Peter C. Burns

Incorporation mechanisms of actinide elements into the structures of U6+ phases formed during the oxidation of spent nuclear fuel

Uranyl oxide hydrate and uranyl silicate phases will form due to the corrosion and alteration of spent nuclear fuel under oxidizing conditions in silica-bearing solution. The actinide elements in the spent fuel may be incorporated into the structures of these secondary U 6÷ phases during the long-term corrosion of the UO 2 in spent fuel. The incorporation of actinide elements into the crystal structures of the alteration products may decrease actinide mobility. The crystal chemistry of the various oxidation states of the actinide elements of environmental concern is examined to identify possible incorporation mechanisms. The substitutions pu6+o U 6÷ and (Pu 5÷, Np 5+) o U 6÷ should readily occur in many U 6÷ structures, although structural modification may be required to satisfy local bond-valence requirements. Crystal-chemical characteristics of the U 6÷ phases indicate that An 4+ (An: actinide) ~ U 6÷ substitution is likely to occur in the sheets of uranyl polyhedra thai: occur in the structures of the minerals schoepite, [(UO2)802(OH)12](H20)12, ianthinite, [U 4+ (UO 2)406(OH)4(H 20)4](H 20)5, becquerelite, Ca[(UO 2)302(OH)312(H 20)8, compreignacite, K2[(UO2)302(OH)312(]~20)8, ct-uranophane, Ca[(UO2XSiO3OH)]2(H20) 5, and boltwoodite, K(H30)[(uo2Xsio4)], all of which are likely to form due to the oxidation and alteration of the UO 2 in spent fuel. The incorporation of An 3+ into the sheets of the structures of ct-uranophane and boltwoodite, as well as interlayer sites of various uranyl phases, may occur.

The crystal structure of ianthinite, [U24+(UO2)4O6(OH)4(H2O)4](H2O)5: a possible phase for Pu4+ incorporation during the oxidation of spent nuclear fuel

Abstract Ianthinite, [U 2 4+ (UO 2 ) 4 O 6 (OH) 4 (H 2 O) 4 ](H 2 O) 5 , is the only known uranyl oxide hydrate mineral that contains U 4+ , and it has been proposed that ianthinite may be an important Pu 4+ -bearing phase during the oxidative dissolution of spent nuclear fuel. The crystal structure of ianthinite, orthorhombic, a = 0.7178(2), b = 1.1473(2), c = 3.039(1) nm, V = 2.5027 nm 3 Z = 4, space group P 2 1 cn , has been solved by direct methods and refined by least-squares methods to an R index of 9.7% and a wR index of 12.6% using 888 unique observed [| F | ≥ 5 σ | F |] reflections. The structure contains both U 4+ . The U 6+ cations are present as roughly linear (U 6+ O 2 ) 2+ uranyl ion (Ur) that are in turn coordinated by five O 2− and OH − located at the equatorial positions of pentagonal bipyramids. The U 4+ cations are coordinated by O 2− , OH − and H 2 O in a distorted octahedral arrangement. The Ur φ 5 and U 4+ | 6 (φ: O 2− , OH − , H 2 O) polyhedra l sharing edges to for two symmetrically distinct sheets at z ≈ 0.0 and z ≈ 0.25 that are parallel to (001). The sheets have the β-U 3 O 8 sheet anion-topology. There are five symmetrically distinct H 2 O groips located at z ≈ 0.125 between the sheets of U φ n polyhedra, and the sheets of U φ n polyhedra are linked together only by hydrogen bonding to the intersheet H 2 O groups. The crystal-chemical requirements of U 4+ and Pu 4+ are very similar, suggesting that extensive Pu 4+ ↔ U 4+ substitution may occur within the sheets of U φ n polyhedra in trh structure of ianthinine.

A new uranyl oxide hydrate sheet in vandendriesscheite: Implications for mineral paragenesis and the corrosion of spent nuclear fuel

Abstract The structure of vandendriesscheite, Z = 8, Pb1.57[(UO2)10O6(OH)11](H2O)11, orthorhombic, a = 14.1165(6), b = 41.378(2), c = 14.5347(6) Å , V = 8490 Å3, space group Pbca, has been solved by direct methods and refined by full-matrix least-squares techniques to an agreement factor (R) of 12.1% and a goodness-of-fit (S) of 1.28 using 4918 unique observed reflections (|F₀| ≥ 4σF) collected with MoKα X-radiation and a CCD (chargecoupled device) detector. The structure contains ten unique U6+ positions, each of which is part of a nearly linear (UO2)2+ uranyl ion that is further coordinated by five equatorial (O2-,OH-) anions to form pentagonal bipyramidal polyhedra. There are two unique Pb positions; one is fully occupied, but site-scattering refinement gives an occupancy factor of 0.573(8) for the other. The Pb positions are coordinated by O atoms of the uranyl ions and by H2O groups. There are 11 unique H2O groups; five are bonded to Pb and the other six are held in the structure by hydrogen bonds only. The U polyhedra link by the sharing of equatorial edges to form sheets parallel to (001). The sheet of U polyhedra is not known from another structure and is the most complex yet observed in a uranyl oxide hydrate. The sheets are structurally intermediate to those in schoepite and becquerelite and are linked by bonds to the interlayer Pb cations and the H2O groups. The extensive network of hydrogen bonds that link adjacent sheets is derived on the basis of crystal-chemical constraints. The high mobility of U6+ in oxidizing fluids, as opposed to Pb21, causes the alteration products of Precambrian uraninite deposits to become progressively enriched in Pb relative to U. In the case of lead uranyl oxide hydrate minerals, there is a continuous sequence of crystal structures that involves a systematic modification of the sheets of U polyhedra and that corresponds to increasing sheet charge and increasing Pb content. Thus, a clear relationship exists between the crystal structures of lead uranyl oxide hydrates and their paragenesis, and this is relevant to the disposal of spent nuclear fuel.

A structural phase-transition in K(Mg1−xCux)F3 perovskite

A complete solid-solution series between cubic (Pm 3 m) KMgF3 and tetragonal (I4/mcm) KCuF3 was synthesized at 730–735 °C in an inert atmosphere. X-ray powder-diffraction at room temperature shows that the transition between the cubic and tetragonal perovskite structures in the series K (Mg1−xCux) F3 occurs at x ∼ 0.6. Rietveld structure-refinements were done for selected compositions. In the cubic phase, all parameters are linear with composition up to the transition point. At the transition point, there is a strong discontinuity in the cell volume; this is strongly anisotropic with expansion along the a axes and contraction along the c axis due to a pronounced axial elongation of the (Mg, Cu) F6 octahedron that increases with increasing Cu content. The phase transition is first-order, with a discontinuity of ≈2% in the symmetry-breaking strain at xC. It is proposed that the phase transition in K (Mg, Cu) F3 is due to the onset of the cooperative Jahn-Teller effect.Compositional relationships for lattice vibrations in this solid solution were established using thin-film infrared spectroscopy. A phase transition occurring above 60 mole % KCuF3 is indicated by the appearance of one of the two modes expected for the tetragonal phase; the weaker mode is not resolved below 80 mole % KCuF3. Modes common to both structures vary smoothly and continuously across the binary; however, frequencies do not depend linearly on composition, nor is mode-softening discernable. Two-mode behaviour is observed only for the bending motion of the cubic phase, because this peak alone has non-overlapping end-member components.

The crystal structure of sinkankasite, a complex heteropolyhedral sheet mineral

The systematics of the columbite group have been studied to quantify variations in composition and structure. Multiple regression methods involving 89 heated samples and five synthetic equivalents of columbite-group minerals give equations that permit prediction of the effects of composition on unit-cell parameters for fully ordered samples. The results are: aD = 14.258 + 0.166Mn/(Mn + Fe) + O.0072Ta/(Ta + Nb) 0.06Ti 0.02Sn + 0.05Sc; ho = 5.7296 + 0.03 1Mn/(Mn + Fe) + 0.0024Ta/(Ta + Nb) 0.024Ti 0.009Sn + 0.02Sc; Co= 5.0495 + 0.033Mn/(Mn + Fe) + O.OllTa/(Ta + Nb) 0.004Ti, where aD, ho, and Coare the cell parameters (A) calculated from unit-cell concentrations of elements. With these equations, crystal-chemical trends, the effects of heating experiments, the degree of cation order, and the structural effects of heterovalent cation substitution can be predicted for samples of columbite-group minerals.

Refinement of the structure of hilgardite‐1A

The fundamental building block of the title compound, dicalcium nonaoxopentaborate chloride hydrate, Ca 2 [B 5 O 9 ]Cl.H 2 O, is the [B 5 O 12 ] 9- polyanion; translationally equivalent polyanions link to form chains parallel to the c axis. These chains are cross-linked, forming a zeolite-type borate framework. There are two Ca sites; Ca(1) has hexagonal bipyramidal coordination and Ca(2) has pentagonal bipyramidal coordination

Rietveld refinement of the crystal structure of α -CoSO 4

The crystal structure of α -CoSO 4 has been refined by the Rietveld method from X-ray powder diffraction data. The structure is orthorhombic, space group Pnma, a = 8.6127(4), b = 6.7058(3), c = 4.7399(2) A, V = 273.75(3) A 3 . Final R B = 2.41%, R P = 5.24%, R WP =6.66%, R WP (expected) =5.74% (WP =weighted profile). The structure consists of edge-sharing octahedral chains parallel to [010] interconnected by SO 4 tetrahedra.

The Crystal Structure of Ianthinite, a Mixed-Valence Uranium Oxide Hydrate

Ianthinite, [U 4+ 2 (UO 2 ) 4 O 6 (OH) 4 (H 2 O) 4 ](H 2 O) 5 , is the only known uranyl oxide hydrate mineral that contains U 4+ , and it has been proposed that ianthinite may be an important Pu 4+ -bearing phase during the oxidative dissolution of spent nuclear fuel. The crystal structure of ianthinite, orthorhombic, a 7.178(2), b 11.473(2), c. 30.39(1) A, V 2502.7 A 3 , Z = 4, space group P2 1 cn , has been solved by direct methods and refined by least-squares methods to an R index of 9.7 % and a wR index of 12.6 % using 888 unique observed [ | F | ≥ 5σ | F | ] reflections. The structure contains both U 6+ and U 4+ . The U 6+ cations are present as roughly linear (U 6+ O 2 ) 2+ uranyl ions (Ur) that are in turn coordinated by five O 2- and OH located at the equatorial positions of pentagonal bipyramids. The U 4+ cations are coordinated by O 2- , OH and H 2 O in a distorted octahedral arrangement. The Ur φ 5 and U 4+ φ 6 (φ: O 2- , OH, H 2 O) polyhedra link by sharing edges to form two symmetrically distinct sheets at z z ≈ 0.0 and z ≈ 0.25 that are parallel to (001). The sheets have the β-U 3 O 8 sheet anion-topology. There are five symmetrically distinct H 2 O groups located at z ≈ 0.125 between the sheets of Uφ n polyhedra, and the sheets of Uφ n polyhedra are linked together only by hydrogen bonding to the intersheet H 2 O groups. The crystal-chemical requirements of U 4+ and Pu 4+ are very similar, indicating that extensive Pu 4+ ↔ U 4+ substitution can occur within the sheets of Uφ n polyhedra in the structure of ianthinite.