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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]

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]