Carboxyl Groups in

Research Article pubs.acs.org/journal/ascecg

Polypropylene Modified with Amidoxime/Carboxyl Groups in Separating Uranium(VI) from Thorium(IV) in Aqueous Solutions Jie Xiong,†,‡ Sheng Hu,† Yi Liu,† Jie Yu,§ Haizhu Yu,§ Lei Xie,† Jun Wen,† and Xiaolin Wang*,†,‡

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†

Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, No. 64 Mianshan Road, Mianyang 621999, China ‡ College of Nuclear Science and Technology, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei 230026, China § Department of Chemistry and Centre for Atomic Engineering of Advanced Materials, Anhui University, No. 111 Jiulong Road, Hefei 230601, China S Supporting Information *

ABSTRACT: A majority of U/Th reprocessing studies are hampered in the production of large amounts of radioactive waste during extraction with organic complexes from highly acidic solutions. Polypropylene grafted with amidoxime/ carboxyl groups (PPAC), which exhibits selective binding toward U(VI), can help reduce organic waste. Batch studies on PPAC separating U(VI) from U(VI)/Th(IV) mixture in acidic to weak alkaline solution with complexing agent EDTA were carried out. The result suggested a much better affinity of PPAC to U(VI) over Th(IV). The pH effect study suggested the largest U/Th mass ratio at pH ∼8. FTIR spectra at different pHs indicated most of the carboxyl and amidoxime functional groups were protonated at lower pH, while with increasing pH, the deprotonating degree increased correspondingly. X-ray diffraction analysis showed U(VI) and Th(IV) could both enter the crystal lattice, resulting in smaller diffraction angle which was associated with U(VI) and Th(IV) adsorption capacity. This work revealed polymer or other material with amidoxime/carboxyl groups is a promising candidate for deployment as a uranium adsorbent in U/Th reprocessing. KEYWORDS: PPAC fiber, Uranium extraction, Uranium/thorium separation, Chelating functional group, Amidoxime ligands

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INTRODUCTION Radionuclides produced in nuclear waste repositories contain actinides such as uranium, thorium, and other fission products.1 Selective separation of thorium and uranium from high-level waste would be advantageous and desirable as a means of recycling or removing these radioactive actinides, as well as decreasing potential human health and environmental risks. Organic extractants have been playing an important role in separating uranium from thorium for a long time due to their chelating properties. Phosphates/phosphonates for U(VI)/ Th(IV) purification are especially widely utilized in the extraction and recovery of hexavalent uranyl (UO22+) and tetravalent thorium from effluents of nuclear power plants.2−6 However, a solution of phosphates/phosphonates (like tri-nbutyl phosphate, TBP) in dodecane or other inert aliphatics is often used in practice, as pure TBP alone is too viscous to operate with.7 This may lead to a massive volume of organic wastes and introduce additional issue to deal with. Not only extracting valuable metals from mixed solutions but also reducing the quantity of disposal waste are challenging tasks. Besides, the aforementioned liquid−liquid extracting system © 2016 American Chemical Society

only works in highly acidic environment. To ensure operational safety, it is of great importance to separate uranium from thorium under mild conditions. Consequently, solid-phase extraction is proposed as a promising separation method for advanced nuclear fuel reprocessing.8−15 Polymer adsorbents decorated with specific functional groups exhibit a great potential for selective removal of heavy metal ions and easy disposal at solid state. In the past few years, new adsorption materials/methods are exploited to detect or enrich uranyl from groundwater and seawater as a uranium source for nuclear energy, of which amidoxime functional materials performed excellently in selectively mining uranyl from complicated systems.16,17 The existences of thorium and uranium in solutions are complicated. For example, in aqueous solution of pH < 2, Th4+ is the major species; at pH > 2, Th4+ is hydrolyzed to Th(OH)3+, Th(OH)22+, Th(OH)3+, Th2(OH)44+, and so on. Received: November 3, 2016 Revised: December 15, 2016 Published: December 17, 2016 1924

DOI: 10.1021/acssuschemeng.6b02663 ACS Sustainable Chem. Eng. 2017, 5, 1924−1930

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Schematic depiction of the functional groups (carboxyl, amidoxime, and cyclic imidedioxime) on the modified PPAC fiber. NH2OH·HCl) for 6 h at 80 °C. The final PPAC product was washed with a large amount of deionized water many times and dried. The chemical structure of PPAC fiber is depicted in Figure 1. Separation. PPAC adsorbent was immersed in a mixed U(VI)/ Th(IV) aqueous solution of 35 mL under desired pHs in a vibrating tube. The tube was then put in a constant temperature vibrating box (35 °C) for 44 h at 100 r/min. Accurate metal concentrations before and after separation were measured by inductively coupled plasmamass spectroscopy (ICP-MS) (Agilent 7700x, Agilent Technologies, Santa Clara, CA, U.S.A.). Characterization. Samples of proper quantities were prepared to be detected by various techniques. Fourier transform infrared (FTIR) spectra were acquired by using a Nicolet 5700 spectrometer (Thermo Electron, U.S.A.). The crystallizable microstructure was examined with X-ray diffraction (XRD) using an X’Pert PRO diffractometer (PANalytical, Netherland). The surface was observed by scanning electron microscopy (S440, Leica Microsystems Cambridge Ltd., U.K.) and transmission electron microscopy (Libra 200FE, Carl Zeise SMT Pte, Germany). Energy dispersive X-ray spectroscopy (EDS) was done by an energy dispersive X-ray spectrometer (Oxford IE450xMax80, Oxford, U.K.). X-ray photoelectron spectroscopy (XPS) was obtained with an ESCALAB250 X system (Thermo, U.K.). The radicals engendered through γ-ray irradiation were measured by electron-spin resonance (E500, Bruker, Karlsruhe, Germany). The specific surface area by BET method and pore size distribution by BJH method were determined on an autoadsorption system (Autosorb-iQ, Quantachrome, U.S.A.). Elemental analysis for C, H, and N were performed by Vario EL CUBE (Elementa, Germany). DFT Methods. All density-functional calculations were implemented in the Gaussian 09 program28 using the exchange-correlation hybrid functional B3LYP.29,30 Geometry optimization was performed in aqueous phase in conjunction with the relativistic effective core potential (ECP) of the SDD pseudopotential basis set for uranium atom31 and 6-311G(d,p) basis set for the light atoms.32 Frequency analysis was performed at the same level of geometry optimization to ensure no imaginary frequency. The most stable configuration is confirmed according to the calculated Gibbs free energies.

When pH is increased to ∼4, thorium precipitates as Th(OH)4.18−20 For uranium in aqueous solutions, the dimer (UO2)2(OH)22+ exists mostly in weak acidic (2 < pH < 5) media with total uranium concentrations above 10−4 mol/L, while UO2(OH)2 is the predominant species in the pH range of 5.1−9.7.18−20 In the pH range of 6−9, schoepite precipitates when the total uranium concentration exceeds 10−5.5 mol/L. In this study, ethylenediaminetetraacetate (EDTA, abbreviated as H4Y) was added into each solution for metal ion stabilization. Thorium may exist as complexes of Th(HY)+, (Th(OH)Y)22−, Th(OH)Y−, and Th(OH)2Y,21 and uranium may exist as UO22+, UO2H4Y2+, UO2H3Y+, UO2H2Y, UO2HY−, (UO2)2Y, UO2(OH)HY2−, and (UO2)2(OH)2H2Y4−.22,23 Though a number of research works on amidoxime type adsorbent have been carried out to investigate the factors that can influence the selective uranium adsorption and various opinions on the mechanism of amidoxime groups coordinated with U(VI) under different conditions are proposed,24−27 it is still not clear how amidoxime groups and other auxiliary groups work in separating U(VI)/Th(IV) in aqueous solution containing EDTA. In the present work, polypropylene unwoven fiber modified with amidoxime/carboxyl groups (PPAC) was synthesized successfully. In order to mimic the application in separation of U(VI)/Th(IV) in reality, unlike most other adsorption experiments, the conditions set in this work were restricted to an excess of PPAC and a shortage of U(VI)/Th(IV), in the hope of getting a reasonable explanation on the behavior of amidoxime materials adsorbing Th(IV) and U(VI).

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EXPERIMENTAL SECTION

Materials. Polypropylene was purchased from WEIJUN nonwoven fiber company in Dongguan, China. Hydroxylamine hydrochloride, sodium hydroxide, 65% nitric acid, acrylonitrile (AN), methacrylic acid (MAA), EDTA, and thorium nitrate [Th(NO3)4], all analyticalreagent-grade, and uranyl nitrate [UO2((NO3)2], of high-purity grade, were used without further purification. All aqueous solutions were prepared with deionized water with a typical resistivity >18.2 MΩ·cm (Millipore Elix5, Millipore, Billerica, MA, U.S.A.). Pure and mixed solutions of U(VI) and Th(IV) were obtained by dissolving proper amount of metal nitrates with EDTA (0.06 mmol/L) in 0.1 mol/L HNO3. Synthesis of PPAC. Polypropylene (PP) was placed in the γ-ray field of 60Co and irradiated in air atmosphere with 70 kGy dose. The material was stored in a refrigerator (−5 °C) for spare after irradiation. The grafting copolymerization of PP and AN/MAA (4/1 in mass ratio) was carried out at 80 °C for 6 h. Copolymerization products were washed by acetone (3 times), ethanol (3 times), and deionized water (many times) successively. After cleaning, the grafted acrylonitrile-co-methacrylic acid copolymer was converted to amidoxime-co-methacrylic acid by reacting with hydroxylamine in a near neutral aqueous solution (by mixing equal moles of NaOH and

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RESULTS AND DISCUSSION Analysis of PPAC Preparation. A photograph of prepared PPAC polymer is shown in Figure S1. During the preirradiation of polypropylene, peroxy radicals were generated in ambient atmosphere, and the characteristic ESR signal of such radicals33,34 were observed (Figure S2). FTIR spectra of PP/ PPAC were recorded before and after grafting as illustrated in Figure S3. PPAC spectrum with new stretches of N−H (3390 cm−1), CN (1650 cm−1), N−O (940 cm−1), and −COO− (1564 and 1396 cm−1) that are absent in PP spectrum demonstrates amidoxime and carboxyl groups grafted onto polypropylene successfully.35,36 SEM and TEM images of PP/PPAC further confirm the functional groups are conglomerated on the surface in sidechains (Figure S4) since the surface of PP is rather smooth and PPAC surface has visible bulges. Elemental analysis gives the 1925


Research Article


goes up at both pHs, though it is always lower than that of uranium (the same as the trend in the pure solution assays). Note the adsorption capacity of thorium at pH 3.97 and 318 K is slightly lower than that at 308 K. This unexpected Th(IV) dot and the nearly constant U(VI) adsorption amount may suggest different adsorption mechanisms of the two metals on PPAC. Adsorption of uranium can be regarded as chiefly chemisorption at both pHs since temperature has not any obvious desorption effect and almost all the initial uranium is adsorbed onto PPAC confirmed by ICP-MS data. For the case of thorium, there seems to be no chemisorption behavior at pH ∼4 (desorption occurs at 318 K). At pH ∼6, the adsorption amount is increasing concomitantly with temperature (all higher than those at pH ∼4), hinting that the majority of thorium adsorption on PPAC changes from physisorption to chemisorption. It suggests that chemical structures (functional sites) of PPAC are altered by pH and in turn influence the adsorption mechanisms of the metals to some extent. We further investigated the effect of pH. The temperature was set at 308 K since it was indicated as the transition point from physisorption to chemisorption. From the results (Figure 2), it can be learned that at the lowest pH studied (∼2) the

percentage contents in PPAC as 66.448% carbon, 5.006% nitrogen, and 10.125% hydrogen. XPS (Figure S5) and EDS (Figure S6) can help determine the chemical composition of PP and PPAC qualitatively. The specific surface area (BET method) of PPAC (0.7773 m2/g) is higher than that of PP (0.3302 m2/g), mainly due to the functional groups grafted on the surface (also reflected on the pore size distribution shown in Figure S7). U(VI)/Th(IV) Adsorption on PPAC. Uranium and thorium will be chelated with EDTA and form multiple complexes. In our study, concentrations of metals were controlled below 11 ppm to ensure their complexed existence in aqueous solutions instead of precipitates or micelles. Note that thorium without EDTA was precipitated as Th(OH)4 when pH goes above 4; therefore, pH ∼4 was chosen as a starting point. Experimental data for thorium and uranium adsorption onto PPAC (QTh and QU, respectively, in mg per gram of PPAC) at pH ∼4 are listed in Table 1, with adsorbent solid(g)/ Table 1. Adsorptive Behaviors of U(VI) and Th(IV) on PPAC at pH ∼4a temperature

QU (mg/g) pH 4.09

QTh (mg/g) pH 4.02

QU/QTh

298 K 308 K 318 K

5.879 ± 0.002 7.953 ± 0.006 8.818 ± 0.008

2.797 ± 0.004 3.256 ± 0.007 3.730 ± 0.003

2.102 2.443 2.364

a

Initial concentrations: thorium(IV), 10.25 ppm; uranium(VI), 6.73 ppm.

liquid(mL) ratio ≈ 1:1500. The affiliation of PPAC to U(VI) is ∼2 times higher than that to Th(IV). As the temperature rises, the adsorption capacities for both metals increase simultaneously, while the ratio of QU/QTh remains ∼2 although the initial concentration of thorium is 1.5 times higher than that of uranium. U(VI)/Th(IV) Separation by PPAC. The above tests only represent single-element adsorption behaviors onto PPAC without competition adsorption or interference between metal ions. To testify to the separation efficiency, additional experiments of PPAC contacted with U(VI)/Th(IV) mixed solution were conducted. Another concern of this work is avoidance of high acidity in typical phosphates/phosphonates extraction of uranium; hence, the separation experiments of U(VI)/Th(IV) mixture were performed in weak acidic (pH ∼4) to near-neutral environment (pH ∼6). The solid/liquid ratio was controlled at ∼1:500, and the ratio of initial uranium and thorium concentration was controlled at 1:1 (both approximately 7 ppm in each temperature test). The extraction results of mixed thorium and uranium solutions are given in Table 2. In the temperature range of 298−318 K, the adsorption amount of U(VI) remains almost unchanged (about 3.7−3.8 mg per gram adsorbent). Meanwhile, the adsorption of thorium rises when the temperature

Figure 2. Adsorption capacity of U(VI) and Th(IV) on PPAC at different pHs.

functional groups do not work well; nonetheless, at pH 4, 6, and 8 the adsorption ability of uranium increases greatly, reaching a plateau of ∼3.7 mg/g. The affinity of thorium to PPAC displays a different trend: first rising then going downward. The selectivity of PPAC for U(VI) over Th(IV) follows the order: pH 6 < pH 4 < pH 8 (the maximum U(VI)/ Th(IV) ratio is 8.98 at pH ∼8). Note at pH ∼2, no thorium is adsorbed, but uranium binds to PPAC weakly (the adsorption quantity is only ∼20% of that at higher pHs). Thus, it cannot

Table 2. Comparison of Adsorption of U(VI) and Th(IV) on PPAC at pH 3.97 and 6.19a pH 3.97

a

pH 6.19

temperature

QU (mg/g)

QTh (mg/g)

QU (mg/g)

QTh (mg/g)

298 K 308 K 318 K

3.299 ± 0.001 3.362 ± 0.001 3.458 ± 0.001

0.053 ± 0.001 0.744 ± 0.001 0.585 ± 0.001

3.421 ± 0.003 3.476 ± 0.001 3.525 ± 0.002

1.196 ± 0.001 2.161 ± 0.001 2.546 ± 0.001

Initial concentrations of thorium(IV) and uranium(VI): ∼7 ppm. 1926


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keeps its structure undisturbed from introduction of actinide metals, protons, or hydroxyl ions. After interacting with U(VI)/Th(IV) solution at pH ∼2 (Figure 3, red line), N−H, CN, and −COO− stretches diminish in regards to those of untreated PPAC (Figure 3, black line), demonstrating hydrolyzed amidoxime groups and protonated carboxyl groups form −COOH partially, which causes the 1700 cm−1 peak for −COOH to be enhanced and broadened. At this pH, thorium cannot be adsorbed, but uranium can bind to amidoxime’s N−OH through η2 complexation24 or tetra-complexation composed of two amidoxime groups and one uranyl ion. As pH goes up, the −COO− peak appears and gradually becomes stronger. Thorium is adsorbed onto PPAC from physisorption to chemisorption with increasing binding sites. In the meantime, uranium may coordinate not only with carboxyl group but also with amidoxime’s N−OH/N−O− and −NH2/−NH−, as can be seen from the huge increase of N−H stretch (3390 cm−1) and CN peak. X-ray diffraction (Figure S8) was conducted to see whether metals were chelated with PPAC by chemical bonding. From XRD analysis (see Supporting Information), it was found U(VI) and Th(IV) could both enter the crystal lattice, resulting in smaller diffraction angle which was associated with U(VI) and Th(IV) adsorption capacity. Then, DFT calculation on several possible chemical structures of U(VI)−PPAC complex was performed to examine the thermodynamically stable coordination configuration. Here we select two models of most probability for PPAC with U(VI) shown in Figure 4 with other two modes from literature.16,24,25

serve as an ideal separation pH. From all the information, pH ∼8 is deduced to be the best condition for U(VI)/Th(IV) separation by PPAC. One possible explanation for weak uranium binding and scarce thorium adsorption at pH ∼2 is the formation of protonated carboxyl groups (−COOH) that cannot provide sufficient coordination sites for competence against EDTA binding with metals. At pH ∼4, the carboxyl groups are partially deprotonated to generate carboxylate −COO− (pKa of carboxylate monomer is ∼4.0).37 At this moment, thorium mainly adsorbs physically on PPAC surfaces; at pH ∼6, most carboxyl groups exist in deprotonated form, promoting thorium to interact chemically and form Th(IV)−PPAC complex. One interesting phenomenon is that thorium adsorption weakens once pH further increases to ∼8. This may be caused by −NH2 in the amidoxime group turning into −NH−, whose binding affinity to thorium is less than that of EDTA at this pH and fails in competition for thorium adsorption. This opinion of the role of −NH2 group is first proposed in all of the studies of amidoxime materials for adsorbing Th(IV) and U(VI). The amine group in amidoxime is different from amine, because amidoximation is an addition reaction with −CN and NH2OH, during which the bond between −NH2 and −OH is broken and carbon and nitrogen are attacked separately (no charge transfer during this addition reaction). The electron of nitrogen in −NH2 is conjugated with the CN bond in amidoxime; hence, the activity of this nitrogen is higher than that in the ordinary amine.38 The PPAC fibers adsorbed with U(VI)/Th(IV) at different pHs were characterized systematically using FTIR (Figure 3) aiming at providing evidence for the proposed mechanism. The peaks at 2870 and 2962 cm−1 belong to −CH3 symmetric and asymmetric stretches; 2850, 2925, and 1462 cm−1 peaks are attributed to stretching and bending modes of backbone −CH2.39 These aliphatic groups’ intensities are nearly invariable in all cases, indicating the backbone carbon chain of PPAC

Figure 3. FTIR spectra of PPAC before and after U(VI)/Th(IV) adsorption at different pHs. The black line shows PPAC untreated with thorium/uranium solutions; other lines with pH in brackets represent PPAC fiber contacted with aqueous solutions of adsorbates at respective pHs. The characteristic stretch modes of N−H (3390 cm−1), CN (1650 cm−1), N−O (940 cm−1), and −COO− (1564 and 1396 cm−1) clearly indicate amidoxime and carboxyl functional groups in PPAC.

Figure 4. Two complexing ligands and possible coordination configurations for U(VI)−PPAC complexes (optimized structures A−D).16,24,25 1927


Research Article

ACS Sustainable Chemistry & Engineering Table 3. Bond Lengths (Å) in Proposed Structures of U−PPAC Complex by DFT Calculation amidoxime ligand

free amidoxime ligand free carboxyl ligand A B C D

carboxyl ligand

C−N (1 or 1′)

CN (2 or 2′)

N−O (3 or 3′)

1.465

1.467

1.378

1.357 1.349 1.354′ 1.343 1.369 1.367′

1.312 1.309 1.308′ 1.300 1.298 1.297′

1.428 1.423 1.420′ 1.383 1.393 1.395′

CO (i)

C−O (ii)

1.380

1.384

1.242

1.281

U(VI)−PPAC Complex U−O(-N) (4 or 4′)

N(H)-U (5 or 5′)

U−O(−C) (iii)

N−H(−−O) (iv)

(N−)H−−O (v)

2.230 2.288 2.285′ 2.224 2.452 2.434′

2.492 2.479 2.503′ 2.331

2.455

1.014

2.362

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2.622

CONCLUSIONS Phosphates/phosphonates are popular materials for U(VI)/ Th(IV) purification in the extraction and recovery of uranyl ion from effluents of nuclear power plants, but the required high acidity and massive organic waste may limit their feasible application. In this work, PPAC fibers are synthesized by a preirradiation−copolymerization method with amidoxime/ carboxyl functional groups, which have shown a good ability in extracting uranyl from a mixed solution of U(VI)/Th(IV). The behavior of PPAC adsorbing these actinides has been analyzed by adsorption capacity measurement, FTIR, XRD, and DFT calculations. The results conclude that amidoxime together with carboxyl groups play an important role in the formation of uranium−PPAC complex. It is of great interest to find PPAC has a good selectivity for uranium over thorium especially in pH ranges of seawater (∼8), promoting the separation and recovery of this valuable element from abundant natural resources in an environmentally compatible manner. This work can provide essential knowledge for rational design of new ligands with enhanced uranyl affinity and selectivity in U(VI)/Th(IV) separation.

From our results, amidoxime has a major role in tetracoordination to uranyl at lower pH (structure A).16 When pH gets higher (4, 6, and 8) and deprotonated carboxyl groups come into existence, the penta-coordination complex with both functional groups is more stable (structure B). This finding is in accordance with the aforementioned fact that affinity of PPAC to uranium is favorable at pH > 4. The η2 coordination (structure C) at low pH which is energetically unfavorable cannot be ruled out since the XRD data shows an exotic behavior. Moreover, cyclic imidedioxime coordination mode (structure D) is recognized as an important component of U(VI)−PPAC complex with the lowest energy in DFT calculation. However, the formation of this cyclic imidedioxime groups is enhanced in KOH solution at 80 °C in the preparation.36 In this work, we only performed amidoximation in near-neutral environment. Therefore, the amount of cyclic imidedioxime functional groups is small (shown in the low intensity of IR spectra), and the penta-coordination configuration (structure B) dominates at higher pH. This kind of structure involving dual functional groups has also been studied by others, for example, when introducing MAA or itaconic acid (ITA) there exists an optimum molar ratio for AN/MAA at ∼2.33 in dimethyl sulfoxide (DMSO)40 and AN/ITA at 7.57− 10.14 in DMSO.41 The calculated bond lengths are listed in Table 3. Compared to the bond lengths in free amidoxime ligand, the N(H)−C bond (1) and CN bond (2) in all complexes are shortened while N−O(H) (3) is lengthened. In the meantime, the U−O bond (4) is shorter than the U−N(H) bond (5) in all cases, meaning that once coordinated the CN bond is slightly strengthened through complexation with uranium in the fivemembered ring and uranium preferably binds to oxygen over nitrogen as evidenced by the changes in lengths of bonds 1, 3, 4, and 5. For carboxyl interaction, we choose isobutyric acid as the free ligand since it is the saturated alkyl form of MAA. The i and ii bonds in deprotonated carboxyl group are intrinsically the same under higher pH. However, the carboxyl group comes into coordination to uranyl ion (structure B) and the i and ii bonds are now discernible, possibly due to one being hydrogenbonded to H atom and another being complexed with U(VI). The U−O bond (iii) length of carboxyl group is much longer than that of U−O bond (4), meaning the amidoxime functional group has a more profound impact on the coordination of PPAC with uranium.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02663. Photograph of PPAC; ESR spectrum of PP after 70 kGy γ-irradiation in air; SEM and TEM images of PP and PPAC; FTIR, XPS and EDS spectra of PP and PPAC; XRD patterns of PPAC before and after adsorption of U(VI)/Th(IV) at four pHs and shifts of diffractive angles for peaks I−V; BJH pore size distributions from desorption branch for PP and PPAC (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. ORCID

Jie Xiong: 0000-0001-6543-6161 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was sponsored by State Administration of Science, Technology and Industry for National Defense under nuclear 1928


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power development project and National Natural Science Foundation of China (Grant Nos. 91326110 and 21401175).

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