K.R. Czerwinski

Mobilization of transuranic radionuclides from disposal trenches by natural organic matter

Abstract Transuranic (TRU) radionuclides in groundwater at the Oak Ridge National Laboratory migrate rapidly and with little retardation of the radionuclides over distances of 80 m. Several interacting hydrogeochemical processes contribute to the observed releases of actinides ( 244 Cm and 241 Am) from the shallow unlined disposal trenches, through the highly weathered, fractured shale (saprolite) and to the surface-water seeps at White Oak Creek. Major releases are promoted when seasonal fluctuations in the water table permit groundwater to contact actinide-contaminated waste. Local recharge of stormwater into the trenches appears to permit minor releases, perhaps due to transient saturation within the trenches but above the local water table. Although the hydrogeology of the site permits contact of the TRU waste with the groundwater, the expected inorganic species of the actinides should strongly adsorb to the layer silicates and mineral oxides of the shale saprolite. Yet the timing of the actinide releases relative to when rising groundwater intercepts the trenches suggests that actinide transport is rapid, and the relative magnitude of peak actinide levels in wells near the trenches and at downgradient seeps suggests that there is very limited retention of the actinides by the formation. Based on anion exchange chromatography of the groundwater and geochemical modeling, the mobilization and transport of the actinides is demonstrated to result from complexation of the actinides by natural organic matter (NOM). Storm events contribute to mobilization by promoting hydrologic links between the TRU waste and groundwater, and by increasing the concentration of NOM in the mobile soil and groundwater. This study demonstrates that even in formations characterized by abundant mineral phases known to strongly adsorb actinides, the actinides can be transported essentially conservatively as NOM complexes.

Selective Separation of Lanthanides with Phenolic Resins: Extraction Behavior and Thermal Stability

Catechol, resorcinol, and their admixtures with 8-hydroxyquinoline were converted into polymeric resins by alkaline polycondensation with formaldehyde. The resins were characterized by FTIR spectroscopy, moisture regain, ion-exchange capacity, and distribution coefficient (D) for Eu3+. Thermogravimetric analysis of the polymer samples was studied, and the effect of the sorption of metal ions on their thermal stability was evaluated. Complexation of Eu3+ to the resins was modeled based on metal ion charge neutralization. The selective uptake of Eu3+ from aqueous solutions containing La3+ was investigated, and the ionoselectivities of the resins were compared. The incorporation of 8-hydroxyquinoline in the molecular matrix of the phenolic resins is shown to exert a significant influence upon the competitive sorption of La3+ and Eu3+, leading to their intragroup separation. The separation factors obtained by phenolic ion-exchange resins from aqueous solutions indicate ion-specific resins can be developed for the specific separation of actinide ions from nuclear waste.

Studies of Polonium Removal from Molten Lead-Bismuth for Lead-Alloy-Cooled Reactor Applications

Abstract The isotope 210Po is the main product of neutron activation in fast reactors cooled by molten lead-bismuth eutectic (LBE). The isotope 210Po is a pure alpha emitter with a half-life of 138.38 days. For typical values of the neutron flux the 210Po concentration in the coolant can reach 1–10 Ci/kg. While exposure of plant personnel to Po is prevented under normal operating conditions because the primary system is sealed, Po does pose a radiological hazard during maintenance activities for which access to submerged structures is required as well as during accidents resulting in breach of the primary-system barrier. Obviously, continuous removal of Po from the LBE reduces this hazard. Therefore, it is important to understand the mechanisms by which Po is formed in and released from the LBE. We summarize research performed at the Idaho National Engineering and Environmental Laboratory and the Massachusetts Institute of Technology to investigate the basic chemistry of four mechanisms of Po release, which could serve as the basis for a coolant cleanup system in LBE-cooled reactors. The mechanisms explored are lead polonide evaporation, formation of polonium hydride, rare-earth filtering, and alkaline extraction. For the key chemical species involved expressions are given for useful quantities such as formation energy, release, and deposition rates. It is concluded that the most promising removal mechanism is alkaline extraction, although a more systematic investigation of this mechanism is needed.

Complexation of transuranic ions by humic substances: Application of laboratory results to the natural system

Environmental investigations show transuranic ions sorb to humic substances. The resulting species are often mobile and are expected to be important vectors in the migration of transuranic ions in natural systems. However, these environmental studies yield no quantitative data useful for modeling. Laboratory complexation experiments with transuranic ions and humic substances generate thermodynamic data required for complexation modeling. The data presented in this work are based on the metal ion charge neutralization model, which is briefly described. When a consistent complexation model is used, similar results are obtained from different experimental conditions, techniques, and laboratories. Trivalent transuranic ions (Cm(III), Am(III)) have been extensively studied with respect to pH, ionic strength, origin of humic acid, and mixed species formation. The complexation of Np(V) has been examined over a large pH and metal ion concentration range with different humic acids. Some data does exist on the complexation ion concentration range with different humic acids. Some data does exist on the complexation of plutonium with humic acid, however further work is needed. Calculations on the Gorleben aquifer system using the thermodynamic data are presented. Critical information lacking from the thermodynamic database is identified. 55 refs., 2 figs., 3 tabs.

Ion Selective Resins: Development and Applications for Nuclear Waste Management

Organic based ion selective resins have some similar attributes: case of synthesis, high metal ion complexation ability, and flexibility for different nuclear waste management applications. For most chelating polymers, the ligand is deemed to be of primary importance for the interaction with the targeted metal ion. The role of the polymer matrix is usually ignored. For ion specific resins, the polymer structure is formed to a specific metal ion. Using the molecular imprinting technique, resins can be formed with functional groups and cavities for a target metal ion. Ion selective resins have been developed for the separation of Cs. The methods and concepts used for the development of the Cs specific resins have been applied to the development of selective resins for Eu (a trivalent actinide model). The resulting resins are characterized by FTIR spectroscopy, moisture regain, and ion exchange capacity. The incorporation of 8-hydroxyquinoline into the resin increases selectivity for Eu over La. The results for the Eu study indicate ion specific resins can be developed for the separation of trivalent actinides from nuclear waste.

Modeling cation exchange in zeolitic nuclear waste form

Zeolites are currently being considered as an encapsulation material for high-level nuclear wastes. As a first step toward understanding the mechanisms of radionuclide migration in such molecular crystals, we apply an atomistic simulation approach to the study of ion exchange, which is assumed to be the underlying chemical process for the release of radioactive cations. Specifically, we investigate the Cs-Na cation exchange isotherm for dehydrated sodalite waste form. Our calculations indicate significant relaxation effects of the sodalite cage, in the form of displacement of cation adsorption sites and cage distortion.

Characterizing Transport and Sorption in Ion-Specific Resin Columns Using Nuclear Magnetic Resonance (NMR) Imaging

The goal of this work is to assess the physical transport properties of Gd through an ion exchange column while determining the sorption properties of the resin. By coupling the physical transport with the chemical sorption, further insight into the behavior of the ion exchange resin can be gained. NMR imaging provides a powerful, non-destructive, means to extract spatial information from complex systems on a near real-time basis. An important example is liquid flow through granular media. With the use of a chemically reactive NMR contrast agent, the chemical speciation can be traced along the physical flow path of the granular media. In this study, trivalent gadolinium (Gd 3+ ) was selected based on its chemical similarity to typical high-level waste components, 241 Am and 244 Cm, and for its paramagnetic contrasting abilities in NMR experiments. NMR imaging results of flow experiments are provided showing a characteristic flow phenomena and resin column loading profiles. ICP-AES data are provided to show resin ion exchange capacities (IECs) and breakthrough curves. The use of NMR imaging with a Gd 3+ tracer will lead to a better understanding of the transport and sorption properties of these ion-specific resins. This technique can be applied to other complex flow systems such as environmental transport.

Characterization of Uranium Speciation in a Metallic Matrix

Three metallic slag samples recovered from the site of their inadvertent creation several decades after the fact were sectioned and analyzed to determine uranium speciation to evaluate environmental behavior as well as assess proliferation resistance of the waste form. Uranium concentration in the highly inhomogeneous samples was up to 5% by weight as determined by gamma spectroscopy. Sample sections were milled in a hardened steel ball mill for x-ray absorption spectroscopy analysis (XAS). Powders were digested in a mixture of heated concentrated nitric acid and peroxide and analyzed for elemental content using ICP-AES and ICP-MS. Though elemental content of the samples varied widely, high concentrations of Al were consistently found. Other metals of significance were Ti, Fe, Ni, Cu, Zn, and Sn. High Pb concentrations were occasionally found. XAS analysis revealed the samples contained primarily uranyl and another phase identified as a uranium-aluminum melt.

Characterization and Dissolution of ZrTh 3 UO 10 and Th 3 UO 8 Ceramics

Thoria-urania-zirconia ceramics were studied in order to investigate the long-term behavior of potential thorium fuels in a repository environment. The ceramics were prepared by coprecipitation of the metal salts. Zirconia was added to determine if further stabilization against dissolution of the thoria-urania system could be achieved. In addition, 0.5 wt% MgO was added to some samples to increase stability and density. The inclusion of Zr in the ceramics did not dramatically decrease the leaching of thorium from the matrix. Material properties of the ceramic were analyzed using electron microscopy techniques such as Energy Dispersive X-ray (EDX) analysis and Electron Energy Loss Spectroscopy (EELS). X-ray diffraction and synchrotron-based x-ray absorption studies including extended xray fine structure (EXAFS) and x-ray absorption near edge spectroscopy (XANES) were also used to reveal elements of the phase structure and chemistry of the ceramics. X-ray diffraction (XRD) and EDX show that these ceramics separate into a zirconium-based phase and an actinide-based phase with low mutual affinity of thorium and zirconium, as well as partial solubilization of uranium in zirconium. The comparison of EELS spectra collected for the ceramics with spectra collected for UO 2 and U 3 O 8 reference materials also allow the assessment of uranium oxidation state independently in the two separate phases. Assessment of the bulk oxidation state using XANES correlated well with the EELS analysis. Interatomic distances and the bulk crystal structure were determined using EXAFS.

REMOVAL OF 243Am WITH PHENOL BASED RESINS

Long-term radiotoxicity of nuclear waste produced during spent nuclear fuel reprocessing could be reduced if the minor actinides (neptunium, americium and curium) contained within the waste are separated into short-lived radionuclides for their subsequent transmutation or for separate storage. Cross-linked phenolic resins based on different substituted phenol were shown to be very efficient for selective uptake of Eu3+ from aqueous solutions containing equal amounts of La3+. In this work, we have prepared and characterized cross-linked phenol based resins to investigate the uptake of 243Am. According to the results obtained for Eu selective extraction and to enhance 243Am sorption, the resins are transformed into their Na+–forms before extraction. The ion selectivities of the cross-linked phenol based resins are then compared as a function of the identity of the ion-exchange phenolic matrix. Radiation stability of the resins was studied and each resin was measured for the effect of ionizing radiations with FTIR spectroscopy, moisture regain and ion exchange capacity.