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The environmental chemistry of actinide elements is of considerable interest since the release of actinides into the environment is considered the primary long-term hazard of nuclear waste disposal in geological repositories. While numerous studies have examined a great variety of organic and inorganic actinide complexes, the possible significance of complexes with orthosilicic acid, found in all natural waters, has been largely ignored. The aqueous complexes formed between trivalent europium or hexavalent uranyl cations and orthosilicic acid were studied in 0.1 M ionic strength solutions to improve the models used to predict actinide migration in the environment. / Ligand competition experiments were employed to measure the stability constants of these f-element-orthosilicate complexes. Europium-orthosilicate complexation was studied by solvent extraction and laser-induced fluorescence spectroscopy. Measurable amounts of the 1:1 complex begin to form at pcH 4 in solutions containing millimolar orthosilicate concentrations. At pcH values greater than 8, europium bisorthosilicate complexes form. The stability constant of the 1:1 uranyl:orthosilicate complex also was measured between pcH 4 and 5 using absorption spectroscopy. / Speciation calculations show that europium orthosilicate complexes are important inorganic species for waters between pcH 6 and 7.5 that are in equilibrium with atmospheric carbon dioxide and contain at least 0.03 mM orthosilicate. Orthosilicate complexes are less important for uranyl cations than for europium cations owing to increased hydrolysis and carbonato complex formation for the former. The presence of 0.1 ppm humic acid completely inhibits formation of aqueous europium and uranyl orthosilicate complexes. / Source: Dissertation Abstracts International, Volume: 55-07, Section: B, page: 2722. / Major Professor: Gregory R. Choppin. / Thesis (Ph.D.)--The Florida State University, 1994.

Source: Dissertation Abstracts International, Volume: 35-08, Section: B, page: 3819. / Thesis (Ph.D.)--The Florida State University, 1974.

<p> Corroding waste tanks at select U.S. Department of Energy's nuclear waste facility have leaked highly alkaline tank waste solutions containing radionuclides and other contaminants into subsurface sediments. These tank wastes react with subsurface sediments to form secondary mineral phase(s) (feldspathoids), which may play key role in the transport of contaminants through the vadose zone and aquifers. Although transformation of secondary precipitates in subsurface sediments has been extensively studied, however, there is lack of knowledge about the role of feldspathoid selectivity in controlling the long-term fate and transport of key anionic radionuclides in the subsurface. The overarching objectives of this dissertation were to (1) determine secondary mineral transformation with aging time, alkalinity, anion identity and selectivity, and (2) quantify the competitive incorporation of ReO₄^- (a chemical analogue for Tc-99) into mineral phase(s) as a function of anion composition, size and selectivity under simulated waste leaks. The key results of this work showed that alkalinity, time and anion composition play important role in mineral transformation that control the mobility of key radionuclide species in the environment. Nitrate in 16 mol OH^-/kg solution favored cancrinite nucleation while in 1 mol OH^-/kg solution fostered mixed cancrinite/sodalite formation. The sequestering capacity of sodalite for ReO₄^- was &sim;5 times higher than that of cancrinite. The immobilized ReO₄^- in the sodalite cages was not easily exchanged with other competing anions. Due to the less distortion to the &beta;-cage, sodalite displayed stronger preference for smaller competing anions relative to the larger ReO₄^- anion. The selectivity of the mixed sodalite cage for ReO₄^- was largely driven by the difference in anion radii (DIR) and increases in the following order: Cl^-&lt;NO₃^-&lt;MnO₄^- and CO₃^2-&lt;SO₄^2-&lt;WO₄^2- for the monovalent and divalent anions respectively. When the DIR between ReO₄^- and competing anions was less than &sim;12%, then ReO₄^- incorporation into sodalite was significant. The results imply that anion size is the major factor that determines sodalite anion compositions and that ReO₄^- is likely to serve as a suitable analogue for TcO₄^-</p>

An electronic system was designed to receive a signal from a Self Powered Neutron Detector (SPND) and measure the initial period (e-folding time) of the TRIGA reactor at the University of Arizona. The design and fabrication of the SPND used was part of this work. The electronic system together with the SPND were used to construct a reactivity worth calibration curve for the transient rod based on pulse measurements.

The redox and complexation interactions of neptunium with humic acid and chemical analogs are studied in this research by solvent extraction and spectrophotometry. / In the redox study, dicarboxylic acids and substituted phenolic compounds are selected as models for humic acids. Np(VI) is found to be reduced to Np(V) by both series of compounds, but phenolic compounds reduce Np(VI) more rapidly under the same experimental conditions. Reaction products and various effects on the reaction rate were studied, including the kinetic solvent isotope, salt, photoexcitation, concentration and micelle effects. Results suggest that the reduction of Np(VI) by dicarboxylic acids proceeds via the formation of 1:1 complex (NpO$\sb2$L) while the formation of 1:2 complex (NpO$\sb2$L$\sb2\sp{2-}$)inhibits the redox reaction. The magnitude of the rate constants depends on the stability of the 1:1 complex and the presence of a methylene proton in those dicarboxylic acids. Results also suggest that the reduction of Np(VI) by phenolic compounds occurs through the interaction of Np(VI) with the hydroxy group, which results in the formation of phenoxy radicals. The rate constant sequence is explained by the electronic effect of various substitution groups. A correlation between the rate constant and the basicity of the hydroxy group is discussed. / Np(VI) is reduced rapidly by humic acid to Np(V), which exists as free Np(V) and Np(V)-humate complex. In the subsequent study of complexation between Np(V) and humic acid, Np(V) is found to form a single type of complex with humic acid in the pH range of 4.5-7.5, which is probably the 1:1 Np(V)-carboxylate species. There is a slight trend that humic acids with lower molecular weights form weaker complexes with Np(V). Different approaches for correction of the apparent binding constants are discussed. The binding strength of Np(V)-humate is compared with those of other metal-humate or Np(V)-dicarboxylate systems. / Source: Dissertation Abstracts International, Volume: 53-10, Section: B, page: 5197. / Major Professor: G. R. Choppin. / Thesis (Ph.D.)--The Florida State University, 1992.

The redox chemistry of UCl$\sb4$ was investigated in the solvent AlCl$\sb3$-1-ethyl-3-methylimidazolium chloride (AlCl$\sb3$-EMIC), a room temperature ionic liquid. In basic solutions ( / Source: Dissertation Abstracts International, Volume: 51-12, Section: B, page: 5857. / Major Professor: Gregory Robert Choppin. / Thesis (Ph.D.)--The Florida State University, 1990.

The complexation of f-elements with ligands in brine is of considerable interest since the release of actinides into the salt-bed environment is considered the primary hazard of radioactive wastes in a long-term geological disposal. To predict the actinide migration in the environment, the aqueous complexes formed between trivalent europium or hexavalent uranyl cations with chloride, nitrate, fluoride and acetate were studied in a wide range of ionic strength, from 0.1 M to 9.0 M. / Ligand competition experiments were employed to measure the stability constants of these f-element complexes: solvent extraction, titrations with pH electrode or ion selective electrode, as well as other techniques. / Based on measured equilibrium constants and literature data, a Parabolic model was developed in this study to correlate equilibrium constants (pKa of acids, log $\beta\sb{\rm i}$ of metal complexes, etc) with ionic strength. This correlation was compared with that by the SIT and the Pitzer models. Possibilities of decomposing the coefficients $\Delta\varepsilon$ and $\Delta\delta$ into $\varepsilon\sb{\rm i}$ and $\delta\sb{\rm i}$ were analyzed with a revision of the Pitzer's subequations. / The speciation diagrams of Eu(III), U(VI) in brines at I = 0.7 M and 5.0 M show that the f-element inorganic complexes studied in this work are less important species for neutral brines at high ionic strengths owing to increased hydrolysis or carbonate complex formation. The influence of ionic strength on f-element speciation in brine solutions is reflected by the increased total concentration of chloride and the variation of log $\beta\sb{\rm MLi}$ and PKa$\sb{\rm HL}$ values with ionic strength. / Source: Dissertation Abstracts International, Volume: 56-08, Section: B, page: 4282. / Major Professor: Gregory R. Choppin. / Thesis (Ph.D.)--The Florida State University, 1995.

<p> Fusion in neutron-rich environments is presently a topic of considerable interest. For example, the optical emission spectrum from the neutron star merger GRB170817A clearly establishes this neutron-rich environment as an important nucleosynthetic site. Fusion of neutron-rich light nuclei in the outer crust of an accreting neutron star has also been proposed as responsible for triggering X-ray super-bursts. The underlying hypothesis in this proposition is that the fusion of neutron-rich nuclei is enhanced as compared to stable nuclei. A good approach to understand how fusion proceeds in neutron-rich nuclei is to measure the fusion excitation function for an isotopic chain of nuclei. Modern radioactive beam facilities provide the opportunity to systematically address this question. An experimental program has been established to measure the fusion excitation function for light and mid-mass neutron-rich nuclei using low-intensity radioactive beams. The technique was initially demonstrated by measuring the fusion excitation functions for <i>18</i>O and ¹⁹O nuclei incident on a ¹²C target. The beam of ¹⁹O ions was produced by the <i>18</i>O(d,p) reaction with an intensity of 2-4 x 10⁴ p/s at Florida State University. Evaporation residues resulting from the de-excitation of the fusion product were distinguished by measuring their energy and time-of-flight. To explore mid-mass neutron-rich nuclei much further from stability, the fusion excitation functions for ^39,47K + ²⁸Si were measured using the ReA3 reaccelerator facility at the National Superconducting Cyclotron Laboratory at Michigan State University. Incident ions were identified on a particle-by-particle basis by &Delta;E-TOF just upstream of the target. Fusion products were directly measured and identified by the E-TOF technique with an efficiency of ~70%. The measured fusion excitation functions for both the light and mid-mass systems have been compared to various theoretical models to elucidate how structure and dynamics impact the fusion of neutron-rich nuclei.</p><p>

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