I. E. Vlasova

Multiscale Speciation of U and Pu at Chernobyl, Hanford, Los Alamos, McGuire AFB, Mayak, and Rocky Flats.

The speciation of U and Pu in soil and concrete from Rocky Flats and in particles from soils from Chernobyl, Hanford, Los Alamos, and McGuire Air Force Base and bottom sediments from Mayak was determined by a combination of X-ray absorption fine structure (XAFS) spectroscopy and X-ray fluorescence (XRF) element maps. These experiments identify four types of speciation that sometimes may and other times do not exhibit an association with the source terms and histories of these samples: relatively well ordered PuO2+x and UO2+x that had equilibrated with O2 and H2O under both ambient conditions and in fires or explosions; instances of small, isolated particles of U as UO2+x, U3O8, and U(VI) species coexisting in close proximity after decades in the environment; alteration phases of uranyl with other elements including ones that would not have come from soils; and mononuclear Pu-O species and novel PuO2+x-type compounds incorporating additional elements that may have occurred because the Pu was exposed to extreme chemical conditions such as acidic solutions released directly into soil or concrete. Our results therefore directly demonstrate instances of novel complexity in the Å and μm-scale chemical speciation and reactivity of U and Pu in their initial formation and after environmental exposure as well as occasions of unexpected behavior in the reaction pathways over short geological but significant sociological times. They also show that incorporating the actual disposal and site conditions and resultant novel materials such as those reported here may be necessary to develop the most accurate predictive models for Pu and U in the environment.

Microdistribution of 241Am in structures of submerged macrophyte Elodea canadensis growing in the Yenisei River

A submerged macrophyte of the Yenisei River, Elodea canadensis, was used to study the microdistribution of the artificial radionuclide (241)Am among different components of the plant. The total amount of (241)Am added to the experimental system was 1850+/-31 Bq/L. The total amount of (241)Am accumulated by the plants was 182 Bq per sample, or 758,333+/-385 Bq/kg dry mass. It has been found that the major portion of (241)Am accumulated by E. canadensis, up to 85%, was bound to solid components of the cells. It is observed that the microdistribution of (241)Am within different components of the submerged plant E. canadensis was not uniform. (241)Am distribution vary depending on the age of the leaf blades, the state of the cells and morphological features of the plant stem.

Partitioning and speciation of Pu in the sedimentary rocks aquifer from the deep liquid nuclear waste disposal

Abstract Plutonium partitioning and speciation was studied under the conditions of a liquid nuclear waste repository site. The dominating effect of corrosion products (Fe and Cr) oxides on plutonium partitioning was established in batch sorption tests at 150 ℃ and by using sequential extraction. Various techniques including Mössbauer spectrometry, XRD and TEM were used to characterize Fe/Cr oxide phases that form both precipitates and surface coatings which sorb plutonium. The sorption and speciation of plutonium was modeled thermodynamically using Pourbaix diagram that takes into account both surface complexation reaction and precipitation of insoluble PuO2.

Speciation of radionuclides in colloidal matter of underground waters taken from observation wells in the zone of impact of Lake Karachai

Associations of radionuclides with colloidal particles of various sizes, isolated from underground waters of the Lake Karachai contamination area, were studied. Analysis by photon correlation spectroscopy showed that the total content of colloidal matter in deeper horizons is higher by an order of magnitude than in near-surface horizons. The mean particle radius also increases with the depth. The major fraction of Pu, Am, and Cm is associated with colloids (40–90%). U and Np are associated with colloidal particles to a lesser extent (2–20%), which determines their higher migration mobility in underground waters. The amount of actinides associated with coarse colloidal particles of size from 450 to 200 nm is insignificant. A considerable fraction of actinides is in the deep-lying water (depth ≥40 m) is associated with colloidal particles of size from 200 to 50 nm. No more than 30% of Pu and Am in water of these horizons is associated with finer colloids (from 10 kDa to 50 nm). With approaching the surface, the amount of actinides in the fraction of nanometer-sized particles (50 nm-10 kDa) increases (to 50%).

Radiography and local microanalysis for detection and examination of actinide-containing microparticles

A procedure was proposed for examination of the spatial microdistribution of fissile actinide isotopes (235U, 239Pu) in various environmental objects (soils, bottom sediments, aerosols, colloid material). Complex analysis of natural actinide-containing microparticles involves α-track radiography (ATR) and neutron-induced fission track radiography (NITR), highly sensitive nondestructive methods for determining the spatial microdistribution of α-emitting and fissile actinides, respectively, and scanning electron microscopy with energy-dispersive spectrometry (SEM-EDS) and secondary ion mass spectrometry (SIMS) for local microanalysis of the general elemental and isotope composition of the particles and determination of the morphology and size of the particles. The actinide-containing particles were localized using an SEM microgrid with electrodeposited 239Pu. Particles characterized by a high concentration of actinides were detected by SEM and SIMS, while those with a low content of actinides, by track radiography followed by fragmentation of the sample for subsequent analysis (γ-ray spectrometry etc.).

Effect of corrosion products on the Pu partitioning in rocks of reservoir horizon upon interaction with acidic solutions under hydrothermal conditions

Plutonium sorption on various types of sands of reservoir horizon depends on the presence of corrosion products in solutions simulating highly saline acidic (pH 2.4) liquid radioactive waste (LRW) under hydrothermal conditions (T = 150°C, 50 h). The effect of corrosion products (Fe, Cr) present in LRW consists not only in considerably increased Pu sorption, but also in increased fraction of its strongly fixed fraction. α-Track radiography revealed essentially heterogeneous distribution of Pu between mineral phases of sands of reservoir horizon after interaction with simulated LRW and predominant association of Pu with separate phases containing Fe and Cr (data of X-ray microanalysis based on scanning electron microscopy, SEM-EDX). Examination by Raman spectroscopy and X-ray diffraction analysis showed that the phases responsible for the predominant sorption of Pu are phases of Cr-containing hematite.

Interaction of plutonium with iron- and chromium-containing precipitates under the conditions of reservoir bed for liquid radioactive waste

The behavior of Pu under the conditions of liquid radioactive waste (LRW) disposal in a reservoir bed was studied. The pH dependence of the Pu sorption is the same in the cases when the radionuclide is introduced together with the simulated LRW solution or after the precipitate formation. The composition and morphology of Fe,Cr-containing precipitates formed under the conditions simulating the radioactive waste disposal were determined. The solid phase was characterized by X-ray diffraction analysis and by transmission and scanning electron microscopy. The phase composition of the precipitates was determined in relation to the presence of acetate ions, Fe concentration in the solution, and storage time and temperature. The main phases formed are ferrihydrite, goethite, hematite, and grimaldiite.

Electrochemical properties and dissolution of UPd3 in nitric acid solutions

Electrochemical properties of the intermetallic compound UPd3 in 0.5–8 M HNO3 solutions were studied by linear voltammetry. In 0.5–2 M HNO3 solutions, the UPd3 surface is in the passive state. At HNO3 concentrations exceeding 4 M, the alloy passivation was not observed. The previously unknown electrochemical characteristics of UPd3 in nitric acid solutions were obtained using the Tafel equation. The values of Е(i = 0) and vcorr increased from 39 mV and 38 μg cm–2 h–1 in 0.5 M HNO3 to 821 mV and 11 mg cm–2 h–1 in 8 M HNO3, respectively. Dissolution experiments have shown that UPd3 can dissolve in HNO3 solutions of concentration exceeding 4 M at room temperature. In 8 M HNO3, the dissolution rate can reach 17 mg cm–2 h–1 at 25°С, with the dissolution being virtually equimolar and accelerating with time.

Behavior of U(VI) under the Conditions of a Reservoir Bed for Liquid Radioactive Waste

The behavior of U(VI) under the conditions of liquid radioactive waste (LRW) disposal in a reservoir bed was studied. Model experiments were performed to determine the radionuclide binding mechanism and the role of hydrolysis products of LRW cations and of sands from the reservoir bed. The U(VI) binding with the reservoir bed rocks depends on pH of the solution. The U(VI) binding with the solid phase under the conditions of LRW reservoir bed is determined by sorption reactions with components of the surrounding rocks. The kinetic of uranium leaching occurring with a decrease in pH of the solution was studied.

Speciation of actinides in groundwater samples collected near deep nuclear waste repositories

Actinide binding to colloidal particles of different nature was studied under oxic and anoxic conditions of an underground nuclear waste disposal site using successive micro- and ultrafiltration techniques. According to the actinide redox speciation, under oxic conditions they were present in high oxidation states except for plutonium, for which a significant part was found in the tetravalent state. In case of the anoxic conditions, the share of An (IV) was proportional to the total U(IV) concentration. This indicated formation of intrinsic U(IV) hydroxocolloids, which bound other actinides. Formation of the intrinsic actinide colloids was proven by the secondary ion mass spectrometry (SIMS) with the submicron resolution. In contrast, under the oxic conditions uranium and plutonium were sorbed by natural colloids (amorphous hydrous ferric oxide and Mn oxides).