Utilization of Slovak bentonites in deposition of high-level radioactive waste and spent nuclear fuel

Research progress on microbial adsorption of radioactive nuclides in deep geological environments

Due to the development and utilization of nuclear energy, the safe disposal of nuclear waste needs to be urgently addressed. In recent years, the utilization of microorganisms' adsorption capacity to dispose of radioactive waste has received increasing attention. When compared with conventional disposal methods, microbial adsorption exhibits the characteristics of high efficiency, low cost, and no secondary pollution. In the long term, microbial biomass shows significant promise as specific chemical-binding agents. Optimization of biosorption conditions, identification of rare earth element binding sites, and studies on the sorption capacities of immobilized cells provide compelling reasons to consider biosorption for industrial applications in heavy metal removal from solutions. However, the interaction mechanism between microorganisms and radioactive nuclides is very complex. This mini-review briefly provides an overview of the preparation methods, factors affecting the adsorption capacity, and the mechanisms of microbial adsorbents.

Sorption and transport characteristics of europium on sandy soils

Radioactive europium can be released as a fission product during nuclear incidents and pose a threat to the human and surrounding environment because of its biological activity and long decay half-lives. For safe design issues and human health protection demands in construction of the planned nuclear power plants (NPPs) at Al-Dabaa site, it is necessary to study the sorption and transport of different radionuclides as europium within the selected area for predicting their fate at any crisis. Many soil samples were collected from different locations at the area selected along the northwestern coast of Egypt. The samples were transported to the laboratory, preserved, and characterized using X-Ray fluorescence (XRF), Fourier transform infrared spectroscopy (FT-IR), and X-Ray diffraction (XRD). Experiments were performed to study the sorption and transport kinetics of Eu(III) ions on two sandy soil samples from the collected ones. The effect of different parameters (e.g. contact time, pH, initial europium concentration, and temperature) on the sorption behavior europium was explored in a static condition. The maximum sorption capacity was determined and found to be 3.4 and 7.0 mg g^-1 for sorption of Eu(III) ions onto soil-1 and soil-2, respectively. Different models were applied to assess the sorption of europium onto the surface of the investigated soils. Data confirmed that Eu retention was attained through a chemisorption process. Further, the thermodynamic parameters were determined and their values confirmed the endothermic nature of the sorption process. The transport of europium radionuclides, with groundwater, through homogeneous porous media with uniform one-dimensional flow in the geosphere was processed and the relative migration velocity was determined in presence of both distilled and seawater media. The transport of Eu(III) radionuclides was higher in presence of seawater than that in presence of distilled water by about two order of magnitude. This obviously clarified the effect of seawater in accelerating the transport of radionuclides with groundwater in the geosphere of studied area. The role of different competing ions have various valances on the relative migration velocity was explored. Further, the time required for studied radionuclides to reach Mediterranean Sea was determined.

Sorption of some radionuclides from liquid waste solutions using anionic clay hydrotalcite sorbent

¹²⁹I and ⁷⁹Se are potentially important anionic radionuclides in safety assessments due to their high mobility, radiotoxicity, and long half life's (1.7 × 10⁷ and 3.27 × 10⁵ years, respectively). This study is interested in the sorption of ¹³¹I and ⁷⁵Se radionuclides onto magnesium iron hydrotalcite (Mg/Fe HTlc). Mg/Fe HTlc was prepared by co-precipitation technique and characterized using different analytical tools such as FT-IR, XRD, XRF, TGA & DTA, SEM, and BET. Results obtained from this study showed that the adsorption process was a very fast equilibrium time (20 min). The distribution coefficient values as a function of pH have high separation factors for ¹³¹I at all different pHs. Reaction kinetic obeys the pseudo-second-order model. Maximum sorption capacity for ¹³¹I and ⁷⁵Se has the values 21.45, and 9.25 mg/g respectively. Sorption isotherms are more relevant to a Langmuir isotherm. The % removal of ¹³¹I is decreased by increasing the concentration of competing species. The investigation evidenced that the prepared sorbent is suitable for the removal of ¹³¹I and ⁷⁵Se from radioactive waste and could be considered potential material for purification of effluent polluted with these radionuclides.

Possibilities of Uranium Deposit Kuriskova Mining and Its Influence on the Energy Potential of Slovakia from Own Resources

Uranium is one of the strategic minerals used mainly in energetics. The main purpose of uranium mining is to achieve maximum production to meet the rapidly growing demand for energies. It needs to become aware that technological progress in mining processes could significantly reduce the negative impacts associated with environmental, economic, and social risks. Uranium mining is one of the most controversial topics. It is dealt with by many experts and scientists around the world. Various methods and technologies of uranium mining are encountered in professional journals, as well as political or socio-economic decisions based on the impact and importance of the energy potential of uranium deposits, or the environmental impacts of uranium mining. The deposit of Kuriskova is one of the most perspective deposits not only in Slovakia but also in the world. The deposit is located near the town of Kosice (with near 240,000 inhabitants) and near the recreational area of Jahodna in the east of the Slovak Republic. The analysis and determination of the energy potential of the deposit of Kuriskova shows that uranium reserves from this deposit would be able to fully cover the needs for nuclear power plants for the production of nuclear fuel, in the Slovak Republic, even in the longer term. With the above-mentioned energy potential of the deposit of Kuriskova at the level of 600 TWh, nuclear power plants in the Slovak Republic are able to be supplied with raw materials from the deposit of Kuriskova for about 40 years with the current amount of electricity produced (approx. 15 TWh). Therefore, for the purposes of this research, a proposal for the extraction of uranium reserves at the deposit of Kuriskova was made. Based on it, it is possible to determine the amount of recoverable uranium reserves from the deposit. A methodology has been determined with mining this proposal, which takes into account the basic criteria of uranium deposit mining, which was used for the selection of a suitable mining technology for the uranium deposit of Kuriskova.

Geological Risk Calculation through Probability of Success (PoS), Applied to Radioactive Waste Disposal in Deep Wells: A Conceptual Study in the Pre-Neogene Basement in the Northern Croatia

The basic principles of geological risk calculation through probability of success (PoS) are mostly applied to numerical estimation of additional hydrocarbon existence in proven reservoirs or potential hydrocarbon discoveries in selected geological regional subsurface volumes. It can be adapted and validated for a comprehensive input dataset collected in the selected petroleum province, by dividing up geological events into several probability categories and classes. Such methodology has been widely developed in the last decades in the Croatian subsurface—mostly in the Croatian Pannonian Basin System (CPBS). Through the adaptation of geological categories, it was also applied in hybrid, i.e., stochastic, models developed in the CPBS (Drava Depression), mostly for inclusion of porosity values. As the robustness of this methodology is very high, it was also modified to estimate the influence of water-flooding in increasing oil recovery in some proven Neogene sandstone reservoirs in the CPBS (Sava Depression). This new modification is presented to be applied to geological risk calculation, intending to assess the safety of geological environment storage in deep wells, where spent nuclear fuel (SPN) would be disposed, a subject of great importance. The conceptual study encompassed the magmatic and metamorphic rocks in the pre-Neogene basement of the CPBS, intended to be used for such purpose. Regionally distributed lithologies are considered for nuclear waste disposal purpose, in order to detect the safest ones, considering petrophysical values, water saturation, recent weathering and tectonic activity.

Influence of colloidal Fe(OH)3 on the adsorption characteristics of strontium in porous media from a candidate high-level radioactive waste geological disposal site

Colloids in groundwater or geological barriers generally play a key role in the migration of special nuclides. Adsorption characteristics of strontium were investigated on porous media in the presence of colloidal Fe(OH)₃ from the Beishan Site, the only high-level radioactive waste disposal site candidate in China. The effects of colloid amounts, solid contents, and pH were determined and studied by batch texts. The results revealed that the presence of colloidal Fe(OH)₃ in porous media contributed to promotion of the sorption effect, and the influencing factors had a significant impact on the adsorption process. The sorption ability increased with increasing colloid amount when the equilibrium time was approximately 10 h under an optimal solid-liquid ratio of 20 g L^-1. The sorption effect in alkaline conditions was better than that under acidic conditions. The sorption kinetics indicated that the strong chemical interaction and/or surface complexation contributed primarily to strontium sorption. The sorption isotherms and model fitting revealed that the sorption of strontium onto porous media in the presence of colloidal Fe(OH)₃ was a monolayer adsorption, and the presence of colloidal Fe(OH)₃ is an important factor that greatly influences the removal of strontium from aqueous solutions. These findings provide useful information for the treatment of strontium in radioactive waste disposal sites.

Direct Shear Experimental Study on the Mobilized Dilation Behavior of Granite in Alxa Candidate Area for High-Level Radioactive Waste Disposal

Dilation behavior is of great importance for reasonable modeling of the stability of the host rock of the repository for high-level radioactive waste disposal. It is a suitable method for carrying out direct shear experiments to analyze the dilation behavior of rock with well understood physical meanings. Based on a series of direct shear experiments on granite samples from the Alxa candidate area under different normal stresses, the shear stress‒shear strain and shear stress‒normal strain relations have been studied in detail. Five typical stages have been divided associated with the fracturing process and deformation behaviors of the granite samples during the experimental process, and the method to determine the typical stress thresholds has been proposed. It has also been found that the increasing normal stress may reduce the maximum dilation angle, and when the normal stress is relatively lower, the negative dilation angle may occur during the post-peak stage. According to the data collected from the direct shear tests, an empirical model of the mobilized dilation angle dependent on normal stress and plastic shear strain is proposed. This mobilized dilation angle has clear physical meanings and can be used in plastic constitutive models of the host rock of the repository, and this analysis can also be put forward to other types of geomechanical problems, including the deformation behaviors related to landslide, earthquake, and so on.

Retention behavior of anionic radionuclides using metal hydroxide sludge

With the speedy growth of nuclear power production, the removal and disposal of radioactive nuclides such as 129I, 99Tc, 79Se, 36Cl, 93Mo, and 137Cs become major environmental security issues. Retention of these radionuclides, especially anionic species such as 129I (t1/2 1.7 × 107 years), 93Mo (t1/2 4 × 103 years) and 79Se (t1/2 3.27 × 105 years) has been challenging. 129I, 93Mo and 79Se bind very weakly to most sorbents and deposits. This study has examined the sorption potential of Metal hydroxide sludge (MHS) for 125I (t1/2 60.2 days), 99Mo (t1/2 2.75 days) and 75Se (t1/2 120 days) as a surrogate for 129I, 93Mo and 79Se, respectively. MHS has been characterized by different techniques and the factors affecting the sorption processes were investigated. The experimental data were analyzed using kinetic models and thermodynamic parameters. The results showed that the kinetics of sorption of 125I and 99Mo on MHS proceeds according to the pseudo-first-order, on the contrary of 75Se sorption follows pseudo second-order kinetic model. The maximum sorption capacity of MHS was found to be 51.2 mg/g, 46.5 mg/g and 40.2 mg/g for 125I, 99Mo and 75Se, respectively. It can be concluded that, in the case of release of anionic radionuclide species to the surroundings the MHS could act as a succeeded and economical sorbent material for retention of different anionic radionuclides such as 133, 129I, 79Se, 36Cl, 93, 99Mo, and 99Tc. To avoid the release of such anionic species from the stored nuclear wastes to the environment.

Sorption behavior of cesium, cobalt and europium radionuclides onto hydroxyl magnesium silicate

The radioactive wastes from different activities have to be safely disposed of and isolated from the human environment. The retardation of radioactive materials by designed barriers is originally controlled by the sorption ability of the mineral compositions. In this work, a naturally available mineral composite, a hydroxyl magnesium silicate (HMS) was investigated as potential natural inorganic sorbent for the retention of long-lived radionuclides (134Cs, 60Co and 152+154Eu) from aqueous solutions. The factors affecting the sorption process, such as contact time and pH were evaluated. Furthermore X-ray fluorescence (XRF), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray diffraction (XRD), differential thermal and thermogravimetry analyses (DTA/TGA) measurements were examined in order to assess the physicochemical properties of the magnesium silicate mineral. Langmuir and Freundlich isotherms fitted the result s substantially better than the Flory–Huggins isotherm and the sorption was found to follow pseudo-first order kinetic model. The proposed mineral has been successfully applied for the sorption of 134Cs, 60Co and 152+154Eu radionuclides from real radioactive waste. The results indicated that about 97.4–99% of 134Cs, 60Co and 152+154Eu radionuclides were efficiently retained onto the HMS mineral. Based on the results obtained, it can be concluded that the HMS mineral is an economic and efficient retaining material for environmental hazardous migration and/or leakage of some radionuclides such as 134Cs, 60Co and 152+154Eu and trivalent actinide (241Am, 242mAm and 243Am) ions. Therefore, this study could be used as a starting point to establish and consider that mineral as an engineered barrier around the disposal facilities at the nuclear activity centres.

Radioactive Barium Ion Trap Based on Metal-Organic Framework for Efficient and Irreversible Removal of Barium from Nuclear Wastewater

Highly efficient and irreversible capture of radioactive barium from aqueous media remains a serious task for nuclear waste disposal and environmental protection. To address this task, here we propose a concept of barium ion trap based on metal-organic framework (MOF) with a strong barium-chelating group (sulfate and sulfonic acid group) in the pore structures of MOFs. The functionalized MOF-based ion traps can remove >90% of the barium within the first 5 min, and the removal efficiency reaches 99% after equilibrium. Remarkably, the sulfate-group-functionalized ion trap demonstrates a high barium uptake capacity of 131.1 mg g(-1), which surpasses most of the reported sorbents and can selectively capture barium from nuclear wastewater, whereas the sulfonic-acid-group-functionalized ion trap exhibits ultrafast kinetics with a kinetic rate constant k2 of 27.77 g mg(-1) min(-1), which is 1-3 orders of magnitude higher than existing sorbents. Both of the two MOF-based ion traps can capture barium irreversibly. Our work proposes a new strategy to design barium adsorbent materials and provides a new perspective for removing radioactive barium and other radionuclides from nuclear wastewater for environment remediation. Besides, the concrete mechanisms of barium-sorbent interactions are also demonstrated in this contribution.