Behavior Of Np Research Articles

Sensitive redox speciation of iron, neptunium, and plutonium by capillary electrophoresis hyphenated to inductively coupled plasma sector field mass spectrometry.

The long-term safety assessment for nuclear waste repositories requires a detailed understanding of actinide (geo)chemistry. Advanced analytical tools are required to gain insight into actinide speciation in a given system. The geochemical conditions in the vicinity of a nuclear repository control the redox state of radionuclides, which in turn has a strong impact on their mobility. Besides the long-lived radionuclides plutonium (Pu) and neptunium (Np), which are key elements in high level nuclear waste, iron (Fe) represents a main component in natural systems controlling redox-related geochemical processes. Measuring the oxidation state distribution for redox sensitive radionuclides and other metal ions is challenging at trace concentrations below the detection limit of most available spectroscopic methods (≥10(-6) M). Consequently, ultrasensitive new analytical techniques are required. Capillary electrophoresis (CE) is a suitable separation method for metal cations. CE hyphenated to inductively coupled plasma sector field mass spectrometry (CE-ICP-SF-MS) was used to measure the redox speciation of Pu (III, IV, V, VI), Np (IV, V, VI), and Fe (II, III) at concentrations lower than 10(-7) M. CE coupling and separation parameters such as sample gas pressure, make up flow rate, capillary position, auxiliary gas flow, as well as the electrolyte system were optimized to obtain the maximum sensitivity. We obtain detection limits of 10(-12) M for Np and Pu. The various oxidation state species of Pu and Np in different samples were separated by application of an acetate-based electrolyte system. The separation of Fe (II) and Fe (III) was investigated using different organic complexing ligands, EDTA, and o-phenanthroline. For the Fe redox system, a limit of detection of 10(-8) M was calculated. By applying this analytical system to sorption studies, we were able to underline previously published results for the sorption behavior of Np in highly diluted concentrations, and we monitored the time-dependent reduction of Pu(VI) by Fe(II). This study clearly shows that CE-ICP-SF-MS is a suitable separation method for the redox states of Pu, Np, and Fe.

Sorption behavior of Np(V) on illite, shale and MX-80 in high ionic strength solutions

Sorption of Np(V) on illite, shale and MX-80 under oxidizing conditions were first studied in two types of high ionic strength solutions: (i) a reference brine solution (SR-270-PW) with an ionic strength of 6.0 M, and (ii) Na–Ca–Cl solutions. The effects of pHc, Na/Ca ratio, and ionic strength on Np(V) sorption in Na–Ca–Cl solutions were investigated. The K d values and the sorption isotherms in SR-270-PW and Na–Ca–Cl solutions were also evaluated.

Leaching behavior of gamma-emitting fission products and Np from neutron-irradiated UO2–ZrO2 solid solutions in non-filtered surface seawater

The gamma ray radionuclides Cs-137, Ba-140, I-131, Ce-141, Ru-103, Zr-95, and Np-239 were produced by neutron irradiation of UO2–ZrO2 solid solutions that were synthesized as simulated fuel debris under reducing and oxidizing conditions. The leaching ratio of radionuclides was investigated under atmospheric conditions at 25 °C for non-filtered natural surface seawater, as well as deionized water after filtration with a membrane of 0.45-µm pore size or that of nominal molecular weight limit of 3 kDa. The uranium molar concentration was affected by the oxidation state in the solid solution samples. The congruent dissolution of Cs, I, and Ba with the hexavalent uranium of U3O8 was facilitated in the seawater samples, whereas a lower leaching ratio of nuclides was observed in the deionized water samples. Neptunium-239, originally produced from uranium-238 in U3O8, showed behavior that was similar to that of Cs, I, and Ba. However, the dissolution of Np (as a parent nuclide of Pu-239) in the debris of UO2 and UO2–ZrO2 was suppressed in the same manner as Zr(IV) and Ce(IV). The concentration exhibited no filtration dependence after 15 d, which shows that most of the leached nuclides can exist in their ionic form in seawater.

Supposed existence of Np4+ in a genuine dissolver solution from the results of extraction simulation by PARC-L code

With a genuine spent fuel solution (a dissolver solution), a laboratory-scale reprocessing experiment of an extraction–separation process was performed using mixer-settlers as extractors. In the experiment, n-butyraldehyde was utilized as a reducing reagent of Np(VI)O22+ to Np(V)O2+ for the purpose to distinguish Np(VI)O22+ from Np4+. From the Np concentration in the aqueous phase, Np would be extracted from the dissolver solution together with U and Pu. The scrutiny of Np behavior was performed utilizing 66 cases of calculation results by a Japan Atomic Energy Agency open extraction simulation code, the Program for Advanced Extraction with Radiation Effect Calculation–Lightened version. From the scrutiny, the authors found that the calculation result with 60% of Np4+ in the dissolver solution represented the best experimental extraction–separation behavior of Np. Therefore, it was supposed that the dissolver solution contained sufficient proportion of Np4+ to affect the extraction–separation behavior of Np.

Studies on the Extraction of Actinides Using a Solvent Containing D2EHiBA in Room Temperature Ionic Liquids: Unusual Extraction of the Tetravalent Ions

The extraction behavior of U(VI), Pu(IV), and Np(IV) from nitric acid medium has been studied using branched chain di(2-ethylhexyl)isobutyramide (D2EHiBA) dissolved in different room temperature ionic liquids (RTILs) [Cnmim][NTf2] (where n = 4, 6, or 8). Uranium extraction (DU) increased gradually with aqueous phase acidity for the three RTILs used in this study suggesting solvation mechanism. There was a reversal in the extraction behavior of Pu(IV) and Np(IV) from nitric acid medium using D2EHiBA dissolved in RTILs as solvents as compared to the behavior reported in the molecular diluent, n-dodecane, which shows negligible extraction of these metal ions. The extraction of Pu(IV) increased with aqueous phase acidity in different RTILs in the order: [C8mim][NTf2] > [C6mim][NTf2] > [C4mim][NTf2]. The distribution ratio values of these metal ions followed the order: DPu(IV) ≥ DNp(IV) > DU(VI) using D2EHiBA as extractant suggesting that RTILs can modify the extraction behavior of extractants.

Behavior of Np(V), Pu(V), and Am(V) in solutions of pyridine-2,6-dicarboxylic (dipicolinic) acid and synthesis of crystalline compounds

The behavior of An(V) (An = Np, Pu, and Am) in aqueous solutions of dipicolinic acid (in particular, the resistance to disproportionation) was studied. Taking these data into account, procedures were developed for preparing crystalline An(V) dipicolinates. Crystals of Li3[AnO2(DPA)2]·nH2O (n = 4 and 6) of X-ray quality were prepared from aqueous solutions with pH 2–3. The trends in variation of the bond lengths in the complex compounds of An(V) and An(VI) with dipicolinic acid, caused by actinide contraction, were analyzed.

A novel Np–Pu separation procedure based on extraction with di-(chlorophenyl)-dithiophosphinic acid in nitric solution

The extraction behavior of Pu(III), Pu(IV), Np(IV) and Np(V) with di(chlorophenyl)-dithiophosphinic acid (DCPDTPA) in toluene from nitric acid solutions was studied systematically. In aqueous solution with high nitric acid concentration, the extraction capability (represented by distribution ratio D) for Pu and Np in different valences with DCPDTPA comes as DNp(IV) > DPu(IV) > DNp(V) > DPu(III). A new radiochemical procedure for Np/Pu separation based on DCPDTPA extraction was proposed and tested with simulated samples. The recoveries of Np and Pu are as high as 80 % after the whole separation procedure, with the decontamination factor of trivalent lanthanide fission product element (e.g. Eu) greater than 1.5 × 104. The decontamination factor of Pu–Np is 2.0 × 103, while the decontamination factor of Np–Pu is greater than 4.8 × 103 after additional purification.

Chemical behavior of neptunium in the presence of technetium in nitric acid media

Abstract The chemical behavior of Np is influenced by many factors, such as U(IV), hydrazine, Pu and Tc, which coexist in U/Pu separation stage of PUREX process. Firstly, the reduction of Np(V) to Np(IV) by Tc(IV) was studied in nitric acid media, and the rate equation could be expressed as − d[Np(V)]/dt = k[Np(V)][Tc(IV)]0.8[HNO3]1.2, k = 28.5 ± 0.9 (L/mol)2/min at 25 ℃, the activation energy was E a = 70.0 kJ/mol. Thereafter, the chemical behavior of Np was investigated in aqueous nitric acid solution containing Tc(VII), U(IV) and N2H4. Np(V) was reduced to Np(IV) quickly, Tc(VII) was reduced to Tc(IV) and U(IV) was oxidized to U(VI) at the same time. Thereafter Np(IV) was entirely reoxidized to Np(V) when U(IV) and hydrazine were completely consumed. The fast reduction of Np(V) was mainly ascribed to Tc(IV). In addition, Tc(VII) and nitric acid had little influence on the fast oxidation of Np(IV).

Role of combinatorial environmental factors in the behavior and fate of ZnO nanoparticles in aqueous systems: A multiparametric analysis

To better understand the environmental behavior, fate, and exposure risks of engineered nanoparticles in aquatic systems, for the first time, combinatorial aqueous systems were established using three-level orthogonal array design (OAD), an OA27 (313) matrix, to assess the effects of six co-varying environmental factors (organic acid type, organic acid concentration, NP concentration, pH, salt content, and electrolyte type) on the aggregation of commercial zinc oxide nanoparticles (ZnO NPs, mean diameter ∼41nm). A separate set of OA27 (313) experiments including temperature was conducted for the dissolution of these NPs. The results showed that the organic acid concentration and the pH were the most significant factors (p<0.001) influencing aggregation and dissolution of ZnO NPs, respectively. The electrolyte type and the salt content were the next most important factors in both the aggregation and dissolution. Based on the kinetics study of the aggregation, a high rate of the NP aggregation resulted in decreased dissolution, such that observed in the presence of calcium chloride. Clear temperature-induced aggregation and reduced dissolution were further observed with increasing temperature. This approach demonstrates that the behavior of ZnO NP may vary substantially under combinatorial environmental conditions.

Separation nanotechnology of diethylenetriaminepentaacetic acid bonded magnetic nanoparticles for spent nuclear fuel

A nanomagnetic separation method based on diethylenetriaminepentaacetic acid (DTPA) conjugated with magnetic nanoparticles (MNPs) is studied for application in spent nuclear fuel separation. The high affinity of DTPA towards actinides aids in separation from the highly acidic medium of nuclear waste. The solubility and magnetization of particles at low pH is protected by encapsulating them in silica. Surface functionalization of silica coated particles with polyamines enhances the loading capacity of the chelator on MNPs. The particles were characterized before and after surface modification using different characterizing tools. The uptake behavior of Am(III), Pu(IV), U(VI), and Np(V) from 0.1M NaNO3 solution was determined. The sorption results show the strong affinity of DTPA towards Am(III) and Pu(IV) by extracting 97% and 80% of actinides, respectively. The high removal efficiency of actinides make the chelator conjugated MNPs an effective method for spent nuclear fuel separation.

Behavior of Np(VI) in HNO3 solutions containing potassium phosphotungstate K10P2W17O61

The behavior of Np(VI) in HNO3 solutions containing potassium phosphotungstate K10P2W17O61 (KPW) at various concentrations of HNO3 (1–3 M) and KPW [(1–3) × 10−3 M] was studied by spectrophotometry. The rate law and the Np(VI) transformation scheme were determined, and the effective rate constants of the Np(VI) reduction and Np(V) reproportionation were calculated.

Redox Behavior of Neptunium(V) in Tributyl Phosphate and N,N-Dihexyl Octanamide Extractants Dissolved in n-Dodecane

Spectrophotometric investigations have been carried out on the disproportionation of Np(V) to form Np(IV) and Np(VI) in 1.1 mol⋅L−1 solutions of tributyl phosphate (TBP) and in N,N-dihexyl octanamide (DHOA) in n-dodecane medium. The Np(V) was found to coordinate with Np(IV) in 1.1 mol⋅L−1 TBP solution in n-dodecane to form a mixed valence “cation–cation” complex by bonding through an axial oxo group on Np(V). By contrast, this interaction was less prominent in the case of 1.1 mol⋅L−1 DHOA solutions. The effect of 1-octanol, added as phase modifier, on the disproportionation behavior of Np(V) was also investigated. An attempt was made to calculate the disproportionation/reduction rate constants for Np(V) under the conditions of these studies. Absorbance measurements on the Np stripped from organic phases revealed the occurrence of Np(V) in the aqueous phase.

Np-incorporation into uranyl phases: A quantum–mechanical evaluation

Density functional theory calculations are used to compare mechanisms of charge-balanced Np 5+-incorporation into boltwoodite [K(UO 2)(SiO 3OH)(H 2O) 1.5]. The charge-balancing mechanisms considered include: (i) H + addition (Np 5+ + H + ↔ U 6+), (ii) interlayer substitution (Np 5+ + Ca 2+/Mg 2+ ↔ U 6+ + K +), and (iii) intra-layer substitution (Np 5+ + P 5+ ↔ U 6+ + Si 4+). The choice of the source (of Np 5+, H +, Ca 2+, Mg 2+, P 5+) and sink (of substituted U 6+, K +, Si 4+) phases, the reference phases, for the cations involved in the incorporation reaction greatly affects the final calculated incorporation energy (Δ E rxn) . The incorporation energies using oxide (Δ E rxn = 183.5 kJ/mol) and silicate (Δ E rxn = 67.5 kJ/mol) reference phases are compared for the interlayer substitution mechanism. Since both the source of Np in environmental systems and the cations released are typically aqueous complexes, a newly-developed approach of combining cluster and periodic simulations was employed to model exchange with aqueous complexes. For the H + addition mechanism, incorporation from oxides reference phases (Δ E rxn = 93.6 kJ/mol) is less favorable than from aqueous (Δ E rxn = 12.5 kJ/mol) reference species. Estimates of the solid-solution behavior of Np 5+/P 5+- and U 6+/Si 4+-boltwoodite and Np 5+/Ca 2+- and U 6+/K +-boltwoodite solid solutions are used to predict the limit of Np-incorporation into boltwoodite. For the interlayer and intra-layer substitution mechanisms, the substitution energies are 67.5 and −2.9 kJ/mol, respectively, resulting in a maximum amount of incorporation at 300 °C of 172 ppm and 768 ppm, respectively.

Physical Properties and Irradiation Behavior Analysis of Np- and Am-Bearing MOX Fuels

The physical properties and irradiation behavior of Np- and Am-bearing MOX were evaluated for the development of advanced fast reactor fuels. The physical properties, lattice parameter, melting temperature, thermal conductivity, and oxygen potential of the fuels were described as functions of Np content, Am content, and oxygen-to-metal (O/M) ratio, and the effect of a few percent Np and Am addition into MOX on those properties was found to be negligibly small. The irradiation tests of fuel pellets having O/M ratios of 1.98 or 1.96 were carried out at a high linear heat rate of about 430 W/cm for 24 h. The redistributions of actinide element and oxygen were analyzed by using the physical properties for Np- and Am-bearing MOX, and the maximum temperatures of the pellets were estimated. The maximum temperatures of the pellets of O/M = 1:96 and 1.98 were estimated to be 2,701 and 2,510 K, respectively. The analysis result showed that the higher maximum temperature in the low-O/M pellet was caused by the low thermal conductivity in the outer region of the pellets, and the maximum temperatures of the pellets did not exceed their melting temperatures. In the results of postirradiation examination, no trace of melting was observed.

Neptunium(V) sorption on kaolinite

Abstract The sorption behavior of neptunium(V) onto the clay mineral kaolinite was studied in batch experiments under different experimental conditions: [Np(V)]=7 × 10−12–8 × 10−6 M, solid-to-liquid ratio 2–20 g L−1, I=0.1 and 0.01 M NaClO4, pH=6–10, ambient air and Ar atmosphere. The short-lived isotope 239Np (T 1/2=2.36 d) was used instead of 237Np (T 1/2=2.14 × 106 a) to study the sorption behavior of Np(V) at environmentally-relevant concentrations, i.e., 7 × 1012 M Np. In addition, 239Np(V) served as tracer to measure sorption isotherms over six orders of magnitude in Np concentration (4.8 × 10−12–1.0 × 10−4 M). The results show that Np(V) sorption on kaolinite is strongly influenced by pH, CO2, and ionic strength. The sorption of 8 × 10−6 M Np(V) at pH 9.0, and ionic strength of 0.1 M NaClO4 was proportional to the solid-to-liquid ratio of kaolinite in the range of 2–10 g L−1. In the absence of CO2, the Np(V) uptake increased continuously with increasing pH value up to 97% at pH 10. Under ambient CO2, the sorption of Np decreased above pH 8 up to zero at pH 10. An increase of Np(V) concentration from 7 × 10−12 to 8 × 10−6 M resulted in a shift of the sorption pH edge by up to one pH unit to higher pH values. The ionic strength influenced the Np(V) sorption onto kaolinite only in the presence of ambient CO2. Under Ar atmosphere the sorption of Np(V) was independent from ionic strength, indicating the formation of inner-sphere complexes of Np(V) with kaolinite. Time-dependent batch experiments at pH 9.0 under ambient CO2 showed that the sorption of Np(V) on kaolinite is fast and fully reversible over six orders in Np(V) concentration.

On the stability of TiN-based electrocatalysts for fuel cell applications

The transition metal compound – titanium nitride (TiN), with its high electrical conductivity and corrosion resistance, can be a potential fuel cell material, particularly in the development of durable electrocatalysts replacing the state-of-the-art Pt/C. Compared to conventional carbon black, TiN nanoparticle (TiN NP) catalyst supports have a lower rate of corrosion under fuel cell conditions. The current research is a follow up study on the stability of TiN based fuel cell electrodes and its impact on the electrochemical activity of the Pt based electrocatalysts when used as an alternative to the state-of-the-art carbon black support in commercial fuel cell catalyst systems under fuel cell operating conditions. Through this paper, we report that an active behavior of TiN NP can be observed at the optimal conditions of 0.5 M H2SO4 and 60 °C. But under increased temperature or acidic conditions, the native layer on TiN tends to dissolve in the electrolyte exposing the underlying nitride surface which then gets passivated with hydroxide groups. Electrochemical and XPS characterization is used to validate our hypothesis of active or passive nature of TiN NP due to presence or absence of surface passivating -OH groups, respectively. The synthesized Pt/TiN electrocatalyst, upon subjecting to accelerated durability test, showed the performance trends which agree well with the active and passive nature of the TiN NP supports.

Influence of Nitric Acid and Nitrous Acid on Oxidation and Extraction of Neptunium with Double Scrub Flow Sheet in Simplified Solvent Extraction Process

A simplified solvent extraction process based on U, Pu and Np co-recovery using tri-n-butylphosphate (TBP) as an extractant has been developed for advanced aqueous reprocessing of fast neutron reactor fuel. The influence of the HNO3 concentration in the feed and scrubbing solutions on the behavior of Np is evaluated experimentally and found to be co-extracted into the TBP with U and Pu. Moreover, the leakage ratio of Np to the raffinate is calculated from the changes in HNO2 concentration in the feed solution. 9.89% of the Np leaked to the raffinate in low HNO3 concentration feed and high HNO3 concentration scrubbing solutions. Almost all the Np in a 4.9 mol/dm3 HNO3 feed solution is recovered with U and Pu, based on the experimental flow sheet with double scrubbing solutions of 9 and 1 mol/dm3 HNO3. The experimental results show a large contribution from the HNO3 concentration in the feed solution and at the extraction section to Np(V) oxidation. On the other hand, the calculation results show that high HNO2 concentrations in the feed solution tended to leak Np into the raffinate. The DF of Cs for U, Pu and Np product achieves 105 in all runs.

Anisotropic Polyhedral Self-Assembly of Ag-CNT Nanocomposites

We report the unique assembling behavior of Ag NP supported CNT nanocomposites. Initially Ag NPs were precipitated homogeneously on the walls of SDS-coated CNTs by a NaBH4 chemical reduction and/or photoreduction directly in an aqueous solution. An additional Ag source was subsequently added into the Ag-CNT dispersion and Ag was further reduced using hydroxylamine in a weakly alkaline condition. The morphology of the resulting Ag-CNT nanocomposites strongly depended on the Ag+ ion concentration. Interestingly, an anisotropic polyhedral self-assembly of Ag-CNT nanocomposites was observed when the Ag concentration was increased to 0.1 M (real concentration).

The Role of Transferrin in Actinide(IV) Uptake: Comparison with Iron(III)

The impact of actinides on living organisms has been the subject of numerous studies since the 1950s. From a general point of view, these studies show that actinides are chemical poisons as well as radiological hazards. Actinides in plasma are assumed to be mainly complexed to transferrin, the iron carrier protein. This paper casts light on the uptake of actinides(IV) (thorium, neptunium, plutonium) by transferrin, focusing on the pH dependence of the interaction and on a molecular description of the cation binding site in the protein. Their behavior is compared with that of iron(III), the endogenous transferrin cation, from a structural point of view. Complementary spectroscopic techniques (UV/Vis spectrophotometry, microfiltration coupled with gamma spectrometry, and X-ray absorption fine structure) have been combined in order to propose a structural model for the actinide-binding site in transferrin. Comparison of our results with data available on holotransferrin suggests some similarities between the behavior of Fe(III) and Np(IV)/Pu(IV)/ Np(IV) is not complexed at pH <7, whereas at pH approximately 7.4 complexation can be regarded as quantitative. This pH effect is consistent with the in vivo transferrin "cycle". Pu(IV) also appears to be quantitatively bound by apotransferrin at around pH approximately 7.5, whereas Th(IV) was never complexed under our experimental conditions. EXAFS data at the actinide edge have allowed a structural model of the actinide binding site to be elaborated: at least one tyrosine residue could participate in the actinide coordination sphere (two for iron), forming a mixed hydroxo-transferrin complex in which actinides are bound with transferrin both through An-tyrosine and through An--OH bonds. A description of interatomic distances is provided.

The reduction of Np(VI) and Np(V) by tit dihydroxyurea and its application to the U/Np separation in the PUREX process

The reduction of Np(VI) and Np(V) by Dihydroxyurea (DHU) was studied by spectrophotometry. The results show that the reduction of Np(VI) to Np(V) by DHU is particularly fast. The apparent rate constant is 1.86 s -1 at 4 °C as [HNO 3 ] = 0.44 M and [DHU] = 7.5 x 10 -2 M. While further reduction of Np(V) to Np(IV) is so slow that no Np(IV) is observed in 2 h. The reduction back-extraction behavior of Np(VI) in 30% tri-butyl phosphate/kerosene was firstly investigated under conditions of different temperature, different concentrations of DHU and HNO 3 and various phase contact time, respectively. The results show that 98% of Np(VI) in the organic phase can be stripped rapidly to the aqueous phase by DHU under the given experimental conditions. As the concentration of HNO 3 in the aqueous phase increases, the stripping efficiency decreases. While the stripping efficiency increases with the increase of the concentration of DHU. Simulating the 1B contactor of the PUREX process using DHU as the stripping agent, the SF U /Np equals to 183 under the given experimental conditions. It indicates that Np will follow with Pu in the U/Pu separation stage in the reprocessing of spent fuels.