Article pubs.acs.org/IECR
Washing of Uranyl Nitrate Hexahydrate Crystals with Nitric Acid Aqueous Solution To Improve Crystal Quality Masaumi Nakahara,*,†,‡ Yasuo Nakajima,† and Tsutomu Koizumi†,‡ †
Nuclear Fuel Cycle Engineering Laboratories and ‡Advanced Nuclear System Research and Development Directorate, Japan Atomic Energy Agency, 4-33 Muramatsu, Tokai-mura, Naka-gun, Ibaraki 319-1194, Japan ABSTRACT: To develop a uranium crystallization process for advanced aqueous reprocessing, two method experiments were investigated to evaluate the effect of crystal washing on uranyl nitrate hexahydrate (UNH). In one experiment, crystal washing was carried out using a uranyl nitrate solution with varying concentrations of HNO3 in washing solutions, and in the other, it was conducted with a dissolver solution of the irradiated fast neutron reactor core fuel. In the crystal washing experiments with the uranyl nitrate solution, the UNH crystals were immersed into a uranyl nitrate solution containing Ce, which was washed out in the mother liquor on the surface of the UNH crystals using an HNO3 solution. The experimental results showed that the decontamination factor (DF) of Ce increased with decreasing HNO3 concentration of the washing solution. However, because the UNH crystals tended to dissolve in low HNO3 concentration solutions, the yield decreased as a result of the washing operation. In the crystallization experiment using a dissolver solution derived from the irradiated fast neutron reactor core fuel, the DFs of the liquid impurities for the UNH crystals were improved with each crystal washing. However, their values approached a constant value because the inclusions within the UNH crystal were not removed by the crystal washing operation. On the other hand, solid impurities such as Cs2Pu(NO3)6 and Ba(NO3)2 remained in the UNH crystals after several crystal washing operations.
1. INTRODUCTION In the fine chemicals and pharmaceutical industries, intermediates and products are obtained as crystals. In addition, demands for high-purity materials have increased recently in various fields, and crystallization methods have been used to synthesize high-purity products. The application of repetitive crystallization processes, such as zone melting, or directional crystallization methods, such as the Czochralski and Bridgman−Stockbarger techniques, is known to be successful for the preparation of high-purity materials.1−3 However, in many cases, organic crystals produced in conventional crystallizers are often contaminated by some impurities. These impurities originate from the mother liquor and are present at the surface and/or within the crystals as inclusions. The removal of inclusions via the application of temperature gradients at 100 °C/cm or pressure gradients in an ultracentrifugal field has been reviewed and shown to be potentially effective. In the atomic energy industry, crystallization processes have been under development for use in the advanced aqueous reprocessing of the fast neutron reactor fuel in Japan.4 A dissolver solution of the irradiated fast neutron reactor mixed oxide (MOX) fuel is cooled to crystallize a part of the uranium as uranyl nitrate hexahydrate (UNH). The dissolver solution derived from the irradiated fast neutron reactor MOX fuel contains transuranium (TRU) elements, fission products (FPs), corrosion products, and other process impurities. Therefore, the UNH crystals produced from the dissolver solution derived from irradiated fast neutron reactor fuel are contaminated by some of these impurities. Because the recovered U is used as a blanket fuel in the crystallization process, it is desirable to decontaminate the accompanying impurities in the UNH crystals in terms of fuel fabrication, storage, and other effective utilization. Therefore, the obtained UNH crystals in the cooling © 2012 American Chemical Society
crystallization process are washed with an HNO3 solution to remove the mother liquor attached to the surface of the crystals after solid−liquid separation. The crystal washing operation is important for the decontamination of impurities in the U crystallization process. In a previous study,5 liquid impurities such as lanthanide elements were removed from UNH crystals by crystal washing with an HNO3 solution. On the other hand, the decontamination factors (DFs) of solid impurities such as Ba were 1−3.5, even after the crystal washing, and crystal washing was not effective for the removal of solid impurities. However, there is very little experimental data on the effect of washing of the UNH crystals. In the crystal washing operation, the HNO3 concentration in the washing solution and the number of crystal washing steps should affect the DFs of the FPs for the UNH crystals, and thus, these variables in the U crystallization process were investigated in this study. The effects of two approaches for crystal washing of UNH were evaluated. In one experiment, the UNH crystals were immersed in a uranyl nitrate solution containing a simulated FP (Ce) and were washed with a solution containing varying concentrations of HNO3. In the other, the effect of the number of washing steps on the UNH crystals recovered from a dissolver solution of the irradiated fast neutron reactor fuel was evaluated. This paper is organized as follows. In section 2, the preparation of the solutions, the experimental procedure, and the analysis methods are described. The results of the crystal washing and crystallization experiments are shown and Received: Revised: Accepted: Published: 15170
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Table 1. Composition of the Feed Solutions with Dissolver Solution of Irradiated Fast Reactor Fuel in the Crystallization Experiment U (g/dm3)
Pu (g/dm3)
Ba (g/dm3)
106 Ru (Bq/cm3)
125 Sb (Bq/cm3)
137 Cs (Bq/cm3)
144 Ce (Bq/cm3)
155 Eu (Bq/cm3)
241 Am (Bq/cm3)
242 Cm (Bq/cm3)
4.43 × 102
6.80 × 101
1.12 × 100
8.44 × 107
3.76 × 107
3.04 × 108
4.40 × 108
5.52 × 107
9.02 × 108
2.20 × 107
mol/dm3 HNO3 solutions at 5.0 ± 0.8 °C for 1 min to wash the UNH crystals. The UNH crystals were then centrifuged to separate them from the washing solution. The washing operation was carried out once in the crystal washing experiments. The crystal size was measured using various sizes of sieves. In the crystallization experiment with the dissolver solution of the irradiated fast neutron reactor core fuel, 82.32 g of the recovered UNH crystals was placed in the basket of a centrifuge and mixed with 100 cm3 of an 8 mol/dm3 HNO3 solution at 5.0 ± 0.8 °C for 1 min to remove the mother liquor attached to its surface. The crystals were then centrifuged for 10 min to separate them from the washing solution. The washing operation was repeated three times. 2.3. Analysis. The radioactive feed solution was measured by optical spectrometry (V-570DS, JASCO Corp.) in the ultraviolet (UV)−visible region to confirm the Pu valence. The acidity of the solution was determined by acid−base titration (COM-2500, Hiranuma Sangyo Co., Ltd.). The concentrations of U and Pu were measured by colorimetry, and the Am and Cm concentrations were determined by α-ray spectrometry (CU017-450-100 detector and NS920-8MCA pulse height analyzer, ORTEC). The concentrations of FPs was analyzed by γ-ray spectrometry (GEN10 detector and 92XMCA pulse height analyzer, ORTEC) and inductively coupled plasma atomic emission spectrometry (ICPS-7510, Shimadzu Corp.).
discussed in section 3. The effect of crystal washing on the DFs of the FPs for the UNH crystals was evaluated in these experiments. Finally, a summary and conclusions are presented in section 4.
2. EXPERIMENTAL SECTION 2.1. Reagents and Feed Solution. HNO3 was purchased from Wako Pure Chemical Industries, Ltd., and used without further purification. The crystal washing experiments were carried out with a uranyl nitrate solution to evaluate the influence of the HNO3 concentration in the washing solution on the UNH crystal purity. The HNO3 and U concentrations in the feed solution were 4 mol/dm3 and 500 g/dm3, respectively. Ce(NO3)3·6H2O (Mitsuwa Chemicals Co., Ltd.) was chosen to simulate an FP, and the Ce concentration in the immersion solution for the UNH crystal was adjusted to 10 g/dm3. In the crystallization experiment, the effect of the number of crystal washing steps on the DFs of the impurities for the UNH crystal was examined using a dissolver solution derived from the irradiated fast neutron reactor core fuel. The irradiated core fuel of fast reactor “JOYO” Mk-III with an average burnup of 53 GWd/t (gigawatt days per metric ton) and a cooling time of 3 years was used for the U crystallization experiment. Sheared pieces of core fuel comprising 166 g of heavy metals were dissolved using 325 cm3 of 10 mol/dm3 HNO3 solution at 100 °C. The Pu valence in the dissolver solution was changed to Pu(IV) by bubbling NOx gas through it. To evaluate the solid impurities, CsNO3 and Ba(NO3)2 solutions were prepared by dissolving CsNO3 (Wako Pure Chemical Industries, Ltd.) and Ba(NO3)2 (Kanto Chemical Co., Inc.) in a 2 mol/dm3 HNO3 solution, and then adding the solution before cooling to the dissolver solution for the U crystallization so that the concentrations of Cs and Ba in the feed solution were 3.7 and 1.12 g/dm3, respectively. The concentrations of U and Pu were adjusted to 443 and 68 g/dm3, respectively, in a 6.1 mol/ dm3 HNO3 solution by adding more or evaporating the HNO3 solution. Table 1 summarizes the composition of the feed solution. 2.2. Procedure. The crystallizer was made of Pyrex glass and had a cooling/heating jacket. The temperature was controlled by a thermostat. The experiments were conducted with uranyl nitrate feed solutions and the dissolver solution derived from the irradiated fast neutron reactor core fuel with volumes of 500 and 170 cm3, respectively. The feed solution was stirred for 30 min at 50.0 ± 1.2 °C and then cooled for 180 min. The uranyl nitrate solution and the dissolver solution of the irradiated fast neutron reactor core fuel were then cooled to 5.0 ± 0.8 °C. The grown crystalline particles were centrifuged (H-112, KOKUSAN Co., Ltd.) at 3000 rpm for 10 min to separate them from the mother liquor. In the crystal washing experiments using a uranyl nitrate solution with varying HNO3 concentrations in the washing solution, 200 g of the obtained UNH crystals was immersed in a 300 g/dm3 uranyl nitrate solution with 6 mol/dm3 HNO3 containing 10 g/dm3 Ce. After solid−liquid separation, the UNH crystals were mixed with 100 cm3 of cooled 1, 5, and 8
3. RESULTS AND DISCUSSION 3.1. Crystallization of Uranyl Nitrate Hexahydrate. Figure 1 shows the solubility of U in different HNO3 solutions,6 and the results are considered to be accurate to ±2 °C. It is known that uranyl ions crystallize to UNH in an HNO3 solution by the following equilibrium reaction:
Figure 1. Solubility of U in HNO3 solution. 15171
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concentration of metal j in the crystal (mg/g), and CP,U is the concentration of U in the crystal (mg/g). After solid−liquid separation from the solution by centrifugation, the DF of Ce for the UNH crystals was 85.6 ± 1.6, and nearly all the Ce was removed from the surface of the UNH crystals. The DF of Ce for the UNH crystals after crystal washing increased with decreasing HNO3 concentration in the washing solution. Because the liquor containing the Ce attached to the surfaces of the UNH crystals was removed with the HNO3 solution and the dissolved UNH crystals by the crystal washing step, the effect of crystal washing was the greatest when the UNH crystals were washed with 1 mol/dm3 HNO3 solution. 3.3. Effect of Number of Crystal Washing Steps Using Dissolver Solution of Irradiated Fast Neutron Reactor Core Fuel. The crystallization experiment was carried out with a dissolver solution of the irradiated fast neutron reactor core fuel, and the effect of the number of washings was examined. Figure 2 shows the UNH crystal yield before and after washing.
(1)
The concentration of U in the solution decreases with decreasing temperature before reaching the eutectic point. Only UNH crystallizes on the right side of the eutectic point, whereas H2O and HNO3 begin to crystallize on the left side of the eutectic point. Thus, it is desirable to perform the crystallization of U in the appropriate region: the right side of the eutectic point. Because the solubility of U is low at high HNO3 concentrations, a high HNO3 concentration increases the yield of the UNH crystals. 3.2. Effect of Nitric Acid Concentration in Crystal Washing Solution Using Uranyl Nitrate Solution. The crystal washing experiments were carried out with a uranyl nitrate solution, and the effect of HNO3 concentration in the washing solution was examined. Table 2 summarizes the Table 2. Crystal Size, Crystal Yield, and DF of Ce for UNH Crystal as a Function of HNO3 Concentration in the Washing Solution in the Crystal Washing Experiments average crystal size (μm)
DF of Ce for UNH crystals
washing solution
before washing
after washing
UNH crystal yield (%)
before washing
after washing
1 mol/ dm3 HNO3 5 mol/ dm3 HNO3 8 mol/ dm3 HNO3
490 ± 5
457 ± 5
82.5 ± 1.1
85.6 ± 1.6
915 ± 3
490 ± 5
472 ± 3
88.0 ± 0.7
85.6 ± 1.6
484 ± 2
490 ± 5
481 ± 4
91.0 ± 1.3
85.6 ± 1.6
471 ± 3
average size of UNH crystals before and after washing. The average crystal size, mass median diameter D50, was 490 ± 5 μm before washing. The average size of UNH crystals affects HNO3 concentration in the washing solution. The crystal size after crystal washing with 8 mol/dm3 HNO3 solution was greater than that under the other washing conditions. The relationship between the HNO3 concentration in the washing solution and the UNH crystal yield after washing is summarized in Table 2. The calculation of the UNH crystal yield was based on the crystal yield before washing and was 100%. The UNH crystals were readily dissolved in the washing solutions with low HNO3 concentration. The experimental results indicate that the difference in the U solubility in HNO3 solution affects the crystal size and the crystal yield of the UNH. Although the U solubility in 1 mol/dm3 HNO3 solution at 5 °C was 430 g/dm3, it was only 180 and 100 g/dm3 in 5 and 8 mol/dm3 HNO3 solutions, respectively.6 The difference in the U solubility affects the amount of dissolution of the UNH crystals. Table 2 also summarizes the DFs of Ce for the UNH crystals before and after washing. Here, the DFs of metals for the UNH crystal were defined by eq 2:
Figure 2. Relationship between the number of washings and the crystal yield in the crystallization experiment.
The crystals were dissolved with an 8 mol/dm3 HNO3 solution during the crystal washing operation, and as a result, the UNH crystal yield decreased to 32.4% after the third washing. Figure 3 shows the relationship between the number of washings and the DFs of the metals for the UNH crystals. The DFs of Ru, Sb, Ce, Eu, Am, and Cm for the UNH crystals increased with increasing number of washings. On the other hand, although Pu, Ba, and Cs were decontaminated slightly by the first washing step, their DFs decreased in the second and third washing operations. The difference in the decontamination performance depends on the characteristics of elements. When the uranyl ions crystallized as UNH by cooling the dissolver solution of the irradiated fast neutron reactor core fuel, Ru, Sb, Ce, and Eu remained in an ionic state in the mother liquor. The mother liquor containing these Ru, Sb, Ce, and Eu ions attached to the surfaces of the UNH crystals were washed out with the HNO3 solution. Their DFs were improved after the UNH crystals were washed. Among the actinide elements, Am and Cm are usually trivalent in an HNO3 solution, and their chemical behavior is similar to that of
C F, j
βj =
C F,U C P, j C P,U
(2)
where βj is the DF of metal j for the UNH crystal, CF,j is the concentration of metal j in the feed solution (g/dm3), CF,U is the concentration of U in the feed solution (g/dm3), CP,j is the 15172
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remained in the mother liquor, even after reacting with Cs. Hence the DF of Pu for the UNH crystals was improved to 14.1 after the first washing. On the other hand, the DF of Cs was still 1.42 under these experimental conditions, even when the UNH crystals were washed with an HNO3 solution. In addition, the DFs of Pu and Cs were not high after the crystal washing operation compared with those of the liquid impurities. The decontamination behavior of Ba was similar to that of Cs in the dissolver solution of the irradiated fast neutron reactor core fuel. The DF of Ba was 4.90 after the first crystal washing step. Therefore, it was difficult to separate Ba from the UNH crystals by crystal washing. Barium is assumed to precipitate as Ba(NO3)2 in the course of U crystallization, because the solubility of Ba(NO3)2 in the nitrate solution decreases with increasing HNO3 concentration in the mother liquor.9 The crystallization of UNH results in high acidity in the mother liquor by cooling the dissolver solution of the irradiated fast neutron reactor core fuel, and Ba tends to precipitate as Ba(NO3)2 in the U crystallization operation. The DFs of solid impurities, such as Cs2Pu(NO3)6 and Ba(NO3)2, are not improved with washing of the UNH crystals, unlike with the liquid impurities. Rather, the DFs of Pu, Cs, and Ba decreased after the second and third washing operations. Figure 4 shows the solubilities of UO2(NO3)2·6H2O, Cs2Pu-
Figure 3. DFs of metals for the UNH crystals as a function of the number of washings in the crystallization experiment.
lanthanide elements. They remain in the mother liquor during the U crystallization, become attached to the surface of the UNH crystals, and then are removed when the UNH crystals are washed. Thus, the DFs of Am and Cm were improved after the crystals were washed. Because they were attached to the surface of the UNH crystals after separation from the mother liquor, liquid impurities such as Ru, Sb, Ce, Eu, Am, and Cm were washed out with the HNO3 solution. Although part of the UNH crystals were dissolved, crystal washing was an effective measure for removing the liquid impurities and for producing high-quality UNH crystals. As can be seen in Figure 3, as the UNH crystals were washed with the HNO3 solution, the DFs of the liquid impurities, such as Ru, Sb, Ce, Eu, Am, and Cm, approached a constant value. This phenomenon is assumed to be caused by the inclusion of liquid impurities during the course of crystallization. The impurities in the inclusions cannot be removed by washing the surface of the UNH crystals. Alkali metal elements such as Cs react with tetravalent actinide elements, and the resultant double salts precipitate in an HNO3 solution. Cesium is one of the most common FPs in the dissolver solution derived from the irradiated fast neutron reactor fuel. It is known that Pu(IV) reacts with Cs and precipitates as Cs2Pu(NO3)6 in an HNO3 solution.7 The chemical reaction of Cs2Pu(NO3)6 is expressed as the following equation: 2Cs+ + Pu(NO3)6 2 − ↔ Cs 2Pu(NO3)6
Figure 4. Solubilities of UO2(NO3)2·6H2O, Cs2Pu(NO3)6, and Ba(NO3)2 in HNO3 solutions at 25 °C.
(3)
(NO3)6, and Ba(NO3)2 in HNO3 solutions at 25 °C prepared by dissolving each of these nitrates in an HNO3 solution.6,9,10 The solubility of UO2(NO3)2·6H2O is greater than those of Cs2Pu(NO3)6 and Ba(NO3)2 in an HNO3 solution at 25 °C. Therefore, the UNH crystals would be easily dissolved in comparison with Cs2Pu(NO3)6 and Ba(NO3)2 in the crystal washing operation. As a result, the abundance ratio of Cs2Pu(NO3)6 and Ba(NO3)2 in the UNH crystals was higher after excessive washing. This behavior is also likely the cause for the decrease in the DFs of the solid impurities after the second and third crystal washing steps. Several washing operations improve the DFs of the liquid impurities by cleaning the surfaces of the UNH crystals. On the other hand, the solid
Pu(NO3)62−
In eq 3, a high abundance of is advantageous for forming Cs2Pu(NO3)6. The crystallization of uranyl ions requires a certain amount of H2O, as shown in eq 1, and this crystallization makes the HNO3 concentration in the mother liquor higher than that in the feed solution. Thus, the U crystallization contributes to the formation of Cs2Pu(NO3)6, because the ratio of Pu(NO3)62− increases with increasing HNO3 concentration in the HNO3 concentration range of the U crystallization process.8 Under the experimental conditions in this study, Cs2Pu(NO3)6 is assumed to precipitate in the course of the U crystallization. Because there was much more Pu than Cs in the feed solution, nearly all the Pu likely 15173
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Table 3. Inclusion Content within the UNH Crystal in the Crystallization Experiment U (μg)
Pu (μg)
Ba (μg)
Ru (μg)
Sb (μg)
Cs (μg)
Ce (μg)
Eu (μg)
Am (μg)
Cm (μg)
−
9.84 × 10−2
9.60 × 10−4
5.32 × 10−5
5.08 × 10−6
8.90 × 10−5
2.36 × 10−4
2.63 × 10−5
1.31 × 10−4
1.24 × 10−5
with an HNO3 solution. Countercurrent continuous equipment, the Kureha Crystal Purifier (KCP),14 has been applied in industrial plants. The KCP has the function of heating and mixing for the crude crystals. A crystal purification method has been proposed that involves the decontamination of the liquid impurities using a sweating phenomenon and washing with a reflux melt.15 On the other hand, the solid impurities, which have different particle sizes, can be separated from the UNH crystals by gravity and mixing using KCP. The countercurrent experiments were carried out with a uranyl nitrate solution containing Sr as the liquid impurity and Ba(NO3)2, SiO2, Al2O3, and CeO2 as the solid impurities for evaluating the dependency of the KCP on the grain size and density of the solid impurities.15 The DFs of 100 were achieved for both impurities in the UNH crystals recovered from the KCP. Although the influence of the density of the solid impurities on the purification performance of the UNH crystals was not very significant, the particle size of the solid impurities measurably affected the decontamination of the solid impurities. The smaller-sized solid impurities tended to pass through the UNH crystal grains toward the bottom of the purifier and were pushed down along the flow of the UNH melt in the purifier. Therefore, the combination of crystal washing with an HNO3 solution and crystal purification using the KCP would be desirable for obtaining high-quality UNH crystals.
impurities are refractory in the HNO3 solution compared with the UNH crystals, and thus, the DFs of the solid impurities for the UNH crystals decreased when the UNH crystals were washed several times. Several previous studies have been reported on the formation of liquid inclusions. Shimizu et al.11 reported that liquid inclusions were formed not only at a contact site, but also at other positions of the crystal surface, using an episcopic differential interference contrast microscope (nondirect contact induced formation). On the other hand, inclusion formation caused by the adhesion of small crystals was observed in an actual suspension crystallizer. Liquid inclusions were also formed at other positions apart from the adhesion point (nondirect adhesion induced formation).12 In these studies, layer inclusion was concluded to occur not only on the impacted surface or the adhesion surface, but also on other surfaces. Structural defects were believed to be generated elsewhere on the crystal surfaces as a result of adhesion or mechanical contacts. New microsteps originated from these detects due to the successive growth of macrostep bunching. More active macrosteps were frequently observed to overlap the preceding, slowly moving macrosteps. At this moment of overlap, the mother liquor inclusions were trapped twodimensionally in parallel with the crystal surface. When a microcrystallite adheres to a crystal in the course of crystal growth, new macrosteps are generated in addition to the existing macrosteps. Therefore, the number of macrosteps increases, and the growth rate of the crystal increases. The total volume of inclusions per crystal was correlated with the crystal size in an agitated crystallization following the empirical equation12 V = 4.0 × 10−6D4
4. CONCLUSIONS The crystal washing and crystallization experiments to confirm the effect of crystal washing with HNO3 solutions were carried out using a uranyl nitrate solution containing Ce and a dissolver solution derived from the irradiated fast neutron reactor core fuel. In the crystal washing experiments, the Ce in the mother liquor that was attached to the surface of the UNH crystals had a tendency to be removed by the washing operation with low HNO 3 concentration solution, because the low HNO3 concentration washing solution also readily dissolved the UNH crystals. In the crystallization experiment, the DFs of the liquid impurities were improved by crystal washing, but approached a constant value after three washings. It was caused by the inclusion of liquid impurities during the U crystallization. On the other hand, the DFs of solid impurities decreased with three washings because UNH is more soluble than the solid impurities in an HNO3 solution. To remove the inclusions of the liquid impurities in the crystals and the solid impurities from the UNH crystals, the crystal purifier KCP was developed and DFs of 100 were achieved. The combination of crystal washing with an HNO3 solution and crystal purification using the purifier would be desirable for obtaining high-quality UNH crystals.
(4)
where V is the total volume of inclusions per crystal (μm3) and D is the crystal size (μm). Equation 4 can be applied regardless of material and crystallization conditions, such as temperature, suspension density, supersaturation level, and agitation speed.12 Therefore, these experimental results indicate that the inclusion of liquid impurities in the UNH crystals cannot be avoided in the U crystallization process. In the crystallization experiment, the total volume of inclusions per crystal calculated by eq 4 was 1.2 × 106 μm3. The content of the elements in the inclusion formed in the UNH crystal from the trapped mother liquor is summarized in Table 3. Here the isotopic composition ratios of the nuclides were calculated by the ORIGEN2 program.13 Cesium, Ba, and Pu precipitated in the form of Cs2Pu(NO3)6 and Ba(NO3)2 in the experiment. However, a portion of these elements remained in the mother liquor after U crystallization, and thus, they are present in the inclusions in the UNH crystals. If the crystallization product must be further decontaminated to remove the TRU elements and FPs, then the inclusions of liquid impurities should be removed from the UNH crystals recovered from the dissolver solution of the fast neutron reactor fuel. 3.4. Further Decontamination of Impurities for Uranyl Nitrate Hexahydrate Crystals. The inclusions of the liquid impurities in the crystals and the solid impurities would not be effectively separated from the UNH crystals via crystal washing
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +81 29 282 1111. Fax: +81 29 282 9290. E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 15174
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ACKNOWLEDGMENTS The authors thank the Mitsubishi Materials Corp. for performing the experimental measurements using uranyl nitrate solution.
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REFERENCES
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