Radionuclide Migration Research Articles

Fe(II) Induced Reduction of Incorporated U(VI) to U(V) in Goethite.

Over 60 years of nuclear activities have resulted in a global legacy of radioactive wastes, with uranium considered a key radionuclide in both disposal and contaminated land scenarios. With the understanding that U has been incorporated into a range of iron (oxyhydr)oxides, these minerals may be considered a secondary barrier to the migration of radionuclides in the environment. However, the long-term stability of U-incorporated iron (oxyhydr)oxides is largely unknown, with the end-fate of incorporated species potentially impacted by biogeochemical processes. In particular, studies show that significant electron transfer may occur between stable iron (oxyhydr)oxides such as goethite and adsorbed Fe(II). These interactions can also induce varying degrees of iron (oxyhydr)oxide recrystallization (<4% to >90%). Here, the fate of U(VI)-incorporated goethite during exposure to Fe(II) was investigated using geochemical analysis and X-ray absorption spectroscopy (XAS). Analysis of XAS spectra revealed that incorporated U(VI) was reduced to U(V) as the reaction with Fe(II) progressed, with minimal recrystallization (approximately 2%) of the goethite phase. These results therefore indicate that U may remain incorporated within goethite as U(V) even under iron-reducing conditions. This develops the concept of iron (oxyhydr)oxides acting as a secondary barrier to radionuclide migration in the environment.

Numerical analysis of safety of a borehole repository for vitrified high-level nuclear waste

We developed a model and carried out transient 3-D computer simulation of radionuclides migration from an underground repository for vitrified high-level radioactive waste (HLW) with long-lived actinides. The repository represents an array of deep boreholes. Canisters with vitrified HLW are disposed one above another in lower parts of the boreholes. The upper parts of the boreholes are sealed. Radionuclides leached from the disposed waste are carried away by thermal convection of groundwater caused by heat generation in HLW. The computer simulation was carried out with use of a refined technique taking into account significant difference between radionuclides concentration in the groundwater in the loaded parts of the boreholes and at the earth surface, dependence of the groundwater properties on pT-parameters, composition of HLW. Values of leaching rate of the waste forms by groundwater depending on time and temperature, as well as role of colloid facilitated transport of radionuclides were determined from previous experiments with Na–Al–P-glass which was used in Russian Federation for HLW vitrification. The results of the calculations allowed us to conclude that distance between containers with HLW and the ground surface of 240 m and more is sufficient for safe disposal of the considered HLW.

Modeling Radiation Exposure from Normal Release of 137Cs Radionuclide to Groundwater for Post-Closure Assessment of Serpong Near Surface Disposal Demo Facility

Near-surface disposal (NSD) has been applied in several countries to dispose of low-level radioactive waste. The demo plant of this disposal type is planned to be constructed in Serpong Nuclear Area, Banten. An assessment of radiation exposure is necessary to ensure the safety requirement of the facility in order to support this program. This study aims to estimate radionuclide migration from the proposed NSD demo facility to the environment and the corresponding total human dose using AMBER mathematical modeling. The representative radionuclide,137Cs, was selected because of its high mobility in the environment and the relatively long half-life in the low-level waste inventory. The scenario considered in the modeling was the normal release to the environment through groundwater. Parameters such as initial radionuclide concentration, soil physical parameters of the study site, and disposal design were entered into AMBER software to be calculated using mathematical formulas. The results show that the radionuclide concentration value in the environment is below the safe limit recommended by the Environmental Supervisory Agency. Likewise, the maximum dose received by the community around the facility is 7.40×10-11 mSv/y, 550 years after the post-closure of the facility, which is also below the regulatory limit of 1 mSv/y for the public.

Anaerobic corrosion of carbon steel in compacted bentonite exposed to natural Opalinus clay porewater: Bentonite alteration study

Abstract. Carbon steel is a potential canister material for the disposal of high-level radioactive waste in deep geological repositories in clays and clay rocks. Bentonite is considered as a potential backfill material for those multi-barrier systems. To predict the long-term performance and for safety assessment the knowledge of canister corrosion behavior is important. The corrosion products formed and mineralogically altered bentonite at the canister/bentonite interface can potentially provide an additional barrier against radionuclide migration. In-situ corrosion experiments were performed at the Mont Terri underground research laboratory. Coupons of carbon steel were embedded in Volclay MX-80 bentonite with controlled densities, installed in a borehole under simulated repository and anaerobic conditions and exposed to natural Opalinus clay porewater for a period up to 5.5 years (Smart et al., 2017). In the present study, the bentonite layer at the canister/bentonite interface was characterized by complementary microscopic and spectroscopic techniques (XPS, SEM-EDX, µXANES) under anoxic conditions. The interface revealed reddish-brown staining up to 2 mm depth into the bentonite in the zone adjacent to the steel in all three obtained samples. The XPS analysis revealed formation of sulfides at the interface consisting of iron and other trace metals present in the steel. The SEM-EDX analyses of the interface (embedded cross-cut with steel removed) showed different degrees of calcium enrichment in the bentonite adjacent to the metal for various samples. The µXRF analysis performed on the bentonite at the interface showed a scarce or distinct calcium-enriched rim up to 100 µm into the bentonite and iron-enriched rim depending on the sample (one sample in Fig. 1), while µXANES analysis revealed formation of iron silicate compounds in the reacted reddish-brown zone. The iron appears to displace calcium from the interlayer sites in montmorillonite. The calcium then precipitates at the interface as calcite. The extent of this process seems to be strongly related to the bentonite density. The steel coupon was removed prior to embedding, with the location marked as resin in Fig. 1. A line scan from the edge towards the bulk bentonite did not indicate any systematic gradient in the Fe2+/3+ ratio. The formation of mixed Fe2+/3+ silicate compounds appears to be heterogeneous. This work contributes to an increasing understanding of steel corrosion mechanisms in clay, which can improve the robustness of canister lifetime predictions.

Impact of the heterogeneity of the sandy Opalinus clay facies at the Mont Terri underground research laboratory on radionuclide migration

Abstract. The assessment of the safety of a deep geological repository (DGR) for high-level radioactive wastes over assessment time scales of up to 1 million years requires an in-depth understanding of the multi-scale coupled processes that affect the repository system evolution over time, to reduce uncertainties and conservatism in safety analyses. This is in particular required with respect to the challenges of a comparative assessment of different repository concepts in different host rocks within the process of a site selection for a DGR for heat-generating radioactive wastes in Germany. The collaborative project “Integrity of nuclear waste repository systems – Cross-scale system understanding and analysis (iCross)” conducted jointly by five research centres of the Helmholtz Association and co-funded by the Initiative and Networking Fund of the Helmholtz Association and the Federal Ministry of Education and Research (BMBF) has been initiated with the overall objective to improve the understanding of coupled thermal-hydraulic-mechanical-chemical-(micro)biological (THMCB) processes and to develop simulation tools that enable a holistic close to reality description of the long-term evolution of the repository system. Geological formations, such as those foreseen as potential host rocks for DGRs, and their surroundings are heterogeneous on various length scales ranging from nanometers to kilometers. Therefore, the aim of this work in the context of iCross is to evaluate the effects of mineralogical, geochemical and microstructural heterogeneities of repository host rocks on radionuclide transport in the repository far field, using the sandy facies of the Opalinus clay (SF-OPA) from the Mont Terri underground research laboratory (St. Ursanne, Switzerland) as an example. Here, we address in particular the migration behaviour of Ra-226 as an important radionuclide to be considered in safety cases for deep geological disposal of spent nuclear fuel. To assess the impact of the heterogeneities in SF-OPA on radionuclide transport, a complementary approach combining microstructural characterisation methods, experimental techniques for the determination of transport parameters of the rock matrix and the mobility of Ra-226 with innovative developments in reactive transport modelling on the pore and continuum scales was pursued. One of the results was that although the limited clay content in SF-OPA decreases the total amount of Ra bound to the illite phase, the solid solutions of sulphate and carbonate compensate for this and provide a major fixation mechanism.

Geochemical benchmark tests to validate the conversion of thermodynamic data for TOUGHREACT

Abstract. According to the ongoing site selection process for a repository for high-level radioactive waste in Germany, rock salt, clay and crystalline rock are possible host rocks. The pore water of these rocks contains saline solutions with high ionic strengths. To model the speciation and/or migration of radionuclides in long-term safety analyses for nuclear waste disposal, a geochemical code that includes thermodynamic data suitable for saline solutions is needed. Thermodynamic equilibrium in saline solutions with high ionic strengths is usually modelled using the Pitzer approach (Pitzer, 1991). Within the context of nuclear waste disposal, the THEREDA project (Moog et al., 2015) provides thermodynamic data for some widely used geochemical codes (PHREEQC, Geochemist's Workbench, ChemApp, and EQ 3/6) using the Pitzer approach; however, for modelling in long-term safety analyses for nuclear waste disposal, another geochemical code, TOUGHREACT, is used. Therefore, scripts were developed to convert thermodynamic data of the THEREDA project to be applicable in TOUGHREACT. The scripts were validated by benchmark tests and by comparing calculations using PHREEQC and TOUGHREACT (Weyand et al., 2021). In total, 50 different benchmark tests were performed considering 3 specific geochemical systems, which are relevant to long-term safety analyses: (1) oceanic salt system, polythermal: K, Mg, Ca, Cl, SO4, H2O(l), (2) actinide system, isothermal: Am(III), Cm(III), Nd(III), Na, Mg, Ca, Cl, OH, H2O(l) and (3) carbonate system, isothermal: Na, K, Mg, Ca, Cl, SO4, HCO3/CO2(g), H2O(l). Each benchmark test considered specific ion concentrations in solution and in gaseous phases in the presence of specific minerals. The benchmark tests derived the geochemical equilibria and the results of both codes were compared to each other and to experimental data. The results of the calculations using both codes showed a good correlation. Remaining deviations can be explained by technical differences of the codes.

Reactive transport modelling of long-term interactions between iron and MX-80 bentonite

Abstract. The geological disposal in deep bedrock repositories is the preferred option for the management of high-level radioactive waste. In some of these concepts, carbon steel is considered as potential canister material and bentonites are planned as backfill material to protect metal waste containers. Therefore, a 1D radial reactive transport model has been developed in order to better understand the processes occurring during the long-term iron–bentonite interaction. The conceptual model accounts for diffusion, chemistry of the porewater and aqueous complexation reactions, mineral dissolution/precipitation and absorption, at a constant temperature of 25 ∘C under anoxic conditions. The geometry of the axisymmetric model reflects the canister–bentonite interface and the bentonite. The primary phases considered are montmorillonitic smectite, quartz, muscovite, albite, illite, pyrite and calcite. We assume that carbon steel is composed only of iron. The potential secondary phases considered are from reported experiments, such as magnetite, nontronitic smectite, greenalite, cronstedtite and siderite. The numerical model results suggest that at the iron–bentonite interface, Fe is adsorbed at the smectite surface via ion exchange in the short term and it is consumed by formation of the secondary phases in the long term. Furthermore, calcite precipitates are due to cation exchange in the short term and due to montmorillonitic smectite dissolution in the long term. The numerical model predicts the precipitation of nontronitic smectite, magnetite and greenalite as corrosion products. Results further reveal a significant increase in pH in the long term, whereas dissolution/precipitation reactions result in limited variations of the porosity. Progressing bentonite dissolution owing to the rising pH and concomitantly increasing silicate concentrations in the porewater induce formation of Fe-silicates as corrosion products at the expense of magnetite. A sensitivity analysis has also been performed to study the effect of selected parameters, such as corrosion rate, diffusion coefficient and composition of the porewater, on the corrosion products. Overall, outcomes suggest that pH and concentration of dissolved Si play an important role in corrosion mechanisms. The predicted main secondary phases in the long term are Fe-silicate minerals. Thus, such phases deserve further attention as possible chemical barriers for radionuclide migration in the repository near-field.

Characteristics of Fluid Flow and Solute Transport in Multicrossed Rough Rock Fractures Based on Three-Dimensional Simulation

Fluid flow and solute transport in rock fractures are important for controlling pollutant migration in groundwater and radionuclide migration in nuclear waste disposal. The quantitative characterization of fluid flow and mass transport in crossed fractures is the key to defining solute transport in a fracture network. In this study, the characteristics of fluid flow and solute transport in rock fractures were investigated through experiments and simulation. The surface topography of these fractures was reconstructed using 3D laser scanning technology. The solute transport in crossed parallel plates and rough fractures with intersecting angles of 30°, 60°, 90°, 120°, and 150° was investigated under different initial conditions. Results show that the intersection angle and fluid velocity significantly affect the fluid flow pattern at the intersection. As the flow velocity and intersection angle increase, streamlines at the intersection of rough fractures are unevenly distributed. The fluid tends to seep into the dominant channel and has different degrees of channeling flow at the intersection of fractures. An eddy phenomenon is observed at the intersection, indicating that the seepage behavior transforms into a non-Darcy flow. As the Peclet number (Pe) increases, solute migration and diffusion gradually decrease; however, the convection part gradually increases. Thus, the solute transport mode gradually changes into a streamline path mode at the intersection of fractures.

EFFECT OF SOIL TREATMENT ON CESIUM-137 ACCUMULATION IN FLOWER POLLEN AND IN ITS PROCESSING PRODUCTS BY HONEY BEES

Over the past few decades, the level of radiation has significantly increased due to artificial sources – radionuclides. In particular, the Chernobyl accident led to the release of about 50 million Кі of various types of radionuclides into the environment. As a result of the accident at the Chernobyl nuclear power plant in Ukraine, a large number of territories of the Zhytomyr region are polluted – 50%, Kiev – 26%. About 26% of the contaminated area falls on the Chernigov, Rivne, Sumy and Volyn regions. The radioactive substances that were part of the nuclear fuel were concentrated in the environment, from where they partially migrate along the soil-plant-living organisms chain, causing a number of negative changes in them. Immediately after the Chernobyl accident, iodine-131 posed a great danger to living organisms; its radioactivity in the contaminated area was 7.3 MКi. Iodine-131 is a beta and gamma emitter with a half-life of 8.04 days. This element is characterized by high activity in the soil-plant-production-organism system. It enters the body by the oral route and accumulates mainly in the thyroid gland. At the same time, cesium-134, cesium-137 and strontium-90 are also dangerous for living organisms, the share of which is about 3% of the total radioactivity. All other radionuclides that were released into the environment during the Chernobyl accident are less dangerous due to the low rate of entry into living organisms. Cesium-137 has a half-life of 30.2 years and is absorbed into the body in three ways: percutaneous, aerial and oral. The largest proportion of cesium-137 is ingested orally. It is a chemical analogue of potassium and is actively involved in the metabolic process. Therefore, it can accumulate in high amounts in body tissues. About 10% of cesium-137 is excreted from the body with indigestible food residues, and the vast majority - with urine. Cesium-137, penetrating into the body, is concentrated mainly in muscle tissue, and comparatively less in bone tissue. It is excreted from bone tissue much more slowly than from muscle tissue. Now in the environment there is still about 50% of cesium-137, which got into the environment as a result of the Chernobyl accident. Radionuclides in the soil are in a moving state, therefore they move to certain layers of it. The speed of such movement of radionuclides in the soil depends on its properties, the content of mineral and organic substances. In particular, it was revealed that the migration of cesium-137 on mineral sod-podzolic soils is ten times lower than on peat and peat-bog soils. This is typical of the soil of the ecological zone of Polesie. The composition of the soils of this zone includes no more than 1.0% of clay, 0.8-1.2% of humus, 3-5% of the silty fraction, causing a high migration of radionuclides. Cesium-137 in these soils is in the sod in a fast-moving form and rather intensively migrates into the vegetation. It has been proven that the mineral part and humus firmly fix cesium-137. The accident resulted in about 1,500,000 pollution in Ukraine. Hectares of forests, most of which have lost their practical value and are withdrawn from use. It has been established that the main amount of radionuclides is concentrated in the upper five-centimeter soil layer and in the forest litter, which is characteristic of pine forests. In pine forest stands, 30-60% strontium-90 and 40-80% cesium-137 are concentrated in the litter, and about a third of radionuclides are concentrated in deciduous forests. Most radionuclides enter plants by the root route. Some authors report that sometimes plants accumulate significantly more radionuclides, even with a lower content of them in the soil. This trend is observed in the Polesie of Ukraine. Over the past 15 years, the strontium-90 content in plant products has not changed significantly. Researchers have studied the peculiarities of the accumulation of radioactive substances by melliferous plants, which can actively accumulate cesium-137. Plants of the family Rosaceae, buckwheat, buttercups are classified as active accumulators of radiocesium. A high content of strontium-90 is characterized by silver cinquefoil, caustic buttercup, and such honey plants as red and white clover, alfalfa, horned lily, sainfoin and mouse peas, common heather, blueberries intensively accumulate both cesium-137 and strontium-90. The intensity of accumulation of cesium-137 in bee pollen, perge and homogenate of drone larvae produced by bees from pollen of winter rape, sunflower and buckwheat in different ways was studied.

A complex network theoretic approach for interdependence investigation: An application to radionuclide behavior in the subsurface

The failure of uranium mine tailings dams results in the infiltration and spreading of tailings in the subsurface. The fate and transport of radionuclides in the subsurface depends on several confounding, complex interdependent factors that describe the elements of the integrated system (i.e., meteorological; hydrological; hydrogeological; and, soil, groundwater, and mine tailings chemistry). The factors describing the integrated system have typically been investigated independently; however, their interdependence and resulting collective influence on the subsurface migration of radionuclides are yet to be explored. The current study develops a complex network theoretical approach to analyze these interdependencies. In this respect, a network of factors (NoF) was developed, and its characteristics (e.g., diameter, density, characteristic pathlength, average clustering coefficient, and factor centrality measures) were evaluated to determine the importance of considering these interdependencies when developing radionuclide fate and transport models. A sensitivity analysis was subsequently performed on the NoF to characterize the propagation of uncertainty associated with the factors in the NoF through a fate and transport model. The sensitivity analysis indicated that microorganisms present in the soil and mine tailings, fraction of organic carbon in the soil matrix, infiltration, and transmissivity must be well characterized (i.e., to minimize their uncertainty) when developing an integrated subsurface radionuclide fate and transport model, as uncertainty in these parameters will be amplified in the model output. The NoF developed in this study can be used to allocate data collection resources strategically in order to minimizing uncertainty in fate and transport models. This improves the reliability of fate and transport models, and ultimately leads to better management and remediation strategies to mitigate impacts from UMTD failures.

Sorption Experiments with HTO, 36-Cl, 125-I and 14-C Labeled Formate on Aged Cement Matrices Retrieved from Long-term In-situ Rock Laboratory Experiments

The long-term performance of a cement-based barrier system, envisaged worldwide in many concepts for deep geological storage of radioactive waste, depends on its material properties and how they evolve with time. Chemical interactions with the service environment may lead to mineralogical alterations and related physico-chemical changes at material interfaces, influencing radionuclide migration. Predictive (reactive) transport modelling therefore requires information on transport properties (e.g. sorption properties, porosity) of aged cement matrices.

Partially open grain and phase boundaries as fluid pathways in metamorphic and magmatic rocks

AbstractTransmission electron microscopy and 3D focused ion beam/scanning electron microscope nanotomography are applied to grain and phase boundaries between quartz, plagioclase, K‐feldspar, clinopyroxene, amphibole, and calcite. The samples come from metamorphic, plutonic and volcanic rocks, and hydrothermal quartz, and experienced cooling and decompression after highly variable P–T peak conditions. Most of the boundaries are partially open in the range of up to several hundred nanometres and partly to totally filled with secondary minerals, such as actinolite, biotite, chlorite, sheet silicates, and quartz, as well as with amorphous matter. Cracking and opening of boundaries are suggested to be related to anisotropic thermoelastic response of crystals to cooling. It starts below the brittle–ductile transition of the involved minerals. The partially open grain and phase boundaries, together with dissolution‐generated cavities, can form porosity of more than 2 vol.% and permeability under conditions of at least lowermost greenschist facies. Such networks of partially open or partially refilled boundaries potentially affect properties of crystalline rocks and processes in the upper crust, such as metasomatism, weathering, migration of radionuclides through bedrock of geological repositories of nuclear waste, and deformation in nature and in experiment.

U- and Th- series disequilibria in separated soil mineral fractions: Insight into the mechanism and timescale of U, Th and Ra redistribution

In recent decades, U- and Th- series nuclides have been successfully used to determine weathering rates in soils and regoliths. However, the redistribution of the nuclides within secondary minerals and/or organic matter may interfere with the recorded U- and Th-series weathering signals and can result in inaccurate and/or poorly constrained weathering rate determinations. In addition, the migration of radionuclides in the environment represents a major current issue. The objective of this study, therefore, was to evaluate the meaning of bulk soil data for weathering investigations and to assess the processes and timescales of U, Th and Ra redistribution in a forested-soil profile, both at the bulk soil scale level and for separated mineral fractions. This study was carried out at the INRAE-Andra experimental beech-forested site of Montiers (Meuse, France), which was developed on acidic detrital sediments inherited from tropical paleosoils erosion. U- and Th-series activity nuclides were measured in bulk soil samples and in separated Fe nodules and clay-size (<2 μm) fractions. Furthermore, the analyses of atmospheric particles allow discussing the potential influence of atmospheric inputs.While bulk soil disequilibria suggested negligible to moderate mobility of the nuclides, an important transfer between mineral fractions was highlighted; this resulted in contrasting redistributions of U, Th and Ra within soil fractions. Fe-oxide nodules controlled the overall mobility of U on the bulk soil scale, while the fine <2 μm fraction controlled the mobility of Ra. A capture of 234U by Fe-oxide nodules (mainly comprised of goethite) was observed within the entire soil profile, most likely occurring for several hundred ky. In the shallow soil layers, a leaching of U from the <2 μm fraction that was not re-adsorbed onto Fe-oxides occurred. In contrast, Fe-oxides appeared to retain all U mobilized from the <2 μm fraction in the deep soil layers, resulting in negligible U mobility and an apparent “closed-system” at the bulk soil scale. This transfer, however, seemed to occur over shorter timescale than the one in the shallowest soil layers. No mobility of 226Ra was observed at the bulk soil scale, despite the large release of this nuclide from Fe-oxides. The measured (210Pb/226Ra) activity ratios suggested that up to 50% of 226Ra in Fe-oxides is available for desorption. However, the <2 μm fraction strongly readsorbed the released Ra and prevented the loss of this nuclide at the bulk soil scale. Considering the short half-life of 226Ra, such transfer from Fe-oxides to the <2 μm fraction occurred over a significantly shorter timescale than U mobilization. In addition, the analysis of the short-lived 228Ra isotope emphasized the flux involved in the biological cycle and the potential impact of vegetation on the soil radionuclide redistribution over a short timescale.Our results suggest that the fine <2 μm fractions might be suitable for probing the residence time of clay minerals in soils by the U-series dating method, if the soil profile can be assumed to consist of homogenous inherited clays.

Radiological Assessment of Environmental Impact of the IF-System Facility of the RAON

Background: The evaluation of skyshine distribution, release of airborne radioactive nuclides, and soil activation and groundwater migration were required for radiological assessment of the impact on the environment surrounding In-Flight (IF)-system facility of the RAON (Rare isotope Accelerator complex for ON-line experiment) accelerator complex.Materials and Methods: Monte Carlo simulation by MCNPX code was used for evaluation of skyshine and activation analysis for air and soil. The concentration model was applied in the estimation of the groundwater migration of radionuclides in soil.Results and Discussion: The skyshine dose rates at 1 km from the facility were evaluated as 1.62 × 10&lt;sup&gt;-3&lt;/sup&gt; μSv·hr&lt;sup&gt;-1&lt;/sup&gt;. The annual releases of &lt;sup&gt;3&lt;/sup&gt;H and &lt;sup&gt;14&lt;/sup&gt;C were calculated as 9.62 × 10&lt;sup&gt;-5&lt;/sup&gt; mg and 1.19 × 10&lt;sup&gt;-1&lt;/sup&gt; mg, respectively. The concentrations of &lt;sup&gt;3&lt;/sup&gt;H and &lt;sup&gt;22&lt;/sup&gt;Na in drinking water were estimated as 1.22 × 10&lt;sup&gt;-1&lt;/sup&gt; Bq·cm&lt;sup&gt;-3&lt;/sup&gt; and 8.25 × 10&lt;sup&gt;-3&lt;/sup&gt; Bq·cm&lt;sup&gt;-3&lt;/sup&gt;, respectively.Conclusion: Radiological assessment of environmental impact on the IF-facility of RAON was performed through evaluation of skyshine dose distribution, evaluation of annual emission of long-lived radionuclides in the air and estimation of soil activation and groundwater migration of radionuclides. As a result, much lower exposure than the limit value for the public, 1 mSv·yr&lt;sup&gt;-1&lt;/sup&gt;, is expected during operation of the IF-facility.

A Review of Geochemical Modeling for the Performance Assessment of Radioactive Waste Disposal in a Subsurface System

Radionuclides are inorganic substances, and the solubility of inorganic substances is a major factor affecting the disposal of radioactive waste and the release of concentrations of radionuclides. The degree of solubility determines whether a nuclide source migrates to the far field of a radioactive waste disposal site. Therefore, the most effective method for retarding radionuclide migration is to reduce the radionuclide solubility in the aqueous geochemical environment of subsurface systems. In order to assess the performance of disposal facilities, thermodynamic data regarding nuclides in water–rock systems and minerals in geochemical environments are required; the results obtained from the analysis of these data can provide a strong scientific basis for maintaining safety performance to support nuclear waste management. The pH, Eh and time ranges in the environments of disposal sites cannot be controlled, in contrast to those under experimental conditions in laboratories. Using a hypothetical error mechanism for the safety assessment of disposal sites may engender incorrect assessment results. Studies have focused on radionuclide reactions in waste disposal, and have offered evidence suggesting that these reactions are mainly affected by the geochemical environment. However, studies have not examined the thermodynamics of chemical reactions or interactions between water and minerals, such as the surface complexation and adsorption of various nuclide-ion species. Simple coefficient models have usually been applied in order to obtain empirical formulas for deriving Kd to describe nuclide distributions in the solid or liquid phase in water–rock geochemical systems. Accordingly, this study reviewed previous research on the applications of geochemical models, including studies on the development of geochemical models, sources of thermodynamic databases (TDBs) and their applications in programs, the determination of the adequacy of TDBs in surface complexation models and case studies, and the selection and application of activity coefficient equations in geochemical models. In addition, the study conducted case studies and comparisons of the activity coefficients derived by different geochemical models. Three activity coefficient equations, namely the Davies, modified Debye–Hückel, and Pitzer equations, and four geochemical models, namely PHREEQC, MINEQL+, MINTEQA2, and EQ3/6, were used in the study. The results demonstrated that when the solution’s ionic strength was &lt;0.5 m, the differences in the activity coefficients between the Davies and modified Debye–Hückel equations were &lt;5%. The difference between the Pitzer and Davies equations, or between the Pitzer and modified Debye–Hückel equations in terms of the calculated activity coefficients was &lt;8%. The effect of temperature on the activity coefficient slightly influenced the modeling outputs of the Davies and modified Debye–Hückel equations. In the future, the probability distribution and uncertainty of parameters of Kd and the equilibrium constant can be used in geochemical and reactive transport models to simulate the long-term safety of nuclear waste disposal sites. The findings of this study can provide a strong scientific basis for conducting safety assessments of nuclear waste disposal repositories and developing environmental management or remediation schemes to control sites marred by near-surface contamination.

Diffusion of tritiated water, 137Cs+, and 125I− in compacted Ca-montmorillonite: Experimental and modeling approaches

Mechanistic understanding and predictive modeling of radionuclide diffusion in Na- and Ca-montmorillonite are essential to evaluate the long-term evolution of the bentonite barrier and their impact on radionuclide migration during geological disposal of radioactive wastes. Thus, the diffusion behavior of 137Cs+, 125I−, and tritiated water (HTO) in compacted Ca-montmorillonite was investigated as a function of porewater salinity and dry density via both experiments and models. The effective diffusion coefficient (De) followed in the order of 137Cs+ > HTO > 125I−. The De of 137Cs+ decreased with increasing salinity, whereas the dependence of De of 125I− on salinity was uncertain. The cation excess and anion exclusion effects for Ca-montmorillonite were lower than those for Na-montmorillonite. The integrated sorption and diffusion (ISD) model, assuming the homogeneous pore structure and the electrical double layer (EDL) theory for 2:1 electrolyte (CaCl2), could account for the observed trends for De in Ca-montmorillonite. The lower dependence of De on the porewater salinity in Ca-montmorillonite was caused by the reduction of the EDL thickness for divalent cations (Ca2+) in comparison with that for monovalent cations (Na+). The multipore model could improve the fit for De of 125I− at low salinity due to the reduction of interlayer pore volumes and anion exclusion effect, however, the disparity at higher densities was considerably larger. From these results, cation diffusion for compacted Ca-montmorillonite could be mainly explained by the electrostatic interactions in the homogeneous pore model; in contrast, anion diffusion was sensitive to both electrostatic interactions and heterogeneous pore structures. The proposed ISD model is an effective tool to evaluate the radionuclide diffusion and sorption behavior in both compacted Ca-montmorillonite and Na-montmorillonite.

The adsorption of U(VI) on magnetite, ferrihydrite and goethite

During the long-term disposal of radioactive waste, ferrous waste containers and building materials are tending to corrode to produce a large number of iron oxides. The formed iron oxides would bring significant effects on the diffusion and migration of radionuclides through adsorption and reduction. This study aimed to evaluate the interaction of U(VI) on iron oxides under anaerobic conditions, where goethite, magnetite, and ferrihydrite were used as adsorbents. The results showed that the adsorption of U(VI) on iron oxides was chemical adsorption, and the adsorption capacity of U(VI) on ferrihydrite was much higher than that on goethite and magnetite. Humic acid (HA) promoted the adsorption of U(VI) on iron oxides under acidic conditions, while U(VI) adsorption was inhibited by HA in the neutral to alkaline conditions. The adsorption of U(VI) on iron oxides was an endothermic process, and higher temperature was beneficial for the adsorption. Surface complexation modeling (SCM) fitting revealed that monodentate inner complexes of ≡ FeOUO 2 + and ≡ FeOUO 2 ( OH ) 0 were mainly formed on the surface of iron oxides. Finally, the reduction of U(VI) to U(IV) on magnetite under anaerobic condition was confirmed by X-ray photoelectron spectroscopy (XPS) and Mössbauer spectroscopy. • Interaction between U(VI) and iron oxides were studied under anaerobic condition. • Ferrihydrite has higher affinity for U(VI) than goethite and magnetite. • U(VI) was reduced to U(IV) on magnetite after reaction for long time.

Computer prediction of the spatial distribution of the Cs-137 concentration in soil

Computer prediction of the migration of radioactive contaminants in soil is a promising scientific direction due to the fact that prediction of the radiation situation on the basis of experimental measurements of radionuclides activity requires the use of mathematical and computer methods of data processing. At the same time, most of the specialized software packages for predicting the spread of radioactive contaminants in soil are based on one- and two-dimensional idealizations of this process, while solving the problem of comprehensive assessment of the biosphere’s state requires the data of the contaminant’s spatial distribution. To obtain such data, the author has developed a software module as a part of SPS (Simulation of Processes in Soil) v2.0 software package, which allows predicting the parameters of the spatial migration of radionuclides in natural dispersed media. To assess the possibility of using the developed software in practice, the prediction results were verified by the experimental data. For this purpose, the spatial distribution of the Cs-137 concentration in the experimental area in the Luninets district of the Brest region was predicted using the measurement data of the Cs-137 activities and the soil characteristics. The comparison of the results obtained on the basis of experimental measurements and the predicted values of the spatial distribution of the Cs-137 concentration showed that the predicted values are within the confidence intervals of the measured values. Consequently, the developed software can be used to solve practical problems in the field of forecasting the migration of radionuclides with obtaining a reliable picture of their distribution over the volume of the modeling area and to comprehensively assess their impact on the environment.

Biomass and salt-dependent effects of Bacillus spores on radionuclide migration from the Waste Isolation Pilot Plant

Spores of a Bacillus sp., isolated from radioactive waste, were tested for their ability to influence the fate and transport of neodymium (Nd3+) under high salt conditions expected at the Waste Isolation Pilot Plant (WIPP) nuclear waste repository. Spores were suspended in neodymium-spiked saline solutions up to 4 M NaCl, and concentrations of Nd and the complexing agent dipicolinic acid (DPA), a component of spores, were monitored along with optical densities and spore numbers. Results support neodymium bioassociation that is dependent upon biomass, with more apparent adsorption occurring at higher spore concentrations. However, probable spore lysis in 2 and 4 M NaCl solutions and possible germination at 0.15 M NaCl appear to drive the release of DPA and subsequent return of Nd to solution. The implications of this work for the WIPP will depend on actual biomass levels and the ionic strength of the repository brines.

Study of cesium sorption by a Brazilian bentonite

Engineered and natural barriers compose the disposal system of near surface repository to store low and intermediate level radioactive wastes. Waterproofing barriers are constituted of clays, such as the bentonite, in order to avoid or to limit the release of radionuclides to with consequences to the environment and humans. Characteristics such as high cation exchange capacity, high adsorption capacity and high sealing capacity, are taken into consideration for choosing this material. The migration of radionuclides through protection barriers can occur in a variety of ways, such as surface water infiltration, groundwater intrusion, among others. Diffusion is the process which governs the contaminant transport through soil barriers. Adsorption is one of the processes that could be considered in the diffusion mechanism, and sorption isotherms are obtained from values measured in the batch-adsorption experiments. Therefore, such experiments were carried out in order to estimate the cesium sorption by the bentonite. The quantitative analysis was performed using atomic absorption and the energy dispersive X-ray fluorescence spectrometry. With the purpose to calculate the true value of cesium sorption, a mass-balance was made considering all the steps of the batch experiment, and it was found a 6% loss in the whole process. The quantity of cesium sorbed by the bentonite was 66.7 mg.g-1. At the moment, eight additional experiments are being performed, using solutions with different cesium concentrations, leading to all the necessary data for the isotherm.