Deep Radioactive Waste Repository Research Articles

Evaluation of adsorption behaviour of selenium onto zeolite-based composite barrier material for intermediate deep radioactive waste repository

The radionuclide 79Se is a potential radio-ecological concern due to its long half-lives and its capacity to be readily incorporated into the natural life cycle from radioactive waste repositories. This study aimed to reveal the role of barrier materials including zeolite in the transfer of selenium from radioactive waste repository areas to the biosphere. The interaction between selenium (IV) and the barrier material was evaluated by examining different parameters. In the event of an accident that may occur, the environmental and health risks that the 79Se may pose were evaluated within the scope of possible dose values using a Monte Carlo-based simulation program. As a result, it has been determined that the composite barrier material used plays an important role in preventing the transfer of selenium from the waste storage area to the biosphere. Total carcinogenic risk (TCR) values for ingestion way were found from 4.93E-17 to 4.03E-16 for adults, and from 6.30E-22 to 5.16E-21 for infants. TCR values for dermal way are from 5.28E-21 to 4.32E-20 for adults and range from 7.74E-31 to 6.34E-30 for infants.

Development of a Low-pH Concrete Intended for Deep Geological Repository for Radioactive Waste

This article deals with the development of concretes intended to be used as construction materials in the Czech deep repository for radioactive waste. The basic requirements for this concrete are a reduced pH value, which must maintain a constant reduced pH with a value of around 11 in the long term, and mechanical properties comparable to conventional concrete. The raw materials for the production of the proposed low pH concrete come exclusively from the Czech Republic. Material characteristics were measured on fresh mixtures and concrete after 28, 56, and 90 days of curing. In addition to the basic raw materials (aggregate, cement, water), plasticizers, microsilica or slag, and defoaming agents were added to the mixture. The aggregate:binder ratio was approximately 5:1 and the w/c water coefficient was approximately 0.6. The mechanical properties of the final concrete were similar to the reference recipe of conventional concrete (the decrease was less than 10%) and the pH value was even below 11 after 90 days. The issue of a sprayed variant of the LPC mixture was solved within the project.

Assessment of Partitioning and Transmutation of High-Level Waste and Hypothetical Implementation Scenarios in Germany

Abstract. In the context of the search for a deep geological repository for high-level radioactive waste from nuclear energy a preliminary waste treatment is repeatedly called into play by partitioning and transmutation (P&T). Proponents of this approach promise that with P&T, the requirements for and the risks posed by a – then still necessary – repository could be significantly reduced. However, such technological promises have to be prospectively, promptly and publicly reasonably verifiable. Partitioning is reprocessing in which, in addition to separating uranium and plutonium from the fission products, other material streams (for example, the minor actinides) are extracted. In transmutation, radionuclides – especially through nuclear fission – are converted into other nuclides. Thus, conversion of the parent nuclides into nuclides with shorter half-lives, lower radiotoxicity, or into stable nuclides could be achieved. For the assessment of P&T, essential aspects are the current degree of maturity of necessary technologies, the requirements for research and development, technological development risks, the basic feasibility and objective, risks of a hypothetical operation of corresponding plants and the possible effects on nuclear waste disposal. More specifically, on the technological side, it is all about development periods, technical security requirements and licensability, proliferation risks and implementation periods. The presentation of the results of some hypothetical P&T scenarios is intended to help to assess the impacts on radioactive waste present in Germany, necessary facilities and operating periods. Thus, pyro-chemical and hydrochemical separation processes, special transuranic fuels based on mixed oxides (MOX) or uranium-free fuel types and critical fast reactors, subcritical (accelerator-driven) reactors, as well as molten salt reactors, are considered. One difficulty is that the multiple recycling of the transuranics changes the fuel composition. Detailed statements about these changes are only possible with complex simulation calculations and their influence on safe reactor operation. So far, this has not happened on an international scale. In the modelling presented here, an attempt was made to represent the restrictions that the reactor design has on the fuel composition more precisely, at least insofar as the element composition of the fuel remains the same for the duration of the scenario. Conclusions presented from the analysis of the hypothetical scenarios affect, among other things, necessary operating periods and the number of plants and changes achieved in the stock of both transuranics and fission products.

Convergence of a finite volume scheme for immiscible compressible two-phase flow in porous media by the concept of the global pressure

This paper deals with development and analysis of a finite volume (FV) method for the coupled system describing immiscible compressible two-phase flow, such as water-gas, in porous media, capillary and gravity effects being taken into account. We investigate a fully coupled fully implicit cell-centered “phase-by-phase” FV scheme for the discretization of such system. The main goal is to incorporate some of the most recent improvements in the scheme and the convergence of the numerical approximation to the weak solution of such models. The spatial discretization uses a TPFA scheme and a new strategy for handling the upwinding. Based on a priori estimates and compactness arguments, we prove the convergence of the numerical approximation to the weak solution. The particular feature in this convergence analysis of the classical engineering scheme based on the “phase-by-phase” upwinding on an orthogonal mesh relies on the global pressure–saturation fractional flow formulation as was defined relatively recently for immiscible compressible flow in porous media. We have developed and implemented this scheme in a new module in the context of the open source platform DuMuX. Two numerical experiments are presented to demonstrate the efficiency of this scheme. The first test addresses the evolution in 2D of gas migration through engineered and geological barriers for a deep repository for radioactive waste. The second test case is chosen to test the ability of the method to approximate solutions for 3D problems modeling scenarios of CO2 injection in a fully water-saturated domain.

Adsorption of Eu(III) at rutile/water interface: Batch, spectroscopic and modelling studies

Knowledge of geochemical behaviors of Eu(III) in natural system is necessary for long-term safety assessment of deep radioactive waste repositories. The mobility or retention of the Eu(III) depends on its adsorption reaction at the mineral/water interface. Here, we studied the Eu(III) adsorption on rutile with batch experiments, zeta-potential analysis and time-resolved fluorescence spectra (TRFS). The results indicate that Eu(III) adsorption is controlled by outer-sphere interaction under acidic conditions, and transforms to inner-sphere interaction under neutral and alkaline conditions. A surface complexation model (SCM) from Rossendorf Expert System for Surface and Sorption Thermodynamics (RES3T) is successfully applied to describe the adsorption behavior of Eu(III) at the rutile/water interface with CHEMSPEC code. The good predictions indicate that the current SCM from RES3T could indeed be used for the estimation of Eu(III) adsorption on rutile surface, though it is not sensitive to the Eu(III) loadings. Overall, this work may provide valuable information on adsorption mechanism of Eu(III) at the rutile/water interface and help us understand the geochemical behaviors of Eu(III) in natural environments.

Homogenization of nonisothermal immiscible incompressible two-phase flow in porous media

In this paper, we consider nonisothermal two-phase flows through heterogeneous porous media with periodic microstructure. Examples of such models appear in gas migration through engineered and geological barriers for a deep repository for radioactive waste, thermally enhanced oil recovery and geothermal systems. The mathematical model is given by a coupled system of two-phase flow equations, and an energy balance equation. The model consists of the usual equations derived from the mass conservation of both fluids along with the Darcy–Muskat and the capillary pressure laws. The problem is written in terms of the phase formulation, i.e. the saturation of one phase, the pressure of the second phase and the temperature are primary unknowns. The major difficulties related to this model are in the nonlinear degenerate structure of the equations, as well as in the coupling in the system. As fluid properties are defined as a function of temperature and pressure, there is a strong coupling between the mass balance and energy balance equations. Under some realistic assumptions on the data, we obtain a nonlinear homogenized coupled system of three coupled partial differential equations with effective coefficients (porosity, permeability, thermal conductivity, heat capacity) which are computed via solving cell problems. We give a rigorous mathematical derivation of the upscaled model by means of the two-scale convergence.

Coupled THMC models for bentonite in an argillite repository for nuclear waste: Illitization and its effect on swelling stress under high temperature

Subsurface manipulations such as those expected from the disposal of heat-emanating radioactive waste in deep repositories can induce strongly coupled Thermal (T), hydrological (H), mechanical (M) and chemical (C) processes. Adequate coupled THMC models are highly desirable or even indispensable for performance assessment of such repositories, for examples for the analysis of bentonite-based engineered barrier system (EBS) surrounding the emplaced waste. In this study, we present coupled THMC model simulations of a generic nuclear waste repository in an argillite with a bentonite-based buffer. The objective is to evaluate the chemical changes in the EBS bentonite and their effect on mechanical behavior under high temperature, attempting to shed light on whether EBS bentonite can sustain temperatures higher than 100°C without significant impact on barrier's performance.Two scenarios were simulated for comparison: a case in which the temperature near the waste canister peaks at 200°C and a case in which the temperature at the same spot culminate with about 100°C. Simulations for a generic case with Kunigel-VI bentonite as backfill and Opalinus Clay as host rock were conducted for 1000years and reported in the previous study (Zheng et al., 2015). In this paper, simulations for 100,000years have been done for two types of bentonite-based buffer materials: Kunigel-VI and FEBEX bentonite. This enables us to evaluate how different types of bentonite behave in terms of the illitization and its impact on swelling stress and whether we can generalize these results to support decision making. The simulations show the occurrence of illitization in the bentonite buffer and the enhancement of illitization under high temperature. However, FEBEX bentonite undergoes less illitization mainly due to the higher ion concentration in pore water and the lower content of K-feldspar in the bentonite mineral composition. Moreover, the reduction of swelling stress by chemical changes is more pronounced for Kunigel-VI bentonite than for FEBEX bentonite. Overall, the results of our model simulations suggest that an argillite repository with a bentonite-based EBS that is similar to FEBEX bentonite could sustain temperatures much higher than 100°C as far as illitization concerns. Model results also reveal that illitization is stabilized after about 2000years in bentonite near the waste package, but continues in bentonite near the bentonite-argillite interface, which manifests the strong effect of geochemical interaction between EBS bentonite and host rock on long term illitization in bentonite.

Influence of temperature on the sorption properties of rocks from the Nizhnekansky massif

The kinetics of radionuclide sorption onto finely divided samples of rocks from the Nizhnekansky massif, taken from the level of the planned arrangement of deep radioactive waste repository, was studied, and the times in which the sorption equilibrium was attained at 20 and 90°С were determined. The distribution coefficients of actinides (233U, 239Pu, 241Am) and fission products (137Cs, 90Sr) were determined. Increasing the temperature from 20 to 90°С intensifies the sorption of the radionuclides studied, except cesium for which the sorption slightly decreases.

Preliminary electrochemical corrosion monitoring of iron in mixture cement paste–bentonite

ABSTRACTIn the context of the French 500-m deep radioactive waste repository, electrochemical impedance spectroscopy and a methodology based on complex impedance analysis were carried out to follow the hydration of grout containing cement–bentonite mixture and iron corrosion together. The grout was initially a paste, and solidified with time. After a 100-day-curing, the temperature of the system (grout + steel) was increased from room temperature to 80°C to simulate the effect of a nuclear waste overpack. At room temperature, the ionic conductance exhibited a continuous decrease followed by an increase at 80°C. After an initial drop, the corrosion rate increased to a quasi-steady state value at around 1.5 µm/year at 21°C. At 80°C, the corrosion rate increased to about 30 µm/year before gradually dropping to much lower values. Eventually, then the passivation of iron seemed to occur since the corrosion rate has decreased continuously down to 0.1 µm/year.This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems.

Hydro-elasto-viscoplastic modeling of a drift at the Meuse/Haute-Marne underground research laboratoratory (URL)

An underground research laboratory (URL) is being constructed by Andra in eastern France, in Callovo-Oxfordian claystone (COx) in which various in situ geomechanical experiments are being undertaken or are to be carried out. The aim of this URL is to characterize the in situ properties of COx claystone and to test disposal technologies in a realistic way in order to assess the short- and long-term safety of a deep radioactive waste repository. In parallel, theoretical and numerical models able to reproduce the phenomena observed under different types of loading paths must be developed.The phenomenological elastic-visco-plastic model developed by Souley et al. (2011) has been enhanced to reflect recent advances in understanding of the mechanical and hydromechanical behavior of COx claystone and the modification of the mechanical and hydraulic properties in the EDZ (Excavation Damage Zone). In particular, the influence of induced damage and fracturing on the delayed strains and strain rates of the COx claystone and the permeability changes observed at the site scale, as well as hydro-mechanical couplings, are discussed and incorporated in a new model. This model is implemented into the commercial code FLAC3D. Short- and long-term test data (Armand et al., 2016) can be used to identify possible key parameters for the model. These tests were also used to identify certain parameters of our model. Some tests were simulated to verify the numerical implementation of the proposed model.Finally, the simulation of the GCS drift excavation (Seyedi et al., 2016) has been performed. Comparisons to in situ measurements are discussed and some accordance and discrepancies were observed.

An improved homogenization result for immiscible compressible two-phase flow in porous media

The paper deals with a degenerate model of immiscible compressible two-phase flow in heterogeneous porous media. We consider liquid and gas phases (water and hydrogen) flow in a porous reservoir, modeling the hydrogen migration through engineered and geological barriers for a deep repository for radioactive waste. The gas phase is supposed compressible and obeying the ideal gas law. The flow is then described by the conservation of the mass for each phase. The model is written in terms of the phase formulation, i.e. the liquid saturation phase and the gas pressure phase are primary unknowns. This formulation leads to a coupled system consisting of a nonlinear degenerate parabolic equation for the gas pressure and a nonlinear degenerate parabolic diffusion-convection equation for the liquid saturation, subject to appropriate boundary and initial conditions. The major difficulties related to this model are in the nonlinear degenerate structure of the equations, as well as in the coupling in the system. The aim of this paper is to extend our previous results to the case of an ideal gas. In this case a new degeneracy appears in the pressure equation. With the help of an appropriate regularization we show the existence of a weak solution to the studied system. We also consider the corresponding nonlinear homogenization problem and provide a rigorous mathematical derivation of the upscaled model by means of the two-scale convergence.

Homogenization of compressible two-phase two-component flow in porous media

The paper is devoted to the homogenization of immiscible compressible two-phase two-component flow in heterogeneous porous media. We consider liquid and gas phases, two-component (water and hydrogen) flow in a porous reservoir with periodic microstructure, modeling the hydrogen migration through engineered and geological barriers for a deep repository for radioactive waste. Phase exchange, capillary effects included by the Darcy–Muskat law and Fickian diffusion are taken into account. The hydrogen in the gas phase is supposed compressible and could be dissolved into the water obeying the Henry law. The flow is then described by the conservation of the mass for each component. The microscopic model is written in terms of the phase formulation, i.e. the liquid saturation phase and the gas pressure phase are primary unknowns. This formulation leads to a coupled system consisting of a nonlinear parabolic equation for the gas pressure and a nonlinear degenerate parabolic diffusion–convection equation for the liquid saturation, subject to appropriate boundary and initial conditions. The major difficulties related to this model are in the nonlinear degenerate structure of the equations, as well as in the coupling in the system. Under some realistic assumptions on the data, we obtain a nonlinear homogenized problem with effective coefficients which are computed via a cell problem. We rigorously justify this homogenization process for the problem by using the two-scale convergence.

Holmehus clay — A Tertiary smectitic clay of potential use for isolation of hazardous waste

The Danish Tertiary Holmehus clay belongs to the series of illite–smectite mixed-layer clays in northern Europe and has a potential to serve as an effective engineered barrier for isolating hazardous waste. Mineralogical specification was made of the components illite/smectite mixed-layer and dioctahedral vermiculite/smectite mixed-layer structures. The lower content of expandable phases than of smectite-rich clays like MX-80 gives somewhat higher hydraulic conductivity and somewhat lower swelling pressure. Both are deemed suitable for isolating canisters with high-level radioactive waste in deep repositories for densities at water saturation of at least 1900kg/m3. Holmehus clay has fulfilled the demanded parameters for swelling pressure with >0.3MPa and hydraulic conductivity with <10 E−12m s−1 under this density. The lower swelling pressure, the potential of being chemically more stable, and the high buffer capacity by providing dissolved Si makes Holmehus clay a candidate for use in both deep and near-surface repositories.

Three-dimensional numerical simulation by upscaling of gas migration through engineered and geological barriers for a deep repository for radioactive waste

AbstractThis paper presents the results of a benchmark study that compares a number of numerical models applied to a specific problem in the context of hydrogen flow and transport in a nuclear waste repository. The processes modelled are two-phase (water and hydrogen) immiscible compressible two-component transient flow in a heterogeneous porous medium under isothermal conditions. The three-dimensional (3D) model represents a module of a repository for high-level waste in a clay host rock. An upscaling technique and a vertex-centred finite-volume method are employed to yield very accurate solutions. Since the full range of results required in the benchmark is too large to be displayed in this paper, we focus on the evolution of the pressures, the saturations, the fluxes and the comparison of the numerical results with the other participants. A homemade C++ upscaling code and the parallel multiphase flow simulator DuMuX have been adopted for this study.

Stability of lanthanum-saturated montmorillonite under high pressure and high temperature conditions

Smectite has been used to capture radioactive cations through adsorption in deep radioactive waste repositories in various parts of the world. Smectite is also important in the transport of water and some trace element cations such as rare earth elements (REE), which are captured in its structure, back to the mantle in subduction environments. Such captures are based on the ionic strength of the surrounding solution and the adsorption coefficient of smectite. However, captured cations can be released from the smectite structure once the ionic strength of the solution changes. In this work, the stability of a particular smectite (montmorillonite) structure saturated with lanthanum was verified at high pressures (up to 12GPa) and room temperature and at high pressure and high temperature (HPHT) concomitantly. La3+-montmorillonite remains stable up to 12GPa at room temperature with a small variance in its vibrational mode. At HPHT, however, the structure becomes muscovite-like and rich in La3+. When in contact with a Ca2+-enriched solution, La3+ is partially replaced by Ca2+ in the new phase, returning to its original Ca2+-montmorillonite phase, whereas another part remains La-muscovite-like. These results were confirmed by X-ray diffraction and scanning electron microscopy with energy dispersive X-ray spectroscopy.

Homogenization of immiscible compressible two-phase flow in highly heterogeneous porous media with discontinuous capillary pressures

This paper presents a study of immiscible compressible two-phase, such as water and gas, flow through highly heterogeneous porous media with periodic microstructure. Such models appear in gas migration through engineered and geological barriers for a deep repository for radioactive waste. We will consider a domain made up of several zones with different characteristics: porosity, absolute permeability, relative permeabilities and capillary pressure curves. Consequently, the model involves highly oscillatory characteristics and internal nonlinear interface conditions. The microscopic model is written in terms of the phase formulation, i.e. where the wetting (water) saturation phase and the non-wetting (gas) pressure phase are primary unknowns. This formulation leads to a coupled system consisting of a nonlinear parabolic equation for the gas pressure and a nonlinear degenerate parabolic diffusion-convection equation for the water saturation, subject to appropriate transmission, boundary and initial conditions. The major difficulties related to this model are in the nonlinear degenerate structure of the equations, as well as in the coupling in the system. Moreover, the transmission conditions are nonlinear and the saturation is discontinuous at interfaces separating different media. Under some realistic assumptions on the data, we obtain a nonlinear homogenized coupled system with effective coefficients which are computed via a cell problem and give a rigorous mathematical derivation of the upscaled model by means of the two-scale convergence.

Analysis of time-dependent deformation measured at deep underground openings

The understanding of time-dependent deformation of rock around underground excavations becomes more and more important with the increase in overburden stress, opening size, and temperature expected in deep underground facilities for various purposes. It is desirable to use in situ measurements instead of laboratory data, because of the difference in time-dependent deformation mechanisms observed in laboratory and in situ measurement. In this study, the deformation measurements from Waste Isolation Pilot Plant, which is a deep underground radioactive waste repository in rock salt and a salt mine, in which yield pillar mining design was adopted, were extensively analyzed. It was possible to determine that the transition time from the primary to the secondary creep stages was around 200–300 days. The influence of adjacent excavation on the deformation measurement could be clearly observed. It was also found that the measuring time interval was important to understand the time-dependent deformation mechanisms.

Homogenization results for a coupled system modelling immiscible compressible two-phase flow in porous media by the concept of global pressure

A model for immiscible compressible two-phase flow in heterogeneous porous media is considered. Such models appear in gas migration through engineered and geological barriers for a deep repository for radioactive waste. The main feature of this model is the introduction of a new global pressure and it is fully equivalent to the original equations. The resulting equations are written in a fractional flow formulation and lead to a coupled degenerate system which consists of a nonlinear parabolic (the global pressure) equation and a nonlinear diffusion–convection one (the saturation) equation with rapidly oscillating porosity function and absolute permeability tensor. The major difficulties related to this model are in the nonlinear degenerate structure of the equations, as well as in the coupling in the system. Under some realistic assumptions on the data, we obtain a nonlinear homogenized problem with effective coefficients which are computed via a cell problem and give a rigorous mathematical derivation of the upscaled model by means of two-scale convergence.

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 (&gt;400 m) radioactive waste repository, even though the samples originated from shallower depths (&lt;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.

Lab-scale performance of selected expandable clays under HLW repository conditions

Smectite clay has been proposed for embedding canisters with highly radioactive waste in deep repositories because of its isolating capacity. Montmorillonite-rich bentonite is a premier buffer candidate for many national organizations that are responsible for disposal of such waste. Experience from the use of drilling mud at large depths indicates that other smectite clay minerals are more stable chemically and saponite is one of them. The physical properties of smectitic mixed-layer minerals like Friedland clay are known to be less sensitive to high salt contents and such clay may also be a buffer candidate. Montmorillonite-rich MX-80 clay, Greek saponite with a minor amount of palygorskite, and Friedland clay were investigated in hydrothermal tests with dense samples confined in oedometers with 95 °C temperature at one end, which was made of copper, and 35 °C at the other, for 8 weeks. A 1 % CaCl2 solution was circulated through a filter at the cold end. At the end of the tests, the samples were sliced into three parts, which were tested with respect to expandability, hydraulic conductivity, and chemical composition. The tests showed that while the saponite was hardly changed at all and did not take up any copper, MX-80 underwent substantial changes in physical performance and adsorbed significant amounts of copper. The Friedland clay sample was intermediate in both respects.