Reactive transport modelling of a low-pH concrete / clay interface

Versión aceptada de https://doi.org/10.1016/j.apgeochem.2020.104562[Abstract:] Cement-based materials are key components in the barrier system and structural support of repositories for disposal of nuclear waste. As such, increased understanding of their long-term performance under repository conditions is paramount for the safety assessment. Quantification of the impact that cement-based materials could have on the surrounding barriers and the host rock is essential to assess long-term safety of the repository system. This interaction can impact the physical properties of the system near the interface and needs to be assessed by means of numerical modelling. A reactive transport modelling study of the interaction between a newly-developed low-pH concrete and a clay host rock (i.e. Callovo Oxfordian) over 100,000 years is presented here. The main goal is to build confidence in the consistency of the different modelling approaches and in the application of different reactive transport codes (iCP, ORCHESTRA, OpenGeosys-GEM, CORE2D, and MIN3P) to analyse the performance of the recently developed low-pH concrete within the CEBAMA project. A common setup of a reference case was established, including precipitation/dissolution reactions, redox and cation exchange processes, building upon preliminary cases of increasing complexity. In addition, a set of sensitivity cases was simulated to test the effect of key geochemical and transport parameters on the results, including the impact of porosity changes on the diffusion coefficient and electrochemical couplings. Different reactive transport codes were used in the benchmark. Overall, the results show not only the high level of understanding of the governing processes but also the good agreement obtained with different codes, which is essential to demonstrate the applicability of reactive transport modelling to support safety assessment. The sensitivity and preliminary cases modelled show that the results obtained are much more sensitive to changes to transport parameters and couplings than to the different modelling tools used in each case. In addition, the impact of including or not the slow kinetics of dissolution of the claystone minerals is shown to be negligible in the studied scenarios.The research leading to these results has received funding from the European Union's European Atomic Energy Community's Horizon 2020 Programme (NFRP-2014/2015) under grant agreement, 662147 – CEBAMA. V. Montoya., J. Poonoosamy and G. Deissmann acknowledge the German Federal Ministry of Education and Research (Grant 02NUK053A) and the Initiative and Networking Fund of the Helmholtz Association (Grant SO-093) within the iCross project for partial funding. The authors would like to thank Barbara Lothenbach for fruitful discussions on modelling cement hydration of the low-pH cement system and the two reviewers for constructive and valuable comments that have helped to improve the manuscript.Alemania. German Federal Ministry of Education and Research; 02NUK053AAlemania. Initiative and Networking Fund of the Helmholtz Association; SO-09

Modelling of the long-term evolution and performance of engineered barrier system

Components of the so-called “multiple-barrier system” from the waste form to the biosphere include a combination of waste containers, engineered barriers, and natural barriers. The Engineered Barrier System (EBS) is crucial for containment and isolation in a radioactive waste disposal system. The number, types, and assigned safety functions of the various engineered barriers depend on the chosen repository concept, the waste form, the radionuclides waste inventory, the selected host rock, and the hydrogeological and geochemical settings of the repository site, among others. EBS properties will evolve with time in response to the thermal, hydraulic, mechanical, radiological, and chemical gradients and interactions between the various constituents of the barriers and the host rock. Therefore, assessing how these properties evolve over long time frames is highly relevant for evaluating the performance of a repository system and safety function evaluations in a safety case. For this purpose, mechanistic numerical models are increasingly used. Such models provide an excellent way for integrating into a coherent framework a scientific understanding of coupled processes and their consequences on different properties of the materials in the EBS. Their development and validation are supported by R&D actions at the European level. For example, within the HORIZON 2020 project BEACON (Bentonite mechanical evolution), the development, test, and validation of numerical models against experimental results have been carried out in order to predict the evolution of the hydromechanical properties of bentonite during the saturation process. Also, in relation to the coupling with mechanics, WP16 MAGIC (chemo Mechanical AGIng of Cementitious materials) of the EURAD Joint Programming Initiative focuses on multi-scale chemo-mechanical modeling of cementitious-based materials that evolve under chemical perturbation. Integration of chemical evolution in models of varying complexity is a major issue tackled in the WP2 ACED (Assessment of Chemical Evolution of ILW and HLW Disposal cells) of EURAD. WP4 DONUT (Development and improvement of numerical methods and tools for modeling coupled processes) of EURAD aims at developing and improving numerical models and tools to integrate more complexity and coupling between processes. The combined progress of those projects at a pan-European level definitively improves the understanding of and the capabilities for assessing the long-term evolution of engineered barrier systems

Synthèse de carbure de titane par combustion auto-propagée (germination, croissance et stabilisation de nanostructures)

L'objectif principal de cette étude est de mieux appréhender les phénomènes physico-chimiques lors de réactions SHS. La trempe nous a permis d'accéder à la microstructure transitoire des produits lors de la synthèse de TiC. Les premiers germes de carbure apparaissent à la surface de particules de C depuis une phase liquide. Les analyses WDS ont permis de montrer que la composition et la taille des grains de TiC augmentent ensuite continûment. La croissance s'accompagne d'autres phénomènes qui conduisent la distribution en taille à s'élargir et à devenir log-normale. La thermographie infrarouge a permis de montrer que la taille finale des grains est contrôlée par la convolution température-temps. Ainsi, la stabilisation de TiC nanostructuré peut être réalisée en augmentant le nombre de sites de germination et en abaissant la température de combustion. Nous avons réussi à stabiliser des nanostructures de TiC en ajoutant des diluants nanostructurés aux mélanges activés mécaniquement.The main goal of this study is to better understand physico-chemical phenomena during SHS reactions. The quench method enabled us to characterize the product microstructure during transitional steps of the TiC synthesis. First seeds of carbide nucleate on C particles from a titanium liquid phase. Thanks to WDS, we showed that TiC grain composition and size continuously increases until it reaches the stoichiometry and its equilibrium size. The grain growth occurs also with other phenomena which lead to the broadening of the grain size distribution but also to a lognormal type distribution. Infrared thermography enabled us to show that the final grain size is controlled by convolution of temperature and time. Thus, the stabilization of nano-TiC can be achieved by increasing the number of nucleation sites and by decreasing the combustion temperature. We showed the ability to stabilize grains of nano-TiC by adding nanostructured diluents to mechanically activated reactant mixtures.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

EURAD-ACED: Modeling of the thermal transient stage and the long-term evolution of the glass / steel / cement / claystone interactive system in a HLW disposal cell

International audienc

EURAD-ACED: Modeling of the thermal transient stage and the long-term evolution of the glass / steel / cement / claystone interactive system in a HLW disposal cell

International audienc

TReacLab: An object-oriented implementation of non-intrusive splitting methods to couple independent transport and geochemical software

International audienceReactive transport modeling contributes to understand geophysical and geochemical processes in subsurface environments. Operator splitting methods have been proposed as non-intrusive coupling techniques that optimize the use of existing chemistry and transport codes. In this spirit, we propose a coupler relying on external geochemical and transport codes with appropriate operator segmentation that enables possible developments of additional splitting methods. We provide an object-oriented implementation in TReacLab developed in the MATLAB environment in a free open source frame with an accessible repository. TReacLab contains classical coupling methods, template interfaces and calling functions for two classical transport and reactive software (PHREEQC and COMSOL). It is tested on four classical benchmarks with homogeneous and heterogeneous reactions at equilibrium or kinetically-controlled. We show that full decoupling to the implementation level has a cost in terms of accuracy compared to more integrated and optimized codes. Use of non-intrusive implementations like TReacLab are still justified for coupling independent transport and chemical software at a minimal development effort but should be systematically and carefully assessed

Impact of fractures on diffusively dominated reactive transport: application to radioactive waste storage studies

International audienceEven in small numbers, fractures must be carefullyconsidered for the geological disposal of radioactivewastes. They critically enhance diusivity, speed upsolute transport, extend mixing fronts and, in turn,modify the physicochemical conditions of reactivityaround possible storage sites. Fractures occur at se-veral places in the cement surrounding the containersand in the Excavation Damaged Zones (EDZ) of thegalleries. They even occur in clays like in the FrenchCallovo-Oxfordian formation mostly because of the de-saturation conditions induced in the operational timeof the galleries.Numerous studies in various elds (e.g. radioactivewaste storage, CO2 sequestration, geothermal storage,hydrothermal alteration) have shown that fracturescannot be simply integrated within an equivalent po-rous medium with a simple enhancement of its petro-physical properties (porosity and permeability). Frac-tures cannot either be accurately identied and fullydeterministic modeling approaches are precluded.We propose a combined numerical and experimentalapproach to determine the influence on reactivity oftypical fracture patterns classically found in radioac-tive waste applications. We investigate the possibilityto apply simplied modeling frameworks on the basisof some key properties :(i) transport is mostly diusive and much faster inthe fractures than in the porous matrix [1],(ii) reactivity occurs predominantly in the matrix be-cause of the large surface to volume ratio favorableto dissolution/precipitation processes,(iii) reactivity within the surrounding matrix is atequilibrium, or equivalently much faster thanthe diusive transport. Reactivity is assumedtransport-limited rather than rate-limited.Based on the separation of the fracture and ma-trix domains, we develop a reactive transport mo-del with diering diusion conditions in the fractureand in the matrix, appropriate flow-rock interactionsat equilibrium in the matrix and fracture-matrix ex-change conditions at their interface. Using preferen-tially existing software, we propose simulation methodsthat comply with much faster diusion in the fracturethan in the matrix, and validate them against elemen-tary fracture structures and simplied reactivity.We intend to use the developed methods on dierentfracture structures to simulation reactivity over longperiods of time. We determine the possible relevanceof the most classical simplied frameworks for fracture-matrix including :(i) fully homogenized models with adapted porosity,permeability and surface to volume ratio to reco-ver localization eects,(ii) models with isolated fractures within "innite ma-trix" assuming implicitly the localization of reac-tivity in the immediate vicinity of the fracture [2],(iii) double porosity models characterized by single ormultiple exchange coecients[3].Following the outcome of the numerical simulations,we will investigate experimentally the most critical li-mitation of reactivity. It might a priori be either thefracture to matrix exchange law especially if fractureis desaturated and matrix saturated.Within the radioactive waste framework, we aim atincluding fractures in the safety assessment workflow.We intend to determine to which extent fractures faci-litate the access to reactive surfaces, the increase of thebulk reactivity, the corrosion potential and the pertur-bation of the chemical conditions.We frame as much aspossible the reference simulations in realistic physicaland chemical conditions including the main operatio-nal phases of the radioactive waste repository. Resultswill be reported as comprehensive evolution scenarios.References[1] D. Roubinet, J. R. Dreuzy, and D. M. Tartakovsky.Semi-analytical solutions for solute transport and ex-change in fractured porous media. Water ResourcesResearch, 48(1), 2012.[2] C. I. Steefel and P. C. Lichtner. Multicomponent reac-tive transport in discrete fractures : I. controls on reac-tion front geometry. Journal of Hydrology, 209(1) :186{199, 1998.[3] T. Xu and K. Pruess. Modeling multiphase non-isothermal fluid flow and reactive geochemical transportin variably saturated fractured rocks : 1. methodology.American Journal of Science, 301(1) :16{33, 2001

Chemical evolution of a HLW cell in Callovo-Oxfordian Claystone: taking into account the oxic transient period

International audienceNumerous calculations have been performed to represent the long-term chemical evolution of theautochthonous (e.g. argillaceous rock) and allochthonous (e.g. cement based materials, carbon steel)materials that may interact within the French underground radioactive waste repository concept. Theoxic transient related to the operating period (due to the access drift ventilation) in High-Level andlong-lived Waste (HLW) repository facilities is often neglected in the simulation of their long-termchemical evolution. However the initial oxidation of the reduced environment prevailing in theclaystone and the oxic corrosion of the carbon steel in the HLW cell head combined with the anoxiccorrosion of the carbon steel and H2 production in the using part of the cell may lead to a complexoxidizing/reducing front (De Windt et al., 2014). The present study intends to simulate the transitionphase between the periods with oxidative and reductive conditions in order to determine:• how the chemical compositions of the metallic materials, clay and cement in the HLW cell arealtered by atmospheric oxygen and carbon dioxide during the operating period;• and how these alterations affect the long term chemical evolution of the system after closureof disposal cell.The modelling strategy relies on a two steps procedure. Two phases flow simulations were carried outwith Comsol Multiphysics, in order to obtain the temperature and water saturation profiles as afunction of the different operating steps. The chemical evolution of the HLW cell was then simulatedwith the reactive transport code CrunchFlow (Steefel et al., 2014) with fixed water saturation andtemperature profiles derived from the thermal-hydrology simulations. The code flexibility enabled thesimultaneous consideration of the irreversible reaction describing the pyrite oxidation by O2, andsubsequent sulphates release, and the reversible reaction of pyrite dissolution/precipitation underanoxic conditions. The simulation of the oxic period led to pyrite oxidative dissolution together withiron oxi(hydr)oxides and gypsum precipitations (Fig. 1). As a result of pyrite dissolution, the pH valuein the pore water decreased, but is rapidly buffered by carbonate dissolution at the wall of the drift.After the sealing of the disposal cell by addition of 3 meters of bentonite and 4 meters of concrete(2009 version of Andra’s concept), iron canister corrosion consumes the O2, which leads to theestablishment of reducing conditions. Once O2 is depleted, the canister corrosion then produces H2.Magnetite and siderite were simulated as being the main corrosion products. The alteration of the claymineralsunder reducing conditions was characterised primarily by a transformation of pyrite intopyrrhotite. In addition, formation of greenalite was simulated at the interface between the claystoneand the metallic material. Those two predictions are in agreement with results obtained on short termexperiments (Truche et al., 2009; Bourdelle et al., 2014). Simulation results indicated also adestabilization of the illite–smectite and quartz minerals of the claystone (Fig. 2)