Uranyl Minerals Research Articles

Hydrothermal Phase Relations Among Uranyl Minerals at the Nopal I Analog Site

AbstractUranyl mineral paragenesis at Nopal I is an analog of spent fuel alteration at Yucca Mountain. Petrographic studies suggest a variety of possible hydrothermal conditions for uranium mineralization at Nopal I. Calculated equilibrium phase relations among uranyl minerals show uranophane stability over a broad range of realistic conditions and indicate that uranyl mineral variety is a consequence of chemical potential heterogeneity.

Chemical Effects at the Reaction Front in Corroding Spent Nuclear Fuel

AbstractPerformance assessment models of the U. S. repository at Yucca Mountain, Nevada suggest that neptunium from spent nuclear fuel is a potentially important dose contributor. A scientific understanding of how the UO2 matrix of spent nuclear fuel impacts the oxidative dissolution and reductive precipitation of Np is needed to predict the behavior of Np at the fuel surface during aqueous corrosion. Neptunium would most likely be transported as aqueous Np(V) species, but for this to occur it must first be oxidized from the Np(IV) state found within the parent spent nuclear fuel. In this paper we present synchrotron x-ray absorption spectroscopy and microscopy findings that illuminate the resultant local chemistry of neptunium and plutonium within uranium oxide spent nuclear fuel before and after corrosive alteration in an air-saturated aqueous environment. We find the Pu and Np in unaltered spent fuel to have a +4 oxidation state and an environment consistent with solid-solution in the UO2 matrix. During corrosion in an air-saturated aqueous environment, the uranium matrix is converted to uranyl (UO22+) mineral assemblage that is depleted in Np and Pu relative to the parent fuel. The transition from U(IV) in the fuel to a fully U(VI) character across the corrosion front is not sharp, but occurs over a transition zone of ∼ 50 micrometers. We find evidence of a thin (∼ 20 micrometer) layer that is enriched in Pu and Np within a predominantly U(IV) environment on the fuel side of the transition zone. These experimental observations are consistent with available data for the standard reduction potentials for NpO2+/Np4+ and UO22+/U4+ couples, which indicate that Np(IV) may not be effectively oxidized to Np(V) at the corrosion potential of uranium dioxide spent nuclear fuel in air-saturated aqueous solutions.

The Application of near Infrared Spectroscopy for the Detection of Secondary Uranium Minerals Using a Fibre-Optic Probe

The proposed increase in usage of nuclear power implies increased nuclear waste resulting in the formation of many uranyl secondary minerals. These minerals can translocate and a method of easy identification is required. Because many of the minerals contain water and/or OH units, the minerals lend themselves to near infrared spectroscopy. Spectra of a range of uranyl carbonate, sulphate and silicate minerals were obtained using a fibre optic-probe with an attenuated total reflectance cell in reflectance mode. The minerals include andersonite, liebigite, rutherfordine, uranopilite, zippeite, boltwoodite, soddyite, sklodowskite, uranophane and weeksite. These uranyl minerals are characterised by intense bands around 7000 cm−1 resulting from the second harmonic of the normal mode of the OH groups. Uranyl silicates show intense bands in the 4200 to 5400 cm−1 region. Two sets of bands, which are electronic in nature, are observed: one centred upon 10100 cm−1 and a second set around 8500 cm−1 in the high wavenumber region.

U6+ MINERALS AND INORGANIC COMPOUNDS: INSIGHTS INTO AN EXPANDED STRUCTURAL HIERARCHY OF CRYSTAL STRUCTURES

The crystal structures of uranyl minerals and inorganic uranyl compounds are important for understanding the genesis of U deposits, the interaction of U mine and mill tailings with the environment, transport of actinides in soils and the vadose zone, the performance of geological repositories for nuclear waste, and for the development of advanced materials with novel applications. Over the past decade, the number of inorganic uranyl compounds (including minerals) with known structures has more than doubled, and reconsideration of the structural hierarchy of uranyl compounds is warranted. Here, 368 inorganic crystal structures that contain essential U6+ are considered (of which 89 are minerals). They are arranged on the basis of the topological details of their structural units, which are formed by the polymerization of polyhedra containing higher-valence cations. Overarching structural categories correspond to those based upon isolated polyhedra (8), finite clusters (43), chains (57), sheets (204), and frameworks (56) of polyhedra. Within these categories, structures are organized and compared upon the basis of either their graphical representations, or in the case of sheets involving sharing of edges of polyhedra, upon the topological arrangement of anions within the sheets.

Molecular structure of the uranyl mineral zippeite - An XRD, SEM and Raman spectroscopic study

Raman spectra at 298 and 77 K and infrared spectra of the uranyl sulfate mineral zippeite, K2[(UO2)6(SO4)3O(OH)6]. 4 H2O, were studied. Observed bands were tentatively attributed to the (UO2)2+ and (SO4)2- stretching and bending vibrations, the OH stretching vibrations of water molecules and hydroxyls, H2O bending vibrations and libration modes, and ￤ U-OH bending vibrations. Empirical relations were used for calculation of U-O bond lengths in uranyl R = f(￮3 or ￮1 (UO2)2+) A. This was found in agreement with U-O bond lengths from the single crystal structure analysis. The number of observed bands supports the conclusion from single crystal structure analysis that at least two symmetrically distinct U6+ (in uranyl) and S6+ (in sulfate), and water molecules and hydroxyls may be present in the zippeite crystal structure. Some O-H…O bond lengths were attributed to the hydrogen-bonding network in zippeite crystal structure.

Fluorescence spectroscopy of U(VI)-silicates and U(VI)-contaminated Hanford sediment

Time-resolved U(VI) laser fluorescence spectra (TRLFS) were recorded for a series of natural uranium-silicate minerals including boltwoodite, uranophane, soddyite, kasolite, sklodowskite, cuprosklodowskite, haiweeite, and weeksite, a synthetic boltwoodite, and four U(VI)-contaminated Hanford vadose zone sediments. Lowering the sample temperature from RT to ∼ 5.5 K significantly enhanced the fluorescence intensity and spectral resolution of both the minerals and sediments, offering improved possibilities for identifying uranyl species in environmental samples. At 5.5 K, all of the uranyl silicates showed unique, well-resolved fluorescence spectra. The symmetric O = U = O stretching frequency, as determined from the peak spacing of the vibronic bands in the emission spectra, were between 705 to 823 cm −1 for the uranyl silicates. These were lower than those reported for uranyl phosphate, carbonate, or oxy-hydroxides. The fluorescence emission spectra of all four sediment samples were similar to each other. Their spectra shifted minimally at different time delays or upon contact with basic Na/Ca-carbonate electrolyte solutions that dissolved up to 60% of the precipitated U(VI) pool. The well-resolved vibronic peaks in the fluorescence spectra of the sediments indicated that the major fluorescence species was a crystalline uranyl mineral phase, while the peak spacing of the vibronic bands pointed to the likely presence of uranyl silicate. Although an exact match was not found between the U(VI) fluorescence spectra of the sediments with that of any individual uranyl silicates, the major spectral characteristics indicated that the sediment U(VI) was a uranophane-type solid (uranophane, boltwoodite) or soddyite, as was concluded from microprobe, EXAFS, and solubility analyses.

A Raman spectroscopic study of the uranyl sulphate mineral johannite

Raman spectroscopy at 298 and 77 K has been used to study the secondary uranyl mineral johannite of formula (Cu(UO 2) 2(SO 4) 2(OH) 2·8H 2O). Four Raman bands are observed at 3593, 3523, 3387 and 3234 cm −1 and four infrared bands at 3589, 3518, 3389 and 3205 cm −1. The first two bands are assigned to OH − units (hydroxyls) and the second two bands to water units. Estimations of the hydrogen bond distances for these four bands are 3.35, 2.92, 2.79 and 2.70 Å. A sharp intense band at 1042 cm −1 is attributed to the (SO 4) 2− symmetric stretching vibration and the three Raman bands at 1147, 1100 and 1090 cm −1 to the (SO 4) 2− anti-symmetric stretching vibrations. The ν 2 bending modes were at 469, 425 and 388 cm −1 at 77 K confirming the reduction in symmetry of the (SO 4) 2− units. At 77 K two bands at 811 and 786 cm −1 are attributed to the ν 1 symmetric stretching modes of the (UO 2) 2+ units suggesting the non-equivalence of the UO bonds in the (UO 2) 2+ units. The band at 786 cm −1, however, may be related to water molecules libration modes. In the 77 K Raman spectrum, bands are observed at 306, 282, 231 and 210 cm −1 with other low intensity bands found at 191, 170 and 149 cm −1. The two bands at 282 and 210 cm −1 are attributed to the doubly degenerate ν 2 bending vibration of the (UO 2) 2+ units. Raman spectroscopy can contribute significant knowledge in the study of uranyl minerals because of better band separation with significantly narrower bands, avoiding the complex spectral profiles as observed with infrared spectroscopy.

Dissolution of uranyl microprecipitates in subsurface sediments at Hanford Site, USA

The dissolution of uranium was investigated from contaminated sediments obtained at the US. Department of Energy (U.S. DOE) Hanford site. The uranium existed in the sediments as uranyl silicate microprecipitates in fractures, cleavages, and cavities within sediment grains. Uranium dissolution was studied in Na, Na-Ca, and NH 4 electrolytes with pH ranging from 7.0 to 9.5 under ambient CO 2 pressure. The rate and extent of uranium dissolution was influenced by uranyl mineral solubility, carbonate concentration, and mass transfer rate from intraparticle regions. Dissolved uranium concentration reached constant values within a month in electrolytes below pH 8.2, whereas concentrations continued to rise for over 200 d at pH 9.0 and above. The steady-state concentrations were consistent with the solubility of Na-boltwoodite and/or uranophane, which exhibit similar solubility under the experimental conditions. The uranium dissolution rate increased with increasing carbonate concentration, and was initially fast. It decreased with time as solubility equilibrium was attained, or dissolution kinetics or mass transfer rate from intraparticle regions became rate-limiting. Microscopic observations indicated that uranium precipitates were distributed in intragrain microfractures with variable sizes and connectivity to particle surfaces. Laser-induced fluorescence spectroscopic change of the uranyl microprecipitates was negligible during the long-term equilibration, indicating that uranyl speciation was not changed by dissolution. A kinetic model that incorporated mineral dissolution kinetics and grain-scale, fracture-matrix diffusion was developed to describe uranium release rate from the sediment. Model calculations indicated that 50–95% of the precipitated uranium was associated with fractures that were in close contact with the aqueous phase. The remainder of the uranium was deeply imbedded in particle interiors and exhibited effective diffusivities that were over three orders of magnitude lower than those in the fractures.

Stability of Peroxide‐Containing Uranyl Minerals.

AbstractFor Abstract see ChemInform Abstract in Full Text.

CRYSTAL CHEMISTRY OF URANYL MOLYBDATES. X. THE CRYSTAL STRUCTURE OF Ag10[(UO2)8O8(Mo5O20)

Crystals of a new silver uranyl molybdate, Ag10[(UO2)8O8(Mo5O20)], have been synthesized by high-temperature solid-state reaction. The structure [monoclinic, C2/c, a 24.672(2), b 23.401(2), c 6.7932(4) A, 94.985(2)°, V 3907.3(4) A 3 , Z = 4] was solved by direct methods and refined to R1 = 0.049 (wR2 = 0.101) on the basis of 4317 unique observed reflections using a crystal twinned on (001). The twinning causes overlap of the reflections with l = 3n. The structure is based upon complex double sheets of UrO5 pentagonal bipyramids and Mo polyhedra. The double sheets are parallel to (100) and consist of two [(UO2)2O2(MoO5)] sheets of UrO5 bipyramids and MoO6 octahedra. Within the [(UO2)2O2(MoO5)] sheets, UrO5 bipyramids share edges to produce complex chains parallel to the c axis and linked via pairs of edge-sharing MoO6 octahedra. The two [(UO2)2O2(MoO5)] sheets are linked into double [(UO2)8O8(Mo5O20)] sheets via the Mo(3)O4 tetrahedron located between the sheets. The MoO4 tetrahedra share two corners with two MoO6 octahedra of the sheet above and two corners with MoO6 octahedra from the sheet below, so that Mo5O20 pentanuclear clusters are formed. The uranyl molybdate sheets found in the structure have not been previously observed in uranyl minerals and inorganic compounds. The Ag(2), Ag(3), Ag(6), Ag(7), and Ag(8) atoms are located within the double sheets, whereas the Ag(1), Ag(4), and Ag(5) are in between the double sheets.

Stability of peroxide-containing uranyl minerals.

Minerals containing peroxide are limited to studtite, (UO2)O2(H2O)4, and metastudtite, (UO2)O2(H2O)2. High-temperature oxide-melt solution calorimetry and solubility measurements for studtite (standard enthalpy of formation at 298 kelvin is -2344.7 +/- 4.0 kilojoules per mole from the elements) establishes that these phases are stable in peroxide-bearing environments, even at low H2O2 concentrations. Natural radioactivity in a uranium deposit, or the radioactivity of nuclear waste, can create sufficient H2O2 by alpha radiolysis of water for studtite formation. Studtite and metastudtite may be important alteration phases of nuclear waste in a geological repository and of spent fuel under any long-term storage, possibly at the expense of the commonly expected uranyl oxide hydrates and uranyl silicates.

Contribution to the mineralogy of acid drainage of Uranium minerals: Marecottite and the zippeite-group

Sulfate-rich acid waters produced by oxidation of sulfide minerals enhance U mobility around U ores and U-bearing radioactive waste. Upon evaporation, several secondary uranyl minerals, including many uranyl sulfates, precipitate from these waters. The zippeite-group of minerals is one of the most common and diverse in such settings. To decipher the nature and crystal chemistry of the zippeite-group, the crystal structure of a new natural hydrated Mg uranyl sulfate related to Mgzippeite was determined. The mineral is named marecottite after the type locality, the La Creusaz U prospect near Les Marécottes, Western Swiss Alps. Marecottite is triclinic, P1, with a = 10.815(4), b = 11.249(4), c = 13.851(6) Å, a = 66.224(7), b = 72.412(7), and g = 69.95(2)°. The ideal structural formula is Mg 3 (H 2 O) 18 [(UO 2 ) 4 O 3 (OH)(SO 4 ) 2 ] 2 (H 2 O) 10 . The crystal structure of marecottite contains uranyl sulfate sheets composed of chains of edge-sharing uranyl pentagonal bipyramids that are linked by vertex-sharing with sulfate tetrahedra. The uranyl sulfate sheets are topologically identical to those in zippeite, K(UO 2 ) 2 (SO 4 )O 2 ·2H 2 O. The zippeite-type sheets alternate with layers containing isolated Mg(H 2 O) 6 octahedra and uncoordinated H 2 O groups. The uranyl sulfate and Mg layers are linked by hydrogen bonding only. Magnesium-zippeite is redefined as Mg(H 2 O) 3.5 (UO 2 ) 2 (SO 4 )O 2 , based on comparison of the powder X-ray diffraction pattern of micro-crystalline co-type material with the pattern of a synthetic phase. Magnesium-zippeite contains zippeite-type uranyl sulfate sheets with Mg located between the layers, where it is in octahedral coordination. In Mg-zippeite, distorted Mg octahedra are linked by sharing vertices, resulting in dimers. The apices of the Mg octahedra correspond to two O atoms of uranyl ions, and four H 2 O groups. Magnesium-zippeite and marecottite co-exist, sometimes in the same sample, at Lucky Strike no. 2 mine, Emery County, Utah (type locality of Mg-zippeite), at Jáchymov, Czech Republic, and at La Creusaz. This study provides insight into the complexity of the zippeite-group minerals containing divalent cations, where different arrangements in the interlayers result in different unit cells and space groups.

Using reactive transport modeling to evaluate the source term at Yucca Mountain

The conventional approach of source-term evaluation for performance assessment of nuclear waste repositories uses the dissolution rate of waste form and the solubility of pure phases of radioactive elements to constrain radionuclide concentrations. This standard practice has obvious shortcomings. First, it is unrealistic to use the dissolution rate of spent fuel to constrain source terms because of its short life under the repository conditions. Second, it is well recognized that most radionuclides may incorporate into secondary uranium minerals as solid solutions. A source-term model without considering the role of secondary minerals would deviate from the reality. As a result, source term predicted by the conventional approach may be several orders of magnitude higher than the concentrations of radioelements measured in spent fuel dissolution experiments. This paper presents the author's attempt of applying reactive-transport modeling approach to realistically evaluate source term by including the precipitation and dissolution of secondary minerals for Yucca Mountain. Based on the general reactive-transport code AREST-CT, a model for spent fuel dissolution and secondary phase precipitation has been constructed. Its predictions have been compared against laboratory experiments and natural analogues. It is found that without calibration, the simulated results match laboratory and field observations very well in many aspects. More important is the fact that no contradictions between them have been found. By considering Np incorporation into uranyl minerals, the model not only predicts a lower Np source term than that given by the conventional model but also produces results that are consistent with laboratory measurements and observations. Moreover, two hypotheses, whether Np enters tertiary uranyl minerals or not, have been tested by comparing model predictions against laboratory observations; the results suggests that the former hypothesis is a better assumption for Np source-term prediction. It is concluded that this non–conventional approach of source-term evaluation not only eliminates over-conservatism in the conventional approach to some extent but also gives a realistic representation of the system of interest. Therefore, it is a promising alternative approach for source-term evaluation.

Stability of U(VI) solid phases in the U(VI)-Ca2+-SiO2-OH system

Abstract Synthetic metaschoepite was altered at 70 °C, for 30 days in silicate solutions of varying compositions to study alteration reactions and to compare experimental observations with a thermodynamic model of stability of the uranyl minerals metaschoepite ([(UO2)8O2(OH)12](H2O)10), becquerelite (Ca[(UO2)3O2(OH)3]2(H2O)8), α-uranophane (Ca[(UO2)(SiO3OH)]2(H2O)5) and soddyite ((UO2)2(SiO4)· (H2O)2). Powder X-ray diffraction analysis, scanning electron microscopy and solution chemistry were used to monitor the alteration of the initial metaschoepite. In general, the secondary phases predicted by the thermodynamic model formed. However, an unexpected mineral (a calcium uranate) was observed above pH 8. Also, within the timeframe of these experiments, U(VI) silicate phases only formed at Si concentrations much higher than the lower field boundaries for uranophane and soddyite.

Advances in Understanding of the Crystal Chemistry of Hexavalent Uranium

ABSTRACTResearch concerning the crystal chemistry of hexavalent U by the Environmental Mineralogy and Crystal Structures research group at Notre Dame has resulted in the description of more than 110 new structures of uranyl compounds (including 36 minerals). New insights into the crystal chemistry of U6+ are presented, with emphasis on recently discovered novel structural connectivities. The structural hierarchy of uranyl minerals and compounds, which was first established for 180 structures in 1996, has been extended to include 145 new structures. The hierarchy is based upon polymerization of polyhedra containing higher-valence cations, and consists of five distinct classes: structures containing isolated polyhedra (7), finite clusters of polyhedra (41), chains of polyhedra (52), sheets of polyhedra (184), and frameworks of polyhedra (41). The dominance of sheets in uranyl compounds (57% of known structures) arises from the unequal distribution of bond-valences within the uranyl polyhedra. Topological relations of the sheets in uranyl compounds are best understood by analysis of the topological distribution of anions within sheets in which sharing of polyhedral edges dominates, and by graphical representation of the connectivity of polyhedra in cases where sharing of vertices of polyhedra dominates the sheet.

Chemical Changes of Minerals Trapped in the Lichen Trapelia involuta: Implication for Lichen Effect on Mobility of Uranium and Toxic Metals

To elucidate development of minerals trapped in a lichen, we examined the lichen Trapelia involuta growing directly on secondary uranyl minerals and U-enriched Fe oxide and hydroxide minerals. Sericite and other minerals in the underlying rock are trapped in the lichen T. involuta during its biological growth and chemically changed by lichen activities. The presence of chemically changed sericite accompanied by an Fe-bearing mineral in the lichen suggests that dissolution of sericite is promoted mainly by polysaccharides excreted by the lichen. Oxalic acid or lichen acids absent in the medulla may not play an important role in the dissolution. Our results suggest that lichens on metal-rich rock surface affect the mobility of uranium and other toxic metals through dissolution followed by trap of minerals from the underlying rock.

Dehydration of synthetic autunite hydrates

The dehydration of uranyl minerals can affect phase structure and stability. Synthetic autunite hydrates, Ca[(UO2)(PO4)]2· × H2O, were studied by X-ray powder diffractometry (XRD) and thermogravimetric analysis (TGA) to address ambiguous or contradictory reports in the literature. Structurally, XRD analysis supported the three well-defined phases commonly reported in the literature, i.e. autunite, metaautunite I, and metaautunite II. In addition, a fourth phase with a basal plane spacing between that of autunite and metaautunite I, designated metaautunite Ia, was identified as an apparent metastable intermediate. TGA analysis confirmed that water loss or accumulation is tolerated to different degrees among the autunite hydrates. Loss of low temperature water appears to initiate collapse of the interlayer spacing from 10 to 9 Å to form metaautunite I and/or Ia, while the lower hydrates accommodate minor water loss and accumulation without significant structural alteration. Our results support previous research indicating the reversibility of the autunite to metaautunite I conversion. The complex dehydration pattern of autunite is not observed in all the 1:1 uranyl phosphates, such as chernikovite (H[(UO2)(PO4)] · 4 H2O).

Field and Laboratory Examination of Uranium Microcrystallization and Its Role in Uranium Transport

ABSTRACTAdsorption is believed to be a dominant mechanism of uranium distribution between solid and solution, and thus, to play a major role in uranium transport. Because iron oxides and hydroxides are abundant at the Earth's surface and are great adsorbents of uranium, we have examined natural rocks that contain iron minerals along with uranium, and also carried out Fe-U coprecipitation and aging experiments to find how uranium is distributed between Fe minerals. Transmission and scanning electron microscopy reveals that microcrystals (10-50 nm) of metatorbernite (Cu(UO2)2(PO4)28H2O) are scattered within nodules consisting of fine-grained (2-50 nm) goethite and hematite, where the ground water is undersaturated with respect to metatorbernite, for the natural rocks from the Koongarra ore deposit, Australia. The microscopy also reveals that microcrystals (a few nm) of dehydrated schoepite ((UO2)O0.25(OH)1.5) are formed among fine-grained hematite after aging coprecipitated Fe-U in the laboratory, and the solution is undersaturated with respect to schoepite. The beam size of microscopes is found to be important for the chemical analysis of such microcrystals. We detect a strong signal of uranium for a beam size < 40 nm; whereas a weak uranium signal is obtained for a beam size > 150 nm. Our results indicate that such a weak uranium signal should not be taken as a result of homogeneously distributed uranium over goethite and hematite surfaces by, for instance, adsorption. The micrcrystallization observed in both the field and laboratory suggests that fine grained uranyl minerals play a major role in uranium transport and migration.

The Gibbs free energies and enthalpies of formation of U (super 6+) phases; an empirical method of prediction

Uranyl minerals form as a result of the oxidation and alteration of uraninite, UO 2+x . These uranyl phases are also important alteration products of the corrosion of UO 2 in used nuclear fuels under oxidizing conditions. However, the thermodynamic database for these phases is extremely limited. The Gibbs free energies and enthalpies for uranyl phases are estimated based on a method that sums polyhedral contributions. The molar contributions of the structural components to ∆G 0 f and ∆H 0 f are derived by multiple regression using the thermodynamic data of phases for which the crystal structures are known. In comparison with experimentally determined values, the average residuals associated with the predicted ∆G 0 f and ∆H 0 f for the uranyl phases used in the model are 0.08 and 0.10%, respectively, well below the limits of uncertainty for the experimentally determined values. To analyze the reliability of the predicted ∆G 0 f values, activity-activity diagrams in SiO 2 -CaO-UO 3 -H 2 O and CO 2 -CaO-UO 3 -H 2 O systems at 298.15 K and 1 bar were constructed using the predicted ∆G 0 f,298.15 values for the relevant U 6+ phases. There is good agreement between the predicted mineral stability relations and field occurrences, thus providing confidence in this method for the estimation of ∆G 0 f and ∆H 0 f of the U 6+ phases.

A Mechanistic Model of Spent Fuel Dissolution, Secondary Mineral Precipitation, and Np Release

AbstractA mechanistic spent fuel dissolution model has been developed, based on the generalreaction-transport code of AREST-CT. It considers the dissolution of spent fuel under flow conditions. The kinetic reactions of spent fuel dissolution and precipitation of schoepite, uranophane, soddyite, and Na-boltwoodite are included in the model. The results of model prediction are compared against the results of drip-tests that simulate the conditions that may occur in the Yucca Mountain Repository. Comparison shows that the modeling results match the laboratory observations very well and no contradiction has been found. It indicates that the model is a reasonably good representation of the real system.After validation, the model was used to investigate the release rate of Np from the dissolution of secondary uranyl minerals by examining various degrees of Np incorporation into secondary uranyl minerals. The predicted Np concentration in the aqueous phase is 3 orders of magnitude lower than the upper-bound of the Np solubility range currently used in DOE performance assessment analyses. It suggests that the Np solubility range currently used is too conservative and could be replaced with more realistic values.