Geological Repositories For Radioactive Waste Research Articles

NIMBY施設の受容に対する補償の交換フレームの効果とTaboo trade-offs―高レベル放射性廃棄物地層処分場のシナリオ実験―

NIMBY施設の受容に対する補償の交換フレームの効果とTaboo trade-offs―高レベル放射性廃棄物地層処分場のシナリオ実験―

Comparison of models to evaluate microbial sulphide generation and transport in the near field of a SF/HLW repository in Opalinus Clay

In a deep geological repository for radioactive waste in the Opalinus Clay, disposal canisters may be corroded due to sulphide produced by sulphate reducing bacteria (SRB). This paper presents two computational models, a reactive transport model (RTM) and a simplified semi-analytical model (SM), to evaluate the potential of SRB to generate elevated sulphide concentrations, to assess sulphide fluxes to the canister and, in a simplified manner, the resulting canister corrosion.Calculations performed in the context of the repository's safety assessment based on a shared conceptual model reveal that the two computational models are complementary. The reactive transport model incorporates state-of-the-art understanding of the system's geochemistry, but is currently too computationally demanding to be applied in probabilistic safety assessment sensitivity analyses. The simplified model is fast and efficient, but some of its assumptions need to be verified, and some parameter values need to be calibrated using the more complete reactive transport model. Nonetheless, given the same set of assumptions, the two models predict comparable magnitudes of sulphide fluxes to the canister and comparable canister corrosion depths.

The shear strength of Opalinus Clay shale in the remoulded state

The Opalinus Clay shale formation is considered as a potential host geomaterial for the Swiss deep geological repository for radioactive waste. It presents different facies and it is characterised by a multi-scale heterogeneous composition, by a typical fissile structure with well-defined bedding planes and by anisotropic hydro-mechanical behaviour. This peculiar complexity makes it difficult to assign a unique set of geomechanical parameters to the material. This paper presents an experimental study aimed at characterising the lowest values of the shear strength parameters. In this sense, the shear behaviour was investigated on remoulded samples where the fabric and the diagenetic bonds of the intact material were eliminated. The results of a triaxial test campaign belonging to different facies of Opalinus Clay shale (“shaly” and “sandy” facies) are presented. Furthermore, with the aim to study the mechanical properties of fault zones in Opalinus Clay, the effect of large cumulated shear displacements on shear strength was also investigated. Ring shear tests were performed to determine the residual shear strength. The main geotechnical property which discriminates the different facies of Opalinus Clay is the grain-size distribution; in this sense, well-defined correlations between this intrinsic characteristic and shear strength angles of the remoulded material are presented.

A positivity-preserving nonlinear finite volume scheme for radionuclide transport calculations in geological radioactive waste repository

Purpose This paper aims to consider numerical simulation for radionuclide transport calculations in geological radioactive waste repository. Design/methodology/approach The nonlinear two-point flux approximation is used to discretize the diffusion flux and has a fixed stencil. The cell-vertex unknowns are applied to define the auxiliary unknowns and can be interpolated by the cell-centered unknowns. The approximation of convection flux is based on the second-order upwind method with a slope limiter. Findings Numerical results illustrate that the positivity-preserving is satisfied in solving this convection-diffusion system and has a second-order convergence rate on the distorted meshes. Originality/value A new positivity-preserving nonlinear finite volume scheme is proposed to simulate the far-field model used in the geological radioactive waste repository. Numerical results illustrate that the positivity-preserving is satisfied in solving this convection-diffusion system and has a second-order convergence rate on the distorted meshes.

Thermo-hydro-mechanical properties of bentonite-sand-graphite-polypropylene fiber mixtures as buffer materials for a high-level radioactive waste repository

In accordance with the conception of repository, the performance of buffer materials around the canister are critical to the long-term safe operation of deep geological repositories for high-level radioactive waste (HLW). To investigate the thermo-hydro-mechanical (THM) properties of bentonite-sand-graphite-polypropylene fiber (BSGF) mixtures as buffer material, the thermal and hydraulic conductivity, swelling and cracking, strength tests were conducted in this paper. From the microscopic, the heat transfer mechanism, penetration mechanism, swelling mechanism and shear mechanism of BSGF mixtures were investigated in detail. Results revealed that the addition of graphite and polypropylene fiber (PPF) can significantly improve thermal conductivity and strength of BSGF mixtures. Compared with bentonite-sand mixtures, the thermal conductivity, the unconfined compressive strength (UCS) and shear strength (cohesion) of BSGF mixtures increased by 20–68%, 12–23% and 21–48%, respectively. Nevertheless, the internal friction angle had little change. The addition of PPF controlled the width, area, and propagation of the cracks through the interlacing effect of the fibers. However, the addition of graphite and PPF can also hinder the development of swelling pressure and improve the hydraulic conductivity of BSGF mixtures. In addition, for BSGF mixtures used as buffer material of HLW storage, the sand content should be retained below 31% and 32% respectively to ensure the maximum swelling pressure and hydraulic conductivity to meet the requirements of the specification. From the view of heat dissipation and strength, sand content shall not exceed 30% and 27%, respectively. Therefore, the effective sand content should not exceed 27% considering the THM properties. This study provides a new buffer material for HLW deep geological repository and enhances the stability and functionality of HLW deep geological repository.

Experimental Study of Pyrite Oxidation at 100 °C: Implications for Deep Geological Radwaste Repository in Claystone

The oxidation of pyrite is one of the near field processes of the chemical evolution of clay rock planned to host a deep geological radioactive waste repository during operation. Indeed, this process can lead to transitory acidic conditions in the medium (i.e., production of sulphuric acid, carbonic acid) which may influence the corrosion kinetics of the carbon steel components of some disposal cells. In order to improve the geochemical modelling of the long-term disposal, the oxidation of pyrite in contact with clays and carbonates at 100 °C must be evaluated. In this study, special attention was paid to the pyrite oxidation rate thanks to an original experimental set-up, involving several pyrite/mineral mixtures and a reactor coupled to a micro gas chromatograph (PO2 and PCO2 monitoring). Although thermodynamic modelling expects that hematite is the most stable phase in a pure pyrite heated system (low pH), experiments show the formation of native sulfur as an intermediate product of the reaction. In the presence of calcite, the pH is neutralized and drives the lower reactivity of pyrite in the absence of native sulfur. The addition of clay phases or other detrital silicates from the claystone had no impact on pyrite oxidation rate. The discrepancies between experiments and thermodynamic modelling are explained by kinetic effects. Two laws were deduced at 100 °C. The first concerns a pure pyrite system, with the following law: r P y = 10 − 4.8 · P O 2 0.5 · t − 0.5 . The second concerns a pyrite/carbonates system: r P y + C a = 10 − 5.1 · P O 2 0.5 · t − 0.5 where PO2 corresponds to the partial pressure of O2 (in bar) and t is time in seconds. Different mechanisms are proposed to explain the evolution with time of the O2 consumption during pyrite oxidation: (i) decrease of the specific or reactive surface area after oxidation of fine grains of pyrite, (ii) decrease of O2 pressure, (iii) growing up of secondary minerals (Fe-oxides or anhydrite in the presence of calcium in the system) on the surface of pyrite limiting the access of O2 to the fresh surface of pyrite, and (iv) change in the pH of the solution.

Investigation of swelling pressure of bentonite/claystone mixture in the full range of bentonite fraction

MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture has been proposed as a sealing/backfilling material in a deep geological repository of radioactive waste in France. A good understanding of the swelling behaviour of this mixture is essential when evaluating the long-term performance of the repository. In this work, the swelling pressure of MX80 bentonite/COx claystone mixture was investigated by the constant-volume method for a full range of bentonite fraction. Results show that the swelling of claystone in the mixture can be inhibited by bentonite and its contribution to the global swelling pressure depends on the bentonite fraction. For the mixture with >70% bentonite, claystone behaves as an inert material, and its contribution to the global swelling pressure can be ignored. However, for the mixture with less bentonite, the swelling of claystone will significantly contribute to the global swelling pressure. A method was proposed allowing the swelling pressure of bentonite/claystone mixture to be predicted in the full range of bentonite fraction.

Mathematical Modelling of Fault Reactivation Induced by Water Injection

Faults in the host rock that might exist in the vicinity of deep geological repositories for radioactive waste, constitute potential enhanced pathways for radionuclide migration. Several processes might trigger pore pressure increases in the faults leading to fault failure and induced seismicity, and increase the faults’ permeability. In this research, we developed a mathematical model to simulate fault activation during an experiment of controlled water injection in a fault at the Mont-Terri Underground Research Laboratory in Switzerland. The effects of in-situ stress, fault shear strength parameters and heterogeneity are assessed. It was shown that the above factors are critical and need to be adequately characterized in order to predict the faults’ hydro-mechanical behaviour.

U(VI) sorption on Ca-bentonite at (hyper)alkaline conditions – Spectroscopic investigations of retention mechanisms

Environmental conditions in deep geological repositories for radioactive waste may involve high pH values due to the degradation of concrete. However, the U(VI) sorption at such (hyper)alkaline conditions is still poorly understood. In this study, batch sorption experiments with Ca-bentonite in the pH range 8–13 at different carbonate concentrations were combined with spectroscopic investigations in order to gain insight into the underlying retention mechanisms. It was found that U(VI) sorption strongly correlates with the aqueous U(VI) speciation determined by time-resolved laser-induced luminescence spectroscopy (TRLFS). Increasing retention with increasing pH was accompanied by a change in aqueous speciation from uranyl carbonates to uranyl hydroxides. The occurrence of luminescence line-narrowing and a decreased frequency of the symmetric stretch vibration, deduced from site-selective TRLFS, indicate the presence of adsorbed U(VI) surface complexes. X-ray absorption fine structure (EXAFS) spectroscopy confirms that surface precipitation does not contribute significantly to the removal of U(VI) from solution but that retention occurs through the formation of two non-equivalent U(VI)-complexes on the bentonite surface. The present study demonstrates that in alkaline environments, where often only precipitation processes are considered, adsorption can provide effective retention of U(VI), despite the anionic character of prevailing aqueous species.

Avoiding unrealistic behaviour in coupled reactive-transport simulations of cation exchange and mineral kinetics in clays

AbstractBentonite clay is often included as a buffer, backfill and/or sealing material in designs for deep geological repositories for radioactive wastes. It is expected that bentonite materials may undergo some mineralogical alteration as they interact with in situ groundwaters over long timescales on the order of thousands to millions of years. Long-term modelling of these materials is therefore important in order to demonstrate confidence that the engineered designs will continue to perform as required over their intended lifetimes (required assessment timescales can be up to 1 million years). The key geochemical processes that must be considered in such modelling are mineral dissolution and precipitation and cation exchange. These processes are expected to occur simultaneously and so modelling of their coupled effects and their rates (kinetics) is necessary. Illustrative reactive-transport models of the geochemical alteration of montmorillonite (the primary mineral in bentonite exhibiting cation exchange) are presented which demonstrate that one possible approach to fully coupling cation exchange and clay mineral dissolution kinetics, referred to here as the ‘all-component coupling’ approach, may lead to unrealistic behaviour due to feedback that may occur in the formulation. This feedback can be avoided if a ‘common-component’ conceptual model for the dissolution of exchanger end members is adopted, where only the saturation of the exchanger ‘structural unit’ is considered when evaluating the potential for dissolution of the mineral. Such considerations have been proposed historically in stability analyses for montmorillonite, but have not been explored widely in the modelling literature.

Effect of Cs and Sr separation on occupied area reduction in current nuclear energy system and its evaluation by CAERA index

ABSTRACTSeparation of Cs and Sr from high-level liquid waste (HLLW) contributes to reducing heat generation and correspondingly to minimizing the required use of space in geological disposal repositories for high-level radioactive waste. To maximize the impact of Cs and Sr separation on reducing the occupied area of a repository, it is necessary to consider both the front-end and back-end stages of the nuclear energy system. Therefore, we quantitatively surveyed factors that potentially affect the reduction in the occupied area – that is, the area required for disposal of one waste package – when introducing Cs and Sr separation. From an evaluation using the Comprehensive Analysis of Effects on Reduction of Disposal Area (CAERA) index, we found that in case of a 15-year cooling period of spent nuclear fuel (SNF) after discharge, which is the current operational condition at the Rokkasho Reprocessing Plant, a maximum reduction of 30% in occupied area was achieved for 25 and 30 wt% waste-loaded vitrified waste with Cs and Sr separation. In that case, separation of more than 70% of Cs and Sr was required based on the current reference case for the Japanese geological disposal concept (the so-called H12 Report). In addition, the effectiveness of Cs and Sr separation for reducing the occupied area was drastically decreased if the cooling period after discharge was extended beyond 15 years; for instance, vitrified waste with a more than 40-year cooling period showed almost no reduction in occupied area. This was because the amount of 241Am, which is generated from the beta decay of 241Pu, and the contribution of 241Am to heat generation in vitrified waste increased as the cooling period became longer. The results suggested that the cooling period of the SNF, the waste loading of the vitrified waste, the Cs and Sr separation ratio and the occupied area of the waste were key factors governing the effectiveness of Cs and Sr separation for geological disposal area reductions.

Economic assessment of the political and technical management of large-scale public projects. The deployment schedule of France’s deep geological repository of radioactive waste

In France, the 2006 act on radioactive waste management stipulated the rapid deployment of a deep geological repository (DGR) for high-level waste (HLW) management. This paper analyzes the impact of different DGR deployment schedules on project costs in order to discern an optimal schedule for the disposal of radioactive waste. It appears more economically favorable to extend the duration of storage rather than to dispose of the waste immediately. However, the French parliament has decided to act quickly, accepting a 1.5% excess cost every year, to avoid leaving the burden to future generations and to continue nuclear development. Secondly, the analysis reveals the economic benefits of shortening the duration of the disposal process.JEL classification: Q01, Q58, Q48.

A resistivity-based approach to determining the rates of groundwater seepage into buffer materials

Abstract Bentonite-based buffer materials play an important safety role in engineered barriers planned for use in geological disposal repositories for radioactive high-level waste (HLW) in Japan. The effectiveness of buffer materials is dependent on the status of groundwater saturation during resaturation of the repository. Accordingly, it is important to determine the behaviour of buffer materials during saturation and predict post-saturation conditions such as the distribution of residual dry density and chemical alteration. In this study, the rate of groundwater uptake into a buffer material was determined to clarify the behaviour of the material during the saturation process. As mechanical changes and chemical alteration of buffer materials are generated by groundwater permeation, knowledge of the water uptake rate is necessary for the prediction of post-permeation conditions. In the experiment reported here, one-dimensional permeation by distilled water and a NaCl water solution at a constant rate was monitored over a period of more than seven years. The results indicated that the seepage and saturation front moved in proportion to the square root of the seepage time. The coefficient of the relationships between the seepage and the saturation fronts with time of the reference bentonite used in Japan was determined.

Observations by Space Geodesy Methods on Objects of Using Atomic Energy

According to the existing international requirements, construction of an underground research laboratory (UGRL) that allows to obtain parameters of a host rock mass is a mandatory initial stage when siting a deep geological repository (DGR) for radioactive waste (RAW). The main idea of the basic international and Russian documents regulating the safety of handling high-level radioactive waste is that geological medium is the main barrier to the spread of radionuclides. The results of the world’s leading research in this area are directly related to the development of methods, algorithms and software modules for predicting the stability of a structural tectonic block containing waste material of an underground HLWR repository and located in the field of action of time-varying and spatially varying tectonic stress fields as well as the heat field from HLRW containers. The results of modeling and implementation of the geodynamic monitoring system based on the use of GPS/GLONASS satellite systems will be used as the basis for the design development of «Rosatom» organizations for the construction of URL, which is created in accordance with IAEA requirements to justify the suitability of the Nizhnekansky massif for underground isolation of radioactive waste. Below we consider the influence of the geodynamic regime of the territory the possible destruction of the rock at different hierarchical levels.

Monte Carlo analysis of montmorillonite particle structures and modeling of dissolution rate reduction

The equilibrium structure of montmorillonite clay particles was calculated using Monte Carlo simulation to evaluate the long-term behavior of alteration for bentonite buffer materials in geological repositories of radioactive waste. It has been experimentally observed that the dissolution rate of montmorillonite strongly decreases as the density increases. One of the primary causes of this reduction in dissolution rate is the reduction in reactive surface area when the edges of particles are covered with other particles (physical masking). In this study, we proposed a geometrical model in which the masking area (the edge surface masked by other particles) is calculated from the edge-to-face distance of particles. The effective edge surface area (ESA) from the equilibrium structure obtained from Monte Carlo simulation was computed using the masking model. The numerical results of the effective ESA are in good agreement with the experimental measurements of the dissolution rate by atomic force microscopy in a wide range of density conditions. Moreover, we propose a prediction expression of the effective ESA of montmorillonite particles over a wide range of density conditions, which produces physically reasonable predictions.

Mineralogy and organic geochemistry of phyllite from the Dewon–Pokrzywna deposit, the Opava Mountains (SW Poland)

Phyllites from the Dewon–Pokrzywna deposit in the Opava Mts., SW Poland, were investigated by XRD (Rietveld method), XRF, EPMA, SEM, and ATR-FTIR from the perspective of their potential usage as a buffer and/or backfill material in a geological repository of radioactive waste. Organic matter dispersed in the phyllite matrix was analysed by GC-MS. Fine-grained Mg-Fe-muscovite (13 to 29 wt.%), Fe-ripidolite (10 to 25 wt.%), detrital quartz (20 to 46 wt.%), and albite (7 to 28 wt.%) ± microcline, illite or illite/smectite, and kaolinite are major minerals in phyllite samples. The chlorite/muscovite ratio ranges from 0.65 to 1.1. Mg-annite inherited from the precursor rock is a minor constituent. Detrital ilmenite is a dominant accessory mineral. Ancylite-(Ce) occurs in quartz-calcite-ripidolite veins. Two types of phyllite have been distinguished based on the proportion of phyllosilicates to silt fraction: argillaceous (47 to 55 wt.% phyllosilicates) and silt-rich (28 wt.% phyllosilicates). Argillaceous phyllite shows elevated content of alumina and moderate concentration of silica. It is highly enriched in Fe compared to phyllites from other localities worldwide. The BET specific surface area of argillaceous phyllite ranges from 1.73 to 3.64 m 2 /g. Whole-rock chemical composition, mineral assemblages, chlorite geothermometry, and the occurrence of aliphatic hydrocarbons suggest that argillaceous phyllite originated from a pelagic pelite protolith under low-temperature (260–370°C) greenschist to subgreenschist facies conditions. Persistent biomarkers are indicative of bacterial degradation of planktonic organic matter suspended in a high water column. Enrichment in Fe-rich chlorite and Mg,Fe-muscovite, low volume of interconnected pores with dominant mesopores suggest that argillaceous phyllite from the Dewon–Pokrzywna deposit is a potential candidate for a buffer and/or backfill material.

Markov chain model for inadvertent human intrusion into geological repositories for high level wastes

Abstract Inadvertent human intrusion into a geological repository for radioactive waste is an incident of low frequency but high consequence. Until now, many studies have considered the frequency of deep borehole drilling for resource explorations in the site of the repository as a time-independent variable. However, future activities and technology of the human race will not be the same as today. Therefore, this study develops a new Markov chain model to estimate the frequency of human intrusion using time-dependent drilling frequencies based on the statistics of oil and gas exploration worldwide. The results showed that the frequency of human intrusion during the early time period is substantially reduced. After a few hundred years, the frequency steadily increases and approaches the value of the time-independent model. The low frequency during the early time period is attributed to enforcement of regulation, continuous knowledge of repository, and low utility of repository area. As these effects disappear, the frequency rapidly increases due to the future demand of resource exploration, which is a significant factor in increasing the risk of human intrusion. In addition, the reduction of average radiotoxicity in final waste is effective in reducing the risk, while the reduction of waste volume is insignificant for the frequency of human intrusion in the long-term.

3D modelling of the Excavation Damaged Zone using a Marked Point Process technique

The Excavation Damage Zone (EDZ) is a key aspect in the assessment of engineering feasibility and long-term safety of deep geological repositories for radioactive waste. The EDZ represents a possible release path for radionuclides dissolved in the porewater and for gases from corrosion and degradation processes which needs to be addressed quantitatively in Safety Assessment (SA). The EDZ is often represented by a discrete fracture network, whose complexity precludes its implementation in conventional SA models. To overcome this problem, two-dimensional abstraction methodologies have been developed that replace the complex geometry of the discrete fracture network by a hydraulically equivalent continuum model. Although such simple 2D models are amenable to SA modelling, they cannot capture the inherent three-dimensional and stochastic nature of the phenomena associated with the EDZ. In this paper we present a methodology based on Marked Point Processes to infer 3D stochastic continuum models of the EDZ from 2D discrete characterizations of the fracture network. The methodology is illustrated with the calibration of a multi-rate injection experiment in Mont Terri (Switzerland).

Hydrogeochemistry in the coastal area during construction of geological repository

A geological repository for low and intermediate radioactive waste (bottom at −130 m a.s.l.) was constructed from 2008 to 2014 in the southeastern coastal area in Korea. This research aimed to evaluate the hydrogeochemistry in this area during the construction period and to determine the characteristics that should be monitored during the operational period. During construction, the groundwater level decreased up to 136 m and the upper groundwater flow in the southeastern area reversed. After the groundwater level dropped below the sea level, the electrical conductivity (EC) in several wells along the coastal line started to increase; 23.2% of the measured water was classified as brackish water and the highest EC observed in groundwater was 18.9 mS/cm, nearly 38% of EC in seawater. The response time of the groundwater chemistry differs depending on the depth, even in wells, because of fracture networks. Among locations that showed drastic changes in EC, only the shallow depth of GM-4 showed a peak pattern in EC, but other locations showed increasing EC patterns or patterns with initial increase and sustained high till the end of the construction period. Based on the Cl/Br ratio, the source of the groundwater salinity was seawater intrusion, and ion exchange played an important role. Compared to Cl concentration, sodium was depleted and calcium was in excess in brackish water; however, the SO4/Cl ratio remained constant at a level similar to seawater. Ca and Fe concentrations showed distinguishable characteristics depending on the location, suggesting differences in geological media. During the operational period, periodical evaluation of the groundwater chemistry in the mixing zone and continuous monitoring of EC patterns and seawater fractions are required.

A new finite volume scheme preserving positivity for radionuclide transport calculations in radioactive waste repository

In order to simulate a far field model used in geological radioactive waste repository, a new finite volume scheme preserving positivity on arbitrary convex polygonal meshes with second order accuracy is proposed in this paper. The model problem describes the coupled process of diffusion, convection and chemical reaction, in which the diffusion tensors are highly anisotropic and heterogeneous. The classic implicit and explicit schemes are used in the temporal discretization. The discretization of diffusive flux in Sheng and Yuan (2016) is utilized, and the discretization of convective flux is based on a new corrected upwind scheme that depends on some available informations of diffusive flux. The resulting scheme is positivity-preserving and locally conservative, and has only cell-centered unknowns. Numerical results are presented to show that the performance of our positivity-preserving scheme for the numerical simulation of radionuclide transport problem.