Intermediate Level Radioactive Waste Repository Research Articles

Study of caesium adsorption onto alluvial sediments from the Italian Po Plain

AbstractThe study investigates the adsorption processes of caesium onto alluvial sediments from the Po Plain (northern Italy). Understanding these adsorption processes is crucial for assessing the safety of low- and intermediate-level radioactive waste repositories, including the proposed Italian repository. Adsorption kinetics and equilibrium experiments on sandy samples were conducted with the aim of evaluating how even small differences in clay content and mineralogy can affect kinetics and equilibrium adsorption behaviour. The obtained data were compared with literature studies and confirmed the significant affinity of caesium for sandy sediments, even for a mud content of less than 5%. Kinetics analysis revealed that a pseudo-second-order model best described the process, suggesting two-site occupancy adsorption kinetics attributed to the presence of illite and characterised by various different sites for caesium adsorption. Samples with higher clay and micaceous minerals content, cation exchange capacity and specific surface area exhibit faster kinetics and higher affinity for caesium. The study shows a significant variation in partition coefficient values, ranging from 57 to 750 mg L−1. This finding emphasises the importance of sediment composition in caesium adsorption, which is crucial for developing accurate environmental protection and safety assessment models.

Effect of supplementary cementitious materials on the degradation of cement-based barriers in radioactive waste repository: A case study in Korea

This study focuses on investigating the chemical degradation characteristics of cementitious barriers used in low- and intermediate-level radioactive waste repository by reactive transport modeling. The impact of the blending with supplementary cementitious materials (SCMs) in the barriers on the chemical degradation was evaluated to find the optimum barrier design. A number of different barrier designs were examined by replacing ordinary Portland cement (OPC) by SCMs (i.e., fly ash, silica fume, and blast-furnace slag). The simulation results showed that silica fume blended barrier has better durability against chemical degradation by rainwater compared to fly ash or blast-furnace slag blended barriers. In addition, the chemical durability of silica fume-based barrier increased with increasing replacement level up to about 20 %. It seems that the amount of formed calcium silicate hydrate (CSH) in the initial cement-based barrier highly affects the overall chemical durability. The newly developed reactive transport model demonstrated its capability for understanding the barrier performance and investigating the optimal design of the barrier system.

Sorption Behavior of Radionuclides on Engineered and Natural Barriers and Prediction of Sorption Distribution Coefficients Using Support Vector Regression

A low- and intermediate-level radioactive waste repository contains various types of radionuclides and organic complexing agents. Their chemical interaction within the repository can lead to the formation of radionuclide-ligand complexes, influencing the limited retention behaviors of radionuclides. This study focuses on the sorption behavior of radionuclides on both engineered (concrete) and natural barriers (sedimentary rock and granite), as well as the prediction of sorption distribution coefficients (Kd) using support vector regression. Batch studies were conducted to determine the K d values for three radionuclides (99Tc, 137Cs, and 238U) under different conditions, including pH, temperature, and the presence of organic ligands (such as ethylenediaminetetraacetic acid, nitrilotriacetic acid, and isosaccharinic acid). The K d values for 238U exhibited a sharp decrease with increasing concentrations of organic ligands. In contrast, the K d values for 99Tc showed only a slight reduction at higher organic ligand concentrations. Meanwhile, the K d values for 137Cs remained relatively unchanged, regardless of the type and initial concentration of organic ligands. This suggests a high level of retention for 137Cs in the rock samples. The support vector regression model with a radial basis kernel function proved effective in predicting the K d values under different experimental conditions. This enhancement in predicting accuracy contributes valuable insights into understanding the sorption processes involved in radionuclide behavior. Overall, this study advances our knowledge of radionuclide behavior on both engineered and natural barriers while providing a reliable prediction tool for estimating sorption distribution coefficients.

Field experiment of cement-modified loess

The construction of a compacted and stabilized layer with local soil from the excavation, mixed with Portland cement, is a soil improvement technique widely applied in foundation works in collapsible loess ground in Bulgaria. Commonly, the role of that cement-modified layer is to replace a part of the collapsible ground, to increase the bearing capacity of the soil base, and/or to be an engineering barrier against migration of harmful substances in the geoenvironment. A multi-barrier near-surface short-lived low- and intermediate-level radioactive waste repository is under construction in Bulgaria. A cement-modified soil layer beneath the disposal cells is going to be built by in-situ compacted mixture of local loess and Portland cement. The cement-modified layer (indicated as loess-cement cushion) is not a continuation of the foundation, but it is a part of the soil base and performs two main functions: to be an engineering barrier against eventual migration of radionuclides in the geoenvironment and to increase the bearing capacity to restrict deferential settlement of the soil base. The present paper describes a field experiment aiming to verify the strength and deformation characteristics of a selected optimum loess–cement mixture by implementation of in-situ cement-modified loess ground. After 28-day curing at in-situ conditions, the loess-cement did not exhibit any fissuring or other disturbances. The allowable bearing capacity qa of the cement-modified loess ground exceeded 900 kN/m2, and it possessed the following strength and deformation characteristics: deformation (plate) modulus EPLT = 500 MPa; coefficient of sub-grade reaction ks = 2158 МPa/m, and unconfined compressive strength qu = 2.00 MPa.

Safety assessment of second-phase disposal facility in Gyeongju low- and intermediate-level radioactive waste (LILW) repository using RESRAD-OFFSITE code

ABSTRACT A safety assessment was performed on the second-phase disposal facility of the Gyeongju low – and intermediate-level radioactive waste repository using the RESRAD-OFFSITE code. On the basis of the acceptance criteria of the repository, 13 radionuclides and gross alpha were selected as the radionuclides to be evaluated. In addition, the thermodynamic database of Japan Atomic Energy Agency and natural groundwater data that reflect the operational condition of the disposal facility were applied to derive the solubility by using the PHREEQC computational code. The distribution coefficient was derived for the component of disposal facility based on the sorption database of Japan Atomic Energy Agency. In addition to the solubility and distribution coefficient for various radionuclides, the data of the disposal facility and repository site were analyzed and applied to the RESRAD-OFFSITE code for the safety assessment modeling. The result of the safety assessment calculation revealed the exposure dose corresponding to 3.8% of the exposure dose limit (0.1 mSv/yr). Moreover, additional sensitivity analysis for the exposure dose was conducted by varying the geochemical parameters. Although the exposure dose changed as the geochemical parameters were varied, the result of the sensitivity analysis confirmed that the exposure dose limit was still satisfied.

Site selection for Croatian low and intermediate level radioactive waste repository

ABSTRACT Radioactive waste disposal and management presents a unique problem at the Krško nuclear power plant as it is built and co-owned by the Republic of Croatia and the Republic of Slovenia. Each country is responsible for the management of half of the low- and intermediate-level radioactive waste (LILW) and spent nuclear fuel (SNF). So far, Slovenia has officially selected the site for LILW disposal and the Croatian programme has a single location left (since 1999), not officially approved as acceptable. This paper contains a new approach to site selection related to the implementation of GIS technology and presents a map of potential areas for LILW disposal including only the remaining Croatian location. It was constructed based on eleven layers that include natural characteristics and anthropogenic pressures. For the construction of the main map, detailed topological checks and spatial analysis of polygons have been made, reducing potential areas to be further evaluated.

Magnesium(II) d-Gluconate Complexes Relevant to Radioactive Waste Disposals: Metal-Ion-Induced Ligand Deprotonation or Ligand-Promoted Metal-Ion Hydrolysis?

The complexation equilibria between Mg2+ and d-gluconate (Gluc–) ions are of particular importance in modeling the chemical speciation in low- and intermediate-level radioactive waste repositories. NMR measurements and potentiometric titrations conducted at 25 °C and 4 M ionic strength revealed the formation of the MgGluc+, MgGlucOH0, MgGluc(OH)2–, and Mg3Gluc2(OH)40 complexes. The trinuclear species provides indirect evidence for the existence of multinuclear magnesium(II) hydroxido complexes, whose formation was proposed earlier but has not been confirmed yet. Additionally, speciation calculations demonstrated that MgCl2 can markedly decrease the solubility of thorium(IV) at low ligand concentrations. Regarding the structure of MgGluc+, both IR spectra and density functional theory (DFT) calculations indicate the monodentate coordination of Gluc–. By the potentiometric data, the acidity of the water molecules is higher in the MgGluc+ and MgGlucOH0 species than in the Mg(H2O)62+ aqua ion. On the basis of DFT calculations, this ligand-promoted hydrolysis is caused by strong hydrogen bonds forming between Gluc– and Mg(H2O)62+. Conversely, metal-ion-induced ligand deprotonation takes place in the case of calcium(II) complexes, giving rise to salient variations on the NMR spectra in a strongly alkaline medium.

Geotechnical parameters of loess-cement mixture for construction of compacted soil-cement cushion

Soil-cement cushions are compacted and stabilized layers of the soil base, built under the foundation. Usually, they are constructed with local soil from the excavation, mixed with Portland cement. In Bulgaria, this soil improvement technique has been applied in foundation works in collapsible loess ground, aiming to replace a part of the collapsible layer, to increase the bearing capacity of the soil base, and/or to play a role of engineering barrier against migration of harmful substances in the geoenvironment. A multi-barrier near-surface short-lived low- and intermediate-level radioactive waste repository is under construction in Bulgaria. A loess-cement cushion beneath repository cells is going to be built by in-situ compacted mixture of local loess and Portland cement. Based on the results from classification and physico-mechanical tests of a set of loess-cement mixtures, it was proposed optimum cement content of the loess-cement cushion beneath the radioactive waste repository to be 5% of Portland cement. The present paper aims to assess the following geotechnical parameters of the selected loess-cement mixture after proper curing: unconfined compressive and flexural strength; shear strength parameters; static and dynamic elastic constants; and hydraulic conductivity. The results obtained prove that the mixture prepared at Wopt and ρds of local loess and 5% (by the dry weight of soil) of Portland cement type CEM I 42.5 N – SR 5 possesses strength and deformation characteristics that completely meet the design stress-strain requirements to the soil-cement cushion beneath the repository foundation.

Sorption of plutonium to bacteria and fungi isolated from groundwater and clay samples

Preliminary studies of Pu(V) sorption to site-specific microorganisms isolated from the Triassic clay selected as an engineered barrier for the near surface low and intermediate level radioactive waste repository and from groundwater at the expected site were carried out. Adsorption of Pu(V) by bacteria attained the maxima values of 82–84 % at pH 6–7. The maximum adsorption values of Pu of about 100 % were found for Penicillium chrysogenum at pH 4.1. Bioreduction of Pu(V) to Pu(IV) by Bacillus cereus was observed, while any Pu(III) was not found in the system.

Effect of anthropogenic organic complexants on the solubility of Ni, Th, U(IV) and U(VI)

The influence of anthropogenic organic complexants (citrate, EDTA and DTPA from 0.005 to 0.1M) on the solubility of nickel(II), thorium(IV) and uranium (U(IV) and U(VI)) has been studied. Experiments were carried out in 95%-saturated Ca(OH)2 solutions, representing the high pH conditions anticipated in the near field of a cementitious intermediate level radioactive waste repository. Results showed that Ni(II) solubility increased by 2–4 orders of magnitude in the presence of EDTA and DTPA and from 3 to 4 orders of magnitude in the case of citrate. Citrate had the greatest effect on the solubility of Th(IV) and U(IV)/(VI). XRD and SEM analyses indicate that the precipitates are largely amorphous; only in the case of Ni(II), is there some evidence of incipient crystallinity, in the form of Ni(OH)2 (theophrastite). A study of the effect of calcium suggests that U(VI) and Ni(II) may form metal-citrate-OH complexes stabilised by Ca2+. Thermodynamic modelling underestimates the concentrations in solution in the presence of the ligands for all the elements considered here. Further investigation of the behaviour of organic ligands under hyperalkaline conditions is important because of the use of the thermodynamic constants in preparing the safety case for the geological disposal of radioactive wastes.

Radionuclide diffusion into undisturbed and altered crystalline rocks

AbstractAn extensive set of porosity, ε, effective diffusion coefficient, De, and hydraulic conductivity, K, data were obtained from 45 granitic samples from the Bohemian Massif, Czech Republic. The measured dataset can be used to define parameter ranges for data to be used in safety assessment calculations for a deep (>400 m) radioactive waste repository, even though the samples originated from shallower depths (<108 m). The dataset can also be used for other purposes, such as evaluating the migration of contaminants in granitic rock (e.g. from shallow intermediate-level radioactive waste repositories and chemical waste repositories).Sample relaxation and ageing processes should be taken into account in research otherwise migration parameters might be overestimated in comparisons between lab results and those determined in situ.

Impact of Water Consumption and Saturation-Dependent Corrosion Rate on Hydrogen Generation and Migration from an Intermediate-Level Radioactive Waste Repository

The corrosion of metals contained in intermediate-level long-lived waste (ILW) under reduced chemical condition will lead to the production of hydrogen gas during the post-closure phase of a deep geological repository for radioactive waste. According to previous investigations by Talandier et al. (Proceedings of TOUGH symposium 2006, Berkeley, 2006), the period of concern covers several 1,000 years after closure of a repository in a clay host rock (Callovo-Oxfordian). The limited hydrogen transport efficiency of the host rock will lead to significant saturation of the concrete waste canister pore space and voids with a gas phase and pressure build-up within the emplacement drifts. On the other hand, the water availability is limited as a result of (i) the low permeability of the clay host rock and (ii) the desaturation of the rock mass close to the drift wall due to the ventilation of the drifts during the operational phase of the repository. In former numerical simulations it was assumed that under the reducing chemical conditions prevailing in the repository, the corrosion rate would be a function of the available metal surface and the temperature only. In this paper, simulation results based on new phenomenological functions are presented, which were implemented in TOUGH2. These allow taking into account (i) a water saturation dependency of the hydrogen generation rate, (ii) the water consumption due to the corrosion process, and (iii) the total metal mass available for corrosion. The paper presents results of 1D radial and 2D vertical simulations of a typical cross-section of a waste emplacement drift and the surrounding rock mass. The interactions between water availability from the low permeable clay, the dependency of the hydrogen generation rates from the water saturation of the waste package and the hydrogen migration in the host rock are demonstrated.

중·저준위 방사성 폐기물 처분장의 지진위험도 평가를 위한 지반운동스펙트럼 산정

In this study, a ground motion response spectrum for the seismic risk assessment of low and intermediate level radioactive waste repositories was developed. For the development of the ground motion response spectrum, a probabilistic seismic hazard analysis (PSHA) was performed. Through the performance of a PSHA, a seismic hazard curve which was based on a seismic bed rock was developed. A uniform hazard spectrum was determined by using a developed seismic hazard curve. Artificial seismic motions were developed based on the uniform hazard spectrum. A seismic response analysis was performed on the developed artificial seismic motion. Finally, an evaluation response spectrum for the seismic risk assessment analysis of low and intermediate level radioactive waste repositories was developed.

An Experiment in Participative Environmental Decision Making

The paper presents a method of participatory decision-making involving the authorities at local level, experts and the public. It studies how to solve the problems caused by the heterogeneity of the parties taking part in the decision-making process. The method builds on mutual learning and consideration among the participants following a systematic approach led by a moderator. The method was tested at a workshop simulating a real-life situation. The test case studied at the workshop was the problem of identifying the best site for a low and intermediate level radioactive waste repository among three alternatives. The learning phase was the Krsko Nuclear Power Plant cooling problem. The results show that the participants were able to reach agreement on the prioritisation of the alternatives and to end up with suprisingly complex and fully reasonable decision models. We believe that this was achieved particularly due to the well defined procedure that guided the decision-making process, and the ability of all participants to see the influence of their views on the decision model.

Test case of the long term preliminary performance assessment for the L&IL radioactive waste repository Baita Bihor, Romania

The paper presents the assessment methodology developed in the framework of the long-term preliminary performance assessment of Baita Bihor Romanian national low-level and intermediate-level radioactive waste repository. Conservative assumptions, input parameters and tools used in this approach reflect limitations imposed by the current assessment capability, as well as limited knowledge and understanding of the disposal system. The resultant dose rates are presented and analysed. Conclusions are drawn relating to the safety of the repository.

The effect of sulfate-reducing bacteria on adsorption of 137Cs by soils from arid and tropical regions

Soils from different climatic regions of Australia were studied to determine their adsorption of 137Cs, and the effect of microbial sulfate reduction on this adsorption. The soils consisted of a surface and regolith samples from the site of a proposed low and intermediate level radioactive waste repository in arid South Australia, and two red earth loam soils from an experimental plot in the tropical Northern Territory. The process of bacterial sulfate reduction substantially decreased the adsorption of 137Cs to the arid and tropical soils, although extended incubation resulted in greater adsorption to the regolith sample. This could have implications for the mobility of radionuclides entering these soil ecosystems.

Coupled modeling of cement/claystone interactions and radionuclide migration

The interactions between cement and a clayey host-rock of an underground repository for intermediate-level radioactive waste are studied with the reactive transport code HYTEC for supporting performance assessment. Care is taken in using relevant time scales (100,000 years) and dimensions. Based on a literature review, three hypotheses are considered with respect to the mineralogical composition of the claystone and the neo-formed phases. In the long term, the pH is buffered for all hypotheses and important mineral transformations occur both in cement and the host-rock. The destruction of the primary minerals is localized close to the cement/claystone interface and is characterized by the precipitation of secondary phases with retention properties (illite, zeolite). However, beyond the zone of intense mineral transformations, the pore water chemistry is also disturbed over a dozen meters due to an attenuated but continuous flux of hydroxyl, potassium and calcium ions. Four interdependent mechanisms control the profile in the whole system: diffusion of the alkaline plume, mineralogical buffering, ion exchange and clogging of the pore space at the cement/claystone interface. The migration of a selected group of radionuclides (Cs, Ra, Tc and U) is explicitly integrated in the simulations of the strongly coupled system. Theoretical profiles of distribution coefficient (Kd) and solubility limit values are derived from the simulations, and their sensitivity with respect to the system evolution is estimated.

The potential effect of cementitious colloids on radionuclide mobilisation in a repository for radioactive waste

Colloid-facilitated transport of contaminants could enhance the release rate of radionuclides from the cementitious near field of a repository for radioactive waste. In the current design of the planned Swiss repository for intermediate-level radioactive waste, a gas-permeable mortar is employed as backfill material for the engineered barrier. The main components of the material are hardened cement paste (HCP) and quartz aggregates. The chemical condition in the backfill mortar is controlled by the highly alkaline cement pore water present in the large pore space. The interaction of pore water with the quartz aggregates is expected to be the main source for colloids. Colloid transport is facilitated due to the high porosity of the backfill mortar. Batch-type studies have been performed to generate colloidal material in systems containing crushed backfill mortar or quartz in contact with artificial cement pore water (ACW) at pH 13.3. The chemical composition of the colloidal material corresponds to that of calcium silicate hydrates (CSH). Batch flocculation tests show that, after about 20 days reaction time, the concentration of the CSH-type colloids is typically below 0.1 mg l −1 due to reduced colloid stability in ACW. Uptake studies with Cs(I), Sr(II) and Th(IV) on a CSH phase (initial C:S ratio=1.09) have been carried out to assess the sorption properties of the colloidal material. The influence of uptake by colloids on radionuclide mobilisation is expressed in terms of sorption reduction on the immobile phase (HCP). Sorption reduction factors can be estimated on the assumption that the sorption properties of the colloidal material are either similar to those of the CSH phase or HCP, and that sorption is linear and reversible. A scaling factor accounts for the higher specific surface area of the colloidal material compared to the CSH phase and HCP. At colloid concentration levels typically encountered in highly alkaline cement pore waters, colloid-induced sorption reduction is predicted to be negligibly small even for strongly sorbing radionuclides, such as Th(IV). Thus, no significant impact of cementitious colloids on radionuclide mobilisation in the porous backfill mortar is anticipated.

Reactive transport modeling of the interaction between a high-pH plume and a fractured marl: the case of Wellenberg

In the context of the proposed low- and intermediate-level radioactive waste repository at Wellenberg (Switzerland), calculations simulating the interaction between hyperalkaline solutions and a fractured marl, at 25 °C, have been performed. The aim of these calculations is to evaluate the possible effects of mineral dissolution and precipitation on porosity and permeability changes in such a fractured marl, and their impact on repository performance. Solute transport and chemical reaction are considered in both a high-permeability zone (fracture), where advection is important, and the wall rock, where diffusion is the dominant transport mechanism. The mineral reactions are promoted by the interaction between hyperalkaline solutions derived from the degradation of cement (a major component of the engineered barrier system in the repository) and the host rock. Both diffusive/dispersive and advective solute transport are taken into account in the calculations. Mineral reactions are described by kinetic rate laws. The fluid flow system under consideration is a two-dimensional porous medium (marl, 1% porosity), with a high-permeability zone simulating a fracture (10% porosity) crossing the domain. The dimensions of the domain are 6 m per 1 m, and the fracture width is 10 cm. The fluid flow field is updated during the course of the simulations. Permeabilities are updated according to Kozeny's equation. The composition of the solutions entering the domain is derived from modeling studies of the degradation of cement under the conditions at the proposed underground repository at Wellenberg. Two different cases have been considered in the calculations. These 2 cases are representative of 2 different stages in the process of degradation of cement (pH 13.5 and pH 12.5). In both cases, the flow velocity in the fracture diminishes with time, due to a decrease in porosity. This decrease in porosity is caused by the precipitation of calcite (replacement of dolomite by calcite) and other secondary minerals (brucite, sepiolite, analcime, natrolite, tobermorite). However, the decrease in porosity and flow velocity is much more pronounced in the lower pH case. The extent of the zone of mineral alteration along the fracture is also much more limited in the lower pH case. The reduction of porosity in the fractures would be highly beneficial for repository performance, since it would mean that the solutions coming from the repository and potentially carrying radionuclides in solution would have to flow through low-conductive rock before they would be able to get to higher-conductive features. The biggest uncertainty in the reaction rates used in the calculations arises from the surface areas of the primary minerals. Additional calculations making use of smaller surface areas have also been performed. The results show that the smaller surface areas (and therefore smaller reaction rates) result in a smaller reactivity of the system and smaller porosity changes.

Rock Mechanical Characterization of an Argillaceous Host Rock of a Potential Radioactive Waste Repository

The physical properties of foliated marls and shales from the Swiss Central Alps were investigated in the laboratory as part of the site characterization of a potential low and intermediate level radioactive waste repository. The rocks were first characterized by density, porosity and water content, then P-wave velocity measurements and uniaxial compression tests were conducted on samples oriented parallel, perpendicular and ∼45° inclined to the foliation. Correlations between P-wave velocities, Young's moduli, and uniaxial strengths and directional dependence of compressibility and permeability (determined at elevated hydrostatic pressures) revealed that two distinct families of microcracks cause the significant anisotropy. The pressure and time dependence of strength was then measured in triaxial compression, creep, and relaxation tests. In triaxial compression at room temperature, confining pressures up to 90 MPa and strain-rates as low as 10−8 s−1 failure occured after small inelastic strains. Failure strength exhibits a strong pressure and a weak strain-rate dependence. The low friction coefficient of clay minerals appears to dominate the bulk frictional properties. Micromechanical modeling of triaxial compression, creep, and relaxation tests suggests that inelastic deformation is accommodated by subcritical crack growth.