Sergei N. Vereshchagin

A novel layered zirconium molybdate as a precursor to a ceramic zirconomolybdate host for lanthanide bearing radioactive waste

Novel layered zirconium molybdates (Mo/Zr = 2) in Na+- and NH4+-exchanged forms with a defective crystalline structure and a specific surface area of up to 100 m2 g−1 were synthesized under mild hydrothermal treatment conditions (150 °C, 3 days) without structure-directing reagents. The NH4+- zirconomolybdate material was tested for its ability to immobilise aqueous radioactive waste containing rare earth elements by a sorption/impregnation–crystallization process using simulant Nd3+/Ln3+ bearing solutions. The motivation for tailoring (Ln,Zr,Mo)-ceramics was the utilization of sediment-forming components of spent nuclear fuel processing solutions, such as Mo and Zr, for immobilization of transuranium radionuclides or an actinide–lanthanide (An–Ln) fraction of high-level effluents. The target (Ln,Zr,Mo)-phase acceptable for incorporation of actinides and lanthanides, Ln2Zr3(MoO4)9 (Ln = La–Tb), was selected on the basis of studying the phase formation in triple oxide systems Ln2O3–ZrO2–MoO3 (Ln = La–Lu, Y, Sc) and analysis of the known promising host phases being developed in the world for the actinide immobilization. X-ray diffraction and thermal analysis methods were used in the study of the thermochemical conversion of the Nd3+/Ln3+-zirconomolybdates resulting from loading the layered NH4+-zirconomolybdate precursor with different quantities of Nd3+/Ln3+. The results on Nd3+ immobilization by the sorption–crystallization route reflected the influence of the acidity of the simulant solutions on the content of the target phase in the solidified (Nd,Zr,Mo)-ceramics. The impregnation/sorption–crystallization procedure provided poly-phase composites including Nd2Zr3(MoO4)9, ZrxNdy(MoO4)2 and ZrO2, the quantity of the target phase depending on the Nd3+ loading. The solidification of the Ln bearing simulant solution at 16% Men+ loading resulted in nearly mono-phase ceramics of the Nd2Zr3(MoO4)9 structural type.

Polyfunctional microspherical materials for long-term disposal of liquid radioactive wastes

The possibility of immobilizing liquid radioactive wastes into polyfunctional microspherical materials of the block and powdered types is demonstrated. These materials are intended for reprocessing radioactive wastes of different compositions and make it possible to perform a multistage process of conditioning radioactive wastes under relatively mild conditions (at temperatures below 1000°C) with the conversion of water-soluble cesium and strontium compounds into water-insoluble mineral forms in the course of solid-phase transformations. Owing to the aluminosilicate composition of microspherical components of energy ashes (cenospheres), the cenospheres can serve as precursors of aluminosilicate granitoid minerals. Different techniques are proposed and conditions are experimentally determined for transforming precursors into final mineral-like materials of the predicted structure types chosen in the framework of the geoecological approach. The framework structures of aluminosilicates and phosphates thus formed can fix cesium and strontium in the crystal lattice and ensure the geochemical equilibrium between the matrix and incorporating granitoid rocks under conditions of long-term disposal.

Porous Materials Based on Cenospheres of Coal Fly Ash for Fixation of Cs-137 and Sr-90 in Mineral-like Aluminosilicates

Among the crystalline matrices being developed for immobilization of 137 Cs and 90 Sr are framework aluminosilicates like feldspars and feldspathoids. A novel approach to obtain mineral-like aluminosilicate forms of 137 Cs and 90 Sr with using porous materials based on hollow aluminosilicate microspheres of coal fly ash (cenospheres) has been demonstrated. Two modifications of microspherical porous materials have been developed, (i) moulded porous matrices based on consolidated cenospheres and (ii) zeolite sorbents obtained by hydrothermal zeolitization of cenospheres. The first includes impregnation of Cs- or Sr-containing solutions into porous matrices, drying and calcining at temperatures (700-900°C) lower than the softening point of the porous matrix material, at which solid phase crystallization takes place with formation of framework aluminosilicates, such as Cs-nepheline, pollucite or Sr-anorthite. Using zeolite sorbents, immobilization of Cs + and Sr 2+ proceeds through the step of trapping ions from the solution followed by drying and thermochemical solid-phase transformation of saturated zeolites into pollucite or Sr-anorthite over the same temperature range. Cenosphere-derived porous matrices and zeolite sorbents were tested in fixation of 137 Cs using spiked simulants of actual radioactive waste. The durability of crystalline aluminosilicate 137 Cs -forms obtained was shown to satisfy Russian (GOST P 50926-96) and international standards.

Nature of the active sites of ferrospheres in the oxidative condensation of methane

The catalytic properties of ferrospheres containing 76–97 wt % Fe2O3 in the oxidative condensation of methane were compared with their phase composition and the distribution of iron cations over the crystallographic positions of iron-containing phases in a steady state. It was established that the reaction route of methane oxidation changed at a Fe2O3 content of 89 wt %. Deep oxidation was the main reaction route on ferrospheres with a Fe2O3 content of <88.8 wt %. At a Fe2O3 content of ≥89 wt %, the yield of C2 hydrocarbons sharply increased and the contribution of deep oxidation decreased. The yield of C2 hydrocarbons correlated with the amount of defects in the structure of iron spinel, which are iron ions with the tetrahedral cation of Ca2+ and the octahedral cation vacancy among the nearest neighbors.

Solid-Phase Transformation of Cs + - and Sr 2+ -Bearing Zeolite Sorbents Derived From Cenospheres to Mineral-Like Forms

The paper describes the studies of the transformation of Cs + - and Sr 2+ -containing zeolite sorbents synthesized from fly ash cenospheres to crystalline mineral composition, suitable for the long-term disposal. Series of Cs + - and Sr 2+ -exchanged NaP1-containing sorbents were subjected to the thermochemical transformation in the temperature range 40-1100°C at atmospheric pressure in air and the progress of reaction was monitored by DSC and XRD analysis. It was shown that initial sodium zeolite undergoes two-step transformation at 736-785°C and 892-982°C forming nepheline as the principle product, with the conversion temperatures being dependant on the heating rate. The thermal treatment of Cs + -bearing zeolite sorbent led to formation of a complex multiphase system, the principal components of which were nepheline and pollucite. Increasing cesium content in the samples led to a monotonous shift of crystallization peak to the higher temperature range (1005-1006°C). A more complicated behavior was observed for Sr 2+- containing samples, for which the crystallization temperature tends to increase (compared with NaP1) at lower Sr contents, but it starts decreasing parallel to the Sr 2+ content at Sr 2+ loadings >10 mg/g. The principal crystalline phases in Sr-NaP1 sample conversion were nepheline and Sr 2+ -containing feldspar, the quantity of which increased parallel to the increase of strontium content in zeolite. Apparent activation energies of thermochemical transformations were calculated and possible approaches to reduce transformation temperature are discussed and experimentally illustrated.

Study of Mobile Oxygen in Ordered/Disordered Nonstoichiometric Sr-Gd-Cobaltate by Simultaneous Thermal Analysis

Simultaneous thermal analysis (DSC/TG/mass-spectroscopy) under variable pressure was applied to study properties of mobile oxygen in cubic disordered and tetragonal A-site ordered Sr-Gd-cobaltate (Sr/Gd=4). It was shown that A-site cation ordering results in a decrease of an enthalpy of oxygen elimination, predominant localization of created anion vacancies only at one of four unequivalent crystallographic O-sites and depletion of amount of mobile oxygen, which can be removed at a reduced pressure. The observed properties of mobile oxygen and catalytic activity in reaction of methane deep oxidation for ordered/disordered perovskite phases are discussed in connection with a local environment of O2crystallographic sites with a different number of Gd3+ cations nearby.

DSC+TG and XRD Study of Order-Disorder Transition in Nonstoichiometric Sr-Gd-Cobaltate

Process of interconversion of tetragonal Sr(0.8)Gd(0.2)CoO(3-d) (with ordered Sr/Gd cations and anion vacancies) to cubic one (with disordered structure) was studied by X-ray structural and thermal analysis at 1100-1473 K. It was shown that the transformation is a first order diffuse phase transition. The ramp rate does not affect cation and anion vacancies disordering which is controlled by thermodynamic parameters of the processes in the solid whereas cubic to tetragonal transition is kinetically controlled. A theory of diffuse phase transition was applied to quantitatively analyze heat capacity temperature dependence.