Article pubs.acs.org/IC
Group Hexavalent Actinide Separations: A New Approach to Used Nuclear Fuel Recycling Jonathan D. Burns*,†,§ and Bruce A. Moyer‡ †
Nuclear Security and Isotope Technology Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, United States ‡ Chemical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, United States S Supporting Information *
ABSTRACT: Hexavalent Np, Pu, and Am individually, and as a group, have all been cocrystallized with UO2(NO3)2·6H2O, constituting the first demonstration of an An(VI) group cocrystallization. The hexavalent dioxo cations of Np, Pu, and Am cocrystallize with UO2(NO3)2·6H2O in near proportion with a simple reduction in temperature, while the lower valence states, An(III) and An(IV), are only slightly removed from solution. A separation of An(VI) species from An(III) ions by crystallization has been demonstrated, with an observed separation factor of 14. Separation of An(VI) species from key fission products, 95Zr, 95Nb, 137Cs, and 144Ce, has also been demonstrated by crystallization, with separation factors ranging from 6.5 to 71 in the absence of Am(VI), while in the presence of Am(VI), the separation factors were reduced to 0.99− 7.7. One interesting observation is that Am(VI) shows increased stability in the cocrystallized form, with no reduction observed after 13 days, as opposed to in solution, in which >50% is reduced after only 10 days. The ability to cocrystallize and stabilize hexavalent actinides from solution, especially Am(VI), introduces a new separations approach that can be applied to closing the nuclear fuel cycle.
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INTRODUCTION For nuclear power to become a major component in the future of a sustainable energy strategy, several barriers have to be overcome to leverage its inherent carbon-free power generation, which has the possibility of curtailing global greenhouse gas emissions.1−3 One major barrier is the complexity of implementing the separations involved in the recycle of used fuel to recover the actinides (Ans), maximize energy utilization of the fuel, minimize the waste going to geologic storage, and additionally serve the needs of nonproliferation.1−3 This will require a technology to recover not only the U and Pu, which are most important in energy generation, but also the minor Ans (MAs, i.e., Np, Am, and Cm), major contributors to the heat load and long-term hazard of geologic storage.4−8 Currently, the most advantageous technological practices employ solvent extraction as a means to separate U and Pu,9 yet these technologies haven proven to be challenging to apply to the MAs in a similar manner. This weakness in dealing with the MAs has spawned a large international research effort.10 Many new technologies are developing in the area of solvent extraction to meet deficiencies; however, the added cost of these advances to separate the MAs is a major limitation and has stifled implementation on a large scale.11 An innovative solution would be to have a technology achieve a single-step separation for all the Ans, known as a group actinide extraction (GANEX) process.12,13 In light of the movement toward a © 2016 American Chemical Society
single-step GANEX process, coupled with the renewed interest in exploiting the challenging Am(VI) oxidation state in nitric acid systems,14,15 attention has turned to the possibility of a group separation of hexavalent Ans U to Am. The challenge has been the instability of the Am(VI), made worse in the presence of organic compounds, even plastic surfaces, and this has caused some trepidation about the viability of a solvent extraction-based GANEX process. It was our hypothesis that a simple and elegant solution would be to cocrystallize actinyl ions as their nitrate salts from nitric acid, which avoids the unwanted effects of organic reductants and could, in principle, accomplish an unprecedented hexavalent actinide group separation. The new extraction system for transuranic (TRU) recovery (NEXT)16−20 has been developed by the Japanese Atomic Energy Agency (JAEA) over the past decade, in which the bulk of the U is removed from solution by crystallization prior to PUREX processing. This helps reduce the effects of the organic phase being overloaded with U during subsequent extraction. JAEA workers observed that Pu(VI) and Np(VI) tend to cocrystallize with uranyl nitrate hexahydrate (UNH), an unwanted interference in producing a pure uranium nitrate product. However, a deliberate An(VI) cocrystallization with Received: June 13, 2016 Published: August 17, 2016 8913
DOI: 10.1021/acs.inorgchem.6b01430 Inorg. Chem. 2016, 55, 8913−8919
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Inorganic Chemistry
L mol−1 cm−1 at 666 nm and 153 L mol−1 cm−1 at 996 nm for Am(VI) were used.30 Crystallization. Small batch crystallization experiments with volumes of 1−2 mL were performed with UO2(NO3)2·6H2O as the carrier species. The starting [U(VI)] was 1−2 M, with other An species (Np, Pu, and Am) spiked in at concentrations of 0.12−3.0 mM, and an acidity of 5.7−6.7 N. The An concentrations selected were designed to balance achieving actual U:An ratios seen in used nuclear fuel while ensuring the safety of the experimenter and keeping the radiation exposure ALARA. In experiments that included multiple TRU species, an aliquot of a Pu(VI) solution was first combined with an aliquot of a U(VI) solution, followed by the addition of an aliquot of a Np(VI) solution, and finally addition of an aliquot of Am(III). In the case of Am(VI), the mixture of U(VI), Np(VI), and Pu(VI) was added to the Am(VI) solution, which contained an excess of NaBiO3, as mentioned above. The system was then cooled to 2−9 °C in a water-jacketed sand bath using a temperature-controlled model 1166D VWR refrigerated circulating bath with a VWR digital temperature controller. The chilling system was optimized concurrently with experimental development, resulting in an overall decrease in the crystallization temperature to 2 °C. After crystallization, the solid and liquid phases were separated by centrifugation with a Costar Spin-X 0.45 μm cellulose acetate centrifuge tube filter with a mini-centrifuge. For experiments involving Am(VI), only glass containers, pipettes, and cuvettes were used, as Am(VI) is a very strong oxidant and will react readily with available organic reductants; this includes plastic surfaces. In these cases, decantation was used rather than centrifugation to separate the phases. Optical spectra of the solution before and after crystallization were recorded, and the separated phases were analyzed by γ-ray spectroscopy. Quantitative analysis for 243Am and 237Np was performed via γ-ray spectroscopy using a calibrated high-purity germanium detector (HPGe) with an active detector volume of ∼100 cm3 and a personal computer-based multichannel analyzer (MCA) (Canberra Industries Inc., Meriden, CT). The detector has an energy resolution of 0.8 at 5.9 keV, 1.0 at 123 keV, and 1.9 at 1332 keV. Relevant nuclear data were obtained from Browne and Firestone.31 All calibrations were determined with standard γ-ray sources that can be traced to the National Institute of Standards and Technology (NIST). 237Np was tracked by the 86.5 keV γ-ray, and 243Am was tracked by the 43.5 and 74.7 keV γ-rays. Of the key fission products, 95Zr was tracked by the 724 and 757 keV γ-rays, 95Nb by the 766 keV γ-ray, 137Cs by the 662 keV γ-ray, and 144Ce by the 133 keV γ-ray. U(VI) was tracked by optical spectra, implementing Beer’s law using an ε of 7.17 L mol−1 cm−1 at 415 nm (see Figure S1), while Np(V), Np(VI), Pu(VI), Am(III), and Am(VI) were tracked as discussed above. Dilutions of 20−35-fold were required to push [U(VI)] below an OD of 1 to obtain an accurate measurement. An error of 10% was assigned to the present precipitation values to account for statistical and systematic errors, as well as to take into account the acid dependence variations of ε for the different species over the course of the experiment.
UNH would be advantageous and could open the door to a new approach to group actinide separations. To realize this group separation, a highly oxidizing environment must be created and maintained, to stabilize the resulting hexavalent states long enough to perform the separation. Am(VI), specifically, is the most problematic to oxidize and stabilize, involving Am(VI)/Am(III) reduction potentials on the order of 1.7 V versus the normal hydrogen electrode (NHE).15 Methods for achieving the oxidation are challenging and under intense investigation.21−29 Of the different methods being explored, Mincher et al.14,15,24−27 have shown that NaBiO3, while suffering from slow kinetics, is an effective chemical oxidant for Am(III). In this work, we explore the fundamental behavior of hexavalent TRUs in cocrystallization with U(VI) and investigate a separation of the An(VI) species from An(III) ions and delve into the separation of An(VI) species from key fission products.
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EXPERIMENTAL SECTION
Materials. Nitric acid (69−70% Omni Trace, HNO3) was purchased from EDM. Sodium bismuthate hydrate (ACS grade, NaBiO3·xH2O) was purchased from Sigma-Aldrich. Depleted uranyl nitrate hexahydrate [ACS grade, UO2(NO3)2·6H2O] was purchased from SPI Supplies. All were used as received. Distilled deionized (ddi) H2O was obtained from a Millipore Milli-Q Academic Ultrapure Water Purification System with a Quantum EX Ultrapure Organex Cartridge filter operated at 18.2 MΩ cm at 25 °C. Neptunium-237 (>99.99%, 237 Np), plutonium-242 (99.98%, 242Pu), and americium-243 (99.80%, 243 Am) were all taken from stock on hand at the Radiochemical Engineering Development Center at the Oak Ridge National Laboratory (ORNL). All were converted from the oxide to the nitrate by dissolving the oxide in 8−10 M HNO3, evaporating gently back to the dryness, and redissolving in HNO3; these steps were repeated several times, with the final dissolution in 0.1−1 M HNO3. A sample of dissolved 237Np oxide pellets that had been irradiated in the High Flux Isotope Reactor (HFIR) at ORNL to produce 238Pu was used to mimic the fission product species present in used nuclear fuel, from which 95Zr, 95Nb, 137Cs, and 144Ce were selected as key isotopes for observation. Care was taken when handling radioactive species to ensure safety, which included the use of radiological facilities, primarily seal, negative-pressure α gloveboxes to reduce the risk of inhalation or ingestion of the TRUs and provide additional shielding of the radiation emitted. The principles of ALARA were maintained throughout the research. Oxidation and Spectral Analysis. Oxidation of Np, Pu, and Am was performed by chemical reaction with NaBiO3. Each metal was oxidized in a separate container and later combined for experiments that included multiple TRU species. In the case of Am(VI), only glass containers and utensils were used, as discussed below, and excess NaBiO3 was always present to ensure the reduction of Am(VI) to Am(III) did not occur. To expedite formation of An(VI) species, the nitric acid was pretreated with ozone from an Absolute Ozone ozone generator, operated at maximum conversation fed by the 25 psig facility air with the output regulated at 4, to 3.2 mM, while [U(VI)] was left relatively constant at 1.4 M; the acidity remained at 6.0 N. This system was cooled to 2−3 °C, leading to crystallization, and 80 ± 8% of the U(VI) and 73 ± 7% of the Pu(VI) were removed from solution (see Figure S3). The Pu(VI), under both sets of conditions, behaved in a manner almost identical to that of the Np(VI) systems, being removed from solution at near proportion to that of U(VI). This suggests that the assumed cocrystallization mechanism is not affected by perturbing the relative amount of the minor component, just as in the Np(VI) systems where adjusting the amount of the major component had little effect on the cocrystallization. It is worth mentioning that the difference in U(VI) removal, 30 ± 3 and 80 ± 8%, respectively, is a result of slight differences in the experimental conditions and is expected. In other words, in the second experiment the initial [U(VI)] was 1.4 M while the first was 1.1 M, roughly 25% more concentrated. In addition, in the second experiment, the system was cooled to a lower temperature, decreasing the solubility limit of U(VI). Having more U(VI) and a lower
Figure 1. Spectra of Am(VI) stability in uranyl nitrate over 10 days.
[Am(VI)] remained constant over the first 66 h, as no change in absorbance was observed; however, after 10 days, only 47 ± 5% of the Am(VI) remained, whereas the rest had been reduced to Am(III). This stability is believed to be caused by excess BiO3− dissolved in solution, acting as a holding oxidant for Am(VI). While Am(VI) eventually is reduced, there seems to be a sufficient working window of nearly 3 days to perform the cocrystallization and separation of the phases. Once the stability was observed to be adequate, a fresh Am(VI) sample was prepared and a system comprising 0.84 M U(VI) and 0.31 mM Am(VI) with an acidity of 6.2 N was prepared and cooled to 3−4 °C. Once crystallization had occurred, 32 ± 3% of the U(VI) and 31 ± 3% of the Am(VI) had been removed from solution, as observed by UV−vis spectroscopy (as shown in Figure 2). Again, care had to be taken to prevent Am(VI) reduction, so instead of spin filtration, the phases were separated by decantation. The separation achieved via decantation, inherently, is not as complete as that realized via spin filtration, so small amounts of the mother liquor were left with the crystalline phase. The two phases were analyzed by γ-ray spectroscopy, showing 35 ± 4% of the 243Am activity was observed to be associated with the crystals, which is in line with the UV−vis spectral data. As previously discussed, the lower yield of 32 ± 3% removal of U(VI) is primarily caused by the low initial [U(VI)] of 0.84 M. The crystalline phases were then investigated to determine if the trace An component remained in the hexavalent oxidation 8915
DOI: 10.1021/acs.inorgchem.6b01430 Inorg. Chem. 2016, 55, 8913−8919
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Figure 3. Crystal structure of the coordination of UO22+ in UNH adapted from refs 32 and 33 . Color legend: O, red; U, green; N, blue; H, white.
Figure 2. Spectra of cocrystallization of UNH with Am(VI) before (blue) and after (red) crystallization diluted 26.6 and 23.2 fold, respectively. The inlay spectra were obtained without dilution.
crystalline phase and improve the separation factors of the actinides from impurities found in dissolved used fuel, e.g., fission products, and reoxidation between crystallizations would not be needed. The results from the studies including only a single TRU species and UO2(NO3)2 are summarized in Table 1. Both of
state within the recovered solids. The crystals containing Am were dissolved in ddi H2O, and the solution was observed by UV−vis spectroscopy to determine the oxidation state of the Am. The resultant solution showed approximately 13% Am(III) and 87% Am(VI). Assuming that most of the Am(III) can be attributed to a less than complete separation between the crystals and the mother liquor and was therefore a part of the mother liquor, the percentage of Am(III) was subtracted from the 243Am activity found in the crystalline phase and added to the activity in the mother liquor; the Am(VI) percentage removed from solution was then recalculated to be 30 ± 3%. This recalculated value is more closely in agreement with the UV−vis spectral data. The mass of Am(VI) in the crystalline phase from the UV−vis spectral data and from the activity corrected γ-ray spectroscopy data was calculated. The two methods result in 25.1 ± 2.5 and 25.7 ± 2.6 μg, respectively, and the values are within the error of measurement of one another. There appears to be an increase in the stability of the Am(VI) species incorporated into the crystalline phase. As mentioned earlier, in solution a significant portion of the Am(VI) was reduced to Am(III) after 10 days, but the crystals were not dissolved until after 13 days. In the dissolved crystals, minimal reduction of Am(VI) to Am(III) was observed, with >97% of the Am(VI) mass removed from solution by cocrystallization with UNH still present as Am(VI) in the dissolved crystals. This increase in stability is most likely caused by steric hindrance provided by the crystalline lattice. In other words, once the Am(VI) is incorporated into the crystalline lattice, access to it by reducing agents is all but restricted. In the case of UNH, the U metal center within the linear dioxo cation is coordinated by two water molecules and two bidentate nitrate ions in the equatorial plane, as shown in Figure 3. A similar configuration can be imagined for Am(VI) bonding as well as for Np(VI) and Pu(VI), which both had results similar to those of the Am(VI)-containing crystals upon dissolution in ddi H2O. Alternatively, in solution, the coordinating ligands are much more labile, giving other ligands more direct access of the metal center, such as nitrite, which is known to grow into nitric acid over time and will act as a reducing agent for Am(VI). The increased stability is very advantageous, especially if a cascade of recrystallization was desired to increase the purity of the
Table 1. Ratios of the Percent Precipitation of Different Single-TRU Species Systems with Respect to UO22+ % precipitation An species Np(VI) Pu(IV) Pu(VI) Am(III) Am(VI)
U 71 83 41 30 80 72 32
± ± ± ± ± ± ±
7 8 4 3 8 7 3
TRU
U:TRU
± ± ± ± ± ± ±
0.96 1.0 10 1.1 1.1 11 1.0
74 82 4.1 28 73 6.4 31
7 8 0.4 3 7 0.6 3
the lower-valent ions, Am3+ and Pu4+, were only slightly removed from solution, approximately 1 order of magnitude less than the amount UO22+ removed. In contrast, all three of the An(VI) species studied were removed in near proportion to that of U(VI). These results favor the likelihood of cocrystallization being the mechanism for removal of the An(VI) ions from solution, where presumably the dioxo cation is occupying one of the UO22+ positions in the lattice structure of UNH. Once the simple systems containing only UO2(NO3)2 and one other hexavalent TRU had been investigated, a more complex system with all three hexavalent TRUs was examined. The system contained 1.2 M U(VI), 0.74 mM Np(VI), 0.84 mM Pu(VI), and 0.99 mM Am(VI) with an acidity of 6.1 N. It was cooled to 2 °C, and crystallization was allowed to occur, at which point 65 ± 7% of the U(VI), 62 ± 6% of the Np(VI), 63 ± 6% of the Pu(VI), and 62 ± 6% of the Am(VI) were removed from solution (see Table 2). As with the single-TRU studies, all the An(VI) species were removed from solution in near proportion of one another, again suggesting cocrystallization. 8916
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from solution, while Zr, Cs, and Ce were almost completely retained in the mother liquor. This suggests that there is no interaction of Zr4+, Cs+, or Ce3+ with the An(VI). Nb, on the other hand, seems to be more complicated, and its behavior will be discussed further below. It must be pointed out that these experiments were designed to maximize crystallization yield and thereby maximize the percent removal of An(VI) species from solution. In other words, the crystals were not washed after the solid and liquid phases were separated. Washing the crystalline phase should increase the separation factor by removing any of the mother liquor remaining on the surface of the crystals, but this may also reduce the amount of solid crystalline phase recovered, as redissolution might be initiated. Once it was determined that few to no interactions were seen between the hexavalent species and the observed fission products, the complexity of the system was increased by including Am(VI). To achieve the hexavalent state of Am, as discussed above, the system must be highly oxidizing, and in fact, all other metals will reach their highest oxidation state in solution prior to the production of Am(VI). The system contained 1.1 M U(VI), 3.0 mM Np(VI), 2.0 mM Pu(VI), and 2.5 mM Am(VI), respectively, at an acidity of 5.7 N, along with a spike in the fission product solution, resulting in 1.4 μCi of 95 Zr, 3.3 μCi of 95Nb, 2.3 μCi of 137Cs, and 31 μCi of 144Ce. This mixture was then cooled to 2 °C, promoting crystallization. Table 5 shows the results for both the An(VI)
Table 2. Results from the Group An(VI) Cocrystallization An species U(VI) Np(VI) Pu(VI) Am(VI)
% precipitation
U:TRU
± ± ± ±
− 1.0 1.0 1.0
65 62 63 62
7 6 6 6
A separation was then performed by creating a system that contained a U(VI) concentration of 1.4 M, Np(VI) and Pu(VI) concentrations of 1.2 and 1.3 mM, respectively, and a Am(III) concentration of 1.6 mM at an acidity of 6.4 N. After crystallization was achieved by cooling the system to 2 °C, 71 ± 7% of the U(VI), 61 ± 6% of the Np(VI), and 62 ± 6% of the Pu(VI) were removed from solutions, while only 5.0 ± 0.5% of the Am(III) was removed. The An(VI) ions were removed from solution 12−14-fold more fully than the Am(III) when all were present within the same system (see Table 3). These Table 3. Results of the Separation of An(VI) from An(III) by Crystallization An species U(VI) Np(VI) Pu(VI) Am(III)
% precipitation
U:TRU
± ± ± ±
− 1.2 1.2 14
71 61 62 5.0
7 6 6 0.5
Table 5. Results of the Separation of U(VI), Np(VI), Pu(VI), and Am(VI) from Key Fission Products
results indeed confirm the potential for a separation of An(VI) species from lower-valent An ions by cocrystallization, as suggested by the single-TRU studies discussed above. After the promising separation of An(VI) species from An(III) ions had been demonstrated, the behaviors of key fission products were investigated. The first step was ensuring the absence of Am(VI) to avoid any concerns that could result from the highly oxidizing environment required to produce the hexavalent Am species. For this, the system consisted of 1.3 M U(VI), 3.0 mM Np(VI), 2.6 mM Pu(VI), and a spike of a fission product solution, resulting in 0.18 μCi of 95Zr, 0.28 μCi of 95Nb, 0.24 μCi of 137Cs, and 1.2 μCi of 144Ce at an overall acidity of 6.0 N. The system was then cooled to 2 °C, and crystallization was allowed to occur, after which 1.1 ± 0.1% of the 95Zr, 12 ± 1.2% of the 95Nb, 1.3 ± 0.1% of the 137Cs, and 1.1 ± 0.1% of the 144Ce were removed from solution. The An(VI) species behaved as previously discussed, all being removed in near proportion, and their results are displayed, along with those of the fission products, in Table 4. The behavior of the fission products was quite different from that of the dioxo An(VI) species, with Nb being only slightly removed
a
a
% precipitation
U:M
U(VI) Np(VI) Pu(VI) 95 Zr 95 Nb 137 Cs 144 Ce
78 ± 8 71 ± 7 (74 ± 7)a 68 ± 7 (1.1 ± 0.1)a (12 ± 1.2)a (1.3 ± 0.1)a (1.1 ± 0.1)a
− 1.1 (1.1)a 1.1 (71)a (6.5)a (60)a (71)a
% precipitation
U:M
U(VI) Np(VI) Pu(VI) Am(VI) 95 Nb 95 Zr 137 Cs 144 Ce
71 ± 7 69 ± 7 (79 ± 8)a 66 ± 7 61 ± 6 (55 ± 6)a (72 ± 7)a (9.7 ± 1)a (9.2 ± 1)a (10 ± 1)a
− 1.0 (0.90)a 1.1 1.2 (1.3)a (0.99)a (7.3)a (7.7)a (7.1)a
Data in parentheses from γ-ray spectroscopy analysis.
species and the selected fission products. Once again, all the hexavalent species behaved like one another, being removed from solution in near proportion. The fission products, however, behaved differently than they did in the system containing no Am(VI), most notably 72 ± 7% of the 95Nb being removed from solution. In this system, Nb is presumably present as an Nb(V) species, which has been known to polymerize and form colloidal Nb2O5·nH2O in acidic media.34 The polymer can be described as linear chains with two inplane bridging oxo ligands. Dangling pendent oxo ligands are doubly bound to the Nb metal center and lie above and perpendicular to the plane, with adduct water molecules coordinated to the Nb below the plane. A schematic of these polymer chains is displayed in Figure S4. While no explicit data were obtained in these studies, it would not be surprising if the decrease in temperature promoted precipitation of the colloidal Nb2O5·nH2O polymer. To a lesser extent, 137Cs and 144Ce were also removed from solution. In this system, Ce is certainly present as Ce4+ and therefore could form a double nitrate salt with Cs+, Cs2Ce(NO3)6. JAEA researchers have observed the double nitrate of Pu4+ to form with Cs+ in a similar system, which they reported being removed quantitatively from
Table 4. Results of the Separation of U(VI), Np(VI), and Pu(VI) from Key Fission Products metal species
metal species
Data in parentheses from γ-ray spectroscopy analysis. 8917
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Inorganic Chemistry solution.16−20,35 Only 10 ± 1% of the Ce was observed to be removed. The difference between Pu4+ and Ce4+ is attributed to differences in the formation constants of the two nitrate species. For example, log K of PuNO33+ at 25 °C and an ionic strength of 0 is 1.8, while that of CeNO33+ is only 0.32;36 therefore, it is not surprising that Pu4+ is quantitatively removed while Ce4+ is only partially removed. Likewise, a similar mechanism for Zr4+ may be occurring, as log K for ZrNO33+ under the same conditions is 0.34.36 Despite the mechanistic determination of these key fission products being outside of the current investigation, it can be seen that a separation factor of at least 7 can be achieved for 95Zr, 137Cs, and 144Ce, whereas 95Nb is not easily separated. Again, these experiments did not investigate the benefit to a separation by washing the crystalline phase, so these results should be considered as conservative values for the separation of the An(VI) from the fission products. From a radiation standpoint, 95Nb, a β− emitter, with an associated γ at 765.8 keV, is somewhat benign, having a halflife of
