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Influence of pH on the speciation and stability of heptavalent neptunium is poorly understood although it is frequently invoked in the literature to explain experimental observations. The present study employs Density Functional Theory (DFT) methodology to assess the thermodynamic feasibility of protonation reactions for the Np(VII) anion complex and the impact on its reduction to Np(VI). This theoretical framework is then explored experimentally through the titration and systematic protonation of Np(VII) in solution and solid-state samples while monitoring them spectroscopically. Computational results reveal that protonation reactions with the axial OH− ligands of the Np(VII) anionic complex, [NpO4(OH)2]3−, are more thermodynamically favorable than the equatorial oxo ligands. In addition, DFT studies indicated that up to four sequential protonation reactions may be feasible before reduction becomes thermodynamically favorable. Experimental results also uncover that protonation leads to distinct changes in the observable vibrational signals and UV-Vis absorption features. Overall, we observed that the protonation of [NpO4(OH)2]3− in solution and in the solid-state occurs before reduction to the Np(VI)O22+ species. [ABSTRACT FROM AUTHOR]

A new method for the direct measurement of 237Np/238U ratio in irradiated UO2 pellets by multicollection inductively coupled plasma mass spectrometry (MC-ICP-MS) is proposed. It allows the determination of ratios down to 10 × 10–6 mol·mol−1 using ion counter and Faraday cup. This approach was validated by intercomparison with the usual two-step-method (Quadrupole ICP-MS for 237Np determination and isotope dilution mass spectrometry (IDMS) for 238U). For ratios between 10 × 10–6 and 100 × 10–6 mol·mol−1, expanded uncertainties (k = 2) varied from 2.75% to 0.81%, twice lower than the uncertainties determined by the usual method. [ABSTRACT FROM AUTHOR]

Neutron total cross-section data for 232Th, 237Np, 235,238U, and 239Pu were measured over the energy range of a few MeV up to 230 MeV at the Weapons Neutron Research Facility of the Los Alamos Neutron Science Center. The results for 235U, 237Np, and 239Pu are the first to be reported over this full energy range. Our data are compared to the ENDF/B-VIII.0 and JENDL-5 evaluations and to selected previous data sets. These results provide neutron cross sections with small statistical uncertainties and low systematic errors to improve theoretical modeling and data evaluation for these nuclei. [ABSTRACT FROM AUTHOR]

In the PUREX (the plutonium uranium reduction extraction) process, a plutonium barrier unit (1BXX) is used to achieve deep plutonium stripping. According to the operating experience of the French reprocessing plant, after the separation of uranium and plutonium in the first cycle (1B + 1BXX), the plutonium barrier unit has excellent stripping effect, such that the removal of plutonium from uranium can already be achieved in the first cycle, and the second cycle only needs to focus on the removal of neptunium from uranium in order to obtain a qualified uranium product. In recent decades, China has also been actively conducting research on the plutonium barrier unit process to reduce the plutonium concentration in the primary uranium product in the first cycle to avoid the need to remove neptunium and plutonium at the same time in the second cycle, and to improve the efficiency and feasibility of reprocessing. Due to the lack of design basis for plutonium barriers to achieve deep plutonium stripping at present, this study conducts a basic study on the plutonium barrier unit, aiming to provide data for the optimization of plutonium barriers in the actual reprocessing process at a later date. In this work, a kinetic study on the reduction and stripping of trace plutonium from dibutyl phosphate-containing organic phases was carried out first, and the kinetic equations for the reduction and stripping of Pu(IV) by U(IV) under flow process conditions were obtained. The effects of U(IV) addition on the extraction loss of U(IV) and the concentration distribution of U(IV) at various stages were investigated by process simulation. Additionally, the oxidation of U(IV) under process conditions was investigated to clarify the process chemistry of U(IV) oxidation and to provide a reference for the oxidation consumption of U(IV). Finally, the process parameters of the plutonium barrier unit were preliminarily designed based on the above research. [ABSTRACT FROM AUTHOR]

A new method for the rapid quantification of neptunium 237 in nuclear effluent was developed. This method is based on the direct measurement of neptunium 237 by ICP-MS/MS in mass-shift mode as NpO and using 20% CO2 as a reaction gas. These measurement conditions allowed the best signal-to-noise ratio at m/z 253 (237Np16O) to be obtained by the efficient elimination of the interferences due to the abundance sensitivity of uranium 238 and hydrides of uranium 235 and uranium 236 while minimizing the elemental fractionation between neptunium and plutonium. Moreover, plutonium 242 can be used as a tracer for neptunium 237 isotopic dilution-based quantification. After an optimization of the measurement parameters, excellent figures of merit were obtained. Indeed, this protocol allows an accurate direct measurement of neptunium 237 in various nuclear effluent samples containing up to 2 mg L-1 (2.4 Bq L-1) of uranium 236 and up to 20 mg L-1 (250 Bq L-1) of uranium 238 with excellent accuracy and uncertainties lower than 15%, for samples containing 10-5 to 10-1 mg L-1 (10-3 to 13 Bq L-1), and a limit of detection for 237Np as low as 4 pg L-1 (10-4 Bq L-1). [ABSTRACT FROM AUTHOR]

The extraction of Np4+ was studied by a series of N-pivot tripodal diglycolamides, with varying substituents (alkyl groups with varying branching and chain length) and varying spacer length, dissolved in an ionic liquid, i.e. [C4mim][Tf2N]. Out of a series of four ligands, LI (propylene spacer) and LII (ethylene spacer) differ only in the spacer length connecting the three diglycolamide units with the tripodal N platform. While LII – LIV have the same spacer length (ethylene spacer), but LII has a hydrogen atom, LIII has methyl group, and LIV has isopropyl group as a substituent at the amidic N atom close to the central N atom. A reversal extraction mechanism of Np4+ was observed when the spacer chain length reduced from a propylene group in LI to an ethylene group in LII – LIV. While a cation-exchange mechanism was observed in case of LI, a solvation extraction mechanism was seen for the other three ligands with the extracted species being Np(NO3)2(L)2+ for LI, and [Np(NO3)4·L] for LII – LIV. The extraction efficiencies of Np4+ with LII – LIV were significantly affected by the nature of the alkyl substituents and follow the order: LII > LIII > LIV. This trend in the extraction pattern affirms that the alkyl substitution at the amidic N atom creates steric strain in the ligand while complexing with the metal ion. [ABSTRACT FROM AUTHOR]

A radioanalytical method was developed for the determination of trace plutonium and neptunium in samples composed primarily of uranium. The procedure uses a neodymium oxalate co-precipitation and a two-column separation using ion exchange resin to achieve high uranium decontamination, high plutonium and neptunium separation factors and low impurity levels. The separation of the three actinides from each other relies on their oxidation–reduction chemistry. Various measurement techniques can be applied to determine plutonium and neptunium in the purified fractions: i.e., alpha spectrometry, and thermal ionization mass spectrometry. The method development, validation, and applications are discussed. [ABSTRACT FROM AUTHOR]

Two new complex Np(VI) nitrates with carbamoylurea (biuret) and with a derivative of terpyridine (Ph-Terpy) of the composition [(NpO2)(biuret)(NO3)2] (I) and [H2(Ph-terpy)(NO3)]2[NpO2(NO3)4] (II) have been synthesized and structurally characterized. The coordination environment of Np atoms in compounds are distorted hexagonal bipyramids with "yl" oxygen atoms in apical positions. The equatorial plane of the bipyramid in complex I is formed by oxygen atoms of two bidentate NO3– anions and an electroneutral ligand. The equatorial plane of the bipyramid in II is formed by oxygen atoms of two bidentate and two monodentate NO3– anions. The twice protonated Ph-terpy is the basis of the complex cation [H2(Ph-terpy)(NO3)]+. [ABSTRACT FROM AUTHOR]

The atomic structure of neptunium (Np) was investigated by two-step resonance ionization spectroscopy. The study involved exploring ground-state transitions as well as following transitions to high-lying states just below the ionization potential (IP) or auto-ionizing states above the IP. That resulted in the identification of two-step ionization schemes, suitable for trace analysis and nuclear structure investigations. The lifetimes of two excited states located at 25,342.48 cm - 1 and 25,277.64 cm - 1 were determined as 230(12) ns and 173(9) ns, respectively. Because of the absence of Rydberg series in wide-ranging spectra recorded, the first IP was determined through the field ionization of high-lying, weakly-bound states using a well-controlled static electric field. By applying the saddle-point model, an IP value of 50,535.54(15) cm - 1 [6.265608(19) eV] was derived. This value agrees with the current literature value of 50,535(2) cm - 1 , while providing a more than ten times higher precision. [ABSTRACT FROM AUTHOR]

Separation of neptunium from an acidic feed containing 0.063 mol/L uranium in 3 mol/L HNO3 was demonstrated in counter-current extraction in mixer-settlers with TBP (tri-n-butyl phosphate) and DHOA (N,N'-dihexyl octanamide) as the organic extractants. With a feed solution of 0.063 mol/L uranium (in the absence of neptunium) and at an organic to aqueous phase ratio (O/A) = 1, >99.9% uranium could be extracted into the organic phase in 6 stages with the raffinate containing <10−5 mol/L of uranium. The extraction results with TBP and DHOA were comparable. However, stripping of uranium with 0.01 mol/L HNO3 from the loaded organic phase was better with DHOA vis-a-vis TBP phase. From the DHOA phase, loaded with 0.063 mol/L uranium, the entire metal ion could be stripped in 6 stages at an O/A = 1. However, ca. 17% uranium remained in the TBP phase even after 6 stages of stripping. With these preliminary results, the extraction of neptunium was systematically studied in mixer-settlers from feed containing uranium, and from simulated high level liquid waste. Our results of neptunium extraction in the presence of 0.021 mol/L uranium yielded excellent result. Similarly, the extraction of neptunium from simulated high level liquid waste was also demonstrated with good extraction efficiency. [ABSTRACT FROM AUTHOR]

Neptunium can exist in multiple oxidation states, including the rare and poorly understood heptavalent form. In this work, we monitored the formation of heptavalent neptunium [Np(VII)O4(OH)2]3− during ozonolysis of aqueous MOH (M=Li, Na, K) solutions using a combined experimental and theoretical approach. All experimental reactions were closely monitored via absorption and vibrational spectroscopy to follow both the oxidation state and the speciation of neptunium guided by the calculated vibrational frequencies for various neptunium species. The mechanism of the reaction partly involves oxidative dissolution of transient Np(VI) oxide/hydroxide solid phases, the identity of which are dependent on the co‐precipitating counter‐cation Li+/Na+/K+. Additional calculations suggest that the most favorable energetic pathway occurs through the reaction of a [Np(V)O2(OH)4]3− with the hydroxide radical to form [Np(VI)O2(OH)4]2−, followed by an additional oxidation with HO⋅ to create [Np(VII)O4(OH)2]3−. [ABSTRACT FROM AUTHOR]

It is critical to develop the methods and technologies of studying the species and concentration in the processing of adsorption, and diffusion of some important radionuclides in spent fuel waste processing such as Beishan granites in domestic institutions and universities for the geological disposal of high-level radioactive wastes. Enrich the models used or constructed and the calculation procedures developed in the past two decades in this field. In order to evaluate the safety of the repository, chemical speciation analysis of nuclides is the premise of research to accurately predict the migration behavior of nuclides. In view of the rapid development of China's nuclear energy industry and the need to establish the identification of nuclides in the environment, it is of great significance to carry out the study of the chemical forms of nuclides around the repository in China. Aiming at the research requirements of element speciation distribution in the environment, a set of B/S architecture element speciation distribution simulation software is developed with JAVA as the development tool, MySQL as the database, and Tomcat as the container. In order to solve the convergence problem of chemical reaction equilibrium nonlinear equations encountered in simulation calculation, the method based on chemical reaction potential energy is introduced to solve the chemical reaction nonlinear equations quickly, and the multi-phase equilibrium calculation can be expanded. SSCS was used to study the species of U in groundwater and the effects of Eh, pH, different ions and concentrations. The calculation results indicate that neptunium mainly exists as NpO2CO-3(aq) in groundwater. Under strong acidic conditions, neptunium maily exists as NpO+2(aq), while in weak acidic, neutral or weak alkaline conditions, the main species are NpO+2(aq) and NpO2OH.When the solution is in strong alkaline conditions, the species are complicated most of neptunium exists as NpO2(CO3)-3(aq), with the possible generation of NpO2(CO3)3-2 or NpO2(CO3)5-3. Np(Ⅴ) is stable in the reduction condition, while Np(Ⅳ) and Np(Ⅵ) are the possible generation. Comparing the neptunium calculation results of the software in the environment with the PHREEQC calculation results, the relative error is within 10 %. For the distribution of trace elements in the environment, a theoretical calculation solution is provided.

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The content of 237Np, 239,240Pu, and 241Am in seawater, suspended matter, and bottom sediment cores of the Ussuri and Amur Bays, sampled in July 2021 and August 2022, was analyzed. The activity concentrations of 237Np, 239,240Pu, and 241Am in the water of Peter the Great Bay were determined for the first time and were found to be equal to (19–105) × 10–3, 2.0–5.3, and 24.1–33.5 mBq/m3, respectively. The activity concentrations as well as the 239Pu/240Pu isotope ratio (⁓0.18) in the bottom sediments definitely indicate that global fallout is the main source of plutonium in the investigated territory. The sedimentation rates were determined for the Amur Bay (in the Razdolnaya River estuary), 0.9 mm/year, and for the Ussuri Bay, 4.1 mm/year. Actinide transfer coefficients were determined for Peter the Great Bay in the system dissolved forms (including colloids)–suspended matter–bottom sediment. [ABSTRACT FROM AUTHOR]

We report the formation of a NpIV complex from the complexation of NpVIO22+ with the redox‐active ligand tBu‐pdiop2−=2,6‐bis[N‐(3,5‐di‐tert‐butyl‐2‐hydroxyphenyl)iminomethyl]pyridine. To the best of our knowledge, this is the first example of the direct complexation‐induced chemical reduction of NpVIO22+ to NpIV. In contrast, the complexation of UVIO22+ with tBu‐pdiop2− did not induce the reduction of UVIO22+, not even after the two‐electron electrochemical reduction of [UVIO2(tBu‐pdiop)]. This contrast between the Np and U systems may be ascribed to the decrease of the energy of the 5 f orbitals in Np compared to those in U. The present findings indicate that the redox chemistry between UVIO22+ and NpVIO22+ should be clearly differentiated in redox‐active ligand systems. [ABSTRACT FROM AUTHOR]

The accurate prediction of the extraction behaviors of various solutes in PUREX reprocessing process is crucial for the operation and implementation of the actual process. In this paper, an integrated model is developed to predict the extraction behaviors of U, Np, Pu, and HNO3 in the co-decontamination step (1A extraction step) in a pulsed extraction column. The model couples several physical and chemical processes, such as countercurrent flow, mass transfer, and chemical reaction. The mass transfer coefficients and the distribution ratios of U(VI), Pu(IV), Np(IV), Np(V), Np(VI), HNO3 and HNO2 can be obtained using this model. In particular, the redox and disproportion reactions of Np are considered in the model, and the flow direction of Np can be judged under various process conditions and the corresponding influential factors can be analyzed. The judgment is based on the yield calculated from the relative concentration profiles of Np(VI), Np(V), and Np(IV) in the two phases. For neptunium to inter 1AP step, it is necessary to select high nitric acid concentration, low nitrite concentration and especially high flow ratio. For neptunium to inter 1AW step, low nitric acid concentration, high nitrite concentration and low flow ratio are needed. Compared with the experimental data, the relative errors of the distribution ratios of various solutes are less than 30%, and the relative errors of the concentrations of U(VI) and Pu(IV) at the outlet of the organic phase are less than 10%, and our model is indicated to be reliable and applicable for co-decontamination step in the PUREX reprocessing process. [ABSTRACT FROM AUTHOR]

The release of radionuclides from high-burnup spent nuclear fuel (SNF) segments was studied at pH = 13.2 as well as the effect of the presence of calcium and silicon. The aim was to ascertain the dissolution of SNF in solutions corresponding to a high-level nuclear waste repository including concrete in different structural parts. The release of uranium at pH = 13.2 was higher than at pH = 8.4 in bicarbonate medium, while the presence of calcium resulted in a decrease of the uranium concentrations in solutions, assumed to be the consequence of the formation of a secondary solid phase such as Ca2U2O7. Caesium release was found higher at pH = 13.2 as well, but it was not influenced by the presence of Ca and Si at long term. On the other hand, actinide elements (plutonium, neptunium and americium) dissolution decreased at pH = 13.2, probably because of the formation of secondary solid phases. On the contrary, ruthenium and technetium release at pH = 13.2 was found to be much higher than the measured at lower pH, perhaps due to the higher dissolution kinetics of the metallic inclusions at such pH. [ABSTRACT FROM AUTHOR]

The study of the redox chemistry of mid‐actinides (U−Pu) has historically relied on cerium as a model, due to the accessibility of trivalent and tetravalent oxidation states for these ions. Recently, dramatic shifts of lanthanide 4+/3+ non‐aqueous redox couples have been established within a homoleptic imidophosphorane ligand framework. Herein we extend the chemistry of the imidophosphorane ligand (NPC=[N=PtBu(pyrr)2]−; pyrr=pyrrolidinyl) to tetrahomoleptic NPC complexes of neptunium and cerium (1‐M, 2‐M, M=Np, Ce) and present comparative structural, electrochemical, and theoretical studies of these complexes. Large cathodic shifts in the M4+/3+ (M=Ce, U, Np) couples underpin the stabilization of higher metal oxidation states owing to the strongly donating nature of the NPC ligands, providing access to the U5+/4+, U6+/5+, and to an unprecedented, well‐behaved Np5+/4+ redox couple. The differences in the chemical redox properties of the U vs. Ce and Np complexes are rationalized based on their redox potentials, degree of structural rearrangement upon reduction/oxidation, relative molecular orbital energies, and orbital composition analyses employing density functional theory. [ABSTRACT FROM AUTHOR]

We present a combined TEVA-DGA chromatography chemistry to purify single-element fractions of Pu, Am, and Np from bulk (mg-level) U materials. Plutonium and Np are first sorbed onto TEVA resin using a 4 + redox adjustment with a hydroxylamine hydrochloride and sodium nitrite treatment, with U + Am eluting directly onto DGA resin. Americium is then purified from U on DGA using 0.1 M HNO3 as a U eluent. A subsequent TEVA column is used to separate Pu from Np using hydrogen peroxide to oxidize Np into the 5 + oxidation state for elution through TEVA. Our presented TEVA-DGA method is able to produce high-purity, single-element fractions of Pu (90% recovery), Am (98% recovery), and Np (62% recovery) from mg-levels of U, and does so in three days of column chemistry. [ABSTRACT FROM AUTHOR]

Increased focus on carbon neutral energy has generated a resurgence of interest in nuclear power, and in particular advanced reactors which are likely to utilize high assay low enriched uranium (HALEU). This in turn could increase the economic attractiveness of recovering still partially enriched uranium from used nuclear fuel. Concomitant to development of advanced reactors, advanced reprocessing schemes should be developed which address the disadvantages to well established reprocessing schemes. The present study focuses on using di-(2-ethylhexyl)-iso-butyramide (DEHiBA) under high metal loading conditions for the reprocessing of used nuclear fuel. The elements examined in the study include the dominant transuranic actinides (Np, Pu, Am) as well as the often-problematic Tc and Zr. By increasing the concentration of the extractant from the more commonly reported 1.0 M – 1.5 M, the extraction of hexavalent actinides is substantially increased, while maintaining effective rejection of tri, tetra, and pentavalent actinides, particularly in the presence of high loadings of uranium. The extraction of Zr by 1.5 M DEHiBA is noted to be negligible by comparison to tributyl phosphate (TBP), however the coextraction of Tc with U is observed to be nominally twice the quantity that is extracted by TBP indicating a need for effective Tc management. [ABSTRACT FROM AUTHOR]

Actinyl‐actinyl interactions are particularly prevalent for the pentavalent neptunyl cation (Np(V)O2)+ where these interactions appear either as a T‐ or D‐shape (diamond‐shape). T‐shaped interactions have been previously identified in high concentration Np(V) solutions containing simple anions (NO3−, ClO4−, Cl−) whereas D‐shaped have only been isolated in the solid‐state in the presence of carboxylate ligands. In this study, Density Functional Theory (DFT) calculations were paired with Raman spectroscopy to evaluate the formation of D‐shaped interactions in the presence of aliphatic (R=H (formate), CH3 (acetate), CH2CH3 (propionate)) and aromatic (R=C6H5 (benzoate), C6H4OH (4‐hydroxybenzoate), C5H4N (isonicotinate)) carboxylate ligands. DFT studies indicate that the ΔG to form hydrated T‐ and D‐shaped forms are not spontaneous but become so with the addition of the carboxylate ligands. Raman spectra of the Np(V) carboxylate solutions contained vibrational modes associated with the D‐shaped interactions, but spectral changes observed over time indicate a dynamic system. Crystallization experiments from the Np(V) carboxylate systems confirmed the presence of D‐shaped dimers for the aromatic carboxylates, suggesting that the choice of the anion in solution favors actinyl‐actinyl interactions even at low concentrations (≤20 mM) of Np(V). [ABSTRACT FROM AUTHOR]

A robust hierarchical model has been demonstrated for monitoring a wide range of neptunium concentrations (0.75–890 mM) and varying temperatures (10–80 °C) using chemometrics and feature selection. The visible–near infrared electronic absorption spectrum (400–1700 nm) of monocharged neptunyl dioxocation (Np(V) = NpO2+) includes many bands, which have molar absorption coefficients that differ by nearly 2 orders of magnitude. The shape, position, and intensity of these bands differ with chemical interactions and changing temperature. These challenges make traditional quantification by univariate methods unfeasible. Measuring Np(V) concentration over several orders of magnitude would typically necessitate cells with varying path length, optical switches, and/or multiple spectrophotometers. Alternatively, the differences in the molar extinction coefficients for multiple absorption bands can be used to quantify Np(V) concentration over 3 orders of magnitude with a single optical path length (1 mm) and a hierarchical multivariate model. In this work, principal component analysis was used to distinguish the concentration regime of the sample, directing it to the relevant partial least squares regression submodels. Each submodel was optimized with unique feature selection filters that were selected by a genetic algorithm to enhance predictions. Through this approach, the percent root mean square error of prediction values were ≤1.05% for Np(V) concentrations and ≤4% for temperatures. This approach may be applied to other nuclear fuel cycle and environmental applications requiring real-time spectroscopic measurements over a wide range of conditions. [ABSTRACT FROM AUTHOR]

Two new Np(V) azide complexes with electroneutral N-donor ligands of terpyridine derivatives have been synthesized and structurally characterized: [(NpO2)(Cl-Terpy)(N3)(H2O)] (1) and a centrosymmetric dimer [(NpO2)(EtO-Terpy)(N3)]2·2CH3OH (2). The coordination polyhedra of Np atoms in compounds are pentagonal bipyramids with "yl" oxygen atoms in apical positions. The equatorial plane of the bipyramid in complex 1 is formed by: a nitrogen atom from an anion, three nitrogen atoms from an electroneutral Cl-Terpy ligand and an oxygen atom of a water molecule. The equatorial plane of bipyramids in dimer 2 is formed by five nitrogen atoms: two atoms from two anions [N3]− and three atoms from EtO-Terpy. In compound 1, the anion [N3]− acts as a monodentate ligand, and in compound 2, the anion [N3]− is a bidentate-bridging ligand. [ABSTRACT FROM AUTHOR]

Uranyl peroxide nanoclusters are an evolving family of materials with potential applications throughout the nuclear fuel cycle. While several studies have investigated their interactions with alkali and alkaline earth metals, no studies have probed their interactions with the actinide elements. This work describes a system containing U60Ox30, [((UO2)(O2))60(C2O4)30]60−, and neptunium(V) as a function of neptunium concentration. Ultra‐small and small angle X‐ray scattering were used to observe these interactions in the aqueous phase, and X‐ray diffraction was used to observe solid products. The results show that neptunium induces aggregation of U60Ox30 when the neptunium concentration is≤10 mM, whereas (NpO2)2C2O4 ⋅ 6H2O(cr) and studtite ultimately form at 15–25 mM neptunium. The latter result suggests that neptunium coordinates with the bridging oxalate ligands in U60Ox30, leaving metastable uranyl peroxide species in solution. This is an important finding given the potential application of uranyl peroxide nanoclusters in the recycling of used nuclear fuel. [ABSTRACT FROM AUTHOR]

Uranyl peroxide nanoclusters are an evolving family of materials with potential applications throughout the nuclear fuel cycle. While several studies have investigated their interactions with alkali and alkaline earth metals, no studies have probed their interactions with the actinide elements. This work describes a system containing U60Ox30, [((UO2)(O2))60(C2O4)30]60−, and neptunium(V) as a function of neptunium concentration. Ultra‐small and small angle X‐ray scattering were used to observe these interactions in the aqueous phase, and X‐ray diffraction was used to observe solid products. The results show that neptunium induces aggregation of U60Ox30 when the neptunium concentration is≤10 mM, whereas (NpO2)2C2O4 ⋅ 6H2O(cr) and studtite ultimately form at 15–25 mM neptunium. The latter result suggests that neptunium coordinates with the bridging oxalate ligands in U60Ox30, leaving metastable uranyl peroxide species in solution. This is an important finding given the potential application of uranyl peroxide nanoclusters in the recycling of used nuclear fuel. [ABSTRACT FROM AUTHOR]

The recovery of neptunium and plutonium play an important role in the spent nuclear fuel reprocessing process. Neptunium and plutonium are multiple oxidation states stable in solution and the varying conditions of the involved processes make it very difficult to effectively restrict them within desired streams in the relevant separation processes, and to precisely depict their species and ideally control their distribution in the environment. In order to control the valence of neptunium and plutonium in nitric acid and 30% TBP-nd, the structures and properties need to be deeply understood, especially the coordination behavior with nitrate. The special 5f orbital electron transition of actinides results in the characteristic absorption spectra of neptunium and plutonium in the visible-near infrared region, therefore, the valence and species structure of neptunium and plutonium in the relevant process points can be studied by absorption spectroscopy. In this paper, the species composition of neptunium in spent fuel reprocessing process, especially the distribution of Np (IV), (V), (VI) in the aqueous and organic phase in dissolution and co-decontamination unit were studied by absorption spectroscopy, the coordination structure of neptunium with nitrate and the spectra in 30% TBP- n-dodecane were also studied. On the other hand, the distribution of Pu (III), (IV), (VI) in the process were studied by absorption spectroscopy method. [ABSTRACT FROM AUTHOR]

NpO2(NO3)2 units are connected by bis(2-pyrrolidone) linker molecules with the trans-1,4-cyclohexyl bridging part (L1) to form a one-dimensional coordination polymer, [NpO2(NO3)2(L1)]n. Molecular and crystal structures of this compound are nearly identical to that of the UO22+ analogue, while its aqueous solubility is greatly enhanced, probably owing to weaker thermodynamic stability of the complexation in NpO22+ compared with that in UO22+. [ABSTRACT FROM AUTHOR]

Pertraction of Np(IV) across a PTFE membrane containing two different aza-crown ether-based multiple diglycolamide (DGA) ligands termed as LI (3 DGA arms) or LII (4 DGA arms) was investigated using nitric acid as feed solution. A few of the studies were also carried out with Pu(IV) for comparison purposes. The kinetics of the extraction of Np(IV) and Pu(IV) was fast at 3 M HNO3 and within 10 min reaches the equilibrium distribution (D) ratio with both ligands. However, the stripping kinetics was slower especially with ligand LII. The extraction and transport studies were carried out using 1.0 × 10−3 M ligand solution in 95% n-dodecane + 5% isodecanol. The transport rates were slower with ligand LII than with LI at higher nitric acid concentrations (>1 M HNO3) and the transport rates decreased with nitric acid concentration with both ligands. The effective diffusion coefficients (Deff) of Np(IV) were estimated using the lag-time method as 6.2 × 10−8 cm2 s−1 (LI) and 4.6 × 10−8 cm2 s−1 (LII). The stability data suggest that the LI containing supported liquid membrane (SLM) is comparatively more stable than that of LII and the transport flux remains the same up to nine days of operation in case of ligand LI. [ABSTRACT FROM AUTHOR]

The production of transuranium isotopes has been demonstrated at the Isotope Separation and ACceleration (ISAC) facility. In particular a laser-ionized 239Pu beam, extracted from a uranium carbide target, was investigated. The experimental work was complemented by G eant 4 simulations, modelling the impact of secondary particles on the creation of transuranium isotopes through inelastic nuclear reactions. Theoretical production cross sections were derived and compared to experimental results, leading to a discussion on boundary conditions for the release of neptunium and plutonium from ISAC uranium carbide targets. [ABSTRACT FROM AUTHOR]

Three new complexes of Np(V) dicyanamide with electroneutral N-donor ligands have been synthesized and structurally characterized: two with terpyridine derivatives: [(NpO2)(Cl-Terpy)(DMA)(N(CN)2)]·0.5DMA (1), [(NpO2)(TPTZ)(N(CN)2)(H2O)]·2H2O·CH3OH (2) and a dimeric complex with bipyridine [(NpO2)(Bipy)(N(CN)2)(H2O)]2·2(CH3OH) (3). In compounds 1–3, the coordination polyhedra of Np atoms are pentagonal bipyramids with "yl" oxygen atoms in apical positions. The equatorial plane of the bipyramids is formed by nitrogen atoms of the anions [N(CN)2]− and N-donor electroneutral ligands (Cl-Terpy, TPTZ, Bipy) and the oxygen atom of water molecule (1, 3) or DMA (2). In compounds 1 and 2, the [N(CN)2]− anions manifest themselves as monodentate ligands, and in compound 3 dicyanamide is a bidentate-bridging ligand. [ABSTRACT FROM AUTHOR]

Raman spectra of three NpO 2 samples and two samples produced from a modified direct denitration (MDD) process were collected. The spectral features of the NpO 2 samples were consistent and indicated only NpO 2. The spectra of the MDD samples indicated the presence of NpO 2 and an additional phase attributed to the neptunium binary oxide Np 2 O 5. These Raman spectra are the first reported of Np 2 O 5 , and the proportions of these neptunium oxide phases varied within the samples, suggesting significant sample inhomogeneity. Peaks in the Raman spectra of Np 2 O 5 at 569 and 782 cm−1 were tentatively assigned to concerted, symmetric stretches of the neptunyl cations. Laser-induced heating of regions in the MDD samples that were rich in Np 2 O 5 showed spectral features that indicated conversion to NpO 2. [Display omitted] Characterizing Np 2 O 5 and NpO 2 by Raman spectroscopy. • First reported Raman spectra of Np 2 O 5. • Assigned neptunyl symmetric stretching modes near 569 and 782 cm−1. • Identified numerous bands in the Np 2 O 5 Raman spectrum to inspire future computations. • Systematic laser-annealing of Np 2 O 5 produced NpO 2. • Reproducible Raman spectra of multiple high fired NpO 2 samples. [ABSTRACT FROM AUTHOR]

In this work, we investigated the stability trends of actinide-bearing hollandites using density functional theory calculations. Incorporating actinides generally decreased the stability of cesium end-members relative to their barium equivalents. Among the actinides, neptunium-containing structures were consistently the least stable. Curium was the most stable actinide in the +3 oxidation state, followed by americium, plutonium, uranium, and neptunium. Conversely, in the +4 state, uranium-bearing hollandites were the most stable. An inverse correlation between actinide content and stability was consistently observed. [ABSTRACT FROM AUTHOR]

σ-Hydrocarbyl complexes of the form [M(η5-PC4Me4)2(μ-η1:η6-CH2Ph)2K(η6-arene)] (M = La, Ce, Pr, U, Np, Pu; arene = benzene or toluene) were synthesised in one-pot reactions from [MI3(THF)4], or [U(BH4)3(toluene)] (M = U). All complexes were examined by multinuclear (1H, 13C{1H}, 31P{1H}) NMR and UV-vis-NIR spectroscopy, as well as single-crystal X-ray diffraction from which molecular metal–phosphorus bonds for Np and Pu, and a σ-hydrocarbyl metal–carbon bond for Pu, have been structurally authenticated. [ABSTRACT FROM AUTHOR]

The sorption properties of mineral phases that constitute the rocks of the Yeniseysky area of the Nizhnekansky granitoid massif toward Np, Pu, Am, Sr, and Cs were studied under conditions of underground radioactive waste disposal. As a result of sorption experiments, sorption kinetic curves were obtained, and the distribution coefficients (Kd) of radionuclides on mineral phases were determined. The resulting data made it possible to evaluate the differences in the sorption properties of minerals towards the studied radionuclides. For each radionuclide, with the exception of americium, it was possible to establish the most effective mineral phases in terms of sorption: for Np(V)—hydroxyapatite and calcite, for Pu(V)—hydroxyapatite and magnetite, for Sr(II)—biotite and hydroxyapatite, for Cs(I)—biotite and muscovite. Americium was sorbed quantitatively on all mineral phases, except for biotite and quartz. [ABSTRACT FROM AUTHOR]

The formal potentials of Np(VI)/Np(V) and Pu(VI)/Pu(V) couples in acetonitrile–water and ethanol–water solutions containing 0.01 M HClO4 at 25°С were measured by the potentiometric method. In acetonitrile–water solutions, the potentials of both couples decrease with decreasing water content from 55.55 to 30 M. This is caused by the hydrolysis of NpO22+ and PuO22+ ions. With a further decrease in the proportion of water, the potentials of the couples do not change. This indicates that the hydration numbers of An(VI) and An(V) are the same and equal to 5. Similar results were obtained in ethanol–water solutions. [ABSTRACT FROM AUTHOR]

This work presents details of the synthesis, properties and structure of a novel neptunium carbonate (NH4)[NpO2CO3], a member of the M[AnO2CO3] (M = K, (NH4), Rb, Cs) class of compounds. Carbonates play an important role in the migration of actinides in the environment, and thus are relevant for handling and disposal of radioactive wastes, including spent nuclear fuel and vitrified raffinates. Knowledge of the crystallographic structure of these compounds is important for models of the environmental migration behavior based on thermodynamic descriptions of such chemical processes. (NH4)[NpO2CO3] crystals were obtained during long-term hydrothermal treatment of Np(VI) in aqueous ammonia at 250 °C. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) show that a single-phase sample containing only Np(V) was obtained. Structural features of (NH4)[NpO2CO3] were elucidated from single crystal X-ray diffraction and confirmed by vibrational spectroscopy. The results obtained are of interest both for fundamental radiochemistry and for applied problems of the nuclear fuel cycle. [ABSTRACT FROM AUTHOR]

Two new approaches to measure Np concentration in dissolved used nuclear fuel simulant (aqueous feed for PUREX process) by spectrophotometry are developed. The first approach is based on chemical reduction of Np in the feed to its tetravalent state using ascorbic acid with simultaneous conversion of Pu(IV) to Pu(III). Interfering effects from light absorbing fission and corrosion products are accounted for by measuring optical absorbance spectrum of aqueous raffinate after extraction of U, Np, and Pu by tributyl phosphate in dodecane. The second approach uses no chemical treatment at all and relies on spontaneous valency adjustment of Np to Np(V) by dilution of the feed with water to reduce its acidity to low decimolar range of nitric acid concentration. Results of Np determination in the feed by spectrophotometry are in good agreement with its concentration measured by ICP-MS. [ABSTRACT FROM AUTHOR]

Liquid–liquid extraction and liquid membrane transport behavior of tetravalent actinide ions viz. Th(IV), Np(IV), and Pu(IV) were investigated for the first time using a diglycolamide (DGA) based dendrimer with a tris(2-aminoethyl)amine (TREN) scaffold as the organic extractant. The generation of 1 dendrimer with six DGA pendent moieties (termed as TREN-G1-DenDGA) extracted Np(IV) more effectively than the other two ions, the trend being Np(IV) > Pu(IV) > Th(IV). The extraction studies of Np(IV) from 3 M HNO3 indicated a 1:1 (metal:ligand) species and the extraction efficiency increased with increasing nitric acid concentration (1–6 M). The transport efficiency of Np(IV) increased with the nitric acid concentration (1–6 M) as well as with the ligand concentration. A very low concentration of 5.75 × 10−4 M ligand, when used as the carrier, resulted in the transport of ca. 25% metal ion transport in 5 h, which increased to >85% with 4.4 × 10−3 M ligand. The transport efficiency of the metal ion across the SLM followed the trend Np(IV) > Th(IV) > Pu(IV). The membrane stability was not satisfactory as seen over a period of 5 days suggesting long-term use may require regular replenishment of the carrier solvent. The effective diffusion coefficient (Deff) of Np(IV)-TREN-G1-DenDGA were determined by the lag-time method and was found to be 5.1 × 10−8 cm2/s. [ABSTRACT FROM AUTHOR]

"Mayak" production association has convincingly demonstrated the possibility of creating a HLW partitioning scheme based on different radical phosphine oxide in heavy fluorinated or chlorinated diluents, including in the presence of tributyl phosphate. The separation system makes it possible to efficiently recover uranium, plutonium, neptunium, REE–TPE, as well as technetium from solutions (up to 2 M) in HNO3 and separate them into at least two fractions: a fraction of trivalent REE/TPE with the bulk of technetium and a fraction of polyvalent actinides. [ABSTRACT FROM AUTHOR]

Fluoride molecular anions have emerged as a potential alternative to oxides for the analysis of actinides in AMS. Research at the Vienna Environmental Research Accelerator (VERA) has focused on mixing samples prepared in an Fe 2 O 3 matrix with PbF 2 for in situ fluoridation during the sputter process. The relative formation probabilities for a range of (oxy-)fluoride molecular anions of uranium, neptunium, plutonium, and americium have been found to be characteristic for each of the actinides investigated. These data can help to identify isobaric interference for the mentioned actinides. A first application is monitoring of the 236U co-production with 236Np during Th irradiation. Extracting these actinides in the form of tetrafluoride anions allows the suppression of U by an order of magnitude relative to Np. [ABSTRACT FROM AUTHOR]

The production of plutonium-238 through irradiation of neptunium-237 (237Np) target materials for the use in radioisotope thermoelectric generators is paramount for continued deep space exploration. This work employs scanning electron microscopy to analyze 237Np materials coupled with a well-developed image analysis framework (Morphological Analysis for Material Attribution, or MAMA) to determine the degree of micron-scale homogeneity in the materials. This work demonstrated how the quantification of particle characteristics can validate production materials and affirm the qualitative similarities observed in micrographs. The 237Np oxide particle analysis determined that the materials from five production runs were quantitatively homogenous (significant at α = 0.05) in particle area, circularity, equivalent circular diameter, and ellipse aspect ratio, with two of the sampling dates having statistically significant different means for one of the four characteristics. These metrics not only confirm general homogeneity of the material but also expand the application of MAMA workflows to 237Np materials, demonstrating the utility of MAMA analysis for a wider breadth of nuclear materials than previously reported. In the open literature, this study is the first time that these microanalytical techniques were applied to 237Np materials to this degree., Competing Interests: Conflict of Interest. The authors declare that they have no competing interest., (Published by Oxford University Press on behalf of the Microscopy Society of America 2024.)

Precision laser spectroscopy of the 229-thorium nuclear isomer transition in a solid-state environment would represent a significant milestone in the field of metrology, opening the door to the realization of a nuclear clock. Working toward this goal, experimental methods require knowledge of various properties of a large band-gap material, such as calcium fluoride doped with specific isotopes of the heavy elements thorium, actinium, cerium, neptunium, and uranium. By accurately determining the atomic structure of potential charge compensation schemes by using a generalized gradient approximation within the ab-initio framework of density functional theory, calculations of electric field gradients on the dopants become accessible, which cause a quadrupole splitting of the nuclear-level structure that can be probed experimentally. Band gaps and absorption coefficients in the range of the 229-thorium nuclear transition are estimated by using the G 0 W 0 method and by solving the Bethe–Salpeter equation. [ABSTRACT FROM AUTHOR]

The Plutonium-238 Supply Program at Oak Ridge National Laboratory has developed the capability to inform process decisions during full-scale 238Pu anion exchange column runs using multivariate chemometrics and visible-near infrared (Vis-NIR) absorption spectroscopy. Multivariate analytical methods provided a suitable option for real-time analysis of complex neptunium (Np) and plutonium (Pu) absorption spectra. Thousands of spectra from multiple production campaigns were analyzed by principal component analysis and Kennard-Stone sample selection to select a training set composed of 60 spectra. Multivariate curve resolution–alternating least squares analysis (MCR-ALS) identified spectral components and component concentrations. A partial least squares regression (PLSR) model was built using the spectra and concentration values determined by these virtually unsupervised methods. Next, a supervised PLSR model was built by scaling the MCR-ALS–selected concentration matrix using molar epsilon values. Both PLSR models predicted the concentrations of Np(IV), Np(V), Pu(III), Pu(IV), and Pu(VI) in the column effluent and identified nearly identical product cut decisions. The time-dependent Np and Pu concentration profiles provided valuable insight into column dynamics and the predictions agreed with traditional methods including alpha spectrometry and inductively coupled plasma mass spectrometry. These results establish a resourceful avenue for modeling multicomponent systems with convoluted absorption bands without significant user input. It is particularly advantageous for production-oriented radiochemical applications in restrictive hot cell environments.

A robust hierarchical model has been demonstrated for monitoring a wide range of neptunium concentrations (0.75–890 mM) and varying temperatures (10–80 °C) using chemometrics and feature selection. The visible–near infrared electronic absorption spectrum (400–1700 nm) of monocharged neptunyl dioxocation (Np(V) = NpO2+) includes many bands, which have molar absorption coefficients that differ by nearly 2 orders of magnitude. The shape, position, and intensity of these bands differ with chemical interactions and changing temperature. These challenges make traditional quantification by univariate methods unfeasible. Measuring Np(V) concentration over several orders of magnitude would typically necessitate cells with varying path length, optical switches, and/or multiple spectrophotometers. Alternatively, the differences in the molar extinction coefficients for multiple absorption bands can be used to quantify Np(V) concentration over 3 orders of magnitude with a single optical path length (1 mm) and a hierarchical multivariate model. In this work, principal component analysis was used to distinguish the concentration regime of the sample, directing it to the relevant partial least squares regression submodels. Each submodel was optimized with unique feature selection filters that were selected by a genetic algorithm to enhance predictions. Through this approach, the percent root mean square error of prediction values were ≤1.05% for Np(V) concentrations and ≤4% for temperatures. This approach may be applied to other nuclear fuel cycle and environmental applications requiring real-time spectroscopic measurements over a wide range of conditions.

The production of 238Pu from 237Np, to the specifications required for space flight, requires the separation of plutonium from neptunium and a range of fission products. The valuable neptunium is then recycled into targets for further irradiation. At Oak Ridge National Laboratory (ORNL), a solvent extraction process has been developed to separate neptunium and plutonium that uses sodium nitrite to control the neptunium oxidation. This results in a neptunium product contaminated with sodium, which needs to be removed prior to recycling. We have developed a separation process that results in a sodium-free neptunium product with very low levels of plutonium contamination, there by reducing the number of purification steps and thus simplifying the process. [ABSTRACT FROM AUTHOR]

Neptunium, with a half life of 2.14 million years is one of the most notorious activation products in the nuclear fuel cycle. It has been more than 5 decades in the reprocessing of nuclear fuels by the well documented PUREX process, but the fate of Np in the PUREX cycle is still not well controlled. Although Np being stable in its pentavalent state in low acid media, its starts to undergo disproportionation at higher acidities. This disproportionation along with the oxidizing conditions of the HNO3 medium makes Np to exits as Np(IV), Np(V) and Np(VI) in the dissolver solution. The overall extractability of Np in the co-decontamination step of the PUREX cycle is dependent on its oxidation state in the medium as Np(VI) and Np(IV) being extractable while Np(V) being least extractable. The present review article discusses about the speciation of Np in HNO3 and its disproportionation. The variety of redox reagents are discussed for their effectiveness towards controlling Np redox behavior in the HNO3 media. The extraction of Np with the different class of extractant has also been discussed and the results are compared for better understanding. Solid phase extraction of Np using both commercially available resin and lab based synthesized resins were discussed. The anion exchange resins with the different cationic centers were shown to behave differently towards the uptake of Np form the acidic medium. The present review also highlight the chemical conditions required for controlling or minimizing the fate of Np in different process streams of the nuclear fuel cycle. [ABSTRACT FROM AUTHOR]