Phase formation in cast stone matrices (CSMs) fabricated through the interaction of comelted basalt with ZrO2 at 1623 K for 5 h on air was studied. Basalt melted under the above conditions contains two spinels (relict and newly formed), clinopyroxene of the composition Mg0.66Ca0.60Fe0.26Ti0.05Al0.66Si1.80O6, and glass as the major phases. When basalt is comelted with ZrO2, taken in a weight ratio of 1 : 1, CSMs containing zircon (ZrSiO4), glass, and baddeleyite (ZrO2) as the major phases are formed. Zirconium is concentrated mainly in two phases, zircon and baddeleyite. The rate of Zr leaching from the synthesized CSMs into H2O after 28 days is 1.0 × 10–9 g/(cm2 day).

Using through diffusion method at room temperature, migration of simulants of RW elements (P, Se, Br, Mo, Cs, U) in compacted samples of clay materials of various mineral compositions was studied during porous diffusion from model solutions: underground water and leachates of phosphate waste forms having a total salt content of up to 500 mg/L. Based on the results of experiments, effective diffusion coefficients and sorption distribution coefficients of elements in barrier materials were determined. Numerical models are proposed to describe diffusion transfer of selenium, cesium, and uranium depending on porosity, mineral composition of materials, and concentration of elements in pore solution. Patterns of diffusion of elements from solutions of different salt composition were revealed.

This work examines cast stone matrices (CSM) fabricated by comelting of basalt with both uranium-bearing perlite (M150 grade) and U3O8 at a temperature of 1623 K in air for 5 h. As a result of comelting of basalt with uranium-free M150 grade perlite, matrices are obtained containing glass and spinel as the major phases. When basalt is comelted with uranium-bearing perlite (M150 grade) CSMs are formed, the major phases of which are two Cr-spinels of different compositions, uranium-bearing glass, and dendritic aluminosilicate crystals. In the case of fusion of basalt and U3O8 taken in the initial weight ratio of 1 : 1, CSM is generated as the major phases containing UO3, CaU3O10, (Al,Cr,Fe)2U2O9, plagioclase, and uranium-bearing glass. The leaching rate of uranium from a basalt melt with uranium-bearing perlite (M150 grade, 7.2 wt

The processes of localization of I2, I–, and [UO2(CO3)3]4– from aqueous solutions under static conditions on metal-containing clay powders from kaolin clays of the Kampanovskoye deposit and from bentonite clays of the 10th Khutor and Dinozavrovoe deposits were investigated. The studies were carried out with Cu-, Ni-, Zn-, and Fe-containing clay powders treated with a 2 M hydrazine hydrate solution. It was shown that the [UO2(CO3)3]4– complex is not sorbed on the synthesized clay materials from aqueous solutions under static conditions. It was established that the synthesized clay materials are capable of not only reducing the amount of the molecular form of iodine in an aqueous solution, but also sorbing the ionic form of iodine from an aqueous solution of KI to almost 100

A method of coupled processes was proposed to maintain concentrations in the model leachate of the radioactive waste phosphate matrix, which served as a source of elements in the study of through-diffusion of P, Se, Br, Mo, Cs, and U in the pore solution of compacted clay materials. The method consisted in adding an leachatable solid phase to the solution in the source chamber of the diffusion cell. The use of this method made it possible to stabilize the boundary conditions and expand the range of element concentrations in the source chamber of diffusion cells. The new as-obtained data on the effective diffusion coefficients of radioactive waste elements in clay rocks were used to refine the empirical models of diffusion transfer. It is shown that in different geochemical systems (model groundwater and phosphate glass leachate) for some elements (Br, Mo, Cs) it is possible to use unified models in the form of effective diffusion coefficients as a function of factors influencing this process: sample porosity, smectite content in the sample, and concentration of radionuclide (element) in pore solution, while for Se and U, diffusion models for various geochemical systems differ. The specificity of diffusion behavior of elements is associated with structural features and physicochemical properties of particles of these elements in aqueous solutions.

The extraction of the tricarbonate complex of uranyl [UO2(CO3)3]4– from aqueous solutions on clay powders from kaolin clays of the Kampanovskoye deposit and from bentonite clays of the 10th Khutor and Dinozavrovoe deposits, as well as their mixtures, was investigated. The studies were carried out with clay powders, both untreated and treated with water, 0.5 M Na2CO3 and NaNO3 solutions, and 2 M NaOH solutions. It was shown that the [UO2(CO3)3]4– complex is not sorbed on clay materials from aqueous solutions under static conditions. The filtration of an aqueous solution of [UO2(CO3)3]4– through columns with clay mixtures was established to enable the extraction up to 87

The study focuses on the synthesis and characterization of three boron-containing glasses, namely, two borosilicate glasses and one borophosphate glass containing mockup RW elements (Se, Sr, Mo, Cs, La, U, etc.) and radionuclides (99Tc, 239Pu, 241Am). It explores the glass-forming elements (Li, B, Na, Si, P, etc.), mockup RW and radionuclides and their leaching from the synthesized glasses in model underground water assuming the presence of barrier bentonite at temperatures of 25, 90 and 150°C. The study evaluates the changes in the elemental composition and density of the leached layer formed upon incongruent dissolution of glasses, as well as the time-dependent changes in the element concentrations of the leachates assuming different temperatures. For boron-containing glasses, it presents the evaluated concentrations of leachate saturation with glass-forming and RW elements under deep disposal conditions. These findings can be used in experimental and computational modeling of radionuclide transport under deep RW disposal conditions.

The work studied the distribution of cesium and strontium in cast stone matrices obtained by co-melting basalt and silica gel (hereinafter SiO2) containing cesium or strontium nitrates at 1623 K during 5 h. The melt cooling rate was similar to 2 degrees C/min. Co-melting basalt and SiO2 containing CsNO3 or Sr(NO3)(2) at a basalt/SiO2 initial mass ratio of 5 : 1 produced cast stone matrices, in which the main phases were Cr-Fe spinel and glass. At the same time co-melting basalt and SiO2 containing CsNO3 or Sr(NO3)(2) at an initial mass ratio of 2 : 1 yielded matrices which, apart from Cr-Fe spinel and glass, included pollucite, clinopyroxene, and Ti-containing magnetite phases for Cs-containing system, magnetite and olivine phases for Sr-containing system. For Cs-containing system at a basalt/SiO2 initial mass ratio of 5 : 1 the glass phase showed almost an even distribution of cesium, whereas the spinel phase was virtually free of cesium. At a basalt/SiO2 initial mass ratio of 2 : 1 the major amount of cesium was found in the pollucite phase, where it mainly occurred in parts containing SiO2. Some amount of cesium was found in the glass phase. The other crystal phases found in the matrix were virtually free of cesium. For Sr-containing system at a basalt/SiO2 initial mass ratio of 5 : 1 the process yielded two types of glass: one rich in Ca and the other in Sr. Strontium was mainly found in the glass phases. The spinel phase was virtually free of strontium. At a basalt/SiO2 initial mass ratio of 2 : 1 strontium was mainly found in the glass phases. The magnetite, olivine, and Cr-Fe spinel phases were virtually free of strontium.

Using the through diffusion method at room temperature, the migration of radionuclides (3H, 99Tc, 137Cs, 233U) in compacted samples of clay materials was studied through pore diffusion from a model leachates of phosphate matrix of radioactive waste with a total salt content of about 400 mg/L. The effective diffusion coefficients and sorption distribution coefficients of radionuclides for the studied clay barrier materials were determined based on the experimental results. The patterns of diffusion transfer of tritium, cesium and uranium were revealed depending on the structure, mineral composition of clay materials and the content of radionuclides in the pore solution. Numerical solutions are proposed for calculating the effective pore diffusion coefficients of these radionuclides. There are not yet enough data to analyze the factors influencing the diffusion of technetium.

The article presents the results of experiments on the interaction of model phosphate waste form materials of various phase compositions containing simulators of RW elements with leaching solutions simulating groundwater and changes in its composition due to the impact produced by bentonite barrier material, radiolysis and alkaline plume from Portland cement concrete at temperatures of 25, 90 and 120°С under static water mode considered typical for deep RW disposal facilities. It provides numerical data on the equilibrium saturation concentrations of leaching solutions saturated with barrier and waste form components and simulators of RW elements during phosphate waste form leaching. The paper also discusses the degree of influence produced by the main leaching factors (the phase waste form composition, elevated temperature, the consequences of radiolysis and groundwater interaction with barrier and structural materials) on the concentrations of elements in RW phosphate waste form leachates under the disposal conditions.

The paper presents initial data and a flowchart supporting the conceptual development of an engineered safety barrier system (EBS) intended for a deep disposal facility (DDF) for RW Class 1 (RW-1) to be established at the Yeniseiskiy site within the Nizhnekanskiy rock mass (NKM). It sets forth the key safety conditions that should be provided when a particular EBS material is selected under DDF designs. The study presents estimated sorption retention of non-mobile and migration of mobile long-lived RW-1 radionuclides in the porous solution of clay-based barrier materials. The study showed that under conservative assumptions (with no account taken of radionuclide retardation by bedrocks given their retention provided solely by the EBS) to provide NKM DDF safety, the clay-based barrier thickness should not be less than 1.5—2 m given the barrier material dry density of not less than 1.4 g/cm3 resulted from its saturation. As for the barrier material dry mass, it should not be less than 100–160 tones per single RW-1 package. The degree of conservatism may be decreased along with the growing knowledge on the protective properties of the rock mass and its consideration in further calculations of radionuclide migration.

The distribution of uranium between different phases of cast stone matrices fabricated by fusion of basalt and uranium-bearing SiO2-based collectors was investigated. It was found that matrices created by fusion of basalt with SiO2 consist of glass, quartz, and spinel as the main phases. A study of the physicochemical properties of collectors based on 30 wt % UO2(NO3)2-bearing SiO2 showed that, after thermal treatment at a temperature of 973 K, they contain uranium only in the form of UO3. The uranium leaching in H2O from SiO2 after its thermal treatment, as well as from a basaltic melt with SiO2, was estimated.

A new approach to the study and description of leaching processes was proposed based on the fundamental difference in water exchange regimes during technological tests of confinement matrices of radioactive wastes (RW) according to standard leaching techniques and in the interaction of conditioned solid RW with groundwater under disposal conditions. According to this approach, in the leaching of matrices in a closed system, the main parameters characterizing the result of the process are the concentrations (specific activities) of saturation of the leachate with matrix elements and radionuclides. The proposed approach is illustrated by the example of calculating the leaching parameters of a model phosphate glass with imitators of radioactive waste, carried out using an appropriate kinetic model based on the data of experiments conducted in a static mode (in the absence of water exchange). The found values of the saturation concentrations of leaching solutions with matrix elements can be used to perform experiments and calculations on the migration of radionuclides in the materials of engineering barriers and host rocks.

Safe disposal of nuclear waste is key to sustainable use of nuclear energy. Exploring safe and reliable disposal methods for nuclear waste has become a research hotspot. The phase distribution of uranium in matrices obtained by co-melting basalt and uranium-containing Al(2)O3 was examined for the development of a potential material for radioactive waste immobilization. The co-melting of basalt with UO3- containing Al(2)O3 produced matrices containing glass, plagioclase, and spinel as major phases. The highest uranium content (97 mass % UO3) was observed on the non-dissolved fragment of the Al(2)O3 grains. The titanium-containing complex oxide phase, supposedly, ilmenite, contained up to 62 mass % UO3. The glass phase exhibited an uneven distribution of uranium, with its content ranging similar to 5 to similar to 10 mass % UO3. Data on the leaching of uranium into H 2 O from the co-melt of basalt and UO3-containing Al(2)O3 were obtained. (C) 2021 Elsevier Ltd. All rights reserved.

The article discusses the types of mineral raw materials that can be used to manufacture clay barrier materials. The paper evaluates the characteristics of materials governing the performance of clay barriers: grain size, mineral and chemical composition, physical, mechanical (in dry state) and colloidal properties, stability in the environment. It considers the methods used to identify these characteristics and provides relevant examples.

The paper elaborates on the characteristics of clays and clay materials governing functional properties (performance) of clay barriers. It considers methods applied to identify these characteristics with relevant examples being provided Criteria were proposed to select the required and sufficient numerical values of these characteristics (requirements) considered appropriate for the barrier materials. The paper discusses operating and envisaged test installations designed for mock-up and field tests of clay barrier materials.