Geological Repositories For Radioactive Waste Research Articles

Two-Phase Flow Modeling with Tough2-Mp of a Deep Geological Repository Within the First Benchmark of the FORGE Project

FORGE (Fate of Repository Gases) is an international research project supported by funding under the European Commission FP7 Euratom program and lasting four years from 2009 to 2013. The project is dedicated to understanding gas generation and migration as part of the quantitative assessment of a geological repository for radioactive waste. Within the FORGE project, Work Package 1 is dedicated to numerical modeling of a two-phase flow system (hydrogen gas due to corrosion and groundwater) in a geological repository for radioactive waste. Several benchmark exercises were proposed that cover the modeling of a deep geological repository from the disposal cell scale to the repository scale with different codes. During the definition of the exercises, special emphasis was given to the roles of the excavation-disturbed zone and of the interfaces between materials, which could act as a conduit for preferential flow. Some changes were made in the TOUGH2 code to enable the implementation of the prescribed conditions, models, and parameters of the benchmark. The results of the calculations performed with different codes show that TOUGH2 gives comparable results under the numerically challenging conditions defined in the exercise. Some differences were observed resulting from the use of different codes and also from some simplifications in the parameters and models adopted by the participating teams. In this paper, the cell-scale benchmark exercise and the results obtained by the Swiss Federal Nuclear Safety Inspectorate (ENSI) with TOUGH2 will be described, together with some difficulties encountered during the simulation, e.g., convergence problems. The results of other teams participating in the benchmark are in good agreement with the ENSI results.

Extension and Tuning of TOUGH2-MP EOS7R for the Assessment of Deep Geological Repositories for Nuclear Waste: Hydrogen, Arbitrarily Long Decay Chains, and Solubility Limits

Numerical simulation of multiphase flow and transport processes forms an important base for the assessment of deep geological repositories for radioactive waste. The finite volume simulation code TOUGH2-MP with its EOS7R equation-of-state module is a good starting point for large-scale simulations of the relevant processes, including solute transport of radionuclides, in and around a geological repository. On this base, we developed the equation-of-state module EOS75Rx that contains optimizations and specific extensions allowing for a much more efficient treatment of the problem at hand. First, hydrogen, which is formed by corrosion of waste containers and by radiolysis of organic wastes, replaces air as the main component of the gas phase. Second, an arbitrary number of variably long decay chains with branching can be considered. Third, solubility limitation and associated precipitation of chemical elements are modeled. Finally, a bug fix related to the source terms has been implemented. The new TOUGH2-MP EOS75Rx module has been validated using unitary tests and benchmark problems for the single- and two-phase flow and transport of radionuclides through porous media and soils. Its performance has been demonstrated by a large-scale, three-dimensional simulation of the performance of a generic deep geological repository in clay host rock.

Time- and density-dependent microstructure features of compacted bentonite

Pre-compacted bentonite bricks are often considered as sealing/backfill elements in deep geological repositories for high level radioactive waste. A good understanding of their microstructure changes upon hydration is essential as the microstructure changes are directly related to the macroscopic hydro-mechanical behaviour. In this study, the microstructure features of the compacted MX80 bentonite used as a sealing material in a field experiment were characterized by means of both mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Emphasis was put on the effects of final dry density (density after swelling) and hydration time. The results obtained show that the changes in soil porosity upon swelling are mainly due to the increase in large-pores of about 50µm diameter and medium-pores of 1µm diameter. In addition, the microstructure changed over time due to the water re-distribution that occurred among each level of pores: the volume of both the large-pores and small-pores decreased along as the volume of the medium-pores increased. A uniform microstructure can be then expected in the long term. Furthermore, it was observed that the higher the final dry density, the slower the microstructure changes.

First shrinkage parameters of Slovak bentonites considered for engineered barriers in the deep geological repository of high-level radioactive waste and spent nuclear fuel

The high potential of bentonites to volume changes depending on the water content is considered as their advantage for the engineered barriers in the deep geological repository of high-level radioactive waste and spent nuclear fuel because of swelling and self-healing of cracks in contact with water. On the other hand, drying may lead to opening of cracks and spaces between the bentonite blocks. This would increase the permeability and contamination risk around the hot container with high-level radioactive waste and spent nuclear fuel, especially if the host rock mass is dry. First shrinkage tests on four Slovak bentonites studied for engineered barriers were carried out. The water content at the shrinkage limit and the relative linear shrinkage are the first available shrinkage parameters received for the bentonite paste. The shrinkage hazard is higher in the best bentonites with high swelling potential—from Kopernica and Jelsový potok. The results indicated the necessity of further shrinkage tests to determine the relative linear and volume shrinkage of bentonite elements pressed of the loose bentonite powder of low water content.

The adsorption of Eu(III) and Am(III) on Beishan granite: XPS, EPMA, batch and modeling study

Granite has been chosen as a promising host rock for geological repository of high-level radioactive waste in many countries. However, the description of radionuclide adsorption on granite is subjected to its complicated composition and still a challenge. We studied the adsorption of Eu(III) and Am(III) on Beishan granite, a preliminary selection of host rock in China, as a function of pH, adsorbate concentration, ionic strength and the composition of background electrolytes. A surface complexation model was set up using Generalized Composite approach based on Eu(III) adsorption in NaCl electrolyte, supported by X-ray photoelectron spectroscopy and verified by Eu(III) adsorption in the presence of CO2 at PCO2=10-3.58atm and in CaCl2 electrolyte. The model was successfully extended to describe Am(III) adsorption in both NaCl and CaCl2 electrolytes. It was also found that temperature effect on Eu(III) adsorption was negligible at 25–80°C, that the adsorption of Eu(III) on freshly crushed granite with “new” surfaces and aged granite with “old” surfaces were identical to each other, and that the presence of fulvic acid of 2–20mg/L significantly declined the distribution coefficients of Eu(III) and Am(III) in high pH range. Electron probe micro analyses indicated that biotite had higher affinity for Eu(III) than other major minerals, such as oligoclase, quartz and orthoclase.

Investigating the anisotropy of the shear modulus of natural Boom Clay

Boom Clay is considered to be a potential host formation for a geological repository of radioactive waste in Belgium. In this context, a thorough understanding of its mechanical behaviour is thus important. In this study, the anisotropy of the shear modulus of natural Boom Clay samples was investigated using the bender element method. Seven samples located at different distances from the connecting gallery of the HADES underground research laboratory were tested for shearing in three different directions with respect to the bedding plane. The results obtained confirm the mechanical anisotropy of Boom Clay. Furthermore, the values of small-strain shear modulus in the near field of the gallery were found to be lower than those in the far field, suggesting that the mechanical properties of Boom Clay remain affected by gallery excavation damage that occurred more than 10 years ago.

Biogeochemical changes at early stage after the closure of radioactive waste geological repository in South Korea

Permanent disposal of low- and intermediate-level radioactive wastes in the subterranean environment has been the preferred method of many countries, including Korea. A safety issue after the closure of a geological repository is that biodegradation of organic materials due to microbial activities generates gases that lead to overpressure of the waste containers in the repository and its disintegration with the release of radionuclides. As part of an ongoing large-scale in situ experiment using organic wastes and groundwater to simulate geological radioactive waste repository conditions, we investigated the geochemical alteration and microbial activities at an early stage (∼63days) intended to be representative of the initial period after repository closure. The increased numbers of both aerobes and facultative anaerobes in waste effluents indicate that oxygen content could be the most significant parameter to control biogeochemical conditions at very early periods of reaction (<35days). Accordingly, the values of dissolved oxygen and redox potential were decreased. The activation of anaerobes after 35days was supported by the increased concentration to ∼50mgL−1 of ethanol. These results suggest that the biogeochemical conditions were rapidly altered to more reducing and anaerobic conditions within the initial 2months after repository closure. Although no gases were detected during the study, activated anaerobic microbes will play more important role in gas generation over the long term.

Thermo-hydro-mechanical-air coupling finite element method and its application to multi-phase problems

In this paper, a finite element method (FEM)-based multi-phase problem based on a newly proposed thermal elastoplastic constitutive model for saturated/unsaturated geomaterial is discussed. A program of FEM named as SOFT, adopting unified field equations for thermo-hydro-mechanical-air (THMA) behavior of geomaterial and using finite element-finite difference (FE-FD) scheme for soil–water–air three-phase coupling problem, is used in the numerical simulation. As an application of the newly proposed numerical method, two engineering problems, one for slope failure in unsaturated model ground and another for in situ heating test related to deep geological repository of high-level radioactive waste (HLRW), are simulated. The model tests on slope failure in unsaturated Shirasu ground, carried out by Kitamura et al. (2007), is simulated in the framework of soil–water–air three-phase coupling under the condition of constant temperature. While the in situ heating test reported by Munoz (2006) is simulated in the same framework under the conditions of variable temperature but constant air pressure.

On the physico-chemical evolution of low-pH and CEM I cement pastes interacting with Callovo-Oxfordian pore water under its in situ CO2 partial pressure

Within the framework of geological repositories for radioactive waste, structural concretes must be adapted to the underground chemical conditions. CEM I cement-based materials are characterised by high pH that may produce an alkaline plume in the near-field of the repository. In order to avoid this problem, low-pH cements have been designed. This study compares the physico-chemical behaviour of a low-pH material with a CEM I cement paste, both being subjected to leaching by an aqueous solution. An original experimental setup was designed to reproduce the underground conditions using a specific CO2 regulation device. Under these conditions, the low-pH material was strongly degraded, which results in coarser porosity, whereas thickness degradation of the CEM I cement paste is limited by the precipitation of a magnesium-calcite crust over the surface, which reduces the exchange of soluble species. This paper also presents a new approach for microstructure characterisation based on high-resolution X-ray microtomography.

On area-specific underground research laboratory for geological disposal of high-level radioactive waste in China

Underground research laboratories (URLs), including “generic URLs” and “site-specific URLs”, are underground facilities in which characterisation, testing, technology development, and/or demonstration activities are carried out in support of the development of geological repositories for high-level radioactive waste (HLW) disposal. In addition to the generic URL and site-specific URL, a concept of “area-specific URL”, or the third type of URL, is proposed in this paper. It is referred to as the facility that is built at a site within an area that is considered as a potential area for HLW repository or built at a place near the future repository site, and may be regarded as a precursor to the development of a repository at the site. It acts as a “generic URL”, but also acts as a “site-specific URL” to some extent. Considering the current situation in China, the most suitable option is to build an “area-specific URL” in Beishan area, the first priority region for China's high-level waste repository. With this strategy, the goal to build China's URL by 2020 may be achieved, but the time left is limited.

EBS behaviour immediately after repository closure in a clay host rock: HE-E experiment (Mont Terri URL)

AbstractThe evolution of the clay-based engineered barrier system (EBS) of geological repositories for radioactive waste has been the subject of many research programmes during the last decade. The early post-closure thermal behaviour is elucidated by the HE-E experiment, a 1:2 scale heating experiment (at the Mont Terri Rock Laboratory), which was implemented in the first semester of 2011, with the initiation of the heating phase in June 2011. A maximum temperature of 140 °C was reached in June 2012. After 15 months of heating, the temperature evolution in the EBS and the Opalinus Clay reflects the design calculations, and thermally induced porewater overpressures are being measured at a few metres’ distance in the Opalinus Clay. Seismic methods proved to be a sensitive tool for the continuous characterization of changes of EBS and Opalinus Clay properties. Design modelling and predictive modelling based on the as-built parameter dataset with established coupled codes (TOUGH, CODE_BRIGHT; using various geometries) are described. The results indicate that the models are generally in agreement with the observations and capable of capturing the evolution of the experiment.

Microbial processes relevant for the long-term performance of high-level radioactive waste repositories in clays

AbstractThe primary purpose of the present work was to qualitatively evaluate the relevance of microbial activity for the long-term performance of a deep geological repository for high-level radioactive waste and spent nuclear fuel utilizing clay and to identify which safety-relevant processes and properties can be potentially influenced by this activity. Deterioration of clay properties accompanying destabilization and destruction of clay mineral structure as a result of microbial actions can be considered as the primary microbial impact on clay. The present analysis identified eight clay properties essential for maintaining safety functions of containment and retardation of the disposal system – swelling pressure, specific surface area, cation exchange capacity, anion sorption capacity, porosity, permeability, fluid pressure and plasticity – which can potentially be influenced by microbial processes in clay-based materials and claystone within a repository. Radioactive waste canisters and over-packs made from cast metal or steel represent a further component of the engineered barrier system which can be strongly affected by microbial activity in the clay buffer or in the adjacent host rock. This work should provide a basis for a follow-up quantitative estimation of the maximum possible effects of microbial processes on the barrier system of a deep geological repository in clay.

Precipitation of chlorite-like structures during OPC porewater diffusion through compacted bentonite at 90 °C

The reactivity between concrete and bentonite is a key issue in the performance of a deep geological repository for radioactive waste. The long-term reaction of alkaline solutions representative of early and late stages of concrete degradation (K–Na–OH, pH 13.5, and Ca–OH, pH 12.5, synthetic solutions) was studied by diffusion experiments with compacted bentonite columns at 90°C. Relevant mineralogical alterations were produced with the K–Na–OH solution and consisted mainly of the precipitation of a non-expandable 14.4–7.4Å chlorite-like phase at the alkaline interface and through the first 2–3mm. The chlorite-like phase, a non-swelling layer silicate, was characterized as a brucite–montmorillonite complex produced by a brucite monolayer intercalation in the montmorillonite interlayer and its predicted formation would thus impact upon the safety functions of the bentonite buffer. The precipitation of discrete brucite was not significant, being this, the main difference found in comparison with previous studies at temperatures lower than 90°C. The high intensity of the 7.4Å X-ray diffraction peak, in the absence of iron components, and the rise of new hk0 trioctahedral bands, determined by X-ray diffraction and electron diffraction on plate-like mineral aggregates, indicate the possible formation of new Mg-layer silicates, although they were not clearly defined. In contrast to K–Na–OH experiments, mineralogical alterations with the Ca–OH solution were almost non-existent, including insignificant changes in the Ca/Mg cation exchange distribution. The buffer capacity of the bentonite against the migration of alkaline fluids is driven by mineralogical reactions affecting a few millimeters thickness from the alkaline solution–bentonite interface. The nature of these reactions is critically affected by the temperature change within the expected designed conditions for radioactive waste repositories.

Hydraulic conductivity and microstructure changes of compacted bentonite/sand mixture during hydration

Compacted bentonite-based materials are often considered as sealing/backfill materials in deep geological repository for high level radioactive waste. A good understanding of their hydration process is essential as this process is directly related to over-pack corrosion and nuclide migration. In this study, the unsaturated hydraulic properties of MX80 bentonite/sand mixture were characterized by carrying out a series of experiments including water retention test, infiltration test as well as microstructure observation. It was found that with suction decrease under constant volume condition, the hydraulic conductivity decreased followed by an increase after a suction threshold. At suctions higher than 12.6MPa, hydration led to progressive large-pore clogging by exfoliation of clay particles. On the contrary, when saturation was approached (suction lower than 4.2MPa), the large-pore quantity increased due to the creation of two-dimensional pores. It was also observed that the soil hydraulic conductivity changed following the same tendency as the large-pore quantity during hydration. In other words, water transfer was primarily governed by the network of large-pores.

A novel technique of in situ phase-shift interferometry applied for faint dissolution of bulky montmorillonite in alkaline solution

The effect of alkaline pH on the dissolution rate of bulky aggregated montmorillonite samples at 23 °C was investigated for the first time by using an enhanced phase-shift interferometry technique combined with an internal refraction interferometry method developed for this study. This technique was applied to provide a molecular resolution during the optical observation of the dissolution phenomena in real time and in situ while remaining noninvasive. A theoretical normal resolution limit of this technique was 0.78 nm in water for opaque material, but was limited to 6.6 nm for montmorillonite due to the transparency of the montmorillonite crystal. Normal dissolution velocities as low as 1 × 10−4 to 1 × 10−3 nm/s were obtained directly by using the measured temporal change in height of montmorillonite samples set in a reaction cell. The molar dissolution fluxes of montmorillonite obtained in this study gave considerably faster dissolution rates in comparison to those obtained in previous investigations by solution analysis methods. The pH dependence of montmorillonite dissolution rate determined in this study was qualitatively in good agreement with those reported in the previous investigations. The dissolution rates to be used in safety assessments of geological repositories for radioactive wastes should be obtained for bulky samples. This goal has been difficult to achieve using conventional powder experiment technique and solution analysis method, but has been shown to be feasible using the enhanced phase-shift interferometry.

Redox front penetration in the fractured Toki Granite, central Japan: An analogue for redox reactions and redox buffering in fractured crystalline host rocks for repositories of long-lived radioactive waste

Redox buffering is one important factor to be considered when assessing the barrier function of potential host rocks for a deep geological repository for long-lived radioactive waste. If such a repository is to be sited in fractured crystalline host rock it must be demonstrated that waste will be emplaced deeper than the maximum depth to which oxidizing waters can penetrate from the earth’s surface via fractures, during the assessment timeframe (typically 1Ma). An analogue for penetration of such oxidizing water occurs in the Cretaceous Toki Granite of central Japan. Here, a deep redox front is developed along water-conducting fractures at a depth of 210m below the ground surface. Detailed petrographical studies and geochemical analyses were carried out on drill core specimens of this redox front. The aim was to determine the buffering processes and behavior of major and minor elements, including rare earth elements (REEs), during redox front development. The results are compared with analytical data from an oxidized zone found along shallow fractures (up to 20m from the surface) in the same granitic rock, in order to understand differences in elemental migration according to the depth below the ground surface of redox-front formation. Geochemical analyses by XRF and ICP-MS of the oxidized zone at 210m depth reveal clear changes in Fe(III)/Fe(II) ratios and Ca depletion across the front, while Fe concentrations vary little. In contrast, the redox front identified along shallow fractures shows strong enrichments of Fe, Mn and trace elements in the oxidized zone compared with the fresh rock matrix. The difference can be ascribed to the changing Eh and pH of groundwater as it flows downwards in the granite, due to reactions with rock forming minerals, in particular feldspar dissolution. These observations give important insights into the processes that control the rates of redox front penetration in fractured crystalline rock. The findings of the study can be used to help build confidence among stakeholders that radioactive waste would be emplaced in such rocks at greater depth than that to which oxidizing water is likely to penetrate in future.

Geometry and Properties of the Excavation-Induced Fractures at the Meuse/Haute-Marne URL Drifts

The performance and safety assessment and technology demonstration are the main objectives of research programs for feasibility studies for deep geological repository of radioactive waste. In this context, the French national radioactive waste management agency (ANDRA) started to develop the Meuse/Haute-Marne underground research laboratory (URL) at Bure, nearly 300 km East of Paris. The host formation consists of a Callovo-Oxfordian claystone found between 420 and 550 m below ground, overlain and underlain by poorly permeable carbonate formations. One of the major concerns related to performance assessment is the excavation-induced fractures which may provide groundwater preferential pathway for radionuclide migration. The extent of the fractures possibly acting significantly in the radionuclide migration is known as the excavation damaged zone (EDZ). A scientific study on the EDZ characterization is performed at the main level of the URL (−490 m). Observations such as structural analysis on core, overcored resin-filled samples, geological survey of the drift face and sidewalls, were made to better understand the fracture network characteristics, extent and its generation. Pulse and constant head test hydraulic conductivity measurements were performed with multi packer system to estimate the extension of the EDZ hydraulic conductivity. Fractures exhibited high transmissivity near the excavation walls, but farther from the exaction walls, shear fractures showed hydraulic conductivity values reflecting values of undisturbed or slightly disturbed rock mass condition. The major findings in terms of geometry and properties of excavation-induced fractures are discussed in detail in this paper. For example, it is observed that the shape of the fracture network depends on the orientation of the drift in relation to the orientation of the in situ stress field.

IMPACT OF DIFFERENT FACTORS ON GAS MIGRATION IN THE DISPOSAL CELL OF CONCEPTUAL GEOLOGICAL REPOSITORY FOR HIGH LEVEL RADIOACTIVE WASTE

The numerical modelling results of different factors impact on hydrogen gas migration in the disposal cell of conceptual geological repository for high level radioactive waste is presented in this paper. The assessment of various factors such as self healing of EDZ, different relative permeability function for geological medium, variation of diffusivity factor of dissolved hydrogen, delay of gas generation, lower intrinsic permeability values for waste interface has been performed. Modelling results showed that decreasing of intrinsic permeability value in the EDZ has major impact on peak pressure. Decreasing of intrinsic permeability value in waste interface closes the preferential flow paths for gaseous hydrogen from disposal cell toward the access drift.DOI: http://dx.doi.org/10.5755/j01.mech.18.6.3172

On persistent primary variables for numerical modeling of gas migration in a nuclear waste repository

Numerical simulation of gas migration driven by compressible two-phase partially miscible flow in porous media is of major importance for safety assessment of deep geological repositories for long-lived high-level nuclear waste. We present modeling of compositional liquid and gas flow for numerical simulations of hydrogen migration in deep geological radioactive waste repository based on persistent primary variables. Two-phase flow is considered, with incompressible liquid and compressible gas, which includes capillary effects, gas dissolution, and diffusivity. After discussing briefly the existing approaches to deal with phase appearance and disappearance problem, including a persistent set of variables already considered in a previous paper (Bourgeat et al., Comput Geosci 13(1):29–42, 2009), we focus on a new variant of the primary variables: dissolved hydrogen mass concentration and liquid pressure. This choice leads to a unique and consistent formulation in liquid saturated and unsaturated regions, which is well adapted to heterogeneous media. We use this new set of variable for numerical simulations and show computational evidences of its adequacy to simulate gas phase appearance and disappearance in different but typical situations for gas migration in an underground radioactive waste repository.

Investigation on compression behaviour of highly compacted GMZ01 bentonite with suction and temperature control

In this paper, an oedometer with suction and temperature control was developed. Mechanical compaction tests have been performed on the highly compacted GMZ01 bentonite, which has been recognized as potential buffer/backfill material for construction of Chinese high-level radioactive waste (HLW) geological repository, under conditions of suction ranging from 0 to 110MPa, temperature from 20 to 80°C and vertical pressure from 0.1 to 80MPa. Based on the test results, suction and temperature effects on compressibility parameters are investigated. Results reveal that: (1) at high suctions, heating induced an expansion, while contraction is induced by heating at low suctions. The thermal expansion coefficient of GMZ01 bentonite measured is 1×10−4°C−1; (2) with increasing suction, the elastic compressibility κ and the plastic compressibility λ(s) of the highly compacted GMZ01 bentonite decrease, while the pre-consolidation pressure increases markedly; (3) with increasing temperature, the elastic compressibility of compacted GMZ01 bentonite changes insignificantly, while the plastic compressibility λ(s) slightly decreases and the yield surface tends to shrink.