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The use of ceramics as radionuclide containment matrices for the safe long-term storage and disposal of high-level nuclear waste is studied with emphasis on chemistry flexibility, structural stability, and radiation tolerance. Here we investigate the flexibility of a particular series of compounds Yb 2 Ti 2-x O 7-2x (x = 0, 0.2, 0.52, 0.8, and 1) to form a solid solution between the end members Yb 2 Ti 2 O 7 of pyrochlore structure through to Yb 2 TiO 5 of fluorite structure. Whilst it is shown that cation disorder and corresponding transition from the ordered pyrochlore to disordered fluorite is facilitated via increasing the ytterbium to titanium ratio, it is also shown that sintering conditions may be controlled in such a way as to encourage the growth of the pyrochlore structure even in Yb 2 TiO 5. The radiation response of these materials was tested in situ via 1 MeV krypton irradiation coupled with transmission electron microscopy characterisation. The critical fluence of ions required to transition the crystalline phase to amorphous was found to increase with increasing disorder from Yb 2 Ti 2 O 7 to Yb 2 TiO 5. However, of the two forms (pyrochlore and fluorite) of Yb 2 TiO 5 studied via ion irradiation that with the greater pyrochlore structure and so more ordered also had the lower critical temperature, 434 K, for maintaining crystallinity during irradiation. [ABSTRACT FROM AUTHOR]

The use of copper canisters in the Swedish KBS-3 concept for spent nuclear fuel disposal could result in the formation of copper-bearing uranyl phases should a canister suffer from defects or if the containment were to fail before reducing conditions are established in the repository. Most uranyl species would be expected to display higher solubility than the original uranium(IV) dioxide fuel, leading to enhanced release, though this would depend on the phase and prevailing groundwater conditions. Secondary alteration products may also be poorly crystalline or even amorphous, making characterization difficult during the pre-closure period owing to the high radiation field close to the canister. Vandenbrandeite, [CuUO2(OH)4], is a rare mineral in nature but known to form by alteration of primary uraninite through interaction with oxidizing groundwater containing dissolved copper. Consequently, an attempt has been made to characterize two vandenbrandeite specimens of varying crystallinity by luminescence and multiple-laser Raman spectroscopy; techniques amenable to remote, robotic deployment and which have proved useful in discriminating other uranyl oxy-hydroxides, silicates, and phosphates. The first reported luminescence emission and excitation spectra for vandenbrandeite revealed near-negligible luminescence, with a slightly enhanced signal for the specimen displaying poorer crystallinity. This observation agrees well with density functional theory calculations. The simulated projected density of state and band structure show an unlikely transition from the U f-orbitals to Cu d-orbitals, or O states, would be required for luminescence to be detectable; this probably improves for poorly crystalline specimens as the spatial overlap between the orbitals increases. Furthermore, negligible differences in the number of peaks and peak positions were detected in the laser wavelength-dependent Raman spectra although again, variation in background noise and peak shape was observed based on the degree of crystallinity. Good agreement was obtained between experimental and simulated Raman spectra, particularly with the environmentally sensitive axial uranyl stretching modes, validating the crystal system derived in this study. The findings of this study suggest luminescence spectroscopy, when combined with Raman spectroscopy, may be able to both identify vandenbrandeite and distinguish between crystalline and amorphous forms based on their relative luminescence intensity. [ABSTRACT FROM AUTHOR]

A common design of a high-level radioactive waste (HLW) disposal system consists of the waste canisters emplaced in tunnels or shafts, with the space between the heat-emitting canisters and the surrounding rock filled with a bentonite-based material. Understanding the behavior of this barrier, in particular the effect of prolonged heating on its properties, is important to assess the barrier's long-term performance. The objective of the present study was to add to this understanding and to supply experimental data about the state of bentonite hydrated and heated for a long period of time. To that end, a 50 cm long column of Wyoming-type bentonite pellets was heated at its base at 140 °C (simulating the waste canister) while a synthetic sodium-chloride-rich groundwater including sulfate, calcium, and magnesium was supplied through its upper surface for almost 10 y. At the end of the experiment the upper half of the column was saturated, but in the lower half the water content decreased sharply toward the heater, where it was close to 0%. No relevant mineralogical changes occurred, and the main component of the bentonite continued to be a predominantly sodium montmorillonite with no structural changes with respect to the initial one. In the area where the temperatures were >60°C and the material very dry, however, the smectite was not able to completely develop the 1-layer hydrate after 48 h of stabilization at RH 55%, although its expandability was fully recovered when the smectite was hydrated with liquid water. The ions solubilized as a result of the water-content increase were transported toward the heater and concentrated in two distinct areas: sodium and calcium chlorides closer to the heater than calcium and sodium sulfates. At the heater contact, the bentonite microstructure was of dense packets with carbon and Na-S-coated cavities. Precipitation of calcite and calcium sulfates and possibly dissolution of silica minerals also took place close to the heater. [ABSTRACT FROM AUTHOR]

We formulate a finite‐size scaling hypothesis to predict the global permeability of fracture networks. To validate the hypothesis, numerous discrete fracture networks are generated, and the permeability is numerically calculated. Results suggest that the dimensionless permeability, scaled by moments of local conductivity and fracture sizes and corrected by two stereological ratios, can capture variations in fracture attributes (orientations, sizes, and apertures). The universal form obtained in this study can also explain the contradictory observations where the permeability decreases or increases with the domain size of fracture networks. We show that how a clear transition point, where the permeability does not change with the domain size, is obtained from this universal form. This study provides a solid theoretical foundation to understand the connection between fracture attributes and field‐scale hydraulic properties. Plain Language Summary: Knowledge of the permeability of fractured rocks is critical for many geo‐engineering applications (such as deep geological disposal of radioactive wastes, mining and dam engineering) and environmental problems (such as ground water and pollutant transport). The influence of fracture attributes (e.g., orientations, sizes, and apertures) on hydraulic properties of rock masses is significant, but the task of correlating the permeability to individual fracture data is still a challenging one with remarkable knowledge gaps. In this letter, instead of using analytical solutions, we develop a finite‐size scaling (FSS) function, inspired by the percolation theory in statistic physics, to describe the permeability of fracture networks. The universality of the scaling function is validated by massive simulation data which cover a wide range of fracture attributes. Results demonstrate that, regardless of how domain sizes and fracture attributes change, the scaling for the permeability is of an invariant form, after considering the finite‐size effect. Our findings motivate further studies of applying the FSS function at the field scale and improve on the information that earth scientists can obtain from fracture statistics. Key Points: A finite‐size scaling hypothesis for the permeability of discrete fracture networks (DFNs) is formulated with several dimensionless quantitiesNumerous DFN representations and flow simulations are implemented to validate the scaling hypothesisThe scaling is found universal for different DFNs with an invariant form [ABSTRACT FROM AUTHOR]

Goiânia, the Goiás State capital, starred in 1987, where one of the largest radiological accidents in the world happened. A teletherapy machine was subtracted from a derelict radiotherapy clinic and disassembled by scavengers who distributed fragments of the 50 TBq 137CsCl source among relatives and acquaintances, enchanted by the blue shine of the substance. During the 15 days before the accident was acknowledged, contaminated recycling materials were delivered to recycling factories in four cities in the state of São Paulo, Brazil, in the form of recycling paper bales. The contaminated bales were spotted, collected, and stored in fifty 1.6 m3 steel boxes at the interim storage facility of the Nuclear and Energy Research Institute (IPEN). In 2017, a check of the content was performed in a few boxes and the presence of high moisture content was observed even though the bales were dry when conditioned and the packages were kept sealed since then. The main objective of this work was to report the fungi found in the radioactive waste after they evolved for 30 years in isolation inside the waste boxes and their role in the decay of the waste. Examination of the microbiome showed the presence of nematodes and fungal communities. The fungi species isolated were Aspergillus quadricinctus, Fusarium oxysporum, Lecanicillium coprophilumi, Scedosporium boydii, Scytalidium lignicola, Xenoacremonium recifei, and Pleurostoma richardsiae. These microorganisms showed a significant capacity to digest cellulose in our trials, which could be one of the ways they survive in such a harsh environment, reducing the volume of radioactive paper waste. These metabolic abilities give us a future perspective of using these fungi in biotechnology to remediate radioactively contaminated materials, particularly cellulose-based waste. [ABSTRACT FROM AUTHOR]

Understanding radiation chemistry is also essential for the sustainable development of the nuclear power industry, in particular, for the processing of nuclear fuel, handling radioactive waste and maintaining the integrity of materials in currently operating nuclear power reactors. Radiation chemistry deals with chemical processes initiated by the interaction of high-energy photons (X-rays) or charged particles (electrons, protons, alpha or heavy ions) with matter. [Extracted from the article]

Gas breakthrough pressure is a significant parameter for the gas exploration and safety evaluation of engineering barrier systems in the carbon dioxide storage, remediation of contaminated sites, and deep geological repository for disposal of high-level nuclear waste, etc. Test for determining gas breakthrough pressure is very difficult and time-consuming, due to the low/ultra-low conductivity of the specimen. It is also difficult to get a comprehensive and high-precision model based on limited results obtained through individual experiments, as the measurements of gas breakthrough pressure were influenced by many factors. In this study, a collected database was built that covered a lot of former test data, and then, two models were developed by the random forest (RF) algorithm and multiexpression programming (MEP) method. The MEP model constructed with explicit expressions for the gas breakthrough pressure overcame the drawbacks of common "black box" models. Meanwhile, five significant indicators were selected from ten common features using the permutation importance algorithm. The RF model was interpreted by the Shapley value and the PDP/ICE plots, while the MEP model was analyzed through the proposed explicit expression, showing strong consistence with that in former studies. Finally, robustness analysis was conducted, and stability of the proposed two models was verified. [ABSTRACT FROM AUTHOR]

Deep geological disposal of high-level radioactive waste is a feasible method for solving the problem of spent fuel storage in China. High-level radioactive waste releases heat during the decay process, which increases the temperature of the surrounding rock in the repository, resulting in a significant increase in radon concentration. In this study, the surrounding rock (granite) of a high-level radioactive waste repository was taken as the research object and, based on the similarity principle, an orthogonal test designed. Similar materials of uranium-containing granite were prepared in the laboratory and the physical and mechanical properties and cumulative radon concentration of granite samples assessed under different temperatures (25, 50, 100, 150, and 200 °C). The results showed that, with increased temperature, the compressive and tensile strengths of samples gradually increased and their pore volume gradually decreased. After heat treatment, the longitudinal wave velocity and thermal conductivity of samples decreased linearly with increased temperature. The radon exhalation rate first increased and then decreased, with the rate reaching a maximum at 100 °C. The radon exhalation rate of single-sided and double-sided samples was 0.00914 and 0.00460 Bq·m−2·s−1, respectively. When the temperature was 25–100 °C, the dominant stage was pore water. The radon exhalation rates of samples were positively correlated with compressive and tensile strengths and negatively correlated with pore volume, longitudinal wave velocity, and thermal conductivity. The temperature of 100–200 °C was range of the dominant stage of pore structure. The conclusions obtained in this study can provide theoretical support for radon reduction and radon control of granite in high temperature environments. [ABSTRACT FROM AUTHOR]

The paper analyzes the effect of tectonic activity on the safety of the geological repository at the Yeniseisky radioactive waste (RW) repository (Nizhnekansky massif, Krasnoyarsk Krai) in the event of the formation of faults in the host rocks following the loading of wastes into the repository. A 3D model of the radioactive contamination transfer from the repository by groundwater is considered, taking into account the inhomogeneous permeability of rocks. The orientation and location of the faults are set based on the modeled stress-strain state of the rocks in this area, as well as data obtained from field and spatial surveys on contemporary tectonic activity in the considered area. Submeridional faults were demonstrated to have no significant effect on the spread rate of the radioactive contamination. However, the activation of the most probable latitudinal fault may lead to the rapid movement of contaminated groundwater in the fault zone towards the Yenisei. The efficacy of plugging various areas in the sublatitudinal fault zone to reduce the migration rate of contaminating radionuclides is considered. [ABSTRACT FROM AUTHOR]

In the context of the deep geological disposal of nuclear waste and to improve the understanding of the homogenisation process of bentonite barriers, in particular with regard to the filling of technological voids, tests were performed with compacted FEBEX bentonite samples hydrated under limited axial swelling conditions. The samples were saturated from the top surface using the vapour transfer technique (imposing suctions of 6 and 0.5 MPa), or with deionised water injected at a low flow rate either from the bottom surface or from the gap on top. The final water content of the samples saturated via vapour transfer was related to the suction imposed during the tests according to the water retention curve. Thus, the final water content of the samples tested under suction 6 MPa was lower and not enough to allow sufficient bentonite swelling to close the gap. In contrast, the samples saturated under suction 0.5 MPa reached higher water contents and were able to fill the gap before the equilibrium water content had been reached. In the tests performed with liquid water supplied through the gap, the samples were able to swell easily into the open void and the gap closed earlier than when hydration took place from the opposite end. In all the tests, the final water content of the bentonite was higher and the dry density lower towards the hydration surface. These gradients were more remarkable and persistent as the initial strain was larger. No completely homogeneous density or water content distribution was observed in any of the tests. These changes were also reflected in the pore size distribution. The pores of size larger than the upper limit of mercury intrusion porosimetry (~ 550 µm) were also quantified, which allowed a better representation of the pore size distribution of the more swollen samples. Although the void ratio corresponding to pores smaller than 200 nm (em) was initially higher, over time the volume of macropores (eM) increased more, giving place to an overall decrease in the em/eM ratio, which tended to be constant along the samples in the longest tests. In the first stages of hydration, the macropore void ratio and size considerably increased close to the gap in the tests with saturation from it. [ABSTRACT FROM AUTHOR]

Flow-through columns were used to assess potential long-term trends in 90Sr biogeochemistry and transport in a Finnish near-surface very low-level waste (VLLW) repository concept. Experiments simulated the effects of water intrusion and flow through the repository barrier and backfill materials, examining impacts on 90Sr migration. Artificial rainwater containing 2.0 mg/L stable Sr (as a proxy for 90Sr) was pumped through column systems that had varying compositions from a matrix of rock flour (backfill material), bentonite (backfill/sealing material), and carbon steel (waste encapsulation material), for 295 days. Effluent geochemistry was monitored throughout. Sr retention behaviour in all column systems was broadly similar. Sr removal from influent rainwater was marked (~95% removed) at the beginning of the experiments, and this degree of removal was maintained for 20 days. Thereafter, Sr concentrations in the effluents began to rise, reaching ~2 mg/L by 295 days. Further, 56%–67% of added Sr was retained in the repository materials over the 295-day reaction period. Analysis of the effluents indicated that colloids did not form; as such, Sr output was likely to be aqueous Sr2+. Upon completion of the experiment, solid-associated Sr distribution and speciation in the columns were assessed through column sectioning and post-mortem analyses, which encompassed the following: total acid digests, sequential extractions, and XAS analysis. The total acid digests and sequential extractions showed that Sr was evenly distributed throughout the columns and that the majority (68%–87%) of solid-associated Sr was in the exchangeable fraction (MgCl2). This suggested that a major part of the solid-phase Sr was weakly bound to the column materials via outer-sphere sorption. Interestingly, a smaller amount of Sr (7%–23%) could only be extracted by aqua regia, suggesting that a proportion of Sr may bind more strongly to the barrier materials. XAS analysis of select samples confirmed that the dominant Sr phase was sorbed to the rock flour and bentonite, but not corroded carbon steel. Columns were also subject to remobilisation experiments using artificial rain- and seawater without added Sr. While rainwater remobilised Sr slowly, high-ionic strength seawater remobilised Sr at much higher rates in the systems containing bentonite. Interestingly, Sr was well retained in the rock flour-only system following rain and seawater intrusion. Overall, the results indicate that the column materials provide reactive surfaces for Sr removal should it be released from waste packages; however, the backfill and barrier materials have limited retention capacity, and the dominant sorption interaction is relatively weak. The safety case for the shallow disposal of radioactive waste should consider the possibility of seawater intrusion and that the bentonite-bound Sr was significantly more susceptible to remobilisation following seawater, despite retaining slightly more Sr during sorption experiments. [ABSTRACT FROM AUTHOR]

Evaluation of the mobility behaviour of radionuclides under highly saline and alkaline conditions is a major concern for the performance assessment of radioactive waste disposal. The aim of this study was to determine the effect of up to 2.8 mol/kgsolution content of NaNO3, on the solubility and the retention of U(VI) at 22 °C onto a hardened cement paste (HCP) prepared from ordinary Portland cement (CEM I). To avoid the interference of the high salt concentration and ionic strength, and because of the expected low solubility of uranium under such alkaline conditions, time-resolved laser fluorescence spectroscopy (TRLFS) was selected to accurately measure U(VI) concentration in solution using the standard addition method in 85% H3PO4. This allows both limiting the dilution and matrix effects and determining the resulting [U(VI)] in solution with acceptable precision for the distribution factor (Rd) in both sorption and desorption experiments. The operational solubility limit measured at high ionic strength lowered by a factor of three compared to the reference cementitious condition, and its Rd values decreased by a factor ca. four. The sorption of U(VI) appears to be reversible under these conditions. [ABSTRACT FROM AUTHOR]

Bentonite, with excellent osmotic swelling performance and low hydraulic conductivity, represents as an ideal material for the final disposal of radioactive wastes. This paper investigates bentonite and three inorganic modified bentonites (Na-Bentonite, K-Bentonite and Mg-Bentonite) in adsorption of Cs+, one of the major hazardous metals in radioactive waste. The effects of solution pH value, sorbent dosage and initial concentration on the adsorption performance of Cs+ were investigated, as well as the effects of temperature and reaction time on the adsorption capacity and removal rate of heavy metal Cs+. The internal structure and chemical composition of bentonite before and after adsorption were also discussed. The results indicate that the maximum adsorption rate is 95.30%, and the estimated best adsorption capacity is almost 28.2 mg/g. 1.1 g bentonite dosage and 1.0 mol/L solution concentration in alkaline environment is the most suitable conditions for cesium adsorption reaction, which can be witnessed in Na-Bentonite. [ABSTRACT FROM AUTHOR]

Preparing similar materials of uranium-bearing granite surrounding rocks in high-level radioactive waste geological disposal repository is the prerequisite for the similar simulation test of the radon exhalation mechanism. The samples of uranium-bearing granite were prepared based on the model similarity principle. The optimized proportion of the samples was obtained by testing their density, compressive strength, elastic modulus, and tensile strength and conducting polar sensitivity analysis and quadratic multiple regression analysis. The radon diffusion coefficient and other properties between the sample and the original rock were verified by similarity ratio, and it meets the requirements. [ABSTRACT FROM AUTHOR]

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Uranium(U) is one of the highly toxic heavy metals and radionuclides that has become a major threat to soil health. There are two types of sources of Uranium in the soil system, natural and anthropogenic. Natural sources of uranium include rock systems and volcanic eruptions while anthropogenic sources include mining activities, disposal of radioactive waste, application of phosphate fertilizers, etc. Uranium accumulation impacts germination, early seedling growth, photosynthesis, metabolic and physiological processes of the plants. Through its accumulation in the aerial parts of the plants, Uranium finds its way to the human body, where it has deleterious health impacts. Different studies have identified the various sources of Uranium, explored, and explained the geochemistry of Uranium in soil, assessed the Uranium uptake and toxicity to the plants, and further studied the impact on human health. Most studies focused on two stages, either soil-plant or plant-human system. However, few studies have critically reviewed and summarized the U in the soil-plant-human system. Thus, the review has been designed to focus on the sources, geochemical behaviour, uptake, and translocation, plant toxicity, food chain entry, and finally, impact on human health. The relationship between the bioavailability of Uranium in the soil-plant system with soil properties like pH, Organic matter, and microorganisms have also been included. The study is further intensified by analyzing the accumulation of Uranium in various parts of the plants. [ABSTRACT FROM AUTHOR]

In Korea, 10 units of nuclear power plants are projected for decommissioning by 2030, which will consequently generate a large amount of decommissioning waste. It could bring about a lack in the capacity of the radioactive waste disposal facility. In this paper, a computational code, REcycling and DIsposal Safety Assessment has been developed for the safety assessment regarding the recycling and disposal of the decommissioning waste. It is composed of two modules: dose assessment module and maximum allowance concentration estimation module. The dose assessment module is intended to evaluate the exposure doses from radioactive nuclides in the wastes for each recycling or disposal scenario. The maximum allowance concentration estimation module has the function to translate the nuclides concentrations in the wastes from the exposure doses received by the end user for each scenario. The concentration of each nuclide provides the basis for the development of the technical criteria in accordance with the annual dose limit. [ABSTRACT FROM AUTHOR]