Thesis (Ph.D.)--Boston University / Due to the necessity of recovering uranium from reactor fuels, a number of separation methods have been investigated in recent years. Of these, the method of solvent extraction has proved to be a practical one. Long chain symmetrical phosphine oxides have been shown to extract uranium and a number of other metals under different conditions. It was the object of the present investigation to establish the nature of the species resulting from uranyl nitrate-R3Po (R =alkyl group) interaction. The two phosphine oxides selected for the study were: (i) Tri-n-octylphosphine oxide (TOPO) and (ii) Tris(2-ethylhexyl)phosphine oxide (TEHPO). An attempt was made to obtain pure TEHPO. Though the attempt has not met with complete success, yet significant conclusions concerning the stoichiometry and relative stability of the uranyl nitrate-TEHPO complex could be drawn. [TRUNCATED]

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Density functional (DFT) methods are first used to study 22 of the most stable solution-phase UN4O12 isomers containing uranyl nitrate, UO2(NO3)2. Based on relative free energy calculations, 4 solution (a6, a5, a8, and a1) and 5 gas-phase isomers (a1, a2, a3, b1, and b2) are identified as the strongest candidates to exist and possibly predominate within their respective environments. DFT is then applied to a new form of binucleating Schiff–base polypyrrolic macrocycles containing actinyl ions [AnO2]n+ (An = U, Np, Pu; n = 1, 2) and 3d transition metals (TM): Mn, Fe, Co, and Zn. Formal bond order evidence is provided for 24 TM to actinyl–endo–oxygen partial bond formations. Special structural cases are discussed. Redox potentials for AnVIO21/AnVO21– couples closely follow the Np > Pu > U trend seen for AnO2(H2O)52+/1+. Predictions of –1.10, 0.25, and 0.01 eV are made for U, Np, and Pu redox potentials.

Density functional (DFT) methods are first used to study 22 of the most stable solution-phase UN4O12 isomers containing uranyl nitrate, UO2(NO3)2. Based on relative free energy calculations, 4 solution (a6, a5, a8, and a1) and 5 gas-phase isomers (a1, a2, a3, b1, and b2) are identified as the strongest candidates to exist and possibly predominate within their respective environments. DFT is then applied to a new form of binucleating Schiff–base polypyrrolic macrocycles containing actinyl ions [AnO2]n+ (An = U, Np, Pu; n = 1, 2) and 3d transition metals (TM): Mn, Fe, Co, and Zn. Formal bond order evidence is provided for 24 TM to actinyl–endo–oxygen partial bond formations. Special structural cases are discussed. Redox potentials for AnVIO21/AnVO21– couples closely follow the Np > Pu > U trend seen for AnO2(H2O)52+/1+. Predictions of –1.10, 0.25, and 0.01 eV are made for U, Np, and Pu redox potentials.

Conversion, the process by which natural uranium ore (yellowcake) is purified and converted through a series of chemical processes into uranium hexafluoride gas (UF6), has historically been excluded from the nuclear safeguards requirements of the 235U-based nuclear fuel cycle. With each step in the conversion process from yellowcake to feedstock for UF6, intermediary uranium oxide and uranium fluoride compounds become progressively attractive products for diversion toward activities noncompliant with international treaties. The diversion of this product material could potentially provide feedstock for a clandestine or undeclared enrichment for weapons development for state or non-state entities. With the realization of this potential, the International Atomic Energy Agency (IAEA) has only recently reinterpreted its policies to emphasize safeguarding this feedstock in response to such diversion pathways. This project employs a combination of simulation models and experimental measurements to develop and validate concepts of nondestructive assay monitoring systems in a natural uranium conversion plant (NUCP). In particular, uranyl nitrate (UN) solution exiting solvent extraction was identified as a key measurement point (KMP), where gamma-ray spectroscopy was selected as the process-monitoring tool. The Uranyl Nitrate Calibration Loop Equipment (UNCLE) facility at Oak Ridge National Laboratory was employed to simulate the full-scale operating conditions of a purified uranium-bearing aqueous stream exiting the solvent extraction process in an NUCP. This work investigates gamma-ray signatures UN circulating in the UNCLE facility and evaluates various gamma-ray detector (HPGe, LaBr3 and NaI) sensitivities to UN.

Des simulations de dynamique moléculaire classique prenant en compte explicitement la polarisation ont été réalisées afin de déterminer les propriétés structurales et thermodynamiques de solutions binaires aqueuses de chlorure, perchlorate et nitrate d'uranyle (UO2Cl2, UO2(ClO4)2 et UO2(NO3)2). A partir d'une étude faite en fonction de la concentration en sels, les propriétés d'hydratation et les interactions ion-ion des solutions aqueuses concentrées de chlorure, perchlorate et nitrate d'uranyle ont été étudiées. Les simulations de dynamique moléculaire permettent de reproduire les propriétés de solvatation de l'uranyle, du chlorure, du perchlorate et du nitrate en accord avec les données expérimentales. Les résultats ont montré les différents modes de coordination du chlorure et du perchlorate dans la deuxième sphère de coordination de l'uranyle et la présence de l'anion NO3- dans la première sphère de coordination à concentration élevée.De plus, nous avons calculé les potentiels de force moyenne des paires ioniques à dilution infinite en fonction de la distance et de l'angle. Les propriétés thermodynamiques des solutions ont été calculées à partir des potentiels des paires ioniques en utilisant la théorie McMillan-Mayer et moléculaire. La constante d'association des complexes UO2Cl+ (Kcal =2,52 L mol−1), UO2ClO4+ (Kcal =2,34 L mol−1) et UO2NO3+ (Kcal =3,02 L mol−1) a été calculée et sont en accord avec les données expérimentales. A partir des potentiels effectifs de McMillan-Mayer et en utilisant une approche multi-échelle basée sur l'approximation MSA, nous avons également calculé les coefficients osmotiques. / Classical molecular dynamics simulations with explicit polarization have been successfully used to determine the structural and thermodynamic properties of binary aqueous solutions of uranyl chloride, perchlorate and nitrate (UO2Cl2, UO2(ClO4)2 and UO2(NO3)2). From a study performed as a function of the concentration of salts, the hydration properties of solutes and the ion-ion interactions of concentrated aqueous solutions of uranyl chloride, perchlorate and nitrate have been studied. The molecular dynamics simulations allow for reproducing the solvation properties of the hydrated uranyl, chloride, perchlorate and nitrate in good agreement with the experimental data. The results point out different coordination modes of Cl− and ClO4− in the UO22+ second hydration shell and the presence of NO3− anion in the UO22+ first coordination shell at high concentration. The ion – ion interaction properties over water configurations have been studied at different concentrations.Furthermore, we investigated the potential of mean force of ion pairs at infinite dilution as a function of the distance and the angle. The thermodynamical properties of the solutions have been calculated from these effective ion-ion pair potentials thanks to the McMillan-Mayer theory and molecular theory. The association constant of complex UO2Cl+ (Kcal =2,52 L mol−1), UO2ClO4+ (Kcal =2,34 L mol−1) and UO2NO3+ (Kcal =3,02 L mol−1) has been determined and compared to the experimental ones. From the effective McMillan-Mayer potentials and using a multi-scale approach based on the MSA approximation, we also calculated the osmotic coefficients.