Geochemical and radiation conditions in coastal landscapes of the Kara Sea Gulf (Novaya Zemlya Archipelago)

This work considers terrestrial coastal landscapes of Abrosimov and Stepovoi gulfs and Yuzhnii (Southern) Island in the Novaya Zemlya Archipelago in the Kara Sea. These areas are dominated by horizons of slightly acidic leptosols and lithic leptosols of 10 cm thick (Stepovoi Gulf) and those of weak skeleton acidic lithic leptosols of 10–15 cm thick (Abrosimov Gulf) covered by moss–shrub assemblages. Kaolinite is formed in a rhizosphere fine earth layer; illite is formed along the leptosol sequence. The studied coastal landscapes are characterized by low accumulation potential of chemical elements, including radionuclides, at higher contents of them. Elements such as Fe and Ti are dispersed in sols, whereas P, S, Cl, Cu, Pb, and Zn are accumulated in soils in minor amounts. Plants accumulate S, P, Cl, Sr, Zn, and ¹³⁷Cs in minor amounts as well. Elements such as Ti, Mn, Fe, Cr, V, Co, Ni, Cu, Rb, Zr, Ba, Th, Y, Nb, Pb, and As are attributed to the group of weak biological adsorption. The specific ¹³⁷Cs activity (Bq kg^–1) amounts to 10–150 in plants, 10–300 in moor leptosol horizons, and 1–40 in mull horizons.     

Interaction of model F-bearing silicic melt with chloride fluid,uraninite, and columbite at 750°C and 1000–2000 bar and its implications for estimation of the ore-forming capability of the upper crustal magma chamber beneath the Strel’tsovka caldera,eastern Transbaikalia

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 10⁻² m 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.     

Uranium Behavior in the Process of Primary Pitchblende Ores Alteration by the Post‐ore Hydrothermal Solutions: An Application to Assessment of Uranium Migration from Underground Spent Nuclear Fuel Repositories

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.     

Safety assessment concept for repositories of spent nuclear materials in crystalline rocks

The methodological issues in safety assessment of repositories of spent nuclear materials (high level wastes and untreatable irradiated fuel) in crystalline rocks are discussed, and a concept of such assessment is proposed. The problem is urgent because of the need for scientific confirmation of the safety assessment procedure for such repositories in Russian territory.     

Model of heat and mass transfer by fluid during formation of Mo-U deposits in the Strel’tsovka ore field,eastern Transbaikal region: Forced convection of solutions generated by a deep source

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.     

The ideas and scientific works of F.I. Wolfson and their current implications for the geology of uranium deposits

The outstanding contribution of Professor F.I. Wolfson to the origination and development of Russian science and practice in the field of geology of uranium deposits is emphasized. Under his guidance, a team of highly skilled specialists on uranium deposits was gathered at the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, in Moscow. Owing to their efforts, all significant uranium deposits in the former Soviet Union and socialist bloc countries have been studied comprehensively. A theory of uranium ore formation was developed on this basis, and elaborated practical recommendations promoted the creation in the Soviet Union of the world’s largest uranium resource sources. Today, Wolfson’s ideas and works are widely used in planning and carrying out complex investigations at operating uranium deposits and in new promising uranium districts.     

Study of the Behavior of Uranium,Niobium, and Tantalum in the Granite Melt–Fluoride Fluid System at 800–950°C, 2300 Bar

Geology of Ore Deposits - Experimental studies were carried out on the solubility of uranium, niobium, and tantalum in acidic melts of Li–F granites and predominantly fluoride fluids at...