Neptunium Incorporation Research Articles

Controls on the Fate and Speciation of Np(V) During Iron (Oxyhydr)oxide Crystallization.

The speciation and fate of neptunium as Np(V)O2(+) during the crystallization of ferrihydrite to hematite and goethite was explored in a range of systems. Adsorption of NpO2(+) to iron(III) (oxyhydr)oxide phases was reversible and, for ferrihydrite, occurred through the formation of mononuclear bidentate surface complexes. By contrast, chemical extractions and X-ray absorption spectroscopy (XAS) analyses showed the incorporation of Np(V) into the structure of hematite during its crystallization from ferrihydrite (pH 10.5). This occurred through direct replacement of octahedrally coordinated Fe(III) by Np(V) in neptunate-like coordination. Subsequent analyses on mixed goethite and hematite crystallization products (pH 9.5 and 11) showed that Np(V) was incorporated during crystallization. Conversely, there was limited evidence for Np(V) incorporation during goethite crystallization at the extreme pH of 13.3. This is likely due to the formation of a Np(V) hydroxide precipitate preventing incorporation into the goethite particles. Overall these data highlight the complex behavior of Np(V) during the crystallization of iron(III) (oxyhydr)oxides, and demonstrate clear evidence for neptunium incorporation into environmentally important mineral phases. This extends our knowledge of the range of geochemical conditions under which there is potential for long-term immobilization of radiotoxic Np in natural and engineered environments.

Structural and Morphological Influences on Neptunium Incorporation in Uranyl Molybdates

The in situ incorporation of pentavalent neptunium has been studied in the structurally related uranyl molybdate frameworks (NH4)4[(UO2)5(MoO4)7](H2O)5 and (NH4)2[(UO2)6(MoO4)7](H2O)2 prepared under similar synthetic conditions. The presence of Np(V) was confirmed by UV–vis–NIR spectroscopy in the first compound, whereas Np(VI) was identified in the second based on the observation of a unit-cell contraction and the lack of a spectral signature for Np(V). The incorporation of neptunium does not affect the overall structure of the host compound based on the crystallographic unit-cell parameters. Neptunium appears to preferentially incorporate in the structure of (NH4)2[(UO2)6(MoO4)7](H2O)2 due to the formation of Np(VI) during synthesis, although higher total uptakes were observed in (NH4)4[(UO2)5(MoO4)7](H2O)5 due to a higher initial concentration of neptunium in solution despite maintaining the same ratio of U:Np.

The incorporation of neptunium and plutonium in thorutite (ThTi2O6)

The incorporation of neptunium (Np) and plutonium (Pu) into the brannerite structured lattice was studied using thorutite (ThTi2O6) as host lattice and sintering in air. The uncompensated Np and Pu doped samples and the low Y-charge compensated Np and Pu doped samples showed main phases as designed together with trace amounts of rutile. Those samples with larger amounts of Y produced yttrium pyrochlores as an additional minor phase. XRD analyses reveal anisotropic changes of the cell parameters; the a-parameter contracts while b- and c-parameters expand with mean cationic radius. This is in reasonable agreement with previous experimental data on ThTi2O6 and Ce0.975Ti2O5.95. Attempts to form Np or Pu valences >4+ by adding Y as a charge compensator were unsuccessful, suggesting that tetravalent Np and Pu ions are favoured in air-fired thorutite.

High-Temperature X-ray Diffraction Study of Uranium–Neptunium Mixed Oxides

Incorporating minor actinides (MAs = Am, Np, Cm) in UO2 fertile blankets is a viable option to recycle them. Despite this applied interest, phase equilibria between uranium and MAs still need to be thoroughly investigated, especially at elevated temperatures. In particular, few reports on the U-Np-O system are available. In the present work, we provide for the first time in situ high-temperature X-ray diffraction results obtained during the oxidation of (U1-yNpy)O2 uranium-neptunium mixed oxides up to 1373 K and discuss subsequent phase transformations. We show that (i) neptunium stabilizes the UO2-type fluorite structure at high temperature and that (ii) the U3O8-type orthorhombic structure is observed in a wide range of compositions. We clearly demonstrate the incorporation of neptunium in this phase, which was a controversial question in previous studies up to now. We believe it is the particular stability of the tetravalent state of neptunium that is responsible for the observed phase relationships.

ChemInform Abstract: Incorporation of Neptunium(VI) into a Uranyl Selenite.

AbstractNpVI is incorporated into compound (IV) with the highest level of Np uptake (12% substitution of NpVI for UVI) observed for an uranyl compound so far.

Synthetic Influences on Neptunium Incorporation in Naturally Occurring Copper Uranyl Phosphates

Neptunium incorporation in the torbernite/meta-torbernite [Cu(UO2)2(PO4)2·nH2O] system has been investigated in this study under both hydrothermal and slow diffusion conditions (at room temperature and 90 °C) to examine the role synthetic conditions play on neptunium uptake by uranyl phases and whether hydrothermal incorporation studies feasibly model environmental incorporation. The hydrothermally prepared crystals contained approximately 19 ± 2 and 73 ± 3 times more neptunium on average than crystals grown during slow diffusion at room temperature and elevated temperature, respectively. UV–vis spectroscopy on the mother solution showed that, on average, 51.53(±0.04)% of the original neptunium(V) remains in solution following hydrothermal synthesis versus 37.24(±0.04)% following slow diffusion at room temperature and 73.62(±0.07)% at 90 °C. Additionally, solid-state UV–vis-NIR indicated that the incorporated neptunium was predominantly present in the +6 oxidation state in the hydrothermal samples despite the fact that the initial oxidation state of neptunium in solution was +5 and no oxidizing species were present in the reactions. The oxidation state of neptunium in the slow diffusion samples was not able to be determined due to the low incorporation levels. These results suggest that neptunium(VI) may play a more significant role than previously expected in geological repositories.

Incorporation of Neptunium(VI) into a Uranyl Selenite

The incorporation of neptunium(VI) into the layered uranyl selenite Cs[(UO(2))(HSeO(3))(SeO(3))] has yielded the highest level of neptunium uptake in a uranyl compound to date with an average of 12(±3)% substitution of Np(VI) for U(VI). Furthermore, this is the first case in nearly 2 decades of dedicated incorporation studies in which the oxidation state of neptunium has been determined spectroscopically in a doped uranyl compound and also the first time in which neptunium incorporation has resulted in a structural transformation.

Mineralogic controls on aqueous neptunium(V) concentrations in silicate systems

The presence of radioactive neptunium in commercially spent nuclear fuel is problematic due to its mobility in environmental systems upon oxidation to the pentavalent state. As uranium is the major component of spent fuel, incorporation of neptunium into resulting U(VI) mineral phases would potentially influence its release into environmental systems. Alternatively, aqueous neptunium concentrations may be buffered by solid phase Np2O5. In this study, we investigate both of these controls on aqueous neptunium(V) concentrations. We synthesize two uranyl silicates, soddyite, (UO2)2SiO4·2H2O, and boltwoodite, (K, Na)(UO2)(SiO3OH)·1.5H2O, each in the presence of two concentrations of aqueous Np(V). Electron microscopy and electron diffraction analyses of the synthesized phases show that while significant neptunyl incorporation occurred into soddyite, the Np(V) in the boltwoodite systems largely precipitated as a secondary phase, Np2O5(s). The release of Np(V) from each system into aqueous solution was measured for several days, until steady-state concentrations were achieved. Using existing solubility constants (Ksp) for pure soddyite and boltwoodite, we compared predicted equilibrium aqueous U(VI) concentrations with the U(VI) concentrations released in the solubility experiments. Our experiments reveal that Np(V) incorporation into soddyite increases the concentration of aqueous U in equilibrium with the solid phase, perhaps via the formation of a metastable phase. In the mixed boltwoodite – Np2O5(s) system, the measured aqueous U(VI) activities are consistent with those predicted to be in equilibrium with boltwoodite under the experimental conditions, a result that is consistent with our conclusion that little Np(V) incorporation occurred into the boltwoodite. In the boltwoodite systems, the measured Np concentrations are likely controlled by the presence of Np2O5 nanoparticles, suggesting an additional potential mobility vector for Np in geologic systems. Our results demonstrate that in systems containing solid phases that cannot incorporate significant concentrations of Np(V), secondary precipitates such as Np2O5(s) are likely to control aqueous neptunium concentrations, and that uranium concentrations are buffered by the uranyl mineral assemblage present. For systems containing uranyl mineral phases such as soddyite, which can incorporate significant concentrations of Np(V), Np2O5(s) precipitation may be suppressed and the Np-bearing uranyl phase may act as a sink and a buffer for aqueous Np(V) in oxidizing environments.

Incorporation of Neptunium(V) and Iodate into a Uranyl Phosphate: Implications for Mitigating the Release of 237Np and 129I in Repositories

The simultaneous incorporation of IO3(-) and NpO2+ into Ba3(UO2)2(HPO4)2(PO4)2 (BaUP), which serves as a model for uranyl alteration phases, was investigated. LA-ICP-MS data demonstrate that the incorporation of both of these species is significantly enhanced when they are present together. The most probable explanation is that charge balance is obtained by the coupled substitutions of NpO2+ <--> UO2(2+) and IO3(-) <--> HPO4(2-). According to the LA-ICP-MS results, in the absence of iodate as much as 2.91 +/- 0.14 to 3.44 +/- 0.25% of the uranium in BaUP can be replaced by neptunium. When iodate is present in the reaction, the amount of uranium substitution by neptunium increases to 6.05 +/- 0.65% to 7.93 +/- 0.83%. The net increase for neptunium is 116 +/- 0.30% to 225 +/- 0.25%. Similarly, in the absence of NpO2+, iodate incorporation into BaUP reaches an I/U level of 0.0021 +/- 0.0004 to 0.0038 +/- 0.0005; whereas in its presence there is an increase to as much as 100 +/- 0.11% to 0.0042 +/- 0.0008.

Re-Evaluating Neptunium in Uranyl Phases Derived from Corroded Spent Fuel

Interest in mechanisms that may control radioelement release from corroded commercial spent nuclear fuel (CSNF) has been heightened by the selection of the Yucca Mountain site in Nevada as the repository for high-level nuclear waste in the United States. Neptunium is an important radionuclide in repository models owing to its relatively long half-life and its high aqueous mobility as neptunyl [Np(V)O2+]. The possibility of neptunium sequestration into uranyl alteration phases produced by corroding CSNF would suggest a process for lowering neptunium concentration and subsequent migration from a geologic repository. However, there remains little experimental evidence that uranyl compounds will, in fact, serve as long-term host phases for the retention of neptunium under conditions expected in a deep geologic repository. To directly explore this possibility, we examined specimens of uranyl alteration phases derived from humid-air–corroded CSNF by X-ray absorption spectroscopy to better determine neptunium uptake in these phases. Although neptunium fluorescence was readily observed from as-received CSNF, it was not observed from the uranyl alteration rind. We establish upper limits for neptunium incorporation into CSNF alteration phases that are significantly below previously reported concentrations obtained by using electron energy loss spectroscopy (EELS). We attribute the discrepancy to a plural-scattering event that creates a spurious EELS peak at the neptunium-MV energy.

Neptunium incorporation into uranyl compounds that form as alteration products of spent nuclear fuel: Implications for geologic repository performance

SummaryAlteration of spent nuclear fuel in a geological repository under oxidizing conditions is likely to result in abundant uranyl compounds. Incorporation of radionuclides into the uranyl alteration phases may significantly reduce their mobility, thereby impacting repository performance. The following compounds have been synthesized from solutions containing from ∼10 to 500ppm Np5+: meta-schoepite, UO3·2H2O; Na-compreignacite, Na2[(UO2)3O2(OH)3]2(H2O)7; uranophane, Ca(UO2)2(SiO3OH)2(H2O)5; and β-(UO2)(OH)2. The structures of each involve sheets of uranyl polyhedra; the interlayers contain cations and H2O groups in Na-compreignacite and uranophane, only H2O in meta-schoepite, and β-(UO2)(OH)2does not contain interlayer constituents. Synthesized powders were characterized by X-ray powder diffractometry, and were analyzed by ICP-AES (U, Na, Ca) and ICP-MS (Np). Aliquots of the powders were washed in 0.5M acetic acid to remove sorbed Np prior to analysis. The powders of meta-schoepite and β-(UO2)(OH)2contained at most a few ppm Np5+. In contrast, powders of Na-compreignacite and uranophane contained Np5+in proportion to the Np5+concentration in the mother solution, and powders of each containing more than 400ppm Np5+(of total Np + U) were obtained. Incorporation of Np5+into uranyl compounds that form in a geological repository appears possible, and may impact the mobility of Np.

Evidence for Neptunium Incorporation into Uranium (VI) Phases

AbstractThis paper examines the ability of electron energy-loss spectroscopy (EELS) combined with transmission electron microscopy (TEM) to detect both low concentrations of Np in a U matrix and to provide evidence for incorporation of Np in U(VI) phases. The case for U(VI) secondary minerals acting as solubility-controlling phases for Np in repository performance assessment models has not been fully established. Direct evidence for incorporation, rather than sorption, continues to be difficult to obtain. Detection of Np with TEM-EELS is hampered by the occurrence of a plural (multiple) scattering event from the more abundant U atoms (U-M5 + U- O4,5), that results in severe overlap on the Np-M5 edge at ∼3665 eV. By examining the energy âgap' between the Np-M5 and Np-M4 edges (184 eV), a method for observing Np independently of the plural scattering event has been established. Clear evidence of Np incorporation into synthetic studtite {(UO2)(O2)(H2O)2](H2O)2} and uranophane {Ca(UO2)2(SiO3OH)2(H2O)5} has been found with TEM-EELS. The EELS technique continues to remain an important tool for examining the potential for transuranic behavior in nuclear waste materials.

Neptunium Incorporation into Uranium(Vi) Compounds formed During Aqueous Corrosion of Neptunium-Bearing Uranium Oxides

ABSTRACTWe report results of experimental studies on the behavior of Np during aqueous corrosion of unirradiated Np-bearing U oxides. Np-doped U oxides were reacted in humid air at 90°C and 150°C for several weeks within sealed stainless-steel vessels. Reacted solids were examined by scanning and transmission electron microscopies (SEM and TEM), electron energy-loss spectroscopy (EELS), and X-ray powder diffraction (XRD). Dehydrated schoepite, (UO2)O0.25-z(OH)1.5+2z (0 ≤z ≤0.15), is the predominant U(VI) compound formed in these experiments. Preliminary EELS analysis on crushed grains verify that dehydrated schoepite formed at 150°C contains up to approximately 2 wt.% Np, corresponding to a maximum Np:U molar ratio of approximately 1:40. These are maximum values because the degree to which surface-sorbed Np is present on the grains analyzed is not yet known. Crystalline NpO2 also precipitated during these experiments, and the concentration of Np in dehydrated schoepite may represent the maximum amount of Np that can be incorporated into dehydrated schoepite under the experimental conditions.