E. A. Luk’yanova

Sorption of Radionuclides by Microorganisms from a Deep Repository of Liquid Low-Level Waste

Sorption of radionuclides on a biomass of aerobic microorganisms isolated from deep repositories of liquid low-level waste was examined. Each strain exhibited substantially different sorption of different radionuclides. In neutral medium, the maximal proportion of the recovered radionuclides was 77, 92, 76, 72, and 33% for Pu, Np, U, Am, and Sr, respectively. None of the isolated strains sorbed Cs. The highest sorption ability was exhibited by Pseudomonas genus bacteria. The results obtained are indicative of different mechanisms of interactions of microorganisms with radionuclides.

Distribution and activity of microorganisms in the deep repository for liquid radioactive waste at the Siberian Chemical Combine

The physicochemical conditions, composition of microbial communities, and the rates of anaerobic processes in the deep sand horizons used as a repository for liquid radioactive wastes (LRW) at the Siberian Chemical Combine (Seversk, Tomsk oblast), were studied. Formation waters from the observation wells drilled into the horizons used for the radioactive waste disposal were found to be inhabited by microorganisms of different physiological groups, including aerobic organotrophs, anaerobic fermentative, denitrifying, sulfate-reducing, and methanogenic bacteria. The density of microbial population, as determined by cultural methods, was low and usually did not exceed 104 cells/ml. Enrichment cultures of microorganisms producing gases (hydrogen, methane, carbon dioxide, and hydrogen sulfide) and capable of participation in the precipitation of metal sulfides were obtained from the waters of the disposal site. The contemporary processes of sulfate reduction and methanogenesis were assayed; the rates of these terminal processes of organic matter destruction were found to be low. The denitrifying bacteria from the deep repository were capable of reducing the nitrates contained in the wastes, provided sources of energy and biogenic elements were available. Biosorption of radionuclides by the biomass of aerobic bacteria isolated from groundwater was demonstrated. The results obtained give us insight into the functional structure of the microbial community inhabiting the waters of repository horizons. This study indicates that the numbers and activity of microbial cells are low both inside and outside the zone of radioactive waste dispersion, in spite of the long period of waste discharge.

Phase equilibria in solid Mg-Rich Mg-Sm-Tb alloys

The structure of Mg-rich Mg-Sm-Tb alloys with samarium and terbium contents up to 30 at % each is studied by metallography, electron microprobe analysis, and electrical resistivity measurements. The magnesium-based solid solution is found to be in equilibrium only with the Mg41Sm5 and Mg24Tb5 phases belonging to the Mg-Sm and Mg-Tb binary systems composing the ternary system, respectively. The individual and total samarium and terbium solubilities in a magnesium-based solid solution at 500 and 300°C are determined. The terbium and samarium solubilities in the Mg41Sm5 and Mg24Tb5 phases, respectively, are also determined. Partial isothermal sections at 500 and 300°C of the Mg-Sm-Tb phase diagram in the Mg- rich region are constructed.

Study of the decomposition of the magnesium-based solid solutions in Mg-Sm-Tb alloys

Kinetics and structural transformations upon the decomposition of a magnesium-based supersaturated solid solution in Mg-Sm-Tb alloys have been studied at various relations between the concentrations of terbium and samarium. It has been established that with increasing terbium content in the alloys the strengthening upon the decomposition of the supersaturated magnesium-based solid solution increases. The decomposition of the supersaturated magnesium-based solid solution in the Mg-Sm-Tb alloys with a percentage ratio (wt %) Tb: Sm of about 2.5 exhibits signs characteristic of the decomposition of the supersaturated solid solution in binary Mg-Tb alloys. With allowance for the equilibrium Mg-Sm-Tb phase diagram, this gives grounds to suppose that samarium is mainly dissolved in the products of the decomposition of the magnesium solid solution that are characteristic of Mg-Tb alloys.

Behavior of the high-strength magnesium alloy IMV7-1 of a Mg-Y-Gd-Zr system in annealing

The changes in the structure and properties of the high-strength magnesium alloy IMV7-1 of a Mg-Y-Gd-Zr system are studied after heating at various temperatures. The alloy is hot-worked before annealing to obtain wrought semifinished products. The high stability of a hot-worked structure in annealing is established. It indicates the possibility of using the alloy as a light structural material at elevated (up to 300°C) temperatures. As the annealing temperature increases to 400°C and the holding time is one or two hours, there is only a slight decrease in the hardness, characterizing the strength properties of the alloy. With a further increase in the annealing temperature, a sharp fall in the hardness is observed. Investigation of the structure shows recrystallization in the alloy after annealing at temperatures up to 400°C. After annealing at higher temperatures of 450 and 500°C, the abrupt fall in the hardness is accompanied by complete disappearance of the elongated deformed grains which exist in the hot-worked state and by a significant grain growth. After recrystallization, the alloy retains the ability to be significantly strengthened upon aging, resulting in decomposition of the magnesium-based supersaturated solid solution.

Liquidus surface of the Mg-Sm-Tb phase diagram

Differential thermal, electron microprobe, and X-ray diffraction analyses and metallography are used to study Mg-Sm-Tb alloys containing up to 30% Sm or Tb. Polythermal sections and the solidification surface of the Mg-Sm-Tb phase diagram are constructed for the Mg-rich region. In the composition range under study, nonvariant transition-type equilibrium L + Mg24Tb5 = (Mg) + Mg41Sm5 is found to exist at a temperature of 539°C.

Texture, Structure, Mechanical Properties, and Deformability of MA2-1pch Magnesium Alloy Rolled Sheets Preliminarily Subjected to Direct Extrusion or Equal-Channel Angular Pressing and Annealing

The textural and structural evolution in MA2-1pch magnesium alloy sheets fabricated from initial workpieces after (1) hot direct extrusion or (2) equal-channel angular pressing performed by route Bc in four passes at 245°C and subsequent recrystallization annealing is investigated during warm rolling and subsequent uniaxial tension. The same sharp basal texture, but different structures with different average grain sizes and fractions of twinned grains form in sheets (made of two different workpieces) after warm rolling. Subsequent uniaxial tension arranges basal planes in the sheets made from workpiece 2 along prismatic directions. Modeling of texture within the framework of a thermoactivation model shows that the texture changes due to the activation of prismatic slip. Structural evolution during uniaxial tension in the sheets made of workpiece 2 is accompanied by more intense twinning than that in the sheets made of workpiece 1. These textural and structural changes are responsible for the enhanced mechanical properties and the deformability parameters of the sheets fabricated from workpiece 2.

Effect of Samarium on the Properties of Mg–Y–Gd–Zr Alloys

The microstructure, the aging kinetics, and the strength properties of Mg–Y–Gd–Zr cast alloys, in particular, a samarium-alloyed IMV7-1 alloy, at room and high (250, 300°C) temperatures after homogenization without and with subsequent aging are studied. Alloying with samarium accelerates the decomposition of the supersaturated magnesium solid solution and enhances the properties of the Mg–Y–Gd–Zr alloys.

A Study of the Structure, Mechanical Properties and Corrosion Resistance of Magnesium Alloy WE43 After Rotary Swaging

The structure and properties of magnesium alloy WE43 are studied after homogenizing and rotary swaging deformation conducted in several stages with step lowering of the temperature from 400 to 325°C and increasing the extrusion ratio to 2.56 – 2.78. The deformation yields an ultrafine-grained (UFG) structure with a mean size of the structural components about 500 – 800 nm and particles of aMg41Nd5 phase with a mean size of about 300 – 400 nm. The formation of the UFG structure raises the strength of the alloy. The best combination of mechanical properties is provided by swaging with the final temperature 325°C. It is shown that the rotary swaging deformation does not affect the corrosion rate of the alloy measured by the methods of potentiodynamic polarization, loss in the mass and emission of hydrogen.

Effect of dysprosium on the kinetics and structural transformations during the decomposition of the supersaturated solid solution in magnesium–samarium alloys

The effect of dysprosium added in the amounts such that it does not form an individual phase in equilibrium with solid magnesium on the decomposition of the supersaturated magnesium solid solution in Mg–Sm alloys is studied. The presence of dysprosium in Mg–Sm alloys is found to retard the decomposition of the supersaturated magnesium solid solution and to increase the hardening effect upon aging. When these alloys are aged, dysprosium is partly retained in the magnesium solid solution and partly enters into the compositions of the phases that form during the decomposition of the solid solution and are characteristic of Mg–Sm alloys.