed lines represent the aqueous stream and organic stream, respectively. umbers in parentheses are stage numbers. The degradation products of DEHDMPA were DEHA, DEHFA, EHEHA, and EHDMPA, and the concentration of each degradation product was 20 mm

5 2.4 The second cycle experiment Experimental conditions for the second cycle are shown in Figure 4. The objective of the second cycle was co-extraction of U and Pu, which was intended to produce a U Pu fraction as the raw material for mixed oxide fuel. Two mixer-settlers, labeled bank 3 and bank 4, were used in this experiment, same as those used in the first cycle. Bank 3 consisted of an 8 stage U Pu extraction step and an 8 stage scrub step, and bank 4 consisted of a 16 stage U Pu back-extraction step. DEHBA diluted in n-dodecane to 0.6 M was fed as the organic phase to the 33rd stage. This organic phase contained DEHA, DEHFA, EHEHA, and EHBA as the degradation products of DEHBA. The concentration of each degradation product was set to 20 mm for the same reason as given in the previous section. Raffinate-1 reserved in the vessel was fed to the 40th stage as the feed for the second cycle. itric acid solutions of 4.5 M and 0.02 M were fed to the 48th and 64th stages, respectively. The U Pu extraction step was used to extract U(VI) and Pu(IV) with DEHBA, and the scrub step was used to back-extract some of the fission products extracted by DEHBA. The U Pu back-extraction step was used to back-extract U(VI) and Pu(IV) by contacting the organic phase from bank 3 with 0.02 M nitric acid, and the aqueous phase from the 49th stage was collected as the U Pu fraction. 0.6 M DEHBA containing degradation products of DEHBA 91 cm 3 /h Raffinate-1 57 cm 3 /h 4.5 M H 3 42 cm 3 /h Bank 3 U Pu extraction 8 stages Scrub 8 stages Raffinate-2 (33) (40) (48) 0.02 M H 3 85 cm 3 /h Bank 4 U Pu back-extraction 16 stages (49) (64) U Pu fraction Used solvent 2 Figure 4. Experimental conditions for the second cycle. Solid and dashed lines represent the aqueous stream and organic stream, respectively. umbers in parentheses are stage numbers. The degradation products of DEHBA were DEHA, DEHFA, EHEHA, and EHBA, and the concentration of each degradation product was 20 mm. 2.5 Analysis The operation of the mixer-settlers for each cycle was continued for ca. 360 min at room temperature, and then the organic and aqueous phases were sampled from the settling part of each stage after stopping the operation of the mixer-settlers. The concentrations of U in both phases and the fission products in the aqueous phase in banks 1 and 3 were measured with an ICP-AES (ICPS-1000IV, Shimadzu). Prior to the quantitative analysis of U in the organic phase using ICP-AES, a 0.1 cm 3 portion of each organic phase was vigorously shaken with 10 cm 3 of 0.1 M nitric acid to back-extract U. The concentrations of Pu in both

6 phases were measured with an alpha-ray spectrometer with a silicone surface barrier detector (CTETE, RTEC). The acid concentration in each sample was measured with a potentiometric titrator (Titrino 798, Metrohm) [17]. 3. Results and Discussion either precipitation nor a third phase was observed at any stage in banks 1 4. The mixture of the organic and aqueous phases clearly separated into two phases at the settling parts in banks 1 and 3, while small aqueous drops were observed in the organic phases at the settling parts in banks 2 and 4. The aqueous drops disappeared after the operation of the mixer-settlers was stopped, and the organic samples taken from the settling part of banks 2 and 4 did not contain any aqueous phase. These phenomena were similar to the ones observed in previous experiments performed without adding degradation products of monoamides [10,11]. These results indicated that the degradation products used in the present study did not have a detrimental effect on the two-phase separation in the settling parts of the mixer-settlers. The concentration profiles of U, Pu, and acid in each stage observed in the first cycle are shown in Figure 5. In the U extraction step, the concentrations of U rapidly increased with the stage number, while those of Pu did not show a remarkable change. In the scrub step of bank 1, the concentrations of U were nearly constant, while those of Pu gradually decreased with the stage number. As a result, the concentrations of Pu in the 15th 32nd stages were on the order of 10-8 M or undetectably low. These results indicate that DEHDMPA selectively extracted U(VI) under the present experimental conditions, and the scrub step in bank 1 effectively worked to back-extract Pu(IV) extracted by DEHDMPA in the U extraction step. The concentration of nitric acid in bank 2 rapidly decreased with the stage number, and accordingly a rapid decrease in the U concentration was observed in this bank. The U concentrations in the 21st 32nd stages for the organic phase and 23rd 32nd stages for the aqueous phase were undetectably low, and thus there are no data points in these stages. U conc. [M] Pu conc. [M] Acid conc. [M] U extraction Bank 1 Scrub rg. phase Bank 2 U back-extraction rg. phase rg. phase Stage number Figure 5. Concentrations of U, Pu, and nitric acid in each stage for the first cycle. The solid and open symbols represent the aqueous and organic phases, respectively. Acid conc. [M] Pu conc. [M] U conc. [M] Bank U-Pu extractio

7 U conc. [M] The concentration profiles of U, Pu, Bank 1 Bank 2 and acid in each stage of the second cycle are U extraction shown in Figure 6. Scrub The concentrations U back-extraction of U and Pu in the organic phase in the U Pu extraction step sharply increased with the stage number, indicating that DEHBA effectively extracted U and Pu. The concentrations of U in the U Pu extraction step were considerably rg. phase lower than those of the fission products. This Pu conc. [M] made 10it -3 impossible to quantitatively analyze U by ICP-AES, and no data for U are rg. availa- phase ble in the 33rd 38th stages. The concentrations of U in the U Pu back-extraction step sharply decreased with the stage number, and were undetectable after the 50th stage in the organic 10 1 phase and 52nd stage in the aqueous phase. The concentrations of Pu in the rg. U Pu phase back-extraction step decreased gradually, and Acid conc. [M] reached ca M at the 64th stage Table 1 shows the recoveries of U and Pu in each stream. The recoveries indicate the respective 1 ratio of 8 U and Pu 16in each 24 outgoing 32 stream to the total U Stage and number Pu amounts outgoing from banks 2 4. The recovery of U in the U fraction was 99.57%, and that of Pu in this fraction was 0.017%. From these results, a value of was obtained as the decontamination factor of U with respect to Pu in the U fraction. The recoveries of U and Pu in the U Pu fraction were, respectively, 0.43% and 99.92%. These results showed the validity of the present separation process for efficient decontamination of U from Pu and recovering Pu. Furthermore, the ratio of U to Pu in the U Pu fraction was obtained as 8.1. This indicates that Pu was not isolated, and agrees with the non-proliferation of nuclear material. The fission products employed in the present experiment were only slightly extracted in banks 1 and 2, and their concentrations decreased to below the detection limit by the last step of bank 4. U conc. [M] Pu conc. [M] Acid conc. [M] Bank 3 U-Pu extraction Scrub Bank 4 U-Pu back-extraction rg. phase rg. phase rg. phase Stage number Figure 6. Concentrations of U, Pu, and nitric acid in each stage for the second cycle. The solid and open symbols represent the aqueous and organic phases, respectively. Table 1. Recoveries of U and Pu in each stream. Stream U % Pu % U fraction U Pu fraction Raffinate Used solvent Used solvent

8 The important point to note is that the present experiments were carried out in the presence of degradation products. Under these conditions, the present process successfully extracted U and Pu and distributed them to the U and U Pu fractions. Furthermore, no detrimental effects on the two-phase separation, such as third phase formation or precipitation, were observed. This confirmed that the degradation products have little effect on the recovery of U and Pu from nitric acid by DEHDMPA and DEHBA under the present experimental conditions. 4. Conclusion A continuous counter-current experiment was performed using DEHDMPA and DEHBA as extractants in the presence of the degradation products of DEHDMPA and DEHBA. Most of the U(VI) in the feed was extracted by DEHDMPA in the first cycle and then recovered in the U fraction. The residual U(VI) and almost all Pu(IV) were extracted by DEHBA in the second cycle, and were recovered in the U Pu fraction. The ratios of U(VI) and Pu(IV) recovered from the U fraction and U Pu fraction were 99.57% and 99.92%, respectively. Third phase formation and precipitation were not observed during the experiments; these results support the applicability of DEHDMPA and DEHBA as extractants for U(VI) and Pu(IV) in the presence of their degraded products. Acknowledgement This study is the result of the study titled Development of the technology dedicated to new reactors for electric power generation which was entrusted to the Japan Atomic Energy Agency (JAEA) by the Ministry of Economy, Trade and Industry (METI). We would like to thank Mr. Yagi, Mr. Shiga, Mr. Kanazawa, Mr. Usami, and other members belonged to the Research Group for Aqueous Separation Process Chemistry for their cooperation with the experiment. References 1) Y. Sasaki, Y. Sugo, M. Saeki, Y. Morita, A. hashi, Solvent Extr. Res. Dev., Jpn., 18, (2011). 2) Y. Sasaki, Y. Kitatsuji, Y. Tsubata, Y. Sugo, Y. Morita, Solvent Extr. Res. Dev., Jpn., 18, (2011). 3) Y. Sasaki, Y. Kitatsuji, Y. Sugo, Y. Tsubata, T. Kimura, Y. Morita, Solvent Extr. Res. Dev., Jpn., 19, (2012). 4) K. Akutsu, S. Suzuki, T. Kobayashi, H. Shiwaku, Y. kamoto, T. Yaita, Solvent Extr. Res. Dev., Jpn., 20, (2013). 5) H. Sugai, ucl. Technol., 98, (1992). 6) H. Sugai, K. Munakata, S. Miyachi, S. Yasu, J. ucl. Sci. Technol., 29, (1992). 7) V. K. Manchanda, P.. Pathak, A. K. Rao, Solvent Extr. Ion Exch., 22, (2004). 8) P.. Pathak, D. R. Prabhu, A. S. Kanekar, V. K. Machanda, Radiochim. Acta, 94, (2006). 9) S. Suzuki, T. Yaita, Y. Sugo, T. Kimura, Proceedings of Global 2007, Boise, U.S.A., (2007). 10) Y. Ban, S. Hotoku, Y. Morita, J. ucl. Sci. Technol., 49, (2012). 11) Y. Ban, S. Hotoku, Y. Tsubata, Y. Morita, Solvent Extr. Ion Exch., 31, (2013). 12) G. M. Gasparini, G. Grossi, Sep. Sci. Technol., 15, (1980)

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