Mark L. Miller

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.

Recoil refinements: Implications for the 40Ar/39Ar dating technique

Integration of the neutron energy distribution for a water-moderated reactor with the most recent cross-section data yields mean recoil energies of 177 keV for 39K (n, p) 39Ar, 969 keV for 40Ca (n, α) 37Ar, and 140 eV for 37Cl (n, γ) 38Cl (β) 38Ar. These estimates are insensitive to the anisotropy of reaction products. Utilizing Monte Carlo simulations of collision cascades, we calculate a mean recoil range of 1620 A for 39K (n, p) 39Ar, 3780 A for 40Ca (n, α) 37Ar, and 11 A for 37Cl (n, γ) 38Cl (β) 38Ar. Rutherford backscatter (RBS) measurements of argon implantation experiments into albite confirm the veracity of these estimates. Integration of the recoil range distributions yields a mean depletion depth in a semi-infinite medium of 820 A for 39Ar, 1950 A for 37Ar, and 6 A for 38Ar. The concentration gradients generated by recoil-redistribution between thin slabs were then incorporated into standard diffusion equations. If the exsolution lamellae are the effective diffusion dimension for argon, then the calculations indicate that the argon release rates and 40Ar/39Ar age spectrum derived from incremental heating of minerals exsolved at the micron to submicron scale are significantly affected by recoil-redistribution. The age spectra will be discordant even if the feldspar has not experienced a complex or slow cooling history. Incremental step apparent ages will increase with the fraction of 39Ar released as the potassium poor lamellae degas. The age spectra will exhibit decreasing apparent ages with increasing fraction 39Ar released as the potassium feldstar lamellae degas. The overall profile of the age spectrum will depend upon the composition of the feldspar and the size distribution of the lamellae, if the lamellae are the effective argon diffusion dimension. In principal, these calculations can be used to discriminate between different models for argon diffusion in minerals. Finally, the 11 A mean recoil distance calculated for 38Ar indicates that it is not a proxy for anion-sited excess argon. Instead, published correlations of 38Ar with excess 40Ar probably reflect the degassing of fine-grained, Cl-rich inclusions.

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.

Image simulation of partially amorphous materials

Abstract The extent to which the aperiodic character of HRTEM images corresponds to amorphous domains in partially amorphous materials has not been determined. Neither the spatial nor the quantitative correspondence between the HRTEM image and the actual structure has been quantified. This complicates the complete interpretation of HRTEM images wherever amorphous domains are present. In this study, periodic continuation has been employed to model large-scale defects (i.e. amorphous domains) within crystalline zircon (ZrSiO 4 ) in order to determine the spatial and quantitative accuracy of the phase-contrast imaging process when applied to partially amorphous materials. HRTEM image simulations of partially amorphous zircon show that increasing proportions of laterally homogeneous layers of amorphous material contribute aperiodic image intensity non-linearly, such that the apparent accumulation of amorphous material is compressed relative to the actual accumulation. Up to thirty percent of a sample can be amorphous before observable disruption to the periodicity occurs in the HRTEM image, and complete loss of the periodic character occurs when the crystalline volume fraction drops below twenty percent. Within this range of amorphous volume fractions, images exhibit a partially periodic character; however, the exact extent of the structural transition which gives rise to partially periodic images depends on sample thickness. Increasing crystal thickness decreases the image sensitivity to amorphous volumes. Laterally discontinuous regions of amorphous material contribute an aperiodic image character which corresponds spatially to the amorphous volume only when the crystalline/amorphous (C/A) interface is vertical (i.e. parallel to the electron beam). Non-vertical C/A interfaces give rise to gradational changes from periodic to aperiodic image character which are consistent with the changing columnar amorphous volume fraction across the interface. This may give rise to significant errors in the volumetric estimate of amorphous material derived from analysis of HRTEM images of laterally inhomogeneous, partially amorphous samples. Proper analysis of the image character in areas exhibiting mixed periodic/ aperiodic character, however, does allow an accurate determination of the amorphous volume fraction for materials which are 30 to 80 volume percent amorphous.

Weathering of Natural Uranyl Oxide Hydrates: Schoepite Polytypes and Dehydration Effects

Partial dehydration of schoepite, U 0 3 · 2H 20, is reported to produce three discrete schoepite polytypes with characteristic unit cell parameters, but this has not been confirmed. The loss of structural water from the schoepite interlayer results in progressive modification to the structure; expansion parallel to schoepite cleavage planes, and extensive fracturing. Dehydration of schoepite commences at grain boundaries and progresses inward until the entire grain is converted to dehydrated schoepite, U 0 3 · 0.8H20. The volume decrease associated with dehydration results in expanded grain boundaries. These gaps can provide pathways for the access of groundwater, and uranyl silicates and uranyl carbonates have precipitated within these gaps, replacing both schoepite and dehydrated schoepite. Schoepite, however, is not observed to re-precipitate where in contact with dehydrated schoepite. Thus, while the formation of schoepite early during the corrosion of uraninite may be favored, schoepite is not a long-term solubility limiting phase for oxidized uranium in natural ground waters containing dissolved silica or carbonate.

HRTEM study of displacement cascade damage in krypton-ion-irradiated silicate — olivine

Abstract With a 1.5 MeV Kr + -ion irradiation at room temperature, olivine [(Mg 0.88 Fe 0.12 ) 2 SiO 4 ] was completely amorphized after a dose of 5.3×10 14 ions/cm 2 , as observed by in situ TEM. After a dose as low as 2.55×10 12 ions/cm 2 , HRTEM revealed damaged domains (approximately 5 nm in dimensions) in which lattice fringes were fuzzy or completely lost at the thin regions of the sample. Multislice image calculations for supercells containing various amounts of amorphous volume and Monte Carlo simulations for displacement cascades indicate that the observed damage domains in the HRTEM image contain 30%–100% amorphous volume through the thickness and are caused by overlapping of the displacement cascades in projection.

Temperature dependent microstructural modification in ion-irradiated Tl-type high temperature superconductors

Abstract Ion irradiation damage creation and recovery were examined in Tl-based high temperature superconductors, HTSC, using TEM, resistivity, and magnetic measurements for irradiation temperatures of 20 to 650 K. During 1.5 MeV Kr + and Xe + ion irradiations of single-crystal Tl-1212 and Tl-1212 TlBaCaCuO HTSC, microstructural modification was observed in situ by electron diffraction and shows a remarkable temperature dependence. At selected sample temperatures, irradiations continued until a critical fluence, D c , was reached where the original structure disappeared. The temperature dependence of D c shows a minimum near the superconducting transition temperature, T c , and is correlated with the temperature dependence of the thermal conductivity, which has a maximum near T c . At an irradiation temperature near this maximum in thermal conductivity, a minimum amount of damage recovery occurs because heat can be dissipated away from the displacement cascade. Ion irradiation suppresses the T c . The rate of decrease in the T c as a function of damage (measured in displacements per atom, dpa) was found to be the same for various incident ions (He + , O 2+ , Au 5+ which shows that the damage accumulation is a result of atomic collisions. Further, the rate of decrease in T c was found to be the same for both transport and magnetization measurements, indicating that the displacements effect the bulk of the samples through point defect creation. An activation energy of 0.4 eV for ion irradiation damage recovery over the temperature range from 100 to 650 K was determined from normal state resistance versus time immediately after irradiation.

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.

Vertically-Stacked InAs Islands Between GaAs Barriers Grown by Chemical Beam Epitaxy

We report vertically-aligned InAs islands separated by GaAs barriers thin enough for electronic coupling. Thinner barriers reduced the InAs critical-thickness for island formation. Transmission electron microscopy revealed well-aligned islands with all detected islands in complete stacks. Atomic force microscopy showed the top islands of uncapped stacks are fully formed. The photoluminescence peak was sharper and shifted to lower energy compared to a single-layer growth. We attribute this shift to island-to-island electronic coupling and to the smaller compressive strain in the center of the composite structure.

Transuranium Element Incorporation into the β-U3O8 Uranyl Sheet

Spent nuclear fuel (SNF) is unstable under oxidizing conditions. Although recent studies have determined the paragenetic sequence for uranium phases that result from the corrosion of SNF, there are only limited data on the potential of alteration phases for the incorporation of transuranium elements. The crystal chemical characteristics of transuranic elements (TUE) are to a certain extent similar to uranium; thus TUE incorporation into the sheets of uranyl oxide hydrate structures can be assessed by examination of the structural details of the β-U 3 O 8 sheet type. The sheets of uranyl polyhedra observed in the crystal structure of β-U 3 O 8 also occur in the mineral billietite (Ba[(UO 2 ) 3 O 2 (OH) 3 ]2(H 2 O) 4 ), where they alternate with α-U 3 O 8 type sheets. Preliminary crystal structure determinations for the minerals ianthinite, ([U 2 4+ (HO 2 ) 4 O 6 (HO) 4 (H 2 O) 4 ](H 2 O) 5 ), and “wyartite II” (mineral name not approved by IMA committee on mineral names), {CaCo 3 }[U 4+ (UO 2 ) 2 O 3 (OH) 2 ](H 2 O) 4 , indicate that these phases also contain β-U 3 O 8 type sheets. The β-U 3 O 8 sheet anion topology contains triangular, rhombic, and pentagonal sites in the proportions 2: 1:2. In all structures containing β-U 3 O 8 type sheets, the triangular sites are vacant. The pentagonal sites are filled with U 6+ O 2 forming pentagonal bipyramids. The rhombic dipyramids filling the rhombic sites contain U 6+ O 2 in billietite, U 4+ O 2 in β-U 3 O 8 U 4+ (H 2 O) 2 in ianthinite, and U 4+ O 3 in “wyartite-II” (in which one apical anion is replaced by two O atoms forming a shared edge with a carbonate triangle of the interlayer). Interlayer species include: H 2 O (billietite, “wyartite II”, and ianthinite), Ba 2+ (billietite) Ca 2+ (”wyartite II”), and CO 3 −2 (”wyartite II”); there is no interlayer in β-U 3 O 8 . The similarity of known TUE coordination polyhedra with those of U suggests that the β-U 3 O 8 sheet will accommodate TUE substitution coupled with variations in apical anion configuration and interlayer population providing the required charge balance.