Swelling Pressure Of Bentonite Research Articles

Towards Reliable Barrier Systems: A Constrained XGBoost Model Coupled with Gray Wolf Optimization for Maximum Swelling Pressure of Bentonite

Bentonite and bentonite mixtures are used as buffer material for deep geological radioactive waste repositories. The proper determination of bentonite maximum swelling pressure is important as it influences the long-term stability and sealing characteristics of the barrier system. We developed a constrained machine learning model based on extreme gradient boosting (XGBoost) algorithm tuned with grey wolf optimization (GWO) to determine the maximum swelling pressure of bentonite and bentonite mixtures. We integrated a penalty term into the XGBoost algorithm’s loss function to constrain the model during the training phase ensuring meaningful predictions. To this end, we compiled a dataset containing 305 experimental data of bentonite maximum swelling pressure and other relevant soil properties including montmorillonite content, liquid limit, plastic limit, plasticity index, initial water content, and soil dry density to develop the model. The performance of the hybrid GWO-XGBoost model was benchmarked against feed-forward and cascade-forward neural network models. The GWO-XGBoost model showed a better performance in estimating the experimental values comparing to the other models, thereby demonstrating its effectiveness and reliability in determining the maximum swelling pressure of bentonite and bentonite mixtures.

Research Progress on the Influence of Thermo-Chemical Effects on the Swelling Pressure of Bentonite

The swelling pressure of bentonite changes dramatically due to diffused nuclear radiation heat and underground osmosis, causing the failure of the buffer isolation layer in deep geological repositories for the disposal of high-level radioactive waste. A detailed overview of the relevant research results on the swelling pressure variation of bentonite under thermo-chemical effects is presented in this paper. The results showed that the values of the swelling pressure obtained by different test methods are dissimilar. The swelling pressure of bentonite decreased with the increasing pore solution concentration; nevertheless, the effect of temperature on the swelling pressure is still controversial. At the micro-level, crystal layer swelling and double- layer swelling are generally considered to be the main factors affecting the swelling pressure; the pore structure and water distribution of bentonite will change owing to thermo-chemical effects. At the macro-level, involving intergranular stress, a mechanical parameter was proposed to explain the mechanism of the changes in the swelling pressure of bentonite. Finally, future research directions for the study of the evolution of bentonite swelling properties under thermo-chemical effects are proposed, based on the current research results.

Hydro-chemo-mechanical modelling of long-term evolution of bentonite swelling

The swelling properties of compacted bentonite have typically been studied by means of hydro-mechanical models. The question of how these properties are affected by changes in its chemical composition has been given much less attention. However, cation exchange reactions in montmorillonite interlayer or changes in salinity are known to affect the swelling properties of bentonite. In this paper, a hydro-chemo-mechanical (HCM) model is proposed to study the effects of reactive transport processes on the long-term evolution of swelling pressure of compacted bentonite under confined and partially saturated conditions. The HCM model is implemented in iCP, an interface between Comsol Multiphysics and PHREEQC and is calibrated using available experimental data. At the constitutive level, the hydro-mechanical Barcelona Basic model is extended to include the effects of cation exchange reactions between interlayer and bulk water, changes in salinity, and montmorillonite dissolution in highly alkaline environments. The geochemistry of bentonite is based on an advanced reactive transport model that couples solute transport by diffusion and advection with chemical reactions. An application case is presented to demonstrate the main HCM model features and its ability to predict long-term changes in bentonite swelling pressure. The performance of the sealing system for closure of the repository for radioactive waste in France is simulated under unsaturated and isothermal conditions over a period of 50,000 years. The HCM interactions of MX-80 bentonite with other near-field materials such as concrete and claystone are simulated to predict the evolution of the swelling pressure of the seal. The results show the potential of the proposed model for the performance assessment of repository components. Overall, the model predicts limited impact of geochemical alteration on the swelling pressure of the bentonite seals, which is mainly driven by cation exchange reactions between the Na-rich bentonite, the Ca-rich claystone, and the calcium and alkalis from concrete porewater.

An experimental and prediction study on the compaction and swell–expansion behavior of bentonite clay containing various percentages of two different synthetic fibers

Reinforcement by fibers is one of the most practical and economic methods to improve some engineering properties and the mechanical behavior of soils in civil engineering. In this paper, the impact of fiber reinforcement on the swelling behavior of bentonite clay was investigated. Virgin homopolymer polypropylene (HPP) and copolymer polypropylene (CPP) with various percentages were used as reinforcement materials, and the influence of fiber contents on one-dimensional swelling pressure was evaluated. At first, sieve and hydrometer analysis and Atterberg limits tests were conducted on bentonite. The standard Proctor compaction test was also used to determine the compaction properties of reinforced and unreinforced bentonite. Then, the oedometer test was carried out on specimens. The results showed that both types of fiber enhanced the swelling potential of bentonite. The optimum amount of fibers was analyzed via test results. Moreover, the most significant improvement in terms of reducing the swelling potential of bentonite was observed due to the use of HPP fiber. The maximum improvement percentages to reduce the swelling pressure of bentonite layers using HPP and CPP fibers were 44.2% and 29.4%, respectively. The statistical analysis was carried out to identify the relationship between the dependent variables (fiber-reinforced samples) and the independent variable (plain bentonite). The result indicated the proper agreement of the model and concerning values. Two significant equations were calculated to estimate the swelling pressure of bentonite with HPP and CPP fibers that showed the use of synthetic fibers additives has a considerable effect on decreasing the swelling pressure of expansive soils.

Swelling behavior of compacted bentonite with the presence of rock fracture

In this study, the influence of rock fracture on the swelling behavior of compacted bentonite was investigated experimentally with bentonite compacted inside a hollow cylinder of fractured Beishan granite from China. During the test, hydration was allowed by letting water flowing into the compacted bentonite from the base plate, and air being expelled from the top plate. Along the sample, swelling pressure and relative humidity were monitored at different positions. Results showed that the presence of rock fracture significantly changed the hydration process of compacted bentonite. The evolutions of swelling pressure were directly related to the relative position to the rock fracture. At the same height, for the position closer to the rock fracture, a larger decrease in local dry density occurred, resulting in a larger loss in swelling pressure. This suggests that in field conditions of radioactive waste repository involving bentonite and rock, local filling of rock fracture by bentonite may occur, and the resulted bentonite swelling pressure decrease must be accounted for in the safety evaluation of the whole storage system.

Estimation of the long term helium production in high burn-up spent fuel due to alpha decay and consequences for the canister

ABSTRACTIn the KBS-3 repository concept and safety analysis, the copper container with a cast iron insert plays a central role in assuring isolation of the waste from the surrounding during long periods of time. All processes that affect its stability are thoroughly analysed, including potential detrimental processes inside the canister. For this reason, an estimation of the helium produced during the long term decay of alpha emitters in the spent fuel is necessary to evaluate if the pressures generated inside can have consequences for the canister.The spent nuclear fuel to be disposed of in Sweden is mainly LWR fuel. The maximum burn-up expected is 60 MWd/kg U for BWR and PWR. A small quantity of BWR MOX is expected to be stored with a maximum burn-up of 50 MWd/kg U.This work has focused on carrying out calculations of the amounts of He generated during more than 1 million years in Swedish spent nuclear fuels with a benchmarking exercise by using both codes AMBER and Origen-ARP. The performance and agreement of the codes in the He generation from α-decay have been checked and validated against data reported in literature [1].In the calculation of the maximal pressure inside the canister, the quantity of helium used to pre-pressurise the fuel rods has been accounted for. The pressure inside the canister due to He generation is at all times much lower than the hydrostatic pressure and/or the bentonite swelling pressure outside the canister.

An alternative approach for the use of DDL theory to estimate the swelling pressure of bentonites

An approach to estimate the swelling pressure of compacted bentonite (Bent) using Gouy–Chapman diffuse double layer theory is presented. Several laboratory swelling pressure tests on compacted Bent (MX80 and Calcigel) under constant volume conditions were performed and compared with the calculated swelling pressure using the original theory. The study revealed that at low dry density the as prepared total void ratios are less than theoretical void ratios using diffuse double layer theory, however, reverse behavior has been noticed for high compaction dry densities. Based on the experimental results of Bent with different valences, the relationship between the theoretical void ratio from diffuse double layer theory and as prepared total void ratio were determined. New equations were derived on the basis of diffuse double layer theory and experimental swelling pressure data. The method categorizes the different Bents into two groups depending upon the weighted average valency. A good agreement was observed in all the cases between experimental and estimated swelling pressure using derived equations. The applicability of the proposed method was investigated by comparing the estimated swelling pressure results with the additional reported in literature laboratory results on swelling pressure of compacted Bent.

Gas migration properties through a bentonite/argillite interface

Among diversified industrial uses, see for instance Koch [Koch, Appl. Clay Sci., (21) 2002], and following positive in situ experiments, compacted bentonite blocks are potential candidates for sealing nuclear waste repositories, thanks to their swelling ability in a wet environment. As requested by Andra (French Agency for Nuclear Waste Management) and complementarily to in situ experiments, an original experimental laboratory set-up was designed in order to reproduce the introduction and swelling of bentonite plugs inside an argillite host rock. Once the argillite/bentonite interface is established, an increase in storage tunnel gas pressure is simulated and the interface gas migration pressure (or gas critical pressure) is evaluated. More precisely, a first experimental set-up provides bentonite swelling pressure and kinetics (i.e. mainly hydraulic cut-off, time to reach asymptotic swelling pressure and value of asymptotic swelling pressure) at given initial compaction and saturation rate. This phase is preparatory to reproducing the introduction and subsequent swelling of a bentonite plug inside the argillite host rock, which uses a similar test rig. Experimental results of water permeability and gas critical pressure are provided for MX80 compacted bentonite associated to Bure Callovo-Oxfordian argillite.

Thermodynamic model on swelling of bentonite buffer and backfill materials

Smectite (montmorillonite) is the major clay mineral constituent of the bentonite buffer and the backfill materials which would be used for geological disposal of a high-level radioactive waste. Swelling pressure of bentonite occurring in the permeation process of moisture was modelled on the basis of thermodynamic theory and data of water sorption by smectite. The activity ( a H 2 O ) of water on smectite surface was measured by a vapor pressure method as a function of water content (0–83%) and temperature (15–40 °C) at smectite densities of 0.6 and 0.9 Mg/m 3. From experimented results, the relative partial molar Gibbs free energy ( Δ G H 2 O ) of water was determined at 25 °C. Purified Na-smectite, of which all interlayer cations were completely exchanged with Na + ions and soluble salts were removed in advance, was used. The a H 2 O and Δ G H 2 O of water decreased with water content when water content is lower than about 40%, which is consistent with the behaviour of Kunipia-F bentonite, for which smectite content was approximately 100 wt.%. Water affected from the surface of smectite was deduced to be up to a distance of approximately two water layers on the average (corresponding to four water layers in the interlayer) from the specific surface area of smectite (ca. 800 m 2/g) and the correlation between a H 2 O (or Δ G H 2 O ) and water content. Swelling pressure against smectite partial density under saturated condition was estimated from the difference ( Δ G H 2 O ) in the chemical potentials of water in equilibrium state between free water and water-saturated smectite, and compared to data measured for various samples of bentonites and different silica sand amounts. The calculated swelling pressures were in good agreement with the measured data for smectite partial density ranging between 0.7 and 2.1 Mg/m 3, covering then most of bentonites and dry densities of compacted bentonites by considering smectite content.

Experimentally determined swelling pressures and geochemical interactions of compacted Wyoming bentonite with highly alkaline solutions

The estimated quantity of cement for construction and sealing purposes is around 9E5 kg in the planned Swedish KBS3 repository for nuclear waste. The highly alkaline cement pore fluid (pH > 12) may affect other components in the repository, and especially the bentonite buffer is of concern. In this study, we simulated possible interactions between cement and bentonite by contacting highly compacted bentonite with high molar hydroxide solutions in a series of laboratory experiments. Wyoming bentonite (MX-80) and purified homo-ionic Na- and Ca-montmorillonite were used for tests with 0.1, 0.3 and 1.0 M NaOH, and saturated Ca(OH) 2 solutions. Pressure cells with permeable filters were loaded with compacted discs of bentonite at the proposed buffer density (2000 kg/m 3 at full water saturation). A hydroxide solution was circulated on one side of the cell and an isotonic chloride solution on the other during a minimum of 45 days. Swelling pressure and solution pH were monitored during the tests and the change in the solution composition and bentonite mineralogy were determined after completed tests. No effect on swelling pressure was observed in tests with 0.1 M NaOH (pH 12.9) or saturated Ca(OH) 2 solutions (pH 12.4) and the mineralogical/chemical changes of the clay were minimal. The bentonite swelling pressure was significantly reduced in the tests with 0.3 (pH 13.3) and 1.0 M (pH 13.8) NaOH solutions. The reduction seems to be due to an instant osmotic effect, and to a continuous dissolution of silica minerals, resulting in mass loss and, consequently, a decrease in density. At these high pH, the release of silica was dominating and the CEC of the clay increased by 20–25%. The structural formula of the smectite and X-ray diffraction tests for non-expandability (Greene-Kelly test) provided strong evidence that the dissolution of montmorillonite proceeds incongruently through an initial step of beidellitization. The calculated rate of silica release from montmorillonite is 1.6E−9 g g −1 clay s −1 for 1.0 M and 5E−10 g g −1 clay s −1 for 0.3 M NaOH solutions. The Si release rate is, however, not a straightforward measure of the montmorillonite dissolution rate due to the non-stoichiometric dissolution. Upon contact between bentonite and NaCl solutions, ion-equilibrium is established between the external solution and the exchangeable cations of the clay. A similar initial pressure response on exposure of bentonite to NaOH solutions indicates that such equilibrium may establish also with an external NaOH solution. If so, the OH-concentration of the clay pore water will be lower than that of the external solution, which would explain that dissolution rates in our experiments with highly compacted bentonite are lower than those reported for batch experiments with hydroxide solutions of the same concentration.

SR-97: Canister Performance under Normal Disposal Conditions

ABSTRACTThe SR-97 disposal canister consists of an inner load bearing cast iron component and an outer corrosion barrier of copper. The mechanical strength of the insert must be such that it withstands, with sufficient safety margins, the combined forces of hydrostatic pressure at the disposal depth (7 MPa) and the bentonite swelling pressure (7 MPa). It must also withstand an increased hydrostatic pressure during a future glaciation. This pressure increase may be as high as 30 MPa. For this load situation, no extra safety margins will be required. Uneven pressure may occur during the water saturation of the buffer and to some extent some uneven pressure may also persist after water saturation. Calculations have been performed for such design basis uneven loads and the present canister design has been shown to withstand them. Under the oxygen-free conditions at the disposal depth, copper will be thermodynamically stable. Only dissolved sulfide will be able to corrode the copper. The supply of sulfide is expected to be low and, consequently, the service life of the canister will be very long. Calculations indicate that no known corrosion processes will lead to canister failure during a shorter time than at least 1 million years.