B. S. Nikonov

Structure of Borosilicate Glassy Materials with High Concentrations of Sodium, Iron, and Aluminum Oxides

Alkali borosilicate glassy materials, which contain high iron and aluminum oxide concentrations and simulate vitrified high-level wastes of the Savannah River Site (United States), are investigated using X-ray powder diffraction, optical and electron microscopies, and infrared spectroscopy. The materials prepared by induction melting in cold crucibles operating in pilot and industrial facilities at the State Unitary Enterprise “Moscow Research and Production Association Radon” consist of a glass matrix with distributed individual or aggregated crystals of spinel similar in composition to trevorite. The maximum content of the crystalline phase in the glassy material from a “dead volume” of the cold crucible with an industrial size reaches ∼13 vol %. The texture of the glass phase is complex and determined by the direction of flows in cold crucibles under the action of eddy currents, the character of outflow of the glass melt stream during pouring into canisters, and the interaction of the stream with the glass solidified in the canister after preceding pourings. The structure of the anionic motif of the glass phase is predominantly built up of metasilicate chains and boron-oxygen fragments with threefold-coordinated boron.

Murataite-based ceramics for actinide waste immobilization

Studying the Synroc, doped with a simulated HLW, the authors have found, along with conventional Synroc phases (zirconolite, perovskite, hollandite), an extra phase with a stoichiometry (Ca,Mn,U,Tr){sub 4}(U,TR,Zr,Ti){sub 2}(Al,Ti){sub 7}O{sub 22}. XRD and TEM study has shown this phase is related to a very rare mineral murataite. In the present work a ceramic based on murataite is studied. The ceramic samples in the system: Ca-Mn-Ti-Zr-U-Ce-Al-Fe-O were produced and examined in details using XRD, SEM/EDS, TEM, and optical microscopy. Total amount of actinide (U) and rare earth (Ce, Gd) elements in the murataite exceeds 20 wt.%. Isomorphic substitution schemes in the structures of synthetic and natural murataites are discussed. High isomorphic capacity of the murataite structure towards actinides and REEs, flexibility of its composition, feasibility of synthesis by melting, including a cold crucible melting, and very high chemical durability under hydrothermal conditions make the murataite-based ceramics very promising for actinides and excess weapon Pu fixation.

Characterization of glassy materials for immobilization of radioactive waste with a high iron oxide content

The influence of the content of oxides in the simulated high-level wastes on the phase composition, the structure, and the water resistance of borosilicate-based glassy materials for immobilization is investigated. An increase in the waste oxide content from 45 to 65 wt % leads to an increase in the fraction of the crystalline phase of the magnetite-type spinel from 3–5 to 20–22 vol %. The glassy materials are characterized by a low leaching rate of the main waste components in water. A considerable increase in the leaching rate of sodium ions and, to a lesser extent, aluminum and uranium ions is observed for the glassy materials containing waste oxides at a content of 55 wt % and more due to the depolymerization of the structural glass network. Under the same conditions, the leaching rate of iron does not increase noticeably because of the high resistance of the iron-containing spinel to water.

Murataite matrices for actinide wastes

Matrices for actinide wastes, consisting of complex murataite-type oxides, were studied. The samples were prepared by sintering of the oxide charge at 1100–1400°C or by fusion at 1450–1600°C followed by crystallization. Along with the structural analog of the natural mineral (murataite 3C), appreciable role in the samples is played by the phases (hereinafter murataites 5C, 7C, and 8C) consisting of pyrochlore and murataite 3C units. The fraction of wastes in the samples is about 10 wt %, which is close to the values for the pyrochlore matrix for Pu immobilization. In the ceramics prepared from the melt, the murataite grains have a zonal structure. They are built of murataite 3C at edges and murataite 5C (rarely 7C) in the center where the actinide content is maximal. This fact accounts for their high capability to isolate radionuclides. Amorphization of the structure only slightly affects the stability of murataite in solution. The optimal procedure for the industrial production of the matrices is their melting by induction heating in cold crucible (IMCC) and crystallization.

Influence of the content of a surrogate of iron aluminate high-level wastes on the phase composition and structure of glassy materials for their immobilization

Alkali borosilicate glassy materials with high iron and aluminum oxide concentrations, which simulate vitrified high-level wastes from the Savannah River Site (United States) at their content ranging from 50 to 70 wt %, have been investigated using X-ray powder diffraction, optical and electron microscopy, and infrared spectroscopy. Quenched and slowly cooled samples containing 50 wt % wastes are glasses. Samples containing 60 and 70 wt % wastes, which were quenched on a metal slab, are predominantly glasses with an insignificant content of the spinel formed in a trevorite-magnetite solid solution. The slowly cooled samples also contain nepheline, and its amount increases with an increase in the waste content in the glassy materials.

Characterization of the glass-ceramic material prepared upon vitrification of an iron-containing surrogate of high-level wastes in a cold crucible

Vitreous materials are prepared by cold crucible induction melting of a surrogate of high-level wastes (from the Savannah River Site, United States) and a borosilicate glass frit taken in mass ratios from 45: 55 to 60: 40. According to the X-ray diffraction and electron microscopic data, the vitreous materials thus produced consist of a glass matrix and a magnetite-type spinel enriched in transition elements. The degree of crystallinity of the materials increases with an increase in the waste oxide content from 6 to 18–20 vol %. The vitreous materials are characterized by a high chemical durability, which decreases only at high contents of the waste oxides (55 wt % and higher) due to the formation of an additional nepheline phase.

Phase Composition and Leach Resistance of Actinide-Bearing Murataite Ceramics

Phase composition of the murataite-based ceramics containing 10 wt.% ThO2, UO2, NpO2 or PuO2 and leaching of actinides using a MCC-1 procedure were studied. The ceramics were prepared by melting of oxide mixtures in Pt ampoules in air at ~1500 C. They are composed of predominant murataite-type phases and contain traces of extra phases (rutile, crichtonite, perovskite). At least two murataite-related phases with fiveand eight-fold elementary fluorite unit cell (5C and 8C) were simultaneously observed. Minor phase 3C (murataite) in the ceramics doped with ThO2 and UO2 was also found. In the Th-bearing sample the 5C phase prevails over the 8C phase. In the U-bearing ceramic they co-exist in comparable amounts. The sample produced at 1500 C contains crichtonite whereas the ceramic produced at lower temperature (1400 C) contained rutile. Higher temperature favors further rutile reactions with formation of crichtonite. The Npand Pu-doped ceramics are also composed of major the 5C and the 8C phases and minor rutile and crichtonite (in Pu-loaded sample only). Unlike the sample prepared under slightly reducing conditions (in glassy carbon crucible) the Pu-doped ceramic produced under neutral conditions (in Pt ampoule) doesn’t contain perovskite-type phase. Occurrence of perovskite in the first sample was supposed to be due to reduction of some Pu(IV) to Pu(III) during experiment. Leach rates (7-day MCC-1 test, 90 C) of the actinide elements from all the ceramics studied are at the level of 10-10 g/(m⋅day).

Effect of Synthesis Conditions on Phase Composition of Pyrochlore-Brannerite Ceramics

Three melted samples of pyrochlore-brannerite-based ceramics produced under different redox conditions were examined. Two of the samples were produced using cold crucible melting at ~1600 C. The third sample was obtained via melting in a microwave oven at 1700-1800 C. All the samples are composed of major pyrochlore and brannerite phases, and minor rutile and UO 2 -based solid solution or pseudobrookite phases. Pyrochlore-structured phases predominate in all three samples and account for 50-60% of the total bulk. Two pyrochlore varieties – Ca-pyrochlore (predominant) and Ba-pyrochlore have been found in these samples. The latter phase is more stable at high temperatures than the Ba-hollandite present in sintered pyrochlore-rich Synroc-F ceramics. Decomposition of the Ba-hollandite results in rutile formation in the melted samples.

Isomorphic Capacity of Synthetic Sphene With Respect to Gd and U

Phase relations in the system: CaO-TiO2-SiO2-(Na2O,Al2O3,Gd2O3,UO2) were studied. This system is of interest due to the formation of sphene, perovskite, and other phases potentially suitable for immobilization of high level waste (HLW) elements. Along with sphene, other phases found in the samples were rutile, chevkinite, anorthite, crystobalite, and pyrochlore-structured phases. Sphene is able to incorporate up to 21.5 wt.% Gd2O3 and 9.3 wt.% UO2 or, in formula units: 0.25 Gd and 0.07 U.

Phase composition, structure, and hydrolytic durability of phosphate glass materials for immobilizing liquid highly level waste rich in-iron-group elements

X-ray diffraction and electron microscopy were used to examine phase composition and the distribution of elements in glass materials which simulated the vitrified high-level wastes. The variations in the compositions of forming glass materials due to the high content of iron in the waste (as well as the low contents of nickel and chrome) from sodium-alumina-phosphate to sodium-iron-phosphate caused variations in phase compositions and distributions of elements between the phases. Sodium-alumina-phosphate glass characterized by an increased tendency for crystallization which declined on the substitution of about half of all Al2O3 for Fe2O3 and NiO, and increased again with a subsequent increase in the content of oxides of transition elements. Chrome oxide(III) served as a crystallization catalyst. Substitution of 6 wt % P2O5 for B2O3 increased somewhat the crystallization durability of the glass containing a high amount of Al2O3, but produced no effect in the case of glass with a high content of iron and nickel oxides. The dependence of the hydrolytic durability of glass materials on composition was complicated, but, in general, the durability declined on transition from alumina-phosphate to iron-phosphate glass.