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Deep geological repositories are the sole option for safeguarding the atmosphere from hazardous radiation from high-level radioactive waste. Hyperalkaline pore fluids are produced when fluids from host rocks penetrate through the concrete/cement layers of such repositories, the layers created as bulkheads in vaults or to facilitate gallery access between buffers and the host rock. Such fluids also disperse through the compacted bentonite layers placed around the steel canisters that contain the waste in the repositories. The corrosion of steel canisters inside such repositories can be observed over a certain time. Chemical corrosion can lead to harmful leachates that can seep through the compacted bentonite layers that surround the steel canisters, altering the bentonite properties. In addition, these compacted bentonite layers are subjected to thermal histories due to the canisters emitting continuous heat (150°C to 200°C). These compacted bentonite layers have to hold up under these three conditions in repositories, all of which can affect the swelling properties of the bentonite. Two bentonite samples (B1 and B2) from Rajasthan's Barmer area were mixed with corrosion products and compacted to dry densities of 1.5, 1.75, and 2.0 Mg/m3 for this investigation. Thermal histories were induced at 110°C and 200°C for 3 h to prepare samples that were then saturated with hyperalkaline cement water. The presence of corrosion products resulted in a decrease in swelling pressure values when compared with samples without adding corrosion products. This decrease was more significant with an increase in the temperature of the induced thermal history. [ABSTRACT FROM AUTHOR]

The use of compacted bentonite around the high-level nuclear waste canister (HLW) inside the deep geological repository (DGR) ensures the prevention of entry of active radionuclides in the atmosphere due to its noteworthy large swelling ability. In the eventual repository, the waste canister has a high (100 °C–200 °C) temperature initially, and it reduces over a vast period, which induces a thermal history over the compacted bentonite layer. The cement/concrete layer is constructed as a bulkhead or in the vaults or to support the access of galleries between a buffer and the host rock, and it degrades over the period. The hyperalkaline fluid is created when it percolates through the cement/concrete layer and comes in contact with the compacted bentonite. The contact of hyperalkaline fluid to compacted bentonite induced with thermal history can hamper the swell pressure characteristic of the bentonite. Therefore to determine the combined effect of hyperalkalinity to the thermal history induced compacted bentonite, swell pressure testing has been conducted on two compacted Barmer bentonites (B1 and B2) specimens with an initial dry density of 1.5 Mg/m3, 1.75 Mg/m3, and 2.0 Mg/m3 and saturated with distilled water as well as with hyperalkaline cement water (W/C = 1 und pH = 12.5) and heated to 110 °C and 200 °C. When the specimens were saturated with hyperalkaline cement water, the swell pressure exerted by both bentonites was noticeably reduced compared to specimens saturated with distilled water. Nevertheless, the time taken to full saturation was longer than distilled water for samples saturated with hyperalkaline cement water. Also, the decrease in swell pressure was observed in the samples subjected to thermal history than samples, which were tested without inducing thermal history in both the cases of hyperalkaline cement water and distilled water. The microstructural observations through XRD, FESEM and EDX revealed the clogging of pores due to the presence of non-swelling minerals. [ABSTRACT FROM AUTHOR]

• Thermo-mechanical characteristics of two bentonites from India are presented. • Variation of swell pressures of bentonite with variation in thermal loading and densities is presented. • The development of time swelling curves for various thermal histories is presented. • The swell pressures of both the bentonite decreased with an increase in the thermal loading. The deep geological disposal system is an internationally adopted option for the disposal of high-level nuclear waste (HLW). It consists of a highly expansive bentonite, which is used as a buffer material and as a part of an engineered barrier system. Bentonite provides sufficient swelling pressure and low hydraulic conductivity to the barrier system giving long-term stability. The high-temperature waste canister in DGR and the variation in temperature due to thermal gradient impose a thermal loading on the compacted bentonite buffer and may alter the swelling pressures of the compacted bentonite. Hence it is necessary to investigate the influence of thermal loading on the swelling pressure of compacted bentonite. The paper discusses an assessments of swell pressures of two compacted bentonites (Barmer 1 (B 1) and Barmer 2 (B 2)) from Barmer district of Rajasthan, India with initial dry densities of 1.5 Mg/m3, 1.75 Mg/m3, and 2 Mg/m3 and subjected to 110 °C and 200 °C, hydrated with distilled water. The swelling load is recorded and compared with non-heated samples of same densities. Experimental and theoretical evidences indicated that the swelling pressure of heated compacted bentonite decreased with an increase in the temperature. Statistical analysis is also presented with the help of analysis of variance (ANOVA). [ABSTRACT FROM AUTHOR]

Abstract The concept of the deep geological repository (DGR) has been widely accepted for the safe disposal of high-level nuclear waste (HLW). Compacted bentonite/bentonite-sand mixture is considered as buffer material in the deep geological repository due to its favorable physicochemical and hydro-mechanical properties. The bentonite buffer is expected to be exposed to high temperature for a long duration because the heat released from the waste canister and due to the geo-thermal gradient inside the deep geological repository. The temperature from the waste canister can go initially as high as 200 °C and probably reduce to ambient temperature upon the decay of the waste material. This long duration temperature variation induces a thermal loading on the bentonite buffer, affecting its index, physicochemical and hydro-mechanical properties. Hence there is a need to critically examine the effect of the induced thermal loading on the long-term performance of the buffer as this study will be quite useful for the preliminary understanding before fixing its design values for the use as a part of engineered barrier system (EBS) in DGR. In the present study, two bentonites (Barmer 1 (B 1) and Barmer 2 (B 2)) from Barmer district of Rajasthan, India, was subjected to four temperature levels, i.e., 60 °C, 110 °C, 150 °C and 200 °C for a time duration of 3, 7, 15, 21, 28 days. Changes in the physicochemical properties of both bentonites were observed and reported. The decrease in the Atterberg's limit, swelling, specific gravity, optimum moisture content, pH and exchangeable monovalent cation and increase in maximum dry density was recorded with an increase in temperature as well as with the duration of heating. Highlights • Influence of thermal loading on index and physico-chemical properties of two bentonites from India is presented. • Index properties such as Atterberg’s limit, specific gravity, free swell index, compaction characteristics are reported. • Physicochemical properties such as pH, cation exchange capacity, and sodium ion concentration are presented. • The mechanism of alterations is explained by observing the mineralogical changes using X-ray diffraction technique. • Results of the present study are compared with the literature and reported with the possible future scope of the study. [ABSTRACT FROM AUTHOR]

The bentonite is compacted around the steel canister containing high-level waste to ensure the sealing of the canister by blocking the entry of harmful radiations emitted due to the reactions of active radionuclides. However, the canister continuously exhibits a high temperature (150 °C–200 °C) due to radioactive waste decay over a long time, which creates a thermal history on the surrounded compacted bentonite. Corrosion can generate changes in the bentonite buffer, which comes in direct contact with the canister and may lead to a change in its swelling ability. This study investigates the influence of corrosion products on the swell pressure of compacted bentonite specimens subjected to thermal history. Two Bentonite samples (B 1 and B 2) from the Barmer district of Rajasthan, India, were mixed with corrosion products and compacted at dry densities 1.5 Mg/m3, 1.75 Mg/m3 and 2.0 Mg/m3. These specimens were subjected to thermal histories at 110 °C and 200 °C and saturated with distilled water. The decrease in the swell pressure values was observed with the corrosion product and the increase in temperature of induced thermal history—comparatively, more time required for saturation in the presence of corrosion products than in its absence. The alteration in montmorillonite minerals might have occurred under the induced thermal history and corrosion reaction. [ABSTRACT FROM AUTHOR]

• The influence of thermal and chemical gradient saturated on bentonite is presented. • A lab-scale set up was fabricated which would create a thermal and chemical gradient. • The degree of saturation decreased towards heating end from the hydration end. • Reduction in cation exchange capacity was noted at various levels of samples. • The micro-structural investigations revealed the mineralogical changes. Compacted bentonite is being used as a buffer material for the safe dumping of high-level radioactive wastes (HLW) in the deep geological repository (DGR) due to its favorable physicochemical properties. However, the temperature variation, probably because of continuous heat emitting canister (initially −150 °C to 250 °C) and geothermal gradient, affect its performance. There would be bentonite iron interface reactions due to the corrosion of steel canister, which might cause reactive transport through the compacted bentonite buffer. Therefore, it is essential to know the influence of a combination of thermal as well as a chemical gradient on the reactive transport through the compacted bentonite buffer in terms of its physicochemical, thermomechanical, mineralogical stability. The purpose of this particular study is to establish the field conditions in the lab by fabricating a small scale set up which would create a thermal gradient on the compacted Barmer bentonite (B 1 and B 2) samples (compacted at 1.5 Mg/m3, 1.75 Mg/m3 and 2 Mg/m3 densities) by heating its one side at 60 °C, 110 °C, 150 °C and 200 °C temperatures for 28 days each and the system was hydrated by distilled water from the other side and was kept in touch with the metallic iron powder. It was observed that the degree of saturation of both bentonites decreased with an increase in temperature as well as density. However, the cation exchange capacity at lower density reduced more than that of higher density. The microstructural observation revealed the presence of impurities between the lamella of bentonite after 28 days that might be hematite. [ABSTRACT FROM AUTHOR]