Impact of Natural Organic Matter on Plutonium Vadose Zone Migration from an NH4Pu(V)O2CO3(s) Source.

We investigated the influence of natural organic matter (NOM) on the behavior of Pu(V) in the vadose zone through a combination of the field lysimeter and laboratory studies. Well-defined solid sources of NH4Pu(V)O2CO3(s) were placed in two 5-L lysimeters containing NOM-amended soil collected from the Savannah River Site (SRS) or unamended vadose zone soil and exposed to 3 years of natural South Carolina, USA, meteorological conditions. Lysimeter soil cores were removed from the field, used in desorption experiments, and characterized using wet chemistry methods and X-ray absorption spectroscopy. For both lysimeters, Pu migrated slowly with the majority (>95%) remaining within 2 cm of the source. However, without the NOM amendment, Pu was transported significantly farther than in the presence of NOM. Downward Pu migration appears to be influenced by the initial source oxidation state and composition. These Pu(V) sources exhibited significantly greater migration than previous studies using Pu(IV) or Pu(III) sources. However, batch laboratory experiments demonstrated that Pu(V) is reduced by the lysimeter soil in the order of hours, indicating that downward migration of Pu may be due to cycling between Pu(V) and Pu(IV). Under the conditions of these experiments, NOM appeared to both enhance reduction of the Pu(V) source as well as Pu sorption to soils. This indicates that NOM will tend to have a stabilizing effect on Pu migration under SRS vadose zone field conditions.

Mobility of Aqueous and Colloidal Neptunium Species in Field Lysimeter Experiments.

Due to its radiotoxicity, long half-life, and potentially high environmental mobility, neptunium transport is of paramount importance for risk assessment and safety. Environmental transport of neptunium through field lysimeters at the Savannah River Site was observed from both oxidized (Np(V)) and reduced (Np(IV)) source materials. While transport from oxidized neptunium sources was expected, the unexpected transport from reduced neptunium sources spurred further investigation into transport mechanisms. Partial oxidation of the reduced neptunium source resulted in significant release and transport into the mobile aqueous phase, though a reduced colloidal neptunium species appears to have also been present, enhancing neptunium mobility over shorter distances. These field and laboratory experiments demonstrate the multiple controls on neptunium vadose zone transport and chemical behavior, as well as the need for thorough understanding of radionuclide source terms for long-term risk prediction.

Uranyl Peroxide Cage Cluster Solubility in Water and the Role of the Electrical Double Layer.

Uranium concentrations as high as 2.94 × 105 parts per million (1.82 mol of U/1 kg of H2O) occur in water containing nanoscale uranyl cage clusters. The anionic cage clusters, with diameters of 1.5-2.5 nm, are charge-balanced by encapsulated cations, as well as cations within their electrical double layer in solution. The concentration of uranium in these systems is impacted by the countercations (K, Li, Na), and molecular dynamics simulations have predicted their distributions in selected cases. Formation of uranyl cages prevents hydrolysis reactions that would result in formation of insoluble uranyl solids under alkaline conditions, and these spherical clusters reach concentrations that require close packing in solution.

Elucidating the Composition and Structure of Uranium Oxide Powders Produced via NO2 Voloxidation.

Voloxidation is a potential alternative reprocessing scheme for spent nuclear fuel that uses gas-solid reactions to minimize aqueous wastes and to separate volatile fission products from the desired actinide phase. The process uses NO2(g) as an oxidant for uranium dioxide (UO2) fuel, ideally producing soluble uranium powders which can then be processed for full recycle. To continue development of the process flowsheet for voloxidation, ongoing examination of the process chemistry and associated process materials is required: discrepancies in the proposed chemical reactions that occur when spent nuclear fuel is exposed to NO2(g) atmospheres must be addressed. The objective of this work is to analyze the intermediate solid phases produced during voloxidation to support verification of the proposed NO2(g) voloxidation reaction mechanisms. This objective was achieved through using (1) powder X-ray diffraction and Raman spectroscopy to identify bulk uranium phases and (2) scanning electron microscopy to describe the morphology and microstructure of the powders at each reaction stage. The initial oxidation of UO2 under NO2(g) reactions produced ε-UO3. Further exposure to NO2(g) did not nitrate the solid to produce uranyl nitrate, as reported in some literature. However, after the powder was hydrated with steam and then further exposed to NO2(g), some traces of uranyl nitrate hexahydrate were found. The results of this study suggest that surface hydration of powders plays a vital role in uranyl nitrate formation under voloxidation conditions and raises questions about the kinetics of the oxide-to-nitrate voloxidation conversion process. Future chemical and engineering design decisions for the voloxidation process may benefit from an improved understanding of these chemical mechanisms.

Vadose-zone alteration of metaschoepite and ceramic UO2 in Savannah River Site field lysimeters.

Uranium dioxide (UO2) and metaschoepite (UO3•nH2O) particles have been identified as contaminants at nuclear sites. Understanding their behavior and impact is crucial for safe management of radioactively contaminated land and to fully understand U biogeochemistry. The Savannah River Site (SRS) (South Carolina, USA), is one such contaminated site, following historical releases of U-containing wastes to the vadose zone. Here, we present an insight into the behavior of these two particle types under dynamic conditions representative of the SRS, using field lysimeters (15 cm D x 72 cm L). Discrete horizons containing the different particle types were placed at two depths in each lysimeter (25 cm and 50 cm) and exposed to ambient rainfall for 1 year, with an aim of understanding the impact of dynamic, shallow subsurface conditions on U particle behavior and U migration. The dissolution and migration of U from the particle sources and the speciation of U throughout the lysimeters was assessed after 1 year using a combination of sediment digests, sequential extractions, and bulk and µ-focus X-ray spectroscopy. In the UO2 lysimeter, oxidative dissolution of UO2 and subsequent migration of U was observed over 1-2 cm in the direction of waterflow and against it. Sequential extractions of the UO2 sources suggest they were significantly altered over 1 year. The metaschoepite particles also showed significant dissolution with marginally enhanced U migration (several cm) from the sources. However, in both particle systems the released U was quantitively retained in sediment as a range of different U(IV) and U(VI) phases, and no detectable U was measured in the lysimeter effluent. The study provides a useful insight into U particle behavior in representative, real-world conditions relevant to the SRS, and highlights limited U migration from particle sources due to secondary reactions with vadose zone sediments over 1 year.

Ultrafiltration of uranyl peroxide nanoclusters for the separation of uranium from aqueous solution.

Uranyl peroxide cluster species were produced in aqueous solution by the treatment of uranyl nitrate with hydrogen peroxide, lithium hydroxide, and potassium chloride. Ultrafiltration of these cluster species using commercial sheet membranes with molecular mass cutoffs of 3, 8, and 20 kDa (based on polyethylene glycol) resulted in U rejection values of 95, 85, and 67% by mass, respectively. Ultrafiltration of untreated uranyl nitrate solutions using these membranes resulted in virtually no rejection of U. These results demonstrate the ability to use the filtration of cluster species as a means for separating U from solutions on the basis of size. Small-angle X-ray scattering, Raman spectroscopy, and electrospray ionization mass spectrometry confirmed the presence of uranyl peroxide cluster species in solution and were used to characterize their size, shape, and dispersity.