Near-field chemistry of a HLW/SF repository in Boom Clay - scoping calculations relevant to the supercontainer design

Transcription

1 EXTERNAL REPORT SCK CEN-ER-17 09/LWa/P-140 Near-field chemistry of a HLW/SF repository in Boom Clay - scoping calculations relevant to the supercontainer design First full draft Lian Wang Report prepared by SCK CEN in the framework of ONDRAF/NIRAS programme on geological disposal, under contract CCHO /00/00 December, 2009 SCK CEN Boeretang Mol Belgium R&D Disposal Unit

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3 EXTERNAL REPORT OF THE BELGIAN NUCLEAR RESEARCH CENTRE SCK CEN-ER-17 09/LWa/P-140 Near-field chemistry of a HLW/SF repository in Boom Clay - scoping calculations relevant to the supercontainer design First full draft Lian Wang Report prepared by SCK CEN in the framework of ONDRAF/NIRAS programme on geological disposal, under contract CCHO /00/00 December, 2009 Status: Unclassified ISSN SCK CEN Boeretang Mol Belgium R&D Disposal Unit

4 SCK CEN Belgian Nuclear Research Centre Boeretang Mol Belgium Phone Fax Contact: Knowledge Centre RESTRICTED All property rights and copyright are reserved. Any communication or reproduction of this document, and any communication or use of its content without explicit authorization is prohibited. Any infringement to this rule is illegal and entitles to claim damages from the infringer, without prejudice to any other right in case of granting a patent or registration in the field of intellectual property. SCK CEN, Studiecentrum voor Kernenergie/Centre d'etude de l'energie Nucléaire Stichting van Openbaar Nut Fondation d'utilité Publique - Foundation of Public Utility Registered Office: Avenue Herrmann Debroux 40 B-1160 Brussel Operational Office: Boeretang 200 B-2400 Mol

5 Abstract In Belgium, the current design of engineering barrier system (EBS) for geological disposal of high level waste (HLW) and spent fuel (SF) plans to use cementitious materials as buffer and backfill in addition to concrete as construction material. This means that the near field (NF) will be composed of essentially cementitious materials and the NF chemistry will be governed by degradation of concrete under the repository conditions. This report presents results of computer simulations aiming at representing some key aspects of NF chemistry relevant to the design under the geochemical conditions of the Boom Clay as a host rock for the repository. Because of the large quantity of concrete used in the design and a very slow ingress of Boom Clay pore water controlled by diffusion, an alkaline condition with a high ph is expected to sustain for a very long time within the NF. Scoping calculations based on a local equilibrium- diffusion transport model were performed and confirmed that the NF will likely remain alkaline for a geological time span. To define compositions of NF pore fluid which is required by laboratory experiments to determine the corrosion rate of vitrified waste and SF, a flow-through model was applied assuming a local equilibrium between Boom Clay pore water and concrete components to derive compositions of concrete pore fluids for three stages in time: young concrete fluid, evolved concrete fluid, and calcium-silicate-hydrates (CSH) fluid. Redox condition within the NF will likely be very reducing before the complete depletion of the metallic barriers due to corrosion. Afterwards, the redox will be governed mainly by the in- diffusing pore water of the Boom Clay host rock. Key factors influencing simulation results are porosity change of concrete, boundary condition, and the value of diffusion coefficient in concrete. Very rapid and significant reduction of porosity was predicted to occur at the concrete- clay interface because of carbonation. Use of more realistic boundary conditions in the modeling was proven to represent the system better but made simulations more difficult and more time consuming to perform. In pursuing modeling with the most realistic boundary conditions possible, simple mass action/balance approach is still indispensable to a fully coupled, quantitative modeling technique in rationalizing the results of computer simulations. Uncertainties remain in thermodynamic data, the value of diffusion coefficient, the treatment of interdependency of porosity and solute transport. Retardation mechanisms of simple anions like chloride and sulfur species are poorly known so the migration of these species within the NF can only be scoped by diffusion and remain uncertain. Although suffice to make a conservative estimation for the intended purpose, boundary conditions used in diffusion simulations should be improved to represent the NF in a more realistic way. 5

6 Table of contents 1 Introduction...7 Background and scope of this report...7 The near field: components and geometry...7 The far field: Boom Clay geochemistry and pore water composition...9 General modeling strategy Diffusion only for solute transport Model composition of the NF: OPC only Simplified CSH model Local equilibrium model without kinetics Temperature effect...12 Computer code and database Input Boom Clay pore water compositions...13 Pore water composition for the reference case...14 Pore water composition for the worst-case A flow-through, local equilibrium model to define NF porewater compositions...15 Selection of hydration products of concrete used in the model...15 Alkali concentrations in the cement pore fluid...16 Model composition of solid phases in concrete...17 Young, evolved, and CSH controlled cement pore water compositions The evolution of ph and concentrations of sulphate and detrimental species from Boom Clay pore water at the overpack and SS envelope...19 Boundary conditions and discretization of model domain...19 The initial model system of NF...21 Definition of different cases...22 Results and discussions Initial pore water composition of the NF Porosity occlusion of concrete ph evolution for a defined conservative case with no porosity clogging Concentration evolution of sulphate and detrimental species at the overpack and the SS envelope Effect of porosity sealing on NF chemistry Effect of boundary conditions on NF chemistry The evolution of redox condition in the NF Uncertainties and limitations Summary References...37 Annex I: Thermodynamic data concerning reactions and constants for minerals considered in the model...41 Annex II: GWB react input and output for calculating cement hydration product composition...42 Annex III: GWB x1t input and output for calculating young, evolved, and CSH concrete pore water compositions

7 1 Introduction Background and scope of this report ONDRAF/NIRAS has recently selected the supercontainer with Ordinary Portland Cement (OPC) buffer as the preferred design for disposal of high level waste (HLW) and spent fuel (SF). In support of the design assessment, experts concerned need to know how long the NF would remain its designed condition for the purpose of radioactive waste containment. This is primarily because: 1. the supercontainer design is based on the Contained Environment Concept (CEC) which considers that a high ph, alkaline environment provided by OPC based concrete will ensure passivity of any carbon or stainless steel embedded in it (ONDRAF/NIRAS, 2004a). It is therefore essential to know how long such a favorable high ph condition might sustain under the expected repository conditions; 2. as vitrified and spent fuel waste forms will be eventually in contact with cementitious materials after the failure of the overpack, NF pore water compositions are needed for laboratory experiment to determine the corrosion rate of waste forms; 3. to be able to estimate the life time of metallic barriers in the NF, corrosion experts need to know the probable time scale at which a critical concentration of detrimental species from Boom Clay might reach the surface of metallic barriers; 4. knowledge of NF chemistry is needed to assess radionuclides retardation within the near field and the influence of the NF chemistry on the retardation of radionuclides in the far field This report was therefore requested by ONDRAF/NIRAS through the research plan DS 251-B62 to carry out scoping calculations to anticipate the evolution of the NF chemistry under the designed and the repository conditions. The report is in the form of first full draft and will either be finalized or updated and contributed to the planned general EBS report at the end of Corrosion experts have identified that the key factors influencing the life time of the metallic barriers are ph and concentrations of chloride and sulfur species (ONDRAF/NIRAS, 2004a). The objective of this report is thus to perform scoping calculations in estimating the evolution of ph and the concentration of anionic species, some are detrimental to the metallic barriers, from Boom Clay pore water within the NF surrounded by the Boom Clay as a host rock. The calculations were done with a geochemical coupled solute transport computer code based on the input data mainly from Wickham (2004). The near field: components and geometry Figure 1 shows the components of the EBS relevant to the supercontainer design for high level vitrified waste (ONDRAF/NIRAS, 2004b). Although this report is dealing with NF chemistry in general, our primary interest is the chemical evolution at the external surface of the overpack and the stainless steel envelope (SS). The NF can thus be defined as the region from the overpack to the interface of Boom Clay and the lining concrete wedge blocks. As the result, the NF is composed of the lining concrete blocks, the cementitious backfill, the SS envelope, and the OPC buffer. Note that this definition of the NF excludes overpack and is for the convenience of the present work only. More detailed modeling might be desirable considering the influence of corrosion products on chemistry of the NF. In that 7

8 case, the NF should include overpack as a reactive component. The definition of the NF can thus be different. Figure 1: Components of the EBS relevant to the supercontainer design for high level vitrified waste (Wickham, 2004). Dimensions correspond to room or surface temperatures are expressed in meters: D ex = 3.23; D g = 2.53; D sc = 1.928; D op = 0.516; and T 2 = Note that a more up-to-date drawing to this one is currently available. The difference between the two drawings is considered minor and will not affect the results and conclusions of the present scoping calculation to significant extent. As the NF components will be constructed in a cylindrical geometry, solute transport simulations were performed in a radial coordinate as represented in Figure 2. The radii of NF components are calculated based on the dimensions given in the caption of Figure 1. r 1 = 0.21 m r 2 = 0.96 m r 3 = 1.62 m r 2 r 3 Boom Clay r 1 OPC buffer overpack stainless steel envelope lining concrete wedge blocks and backfill Figure 2 Schematic illustration of the model domain representing the NF geometry for a model simulation of the NF chemical evolution as a function of time. In Figure 2, r1, r2, and r3 are the larger radius of the overpack, the cementitious buffer, and the concrete wedge blocks respectively. The extent of the NF is therefore r3 r1 = 1.41 meters. 8

9 The far field: Boom Clay geochemistry and pore water composition The far field is the Boom Clay which provides the inward pore water to the NF. The pore water is considered in chemical equilibrium with reactive minerals present in the Boom Clay through solubility and cation exchange reactions. The mineralogical composition of Boom Clay is provided in Table 1 based on the data presented by De Craen et al., (2004). Table 1: Mineralogical composition of Boom Clay. Value in wt% dry weight Clay minerals Illite Smectite + illite/smectite ML Kaolinite 5-20 Chlorite 0-5 Chlorite/smectite ML 0-5 Quartz K-Feldspars 1-10 Albite 1-10 Carbonates 1-5 Calcite 1-5 Siderite present Dolomite present Ankerite present Pyrite 1-5 Organic Carbon 1-5 Others: Glauconite, apatite, rutile, anatase, Ilmenite, zircon, monazite, xenotime present present Among Boom Clay minerals, reactive components, i.e., with relative fast kinetics, are calcite, siderite, and pyrite. Calcite dissolution/precipitation controls the calcium concentration of Boom Clay pore water while the iron concentration and the redox potential are determined by solubility of siderite and pyrite. Cation exchange is considered responsible for regulating concentrations of sodium, potassium, and magnesium in pore water. Majority of clay minerals are not reactive in terms of fixing pore water composition by solubility reactions except that the dissolved aluminum concentration is constrained by the solubility of kaolinite. Solubility of chalcedony (quartz) seems in agreement with the measured concentration of dissolved silica. A reference Boom Clay pore water composition is given in Table 2. Detailed experimental and modeling procedures for deriving this composition were presented by De Craen et al., (2004). An important aspect of Boom Clay pore water geochemistry is the mechanism controlling the partial pressure of carbon dioxide P CO2 (g) since it affects the ph of the pore water. It is the current understanding (De Craen et al., 2004) that the P CO2 (g) in Boom Clay would depend merely on temperature and is expected to be fixed by water-rock interactions between the pore water and Boom Clay forming minerals such as carbonates, kaolinite, chlorite, and silica. In other words, P CO2 in Boom Clay should be well buffered as long as these controlling minerals are present at a constant temperature. This has an important implication in using a fixed concentration as the boundary condition at the NF- Boom Clay interface to calculate diffusion (Section 4.1 and 4.4.6). However, if 9

10 temperature of the Boom Clay varies, the P CO2 would change accordingly. Another mechanism might influence the Boom Clay P CO2 is the degradation of natural organic matter at elevated temperature (Deniau et al., 2005). Both mechanisms would lead to an increase in P CO2, hence a decrease in ph, when temperature in the Boom Clay increases. A fully coupled modeling taking temperature effects into account is not performed within the frame of this report. For a purpose of scoping the system mass balance in terms of the ph buffering capacity of the NF, a very conservative case has been modeled and illustrated that an expected decrease in ph due to temperature increase would slightly decrease the life time of the NF concrete but not to a significant extent (see Section and 4.4.3). From Table 2, it is seen that a undisturbed Boom Clay pore water is a diluted NaHCO 3 solution of 15 mmolal with a ph of 8.5 and redox potential (E h ) of around -300 mv. Presence of a relative high concentration of dissolved organic matter (DOC) in pore water is one of the outstanding characteristics of the Boom Clay. Table 2: A reference Boom Clay pore water composition (De Craen et al., 2004) Species or parameter Concentration Units Na K Ca Mg Fe Si 0.1 Al 2.40 E-05 mmole litre -1 - HCO TIC (mg C/litre) 15.1 DOC (mg C/litre) Cl Total S SO ph pco atm Eh -274 mv Temperature 16 C General modeling strategy The objective of the modeling is to determine through model simulation the evolution of ph and the concentration of anionic species from the Boom Clay at the overpack and the SS envelope as a function of time. Since the anticipated chemical evolution within the NF will be regulated by both chemical degradation of the NF materials and time dependent solute transport, a chemical coupled reactive transport model was employed. Note that only chemical components of concrete were treated as reactive. The transport of corrosion products of metallic barriers was not taken into account as the overpack and the SS envelope were not considered as part of the NF (Section 1.2). 10

11 Future modeling may consider the influence of the reactive transport of corrosion products on evolution of NF chemistry. Dissolution/precipitation reactions of cementitious materials are calculated together with diffusion of dissolved species within the NF. Retardation of chemical species is due only to precipitation in space. No sorption is mechanistically taken into account, for example, by surface complexation or formation of a solid solution. Linear sorption of chloride was considered in one case by simple distribution coefficient K d. No reactive transport was attempted in terms of up take of chemical species by concrete. The modeling accounts for changes of chemical speciation of Boom Clay pore water and cementitious components along the diffusive pass ways. In that term, the modeling is fully reactive. When calculating the ingress of chloride, sulfate, thiosulphate, and sulfide species, only diffusion was taken into account because of the lack of reaction mechanisms describing the interactions of these species with cement. In those cases, the modeling was not reactive and the concentration evolution of species was only determined by diffusion driven by concentration gradient Diffusion only for solute transport Molecular diffusion is the only solute transport process taken into account in this work. Advection might occur at the stage of re-saturation of the NF by Boom Clay pore water but the period is expected to be extremely small compared to the time scale of interest for scoping the evolution of NF chemistry. Weetjens et al., (2005) concluded that only a few years is needed to re-saturate completely the NF. After the re-saturation, the solute transport within the NF will be purely diffusive. The applied computer code (see Section 1.5) calculates diffusion with Fick s law for the dispersive flux q D (mol/cm 2 sec) of a chemical component in solution: q D C = φ D (1) x where φ is the domain porosity, D is the coefficient of hydrodynamic dispersion and becomes the pore diffusion coefficient in a pure diffusion case where water velocity is zero, and C is the volumetric concentration of the component in question. The product of φ and D can be considered as an effective diffusivity or an effective diffusion coefficient. The diffusive flux varies as the porosity changes by chemical reactions as shown in Equation (1). The pore diffusion coefficient D remains constant throughout in the present simulation. It is however possible to link the pore diffusion coefficient with the porosity evolution and it could be one of the future aspects to consider Model composition of the NF: OPC only To simplify the computer simulation, the NF composition is considered as homogeneous at this stage, i.e., the entire NF is consist of only OPC based concrete. No distinction is made between the buffer, the backfill, and the lining wedge block materials. In terms of chemistry, difference between these cementitious materials is expected to be small for the purpose of this modeling work. In terms of solute transport properties, the buffer, the backfill, and the domain of wedge blocks may differ in initial porosity, permeability, and diffusion coefficient. Future simulation might be interesting to estimate the influence of using different chemical compositions and solute transport characteristics on the results of simulation. 11

12 One important decision must be made before the modeling work is whether or not the SS envelope is water tight, i.e., if the SS envelope is a barrier against water ingress and diffusion. In this report, we assume that the SS envelope does not exist, meaning that the SS envelope has neither a barrier function nor influence on chemical evolution of the NF. This may be considered as equivalent to the case where very early perforation of the SS envelope occurs or the SS envelope is not water tight at the beginning of the repository operation. Note that if the SS envelope is water tight, no solute transport will be possible across the interface of the backfill and the buffer. The chemistry within the buffer would remain unchanged until the perforation of the SS envelope Simplified CSH model Mechanism of CSH dissolution in cement is complicated because reaction stoichiometry varies as a function of Ca/Si ratio. Berner (1992) has reorganized that modeling CSH dissolution with a classic speciation model is difficult because of incongruent dissolution of CSH phases when solution composition changes. Lothenbach and Winnefeld (2006) recently modeled hydration of Portland cement with a CSH solid solution model considering kinetics of clinker dissolution. At present, solid solution model for CSH might be the state of the art in predicting a pore water chemistry of cement at initial stage of hydration (days to months), implementing a solid solution model into a reactive transport simulation is not straightforward. In our current scoping calculations, we decided not to use a solid solution model mainly because the relative fast kinetics of clinker dissolution (days to months) compared to the time scale of our interest (thousands to tens of thousands of years). In other words, our primary interest is on the mass balance of a system involving end members of hydration products of cement for a long term. In addition, the expected NF chemistry would be controlled by dissolution of alkalis and portlandite for a very long time before CSH phases start to dominate. For our intended purpose, we modeled CSH with a congruent dissolution mechanism using phases with a fixed stoichiometry that is independent of Ca/Si ratio in solution Local equilibrium model without kinetics Chemical models are based on local equilibrium assumption without kinetics. In other words, chemical reactions within the NF occur instantaneously without time constrain and thus the only factor relating the chemistry evolution to time is diffusion of the Boom Clay pore water into the NF. In reactive transport modeling, solution composition predicted is in general independent of kinetics if sufficiently long time spans are considered (Lichtner, 1996). This is mainly because the long time spans allow chemical equilibrium to be reached between the pore fluid and solid phases. In our case, a local equilibrium assumption seems reasonable because of the slow ingress of Boom Clay water components and relative fast kinetics of water-cement interactions Temperature effect The NF may experience an elevated temperature in the thermal phase of the repository operation. Weetjens, et al., (2005) calculated the heat production and transport in Boom Clay for a vitrified waste repository and concluded that the temperature at the interface of the NF and far field can reach 80 C at the first couple of years after the emplacement of the waste. It is therefore desirable if the temperature effect on the NF chemistry can be scoped. Solubility calculations were performed at different temperatures and confirmed that the ph will decrease within the concrete buffer at a temperature higher than 25 C, in agreement with literature data. Systematic accounting for temperature effect is difficult because of both uncertainty in thermodynamic data for temperatures other than 25 C and a lack of mechanistic descriptions on system alteration. 12

13 Computer code and database The computer code applied in this work is GWB - The Geochemist s Workbench professional, release (Bethke, 2006). GWB is a bunch of computer programs for manipulating chemical and geochemical reactions, calculating stability diagrams and the equilibrium state of natural waters, tracing reaction processes, modeling reactive transport, and plotting the results of these calculations. Two programs were used in the current scoping: the react was employed for equilibrium calculation and the x1t was applied for reactive-diffusion simulations. Activity coefficients of aqueous species are computed with the B-Dot model (Bethke, 1996): 2 Azi I logγ i = + b I (2) 1 + a B I i which is an extension of the Debye-Hückel equation. Coefficients A, B, and b vary with temperature, whereas the ion size parameter a i for each species remains constant and z i the charge of the species. The thermodynamic database is the LLNL v8r6 database (Wolery, 1992) provided with the code plus some improvements and modifications. With all basis data unchanged within the LLNL database, most data of cement phases were taken from Bourbon (2003); the data of calcium-silicate-hydrate (CSH) phases was from Stronach and Glasser (1997); the solubility constant for portlandite came from the work of Duchesne and Reardon (1995); and some stability constants of aluminum aqueous species were adapted from Nordstrom et al., (1990). Reaction constants of considered minerals are provided in annex 1. These data were selected from different literatures as cited. The general strategy is to use the data produced or reviewed by well accepted experts or groups in the related field. However, we are aware of the fact that by mixing thermodynamic data from different sources, there is possibility that the newly compiled database becomes internally inconsistent. A database project is in progress aiming at compilation and quality checking of thermodynamic database used for safety case development of radioactive waste disposal in the Boom Clay. At the present time, we check the database by comparing our simulation results with literature data. For example, solubility of individual cement components were computed and compared with published values under similar conditions. The database can only be used if agreements were reached between our own calculations and literature data. Even though one must note that there has not been a systematic consistency checking done to the thermodynamic database used in the present scoping. 2 Input Boom Clay pore water compositions For predicting the ph of the NF, it is sufficient to consider the in-diffusing Boom Clay pore water as a 15 mmolal NaHCO 3 solution. In terms of corrosion of metallic barriers in the NF, dissolved anionic species to be considered are chloride (Cl - ), sulfur species (SO 4 2-, S 2 O 3 2-, HS - /S 2-, and polysulfide), and the dissolved inorganic carbon species (HCO 3 - and CO 3 2 ). Except SO 4 2-, these anionic species are considered detrimental to metallic barriers. It is accepted that the concentrations of chloride and sulfur species in an undisturbed Boom Clay pore water are too low to cause a significant corrosion of metallic barriers under the expected NF conditions (Wickham, 2005). Increased concentrations of detrimental species are however possible when the Boom Clay is perturbed. It should be noted that although Boom Clay pore water is a bicarbonate solution, when ph 13

14 remains high (> 10) in the NF, the in-diffusing bicarbonate species from the Boom Clay will turn to carbonate (CO 3 2 ) because of change in speciation. In Section 1.3, a reference Boom Clay pore water composition was already given (Table 2). For the purpose of reactive transport modeling, a slightly different composition was adapted to represent a undisturbed water composition, which is defined as a reference case. A worst-case water composition was also considered to replicate a pore water under perturbed conditions with the highest possible concentrations of detrimental species. Pore water composition for the reference case In the reference case, an undisturbed Boom Clay pore water composition was defined as input and is provided in Table 3. Table 3: Input Boom Clay pore water composition for the reference case Species or parameter Concentration Units Na + charge balance K Ca Mg Fe Si 0.1 mmol litre -1 Al 2.40E-05 - HCO Cl SO ph Temperature 25 C Table 3 is almost identical to Table 2. The only differences are temperature, redox constraint, and the sodium concentration. Temperature is 25 C for the input pore water composition instead of 16 C. Redox potential is not constrained for the water composition in Table 3, i.e., assuming there is no redox change within the NF 1. For scoping the evolution of ph and concentration of detrimental chemical species at the current stage of modeling, redox potential should have minor effect on the most of issues considered. Sulfur species may undergo redox reactions when diffusion occur in the NF. We however do not have sufficient understandings or data to model sulfur redox behavior in a cementitious environment. Different from Table 2 where the sulphate concentration was in a redox equilibrium with pyrite in Boom Clay (E h = -274 mv), sulphate in the NF is in chemical equilibrium with a cement hydration product ettringite Ca 6 Al 2 (SO 4 ) 3 (OH) 12 :26H 2 O so no redox reaction is involved. Also, no P CO2 is imposed for the water composition in Table 3 and the Na + is defined as the charge balancing species instead of given concentration. Comparing Table 3 to Table 2, all simplifications were made to facilitate the simulation and won t change pore water chemistry for the intended purpose. 1 redox was not modeled as explained in Section 5. 14

15 Pore water composition for the worst-case We use the worst-case water (Table 4) defined by Wickham (2004) to include an oxidized, disturbed Boom Clay pore fluid. It was considered as the most unfavorable composition of anions to the integrity of the overpack, i.e., the worst case for the corrosion of the metallic overpack. Note that Table 4 lists concentrations measured in different water samples by different water extraction techniques. Thus the cause of the observed high concentrations compared to the undisturbed water may not be due only to oxidation. For example, the worst-case chloride concentration was measured in a batch experiment where only limited oxidation was noticed. Also, although named as the worst-case, anion concentrations in Table 4 are not the highest ever measured in Boom Clay pore water. Van Geet et al., (2006) recently summarized a composition under the worst conditions ever measured which shows substantial higher concentrations of sulfur species (see Table 10). Table 4: A reference composition and parameter ranges for a worst-case disturbed Boom Clay pore water, based on data of Wickham, (2004) Species or parameter Concentration units 2- SO S 2 O mmol litre -1 HS - /S Cl - 12 ph 6-3 A flow-through, local equilibrium model to define NF porewater compositions In this section, concrete pore fluid compositions are derived by a flow-through modeling assuming the pore water and the selected hydration products of cement are in chemical equilibrium. A flowthrough model simulated the evolution of the NF chemistry by allowing Boom Clay pore water to flow through the model domain. As a function of time, effective components of concrete were gradually eluted by the incoming Boom Clay pore water and the composition of the concrete pore fluid varied. This is similar to the so called mixing tank model (Neall, 1994) in which mass balance calculation can be made based on the amount of water cycles passing through the NF and washing out the buffer materials. Hydration products of cement were selected mainly based on literature data. The volume percentage of these hydration products and concrete aggregates were determined through equilibrium calculation with selected thermodynamic database (annex I). Water content and porosity of the concrete buffer were calculated with the known water to cement ratio. Alkali concentrations in concrete pore fluid were fixed based on literature data and the total alkali content of the cement. Pore fluid with dissolved alkalis were assumed in chemical equilibrium with the selected mineral assemblage of concrete to derive concrete pore water compositions for three stages: young concrete water, evolved concrete water, and CSH water. Selection of hydration products of concrete used in the model According to the composition of supercontainer concrete given in Table A.4.1 by Wickham (2004), the supercontainer is composed of 350 kg m -3 cement CEM I, 150 kg m -3 water (W:C = 0.43), and 15

16 the rest calcium carbonate aggregates. We therefore consider that the model domain contains a mixture of cement, water, and calcite (representing aggregates but as pure mineral). Chemical composition of cement is taken from Atkins and Glasser (1992) and is given in Table 5 as an averaged oxide composition of Portland cement. Table 5: Chemical composition of Portland cement used in the model oxide wt% CaO 63 SiO 2 20 Al Fe 2 O 3 3 MgO 2 SO 3 3 To set up a chemical composition for the model domain, the hydration products of cement must be defined first. Based on classic literatures in cement chemistry, e.g., Berner (1992), Readon et al (1992), Nielsen et al., (2005b), and Bourbon (2003), common phases of hydration products were selected as the followings: portlandite, Ca(OH) 2 afwillite Ca 3 Si 2 O 4 (OH) 6 or CSH_1.8 (Ca 1.8 SiO 4.6 H 3.6 ) ettringite, Ca 6 Al 2 (SO 4 ) 3 (OH) 12 :26H 2 O hydrogarnet, Ca 3 Al 2 (OH) 12 hydrotalcite, Mg 4 Al 2 (OH) 14 :3H 2 O hematite, Fe 2 O 3 We selected the well crystalline phase afwillite to represent CSH in hydrated cement partly because afwillite has been shown stable in presence of portlandite at high Ca/Si ratio (Wickham, 2005). Although amorphous phases of CSH solid solution with varying compositions might be more realistic in a real system, without applying an incongruent dissolution model (Section 1.4.3), there is little advantage using amorphous CSH instead of well crystalline phases. Also CSH only start to regulate concrete pore water chemistry after alkalis and portlandite initially present in concrete are completely dissolved and diffused away, the time when CSH phases become important is probably out of our primary interest. For a purpose of comparison, we did nevertheless use the solubility data of a so called CSH_1.8 phase in some equilibrium calculations to derive the concentration of dissolved silica present in the initial stage of concrete pore water. The concentration of dissolved silica is supposed to be an important parameter for a laboratory leaching experiment on vitrified waste forms. For flow-through and diffusion simulations however, only afwillite data is used. Alkali concentrations in the cement pore fluid Alkali Na and K exist in cement primarily as alkali sulphate salts. These salts are very soluble and as a result high concentrations of alkali sulfates are normally present in a concrete pore fluid. Sulfate form sparingly soluble salt such as ettringite in cement so a cement pore fluid at the early stage of hydration contains high concentrations of free Na and K. In charge balancing the system, equivalent concentration of hydroxyl ion will be produced thus a cement pore fluid at the initial stage of hydration has normally a high ph >

17 The current supercontainer design plans to use a low alkali cement (< 0.6 wt%) so the total alkali content will be % = 2.1 kg m -3. Without knowing the Na 2 O/K 2 O ratio in the to be used cement for supercontainer, we take a ratio of 0.1 according to the composition of a CEM I 42.5N cement reported by Lothenbach and Winnefeld (2006). So knowing that: % Na2O % Na 2 O % K 2 O 0.6 % and = 0. 1 % K O we obtain that the maximum content of alkalis is 0.08% and 0.8% of the total quantity of cement for Na 2 O and K 2 O respectively. Thus the supercontainer concrete should contain 280 g of Na 2 O and 2800 g of K 2 O per cubic meter of concrete. Such amounts of alkali oxides occupy about 0.01 vol% and 0.1 vol% of concrete if mole volume of Na 2 O and K 2 O is 25 and 40 cm 3 mole -1 respectively. Alkali oxides in cement do not form stoichiometrically sparingly soluble minerals as other clinkers and also do not dissolve completely in pore fluid. Taylor (1990) reported in general only 40% and 70% of Na 2 O and K 2 O are soluble in cement pore fluid. We at the moment do not know the amount of alkali oxides in supercontainer concrete that might dissolve easily in pore fluid. For the purpose of deriving cement pore fluid compositions, we start from soluble concentrations of alkalis in pore fluid. Brouwers and van Eijk (2003) derived from analyzing and modeling many sets of experimental data that an average Na and K concentration in an OPC pore fluid is 0.14 and 0.37 molal respectively and we apply these values in our modeling. To define the initial alkali rich pore fluid with an equilibrium modeling, alkali oxides were allowed to be in chemical equilibrium with a fluid containing 0.14 and 0.37 molal of dissolved Na and K respectively. An immediate problem of doing so is that the added amount of alkali oxides will dissolve completely and the resulting pore water will have much higher concentrations of dissolved alkalis than intended. To impose an equilibrium between alkali oxides and a pore water with a targeted concentrations of dissolved Na and K, values of stability constant of Na 2 O and K 2 O were modified as the following: 2 Na 2 O + 2 H + = H 2 O + 2 Na + logk (25 C) instead of K 2 O + 2 H + = H 2 O + 2 K + logk (25 C) instead of The values of stability constants of alkali oxides were chosen so that dissolution of Na 2 O and K 2 O in pore water will result in dissolved Na and K concentrations to be 0.14 an d0.37 molal respectively, i.e., the intended alkali concentrations. Such manipulation allowed that the concrete containing realistic amount of alkali oxides is in chemical equilibrium with a pore fluid having the intended alkali concentrations. Model composition of solid phases in concrete A react calculation was set up by mixing 150 kg of water with 350 kg of cement with proportions of oxides given in Table 5 plus 1,950 kg of calcite as aggregate to form 1 m 3 of concrete. Note that only the selected hydration products were allowed to precipitate when oversaturated. The total alkali content used in the calculation was 0.6 wt% of total Na 2 O equivalent. The volume percentages of cement hydration products, calcite aggregates, and alkali oxides were derived based on mineral molar volume data in the thermodynamic database and the results are listed in Table 6. 17

18 Table 6: Calculated volume percentage of 1 m 3 concrete containing cement hydration products, calcite, and alkali oxides for 25 C in dissolved alkali concentrations of K 0.37 and Na 0.14 molal (ph 13.5) minerals vol% calcite (as agg.) afwillite or CSH_1.8 portlandite ettringite hydrogarnet hydrotalcite hematite ~ K 2 O Na 2 O free H 2 O ~5 Results in Table 6 indicate that there is about 5 volume percent of free water in concrete at the end of hydration and the sum of the individual volume percentages of minerals and water is about 96 % so about 4 % in deficit. This can be considered as the degree of water un-saturation of concrete. The total porosity is therefore 5 % (free water) + 4 % (unfilled void) = 9 %. Wickham (2004) recommended a total porosity of 6.3 % for a concrete with calcareous aggregate at 20 C. We used a total porosity of 7 % as the initial concrete porosity in our diffusion calculations throughout. Young, evolved, and CSH controlled cement pore water compositions With the known assemblage of cement hydration products and the alkali concentrations in the initial concrete pore fluid, equilibrium composition of concrete fluid can be calculated. A x1t simulation was set up to let Boom Clay pore water flow-through the model concrete system as given in Table 6 with a total porosity of 7 %. The simulation proceeded as a function of time and concrete pore water were step by step replaced by inward Boom Clay water. As expected and in agreement with results of similar type of simulations in literature, e.g., Atkinson, et al., (1989; 1989a; 1989b) and Neall, (1994), three types of concrete pore water were illustrated as a result of concrete-boom Clay water interaction: young concrete water: this water is still in equilibrium with dissolved alkalis and has a high ph value of around 13.5; evolved concrete water: after leaching out of dissolved alkalis, this water has a ph value 12.5 controlled by dissolution of portlandite; CSH concrete water: with increasing number of pore volumes of original concrete pore water is replaced by Boom Clay water, portlandite dissolves away completely and ph drops to below 12 Table 7 summarizes the resulting concrete pore fluid compositions at different stages. Detailed input and output files are provided in annex III of this report. 18

19 Table 7: Concrete pore fluid compositions at different stages, 25 C element concentration, mmol litre -1 Ca Na K Al Si * Mg Fe S C young concrete water ph /0.3 ~ evolved concrete water ph / CSH concrete water * ph 11.7/ / / / * controlled by afwillite and CSH_1.8 if two values are given Note that results given in Table 7 are calculated for 25 C. An expected increase of temperature at the thermal phase of the repository operation will decrease the ph in concrete owing to the effect of temperature on hydrolysis properties of the system. At a temperature of 80 C, the expected maximum temperature at the interface of the NF and Boom Clay in the thermal phase, the ph controlled by KOH/NaOH is about 12 instead of 13.5 and the ph controlled by the dissolution of portlandite is 11 instead of The evolution of ph and concentrations of sulphate and detrimental species from Boom Clay pore water at the overpack and SS envelope In this section, derived model system containing concrete and pore fluid are coupled with diffusion to scope the evolution of ph and concentrations of sulphate and detrimental species from Boom Clay pore water at the overpack and the SS envelope. The same 1D radial model as in the flow-through case was applied but with only diffusion as the solute transport mechanism. Boundary conditions and discretization of model domain In terms of diffusion calculation, boundary conditions were applied to the left boundary, i.e., the overpack- the NF interface, and the right boundary, i.e., the NF- Boom Clay interface. The left boundary was set as an sealed outlet boundary across which no diffusion takes place. The right boundary was imposed as an inlet boundary where diffusion may occur and the concentration was fixed by in-diffusing Boom Clay pore water composition, i.e., a constant concentration at the right boundary. To test the influence of boundary conditions on simulation results, the right boundary was put in two different positions as shown in Figure 3. Majority of diffusion simulations were performed with A boundary condition where the right bound was the NF- Boom Clay interface. The A condition is relatively easy to be implemented in a x1t simulation since the model domain, i.e., the NF has a homogeneous concrete composition. The A condition however ignores the fact that concentration at the right bound is not constant in reality. For example, diffusion of concrete components to clay will increase the concentration at the bound as a function of time so the diffusion gradient across the boundary drops faster than the A condition would predict. A more realistic boundary condition is 19

20 the B where the right boundary was extended towards the Boom Clay. The thickness of the clay should be long enough to honor the constant boundary condition intended at the right side. Although the B condition is preferred, it involves setting the model domain with different assemblage of mineral compositions, i.e., a concrete assemblage at the left and a Boom Clay assemblage at the right. Although x1t is in principle capable of swapping different minerals to construct a heterogeneous model domain, defining different quantity of minerals in different part of the domain is still difficult in practice. Also, application of the B condition needs to implement a Boom Clay model to describe clay alterations by an alkaline plume coming from the NF and that is not planned within the framework of the current scoping. Nevertheless, one simulation was carried out with the B boundary condition to estimate the sensitivity of boundary condition on the time needed for the amount of dissolved alkalis to diffuse out of the NF. From the foregoing discussion, one may expect that a simulation with the A boundary condition should underestimate the buffering capacity of concrete to a change of NF chemistry. In other words, the A boundary condition would predict a faster depletion of concrete components than reality because of an exaggerated concentration gradient across the boundary. In terms of a life time prediction for the concrete buffer, the A boundary condition is conservative as it tends to anticipate a sooner break down of the buffer than reality. The model domain was discredited in both homogeneous and heterogeneous ways (Figure 4). In order to make a simulation run to complete within a reasonable computational time, majority of simulations were done with a homogeneous gridding of 20 nodes over the 1.4 meter thickness of the NF. Some runs were executed with 50 and 100 grids to demonstrate the influence of gridding on the diffusive distance of inward Boom Clay water. Also one calculation was done with heterogeneous gridding where less than one centimeter nodal width was used near the NF and the Boom Clay boundary to illustrated that gridding affects the modeling results strongly if a local equilibrium assumption is adopted. A 1.4 m Boom Clay host rock NF concrete diffusion Boom Clay water left boundary right boundary B 1.4 m Boom Clay host rock diffusion Boom Clay water NF concrete left boundary right boundary Figure 3 Illustration of boundary conditions applied for a 1D radial diffusion simulation. 20

21 26 nodes Boom Clay Boom Clay 20 or 100 nodes cm...1, 0.5, 0.4, 0.3, 0.2, 0.1 Figure 4 Illustration of discretization of the model domain. The initial model system of NF A model domain with dimension as shown in Figure 2 was set up in x1t and consist of concrete of composition given in Table 6. Concrete properties relevant to diffusion calculation are given in Table 8. The initial total porosity was assigned to 7 % (see discussion in Section 3.3). This total porosity was considered as being filled by 5 % of concrete fluid at the hydration stage and 2 % of Boom Clay pore water after the re-saturation of concrete. The 2 % of un-saturation is simply the difference between the total porosity and the porosity filled by free water. The degree of un-saturation might be higher at a higher temperature of thermal phase. The pore diffusion coefficient used was m 2 s -1. The value is assigned based on literature data concerning the effective diffusivity in concrete. A review in the recent NF-PRO project (2004) concluded that an effective diffusivity in concrete is in a range of to m 2 s -1. To be conservative, we picked the higher value and convert it to a pore diffusion coefficient using a porosity of 0.1. Table 8: Properties of the model domain representing the NF concrete used in reactive diffusion simulations pore diffusion coefficient (m 2 /s) porosity (-) 0.07 dispersivity (m) 0 Temperature ( C) 25 The initial pore water composition is expected to be determined by cement hydration and resaturation of unfilled porosity by Boom Clay pore water. For elements controlled by cement hydration reactions, i.e., dissolution and precipitation of hydration products, their concentrations are porosity independent so the concrete pore fluid composition given in Table 7 were used. For anion species, except sulphate, which are not regulated by solubility of hydration products of cement, their initial concentrations present in concrete are the concentrations in Boom Clay pore water diluted by a factor of 0.3, the ratio between the unfilled porosity to the total porosity. 21

22 Definition of different cases Three different cases were defined in terms of composition of in-diffusing Boom Clay water: the reference case: Boom Clay pore water with the reference composition as given in Table 3 diffuse into the NF; the worst case: Boom Clay pore water with the worst-case composition as given in Table 4 diffuse into the NF; the mixed case: the worst-case water diffuse into the NF for 20 years followed by the reference water The reference case considers the degradation of the NF concrete due to ingress of undisturbed Boom Clay water. This case is relevant for calculating the ph evolution and system mass balance for a relatively long term after repository operation. The worst case assumes that the in-diffusing Boom Clay pore water is oxidized due to repository excavation and operation. Note that as an excavated disturbed zone (EDZ) would be limited in reality and the Boom Clay would not provide the worst-case water for long time, this case unrealistically assumes that the worst-case water would persist for an unlimited time span. This case is therefore overly conservative. The mixed case considers arbitrarily that diffusion of the worst-case water would last only for the first 20 years after the backfill of the repository. This is only to demonstrate a case between the two extreme situations defined for the reference and the worst cases. Only diffusion of anion species was calculated. Diffusion of 20 years of the worst-case water does not suppose to affect the ph evolution and the total mass balance of the system to a big extent. Results and discussions Initial pore water composition of the NF Following the procedures given in Section 3.4 and 4.2, initial pore fluid composition of concrete was derived for unsaturated and saturated stages (Table 9). 22