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Abstract The possibility of a rigorous theoretical solution of the three-dimensional problem of scattering of elastic waves at the tip of a vertical crack in a weld is analyzed. It is shown that in the general case of ultrasound scattering on a flat target, the three-dimensional problem can be reduced to a two-dimensional one. On this basis, for an arbitrary direction of sounding a crack in a weld, the dependence of the signal from the crack tip on the direction to the receiver was assessed. It is noted that the wave size of the crack tip is much smaller than the wave size of the end of the grooves that can be made to simulate defects in welded joints. As a result, it was found that the nature of the scattering of elastic waves by the tip of the crack differs from the nature of scattering on the grooves, including in cases where the opening of the grooves is minimal, based on the possibility of their manufacture. The results of an experimental verification of the obtained theoretical estimates on samples of steel and aluminum are presented. It is shown that at the ends of the grooves (in contrast to the crack tips), even with their minimal openings, conditions are created for the formation of a scattered field of the type of specular reflection from a volumetric cavity—a side-drilled hole.

The urgent task is improvement of the accuracy of diagnosing the linear part of pipelines, structures and equipment of pumping and compressor stations, tank batteries, tank farm complexes. Therewith, it is required not only to identify a defect with a given probability, but also to ensure measurement of its shape and dimensions with accuracy to be sufficient to perform analysis of structures for strength and durability. The article deals with certain possibilities for increasing the detectability of volumetric defects, such as knots, through improvement of the methods for setting parameters of ultrasound pulse-echo testing. The results of research in the field of improvement of the technology for setting advanced ultrasonic systems using phased array antennas are presented. Cylinder-shaped drilled holes of different orientations have been reviewed as artificial defects for setting sensitivity and time base of the flaw-detecting instrument. Following on the earlier published works, the expediency of using vertical cylindrical drilling for setting the equipment is additionally justified: such target is the easiest to manufacture, its parameters are verified much easier than, for example, the parameters of flatbottomed holes. Examples are given of the practical use of drills of various orientations for adjusting sensitivity of monitoring and determination of the indication length of local bulk defects of the base metal and welds. Актуальной является задача повышения точности диагностирования линейной части трубопроводов, сооружений и оборудования перекачивающих и компрессорных станций, резервуарных парков, нефтебаз. При этом требуется не просто выявить дефект с заданной вероятностью, но и обеспечить измерение его формы и размеров с точностью, достаточной для выполнения расчетов конструкций на прочность и долговечность. В статье рассмотрены некоторые возможности повышения выявляемости объемных дефектов типа свищей за счет совершенствования способов настройки параметров ультразвукового контроля эхо-методом. Представлены результаты исследований в области совершенствования технологии настройки современных ультразвуковых систем, использующих фазированные антенные решетки. В качестве искусственных дефектов для настройки чувствительности и временной развертки дефектоскопа рассмотрены цилиндрические сверления разной ориентации. В развитие ранее опубликованных работ обоснована целесообразность применения для настройки оборудования вертикального цилиндрического сверления: такая мишень наиболее проста в изготовлении, ее параметры поверяются значительно проще, чем, например, параметры плоскодонных отверстий. Приведены примеры практического использования сверлений различной ориентации для настройки чувствительности контроля и определения условной протяженности локальных объемных дефектов основного металла и сварных швов.

Agglomerations of oxide films are some of the typical defects of friction stir welding. The structure of such defects and their small opening do not allow finding them using the conventional ultrasonic nondestructive testing methods (in particular, by the pulse-echo technique), among other things because of their low reflectivity. We have proposed a technique of artificial introduction of oxide films into a welded joint. We have shown that, to reveal such defects, it is promising to use the diffraction methods of ultrasonic nondestructive testing. The diffraction methods make it possible to detect signals from defects with minimum opening as well as from defects with a partially metallic bonding, which are semitransparent to ultrasonic waves and revealing of which is impossible using other methods of ultrasonic testing, in particular, the pulse-echo technique, along with X-ray testing.

The problems of detecting operational plane surface defects by the ultrasonic method of nondestructive testing with the use of Rayleigh surface waves generated by an electromagnetic-acoustic transducer are considered. The paper presents an experimental study of the detection of plane defects simulated by an artificial reflector of the “groove” type with different opening, depth, and angle of slope. The dependences of the signal amplitude on the listed parameters are constructed and their nature is evaluated. The optimal amplitude models for constructing the curves of the probability of detection of defects (PoD) have been determined. A conclusion is made about the minimum dimensions of a plane defect of the operational type detected by the considered method with a probability of 90% taking into account the confidence interval of 95%.

—The paper presents the results of studying the detectability of surface volumetric defects by ultrasonic method of nondestructive testing. The possibility of using Rayleigh surface waves excited by an electromagnetic-acoustic transducer (EMAT) for these purposes is considered. Blind vertical drillings of various diameters and depths were used as artificial defects to simulate defects in low carbon steel samples. Based on the results of measurements, the dependence of the amplitude of the received signals on drilling parameters has been constructed. When statistically processing the results, the signal-to-noise ratio during the excitation of the Rayleigh wave with the help of the EMAT on defect-free areas of samples was taken into account. Optimal models of the distribution of signal amplitudes were determined to construct the curves of probability of detection (PoD) of a defect. Under the conditions of the experiments, based on the PoD curves constructed, conclusions have been drawn about the minimum dimensions of surface volumetric defects identified with 90% probability with allowance for the confidence interval of 90% and about the possibility of adjusting the parameters of ultrasonic testing by signals reflected from vertical drillings.

The current goal in uranium mining is to ensure the maximum effect at minimized costs. One of the problems when using the in-situ leaching (ISL) technology consists in the incomplete mining of the process blocks due to premature uranium content reduction in the pregnant solutions, leading to increased mining times and excessive reagent consumption. The required complete block mining shall be ensured with the highest possible efficiency. The ISL technology is currently used for the mining of all hydrogenous uranium deposits in the Republic of Kazakhstan, which are often characterized by complex geological and hydrogeological conditions. One of these deposits, Semizbay, rates as complexity type III. The conditions at the deposit require increasing the redox potential of the solution by oxidizing Fe2+ into Fe3+. The article considers the addition of hydrogen peroxide to the leaching solutions to improve the redox potential with due account of the mining and geological features of the Semizbay deposit. The pilot tests have established that this oxidizing agent has a positive effect on the uranium content in the pregnant solution with the concentrations of 0.2–0.3 l/m3, provided that the concentration of sulfuric acid in the leaching solutions is within the range of 13–15 g/l. The studies allowed increasing the concentration of uranium in the pregnant solutions from 74 to 114 g/l (i.e., by over 50 %). The authors are grateful to the staff of the geotechnology department of the Semizbay mine, Semizbay-U LLP, for collecting the data and to the staff of the IVT-Zerde LLP laboratory (branch of the Semizbay mine) for the high-quality laboratory experiments during the pilot tests at the mine.

The reliability of ultrasonic testing (UST) of the quality of welded joints of polyethylene pipelines, made end-to-end with а heated tool, is considered in comparison with mесhапiсаl tests and radiography. The greatest detection of solid defects is provided bу u|trasonic inspection with the use of chord transducers (not less than 90%). When detecting defects translucent for ultrasound (lack of penetration, lack of fusion, etc.), the reliability decreases to 70–80%.

Small-opening slots have been used to simulate cracks in butt welds. The influence of the orientation of the slot ribs on their detectability with pulse echo and diffraction methods of ultrasonic testing has been studied. It is shown that the processes occurring during the scattering of elastic waves on cracks should be modeled by a three-dimensional problem. However, theoretical analytical and numerical studies in this area most often concern two-dimensional problems of diffraction of elastic waves, where the target edge under study is located on the acoustic axis of the ultrasound source and/or receiver and is oriented perpendicular to the incident beam. The article presents experimental results illustrating the influence of the orientation of the edge (rib) in three-dimensional problems on the received signals both when using diffraction schemes of the TOFD type, and in classical ultrasonic testing by the pulse echo method according to the “tandem” scheme and with the transducers turned according to the “duet” scheme.

The requirements for the speed, pitch and scanning direction, and also for ensuring acoustic contact maintenance and taking into account the anisotropy of rolled products during mechanized ultrasonic testing of extended welds of steel structures are formulated based on the research conducted and the analysis of the literature data on the conditions of diagnostics for main pipelines facilities. The relevance of the work is determined by a large amount of welded joints inspection, including butt and longitudinal pipe welds, casings of pig-trap stations, filters of different application purposes, and steel tank wall structures. The article formulates the general principles of choosing mechanized scanning parameters and shows the need to consider ultrasound scattering indicatrix on various defects. The issues of setting the monitoring parameters are considered, including the possibility of using vertical drilling (vertical cylindrical reflector) for the adjustment and check of the equipment during mechanized scanning.

The reliability of ultrasonic testing (UT) of the quality of welded joints of polyethylene pipelines, made end-to-end with a heated tool, is considered in comparison with mechanical tests and radiography. The greatest detection of solid defects is provided by ultrasonic inspection with the use of chord tipe probes (not less than 90 %). When detecting defects translucent for ultrasound (lack of penetration, lack of fusion, etc.), the reliability decreases to 70÷80 %. Keywords: welding, polyethylene pipeline, quality control, ultrasonic testing, chord tipe probe. aleshin@bmstu.ru

Issues of quality control of welded joints in hot-tool butt-welded polyethylene pipelines are considered. It is indicated that the main scope of control currently falls on inspections based on the parameters of welding and on selective mechanical testing of welded joints. Physical nondestructive post-welding testing methods are used little, which is generally not typical for the practice of testing welded joints. The reason for this is the general problem of ultrasonic methods, viz., the low detectability of contracted defects, partially transmitting ultrasound. To ensure the detectability of such defects, we evaluate the possibility of increasing the sensitivity of ultrasonic testing by using the ultrasonic time-of-flight-diffraction (TOFD) technique. It is proposed to use the tip of vertical drillings to simulate defects weakly scattering ultrasound. It is shown that when tuning is performed based on such targets, the inspection sensitivity can be increased by at least 6 dB, i.e., two times or more. In this case, the drilling tip can simulate the upper and lower edges of a planar defect vertically oriented in the section of the weld. The results of experimental studies aimed at revealing multidirectional drills with a diameter of 1 mm in welded seams of polyethylene pipes, including pipes with an additional removable outer layer, are presented. The expediency of gathering statistical data on the identification of real defects in welded butt joints of polyethylene pipes using the considered testing method is shown.

The use of chromium carbides to harden surfaced metal that is subject to abrasive wear is considered. With correct hardening of the applied layer, carbide formation extends the working life.

The issues of identifying discontinuities that partially transmit ultrasonic waves are examined. Such defects are widespread in various products and welded joints of metals and plastics. Examples of “semitransparent” defects include lacks of fusion, “dead spots”, slag and other foreign inclusions in welded joints, flokens and oxide films in forgings and rolled products, etc. Moreover, even such well-studied defects as cracks can also partially transmit ultrasonic waves. However, to calculate and adjust the parameters of ultrasonic testing, hollow model reflectors (various drills, grooves, etc.), which do not transmit ultrasound, are traditionally used. The present article states that for the purposes of defect sizing, it is necessary to correctly calibrate and configure the flaw-detector–transducer kit taking into account the peculiarities of detecting defects of various types, including ones that are “semitransparent” for ultrasonic waves. To develop models of artificial reflectors imitating defects semitransparent for ultrasound, it is proposed to classify such semitransparent defects into three groups, viz., defects with filling, intermittent structures, and clogged weld. The main features of defects of these types are considered. Possible approaches to calculating the acoustic paths of ultrasonic flaw detectors when identifying defects of the three indicated types are described. It is noted that analytical and numerical methods can be used for calculating defects with filling; numerical methods should be preferred for intermittent structures; and analytical methods are better suited to defects of the clogging type.

A model is developed to describe the refraction of an elastic transverse wave by an immersed concave cylindrical surface. The wave may be incident on the liquid surface at any angle. Immersional monitoring of a small-diameter pipeline is considered.

Due to their high antifriction characteristics, the Sn-Sb-Cu alloys (referred to as babbits) are widely used to coat sliding bearings. However, some limitations of the present techniques for applyi...

The influence of nanoparticles introduced into a molten weld pool using a flux-cored wire on the structure and impact toughness of the weld metal is considered. The use of a flux-cored wire with tungsten carbide particles is shown to decrease the scatter of the impact toughnesses of the weld metal. The use of titanium carbide allows one to increase the average impact toughness.

The influence of tungsten carbide and titanium nitride nanoparticles on the structure and properties of the weld metal of welded joints made by automatic twin submerged arc welding is considered. The nanoparticles have been introduced into the weld pool as a part of the “master alloy” based on nickel powder (PNE-1 according to GOST 9722). It has been shown that modifying the weld metal with tungsten carbide nanoparticles holds promise for enhancing the impact strength. In addition, it has been found that titanium nitride is prone to dissociation under the same conditions. However, microalloying with titanium, which is due to the release of titanium from the nitride, leads to an increase in the impact strength of the weld metal.

Results of the tomographic inspection of a prototype sample additive manufactured by selective laser alloying have been evaluated. Specific features of revealing “incomplete fusion” flaws and the influence of various parameters of inspection on its results are considered.

The possibilities and limitations of applying head and surface waves to ultrasonic inspection of articles manufactured by selective laser alloying are considered. The optimum angles have been determined for the generation of head and surface waves in a product made of the domestic EP648 powder. The peculiarities of revealing crack-like flaws by the ultrasonic echo method are demonstrated.

A wide range of the temperatures of hydrothermal fluid regime (430-103 °C) has been revealed in the result of microthermometric study of the fluid inclusions in the minerals of pre-ore and post-ore stages developed at the Elkon deposit. Average temperatures, as well as the temperature variation are similar for both pre-ore and post-ore stages. It allows sure determination of temperatures of uranium ore formation as 400-120 °C. Fluids possessed Na-chloride, rarely Na-chloride-carbonate composition and moderate salinity (6-16 wt. % · NaCl equiv.). The wide range of tem-peratures confirms an assumption made before that various temperatures were the reason of formation of uranium mineralization both in the form of predominantly amorphous phase (U-Ti-metagel), as well as the rarely crystalline form (brannerite). Abrupt temperature decrease was apparently the main factor of the ore precipitation.

This article discusses the possibility of introducing nanosized particles into a molten pool using a flux-cored wire. A flux-cored wire has been proposed with added composite granules obtained upon the combined treatment of nickel powder and nanosized particles. In order to avoid the overheating of particles, it has been proposed to add a flux-cored wire to a molten-pool tail using preheating. The following hardmelting particles have been tested: tungsten carbide (WC), titanium nitride (TiN), and aluminum oxide (Al2O3). It has been demonstrated that the addition of tungsten carbide to a molten pool improves the stability of weld-metal (WM) impact toughness while retaining its average value and the addition of aluminum oxide to a molten pool leads to increased average values of WM impact toughness while retaining its stability. Meanwhile, nanosized particles of titanium nitride in a molten pool are prone to dissociation, which leads to the formation of porosity, thus limiting the field of application of this modifier.

Results of ultrasonic testing of parts that are fabricated by selective laser melting are assessed using radiography and X-ray computed tomography. Peculiarities of revealing incompletefusion flaws with the above methods are considered.

Possibilities and peculiarities of ultrasonic testing of parts manufactured by selective laser melting are considered. Special features of revealing solidity violation flaws with ultrasonic echo-pulse method are demonstrated.

The possibility of using hydrothermal fluorite as an Sm–Nd geochronometer is based on the results of an REE pattern study of this mineral (Chernyshev et al., 1986). As a result of REE fractionation, in many cases, the Sm/Nd ratio achieves a multifold increase compared with its level in terrestrial rocks, and the radiogenic shift of the 143Nd/144Nd isotope ratio reaches 10–20 eNd units over a short time interval (as soon as tens of Ma). This is a necessary prerequisite for Sm–Nd isochron dating of fluorite. Zonal polychrome fluorite from a vein referred to the final stage of large-scale uranium mineralization at the Sterl’tsovka deposit in the ore field of the same name located in the eastern Transbaikal region has been dated using the 143Nd/144Nd method. To optimize isochron construction, local probes with high and contrasting Sm/Nd ratios have been sampled from the polished surfaces of two samples, taking into account the REE pattern of zonal fluorite. Sm–Nd isochron dating has been carried out separately for each sample. The 147Sm/144Nd и 143Nd/144Nd ratios vary within the intervals 0.5359–2.037 and 0.512799–0.514105, respectively. Two isochrons, each based on six fluorite probes, have been obtained with the following parameters, which coincide within 2σ uncertainty limits: (1) t = 134.8 ± 1.3 Ma, (143Nd/144Nd)0 = 0.512310 ± 13, MWSD = 0.43 and (2) t = 135.8 ± 1.6 Ma, (143Nd/144Nd)0 = 0.512318 ± 10, MWSD = 1.5. The mean age of fluorite based on two isochron datings is 135.3 ± 1 Ma. Comparison of this value with the most precise dating of pitchblende related to the ore stage in the Strel’tsovka ore field (135.5 ± 1 Ma) shows that four mineralization stages, distinguished by geological and mineralogical data, that were completed with the formation of polychrome fluorite veins 135.3 ± 1 Ma ago, represent a single and indivisible hydrothermal process whose duration does not exceed 1 Ma.

Possibilities, application domains, advantages, and limitations are considered for currently used physical methods of nondestructive testing of additive manufactured parts. The methods are classified according to the physical principle of operation.

Telescopes with aperture higher than 3 m and with travel rate of optical axis of more than 4 angular degrees per second are examined. These telescopes are used for estimating the situation in near-Earth space. Up to now, there are three telescopes with such parameters: SOR, AMOS, SST (all in the United States). A telescope of the same class with aperture of 3.12 m (information telescope TI-3.12) is under completion at Altai Optical Laser Center. The main technical performances of the TI-3.12 telescope are described in the paper. Current problems are defined and ways for their solution are presented.

Peculiarities of additive technologies are considered, and the most typical flaws are listed. The possibility of layer-by-layer nondestructive testing of parts immediately in the course of additive manufacturing is demonstrated. Based on the review of domestic and international publications, possibilities that are offered by various physical methods of nondestructive evaluation are analyzed and potential of their usage is reviewed.

Large-scale reserves of uranium in Au-U deposits of the Elkon ore district are composed of newly discovered polyphase uranium-titanium metagel (UTM). Before, it was assumed that brannerite is the major mineral of these ores. However, it has been revealed that brannerite and uranium oxides and silicates are minor components of mineralization. Succession of UTM phases selection, their relations with brannerite are characterized. Few groups of UTM has been revealed by composition of major and trace elements. The filter cake of acidic leaching of a technological sample with residual uranium concentration of 0,02 wt.% and grain size of -0.16 mm was investigated. As it was established, the major part of uranium is in the fine grained UTM phases submerged in quartz-feldspar matrix of the cake particles. Taking into account a small size of the uranium phases (first microns, rarely n-10 mkm), recrusing of the ore to smaller grades will unlikely lead to a noticeable uranium yield into final product.

Modification of Babbitt coatings by carbon nanotubes in plasma–powder application is considered. A model is proposed for the interaction of a graphene-like surface and atoms from the Babbitt alloy. The influence of carbon nanotubes obtained by different means on the performance of antifrictional coatings is studied.

Flaws in metal parts that are typical for the selective laser melting technique are considered. The flaws are divided into four groups by the degree of their danger. The quantitative and qualitative indices of the flaws are given and the causes of their formation are described.

Compositions and manufacturing technology are developed for manufacturing a crucible lining for master alloy preparation by an aluminothermic method. Slag is the main material used for lining manufacture, formed during reduction of aluminum metal oxides containing 80 – 84% α-Al2O3. Charge compositions are developed within which soda and high-alumina cement are tested as binder. The technology is introduced into OAO Uralredmet.

Micro to nano-scale structures of pockets of solidified uranium oxide gel were found for the first time and deciphered ontogenetically during electron-microscopic mineralogical study of four samples of high-grade uranium ores from the Dalmatovskoe deposit. Mineralogical-geochemical features of paragenesis and structural features are reported for unique hydrothermal veinlets consisting of variably crystallized gelpyrite and subveinlets of gel-pitchblende of several generations. Mineralogical features indicate repeated opening of veinlets during hydrothermal ore formation and provide insight into physical and chemical features of non-solidified gel, its mobility, and aggressive behavior during metasomatism. It was proposed that the veinlets are genetically related to the main hydrothermal sources and possibly to the endogenous uranium occurrence, which was formed owing to opening of main fault marked by paleoriver bed. The find of such veinlets is considered as evidence for repeated influx of hydrothermal uranium in the uranium ores of the sedimentary-epigenetic deposit simultaneously with accumulation and transformation of sediments, which made it possible to ascribe this deposit to the hydrothermal-sedimentary type. The depth of core sampling of high-grade ores corresponds to the “ore zone” of the Dalmatovskoe Deposit, near the Paleozoic basement. For this reason, the entire “zone” was ascribed to the specific endogenous metasomatic epihydrothermal occurrences, possibly, of elision nature, with approximate temperature of ore formation of 150–250°C.

A device and a principle of operation of some ultrasound scanners-flaw detectors for diagnosing the basic metal mass of main oil and gas pipelines in reinsulation is considered. Different methods, materials, and devices for efficient excitation and reception of ultrasound waves in pipelines with wall thicknesses of 6–20 mm and the selection of Lamb and Rayleigh wave modes for testing as a function of the pipe-wall thickness are discussed. The principle of operation of a unidirectional primary EMA transducer is described.

Aluminothermic smelting of Al-Zr and Al-Zr-Mo-Sn alloys has been simulated in terms of thermodynamics. The thermodynamic properties of molybdenum and zirconium intermetallic compounds are calculated. It is demonstrated that, with consideration for their formation, the calculated compositions of aluminothermic blends and the forecasted extraction of target metals into an alloy are in good agreement with the data obtained during pilot tests.

The microstructure and the element and phase compositions of the zone of deposited metal, of the fusion zone, and of the heat-affected zone in welds of Fe3Al intermetallic with austenitic steels are studied by methods of light and scanning microscopy, microscopic x-ray spectrum analysis, and x-ray diffraction analysis. Argon arc welding is used to obtain beads of corrosion-resistant austenitic steels 08Kh18N10, 10Kh17N13M2T, 20Kh23N13, and 20Kh23N18 on the surface of a Fe3Al intermetallic.

The differential positron annihilation spectroscopy (DPAS) is used for the nondestructive investigation of thin metallic coatings on metallic substrates when the defectiveness of the coating applied is judged from the difference between the spectra of the individual substrate and the substrate covered with the coating. Using the DPAS angular correlation curves of positron annihilation radiation (ACAR), the presence of nanoscale void clusters in the X-ray amorphous galvanic chromium coatings with a weight thickness of 9.15 mg/cm2 (1.3 μm) is determined on the copper substrate. The ACAR curves of the coatings have a complex structure compared to those of the specimens of Cr3C2 and Cr23C6 carbide powders and involve a narrow peak with a half width FWHF = 3 mrad and an intensity about 2% assigned to void clusters with a size of 0.86 nm, which act as positron traps, in the coating.

The Strel’tsovka and Antei uranium deposits located in the Strel’tsovka caldera are unique in ore resources. According to the considered mathematical model, the uranium source of these deposits was related to the middle-lower crustal silicic magma chambers or had mantle origin. Boundary conditions of the model are based on modern views of physicochemical conditions of hydrothermal process in the Strel’tsovka ore field and factors governing ore deposition therein. Modeling results are consistent with morphology of orebodies and ultimate uranium resources of the deposits and thus confirm indirectly that the physicochemical parameters of the ore-forming system are coherent. The maximal duration of uranium ore deposition is estimated at 500 ka.

It has been shown that the main uranium ore mineral, pitchblende (uranium dioxide), is a natural analog of synthetic uraninite (also uranium dioxide), which constitutes 96% of spent nuclear fuel (SNF). Geochronological studies of the U-Pb isotope systems in unaltered pitchblende from the orebodies reveal that these systems remained completely closed over the entire period (approximately 135 Ma) since the formation of the deposits. The bulk of the primary uranium ores within the Streltsovskoye ore field was influenced to various degrees by post-ore hydrothermal solutions that led to pitchblende spherulites being replaced by pseudomorphs of an amorphous phase with a U-Si composition; this phase also re-precipitated in veinlets proximal to the pitchblende pseudomorphs. A technique specially developed by the authors was used to carry out quantitative counts of the abundance of uranium minerals by calculating the uranium mass balance in one of the orebodies subjected to hydrothermal alteration. The calculations reveal minimal uranium loss from the orebody. Uranium liberated in the process of the pseudomorphic replacement of pitchblende was immediately fixed, in situ, in the newly formed coffinite-like amorphous U-Si phase as a result of the development of an efficient geochemical barrier that prevented the long-distance migration of uranium. In assessing the long-term safety of underground SNF repositories, the results of the present study give us confidence that SNF uraninite, in terms of the preservation of its integrity as a mineral phase, provides for the reliable long-term isolation of uranium, transuranium elements, and fission products that are “sealed” in the uraninite matrix. In the case of the mineral transformation of the uraninite matrix by hydrothermal solutions, the liberated uranium would be efficiently immobilized by the newly formed amorphous U-Si phase.

The experimental study of an F-bearing silicic melt—U, Nb, Ta minerals—chloride-fluoride fluid system is focused on ascertaining the origin of uranium deposits spatially related to intraplate silicic volcanism. The first series of experiments on uranium solubility in silicic melts close in composition to ore-bearing rhyolite of the unique Strel’tsovka Mo-U ore field has been performed in order to determine more precisely the ore genesis. As starting solid phases, model homogeneous glass of the chemical composition (wt %) 72.18 SiO2, 12.19 Al2O3, 1.02 FeO, 0.20 MgO, 0.33 CaO, 4.78 Na2O, 3.82 K2O, 1.44 Li2O, and 2.4 F (LiF, NaF, KF, CaF2, MgF2); synthetic UO2 and UO3·0.33H2O; and natural columbite were used. The starting solutions contained 1.0 m Cl and 10−2m F. The runs were conducted in a gas vessel at a pressure of 1000 bar and in a high-pressure hydrothermal vessel at 2000 bar. The O2 (H2) fugacity was set by Ni-NiO, Co-CoO, Fe3O4-Fe2O3, and Cu-Cu2O buffers. The equilibrium between melt and solution for major elements is reached during the first day, whereas 5–7 days are required for ore elements (U, Nb, Ta) to come into equilibrium. The solubility of Nb and especially Ta in Cl-F solutions equilibrated with F-bearing melt is extremely low. The solubility of U is much higher (10−4−10−5 mol/kg H2O). The energy dispersive spectroscopy of run products allowed us to establish that columbite dissolved incongruently with formation of U- and F-bearing pyrochlores. The performed experiments have shown that a silicic melt close to the rhyolitic magma of the Strel’tsovka caldera in composition is not able to generate postmagmatic ore-forming solutions containing more than 10−6−10−5 mol U/kg H2O under the relatively low pressure necessary for the existence of the first type of fluid. The amount of uranium that could have precipitated from this fluid in the zone of ore deposition is estimated at 216–9000 t. This estimate is two orders of magnitude lower than the total uranium resources of the deposits localized in the Strel’tsovka caldera. Thus, the upper crustal silicic magma chamber hardly was a source of uranium for Mo-U deposits of the Strel’tsovka ore field.

This paper presents an analysis of the radioactivity distribution in the Bazhenovo Formation rocks on the western slope of the Surgut Dome. The maximum uranium concentrations are confined to accumulations of organic matter and are proportional to its concentration in the rocks. The lowermost part of the section has lower radioactivity values than the upper one. The value of radioactivity varies throughout the area of the Surgut Dome. Coeval analogs of the Bazhenovo Formation show lower radioactivity values.

The hydrothermal Mo-U deposits of the Strel’tsovka ore field, unique in reserves, are localized in the Late Mesozoic caldera of the same name. The consideration of geochemical processes that controlled uranium transfer by ore-bearing fluids and its precipitation in orebodies has shown that a nonstationary temperature distribution could have exerted a substantial effect on ore formation. The temperature field in the Strel’tsovka caldera, which was caused by a shallow-seated magma chamber that existed beneath the caldera by the onset of the ore stage, was simulated by mathematical modeling. A one-dimensional nonstationary model of conductive heat transfer taking into account the latent heat of magmatic melt crystallization was used. The problem was solved with the finite difference method. It has been established that, at optimal parameters of the model, the magma chamber would have completely crystallized in 56 ka; the maximum estimate is 133 ka. Three million years after emplacement of the granitic intrusion, the related thermal anomaly in the upper crust should have disappeared. The results obtained indicate that granitic melt of this chamber could not have been a source of uranium-bearing solutions that formed deposits 5 Ma after the cessation of magmatic activity.