Sorbing Radionuclides Research Articles

Учет насыщения инженерных барьеров, коррозии стальных контейнеров, образования водорода и миграции в трещиноватых горных породах в оценках выхода слабо сорбируемых радионуклидов из ПГЗРО

The paper discusses the influence of bentonite saturation, package failure and water conductive fracture network on the transport of long-lived low sorbing radionuclides in geological disposal facilities (DGR). Based on the example of 129I, the study evaluates the influence of space and time dispersion of radionuclide transport and processes mentioned above on the long-term DGR safety. It demonstrates that these effects can cause significant radionuclide retention, nevertheless having minor impact on the long-term DGR safety as regards low sorbing nuclides with their half-lives amounting to over hundreds of thousand years. The paper also considers the opportunities for enhancing the GDF safety considering such radionuclides.

Modeling cesium migration through Opalinus clay: a benchmark for single- and multi-species sorption-diffusion models

Sophisticated modeling of the migration of sorbing radionuclides in compacted claystones is needed for supporting the safety analysis of deep geological repositories for radioactive waste, which requires robust modeling tools/codes. Here, a benchmark related to a long term laboratory scale diffusion experiment of cesium, a moderately sorbing radionuclide, through Opalinus clay is presented. The benchmark was performed with the following codes: CORE2DV5, Flotran, COMSOL Multiphysics, OpenGeoSys-GEM, MCOTAC and PHREEQC v.3. The migration setup was solved with two different conceptual models, i) a single-species model by using a look-up table for a cesium sorption isotherm and ii) a multi-species diffusion model including a complex mechanistic cesium sorption model. The calculations were performed for three different cesium boundary concentrations (10−3, 10−5, 10−7 mol / L) to investigate the models/codes capabilities taking into account the nonlinear sorption behavior of cesium. Generally, good agreement for both single- and multi-species benchmark concepts could be achieved, however, some discrepancies have been identified, especially near the boundaries, where code specific spatial (and time) discretization had to be improved to achieve better agreement at the expense of longer computation times. In addition, the benchmark exercise yielded useful information on code performance, setup options, input and output data management, and post processing options. Finally, the comparison of single-species and multi-species model concepts showed that the single-species approach yielded generally earlier breakthrough, because this approach accounts neither for cation exchange of Cs+ with K+ and Na+, nor K+ and Na+ diffusion in the pore water.

Upscaling of radionuclide transport and retention in crystalline rocks exhibiting micro-scale heterogeneity of the rock matrix

In different in-situ diffusion experiments carried out in fractured crystalline rocks, sorbing radionuclides have shown a behaviour that strongly differs from what is predicted by homogeneous-based models. Their breakthrough curves are in fact often characterised by a fast first-arrival and these radionuclides can penetrate surprisingly long distances deep into the matrix. The heterogeneous structure of mineral distribution and porosity geometry had been offered as an explanation for these discrepancies. Here, we use reactive transport simulations to investigate the effect of the sparse distribution of sorption sites on the breakthrough curves of sorbing radionuclides. At small scale, the computed breakthrough curves significantly differ from those predicted using homogeneous models. For instance, the early part of these curves does not show any clear separation with the corresponding part of the curve of a non-sorbing tracer and a long transition zone is observed, with a very smooth slope of the tailing. Two different upscaling strategies, aimed at propagating the signal of heterogeneous retention over larger scales, are proposed and demonstrated against independent solutions computed at intermediate scales. The upscaling strategies are also used to show that at large scales (e.g. the scale of interest in a safety assessment study for a deep geological repository for nuclear waste) the signature of mineralogical heterogeneity is smoothed out and the heterogeneous breakthrough curve is well approximated by a homogeneous solution where the radionuclide distribution coefficient for the pure mineral phase is scaled by the mineral volume fraction. However, the spatial persistence of the heterogeneous signature is significant when the sorbing mineral is present in a low amount.

Microtomography-based Inter-Granular Network for the simulation of radionuclide diffusion and sorption in a granitic rock

Field investigation studies, conducted in the context of safety analyses of deep geological repositories for nuclear waste, have pointed out that in fractured crystalline rocks sorbing radionuclides can diffuse surprisingly long distances deep into the intact rock matrix; i.e. much longer distances than those predicted by reactive transport models based on a homogeneous description of the properties of the rock matrix. Here, we focus on cesium diffusion and use detailed micro characterisation data, based on micro computed tomography, along with a grain-scale Inter-Granular Network model, to offer a plausible explanation for the anomalously long cesium penetration profiles observed in these in-situ experiments. The sparse distribution of chemically reactive grains (i.e. grains belonging to sorbing mineral phases) is shown to have a strong control on the diffusive patterns of sorbing radionuclides. The computed penetration profiles of cesium agree well with an analytical model based on two parallel diffusive pathways. This agreement, along with visual inspection of the spatial distribution of cesium concentration, indicates that for sorbing radionuclides the medium indeed behaves as a composite system, with most of the mass being retained close to the injection boundary and a non-negligible part diffusing faster along preferential diffusive pathways.

APPLICATION OF SURFACE COMPLEXATION AND ION EXCHANGE MODELLING FOR THE ESTIMATION OF BENTONITE SORPTION CAPACITY ON SELECTED RADIONUCLIDES UNDER POSSIBLE GEOLOGICAL DISPOSAL CONDITIONS IN LITHUANIA

Designing a repository for radioactive waste as a multi-barrier system with predefined multiple safety functions provides for the protection of people and environment from the harmful effect of the ionising radiation of radionuclides once released from high-level radioactive waste. The multi-barrier system is comprised of the engineered barrier system (EBS) and its surrounding natural environment. Because of the high retention capacity for the radionuclides, various forms of bentonite are usually considered as a part of the EBS in the design of radioactive waste repositories. Laboratory batch experiments with natural sediments illustrate that the sorption (and Kd values) is sensitive to aqueous chemical conditions, including the pH and the dissolved carbonate concentration as well as the mineralogy and surface area of the bentonite buffer. The cations considered for the study were chosen as covering a representative range of cations behaviour: from a weakly sorbed cation Cs(I) to moderately sorbed cation the Ni(II) and strongly sorbed cation, Eu(III). In this study, a potential retardation of sorbing radionuclides (Cs, Ni, Eu) as a function of aqueous chemical conditions in the groundwater of the Lithuanian crystalline basement contacting the bentonite material was estimated for the first time. The ion exchange and surface complexation models (SPNE/CE) and the computer program PHREEQC (USA) were applied for modelling the interaction within the radionuclide/water/bentonite system. The modelling of the Kd values under different pH conditions and a given partial pressure of carbon dioxide in the gas phase were performed. The highest value of Kd in the pCO2 open system and the strongest sorption dependence on pCO2 was estimated for Eu(I) among radionuclides being studied. There was estimated only a slight change in Kd values with respect to the temperature.DOI: http://dx.doi.org/10.5755/j01.ct.68.1.18875

Effect of transport-pathway simplifications on projected releases of radionuclides from a nuclear waste repository (Sweden)

The Swedish Nuclear Fuel and Waste Management Company has recently submitted an application for a license to construct a final repository for spent nuclear fuel, at approximately 500 m depth in crystalline bedrock. Migration pathways through the geosphere barrier are geometrically complex, with segments in fractured rock, deformation zones, backfilled tunnels, and near-surface soils. Several simplifications of these complex migration pathways were used in the assessments of repository performance that supported the license application. Specifically, in the geosphere transport calculations, radionuclide transport in soils and tunnels was neglected, and deformation zones were assumed to have transport characteristics of fractured rock. The effects of these simplifications on the projected performance of the geosphere barrier system are addressed. Geosphere performance is shown to be sensitive to how transport characteristics of deformation zones are conceptualized and incorporated into the model. Incorporation of advective groundwater travel time within backfilled tunnels reduces radiological dose from non-sorbing radionuclides such as I-129, while sorption in near-surface soils reduces radiological doses from sorbing radionuclides such as Ra-226. These results help quantify the degree to which geosphere performance was pessimistically assessed, and provide some guidance on how future studies to reduce uncertainty in geosphere performance may be focused.

Sorption reversibility kinetics in the ternary system radionuclide–bentonite colloids/nanoparticles–granite fracture filling material

Abstract The kinetics of radionuclide desorption from bentonite colloids and subsequent sorption onto fracture filling material can influence colloid-facilitated radionuclide migration in ground water. To shed light on the significance of these issues batch-type experiments using a cocktail of strong and weak sorbing radionuclides as well as FEBEX bentonite colloids in the presence of fracture filling material from Grimsel (Switzerland) under Grimsel ground water conditions have been conducted. Results show that tri- and tetravalent radionuclides, 232Th(IV), 242Pu(IV) and 243Am(III) are clearly colloid associated in contrast to 233U(VI), 237Np(V) and 99Tc(VII). Concentrations of colloid-borne 232Th(IV), 242Pu(IV) and 243Am(III) decrease after ∼100 h showing desorption from bentonite colloids while 233U(VI) and 99Tc(VII) concentrations remain constant over the entire experimental time of 7500 h thus showing no interaction either to colloids or to the fracture filling material. 232Th(IV) and 242Pu(IV) data yield a slower dissociation from colloids compared to 243Am(III) indicating stronger RN–colloid interaction. In the case of 237Np(V), a decrease in concentration after ∼300 h is observed which can be explained either by slow reduction to Np(IV) and subsequent sorption to mineral surfaces in accordance with the evolution of pe/pH and/or by a slow sorption onto the fracture filling material. No influence of the different fracture filling material size fractions (0.25–0.5 mm, 0.5–1 mm and 1–2 mm) can be observed implying reaction independence of the mineral surface area and mineralogical composition. The driving force of the observed metal ion desorption from colloids is binding to fracture filling material surfaces being in excess of the available colloid surface area (76:1, 55:1 and 44:1 for the 0.25–0.5 mm, 0.5–1 mm and 1–2 mm size fraction of the FFM, respectively).

Sorption of 14C, 99Tc, 137Cs, 90Sr, 63Ni, and 241Am onto a rock and a fracture-filling material from the Wolsong low- and intermediate-level radioactive waste repository, Gyeongju, Korea

Sorption experiments for radionuclides such as 14C, 99Tc, 137Cs, 90Sr, 63Ni, and 241Am were conducted using two different groundwaters (GM-1 and SS-5) and solid materials (granodiorite and fracture-filling material) sampled from the Wolsong low- and intermediate-level radioactive waste (LILW) repository, Gyeongju, Korea. The distribution coefficients of the radionuclides, Kd’s, were obtained and their sorption properties were discussed for each radionuclide. For all sorbing radionuclides, the Kd values for the fracture-filling material were observed to be higher than those for granodiorite regardless of the groundwater. The Kd values were increased in the sequence 99Tc < 14C < 90Sr < 137Cs < 63Ni < 241Am regardless of sorbent types implying that the sorption of radionuclides onto geological media is affected by their chemical behavior in accordance with geochemical environments. Anionic radionuclides such as 14C and 99Tc showed very low Kd values both for the granodiorite and fracture-filling material. The mineralogical composition of the geological media and groundwater conditions was also observed to be important in the sorption of sorbing radionuclides, especially in the case of strongly sorbing radionuclides.

Large-scale laboratory diffusion experiments in clay rocks

Clay formations are potential host rocks for high-level radioactive waste repositories. In clay materials, the radionuclide diffusion is the main transport mechanism, thus, the understanding of the diffusion processes and the determination of diffusion parameters under conditions as similar as possible to the real ones, are critical for the performance assessment of deep geological repository. A large-scale laboratory diffusion experiment in a large cylindrical block of consolidated clay is presented in this work, trying to approach to the real conditions. In this experimental set-up the radionuclides were mixed with the clay obtained from the drilling of a central hole in the large sample, and re-compacted into the hole at approximately the same density as the consolidated block, forming a tracer “solid source”. The system, that was sealed after the addition of the tracer, did not need additional treatment of the sample nor external monitoring. After the experimental time selected for diffusion to take place (estimated by scoping calculations), the block was sampled to obtain a 3D distribution of the tracer concentration. The modelling of the tracer distribution allowed the estimation of the diffusion parameters on different geometrical planes. The proposed methodology was tested in the Opalinus clay from Mont Terri (Switzerland) with preliminary diffusion tests of neutral (HTO), anionic ( 36Cl), and weakly sorbing ( 85Sr) radionuclides and very promising results were obtained. It is interesting noticing that such a configuration could be used as an “ in situ” methodology for diffusion tests without any modification.

A high-resolution abrasive method for determining diffusion profiles of sorbing radionuclides in dense argillaceous rocks

The diffusion of 134Cs + and 22Na + in Opalinus Clay (OPA) was studied by in-diffusion laboratory experiments. The diffusive tracer profiles in the rock were determined using a high-resolution abrasive peeling method. The radionuclide activities in the grinding swarf were measured directly via γ -spectrometry. By choosing the appropriate abrasive paper, a resolution down to 15 μm can be achieved. This is important when analysing strongly sorbing radionuclides such as tri- and tetravalent actinides that show steep, shallow diffusion profiles. In this study, a resolution between 20 and 90 μm was obtained which was sufficient for a good spatial resolution of the diffusion profiles. Both the effective diffusion coefficients and the distribution coefficients of the radionuclides could be determined by applying a single reservoir with decreasing source concentration analysis for a semi-infinite case. In the case of 22Na +, effective diffusion coefficients of D e = 2.0 × 1 0 - 11 m 2 s - 1 and D e = 1.5 × 1 0 - 11 m 2 s - 1 for Benken (Zürcher Weinland) OPA and Mont Terri OPA, respectively, were derived. The distribution coefficients were K d = 3.1 × 1 0 - 4 and 0.9×10 −4 m 3 kg −1, respectively. For 134Cs + the effective diffusion coefficients were higher, i.e. D e = 3.1 × 1 0 - 11 m 2 s - 1 for OPA from Benken and D e = 3.0 × 1 0 - 11 m 2 s - 1 for OPA from Mont Terri. The distribution coefficients determined were K d = 0.16 m 3 kg - 1 for Benken and 0.23 m 3 kg −1 for Mont Terri. Comparison of the data obtained for the weakly sorbing 22Na + with those from earlier through-diffusion experiments showed that there is good agreement between the two methods. In the case of 134Cs + such a comparison was not possible because through-diffusion data are not available. Because through-diffusion methods cannot be applied to strongly sorbing tracers in reasonable time periods, in-diffusion combined with high-resolution abrasive peeling offers an excellent alternative for measuring the diffusion properties of strongly sorbing tracers in dense argillaceous rocks.

Fracture flow and radionuclide transport in block-scale laboratory experiments

Summary Block-scale migration experiments were introduced to evaluate the simplified radionuclide transport concept used in assessing the safety of underground spent nuclear fuel repositories. The experiments were aimed to demonstrate visually the fracture flow, and to determine the hydraulic characteristics of a natural planar fracture and the transport behaviour of non-sorbing and sorbing radionuclides. For drill holes orthogonal to the fracture and equipped with injection or sealing packers flow rates in this study were measured as a function of hydraulic head. The outflow positions of water at each four side of the block were determined using uranine dye tracer. Tracer tests were performed using uranine, 99mTc and 22Na. Transport of a non-sorbing tracer through one of the flow channels was interpreted using an advection-dispersion model that on the generalised Taylor dispersion. Characterisation of the hydraulic properties of the fracture indicated that some drill holes were located in the region where the fracture was open and water conductive. No water conductivity was observed in two drill holes indicating closure of the fracture. Reasonably low flow rates obtained from three drill holes indicated their suitability for further radionuclide transport experiments. Elution times of technetium and uranine were fairly similar. Sodium was slightly retarded and was spread over a wider area than uranine and technetium. High water flow rates suggest that advective flow field dominated tracer transport. Experimental and calculated elution curves substantiate the suitability of our experimental set-up for further radionuclide transport experiments.

Mechanical interaction between swelling compacted clay and fractured rock, and the leaching of clay colloids

We consider the interaction between a saturated clay buffer layer and a fractured crystalline rock engineered disturbed zone. Once saturated, the clay extrudes into the available rock fractures, behaving as a compressible non-Newtonian fluid. We discuss the modelling implications of published experiments carried out in Sweden and more recently in Japan. Extrusion is halted either when the advancing clay front reaches a narrow enough aperture along the fracture (relative to the yield stress of the gel-like front), or when enough of the clay mass has extruded so as to reduce the density back in the buffer, and hence the swelling pressure (which is exponentially dependent upon density). In the latter case a relatively small reduction in density may be sufficient. As the clay extrudes, the gel-like front may be a source of clay colloids, being sufficiently hydrated so as to allow clay platelets to escape the matrix and diffuse away. We show that such mass loss is limited as a mechanism for leaching away the emplaced barrier, yet may still be significant in mobilising otherwise highly sorbing radionuclides within the buffer.

Diffusion of Cs and Sr in Compacted Bentonites Under Reducing Conditions and in the Presence of Corrosion Products of Iron

ABSTRACTIn high-level waste repositories, a carbon steel overpack will be corroded by consuming oxygen trapped in the repository after closure. Iron corrosion products are expected to interfere with migration of radionuclides by filling the pore in bentonite and sorbing radionuclides. In this study the apparent diffusion coefficients of cesium and strontium were measured in compacted Na-bentonites (Kunigel VI® and Kunipia F®, JAPAN) contacted with carbon steel and its corrosion products under reducing conditions or without carbon steel under oxidizing conditions for comparison. The apparent diffusion coefficients of cesium with and without corrosion product were 2.2 to 13 × 10−12m2/s. The apparent diffusion coefficients of strontium with and without corrosion product were 3.1 to 25 × 10−12m2/s. There were significant effects of dry density (0.8 to 2.0 g/cm3) and montmorillonite contents (50% for Kunigel V1 or 100% for Kunipia F). The presence of corrosion product decreased the apparent diffusion coefficients of Cs in both bentonites and that of Sr in Kunigel V1, especially at low dry density. This may be due to corrosion product filling the pore in the bentonite, decreasing the free pore size and density for diffusion.

Radionuclide Migration Studies on Tonalite

ABSTRACTMigration of water, chloride, sodium, and calcium in tonalite was studied, using dynamic column and static through-diffusion methods. Autoradiography of rocks impregnated with 14C-methylmethacrylate was introduced in order to determine the spatial porosity distribution, as well as to identify and visualize the migration pathways of non-sorbing radionuclides in tonalite matrix at the mm-cm scale. The migration routes of sorbing radionuclides and the sorptive minerals in tonalite were determined by autoradiographic methods, using 45Ca as a tracer. Transport of radionuclides was interpreted, using models for hydrodynamic dispersion with diffusion into the rock matrix. In tonalite, porous minerals were distributed homogeneously in matrix and, therefore, retardation capacity of the rock matrix was found to be high.