Three dimensional multi-pass repair weld simulations

Full 3-dimensional (3-D) simulation of multi-pass weld repairs is now feasible and practical given the development of improved analysis tools and significantly greater computer power. This paper presents residual stress results from 3-D finite element (FE) analyses simulating a long (arc length of 62°) and a short (arc length of 20°) repair to a girth weld in a 19.6 mm thick, 432 mm outer diameter cylindrical test component. Sensitivity studies are used to illustrate the importance of weld bead inter-pass temperature assumptions and to show where model symmetry can be used to reduce the analysis size. The predicted residual stress results are compared with measured axial, hoop and radial through-wall profiles in the heat affected zone of the test component repairs. A good overall agreement is achieved between neutron diffraction and deep hole drilling measurements and the prediction at the mid-length position of the short repair. These results demonstrate that a coarse 3-D FE model, using a ‘block-dumped’ weld bead deposition approach (rather than progressively depositing weld metal), can accurately capture the important components of a short repair weld residual stress field. However, comparisons of measured with predicted residual stress at mid-length and stop-end positions in the long repair are less satisfactory implying some shortcomings in the FE modelling approach that warrant further investigation

Formation and Characterization of the Uranyl–SO₂ Complex, UO₂(CH₃SO₂)(SO₂)⁻

The uranyl–SO₂ adduct, UO₂(CH₃SO₂)­(SO₂)⁻, was prepared and characterized by mass spectrometric studies as well as by density functional theory. Collision induced dissociation of UO₂(CH₃SO₂)₂^– in an ion trap resulted in the formation of UO₂(CH₃SO₂)­(SO₂)⁻, which spontaneously reacted with O₂ to give UO₂(CH₃SO₂)­(O₂)⁻, with SO₂ released. The UO₂(CH₃SO₂)­(SO₂)⁻ complex is computed to have a triplet ground state at the B3LYP level, and the SO₂ ligand is coordinated to uranium through two oxygen atoms, similar to the coordination mode of SO₂ in its complexes with hard metals. On the basis of the calculated geometric parameters and vibrational frequencies of the SO₂ ligand, the UO₂(CH₃SO₂)­(SO₂)⁻ complex can be considered as a U^VO₂⁺ cation coordinated by SO₂^– and CH₃SO₂^– anions. The UO₂(CH₃SO₂)­(O₂)⁻ complex is computed to have a peroxo ligand, suggesting that U^V in UO₂(CH₃SO₂)­(SO₂)⁻ is oxidized to the U^VI state upon O₂ substitution for SO₂

Crown Ether Complexes of Uranyl, Neptunyl, and Plutonyl: Hydration Differentiates Inclusion versus Outer Coordination

The structures of actinyl–crown ether complexes are key to their extraction behavior in actinide partitioning. Only UO₂(18C6)²⁺ and NpO₂(18C6)⁺ (18C6 = 18-Crown-6) have been structurally characterized. We report a series of complexes of uranyl, neptunyl, and plutonyl with 18-Crown-6, 15-Crown-5 (15C5), and 12-Crown-4 (12C4) produced in the gas phase by electrospray ionization (ESI) of methanol solutions of AnO₂(ClO₄)₂ (An = U, Np, or Pu) and crown ethers. The structures of 1:1 actinyl–crown ether complexes were deduced on the basis of their propensities to hydrate. Hydration of a coordinated metal ion requires that it be adequately exposed to allow further coordination by a water molecule; the result is that hydrates form for outer-coordination isomers but not for inclusion isomers. It is demonstrated that all the actinyl 18C6 complexes exhibit fully coordinated inclusion structures, while partially coordinated outer-coordination structures are formed with 12C4. Both inclusion and outer-coordination isomers were observed for actinyl–15C5 complexes, depending on whether they resulted from ESI or from collision-induced dissociation. Evidence for the formation of 1:2 complexes of actinyls with 15C5 and 12C4, which evidently exhibit bis-outer-coordination structures, is presented

Gas Phase Uranyl Activation: Formation of a Uranium Nitrosyl Complex from Uranyl Azide

Activation of the oxo bond of uranyl, UO₂²⁺, was achieved by collision induced dissociation (CID) of UO₂(N₃)­Cl₂^– in a quadrupole ion trap mass spectrometer. The gas phase complex UO₂(N₃)­Cl₂^– was produced by electrospray ionization of solutions of UO₂Cl₂ and NaN₃. CID of UO₂(N₃)­Cl₂^– resulted in the loss of N₂ to form UO­(NO)­Cl₂^–, in which the “inert” uranyl oxo bond has been activated. Formation of UO₂Cl₂^– via N₃ loss was also observed. Density functional theory computations predict that the UO­(NO)­Cl₂^– complex has nonplanar <i>C_s</i> symmetry and a singlet ground state. Analysis of the bonding of the UO­(NO)­Cl₂^– complex shows that the side-on bonded NO moiety can be considered as NO^3–, suggesting a formal oxidation state of U­(VI). Activation of the uranyl oxo bond in UO₂(N₃)­Cl₂^– to form UO­(NO)­Cl₂^– and N₂ was computed to be endothermic by 169 kJ/mol, which is energetically more favorable than formation of NUOCl₂^– and UO₂Cl₂^–. The observation of UO₂Cl₂^– during CID is most likely due to the absence of an energy barrier for neutral ligand loss

Dissociation of Diglycolamide Complexes of Ln³⁺ (Ln = La–Lu) and An³⁺ (An = Pu, Am, Cm): Redox Chemistry of 4f and 5f Elements in the Gas Phase Parallels Solution Behavior

Tripositive lanthanide and actinide ions, Ln³⁺ (Ln = La–Lu) and An³⁺ (An = Pu, Am, Cm), were transferred from solution to gas by electrospray ionization as Ln­(L)₃³⁺ and An­(L)₃³⁺ complexes, where L = tetramethyl-3-oxa-glutaramide (TMOGA). The fragmentation chemistry of the complexes was examined by collision-induced and electron transfer dissociation (CID and ETD). Protonated TMOGA, HL⁺, and Ln­(L)­(L–H)²⁺ are the major products upon CID of La­(L)₃³⁺, Ce­(L)₃³⁺, and Pr­(L)₃³⁺, while Ln­(L)₂³⁺ is increasingly pronounced beyond Pr. A C–O_ether bond cleavage product appears upon CID of all Ln­(L)₃³⁺; only for Eu­(L)₃³⁺ is the divalent complex, Eu­(L)₂²⁺, dominant. The CID patterns of Pu­(L)₃³⁺, Am­(L)₃³⁺, and Cm­(L)₃³⁺ are similar to those of the Ln­(L)₃³⁺ for the late Ln. A striking exception is the appearance of Pu­(IV) products upon CID of Pu­(L)₃³⁺, in accord with the relatively low Pu­(IV)/Pu­(III) reduction potential in solution. Minor divalent Ln­(L)₂²⁺ and An­(L)₂²⁺ were produced for all Ln and An; with the exception of Eu­(L)₂²⁺ these complexes form adducts with O₂, presumably producing superoxides in which the trivalent oxidation state is recovered. ETD of Ln­(L)₃³⁺ and An­(L)₃³⁺ reveals behavior which parallels that of the Ln³⁺ and An³⁺ ions in solution. A C–O_ether bond cleavage product, in which the trivalent oxidation state is preserved, appeared for all complexes; charge reduction products, Ln­(L)₂²⁺ and Ln­(L)₃²⁺, appear only for Sm, Eu, and Yb, which have stable divalent oxidation states. Both CID and ETD reveal chemistry that reflects the condensed-phase redox behavior of the 4f and 5f elements

Tetrapositive Plutonium, Neptunium, Uranium, and Thorium Coordination Complexes: Chemistry Revealed by Electron Transfer and Collision Induced Dissociation

The Pu⁴⁺, Np⁴⁺, and U⁴⁺ ions, which have large electron affinities of ∼34.6, ∼33.6, and ∼32.6 eV, respectively, were stabilized from solution to the gas phase upon coordination by three neutral tetramethyl-3-oxa-glutaramide ligands (TMOGA). Both collision induced dissociation (CID) and electron transfer dissociation (ETD) of Pu­(TMOGA)₃⁴⁺ reveal the propensity for reduction of Pu­(IV) to Pu­(III), by loss of TMOGA⁺ in CID and by simple electron transfer in ETD. The reduction of Pu­(IV) is in distinct contrast to retention of Th­(IV) in both CID and ETD of Th­(TMOGA)₃⁴⁺, where only the C–O_ether bond cleavage product was observed. U­(TMOGA)₃⁴⁺ behaves similarly to Th­(TMOGA)₃⁴⁺ upon CID and ETD, while the fragmentation patterns of Np­(TMOGA)₃⁴⁺ lie between those of Pu­(TMOGA)₃⁴⁺ and U­(TMOGA)₃⁴⁺. It is notable that the gas-phase fragmentation behaviors of these exceptional tetrapositive complexes parallel fundamental differences in condensed phase chemistry within the actinide series, specifically the tendency for reduction from the IV to III oxidation states

Periodic Trends in Actinyl Thio-Crown Ether Complexes

In-cavity complexes and their bonding features between thio-crown (TC) ethers and f-elements are unexplored so far. In this paper, actinyl­(VI) (An = U, Np, Pu, Am, and Cm) complexes of TC ethers have been characterized using relativistic density functional theory. The TC ether ligands include tetrathio-12-crown-4 (12TC4), pentathio-15-crown-5 (15TC5), and hexathio-18-crown-6 (18TC6). On the basis of the calculations, it is found that the “double-decker” sandwich structure of AnO₂(12TC4)₂²⁺ and “side-on” structure AnO₂(12TC4)²⁺ are changed to “insertion” structures for AnO₂(15TC5)²⁺ and AnO₂(18TC6)²⁺ due to increased size of the TC ether ligands. The actinyl monocyclic TC ether complexes are found to exhibit conventional conformations, with typical An–O_actinyl and An–S_ligand distances and angles. Chemical bonding analyses by Weinhold’s natural population analysis (NPA), natural localized molecular orbital (NLMO), and energy decomposed analysis (EDA), show that a typical ionic An–S_ligand bond with the extent of covalent interaction between the An and S atoms primarily attributable to the degree of radial distribution of the S 3p atomic orbitals. The similarity and difference of the oxo-crown and TC ethers as ligands for actinide coordination chemistry are discussed. As soft S-donor ligands, TC ethers may be candidate ligands for actinide recognition and extraction

Reliable Potential Energy Surfaces for the Reactions of H₂O with ThO₂, PaO₂⁺, UO₂²⁺, and UO₂⁺

The potential energy surfaces for the reactions of H₂O with ThO₂, PaO₂⁺, UO₂²⁺, and UO₂⁺ have been calculated at the coupled cluster CCSD­(T) level extrapolated to the complete basis set limit with additional corrections including scalar relativistic and spin–orbit. The reactions proceed by the formation of an initial Lewis acid–base adduct (H₂O)­AnO₂^0/+/2+ followed by a proton transfer to generate the dihydroxide AnO­(OH)₂^0/+/2+. The results are in excellent agreement with mass spectrometry experiments and prior calculations of hydrolysis reactions of the group 4 transition metal dioxides MO₂. The differences in the energies of the stationary points on the potential energy surface are explained in terms of the charges on the system and the populations on the metal center. The use of an improved starting point for the coupled cluster CCSD­(T) calculations based on density functional theory with the PW91 exchange–correlation functional or Brueckner orbitals is described. The importance of including second-order spin–orbit corrections for closed-shell molecules is also described. These improvements in the calculations are correlated with the 5f populations on the actinide

Experimental and Theoretical Studies on the Fragmentation of Gas-Phase Uranyl–, Neptunyl–, and Plutonyl–Diglycolamide Complexes

Fragmentation of actinyl­(VI) complexes U^VIO₂(L)₂²⁺, Np^VIO₂(L)₂²⁺, and Pu^VIO₂(L)₂²⁺ (L = tetramethyl-3-oxa-glutaramide, TMOGA) produced by electrospray ionization was examined in the gas phase by collision induced dissociation (CID) in a quadrupole ion trap mass spectrometer. Cleavage of the C–O_ether bond was observed for all three complexes, with dominant products being U^VIO₂(L)­(L-86)⁺ with charge reduction, and Np^VIO₂(L)­(L-101)²⁺ and Pu^VIO₂(L)­(L-101)²⁺ with charge conservation. The neptunyl and plutonyl complexes also exhibited substantial L⁺ loss to give pentavalent complexes Np^VO₂(L)⁺ and Pu^VO₂(L)⁺, whereas the uranyl complex did not, consistent with the comparative An 5f-orbital energies and the An^VIO₂²⁺/An^VO₂⁺ (An = U, Np, Pu) reduction potentials. CID of Np^VO₂(L)₂⁺ and Pu^VO₂(L)₂⁺ was dominated by neutral ligand loss to form Np^VO₂(L)⁺ and Pu^VO₂(L)⁺, which hydrated by addition of residual water in the ion trap; U^VO₂(L)₂⁺ was not observed. Theoretical calculations of the structures and bonding of the An^VIO₂(L)₂²⁺ complexes using density functional theory reveal that the metal centers are coordinated by six oxygen atoms from two TMOGA ligands

Heptavalent Actinide Tetroxides NpO₄^– and PuO₄^–: Oxidation of Pu(V) to Pu(VII) by Adding an Electron to PuO₄

The highest known actinide oxidation states are Np­(VII) and Pu­(VII), both of which have been identified in solution and solid compounds. Recently a molecular Np­(VII) complex, NpO₃(NO₃)₂^–, was prepared and characterized in the gas phase. In accord with the lower stability of heptavalent Pu, no Pu­(VII) molecular species has been identified. Reported here are the gas-phase syntheses and characterizations of NpO₄^– and PuO₄^–. Reactivity studies and density functional theory computations indicate the heptavalent metal oxidation state in both. This is the first instance of Pu­(VII) in the absence of stabilizing effects due to condensed phase solvation or crystal fields. The results indicate that addition of an electron to neutral PuO₄, which has a computed electron affinity of 2.56 eV, counterintuitively results in oxidation of Pu­(V) to Pu­(VII), concomitant with superoxide reduction