Nuclide Migration Research Articles

Kinetic simulation of uranium migration in granite fissure media of beishan, gansu, China: A case study based on the Laplace transform and inverse transform methods

Deep geological disposal is currently considered the most practical and feasible method for disposing high-level radioactive wastes (HLWs). One of its key scientific issues is the migration of nuclides in fissure media. However, studies on the migration of nuclides like U-238 are relatively limited. In this study, the granite rock masses in Beishan, Gansu were selected to construct a physical and mathematical model of U-238 migration using the Laplace transform and inverse transform methods. Based on the numerical methods, the kinetic migration of nuclide U-238 in the fissure media of granite was simulated, and the effects of parameters such as fissure width, hydraulic gradient, diffusible area ratio and rock porosity on the migration of U-238 were investigated. The following insights were obtained: (1) After 100,000 years, U-238 has a very limited diffusion depth in the matrix domain, with a diffusion range of only a few tens of millimeters, whereas it migrates relatively quickly in the fissure domain, with a maximum migration distance of about 1500 m. (2) Under the same migration time and distance, the relative concentration of nuclide U-238 in the fissure domain increases with larger gap width, hydraulic gradient, and diffusible area ratio, but decreases with higher rock porosity. (3) In the same time range, the rock masses with larger gap widths, hydraulic gradients, and diffusible area ratios have larger migration ranges, while those with higher porosities have smaller migration ranges. (4) While selecting a site for diposal of HLWs, it is recommended to choose rock masses in the granite area of Beishan with no fissures or few fissures; additionally, areas with smaller fissure widths, hydraulic gradients, and diffusible area ratios but higher rock porosity should be prioritized. This study can provide important theoretical support for understanding nuclide migration in the future geological disposal of HLWs.

Evidence of nuclide migration from high-level radioactive waste glass via geogas

Characteristics of high-level radioactive waste transport in glass as gases were studied. A transmission electron microscope was used to observe the morphology of the substance after penetrating the high-level radioactive waste glass and the bentonite soil column, and the composition of the substance was analyzed using electron probe energy spectroscopy. The results show that the radionuclides in the high-level radioactive waste glass were transported out of the glass, the bentonite was penetrated by the geogas, and the transported substance occurred as nanometer-size particles and particle aggregates.

Parameter Sensitivity Analysis for Long-Term Nuclide Migration in Granite Barriers Considering a 3D Discrete Fracture–Matrix System

As a geological barrier for high-level radioactive waste (HLW) disposal in China, granite is crucial for blocking nuclide migration into the biosphere. However, the high uncertainty associated with the 3D geological system, such as the stochastic discrete fracture networks in granite, significantly impedes practical safety assessments of HLW disposal. This study proposes a Monte Carlo simulation (MCS)-based simulation framework for evaluating the long-term barrier performance of nuclide migration in fractured rocks. Statistical data on fracture geometric parameters, on-site hydrogeological conditions, and relevant migration parameters are obtained from a research site in Northwestern China. The simulation models consider the migration of three key nuclides, Cs-135, Se-79, and Zr-93, in fractured granite, with mechanisms including adsorption, advection, diffusion, dispersion, and decay considered as factors. Subsequently, sixty MCS realizations are performed to conduct a sensitivity analysis using the open-source software OpenGeoSys-5 (OGS-5). The results reveal the maximum and minimum values of the nuclide breakthrough time Tt (12,000 and 3600 years, respectively) and the maximum and minimum values of the nuclide breakthrough concentration Cmax (4.26 × 10−4 mSv/a and 2.64 × 10−5 mSv/a, respectively). These significant differences underscore the significant effect of the uncertainty in the discrete fracture network model on long-term barrier performance. After the failure of the waste tank (1000 years), nuclides are estimated to reach the outlet boundary 6480 years later. The individual effective dose in the biosphere initially increases and then decreases, reaching a peak value of Cmax = 4.26 × 10−4 mSv/a around 350,000 years, which is below the critical dose of 0.01 mSv/a. These sensitivity analysis results concerning nuclide migration in discrete fractured granite can enhance the simulation and prediction accuracy for risk evaluation of HLW disposal.

Investigating advection–dispersion behavior for simulation of HTO and 238Pu transport in argillaceous shale with different varying degrees of weathering

Abstract Exploring the migration behavior of radioactive nuclides in the surrounding rock media of the disposal site is the basis for the safety evaluation of radioactive waste disposal sites. In this study, the column experiment was employed to evaluate the nuclide migration behavior in the surrounding rock medium of a near-surface disposal site in China and to investigate the advection–dispersion behavior of tritium (HTO) and plutonium-238 (238Pu) in highly weathered argillaceous shale with different degree of weathering. A reasonable numerical model was selected to fit the experimental breakthrough curves (BTCs) and obtain relevant migration parameters. The results showed that: (1) the breakthrough curves of HTO and 238Pu in fractured media with varying degrees of weathering exhibited clear “peak forward” and “tail dragging” phenomena; (2) the stream tube model can better fit the BTCs of the nuclides in the highly weathered fractures and obtain the average dispersion coefficient <D>, average distribution coefficient <K d>, and other parameters; (3) the two-region non-equilibrium model can better fit the BTCs of the nuclides in the penetrating fractures and weak weathering fractures and obtain the volumetric water content of the mobile liquid region (θ m), distribution coefficient (K d), and other parameters.

Speciation simulation and sorption mechanism of 238Pu in radioactive waste repository lithosphere

Abstract The assessment of the migratory and environmental behavior of radioactive nuclides escaping from waste treatment facilities heavily relies on the use of numerical models capable of simulating and characterizing all significant processes of nuclides in complex geological environments. Adsorption models typically encompass the chemical properties of the nuclides themselves and their chemical reactions with the surrounding environment, as well as processes such as ion exchange or physical adsorption. These processes must be taken into consideration in the long-term safety assessment of radioactive waste repositories. The redox-sensitive nuclide 238Pu, a critical member among transuranic elements, exhibits a diverse range of aqueous forms, and concurrently, it possesses high toxicity. The chemical behavior of 238Pu shows strong spatial variability with changes in environmental conditions. In this study, we constructed a theoretical model for the migration of nuclides in soil and groundwater environments through indoor static batch experiments and hydrogeochemical simulations. Experimental methods were employed to dissect the micro-scale, irreversible adsorption reaction processes of nuclides and identify their primary existing forms. According to field measurements, the pH of groundwater was recorded as 7.48, with an Eh of 125.7 mV. Introducing a solid-to-liquid ratio of 1:10 g/mL in centrifuge tubes, we measured the radioactive nuclide concentration after achieving adsorption and desorption equilibrium, obtaining adsorption and desorption isotherms. The PHREEQC software was employed to investigate the changes in 238Pu forms under varying conditions of pH and redox potential. Field measurements provided groundwater pH and Eh values. The activity concentration of the nuclide was measured after reaching adsorption and desorption equilibrium. The results show that the adsorption isotherms of 238Pu differ from its desorption isotherms, indicating an irreversible adsorption-desorption process. Ion exchange and surface complexation were identified as the main modes of adsorption. PHREEQC simulations revealed that 238Pu primarily existed in forms such as tetravalent Pu(OH)4 and trivalent Pu(SO4)2 −, PuSO4 +. Pu(OH)4 accounted for the largest proportion (97%) in the groundwater solution system, while a minimal amount of pentavalent PuO2 + was present. Environmental factors, such as pH and the presence of ions like SO4 2− and HCO3 −, influenced the forms of 238Pu.

The Impact of Tides and Monsoons on Tritium Migration and Diffusion in Coastal Harbours: A Simulation Study in Lianyungang Haizhou Bay, China

Many nuclear power plants have been built along China’s coasts, and the migration and diffusion of radioactive nuclides in coastal harbours is very concerning. In this study, considering the decay and free diffusion of radioactive nuclides, a local hydrodynamic model based on the FVCOM was built to investigate the migration and diffusion of the radioactive nuclide tritium in Haizhou Bay, China. This model was calibrated according to the observed tidal level and flow velocity and direction, which provide an accurate background. This study aimed to evaluate the impact of tides and monsoons on the migration path and concentration variations in tritium over time. The results demonstrated that the simulated flow field can reflect real-life receiving waters. The distribution of the tritium concentration is affected by the flow field, which is related to the tides. Moreover, more severe radioactive contamination was exhibited in winter than in summer because monsoons may have hindered the migration and diffusion of tritium within the harbour. Given the poor hydrodynamic conditions and slow water exchange in the open ocean in Haizhou Bay, the diffusion rate of radioactive nuclides outside the bay area was higher than that within it.

Impact on repository footprint of blending spent mixed oxide fuels with UO2 fuels during reprocessing

The heat generated by Am-241 poses a challenge in the vitrification of spent mixed oxide (MOX) fuel after reprocessing. When the buffer material temperature exceeds the limit temperature of 100℃ due to the heat generated by vitrified waste, the buffer material quality is degraded, thereby affecting nuclide migration. Therefore, the waste-loading ratio of vitrified waste is estimated by heat generation, which determines the number of glass canisters and repository footprint. An option to reduce heat generation is to blend MOX high-level waste (HLW) with UO2 HLW. This study evaluated the thermal impact of blending MOX HLW with UO2 HLW while considering the cooling period and blending ratio. The maximum loading ratio was calculated for each blending condition in the range of buffer material temperatures below 100 °C, and the results were used to estimate the number of glass canisters and assess the repository footprint.Graphical abstract

Combustion characteristics and migration behavior of nuclides during incineration of low-level radioactive waste

Incineration is a main method of disposal low-level radioactive waste (LLRW). However, the combustion characteristics and migration of nuclides during incineration is still unclear. This study first investigated the effect of package mass, feeding time interval, air distribution, and water vapor supply on the combustion characteristics of LLRW and NOx emission. Optimal parameters were obtained based on combustion efficiency and NOx concentration. Then, the migration of nuclides was studied in conditions with the optimal parameters. The results showed that the combustion efficiency and NOx concentration were affected by furnace temperature, redox atmosphere and their distribution inside the furnace. Package mass, feeding time interval, and air distribution were optimized to balance the thermal decomposition rate of the waste and the oxidation rate of the combustible gas generated from thermal decomposition of the waste. Water vapor supply can decrease the furnace temperature and the thermal decomposition rate of the waste. The optimal parameters of package mass, feeding time interval, air distribution, and the feeding rate of water vapor is 100 g/bag, 40 seconds, 0.5/0.23/0.23/0.23 (excess air coefficient) for the first layer to fourth layer of air inlet, and 25 kg per hours, respectively. During incineration, more than 99% nuclides were fixed in bottom ash. The order of the ratio of nuclides in flue gas, fly ash, and bottom ash is bottom ash > flue gas > fly ash. Based on the ratio of nuclides in fly ash and flue gas, the order of volatility of nuclides during incineration is Cr > C s> Mn > Co > Nb > Zr.

Basic Study on Applicability to Artificial Barriers for Radioactive Waste Disposal Facility for Various Zeolites

Artificial barriers are being considered for the burial disposal of low-level radioactive waste in Japan. The artificial barrier consists of i) a low permeable layer made of bentonite materials that suppresses migration of nuclides due to advection, and ii) a low diffusion layer that uses cementitious materials that suppresses migration of nuclides due to diffusion. In addition, as methods of disposing of radioactive waste generated by the accident at the Fukushima Daiichi Nuclear Power Station, controlled disposal and isolation-type disposal are being considered. We thought that it would be effective to add adsorption performance to the performance of suppressing migration of such radioactive substances. In this study, we designed and experimented on the adsorption performance of mortar mixed with various zeolites, which had never been examined. As evaluation methods, i) fluidity, ii) strength, iii) low diffusivity, and iv) distribution coefficient of cesium and iodine were obtained. Based on the results, we reexamined the composition and conducted experiments to examine the applicability to high-adsorption artificial barriers. As a result, it was confirmed that the mass transfer resistance tends to be low due to the zeolite being porous. It was also shown that the strength of mortar mixed with zeolite depends on the cement matrix. As a result of this research, the distribution coefficient of cesium in cementitious materials using zeolite with a high cation exchange capacity designed in this research is higher than that of cementitious materials for low-level radioactive waste disposal facilities currently in operation 1,500 times was obtained. On the other hand, we were able to discover the possibility of examining construction on a full-scale regarding fluidity and compressive strength. As a future task, it was found that it is necessary to investigate a more practical artificial barrier by increasing the strength and densification of the cement paste.

Experimental study on mass transfer behavior and mechanism in nuclide-containing sandy soil under temperature gradient

The level of nuclear waste emergency disposal is an important factor restricting the rapid development of the nuclear energy industry under the trend of global energy revolution. To promote the effective application of artificial ground freezing (AGF) for the treatment of nuclear wastewater leakage, the mass transfer and migration mechanism of key nuclides (137Cs, 90Sr, and 129I) during unidirectional freezing of saturated silty sand were elucidated in this study. Temperature sensors and a nuclear magnetic resonance (NMR) were used to explore the internal temperature and moisture variation characteristics of nuclide-containing sandy soil during the freezing. Furthermore, using the designed device, a unidirectional freezing of different nuclide-containing sandy soils was performed to establish a relationship between the development characteristics of the freezing front and the mass transfer of nuclide. The results show that the T2 spectrum of nuclide-containing sandy soils has the most significant change from 0 °C to -1 °C, and the bound water content of I-containing sandy soil remains evident change when the temperature decreases below -1 °C. During unidirectional freezing, owing to the influence of the freezing front on nuclide migration, the resistivity curve exhibits a V-shaped jump, and the concentration peaks appear in turn along the freezing direction and show a stepwise increase. Nuclide mobility is closely related to the freezing rate of the corresponding location. With an increase in freezing thickness, the freezing rate decreases and the nuclide mobility increases. Owing to the combined influence of water-ice phase transition, concentration gradient and matrix suction in sandy soil, the concentration only increases within a small area at the face of the freezing front. The differences in migration behavior of nuclides in sandy soil driven by temperature gradient is mainly due to the different degrees of compression of nuclide ion on the diffusion double electric layer of soil particles, which results in different forms of unfrozen water within the sandy soils. The overall mobility of the three nuclides shows the following sequence: Cs>I>Sr. In the practical application of AGF, attention should be paid to the effects caused by the rapid migration of low-valence cationic nuclides and the retention of high valence cationic nuclides in the frozen wall.

Effects of iron corrosion products on the degradation of bentonite structure and properties

Bentonite is a key material for engineering barriers to prevent groundwater and nuclide migration in the multi-barrier system of high-level radioactive waste (HLW) geological disposal. However, its barrier property will be degraded under the action of iron corrosion products of steel disposal containers. In this paper, the effects of iron corrosion products on the degradation of bentonite structure and properties were investigated in the simulated environments for HLW geological disposal. The results showed that Fe2+/Fe3+ dissolved from iron powder could enter montmorillonite (Mt) interlayer and substituted part of Na+, which caused the decrease of the volume and interlayer spacing of Mt, and the structural integrity of Mt was destroyed. Macroscopically, the water absorption and swelling property of bentonite were significantly decreased. The degradation mechanism of Mt structure was mainly that Fe2+/Fe3+ generated by iron corrosion entered the interlayer domain of Mt to compensate for the interlayer charge deficit.

Investigation of nuclide migration in complex fractures with filling properties

Natural fractures in rock masses commonly feature filling properties, which greatly impact the process of fluid flow and mass transport in the geological disposal of high-level radioactive waste. This study develops a unified pipe-network method to numerically investigate the mechanism of nuclide migration in rough-walled fractures with filling properties based on mobile–immobile models (MIMs). Benchmark tests are conducted against analytical results with respect to the concentration distribution along a single filled fracture to demonstrate the numerical reliability. A case study is performed at the Xinchang (Beishan) selected site as the geological disposal repository for high-level radioactive waste. The migration process of U-238 in a fracture-network system close to a water-conducting zone is investigated with varying fracture-filling properties, fracture-filling proportions in mobile and immobile domains, and hydrodynamic properties. A safety classifier is developed using a support vector machine (SVM) based on 500 sets of numerical results for the concentration evolution of nuclides in complex fractures. This is to effectively evaluate the avoidance distance between the repository and the water-conducting zone with respect to factors including the fracture delay coefficient, the porosity of the fracture mobile domain, the hydraulic gradient, and the equivalent hydraulic aperture.

Advection–Dispersion Behavior for Simulation of H-3 and Pu-238 Transport in Undisturbed Argillaceous Shale of a Near-Surface Repository

In this study, a column experiment was employed to evaluate the nuclide migration behavior in the surrounding rock medium of a near-surface disposal site in China and to investigate the advection-dispersion behavior of tritium (H-3) and plutonium-238 (Pu-238) in highly weathered argillaceous shale. A reasonable numerical model was selected to fit the experimental breakthrough curves (BTCs) and to obtain the relevant migration parameters. The results show the following: (1) the internal structure of the highly weathered argillaceous shale exhibited heterogeneity, and the nuclide migration BTC showed characteristics of a "curve peak moving forward" and a "tail curve trailing"; (2) compared with other models, the stream tube mode could better fit the BTCs and obtain the average dispersion coefficient <D>, average distribution coefficient <Kd>, and other parameters; (3) compared to the results of the batch experiment, the distribution coefficient Kd obtained from the column experiment was smaller than that obtained from the batch experiment, which is speculated to be due to the influence of contact time and the contact area between the nuclide and the medium.

РЕЗУЛЬТАТЫ ЭКСПЕРИМЕНТАЛЬНЫХ ИССЛЕДОВАНИЙ НЕФТЕСОДЕРЖАЩИХ ОТХОДОВ С ПОВЫШЕННОЙ РАДИОАКТИВНОСТЬЮ КАК ОБЪЕКТА ЭКОЛОГИЧЕСКОГО РИСКА

In this paper peculiarities and results of carrying out of experimental research of determination of the main dependencies of migration and distribution of radioactive nuclides during the treatment of oil-containing waste are considered, including selection of samples of oil sludges to set up an experiment, planning and direct execution of the experiment. For the execution of the experiment five samples of oil sludges from sludge accumulators on the territory of Samara region were taken. Measurements were carries out according to the method developed by the authors and software application guide for PC «SPTR». Suggested method is allowing us to establish the nature of the distribution of natural radioactive nuclides of oil-containing waste, to determine the coefficients of migration of radioactive nuclides and it dependence from the characteristic of oil-containing waste. Using of results of experimental researches is allowing us to take timely and qualitative measures to reduce radioactive nuclides negative impact in oil-containing sludges as a factor of ecological risk.

МЕТОДИКА ЭКСПЕРИМЕНТАЛЬНЫХ ИССЛЕДОВАНИЙ НЕФТЕСОДЕРЖАЩИХ ОТХОДОВ С ПОВЫШЕННОЙ РАДИОАКТИВНОСТЬЮ КАК ОБЪЕКТА ЭКОЛОГИЧЕСКОГО РИСКА

In this paper the problems of negative impact of oil-containing waste with increased radioactivity to biosphere are considered. Review of the sources of increased radioactivity of oil-containing waste as a factor of negative impact to biosphere is carried out. Method of carrying out of experiment for determination of specific effective activity of natural radioactive nuclides is described. Suggested method is allowing us to establish the nature of the distribution of natural radioactive nuclides of oil-containing waste, to determine the coefficients of migration of radioactive nuclides and it dependence from the characteristic of oil-containing waste. Results of work are allowing us to carry out more efficient and high quality reduction of negative impact of oil-containing waste to the biosphere and to take timely and qualitative measures to reduce radioactive nuclides negative impact in oil-containing sludges as a factor of ecological risk.

A case study on the migration of nuclides in the coupled vadose zone-groundwater system at a proposed spent fuel reprocessing site in Gansu, China

Nuclear energy represents a relatively novel clean energy source, and potential nuclear contamination must be considered while developing the nuclear industry. In the context of a hypothetical nuclear accident, a nuclide migration model was established based on regional geological and hydrological data to investigate the migration characteristics of the radionuclides 3H, 99Tc(VII), 137Cs, and 79Se(IV) in a proposed spent fuel reprocessing site (SFRS) in Gansu, China. The temporal and spatial evolution of these four nuclides as well as their effects on the environment and local population were discussed in detail. The model of nuclide migration in the vadose zone was coupled to a large-scale regional nuclide migration model of groundwater in the form of boundary conditions. The results under the least favorable conditions showed that nuclides with high adsorption rates (137Cs and 79Se(IV)) remained in the vadose zone and decayed out completely. In contrast, nuclides with low adsorption rates (3H and 99Tc(VII)) penetrated the vadose zone and polluted the groundwater. The vadose zone delayed the arrival of 3H and 99Tc(VII) to the phreatic surface and reduced their concentrations. It was predicted that the concentration of 3H in the groundwater would fall below the threshold after 40 years, and the distance from the leakage area to the leading edge of the contamination plume would be 374 m. 99Tc(VII), which has a slightly higher adsorption rate and a lower decay rate than 3H, spread 833 m after 3000 years when its concentration was below the threshold. 3H and 99Tc(VII) did not migrate to downstream farms to threaten the safety of residential drinking water. This research presents a safety assessment of a nuclear area in Gansu, China, and provides a significant reference for the future construction of nuclear facilities and nuclear contamination control.

Experimental study on the physico-mechanical properties of Tamusu mudstone – A potential host rock for high-level radioactive waste in Inner Mongolia of China

Tamusu mudstone, located in Bayin Gobi Basin in Inner Mongolia of China, has been selected as a potential host rock for high-level radioactive waste (HLW) disposal in China. A series of tests has been carried out, including X-ray diffraction (XRD) tests, scanning electron microscopy (SEM) tests, disintegration tests, permeability tests and triaxial compression tests, to estimate the physico-mechanical properties of Tamusu mudstone in this work. The mineral composition of Tamusu mudstone was analyzed and it was considered as a stable rock due to its low disintegration rate, i.e. approximately 0.11% after several wet/dry cycles. Based on the results of permeability test, it was found that Tamusu mudstone has a low permeability, with the magnitude of about 10 –20 m 2 . The low permeability makes the mudstone well prevent nuclide migration and diffusion, and might be influenced by temperature. The triaxial tests show that Tamusu mudstone is a stiff mudstone with high compressive strength, which means that the excavation disturbed zone would be smaller compared to other types of mudstone due to construction and operation of HLW repositories. Finally, the properties of Tamusu mudstone were compared with those of Opalinus clay, Callovo-Oxfordian (COx) argillite, and Boom clay to further discuss the possibility of using Tamusu mudstone as a potential nuclear waste disposal medium.

Interaction between Gaomiaozi bentonite colloid and uranium

In a deep geological repository system, the presence of clay colloids in the geological disposal medium is considered to be a potential factor promoting nuclide migration. In this study, Gaomiaozi bentonite colloid (GMZC) was extracted using the gravity sedimentation method, and its interaction with U(VI) at neutral pH was studied. Small angle X-ray scattering (SAXS), X-ray photoelectron spectroscopy (XPS), dynamic light scattering, and other characterizations methods were used to analyze and observe the microstructure and agglomeration morphology of the bentonite colloidal particles before and after the reaction with U(VI) at the macro-meso-microscopic level. The XPS results indicate that the U(VI) reacted with the Si–O and Al–O on the GMZC particle surfaces at room temperature, and the hydrolyzed product of the U(VI) covered the colloidal surfaces in the neutral water environment, resulting in a decrease in the absolute zeta potential (from ∼31 mV to ∼19 mV). The hydrodynamic diameter of the colloidal particles did not change significantly (∼336 nm–∼360 nm), but the small-angle scattering data revealed that the fractal dimension was larger after the reaction (∼2.8–∼3.1), indicating particle agglomeration. About 14% of the GMZC particles were precipitated after reacting with the U(VI). When the temperature was increased to 55 °C and 85 °C, the precipitation did not change significantly, but the fractal dimension of the mixed system increased (from ∼3.1 to ∼3.6), and the d001 peak was not observed in the SAXS results. When the temperature was decreased by 25 °C, the hydrodynamic diameter of the zeta potential and SAXS image both returned to the level before the temperature increase, indicating that the unstable change in the colloidal system caused by the temperature increase was reversible.

Investigating diffusion mechanism for HTO and Se(IV)/Se(VI) in compacted Tamusu clay rock with different column lengths

Abstract Due to continuous self-sealing and good mechanical properties, the Tamusu clay rock of Inner Mongolia has been identified as the pre-selected site for high-level radioactive waste geological disposal site in China. The study of chemical behaviors related to Tamusu clay rock, such as nuclide migration, will be an important content of the performance assessment and safety assessment of the disposal repository in the future. The diffusion behavior of HTO and Se(IV)/Se(VI) with different compacted column lengths in Tamusu clay rock is discussed by the through-diffusion method. The diffusion coefficient, rock capacity factor, effective porosity, and other diffusion parameters closely related to nuclide migration are calculated, and the mechanism of nuclide diffusion is preliminarily discussed. The results show that D a (6.23 × 10−11∼17.96 × 10−11 m2 s−1), D e (1.62 × 10−11∼4.67 × 10−11 m2 s−1) for HTO increase with the increase of the compacted column length, and it is proposed that the diffusion process of HTO is affected by the change of geometrical factor and path tortuosity. D a (7.29 × 10−13∼1.74 × 10−13 m2 s−1), D e (5.15 × 10−12∼2.15 × 10−12 m2 s−1) for Se(IV), D a (3.11 × 10−12∼1.09 × 10−12 m2 s−1), D e (2.53 × 10−12∼1.09 × 10−12 m2 s−1) for Se(VI), which decrease with the increase of the compacted column length, it is mainly due to the existence of anion repulsion effect.

A Study of Sr Sorption Behavior in Claystone from a Candidate High-Level Radioactive Waste Geological Disposal Site under the Action of FeOOH Colloids.

Colloids have a significant influence on the migration of nuclides in claystone, which is an important geological barrier. The sorption of strontium on claystone in the presence of FeOOH colloids was investigated in samples from the Suhongtu site, a candidate high-level radioactive waste disposal site in China. The effects of colloid amount, solid content, and pH were investigated by batch tests, and the sorption reaction mechanism was analyzed by kinetic modeling and microscopic characterization techniques. The results indicate that the sorption of Sr by claystone increased with the solids content, and the claystone had a stronger Sr sorption capacity under alkaline conditions. The Sr sorption kinetics were best described by the pseudo-first-order and pseudo-second-order models, which revealed that the progress is affected by physical diffusion and chemical sorption. Furthermore, the microscopic characterization results demonstrate that cation exchange reactions and surface complex reactions are the main sorption mechanisms for Sr sorption on claystone. Ca and Mg plasmas in claystone minerals can have cation replacement reactions with Sr, and functional groups such as -OH and [CO3]2− can have complexation reactions with Sr to adsorb Sr on the surface of the claystone. Additionally, the presence of the FeOOH colloid inhibited the sorption effect of claystone slightly. The FeOOH colloid could occupy sorption sites on the claystone surface, which reduces the activity of the functional groups and inhibits the sorption of Sr on claystone.