Actinide Incorporation and Radiation Effects in U(VI) Solids Actinide sorption and reduction on sulfides, oxides, and clay minerals Photoexcitation of Mn-oxide Minerals and Actinide Metal-organic Frameworks for Catalysis of Actinyl Complexes and Nanoclusters. Final Report
- Univ. of Michigan, Ann Arbor, MI (United States)
A topic that we addressed in the earlier part of this grant was the radiation effects in minerals. While radiation effects can be used in the Earth sciences, for example, for dating exhumation rates, they are also important to understand the behavior of mineral-like crystalline phases under irradiation and if they would be good candidates as waste forms for radioactive materials. Therefore we have tested different materials in how they behave under different radiations. A uranyl-peroxide containing mineral, studtite, is expected to form as a consequence of α radiolysis of water in contact with spent nuclear fuel (SNF) in a geologic repository. Studtite was exposed to different fluences of radiation, and amorphization was analyzed as a function of these doses. In a different set of experiments, two other uranium-containing minerals, soddyite and uranophane, were studied under electron irradiation and over the temperature range of 25-300 °C. Under certain conditions the formation of nanocrystalline U02 was observed. Related to this we synthesized different uranium -containing compounds, for example, coffinite, USiO4, has been produced by hydrothermal synthesis and was analyzed using different electron microscopy and diffraction methods. Related to this, phosphorous-rich coffinite, U(Si,P)O4 $$\cdot$$ H2O, from the natural nuclear reactor at Bangombe, Gabon, has been examined as an important primary mineral and alteration product of uraninite under reducing conditions. In addition, a mineral assemblage of coffinite, USiO4$$\cdot$$ nH2O, carbonate-fluorapatite and (Ca, Sr)-(meta)autunite from the Woodrow Mine, Grants uranium region, New Mexico, has been investigated in order to understand the influence of a P-rich micro-geochemical environment on precipitation of coffinite and its subsequent alteration under oxidizing conditions. Finally, the response of synthetic coffinite to energetic ion beam irradiation was investigated. The second complex topic was the mechanism of redox reactions of actinides, with special emphasis on the kinetics. This topic led to the development of computational and experimental techniques to approach the redox mechanism in a more fundamental way, specifically to conceptualize a standard set of kinetic roadblocks that could be built into a more fundamental kinetic theory that is less based on a black-box model. Here, we have combined experimental techniques such as electrochemistry, electrochemical atomic force microscopy, x-ray photoelectron and Auger spectroscopy, electron microscopy, and a new set of quantum mechanical calculations that allow for the derivation of reaction paths. In addition, the role of mineral surfaces was evaluated in catalyzing reactions as they might be doing it in natural environments, and geologic barriers, but also in the near field of nuclear waste repositories. In addition, we have developed an experimental setup to quantify the synergistic interaction of different organic ligands as e- donors and TiO2 as a photocatalyst, with light at different wavelengths, for uranyl reduction, showing that all components are necessary for efficient photoreduction. In addition, electronic and atomic structure, thermodynamics, and reaction pathways were investigated for light -induced redox reactions. The third complex topic developed in the grant period was the incorporation of actinide ions into minerals as well as actinide metal organic frameworks. Step by step, we have developed a more comprehensive picture of the incorporation energies, structures, and electronic properties of actinyls incorporated into sedimentary minerals, such as carbonates and sulfates, iron oxides that, in addition to being present in natural environments, maybe a corrosion product of nuclear waste containers, and jarosites that are common waste products in acid mine setting s and also occur in uranium mines. There is also a progression of theoretical concepts within this series of papers: while our earlier papers lay the ground rules for combining aqueous ions and periodic minerals within the same chemical equation (typically a roadblock for traditional quantum-mechanical models), and we have extended the usage of endmember minerals by complex solid solutions series on the cation and anion sites. Furthermore, while typically only equilibria between dissolved species and incorporation into bulk minerals are considered, our latest contribution analyzes the thermodynamics along the entire reaction path from solution, to surface adsorption, to incorporation into the uppermost surface layer, and finally to incorporation into the bulk solid.
- Research Organization:
- Univ. of Michigan, Ann Arbor, MI (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences, and Biosciences Division
- DOE Contract Number:
- FG02-06ER15783
- OSTI ID:
- 1615667
- Report Number(s):
- DOE-UMICH-15783-1; TRN: US2104883
- Country of Publication:
- United States
- Language:
- English
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