Buffer, backfill and closure process report for the safety assessment SR-Site

Technical Report

TR-10-47

Buffer, backfill and closure
process report for the safetyassessment SR-Site
Svensk Kärnbränslehantering AB

November 2010

Svensk Kärnbränslehantering AB

Box 250, SE-101 24 Stockholm
Phone +46 8 459 84 00

TR-10-47

CM Gruppen AB, Bromma, 2010

Swedish Nuclear Fueland Waste Management Co

ISSN 1404-0344
Tänd ett lager:
SKB TR-10-47
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Buffer, backfill and closure
process report for the safetyassessment SR-Site
Svensk Kärnbränslehantering AB

November 2010

A pdf version of this document can be downloaded from www.skb.se.

Preface
This report gives an account of how processes in buffer, deposition tunnel backfill and the closure
important for the long-term evolution of a KBS-3 repository for spent nuclear fuel, will be documented
in the safety assessment SR-Site.
Most of the material in the report was contributed by selected experts in the field, in particular
Ola Karnland and Lennart Börgesson, Clay Technology AB. See Section 1.2 for a complete list ofcontributors. The sub-sections describing the handling in the safety assessment of the various processes
were written by Patrik Sellin, SKB in collaboration with the concerned experts. Patrik Sellin hasedited the report.
This report has been reviewed by Geert Volckert, SCK-CEN, David Read, Enterpris ltd andEsther Jonsson SKB according to the instructions for the SR-Site project. The reviewers’ comments with answers will be made available as project documentation.
Stockholm, November, 2010
Allan Hedin
Project leader, SR-Site

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Contents
1
1.1

1.2
1.3
1.4
2
2.1
2.2

2.3

2.4

2.5

2.6

2.7

2.8

3
3.1
3.2
3.3

3.4

Introduction
Role of this report in the SR-Site assessment
1.1.1 Overall methodology
1.1.2 Identification of processes
1.1.3 Relation to specific sites
1.1.4 Intended audience of this report
Structure for process descriptions
Participating experts
Handling of FEP’s mapped to the buffer, backfill and closure process system

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Definition of system components and summary of handling of processes
in SR-Site
Overview
Buffer
2.2.1 Initial state of the buffer
2.2.2 Definition of buffer variables
2.2.3 Buffer performance and safety
2.2.4 Summary of handling of buffer processes in SR-Site
Backfill
2.3.1 Initial state of the backfill in SR-Site
2.3.2 Definition of backfill variables
2.3.3 Backfill performance
2.3.4 Summary of handling of backfill processes in SR-Site
Tunnel plug
2.4.1 Initial state of the tunnel plug
2.4.2 Definition of tunnel plug variables
2.4.3 Summary of handling of tunnel plug processes in SR-Site
Central area
2.5.1 Initial state of the central area
2.5.2 Definition of central area variables
2.5.3 Summary of handling of central area processes in SR-Site
Top Seal
2.6.1 Initial state of the top seal
2.6.2 Definition of top seal variables
2.6.3 Summary of handling of top seal processes in SR-Site
Bottom plate
2.7.1 Initial state of the bottom plate2.7.2 Definition of bottom plate variables
2.7.3 Summary of handling of bottom plate processes in SR-Site
Borehole seals
2.8.1 Initial state of the borehole seals
2.8.2 Definition of variables for borehole seals
2.8.3 Summary of handling of borehole seal processes in SR-Site

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Processes in the buffer
Radiation-related processes
3.1.1 Radiation attenuation/heat generation
Thermal processes
3.2.1 Heat transport
3.2.2 Freezing
Hydraulic processes
3.3.1 Water transport under unsaturated conditions
3.3.2 Water transport under saturated conditions
3.3.3 Gas transport/dissolution
3.3.4 Piping/Erosion
Mechanical processes
3.4.1 Swelling/mass redistribution
3.4.2 Liquefaction

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3.5

3.6

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4.1
4.2

4.3
4.4

4.5

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5.1
5.2

5.3
5.4

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Chemical processes
3.5.1 Introduction
3.5.2 Advective transport of species
3.5.3 Diffusive transport of species
3.5.4 Colloid transport
3.5.5 Sorption (including ion-exchange of major ions)
3.5.6 Alterations of impurities
3.5.7 Aqueous speciation and reactions
3.5.8 Osmosis
3.5.9 Montmorillonite transformation
3.5.10 Iron – bentonite interaction
3.5.11 Montmorillonite colloid release
3.5.12 Radiation-induced transformations
3.5.13 Radiolysis of porewater
3.5.14 Microbial processes
3.5.15 Cementation
Radionuclide transport processes
3.6.1 Speciation of radionuclides
3.6.2 Transport of radionuclides in the water phase
3.6.3 Transport of radionuclides in a gas phase

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Processes in the tunnel backfill
Thermal processes
4.1.1 Heat transport
4.1.2 Freezing
Hydraulic processes
4.2.1 Water uptake and transport under unsaturated conditions
4.2.2 Water transport under saturated conditions
4.2.3 Gas transport/dissolution
4.2.4 Piping/erosion
Mechanical processes
4.3.1 Swelling/mass redistribution
4.3.2 Liquefaction
Chemical Processes
4.4.1 Advective transport of species
4.4.2 Diffusive transport of species
4.4.3 Sorption (including exchange of major ions)4.4.4 Alterations of backfill impurities
4.4.5 Aqueous speciation and reactions
4.4.6 Osmosis
4.4.7 Montmorillonite transformation
4.4.8 Backfill colloid release
4.4.9 Radiation-induced transformations
4.4.10 Microbial processes
Radionuclide transport processes
4.5.1 Speciation of radionuclides
4.5.2 Transport of radionuclides in the water phase

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Processes in the tunnel plugs
Thermal processes
5.1.1 Heat transport
5.1.2 Freezing
Hydraulic processes
5.2.1 Water uptake and transport under unsaturated conditions
5.2.2 Water transport under saturated conditions
5.2.3 Gas transport/dissolution
5.2.4 Piping/erosion
Mechanical processes
5.3.1 Swelling/mass redistribution
Chemical processes

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6.1
6.2

6.3
6.4

6.5

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7.1
7.2

7.3
7.4

5.4.1 Advection
5.4.2 Diffusive transport of species
5.4.3 Sorption (including exchange of major ions)5.4.4 Alteration of concrete
5.4.5 Aqueous speciation and reactions
5.4.6 Osmosis
5.4.7 Montmorillonite transformation
5.4.8 Montmorillonite colloid release
5.4.9 Microbial processes
Radionuclide transport processes
5.5.1 Speciation of radionuclides
5.5.2 Transport of radionuclides in the water phase
5.5.3 Transport of radionuclides in a gas phase

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Processes in the central area
Thermal processes
6.1.1 Heat transport
6.1.2 Freezing
Hydraulic processes
6.2.1 Water uptake and transport under unsaturated conditions
6.2.2 Water transport under saturated conditions
6.2.3 Gas transport/ dissolution
6.2.4 Piping/erosion
Mechanical processes
6.3.1 Swelling/mass redistribution
6.3.2 Liquefaction
Chemical Processes
6.4.1 Advective transport of species
6.4.2 Diffusive transport of species
6.4.3 Sorption
6.4.4 Alteration of the central area backfill
6.4.5 Alteration of concrete components
6.4.6 Aqueous speciation and reactions
6.4.7 Osmosis
6.4.8 Corrosion of steel components
6.4.9 Microbial processes
6.4.10 Colloid release
6.4.11 Radiation induced transformation
Radionuclide transport processes
6.5.1 Speciation of radionuclides
6.5.2 Transport of radionuclides in the water phase

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Processes in the top seal
Thermal processes
7.1.1 Heat transport
7.1.2 Freezing
Hydraulic processes
7.2.1 Water uptake and transport under unsaturated conditions
7.2.2 Water transport under saturated conditions
7.2.3 Gas transport/dissolution
7.2.4 Piping/erosion
Mechanical processes
7.3.1 Swelling/mass redistribution
7.3.2 Liquefaction
Chemical Processes
7.4.1 Advective transport of species
7.4.2 Diffusive transport of species
7.4.3 Sorption (including exchange of major ions)7.4.4 Alteration of concrete components
7.4.5 Aqueous speciation and reactions

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7.5

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8.1
8.2

8.3
8.4

8.5

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9.1
9.2

9.3
9.4

9.5

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7.4.6 Colloid release
7.4.7 Steel corrosion
7.4.8 Microbial processes
7.4.9 Radiation induced transformation
Radionuclide transport processes
7.5.1 Speciation of radionuclides
7.5.2 Transport of radionuclides in the water phase

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Processes in the bottom plate
Thermal processes
8.1.1 Heat transport
8.1.2 Freezing
Hydraulic processes
8.2.1 Water uptake and transport under unsaturated conditions
8.2.2 Water transport under saturated conditions
8.2.3 Gas transport/dissolution
8.2.4 Piping/erosion
Mechanical processes
8.3.1 Swelling/mass redistribution
8.3.2 Liquefaction
Chemical processes
8.4.1 Advective transport of species
8.4.2 Diffusive transport of species
8.4.3 Sorption (including exchange of major ions)
8.4.4 Alteration of concrete
8.4.5 Aqueous speciation and reactions
8.4.6 Corrosion of copper
8.4.7 Microbial processes
Radionuclide transport processes
8.5.1 Speciation of radionuclides
8.5.2 Transport of radionuclides in the water phase

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Processes in borehole seals
Thermal processes
9.1.1 Heat transport
9.1.2 Freezing
Hydraulic processes
9.2.1 Water uptake and transport under unsaturated conditions
9.2.2 Water transport under saturated conditions
9.2.3 Gas transport/dissolution
9.2.4 Piping/erosion
Mechanical processes
9.3.1 Swelling/mass redistribution
9.3.2 Liquefaction
Chemical processes
9.4.1 Advective transport of species
9.4.2 Diffusive transport of species
9.4.3 Sorption (including exchange of major ions)9.4.4 Alteration of concrete
9.4.5 Aqueous speciation and reactions
9.4.6 Copper corrosion
9.4.7 Alteration of impurities in bentonite
9.4.8 Osmosis
9.4.9 Montmorillonite transformation
9.4.10 Montmorillonite colloid release
9.4.11 Microbial processes
Radionuclide transport processes
9.5.1 Speciation of radionuclides
9.5.2 Transport of radionuclides in the water phase

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References351
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1

Introduction

1.1

Role of this report in the SR-Site assessment

1.1.1 Overall methodology
This report documents processes related to the buffer, backfill and closure identified as relevant to
the long-term safety of a KBS-3 repository, and forms an important part of the reporting of the safetyassessment SR-Site. The detailed assessment methodology, including the role of the process reports
in the assessment, is described in the SR-Site Main report, /­SKB 2011/. The following excerpts
describe the methodology, and clarify the role of this process report in the assessment.
The repository system, broadly defined as the deposited spent nuclear fuel, the engineered barriers
surrounding it, the host rock and the biosphere in the proximity of the repository, will evolve overtime. Future states of the system will depend on:• the initial state of the system,• a number of radiation-related, thermal, hydraulic, mechanical, chemical and biological processes
acting within the repository system over time, and• external influences acting on the system.
A methodology in eleven steps has been developed for SR-Site, as summarised in Figure 1-1 anddescribed below.
1. Identification of factors to consider (FEP processing).
This step consists of identifying all the factors that need to be included in the analysis. Experiencefrom earlier safety assessments and KBS-3 specific and international databases of relevant features,
events and processes (FEP’s) influencing long-term safety are utilised. An SKB FEP database isdeveloped where the great majority of FEP’s are classified as being either initial state FEP’s,
internal processes or external FEP’s. Remaining FEP’s are either related to assessment method­
ology in general or deemed irrelevant for the KBS-3 concept. Based on the results of the FEP
processing, an SR-Site FEP catalogue, containing FEP’s to be handled in SR-Site, has been
established. This step is documented in the SR-Site FEP report /­SKB 2010a/.
2. Description of the initial state.
The initial state of the system is described based on the design specifications of the KBS-3
repository, a descriptive model of the repository site and a site-specific layout of the repository.
The initial state of the fuel and the engineered components is that immediately after deposition,
as described in the respective SR-Site Production line reports /­SKB 2010b, c, d, e, f, g/. The
initial state of the geosphere and the biosphere is that of the natural system prior to excavation,
as described in the site descriptive model /­SKB 2008/. The repository layout adapted to theForsmark site is provided in an underground design report /­SKB 2009a/.
3. Description of external conditions.
Factors related to external conditions are handled in the categories “climate-related issues”,“large-scale geological processes and effects” and “future human actions”. The handling ofclimate-related issues is described in the SR-Site Climate report /­SKB 2010h/, whereas the
few external, large-scale geosphere processes are addressed here in the Geosphere process report/­SKB 2010i/. The treatment of future human actions in SR-Site is described in the SR-Site FHA
report /­SKB 2010j/.
4. Compilation of process reports.
The identification of relevant processes is based on earlier assessments and FEP screening. Allprocesses identified within the system boundary relevant to the long-term evolution of the system
are described in dedicated Process reports, i.e. this report and process reports for the fuel andcanister /­SKB 2010k/ and for the geosphere /­SKB 2010i/. Also short-term geosphere processes/
alterations, due to repository excavation, are included. For each process, its general characteristics,
the time frame in which it is important, the other processes to which it is coupled and how
the process is handled in the safety assessment are documented.

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5. Definition of safety functions, function indicators and function indicator criteria.
This step consists of an account of the safety functions of the system and of how they can beevaluated by means of a set of function indicators that are, in principle, measurable or calculable
properties of the system. Criteria for the safety function indicators are provided. The Processreports are important references for this step. A FEP chart is developed, showing how FEP’s are
related to the function indicators.
6. Compilation of input data.
Data to be used in the quantification of repository evolution and in dose calculations are selected.
The process of selection and the data adopted are reported in a dedicated Data report /­SKB 2010l/.
Also, a template for discussion of input data uncertainties has been developed and applied.
7. Definition and analysis of reference evolution.
A reference evolution, providing a description of a plausible evolution of the repository system,
is defined and analysed. The isolation potential of the system over time is analysed, yielding adescription of the general system evolution and an evaluation of the safety function indicators.
Each process is handled in accordance with the plans outlined in the process reports. Radiologicalconsequences of potential canister failures are not analysed in this step.
8. Selection of scenarios.
A set of scenarios for the assessment is selected. A comprehensive main scenario is defined inaccordance with the Swedish Radiation Safety Authority’s, SSM’s, regulations SSMFS 2008:21/­SSM 2008/. The main scenario is closely related to the reference evolution analysed in step 7.
The selection of additional scenarios is focused on the safety functions of the repository, andthe safety function indicators defined in step 5 form an important basis for the selection. For
each safety function, an assessment is made as to whether any reasonable situation where it is
not maintained can be identified. If this is the case, the corresponding scenario is included in
the risk evaluation for the repository, with the overall risk determined by a summation over such
scenarios. The set of selected scenarios also includes scenarios explicitly mentioned in applicableregulations, e.g. human intrusion scenarios, and scenarios and variants to explore design issues
and the roles of various components in the repository.
9. Analysis of scenarios.
The main scenario is analysed essentially by referring to the reference evolution in step 7, comple­
mented by consequence calculations for potential canister failures in the reference evolution
yielding a calculated risk contribution from the main scenario. The additional scenarios areanalysed by focussing on the factors potentially leading to situations in which the safety functionin question is not maintained. In most cases, these analyses are carried out by comparison withthe evolution for the main scenario, meaning that they only encompass aspects of repository
evolution for which the scenario in question differs from the main one. If the scenario leads tocanister failures, consequence calculations are carried out. If the likelihood of the scenario is
non-negligible, a risk contribution is also calculated.
10. Additional analyses.
In this step, a number of additional analyses, required to complete the safety assessment, arecarried out. These comprise e.g. sensitivity analyses of the outcome of the scenario analyses,
analyses required to demonstrate optimisation and use of best available technique, analyses ofdesign options alternative to the reference design, analyses supporting risk evaluation for the
initial 1,000 years and an account of supporting arguments based on natural analogues.
11. Conclusions.
This step includes integration of the results from the various scenario analyses, development ofconclusions regarding safety in relation to acceptance criteria and feedback concerning design,
continued site investigations and the R&D programme.
This Buffer, Backfill and Closure process report is one of the process reports required to complete
step 4. The purpose of the process reports is to document the scientific knowledge of the processes
to a level required for their adequate treatment in the safety assessment. The documentation is not
exhaustive from a scientific point of view, since such a treatment is neither necessary for the purposesof the safety assessment nor possible within the scope of an assessment. However, it must be sufficiently
detailed to facilitate, by arguments founded on scientific understanding, the treatment of each process10

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Reference
designSite description

R&D results

Results of earlier
assessments

FEP databases

1 FEP processing
Initial sate
Internal processes
External factors
2a Description of site
initial state

2b Description of engineered
barrier system (EBS)
initial state

3 Description of external
conditions– Climate and climate related
issues– Future Human Actions

2c Description of repository
layouts– with site adaptations

4 Compilation of Processreports
with handling prescriptions,
including models

5 Definition of safety functions andfunction indicators
Define– safety functions of the system,– measurable/calculable safety function
indicators and– safety function indicator criteria

6 Compilation of
input data

7 Definition and analyses of reference evolution
Study repository evolution for– repetition of most recent 120,000 year glacial cycle and– variants assuming global warming due to increased greenhouse effect
8 Selection of scenarios
based on– results of reference evolution– FEP analyses– safety functions

9 Analyses of selected scenarioswith respect to– isolation– retardation

10 Additional analyses– sensitivities– optimization and BAT– design alternatives– relevance of excluded FEPs
– barrier functions

11 Conclusions– compliance withregulatory requirements– feedback to design,
R&D, site investigation

Figure 1-1. An outline of the eleven main steps of the SR-Site safety assessment. The boxes above the dashedline are inputs to the assessment.

in the safety assessment. The purpose is to determine how to handle each process in the safety assessment at an appropriate degree of detail, and to demonstrate how uncertainties are taken care of, given
the suggested handling. The means of handling processes established in this report are used in the analysisof the reference evolution, step 7, and in the analyses of scenarios, step 9.

1.1.2 Identification of processes
The process documentation in this SR-Site process report is an update and extension of the descriptionsin the SR-Can version of the Process report /­SKB 2006a/. The extension concerns processes for thesystem components not treated in detail in SR-Can, i.e. tunnel plugs, central area, top seal, bottom platein deposition holes and borehole seals. The SR-Can version of the report, in turn, builds on the process
documentation in the SR 97 version of the process report /­SKB 1999/ and the FEP processing carriedout in SR-Can. The complementary FEP processing carried out in SR-Site (step 1 above) did not identify
any need to modify the list of relevant processes that were included in the SR-Can version of the process
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report /­SKB 2006a/, see the SR-Site FEP report /­SKB 2010a/ for details. However, some modifications
have been made to improve the structure and logic of the process descriptions. For example, to improvethe clarity of the documentation and the handling of uncertainties in the geochemical evolution of thebuffer, some mechanisms included in integrated descriptions in SR-Can are included as separate processes in SR-Site, e.g. iron-bentonite interactions and cementation. For the system components nottreated in detail in SR-Can, the list of processes has been established largely based on those definedfor the buffer and backfill. A complete list of processes described in the SR-Site process reports canbe found in the abovementioned FEP report.

1.1.3 Relation to specific sites
SKB has undertaken site characterisation at two different locations, Forsmark and Laxemar-Simpevarp,
as candidate sites for a final repository for spent nuclear fuel. The SR-Site assessment builds on sitespecific data for, and site-descriptive models of, the selected Forsmark site /­SKB 2008/, but site-specificdata for, and site-descriptive models of, the Laxemar site /­SKB 2009a/ are also used in evaluations to
support the site selection /­SKB 2010m/. Therefore, the process descriptions are of a sufficiently general nature to be applicable to both of these sites. The result of quantitative evaluation of the processes
in the different scenarios analysed in SR-Site will, in many cases, be dependent on site-specific data.
These data are not given here, but in dedicated modelling studies. In addition, the most essential data
for the safety assessment are thoroughly evaluated in the SR-Site Data report /­SKB 2010l/, step 6 above.
1.1.4 Intended audience of this report
This report is written by, and for, experts in the relevant scientific fields. It should though be possible
for a generalist in the area of long-term safety assessments of geologic nuclear waste repositories
to comprehend the contents of the report. The report is an important part of the documentation of the
SR-Site project and an essential reference within the project, providing a scientifically motivated planfor the handling of processes related to the buffer, backfill and closure. It is, furthermore, foreseen thatthe report will be essential for reviewers scrutinising the handling of buffer, backfill and closure issues
in the SR-Site assessment.

1.2

Structure for process descriptions

All processes identified are documented using a template, where, in essence, all of the headings are
the same as those used in the SR-Can version of the report. These are described below.
Overview/general description

Under this heading, a general description of current knowledge regarding the process is given. For
most processes, the basis for this is the content of the SR-Can version of the buffer and backfill processreport /­SKB 2006a/, reviewed and updated as necessary.
Dependencies between process and system variables

For each system component, in this case the buffer, backfill, tunnel plugs, central area, top seal,bottom plate in deposition holes and borehole seals, a set of physical variables that defines the stateof the system is specified. For each process, a table is presented under this heading with documentation of how the process is influenced by the specified set of physical variables and vice versa, i.e.
how the process influences the variables. In addition, the handling of each influence in SR-Site isindicated in the table.
Boundary conditions

The boundary conditions for each process are discussed. These refer to the boundaries of the buffer,
backfill, tunnel plugs, central area, top seal, bottom plate in deposition holes and borehole seal systems,
respectively. The processes for which boundary conditions need to be described are, in general, related
to transport of material or energy across the boundaries.
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Model studies/experimental studies

Modelling and experimental studies of the process are summarised. This documentation constitutes
the major source of information for many of the processes.
Time perspective

The timescale or timescales over which the process occurs are documented, if such timescales can be
defined.
Natural analogues/observations in nature

If relevant, natural analogues and/or observations in nature that contribute to the present understandingof the process are documented under this heading.
Handling in the safety assessment SR-Site

Under this heading, the handling of the process in the safety assessment SR-Site is described.
Typically, the process is either• neglected on the basis of the information under the previous headings, or• included by means of modelling.
The following aspects need to be covered, although no prescribed format for the documentation is given:
Boundary conditions: The handling of boundary conditions is discussed, especially any spatial andtemporally varying chemical and hydraulic conditions.
Influences and coupling to other processes: The handling of the documented influences is discussed,
as is coupling to other processes within that part of the system.
The method of treatment for all processes is discussed and where relevant,, the models that have beendeveloped for each process are described.
Handling of uncertainties in SR-Site

The handling of different types of uncertainties associated with each process is summarised.
Uncertainties in mechanistic understanding: The uncertainty in the general understanding of the process
is discussed based on the available scientific literature and with the aim of addressing whether the basic
scientific mechanisms governing the process are understood to the level necessary for the suggested
handling.
Model simplification uncertainties: In most cases, the quantitative representation of a process willcontain simplifications. These may be a significant source of uncertainty in the description of systemevolution. These uncertainties are discussed and approaches to addressing them are identified including
alternative models or alternative approaches to simplification of a particular conceptual model.
Input data and data uncertainties: The set of input data necessary to quantify the process for thesuggested handling is documented. The further treatment of important input data and input data
uncertainties is described in a separate report, the SR-Site Data report /­SKB 2010l/, to which reference
is made if relevant.
Adequacy of references supporting the handling in SR-Site

Under this heading, statements are provided concerning the adequacy of the references from a quality
assurance perspective. These statements are restricted to the references supporting the selectedhandling and are evaluated in the factual review of the process report together with the arguments
and justification for the selected handling provided in the preceding subsections.
References

A list of references used in the process documentation is given at the end of the report.
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1.3

Participating experts

The experts involved in assembling the basic information on the processes are listed in Table 1-1.
All these experts are included in the SR-Site list of experts as required by the SR-Site QA plan ,see further the SR-Site Main report /­SKB 2011/.
Table 1-1. Experts responsible for the process documentation.
Process

Expert author, affiliation

Radiation attenuation/heat generation

Patrik Sellin, SKB

Heat transport

Harald Hökmark, Clay Technology

Freezing

Martin Birgersson, Clay Technology

Water uptake and transport for unsaturated conditions

Lennart Börgesson Clay Technology

Water transport for saturated conditions

Lennart Börgesson Clay Technology

Gas transport/dissolution

Patrik Sellin, SKB

Piping/Erosion

Lennart Börgesson Clay Technology

Swelling/mass redistribution

Lennart Börgesson Clay Technology

Liquefaction

Lennart Börgesson Clay Technology

Advective transport of species

Ola Karnland, Clay Technology

Diffusive transport transport of species

Michael Ochs, BMG

Sorption (including ion-exchange)

Michael Ochs, BMG

Alterations of impurities

David Arcos, Amphos XXI

Aqueous speciation and reactions

Patrik Sellin, SKB

OsmosisOla Karnland, Clay Technology

Montmorillonite transformation

Ola Karnland, Clay Technology

Iron-bentonite interaction

Daniel Svensson, SKB

Montmorillonite colloid release

Magnus Hedström, Clay Technology

Radiation-induced transformations

Patrik Sellin, SKB

Radiolysis of porewater

Patrik Sellin, SKB

Microbial processes

Karsten Pedersen, University of Göteborg

Cementation

Ola Karnland, Clay Technology

Colloid transport

Patrik Sellin, SKB

Speciation of radionuclides

Patrik Sellin, SKB

Transport of radionuclides in water phase

Patrik Sellin, SKB

Transport of radionuclides in gas phase

Patrik Sellin, SKB

Copper corrosion

Patrik Sellin, SKB

Steel corrosion

Patrik Sellin, SKB

Concrete degradation

David Arcos, Amphos XXI

14

TR-10-47

1.4

Handling of FEP’s mapped to the buffer, backfill and closure
process system

Processing of features, events and processes, FEP’s, of relevance for the long-term safety of a KBS-3
repository was carried out for the safety assessment SR-Can /­SKB 2006b/. For SR-Site, a comple­
mentary processing was undertaken , focussing on new NEA project FEP’s included in a later
version of the international NEA FEP database. This work is further described in the SR-Site FEP
report /­SKB 2010a/.
As mentioned above, the complementary FEP processing has not led to any revision of the listof buffer and backfill processes compared with those treated in the previous SR-Can assessment.
For the system components not treated in detail in SR-Can, i.e. tunnel plugs, central area, top seal,bottom plate in deposition holes and borehole seals, it was found appropriate to establish the list ofprocesses based on those defined for the buffer and backfill.
The complementary FEP processing has also provided lists of project FEP’s in the international
NEA FEP database version 2.1 that are related to the different system processes. For the buffer andbackfill system, these lists are essentially the same as those developed for SR-Can based on version
1.2 of the NEA FEP database, except that the updated lists also contain all new project FEP’s in thelater version (2.1) of the NEA FEP database. As in SR-Can, these FEP lists have been used by theexperts developing the process descriptions, in order to ensure that all relevant aspects of a process
are addressed in the process descriptions and handled appropriately in the SR-Site assessment.
The handling of each FEP has been documented by the experts involved in developing the processdescriptions in tables created for this purpose. These tables are provided in the SR-Site FEP report/­SKB 2010a/ and the handling is also documented in the SKB FEP database.
In general, the results of these checks showed that all relevant NEA project FEP’s were already coveredby the process descriptions and included in existing plans for handling the associated processes in
the SR-Site assessment.

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15

2

Definition of system components and summaryof handling of processes in SR-Site

2.1

Overview

This section defines and describes the system components and gives a summary of their handling in
the SR-Site safety assessment. The components have been defined as:• Buffer• Backfill• Tunnel plug
• Central area
• Top seal
• Bottom plate• Bore hole seals
In describing the production and installation of the components in the Production reports /­SKB2010d,
e, f, g/ that also serve as supporting documentation for SR-Site, slightly different definitions havebeen used. The Backfill Production Report /­SKB2010e/ covers the backfill and the tunnel plugs,
the Closure Production Report /­SKB2010f/ covers the central area, top seal and bore hole seals andthe bottom plate is included in the Underground openings construction report /­SKB2010g/. The titleof this report is based on the titles of the production reports.

2.2

Buffer

2.2.1 Initial state of the buffer
The following is an overview description of the initial state of the buffer, i.e. its state at the time ofdeposition. A more formal and exhaustive account is given in the Buffer Production Report /­SKB
2010d/. A specified set of variables, see Section 2.2.2, is used to describe the buffer.
In the deposition hole, the copper canister is surrounded by a buffer of clay. The buffer is installed
as bentonite blocks and rings. The blocks are placed below and above the canister and the bentonite
rings surround the canister.
Two different types of bentonite are considered as reference buffer material for the purpose of SR-Site.
One is a natural Na-bentonite of Wyoming type (MX-80) supplied by the American Colloid Company
and the other is IBECO RWC (also called Calziumbentonit IBECO DEPONIT CA-N), a natural
Ca-bentonite from the Milos island, Greece supplied by Silver and Baryte (non-converted Milosbentonite). These bentonites consist mainly of the smectite mineral montmorillonite (about 85%)
with the characteristic property that it swells when unconfined and in contact with water. The mont­
morillonite content is not expected to vary by more than a few percent. For an actual repository there
may be many possible choices for a suitable buffer material.
MX-80 contains accessory minerals in addition to smectite: albite, quartz, cristobalite, muscovite
and gypsum, and also, grains of pyrite, calcite, siderite, barite and iron hydroxides (see Table 2-1).
The mean chemical composition, as analysed by ICP/AES, of MX-80 expressed as oxides is: 57%
SiO2, 18.5% Al2O3, 3.6% Fe2O3, 2.3% MgO, 1.3% CaO, 2.0% Na2O, 0.5% K2O, 0.2% TiO2, 0.3%
total carbon, 0.3% total sulphur, and 13.7% loss of ignition.
Minerals other than smectite in Milos-bentonite are mainly calcite, dolomite, quartz, and cristobalite,
see Table 2-1. The mean chemical composition, expressed as oxides as analysed by ICP/AES, of thebentonite is 48% SiO2, 15.7% Al2O3, 4.56% Fe2O3, 2.92% MgO, 5.4% CaO, 0.7% Na2O, 0.8% K2O,
0.7% TiO2, 1.0% total carbon, 0.7% total sulphur, and 20% loss of ignition.
There are primarily two methods available for fabricating bentonite blocks and rings; uniaxial pressing
and isostatic pressing. Here is assumed that all blocks will be manufactured by uniaxial pressing.
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17

Table 2-1. Composition of MX-80 and Deponit CA-N bentonite including, impurities and accessory
minerals in /­Karnland et al. 2006/.
Component

MX-80 (wt-%)

Deponit-CaN (wt-%)±

AlbiteAnorthoclase
Calcite + Siderite
Cristobalite
Dolomite
Gypsum + anhydrite
Mica
Montmorillonite
PyriteQuartz
CEC (meq/100g)
Na+
Ca2+
Mg2+
K+

3
0
0
2
0
0.7
4
87
0.07
3
75
72%
18%
8%
2%

0
2
10
1
3
1.8
0
81
0.5
1
70
24%
46%
29%
2%

1
1
1
0.5
1
0.2
1
3
0.05
0.5
2
5%
5%
5%
1%

The bentonite, bought in bulk form and transported by ship, is subject to quality control both before
loading onto the ship and at reception. This is done to ensure that the quality is within the specifications and that there is no contamination during transport. Quality control is also undertaken during
manufacture of the blocks and rings; one important check is the water content before pressing so that
this can be adjusted to obtain the correct water content for the manufacturing process.
The primary goal in the manufacture of bentonite blocks and rings and the subsequent deposition
process is to achieve a pre-defined final density in the water-saturated buffer once post-emplacement
equilibration has occurred. The currently defined range of density required for the saturated buffer
is 1,950–2,050 kg/m3. The bulk density is dependent on the gaps left between the canister and buffer
and between buffer and rock. The gaps are a results of the space needed to install the blocks. The
slot between the canister tube and the buffer is 10 mm wide and the slot along the circumferential
boundary between the buffer and the rock is 50 mm in the Buffer Production Report/SKB 2010d/.
The slots are filled with bentonite pellets.
Buffer emplacement in the deposition holes of each tunnel may take place several months after
the deposition holes were drilled. The deposition holes are assumed to have filled with water inthe period between drilling and buffer emplacement, which is why draining is the first step inthe preparation of the holes. The buffer is put into position by a specially designed buffer filling
vehicle. The buffer positioning is thereafter checked and adjusted if necessary. The emplacementof the copper canister is done with a specially designed deposition machine. A top bentonite blockis installed immediately after the canister is emplaced with the buffer installation equipment. The
final handling procedures and the final design of the buffer installation equipment and the deposition
machine have not yet been decided, but do not affect this description of the general work procedures.
Small geometric tolerances in the deposition holes mean a small risk for faulty emplacement of
the buffer and canister.
The buffer must be protected from water or high humidity until the tunnel is backfilled. The reason isthat the buffer may start swelling before deposition of the canister and before the tunnel backfill canapply its counterweight on the buffer. One possible method is to insert a drain tube in the deposition
hole and to protect the whole buffer with a temporary rubber protection sheet that is kept sealed
until backfilling of the tunnel starts. The temporary rubber protection sheet and drain tube would
be removed after use.

2.2.2 Definition of buffer variables
Each component in the EBS initial state is described by a specified set of physical variables, selected
to allow an adequate description of the long-term evolution of the component in question in the safetyassessment.
18

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The buffer is bounded on the inside by an interface with the canister, on the outside by the interface
with the rock surface of the deposition hole, at the bottom by an interface with the copper plate on
top of the concrete plate at the base of the borehole and on the top by its interface with the backfill.
The buffer is characterised thermally by its temperature distribution and with respect to radiation by
the intensity of the radiation field it is exposed to, mainly γ and neutron radiation.
Hydraulically, the buffer is characterised by its water content, sometimes by gas content (desaturation)and by hydrovariables (pressure and flows), which are mainly of interest in the transient period when
the buffer is being saturated with water.
The buffer is characterised mechanically by its stress state.
The chemical state of the buffer is defined by its mineralogy, including the montmorillonite composition,other clay minerals, and any other impurities. The chemical state is also defined by the porewatercomposition and the occurrence of structural and stray materials in the deposition hole.
The variables are defined in Table 2-2. The values of some of the variables are dependent on the densityof the different phases. The following values have been used: density of water (ρw) 1,000 kg/m3 anddensity of clay solids (ρcs) 2,780 kg/m3.
The initial values of the variables, i.e. the values at the time of deposition, are given in the Buffer
Production Report /­SKB 2010d/.

2.2.3 Buffer performance and safety
In order to evaluate buffer performance in the safety assessment, a number of so called function
indicators have been formulated together with criteria that these should fulfil over time, , see further
the SR-Site main report, /­SKB 2011/. The criteria are summarised in Table 2-3, with reference to thesections in this report where the criteria are discussed. Some of the criteria does not directly relate tobuffer properties (e.g. temperature).
The content of canister corroding agents in the buffer should be low. Apart from unavoidable initialamounts of oxygen, the pyrite content could pose a long-term problem, as pyrite, if not oxidisedby initially present or intruding oxygen, will release sulphide, a canister corroding agent. There is,however, no absolute criterion placed on this amount.
Table 2-2. Definition of buffer variables.
Variable

Definition

Buffer geometry

Geometric dimensions for buffer. An example is description of interfaces (on the inside
towards the canister and on the outside towards the geosphere).

Pore geometry

Pore geometry in buffer as a function of time and space. The total porosity, (the fractionof the volume that is not occupied by solid material) is often given.

Radiation intensity

Intensity of (α, β,) γ and neutron radiation as a function of time and space in buffer.

Temperature

Temperature as a function of time and space in buffer.

Water content

Water content as a function of time and space in buffer.

Gas content

Gas content (including any radionuclides) as a function of time and space in buffer.

Hydrovariables (pressureand flows)

Flows and pressures of water and gas as a function of time and space in buffer.

Stress stateStress conditions as a function of time and space in buffer.

Bentonite composition

Chemical composition of the bentonite (including any radionuclides) in time and spacein buffer, levels of impurities in time and space in buffer.

Montmorillonite composition The mean molecular structure of montmorillonite including the type of charge
compensating cations.
Porewater composition

Composition of the porewater (including any radionuclides and dissolved gases) in timeand space in the buffer.

Structural and straymaterials

Chemical composition and quantity of any stray materials accidentaly left in the buffer.
At this stage, no structural materials are defined for this component.

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19

Table 2-3. Summary of the buffer function indicators and the criteria they should fulfil.
Function indicator

Criterion

Rationale

Reference

Bulk hydraulic conductivity

kBuff < 10–12 m/s

Limit mass transport to a diffusion
dominated process

3.3.2

Swelling pressure> 1 MPa

Ensure tightness, self sealing

3.4.1

Maximum temperature1

TBuffer < 100°C

Ensure that the buffer will retain its
favourable properties in the long term

3.5.9

Minimum temperature

TBuffer > –4°C

Avoid freezing

3.2.2

Swelling pressure aroundentire canister> 0.2 MPa

Avoid canister sinking (criterion to be
determined)

3.4.1

Buffer saturated density
around entire canister> 1,800 kg/m3

Reduce microbial activity

3.5.14

Buffer saturated density
around entire canister> 1,650 kg/m3

Prevent colloid transport through buffer 3.5.4

Buffer density aroundentire canister< 2,050 kg/m3

Ensure protection of canister againstrock shear

3.4.1

Several characteristics of the groundwater composition are essential for providing chemicallyfavourable conditions for the repository. These are discussed in the Geosphere Process Report/­SKB 2010i/, but some of them are directly related to the performance of the buffer:• The salinity of the groundwater should neither be too high, nor too low. The total charge concentration of cations should exceed 4 mM in order to avoid colloid release from buffer and backfill,hence: Σq[Mq+]GW > 4 mM (Section 3.5.11). Groundwaters of high ionic strengths would have anegative impact on the buffer properties (Section 3.5.8) In general, ionic strengths corresponding
to NaCl concentrations of around 100 g/L (1.7 M NaCl) is tolerable for the buffer, but no specificcriterion is given.
• Regarding pH, a criterion can be formulated from the point of view of buffer and backfill stability
pH < 11 (Section 3.5.9). This is fulfilled for any natural groundwater in Sweden. However,construction and stray materials in the repository, in particular concrete, could contaminate
the groundwater such that high pH values are reached.

2.2.4 Summary of handling of buffer processes in SR-Site
Table 2-4 summarises the handling of buffer processes in the safety assessment SR-Site, as suggested
in this report. In the table, the process is either “mapped” to a model by which it will be quantified
or, alternatively a brief verbal description of how it will be handled is provided. Since the initial evolution, characterised by unsaturated conditions and elevated temperatures, is in many respects different
from the long-term, saturated period, the description in the table has been divided accordingly.
The information in the table can be summarised as follows:
For the initial saturation phase, the peak canister and buffer temperatures and the THM evolution
as the buffer saturates need to be quantified. Modelling of the thermal evolution of the entire nearfield will be performed with the near field evolution model to evaluate peak canister and buffer temperatures. Coupled THM modelling of the buffer will be performed to clearly describe the hydraulicevolution for different hydraulic conditions in the bedrock. The swelling at the end of the saturation
phase is addressed by both the system model and the THM model, where the former can be used for
rapid evaluation of the final result of the swelling for a number of input data combinations, including
osmotic effects due to intruding saline water.
Chemical evolution during the thermal phase of elevated and varying temperature in the buffer will
be addressed by the chemistry model.
Most other processes are not affected to any significant extent by the thermal and unsaturated conditions and are therefore neglected during the early saturation and thermal phases. One typical example
would be the process “freezing”.

20

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Table 2-4. Process table for the buffer describing how buffer processes are handled in different
time frames and for the special case of an earthquake. Green fields denote processes that are
neglected or not relevant for the time period of concern. Red fields denote processes that arequantified by modelling in the safety assessment. Orange fields denote processes that are
neglected subject to a specified condition.
Resaturation/“thermal” period

Long-term after
saturation and“thermal” period

Earthquakes

Notes

Bu1
Radiation attenuation/
heat generation

Neglected sincedose rate is too low
to be of importancefor the buffer

Neglected sincedose rate is too low
to be of importancefor the buffer

Not relevant

Bu2
Heat transport

System model

System modelNot relevant

Bu3Freezing

Neglected, since thisrequires permafrost
conditions

Neglected if buffertemperature > −4°C.
Otherwise bounding
consequencecalculation

Not relevant

Bu4
Water uptake
and transport
for unsaturated
conditions

THM model

Not relevant bydefinition

Not relevant

Bu5
Water transport for
saturated conditions

Neglected underunsaturated
conditions, For
saturated conditions
the treatment is
the same as for“Long-term”

Neglected if
hydraulicconductivity< 1012 m/s sincediffusion would
then dominate

Consider pressuretransients

Bu6
Gas transport/dissolution

Through dissolution(Through dissolution)
No gas phase isassumed to bepresent(Through dissolution)
No gas phase isassumed to bepresent

Bu7
Piping/Erosion

Model study

Not relevant, seealso Bu17

Not relevant

Loss of buffer mass iscalculated

Bu8
Swelling/mass
redistribution

THM modelling
including interaction
buffer/backfill andthermal expansion
System model (final
swelling)

Integrated evaluationof erosion,
convergence,corrosion products,
creep, swellingpressure changes
due to ion exchange
and salinity, canister
sinking

Part of integrated
assessment of
buffer/canister/rock

Need to also
consider deviationsin amount of buffer
initially deposited

Bu9
Liquefaction

Not relevant in an
unsaturared material

Neglected sinceliquefaction from ashort pulse cannot
occur in a highdensity bentonite,
due to high effective
stresses

Neglected sinceliquefaction from ashort pulse cannot
occur in a highdensity bentonite,
due to high effective
stresses

Bu10
Advective transportof species

Simplified
assumptions ofmass transport ofdissolved species
during saturation

Neglected if
hydraulicconductivity< 10−12 m/s

Consider pressuretransients

Intact canisterTR-10-47

Repository
temperature in longterm obtained frompermafrost depth
modelling

The consequencesof a buffer with highhydraulic conductivity
are evaluated. Two
different cases arestudied:the buffer acts as aporous medium withhigh conductivity(mass loss)
the buffer is fractured(alteration)

See “Water transport
for saturated
conditions”

21

Resaturation/“thermal” period

Long-term after
saturation and“thermal” period

Earthquakes

Notes

Bu11
Diffusive transporttransport of species

Chemistry model(thermal, saturated
phase; unsaturated
phase disregarded)

Chemistry model

Consider alteredgeometry (diffusion
pathways)

Consider varying
groundwatercompositions
The initial thermal
gradient is considered

Bu12
Sorption (including
ion-exchange)

Chemistry model(thermal, saturated
phase; unsaturated
phase disregarded)

Chemistry model

Not specificallytreated

Bu13
Alteration of
impurities

Chemistry model(thermal, saturated
phase; unsaturated
phase disregarded)

Chemistry model

Not specificallytreated

Bu14
Aqueous speciation
and reactions

Chemistry model(thermal, saturated
phase; unsaturated
phase disregarded)

Chemistry model

Not specificallytreated

Bu15
OsmosisSystem model (initialswelling)

Evaluation throughcomparison with
empirical data

Not specificallytreated

Bu16
Montmorillonitetransformation

Model calculations(thermal, saturated
phase; unsaturated
phase disregarded)

Estimate based onevidence from naturePart of the integrated
treament buffer/canister/rockBu17
Iron-bentonite
interaction

Neglected since noiron will be in contact
with the bentonite

Only considered
for defective canister:
Included in a
separate scenario

Only considered
for failed canister.
Possible loss of
buffer efficiency

Bu18
Montmorillonite
colloid release

Neglected if total
cation charge is> 4 mM Otherwise
modelled

Neglected if total
cation charge is> 4 mM Otherwise
modelled

Not specificallytreated

Bu19
Radiation-induced
transformations

Neglected sincedose rate outside
canister is too low
to have any effect

Neglected sincedose rate outside
canister is too low
to have any effect

Neglected sincedose rate outside
canister is too low
to have any effect

Bu20
Radiolysis ofporewater

Neglected sincedose rate outside
canister is too low tohave any effect

Neglected sincedose rate outside
canister is too low
to have any effect

Neglected sincedose rate outside
canister is too low
to have any effect

Bu21
Microbial processes

Neglected underunsaturated
conditions, since theextent of aqueous
reactions is limited.
For saturated conditions the treatment
is the same as for“Long-term”

Quantitative
estimate of sulphate
reduction, limited
by supply of
microbe nutrientsin groundwater

Not specificallytreated

Bu22 Cementation

Discussed together
with Process Bu16
“Montmorillonite
alteration”

Discussed together
with Process Bu16
“Montmorillonite
alteration”

Part of the integrated
treament buffer/canister/rock22

Handling of long-term
intrusion of saline
water

Loss of buffer massis calculated
See also watertransport undersaturated conditions

TR-10-47

Resaturation/“thermal” period

Long-term after
saturation and“thermal” period

Earthquakes

Bu6
Failed canister. Gas
transport/ dissolution

Quantitative
estimate based on
empirical data (nofailures are expected
this period)

Quantitative estimatebased on empirical
data

Quantitative estimatebased on empirical
data

Bu19
Failed canister.
Radiation-induced
transformations

Neglected sincedose rate outside
canister is too low
to have any effect

The effect of
α-radiation fromnuclides froma failed canister
is estimated

The effect of
α-radiation fromnuclides from a failed
canister is estimated

Bu23
Colloid transport

Neglected if
density at saturation> 1,650 kg/m3,
otherwise bounding
calculation (nofailures are expected
this period)

Neglected if
density at saturation> 1,650 kg/m3,
otherwise bounding
calculation

Neglected if
density at saturation> 1,650 kg/m3,
otherwise bounding
calculation

Bu24
Speciation ofradionuclides

Assumptions basedon empirical data(no failures are
expected this period)

Assumptions basedon empirical data

Assumptions basedon empirical data

Bu25
Transport of radionuclides in water
phase

COMP23
Analytic (no failures
are expected thisperiod)COMP23

COMP23
Reduced diffusion
path

Bu26
Transport ofradionuclides in gas
phase

Neglected (nofailures are expected
this period)

Quantitative estimate

Quantitative estimate

Notes

Failed canister

The effect of
α-radiation fromnuclides is estimatedin Section 2.5.11.
There, it is concluded
that the consequences
can be neglected.

The long-term chemical evolution following the thermal phase will be addressed by both thechemistry model and the system model and for the varying boundary conditions expected as a resultof climate change etc. The former model provides a spatially resolved result and involves morechemical reactions whereas the latter can be used for rapid evaluations for a number of input data
combinations. Montmorillonite transformation is handled by separate modelling. Erosion due tointrusion of dilute groundwater during glacial conditions must be addressed if the hydrogeochemical
analyses imply that ionic strengths below the given criterion (Section 3.5.11) cannot be excluded.
Colloidal release needs to be modelled accordingly.
The effects of chemical evolution on key properties such as swelling pressure and hydraulicconductivity will be evaluated using empirical relationships. A number of issues related to mass
redistribution in the buffer still need to be evaluated for the long-term evolution.
In the case of a canister failure, the release of gas from the corroding cast iron canister insert will behandled by quantitative estimates based on experimental studies of gas transport through bentonite.
Diffusion of radionuclides through the buffer is calculated taking into account radionuclide
speciation, necessary for the selection of diffusion and sorption data and estimated on the basis ofexperimental data. Transport of fuel colloids through the bentonite is neglected if the buffer density
exceeds a specified value; otherwise the effect of this process on potential dose consequences is
estimated by a bounding calculation case.

TR-10-47

23

2.3

Backfill

2.3.1 Initial state of the backfill in SR-Site
The following is an overview description of the initial state of the backfill, i.e. its state at the timeof deposition. A more formal and exhaustive account is given in the SR-Site Production line reportfor backfill /­SKB 2010e/. In that description, the specified set of variables describing the backfill isutilised.
When the holes in a deposition tunnel have been filled with canisters and the buffer the tunnel will
be backfilled. Before backfilling, all tunnel installations including concrete on the floor of the tunnel
will be removed. In SR-Site, the deposition tunnels as well as the transport tunnels and the lower
part of the ramp and shaft, extending from 200 m down to the repository is assumed to be filled withbackfill material and will be considered as “backfill”.
One backfill concept will be analysed in SR-Site:• Precompacted blocks of a natural swelling clay (not necessarily a bentonite), IBECO-RWC-BF(manufacturer’s name) or Milos-B (SKB term) bentonite is used as an example of such a material
in SR-Site. The entire tunnel is filled with pre-compacted blocks. The gaps between the rock andthe blocks are filled with pellets of the same material. An example of the block filling is given
in Figure 2-1. The backfilling in the top part of the deposition hole is shown in Figure 2-2. The
estimated volumes and proposed densities are given in Table 2-5.
The choice of IBECO-RWC-BF is based on a large number of tests /­Gunnarsson et al. 2006/ whichshowed that the material together with a feasible backfilling technique fulfils the requirements withan acceptable margin. The composition of IBECO-RWC-BF is given in Table 2-6.
Nominal tunnel cross section
Rock surface

Block filled volume
Pellet filled volume
Bottom bed

Figure 2-1. Example of block configuration in a fictive tunnel section /­SKB 2010e/.

Nominal tunnel floor
Upper part ofdeposition hole

Pellets
BlocksPart where buffer
is installed

Figure 2-2. Backfill in the top section of the deposition hole /­SKB 2010e/.

24

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Table 2-5. Calculated installed dry density for the reference design of backfill components andinstalled backfill. The tunnel volume is set to the largest acceptable. From the Backfill Production
Report /­SKB 2010e/.
Parameter

Tunnel cross section

Upper part of deposition hole

Dry density of blocks (ρdb)

1,700 kg/m3

1,710 kg/m3

Dry density of pellets (ρdp)1

1,000 kg/m3

1,200 kg/m3

Volume fraction of slots between blocks (dVs)2%0Volume of pellets filling and bottom bed

Total volume minus block and void
part of the volume
Nominal: 25–16,8*(1+0.02) m3/m
Accepted: 25–0.60*25(1+0.02) m3/m

2.56 m3

Total volume

Maximum allowed tunnel blasting
volume25 m3/m

5.17 m3

Calculated installed dry density(nominal block part of cross section andlargest acceptable tunnel volume)1,458 kg/m3

1,457 kg/m3

Calculated installed dry density(acceptable block part of cross section andlargest acceptable tunnel volume)

1,408 kg/m3–

For the tunnel cross section the same density is used for the pellets filling between blocks and tunnel walls and bottom bed.

Table 2-6. Composition of IBECO RWC-BF.
Component

Reference design (wt-%)

Accepted variation

Montmorillonite

50–60%

45–90%

cationCaNa

CEC (meq/100g)

75

> 60

Dominant exchangeable

2.3.2 Definition of backfill variables
The deposition tunnels are constrained by the rock surrounding the tunnel but also by the buffer inthe deposition holes and the plugs at the tunnel ends. In the case of rejected deposition holes /­SKB
2010e/, the subsystem is constrained also by the rock around rejected deposition holes.
The transport tunnels are constrained by the surrounding rock as well as tunnel plugs separatingthem from the deposition tunnels and the central area.
The backfill in the tunnels is characterised thermally by its temperature. Hydraulically it is characterised by its pore geometry, water content, gas content and the hydrovariables (pressure and flow).
Mechanically, the backfill is characterised by the stress state. The chemical state is also defined by theporewater composition and the occurrence of structural and stray materials in the deposition tunnel.
The radiation intensity (dose rate) in the backfill has not been calculated, since it is considerably
lower than on the outside of the buffer and is of no importance in the safety assessment.
All variables are defined in Table 2-7.
The initial values of the variables are given in the Backfill Production Report /­SKB 2010e/.

2.3.3 Backfill performance
In order to evaluate the performance of the tunnel backfill, a few so called function indicators havebeen established together with criteria these should fulfil over time, see further the SR-Site main
report. These criteria are summarised in Table 2-8, referencing sections in this report where thereasoning for these criteria are given.

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25

Table 2-7. Variables for the tunnel backfill.
Variable

Definition

Backfill geometry

Geometric dimensions for backfill. A description of e.g.
interfaces towards buffer and towards the geosphere.

Backfill pore geometry

Pore geometry as a function of time and space in backfill.
The total porosity, i.e. the fraction of the volume that is not
occupied by solid material is often given.

Temperature

Temperature as a function of time and space in deposition
tunnels.

Water content

Water content as a function of time and space in deposition
tunnels.

Gas content

Gas content (including any radionuclides) as a function of timeand space in deposition tunnels.

Hydrovariables(pressure and flows)

Flows and pressures of water and gas as a function of timeand space in deposition tunnels.

Stress stateStress state as a function of time and space in backfill.

Backfill materials – composition and content

Total chemical composition and content of the backfill material(including any radionuclides) in time and space.

Backfill porewater composition

Composition of the porewater (including any radionuclides anddissolved gases) in time and space in backfill.

Structural and stray materials

Chemical composition and quantity of structural materials(rock bolts, filling material in boreholes for grouting, nets etc)
and stray materials in deposition tunnels.

Table 2-8. Summary of the Tunnel Backfill function indicators and the criteria they should fulfil.
Function indicator

Criterion

Bulk hydraulic conductivitykBackfill

< 10

–10

m/s

Backfill> 0.1 MPa
Swell

Rationale

Reference

Limit advective transport

4.2.2
4.3.1
4.1.2

Swelling pressure

P

Ensure homogeneity andavoid preferential pathways

Minimum temperature

TBackfill > –2°C

Avoid freezing

2.3.4 Summary of handling of backfill processes in SR-Site
Table 2-9 summaries the handling of backfill processes in the safety assessment SR-Site, assuggested in this report. In the table, the process is either “mapped” to a model by which it will bequantified or a brief verbal description of how it will be handled is provided. Since the initial evolution, characterised by unsaturated conditions and elevated temperatures is in many respects different
from the long-term, saturated phase, the description in the table has been divided accordingly.
The information in the table can be summarised as follows:• For the initial saturation phase, the saturation and swelling of the backfill in the deposition tunnels
and its interaction with the buffer needs to be quantified.
• If hydraulic gradients are present in the early phase, the potential effect of piping and erosion has
to be evaluated.
• The temperature increase in the backfill will be moderate and therefore no effects of elevatedtemperature will be evaluated.
The most important processes in the long-term are physical and chemical processes that could leadto an undesirable change in backfill properties. These include ion-exchange, osmosis, colloid releaseand montmorillonite transformation.
In the case of a canister failure, radionuclide transport in the backfill will be calculated both in
the near- and far field models, including advective and diffusive transport as well as sorption.

26

TR-10-47

Table 2-9. Process table for the backfill describing how backfill processes will be handled
in different time frames and in the special cases of earthquakes and failed canisters.
(Green fields denote processes that are neglected or irrelevant for the time period of concern.
Red fields denote processes that are quantified by modelling in the safety assessment.
Orange fields denote processes that are neglected subject to a specified condition.)
Resaturation/ “thermal”
periodLong-term after saturation
and “thermal” period

Notes

BfT1.
Heat transport

Simplified assumption

Simplified assumption

BfT2.
Freezing

Neglected, since thisrequires permafrost
conditions

Neglected if backfill
temperature > –2°C.
Otherwise discussed

Less severe
consequences
than for buffer

BfT3.
Water uptake and transport
for unsaturated conditions

THM model

Not relevant by definition

The pellets are includedin the model

BfT4.
Water transport for saturated
conditions

Neglected underunsaturated conditions, forsaturated conditions thetreatment is the same as for“Long-term”

Included in geospheremodelling

Evaluate effects on
conductivity of chemical
evolution and mass
redistribution/loss andof possible changesof hydraulic gradientsfor permafrost andglaciation

BfT5.
Gas transport/ dissolution

THM model(Through dissolution)

The presence of atrapped gas phase isconsidered in the modelling of the saturationof the backfill (not thecase for the buffer)

BfT6.
Piping/erosion

Quantitative estimate with
an empirical modelNot relevant, see also
BfT16

See also water transport
for saturated conditions

BfT7.
Swelling/Mass redistribution

THM modelling includinginteraction buffer/backfill andhomogenisation in tunnel

Integrated evaluation oferosion, convergence,
creep, swelling pressurechanges due to ion
exchange and salinity
and transformation

Need to also consider
deviations in amountof buffer and backfill
initially deposited andbuffer saturating before
tunnel backfill

BfT8.
Liquefaction

Not relevant

Not relevantLess severe consequences than for buffer.

BfT9.
Advective transport ofspecies

Simplified assumptions ofmass transport of dissolved
species during saturation

Included in geospheremodelling. Cases without
the backfill path will beconsidered

See “Water transport for
saturated conditions”

BfT10.
Diffusive transport of species

The early stage is notstudied specifically, since
the conditions in the backfill
will be about the same as
for the long-term evolution

Chemistry model
System model?

Consider varying
groundwatercomposition

BfT11.
Sorption (including
ion-exchange)

The early stage is notstudied specifically, since
the conditions in the backfill
will be about the same as
for the long-term evolution

Chemistry model

BfT12.
Alteration of impurities

The effect on inorganic
reduction of oxygen ismodelled

Chemistry model

BfT13.
Aqueous speciation
and reactions

The early stage is notstudied specifically, since
the conditions in the backfill
will be about the same as forthe long-term evolution

Chemistry model

Intact canisterTR-10-47

27

Resaturation/ “thermal”
periodLong-term after saturation
and “thermal” period

Notes

BfT14.
OsmosisHydraulic conductivity in
THM model chosen so
as to handle osmosis

Evaluation throughcomparison with
empirical data

Handling of long-term
intrusion of saline water

BfT15.
Montmorillonitetransformation

Model calculations (thermal,
saturated phase; unsaturated phase disregarded)

Model calculations

BfT16.
Colloid release

Neglected if total cation
charge is > 4 mM
Otherwise modelled

Neglected if total cation
charge is > 4 mM
Otherwise modelled

BfT17.
Radiation-induced
transformations

Neglected, since dose
rate in backfill is too low
to have any effect

Neglected, since dose
rate in backfill is too low
to have any effect

BfT18.
Microbial processes

Excluded, (the effect on
oxygen consumption is
not considered)

Mass balance
considerations

BfT5
Failed can. Gas transport/dissolution

Neglected, since gas
volumes (from buffer)assumed to be too low
to reach backfill during
this period

Neglected, pessimisticallysince transport would delay
radioactive releases anddecrease buffer pressure.
The backfill would act asa sink for gas.

Gas release fromcanister

BfT19.
Colloid formation andtransport

See geosphere (no failures
are expected this period)

See geosphere

Called “colloid transport”
for buffer
Reference to corresponding geosphereprocess

BfT20.
Speciation of radionuclides

Assumptions based on
empirical data (no failures
are expected this period)

Assumptions based on
empirical data

BfT21.
Transport of radionuclides
in water phase

COMP23
Analytic (no failures are
expected this period)

COMP23

BfT22.
Transport of radionuclides
in gas phaseBy-passed (no failures are
expected this period)

By-passed

Loss of backfill is
calculated

Failed canister

2.4

Tunnel plug

2.4.1 Initial state of the tunnel plug
The plug is in itself not a barrier but it is a necessary ingredient to help the backfill in the deposition
tunnel to maintain its barrier function. The main requirements of the plug are listed below.
• The plug shall seal the deposition tunnel and keep the backfill in place during the operational
phase until the deposition and transport tunnels have been backfilled and water saturated, andhave regained their hydrostatic water pressure.
• The plug shall resist the hydrostatic water pressure at repository level and the swelling pressure
from the backfill and the bentonite seal.
• The plug shall limit water flow from the deposition tunnel past the plug to such an extent that noharmful backfill erosion takes place from the deposition tunnel.
• The plug shall not significantly impair the barrier function of the other barriers.
• The movement of the plug due to pressure shall be within sufficiently small to avoid a drop in
backfill density in the vicinity of the plug.

28

TR-10-47

The lifetime of the plug can be divided into three phases with different requirements:
1. Installation phase.
2. Sealing phase.
3. Degradation phase.
There are five different components that constitute the tunnel plugs (Figure 2-3). These are (counted
from inside the tunnel and outwards):
1. Drainage material (crushed rock filter) that is used for keeping low water pressure on the concrete
plug until it has cured.
2. A bentonite seal built of bentonite blocks.
3. Two walls made of concrete beams with the purpose of supporting the bentonite seal and separating
it from the drainage material and the concrete plug.
4. The concrete plug with grouting pipes.
5. Drainage arrangements for pressure control in the drainage material.
The entire plug is thus a composite of different materials with different functions.
1. The drainage material shall keep the pressure from the concrete plug until it has cured. Then it has
no function.
2. The bentonite seal shall prevent leakage past the plug after closing the drainage system until fullwater pressure is reached on both sides.
3. The walls are only required to separate the materials during construction.
4. The concrete plug shall mechanically withstand the water and swelling pressure inside the plug
until full water pressure is reached on both sides.
5. The drainage system shall keep the water pressure low in the drainage material.
The concrete part of the plug is constructed from “low-pH” concrete according to the recipes inTable 2-10.
Grouting pipes

Concrete beams Backfill end zone

Drainage

Concrete plug

Watertight seal

FilterFigure 2-3. Schematic section of the reference design of the plug /­SKB 2010e/.
TR-10-47

29

Table 2-10. Reference concrete formulation for the concrete in the plug.
Design parameter

Nominal design[kg/m3 if not specified]

BinderCement CEM I 42.5 MH/LA/SR

120

Silica fume (densified)80

Other components

Water
Limestone filler L25
Sand 0–8 mm

165
369
1,037

Gravel 8–16 mm

558

Glenium 51

6,38

water/cement

1,375 kg/kg

water/binder

0,825 kg/kg

water/powder

0.29 kg/kg

2.4.2 Definition of tunnel plug variables
The tunnel plugs are bounded on the inside by the backfill in the deposition tunnels and on theoutside by the backfill in transport tunnels. The plugs are surrounded by the surface of the rock.
There will be tunnel plugs separating the transport tunnels from the central area as well.
The tunnel plugs are characterised thermally by their temperature.
No radiation variable is defined for the plugs since the levels of radiation caused by the repository
always will remain very low at the positions of the plugs.
Hydraulically, the plugs are characterised by their pore geometry, hydrovariables (pressure and flows),
water content, and sometimes by gas content, which are mainly of interest during the period when
the plug is being saturated with water.
The plugs are characterised mechanically by their stress state.
The chemical state of the plugs is defined by the composition of the concrete plug, the concretebeams, the bentonite, the drainage and the filter material. The chemical state is also defined by
the porewater composition.
The variables are defined in Table 2-11. The initial values of the variables, i.e. the values at the timeof deposition, are given in the production line report for the closure /­SKB 2010f/.
Table 2-11. Definition of tunnel plug variables.
Variable

Definition

Plug geometry
Plug pore geometry

Geometric dimensions of the plug
Pore geometry as a function of time and space in the components. The porosity(the fraction of the volume that is not occupied by solid material) is often given
Temperature as a function of time and space in the plug
Water content as a function of time and space in the plug
Gas content as a function of time and space in the components
Flows and pressures of water and gas as a function of time and space in the plug

Temperature
Water content
Gas content
Hydrovariables (pressureand flows)
Stress state
Plug materials – compositionand content
Plug porewater composition
Structural and stray materials

30

Stress conditions as a function of time and space in the plug
Composition of the concrete plug, the concrete beams, the bentonite, the drainage
and the filter material in space and time
Composition of the porewater in time and space in the plug
(undefined) The structural materials in the plug are already included in the “Plugmaterials” variable – stray materials are assumed to be of no concern since nolong-term performance is expected from the plug

TR-10-47

2.4.3 Summary of handling of tunnel plug processes in SR-Site
Table 2-12 summarises the handling of tunnel plug processes in the safety assessment SR-Site, assuggested in the Process Report. In the table, the process is either “mapped” to a model by which it
will be quantified or a brief verbal description of how it will be handled is provided. Since the initial
evolution, characterised by unsaturated conditions and elevated temperatures is in many respects different from the long-term, saturated phase, the description in the table has been divided accordingly.
Table 2-12. Process table for the tunnel plugs describing how tunnel plug processes are handled
in different time frames. Green fields denote processes that are neglected or not relevant for thetime period of concern. Red fields denote processes that are quantified by modelling in the safetyassessment. Orange fields denote processes that are neglected subject to a specified condition.
Resaturation/ “thermal” period

Long-term after saturation and“thermal” period

Pg1
Heat transport

Temperature as a functionof distance from deposition
holes is calculated. However,no specific estimate of thetemperature in the plugs isdone. The temperature inthis component will under
all circumstances remain
relatively low

Temperature as a function of
distance from deposition holes
is calculated. However, no
specific estimate of thetemperature in the plugs isdone. The temperature inthis component will under
all circumstances remain
relatively low

Pg2
Freezing

Neglected, since this requires
permafrost conditions

Neglected, since there are
no long-term performancerequirements on the plugs

Pg3
Water uptake andtransport underunsaturated conditions

THM model

Not relevant by definition

Pg4
Water transport undersaturated conditions

Assumed to be a part of thetunnel with high hydraulicconductivity

Assumed to be a part of thetunnel with high hydraulicconductivity

Pg5
Gas transport/ dissolution

THM model

Neglected – any effect from theplugs on gas migration would
be positive, since they could
act as sinks. However, theeffect is impossible to quantify

Pg6
Piping/erosion

The plug is a part in the
integrated treatment of piping.
However, no assessment of
piping in the plug itself is done

Neglected, since piping only
occurs during the early part of
the repository evolution

Pg7
Swelling/mass redistribution

THM-model

Neglected, since there are
no long-term performancerequirements on the plugs

Pg8
Liquefaction

Neglected, based on same
arguments as for the buffer

Neglected, since there are
no long-term performancerequirements on the plugs

Pg9
Advective transport ofspeciesAssumed to be a part of thetunnel with high hydraulicconductivity

Assumed to be a part of thetunnel with high hydraulicconductivity

Pg10
Diffusive transport of species

Neglected since advection will
dominate

Neglected since advection will
dominate

Pg11
Sorption

Sorption will occur in the plugs,
but is pessimistically neglected

Sorption will occur in the plugs,
but is pessimistically neglected

Pg12
Alteration of concrete

Separate modelling of
concrete degradation

Separate modelling of concretedegradation

Pg13
Aqueous speciation andreactions

Separate modelling of
concrete degradation

Separate modelling of concretedegradation

Notes

Intact canisterTR-10-47

31

Resaturation/ “thermal” period

Long-term after saturation and“thermal” period

Pg14
OsmosisOsmotic effects are neglected
due to the lack of an osmotic
flow in the tunnel

Osmotic effects are neglected
due to the lack of an osmotic
flow in the tunnel

Pg15
Montmorillonite transformation

Neglected, since there are
no long-term performancerequirements on the plugs

Neglected, since there are
no long-term performancerequirements on the plugs

Pg16
Montmorillonite colloid
releaseNeglected, since there are
no long-term performancerequirements on the plugs

Neglected, since there are
no long-term performancerequirements on the plugs

Pg17
Microbial processes

Neglected under unsaturated
conditions, since the extent of
aqueous reactions is limited.
For saturated conditions thetreatment is the same as for“Long-term”

Microbial effects in tunnelplugs will be included in thedescription of the geochemistry
in the repository

Pg5 Failed canister. Gas
transport/dissolution

Assumed to be a part of
the tunnel with high hydraulicconductivity (no failures
expected in this period)

Assumed to be a part of thetunnel with high hydraulicconductivity

Pg18 Speciation of radionuclides

Assumptions based on
empirical data (no failures are
expected this period)

Indirectly accounted for throughthe selection of parameters forradionuclide transport

Pg19 Transport ofradionuclides inwater phase

Assumed to be a part of thetunnel with high hydraulicconductivity (no failures are
expected this period)

Assumed to be a part of thetunnel with high hydraulicconductivity

Notes

Failed canister

2.5

Central area

2.5.1 Initial state of the central area
The central area is the assembled part of the sub-surface of the KBS-3 repository facility comprisingrock cavities for operation, logistics and maintenance. The rock cavities of the central area shallhouse transfer of canisters, storage and transhipment of rock masses, buffer, backfill and closurematerial, storage of equipment etc, garages and workshops, spaces for personnel and visitors andspaces for technical systems.
The central area will be filled with crushed rock. The following is an overview description of the initialstate of the central area (Figure 2-4). The initial state for this component is defined as the state at theclosure of the repository. A more formal and exhaustive account is given in the production line reportfor the closure /­SKB 2010g/. The repository part that is included in this description is the central areaat the repository level where the ramp and shafts for transporting material and personnel start.
The total volume of this area is 125,000 m3. The entire volume is filled with crushed rock from thesite. The final density and porosity will be 1,900 kg/m3 and 27% (in the compacted rock fill; 100%in the crown space) respectively. Since crushed rock is a non-swelling material there will be an open
gap of about 10 cm in the top of the central area.

2.5.2 Definition of central area variables
Each component in the EBS is described by a specified set of physical variables, selected to allow anadequate description of the long-term evolution of the component in question in the safety assessment.
The central area is bounded on one side by the plugs separating the transport tunnels from the central
area and on the other side by the plugs separating the backfill in the ramp and shaft from the central
area. The central area is surrounded by the surface of the rock.
32

TR-10-47

Operation area
Rock heap

Ventilation

Ramp

Shaft

Ventilation

Repository area

Central area

Figure 2-4. The central area of the repository with ramp and shaft /­SKB 2010f/.

The central area is delimited by the variable component geometry, which in this case means actual
size of the cavern. It is characterised thermally by its temperature.
No radiation variable is defined for these components since the levels of radiation caused by therepository always will remain very low.
Hydraulically, the central area is characterised by its pore geometry, hydrovariables (pressure andflows), water content, and sometimes by gas content, which are mainly of interest in the phase when
the area is being saturated with water.
The central area is characterised mechanically by its stress state.
The chemical state of the central area is defined by the composition of the crushed rock and the structural and stray materials within the component. The chemical state is also defined by the porewatercomposition.
The variables are defined in Table 2-13. The initial values of the variables, i.e. the values at the timeof deposition, are given in the closure production report /­SKB 2010f/, the underground openings
construction report /­SKB 2010g/ and the design report /­SKB 2009b/.
Table 2-13. Definition of central area variables.
Variable

Definition

Central area geometry
Central area pore geometry

Geometric dimensions of the component /­SKB 2009b/
Pore geometry as a function of time and space in the component. The porosity(the fraction of the volume that is not occupied by solid material) is often given
Temperature as a function of time and space in the component
Water content as a function of time and space in the component
Gas content as a function of time and space in the component
Flows and pressures of water and gas as a function of time and space inthe component
Stress conditions as a function of time and space in the component
Composition of the crushed rock in time and space in the component /­SKB 2008/

Temperature
Water content
Gas content
Hydrovariables (pressureand flows)
Stress state
Central area materials –composition and content
Central area porewater composition Composition of the porewater in time and space in the component
Structural and stray materials

TR-10-47

Composition and quantity of construction, reinforcements and stray materials
in the component as a function of time and space. /­SKB 2010f/

33

2.5.3 Summary of handling of central area processes in SR-Site
Table 2-14 summarises the handling of central area processes in the safety assessment SR-Site, assuggested in the Process Report. In the table, the process is either “mapped” to a model by which it
will be quantified or associated with a brief verbal description of how it will be handled. Since theinitial evolution, characterised by unsaturated conditions and elevated temperatures is in many respects
different from the long-term, saturated phase, the description in the table has been divided accordingly.
Table 2-14. Process table for the central area describing how the central area processes are handled
in different time frames. Green fields denote processes that are neglected or not relevant for the time
period of concern. Red fields denote processes that are quantified by modelling in the safety assessment. Orange fields denote processes that are neglected subject to a specified condition.
Resaturation/ “thermal” period

Long-term after saturation and“thermal” period

CA1
Heat transport

Temperature as a function of
distance from deposition holes iscalculated. However, no specificestimate of the temperature in
the central area is made. The
temperature in this component will
under all circumstances remain
relatively low

Temperature as a function of
distance from deposition holes iscalculated. However, no specificestimate of the temperaturein the area is done. The
temperature in this component
will under all circumstances
remain relatively low

CA2Freezing

Neglected, since this requires
permafrost conditions

Neglected, due to small impact

CA3
Water uptake and transport
under unsaturated conditions

THM model

Not relevant by definition

CA4
Water transport undersaturated conditions

Assumed to be a part of therepository with high hydraulicconductivity

Assumed to be a part of therepository with high hydraulicconductivity

CA5
Gas transport/ dissolution

THM model

Neglected, could potentially bea sink for gas

CA6
Piping/erosion

Piping in the crushed rock cannot
be excluded. However, the central
area is always assumed to be apart of the repository with highhydraulic conductivity

Piping in the crushed rockcannot be excluded. However,the central area is always
assumed to be a part of therepository with high hydraulicconductivity

CA7
Swelling/mass redistribution

Most mechanical processes canbe neglected since the impact andthe requirements on the material
are low. However, the interaction
between plugs and the central
area will be treated in the THMmodelling

Neglected, since there are
no long-term performancerequirements on the central area

CA8
Liquefaction

Neglected, since impact is low –if the process occurs at all

Neglected, since impact is low –if the process occurs at all

CA9
Advective transport ofspeciesAssumed to be a part of therepository with high hydraulicconductivity

Assumed to be a part of therepository with high hydraulicconductivity

CA10
Diffusive transport of species

Neglected since advection will
dominate

Neglected since advection will
dominate

CA11
Sorption

Sorption will occur inthe central area, but ispessimistically neglected

Sorption will occur inthe central area, but ispessimistically neglected

CA12
Alteration of the central area
backfill

Modelling of the geochemical
evolution

Modelling of the geochemical
evolution

CA13
Aqueous speciation andreactions

Separate modelling of concretedegradation

Separate modelling of concretedegradation

Notes

Intact canister

34

TR-10-47

Resaturation/ “thermal” period

Long-term after saturation and“thermal” period

CA14
OsmosisOsmotic effects will be insignificant
in the crushed rock material

Osmotic effects will be insignificant
in the crushed rock material

CA15
Alteration of concrete
components

Separate modelling of concretedegradation

Separate modelling of concretedegradation

CA16
Corrosion of steel
components

Modelling of the geochemical
evolution

Modelling of the geochemical
evolution

CA17
Microbial processes

Neglected under unsaturated
conditions, since the extent of
aqueous reactions is limited. For
saturated conditions the treatment
is the same as for “Long-term”

Microbial effects in the central
area will be included in thedescription of the geochemistry
in the repository

CA18
Speciation of radionuclides

Assumptions based on empirical
data (no failures are expected thisperiod)Indirectly accounted for throughthe selection of parameters forradionuclide transport

CA19
Transport of radionuclides
in water phase

Assumed to be a part of therepository with high hydraulic conductivity (no failures are expected
this period)

Assumed to be a part of therepository with high hydraulicconductivity

Notes

Failed canister

2.6

Top Seal

2.6.1 Initial state of the top seal
The main purpose of the top seal in the upper part of the ramp and shafts is to make unintended
intrusion in the repository substantially more difficult. The top seal is defined as the filling in theuppermost 200 m of the ramp and the shafts in the repository. The lower part of the ramp and shaft,extending from 200 m down to the repository is assumed to be filled with backfill material and will
have the same properties as the backfill (Section 2.3):• From 200 to 50 m depth from the ground surface the average hydraulic conductivity of the rock issignificantly higher and here is where one can suitably fill the ramp and shafts with crushed rock thathas to be effectively compacted to minimise self-compaction under its own weight and overburden.
• The uppermost 50 m of the ramp and shafts should be backfilled with a rock-like material formechanical protection of the tight, lower parts. It is proposed that residues left over by the quarrying
industry forming big heaps at many quarries be trimmed to form a “jig-saw puzzle” with silica
concrete poured in the joints. By this means, the ramp and shafts will be sealed with material that
resembles host rock with respect to appearance, joints, porosity and resistance to excavation.
The hydraulic properties of these materials have not been measured, but the conductivity can beassumed to be very high, up to ~0.1 m/s.

2.6.2 Definition of top seal variables
Each component in the EBS is described by a specified set of physical variables, selected to allow anadequate description of the long-term evolution of the component in question in the safety assessment.
The top seal is bounded on one side by the backfill in the lower part of the ramp and shaft and on theother side by the ground surface. The top seal is surrounded by the surface of the host rock.
The top seal, as it is delimited by the variable geometry, is characterised thermally by its temperature.
No radiation variable is defined for this component since the levels of radiation caused by the repository
always will remain very low at the locations of the top seals.

TR-10-47

35

Hydraulically, the top seal is characterised by its pore geometry, hydrovariables (pressure and flows),
water content, and sometimes by gas content, which are mainly of interest in the phase when thefilling is being saturated with water.
The top seal is characterised mechanically by its stress state.
The chemical state of the top seal is defined by the composition of the crushed rock and the structural andstray materials within the component. The chemical state is also defined by the porewater composition.
The variables are defined in Table 2-15. The initial values of the variables, i.e. the values at the timeof deposition, are given in the production line report for the closure /­SKB 2010f/, the production line
for the underground openings /­SKB 2010g/ and the design report /­SKB 2009b/.

2.6.3 Summary of handling of top seal processes in SR-Site
Table 2-16 summarises the handling of top seal processes in the safety assessment SR-Site, assuggested in the process report. In the table, the process is either “mapped” to a model by which it
will be quantified or a brief verbal description of how it will be handled is provided. Since the initial
evolution, characterised by unsaturated conditions and elevated temperatures is in many respects different from the long-term, saturated phase, the description in the table has been divided accordingly.
Table 2-15. Definition of top seal variables.
Variable

Definition

Top seal geometry
Top seal pore geometry

Geometric dimensions of the component /­SKB 2009b/
Pore geometry as a function of time and space in the components. The porosity(the fraction of the volume that is not occupied by solid material) is often given
Temperature as a function of time and space in the component
Water content as a function of time and space in the component
Gas content as a function of time and space in the component
Flows and pressures of water and gas as a function of time and space inthe component
Stress conditions as a function of time and space in the component
Composition of the crushed rock in time and space in the component /­SKB 2008/

Temperature
Water content
Gas content
Hydrovariables (pressureand flows)
Stress state
Top seal materials –composition and content
Top seal porewater composition Composition of the porewater in time and space in the component
Structural and stray materials
Composition and quantity of construction, reinforcements and stray materials
in the component as a function of time and space

Table 2-16. Process table for the top seal describing how top seal processes are handled in different time frames. Green fields denote processes that are neglected or not relevant for the time
period of concern. Red fields denote processes that are quantified by modelling in the safetyassessment. Orange fields denote processes that are neglected subject to a specified condition.
Resaturation/ “thermal” period

Long-term after saturation and“thermal” period

TS1
Heat transport

Temperature as a functionof distance from deposition
holes is calculated. However,no specific estimate of thetemperature in the top seal
is madeTemperature as a functionof distance from deposition
holes is calculated. However,no specific estimate of thetemperature in the top seal
is doneTS2
Freezing

Neglected, since this requires
permafrost conditions (probably
not entirely true, since the topof the top seal may freeze
during winter, see “Long-term”)

The top seal will freeze
during permafrost periods.
However, the consequences
can be neglected due to thehigh hydraulic conductivity ofthe material

TS3
Water uptake andtransport underunsaturated conditions

Rough guess of time to reachsaturation

Not relevant by definition

Notes

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Resaturation/ “thermal” period

Long-term after saturation and“thermal” period

TS4
Water transport undersaturated conditions

Assumed to be a part of therepository with high hydraulicconductivity

Assumed to be a part of therepository with high hydraulicconductivity

TS5
Gas transport/ dissolution

Rough guess of time to reachsaturation

Neglected – the top seal will
have limited effect on gas
movement in the repository

TS6
Piping/erosion

Neglected, since piping cannot
occur in the material

Neglected, since piping cannot
occur in the material

TS7
Swelling/mass redistribution

Mechanical processes in the
top seal are of no concern
for the performance of therepository

Mechanical processes in the
top seal are of no concern
for the performance of therepository

TS8
Liquefaction

Neglected, cannot occur in thistype of material

Neglected, cannot occur inthis type of material

TS9
Advective transport ofspeciesAssumed to be a part of therepository with high hydraulicconductivity

Assumed to be a part of therepository with high hydraulicconductivity

TS10
Diffusive transport ofspecies

Neglected, since advection will
dominate

Neglected, since advection will
dominate

TS11
Sorption

Sorption may occur in the top
seal, but is pessimisticallyneglected

Sorption may occur in the top
seal, but is pessimisticallyneglected

TS12Alteration of concrete

Neglected, due to the distancechemical processes in thiscomponent will have limited
impact on the performance ofthe rest of the repository.
The hydraulic conductivity ofthis component will under all
circumstances be highNeglected, due to the distancechemical processes in thiscomponent will have limited
impact on the performance ofthe rest of the repository.
The hydraulic conductivity ofthis component will under all
circumstances be high

TS13
Aqueous speciation andreactions

Neglected, due to the distancechemical processes in thiscomponent will have limited
impact on the performance ofthe rest of the repository

Neglected, due to the distancechemical processes in thiscomponent will have limited
impact on the performance ofthe rest of the repository

TS14
Colloid release

Neglected, will have no effecton the performance

Neglected, will have no effecton the performance

TS15
Steel corrosion

Neglected, due to the distancechemical processes in thiscomponent will have limited
impact on the performance ofthe rest of the repository

Neglected, due to the distancechemical processes in thiscomponent will have limited
impact on the performance ofthe rest of the repository

TS16
Microbial processes

Neglected under unsaturated
conditions, since the extent of
aqueous reactions is limited.
For saturated conditions thetreatment is the same as for“Long-term”

No specific treatment of
microbial processes in the
top seal will be done – is notexpected to differ from the restof the repository

TS17
Speciation of radionuclides

Assumptions based on
empirical data (no failures are
expected this period)

Indirectly accounted forthrough the selection ofparameters for radionuclidetransport

TS18
Transport of radionuclides
in water phase

Assumed to be a part of therepository with high hydraulicconductivity (No failures are
expected this period)

Assumed to be a part of therepository with high hydraulicconductivity

Notes

There will be aninteraction between
top seal and ramp/
shaft backfill – thisis discussed in
Section 7.3.1

Failed canisterTR-10-47

37

2.7

Bottom plate

2.7.1 Initial state of the bottom plate
The bottom plate (Figure 2-5), which consists of a 2 mm thick copper plate on top of a 15 cm thickconcrete layer, has only the following two purposes:• to be a plane, horizontal foundation for the bentonite blocks in the buffer,• to fasten the temporary bentonite protection (rubber protection sheet and drain tubes, see Figure 2-5)
before the pellets are installed. The purpose of the rubber protection sheet and drain tubes is toprotect the buffer from water during the installation. They will be removed before the backfill
is installed.
There are thus no other demands on the bottom plate after installation other than not to affect the buffer
in a negative way. There are, however, several; such effects during the lifetime of the repository thatneed to be considered.

2.7.2 Definition of bottom plate variables
The bottom plate is bounded on the top by the buffer in the deposition hole and by the bottom andsides of the rock wall.
The bottom plate, as it is delimited by the variable geometry, are characterised thermally by its
temperature.
No radiation variable is defined for the bottom plate since the levels of radiation caused by the
spent fuel in the canister always will remain low at this location and the radiation will not affectthe intended performance of the bottom plate.
Hydraulically, the bottom plate is characterised by its pore geometry, hydrovariables (pressure andflows), water content, and sometimes by gas content, which is of main interest in the phase when
the deposition hole is being filled with water.
The bottom plate is characterised mechanically by its stress state.
The chemical state of the bottom plate is defined by the composition of the concrete part andthe copper plate. The chemical state is also defined by the porewater composition of the concrete.
The variables are defined in Table 2-17. The initial values of the variables, i.e. the values at the timeof deposition, are given in the production line report for the buffer /­SKB 2010d/.

Figure 2-5. The bottom plate (with the temporary rubber protection sheet and drain tubes) /­SKB 2010g/.

38

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Table 2-17. Definition of bottom plate variables.
Variable

Definition

Bottom plate geometry

Geometric dimensions of the bottom plate

Bottom plate pore geometry

Pore geometry as a function of time and space in the plate components. The porosity(the fraction of the volume that is not occupied by solid material) is often given

Temperature

Temperature as a function of time and space in the bottom plate

Water content

Water content as a function of time and space in the bottom plate

Gas content

Gas content as a function of time and space in the bottom plate

Hydrovariables (pressureand flows)

Flows and pressures of water and gas as a function of time and space inthe bottom plateStress stateStress conditions as a function of time and space in the bottom plate

Bottom plate materials –composition and content

Composition of the concrete plate and the copper plate

Bottom plate porewatercomposition

Composition of the porewater in time and space in the bottom plate

Structural and stray materials(undefined) The structural materials in the bottom plate are already included in
the “Bottom plate materials – composition and content” variable – stray materials
are assumed to be of no concern since no long-term performance is expected fromthe bottom plate

2.7.3 Summary of handling of bottom plate processes in SR-Site
Table 2-18 summarises the handling of bottom plate processes in the safety assessment SR-Site, assuggested in the Process Report. In the table, the process is either “mapped” to a model by which it
will be quantified or a brief verbal description of how it will be handled is provided. Since the initial
evolution, characterised by unsaturated conditions and elevated temperatures is in many respects different from the long-term, saturated phase, the description in the table has been divided accordingly.
Table 2-18. Process table for the bottom plate describing how bottom plate processes are handled
in different time frames. Green fields denote processes that are neglected or not relevant for thetime period of concern. Red fields denote processes that are quantified by modelling in the safetyassessment. Orange fields denote processes that are neglected subject to a specified condition.
Resaturation/ “thermal” period

Long-term after saturation and“thermal” period

Notes

Intact canister
BP1
Heat transport

Temperature as a function of
distance from deposition holes iscalculated. However, no specificestimate of the temperaturein the bottom plate is made,since thermal effects on theproperties of the bottom plate
are disregarded

Temperature as a function of
distance from deposition holes
is calculated. However, no
specific estimate of thetemperature in the bottom
plate is made since thermal
effects on the properties of thebottom plate are disregarded

BP2
Freezing

Neglected, since this requires
permafrost conditions

Of no significance, since no longterm performance is expectedfrom the bottom plate

BP3
Water uptake and transport
under unsaturated conditions

The bottom plate is includedin the THM modelling of thesaturation of the buffer

Not relevant by definition

BP4
Water transport undersaturated conditions

The concrete is included as acomponent with high hydraulicconductivity

The concrete is included as acomponent with high hydraulicconductivity

BP5
Gas transport/ dissolution

No specific assessment of thesaturation of the bottom plate will
be doneNeglected – any effect from thebottom plate on gas migration
would be positive, since it could
act as sink for released gas.
However, the effect is impossible
to quantify

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39

Resaturation/ “thermal” period

Long-term after saturation and“thermal” period

BP6
Piping/erosion

The process does not take placein the bottom plate – but piping in
the buffer may be of importancefor the interaction with the bottom
plate and will be assessed. Thisis treated in Process BP7

Does not occur

BP7
Swelling/mass redistribution

Included in THM-modelling

Neglected, of no concern

BP8
Advective transport of species

Included as a component withhigh hydraulic conductivity

Included as a component withhigh hydraulic conductivity

BP9
Diffusive transport of species

Neglected since advection is
likely to dominate – if transport
occurs at all

Neglected since advection is
likely to dominate – if transport
occurs at all

BP10
Sorption

Sorption may occur in the bottom
plate, but is pessimisticallyneglected

Sorption may occur in the bottom
plate, but is pessimisticallyneglected

BP11
Alteration of concrete

Separate modelling of concretedegradation

Separate modelling of concretedegradation

BP12
Aqueous speciation andreactions

Separate modelling of concretedegradation

Separate modelling of concretedegradation

BP13
Copper corrosion

Neglected, since no long term
performance is required fromthe copper

Neglected, since no long term
performance is required fromthe copper

BP14
Microbial processes

Neglected under unsaturated
conditions, since the extent of
aqueous reactions is limited.
For saturated conditions thetreatment is the same as for“Long-term”

Separate assessment of thereactions that can take placein the bottom plate

BP15
Speciation of radionuclides

Assumptions based on empirical
data (no failures are expected
this period)

Indirectly accounted for throughthe selection of parameters forradionuclide transport

BP16
Transport of radionuclides inwater phase

Included in COMP23 (No failures
are expected this period)

Included in COMP23

Notes

Failed canister

2.8

Borehole seals

2.8.1 Initial state of the borehole seals
A number of more or less vertical investigation or surface-based characterisation boreholes were
drilled during the site investigations in order to obtain inter alia data on the properties of the hostrock. These boreholes will be sealed no later than at the closure of the final repository. Holes will
also be drilled from the repository tunnels during the construction phase, implying that horizontal
and upwards directed holes also have to be sealed. Longevity (chemical stability over time) is an
essential property of borehole plugs.
Borehole seals will be constructed from compacted bentonite in perforated copper tubes (Figure 2-8).
Borehole sections intersected by fracture zones will be filled with concrete (Figure 2-8). In orderto minimise the negative impact on contacting clay plugs, the cement content will be very low andlow-pH cement will be utilised. Likewise, the amount and type of super-plasticiser, which is required
for making the concrete sufficiently fluid, will be kept to a minimum. The cement is not relied on
for long periods and it is pessimistically assumed that it will be dissolved and lost, which requiresthat the physical stability of the remaining sand fill is sufficiently stable to provide the rock andneighbouring clay plugs with adequate support.

40

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The sealing of the upper end of deep boreholes needs to be effective and lasting. Further down in theholes, a “mechanical lock” in the form of a silica concrete plug anchored in a reamed recess will beplaced (Figure 2-7). The “lock” will be covered by well compacted till, and trimmed, well-fitting rockcylinders. These covering materials can be allowed to degrade and be lost by glacial erosion, whereas
the “locks” need to remain largely intact for protecting the tight seals deeper down. The chemical longevity of the plug materials shall be sufficient to provide adequate sealing of the boreholes for at least
100,000 years. They must have the necessary compressive and shear strengths to withstand the pressure
from the tight seals in the holes as well as that arising from potentially high hydraulic gradients.

Clay plug in perforated copper tubes

Concrete plug

Fracture zone

Stabilized part of
the fracture zone
Clay plug in perforated copper tubes
Borehole axis

Figure 2-6. Schematic sketch of the construction of concrete plugs in parts where the borehole passes
water-conducting fractures /­SKB 2010f/.

About 100 m

Ground surface
Trimmed, well fittingrock cylinders
Concrete plug anchored inreamed recesses
Compacted till

ne

e zo

tur
Frac

Concreteplugged partClay-plugged part
except for intersected
fracture zones

Figure 2-7. The principle for closure of the upper part of a borehole with well compacted till below
concrete and fitted rock block /­SKB 2010f/.
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41

Rock cylinder
Concrete plug~100 mCompacted till

Highly compacted bentonite
in perforated copper tubes

Concrete plug

Figure 2-8. Reference concept for sealing investigation boreholes from the surface /­SKB 2010f/.

2.8.2 Definition of variables for borehole seals
Each component in the EBS is described by a specified set of physical variables, selected to allow anadequate description of the long-term evolution of the component in question in the safety assessment.
The borehole seals are bounded on one side either by the ground surface or by the tunnel backfill.
The borehole seals are entirely surrounded by the rock.
The borehole seals, as delimited by the variable geometry, are characterised thermally by their temperature.
No radiation variable is defined for this component since the levels of radiation caused by the repository
always will remain very low at the locations of the borehole seals.
Hydraulically, the borehole seals are characterised by their pore geometry, hydrovariables (pressureand flows), water content, and sometimes by gas content, which are mainly of interest in the phasewhen the bentonite in the seals is being saturated with water.
The borehole seals are characterised mechanically by their stress state.
The chemical state of the borehole seals is defined by the composition of the bentonite, the copper
casing and the concrete mixture. The chemical state is also defined by the porewater composition.
The variables are defined in Table 2-19. The initial values of the variables, i.e. the values at the timeof deposition, are given in the closure production report /­SKB 2010f/.

2.8.3 Summary of handling of borehole seal processes in SR-Site
Table 2-20 summarises the handling of borehole seal processes in the safety assessment SR-Site, assuggested in the Process Report. In the table, the process is either “mapped” to a model by which it
will be quantified or a brief verbal description of how it will be handled is provided. Since the initial
evolution, characterised by unsaturated conditions and elevated temperatures is in many respects
different from the long-term, saturated phase, the description in the table has been divided accordingly.
42

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Table 2-19. Definition of borehole seal variables.
Variable

Definition

Borehole geometry

Geometric dimensions of the components

Pore geometry

Pore geometry as a function of time and space in the component. The porosity (thefraction of the volume that is not occupied by solid material) is often given

Temperature

Temperature as a function of time and space in the components

Water content

Water content as a function of time and space in the components

Gas content

Gas content as a function of time and space in the components

Hydrovariables (pressureand flows)

Flows and pressures of water and gas as a function of time and space inthe component

Stress stateStress conditions as a function of time and space in the component

Sealing materials –composition and content

Composition of the bentonite and the concrete in time and space in the component

Porewater composition

Composition of the porewater in time and space in the component

Structural and stray materials

Composition and quantity of construction, reinforcements and stray materials in
the components as a function of time and space. This includes the copper tube,
the till, rock cylinders and the anchor materials

Table 2-20. Process table for the borehole seals describing how borehole seal processes
are handled in different time frames. Green fields denote processes that are neglected or not
relevant for the time period of concern. Red fields denote processes that are quantified bymodelling in the safety assessment. Orange fields denote processes that are neglected subject
to a specified condition.
Resaturation/ “thermal” period

Long-term after saturation and“thermal” period

BHS1
Heat transport

Temperature as a function of
distance from deposition holes iscalculated. However, no specificestimate of the temperature in
the borehole seals is madeTemperature as a function of
distance from deposition holes iscalculated. However, no specificestimate of the temperature in
the borehole seals is made

BHS2
Freezing

A specific assessment of theevolution of the borehole seals
during freezing conditions will
be doneA specific assessment of theevolution of the borehole seals
during permafrost conditions
will be done

BHS3
Water uptake andtransport underunsaturated conditions

Assessment of the timescale ofbore hole seal hydration

Not relevant by definition

BHS4
Water transport undersaturated conditions

Can be neglected as long as
the bentonite component
remains in place.
The case of lost sealing will beassessed in the modelling of
the geosphereCan be neglected as long as the
bentonite part remains in place.
The case of lost sealing will beassessed in the modelling of
the geosphere

BHS5
Gas transport/ dissolution

No specific assessment of thesaturation of the borehole seals
will be done. The process is
expected to be rather fast
since the diameter is small

Neglected – no gas is expected toenter the borehole seals

BHS6
Piping/erosion

Neglected since the holes are initially water filled and no
gradients are expected

Neglected, since piping only
occurs during the early part of
the repository evolution

BHS7
Swelling/mass redistribution

Separate modelling of
the homogenisation

Assessment of the consequencesof a local loss of bentonite

BHS8
Liquefaction

Neglected, based on same
arguments as for the buffer

Neglected, based on same
arguments as for the buffer

Notes

Intact canisterTR-10-47

43

Resaturation/ “thermal” period

Long-term after saturation and“thermal” period

BHS9
Advective transport ofspeciesCan be neglected as long as the
bentonite part remains in place.
The case of lost sealing will beassessed in the modelling of
the geosphereCan be neglected as long as the
bentonite part remains in place.
The case of lost sealing will beassessed in the modelling of
the geosphere

BHS10
Diffusive transport of species

Neglected since diffusivetransport in the borehole
seals will be very slow

Neglected since diffusivetransport in the borehole
seals will be very slow

BHS11
Sorption

Sorption will occur in theborehole seals, but ispessimistically neglected

Sorption will occur in theborehole seals, but ispessimistically neglected

BHS12
Alteration of concrete

Separate modelling of concretedegradation

Separate modelling of concretedegradation

BHS13
Aqueous speciation andreactions

Separate modelling of concretedegradation

Separate modelling of concretedegradation

BHS14
Copper corrosion

Neglected for the tubes, since
no long term performance isrequired from the copper there

Neglected tubes, since no longterm performance is required
from the copper there

BHS15
Alteration of impurities
in bentonite

Neglected, since no thermal
gradients will exist

No specific assessment – theresults from the modelling of thebuffer will be used as “analogue”

BHS16
OsmosisNo specific assessment of thesaturation of the bore holeseals
will be done. The process is
expected to be rather fast since
the diameter is small. For post
saturation, see “long term”

Evaluation of swelling pressureand hydraulic conductivity of thebentonite part of the seals throughcomparison with empirical data

BHS17
Montmorillonitetransformation

Neglected, process will notoccur on short time scale(see long term)

Estimate based on comparison
with natural systems. Separate
discussion on the interaction
between bentonite and concrete

BHS18
Montmorillonite colloid
releaseNeglected if total cation charge is> 4 mM Otherwise discussed

Neglected if total cation charge is> 4 mM Otherwise discussed

BHS19
Microbial processes

Neglected under unsaturated
conditions, since the extent of
aqueous reactions is limited.
For saturated conditions thetreatment is the same as for“Long-term”

No specific treatment of microbial
process will be done – is notexpected to differ from the restof the repository

BHS21
Speciation of radionuclides

Assumptions based on empirical
data (no failures are expected
this period)

Indirectly accounted for throughthe selection of parameters forradionuclide transport

BHS22
Transport of radionuclides inwater phase(No failures are expected thisperiod)Can be neglected as long as the
bentonite part remains in place
The case of lost sealing will beassessed in the modelling of thegeosphere

Notes

Failed canister

44

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3

Processes in the buffer

3.1

Radiation-related processes

3.1.1

Radiation attenuation/heat generation

Overview/general description

Gamma and neutron radiation from the canister are attenuated in the buffer. The process is describedin the Fuel and canister process report /­SKB 2010k/. The maximum dose outside the canister has been
calculated to be less than 500 mGy/h at time of encapsulation. The dose is dominated by Cs-137, which
has a half life of ~ 30 years. The radiation that is not attenuated reaches as far as near-field rock. The
main part of the gamma radiation from the spent fuel is shielded by the iron and copper in the canister.
Only a minor fraction will reach the buffer.
Radiation is of importance owing to its effect on the chemical processes, principally radiation-induced
montmorillonite decomposition and gamma radiolysis of porewater.
Dependencies between process and buffer variables

Table 3-1 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Table 3-1. Direct dependences between the process “Radiation attenuation/heat generation” andthe defined buffer variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
(Yes/No)
Description

Process influence on variable

Handling of influence? Influence present?
(How/Why not)(Yes/No)
Description

Buffer geometryYes. Determines
Included in dose ratemass/volume available calculation

No

Pore geometry

No

No

Radiation intensity

Yes. Determines
energy deposited

Temperature

No

Water content

Yes. Determines how
energy is deposited

Included in dose rate
calculationNo

Gas content

No, but indirectly
through water content

See row above

Indirectly through3.5.13

Hydrovariables(pressure and flows)

No

No

Stress stateNo

No

Bentonitecomposition

Yes. Determines how
energy is deposited

Included in dose rate
calculation

No

Montmorillonite
composition

Yes. Determines how
energy is deposited

Included in dose rate
calculation

Yes

Porewater
composition

Yes. Determines how
energy is deposited,
but the effect is minor

Neglected, Water
density does not varyvery much

No

Structural and straymaterials

No

TR-10-47

Included in dose rate
calculation

Handling of influence?
(How/Why not)Yes. Shielding

Included in dose rate
calculation

Yes. Energy isdeposited

Neglected since it isnegligible comparedto other processes
influencing temperature

Negligible

Negligible, see 3.5.11

Of no concern

45

The result is a radiation field in the buffer that can lead to radiolysis and that has a marginal impact
on the montmorillonite, see further Section 3.5.12.
Attenuation of gamma and neutron radiation will raise the temperature of the buffer, but the effect isnegligible compared with other temperature-raising processes.
Boundary conditions

The boundary condition is the radiation intensity to which the buffer is exposed, i.e. the flux leaving
the outer surface of the canister.
Model studies/experimental studies

Attenuation of gamma and neutron radiation can be calculated theoretically for an arbitrary material
if geometry and composition are known, see e.g. the model studies mentioned for the corresponding
process in the canister. Experimental studies of radiation attenuation in bentonite are reported in
Section 3.5.12.
Natural analogues/observations in nature

Not applicable.
Time perspective

The time perspective is determined by the decay properties of the spent fuel. The gamma and neutronradiation is significant over the first 1,000 years. Details of the fuel decay characteristics are provided
in Spent Fuel Report /­SKB 2010b/.
Handling in the safety assessment SR-Site

Quantification of this process is needed to evaluate the processes 3.5.12 Radiation-induced transformations and 3.5.13 Radiolysis of porewater. The former is shown to be negligible in Section 3.5.12,
for the relevant radiation intensities. The latter could affect the canister surface, but also this effect isshown to be insignificant, see further Sections 3.5.13 and the relevant canister process. The radiation
field outside a canister has been calculated in /­Lundgren 2004/. The calculated dose rate is lower
than 500 mGy/h (100–150 mGy/h) at the time of deposition.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

See Fuel and canister process report /­SKB 2010k/.
Uncertainties

Uncertainties in mechanistic understanding
The understanding of the process is deemed sufficient for the needs of the safety assessment.
Model simplification uncertainties in SR-Site
Not relevant.
Input data and data uncertainties in SR-Site
In general, data for a quantitative description of the process are known with sufficient accuracy for theneeds of the safety assessment, considering the small effects of this process on the repository system.

46

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3.2

Thermal processes

3.2.1

Heat transport

Overview/general description

Heat is transported from the canister surface to the buffer, through the buffer and finally from thebuffer to the rock both directly and via the tunnel backfill. Initially after deposition there will be gaps
between the canister and the buffer and between the buffer and the rock. During this time, heat will betransported by a combination of radiation and conduction across the two gaps and by pure conduction
across the bentonite that will be unsaturated. There may be some convection, but because of the low
permeability of the bentonite and because of the narrow gaps, this contribution can be ignored. In
particular the heat resistance of the gap between canister and bentonite will be high because of the low
emissivity of the copper surface and the associated low radiant heat transfer. The heat resistance of thepellet-filled gap between buffer and rock will be lower and will not have such a large impact on thecanister temperature as the canister/buffer gap. In addition, the pellet-filled gap will start to close soonafter deposition while the canister/buffer gap will remain open longer. The efficiency of heat transport
through the buffer region is important for the performance of the system, since it affects the maximum
buffer temperature for which a design threshold (< 100°C) has been established.
When the buffer has been water-saturated and has swelled so that all gaps and joints are filled, heattransport will take place by conduction through water-saturated bentonite.
Shortly after deposition, heat transport through the buffer is largely independent of the buffer heat
capacity, and heat conduction in the buffer can be approximately described by the time-independent
heat conduction equation:∇(λ∇T) = 0									(3-1)
The most important parameter is thus the thermal conductivity, λ, of different parts of the system.
The thermal conductivity of bentonite is primarily dependent on its density, water saturation andmineral composition. The thermal conductivity of water-saturated MX-80 bentonite that has swelled
to its intended density of 2,000 kg/m3, is about 1.3 W/(m·K) according to laboratory experiments.
The thermal conductivity of the pellets-filled gap between the bentonite blocks and the surroundingrock is 0.3–0.4 W/(m·K) /­Hökmark et al. 2009/. For the hot air filling the canister-bentonite space,
the conductivity is about 0.03 W/(m·K), but radiant heat transfer contributes such that the resulting
effective conductivity can be set at 0.04 W/(m·K) /­Hökmark et al. 2009/.
Dependencies between process and buffer variables

Table 3-2 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Buffer geometry: The resistance to heat transport in the buffer prior to its achieving water saturation
is dependent on the distance between the canister and the deposition hole boundary, which entailschanges in the temperature of the buffer if the dimensions of the canister hole or the location of thecanister in the hole should be altered. After water saturation, however, the variations in geometrythat could occur due to block fallout or a poorly centred canister have negligible effects. This is also
true of the effects of instantaneous or time-dependent deformations that can alter the geometry of thecanister hole. Before water saturation, a poorly centred canister can be of importance for the distribution of gas-filled volumes and therefore for the temperature level and temperature distribution in thebuffer. The thermal expansion of the buffer material is insignificant.
Pore geometry: The geometry of the pore system will change slightly because of thermal expansion.
The porosity, i.e. the volume fraction taken up by pores, influences the thermal conductivity. In theunsaturated state, i.e. before swelling and homogenization has started to even out porosity differences,
the nominal conductivities of perfectly dry bentonite blocks and perfectly dry bentonite pellets differ
significantly. The differences found between the block thermal conductivity (around 1.25 W/(m·K))
and the pellet slot conductivity (around 0.4 W/(m·K)) in dry Prototype Repository deposition holes/­Hökmark et al. 2009/, is however, also due to differences in initial saturation.

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47

Table 3-2. Direct dependencies between the process “Heat transport” and the defined buffer
variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Buffer geometry

Yes

Given as data in thethermal calculation

Yes, but insignificant

Pore geometry

Yes

Given as data in thethermal calculationNo, but indirectly via
temperature

Radiation intensity

No

No

Temperature

Yes. In the unsaturated Included in model
state, thermally driven where appropriate
vaporization/diffusion/condensation contributes to heat transport

Yes

Result from the model(in time and space)

Water content

Yes

Yes

Treated in 2.3.1

Gas content

No, but indirectly via
water content

No, but indirectly via
water content

Hydrovariables(pressure and flows)NoNo, but indirectly
via temperature andinsignificant

Stress stateNo

No

Bentonitecomposition

Yes

Montmorillonite
composition

No

No

Porewater
composition

No

No

Structural and straymaterials

Yes. May affectthermal conductivity

Given as data in thethermal calculation

Given as data in thethermal calculation

Neglected, since tobe of any importanceto the heat transport,
there must be verysignificant amounts of
non-buffer materials

IgnoredIgnoredNo

No

Temperature: The influence of heat transport on the buffer temperature is obvious. If the heat transport
is efficient, the temperature in the inner parts of the system will be lower than if there is a high thermal
resistance. The temperature in the outer parts, however, is practically independent of the conditions in
the interior of the deposition holes. Different models have been proposed for estimating the influenceof the temperature on the thermal conductivity of bentonite /­Knutsson 1983/. The overall effect is asmall increase, which can be ignored for practical purposes. Under unsaturated conditions, vaporization,
vapour diffusion and condensation contribute to the heat transport. The contribution is however small
and not noticeable other than in the low porosity pellet slot /­Kristensson and Hökmark 2010/.
Water content: The thermal conductivity of bentonite depends on the degree of saturation Sr (cf.
experimental results shown under “Model studies/experimental studies” below. The conductivity is
about 1.3 W/(m·K) for fully saturated bentonite with a KBS-3 buffer density and about 1.0 W/(m·K)
if the degree of saturation is 60%. The sensitivity to saturation variations is larger if the saturation
drops below 50–60%.
As long as there is heat transport and incomplete saturation, thermal gradients in the buffer may
influence the state of saturation, such that the water content of the innermost parts of the buffer
decreases temporarily due to moisture relocation to cooler regions further away from the canister.

48

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Gas content: Heat transport will not affect the gas content. The gas content in itself does not have
any influence on the heat transport. If there is a gas phase, the effects of this will be just those of
the incomplete saturation.
Hydro variables: Temperature variations will cause insignificant changes to the porewater pressure.
During the unsaturated changes there is no or little pressure. During the saturated phase, temperaturechanges are too slow to impact on the pressure.
Stress state: Different models have been proposed for estimating the influence of increasing pressure
on the conductivity of bentonite /­Knutsson 1983/. They show a slight increase of the thermal
conductivity, but the effect is not sufficiently verified to be credited /­Börgesson et al. 1994/.
Bentonite composition: There is no influence on the composition of the smectite. The presenceof impurities may affect the thermal conductivity. To be of any importance, the impurity content
must be high and the thermal properties of the impurities significantly different from those of thebentonite. High contents of quartz, for instance, will increase the conductivity.
Montmorillonite composition: Variability of the internal structure of the montmorillonite is
unimportant to thermal conductivity.
Structural and stray materials: To be of any importance to the heat transport, there must be verysignificant amounts of non-buffer materials.
Boundary conditions

There are two boundary conditions:• The heat flux from the local canister, determined by the output at the time of deposition and by
the heat decay characteristics of the waste.
• The temperature at the walls of the deposition hole.
The distribution of the total heat output over the surface of the canister will depend on the way the
initial canister-bentonite clearance develops. There is also a dependence on the vertical distributionof the heat generation within the canister, i.e. on the burn-up ratio between the end parts and the midparts of the fuel elements. As long as the canister-buffer clearance, or part of that clearance, remains
open there will be some redirection of the heat flow towards regions with direct canister-bentonite
contact, for instance at the canister base and top. In regions where the clearance is open, part of theflux will be due to heat radiation.
The temperature at the rock wall depends not only on the heat output from the local canister but also on
the contribution from other canisters, on the rock’s thermal properties and on the repository layout.
Model studies/experimental studies

Model studies. Predictions of the temperature evolution in the near field have been done analytically/­Claesson and Probert 1996, Hökmark and Fälth 2003, Hedin 2004/ and numerically /­Thunvik andBraester 1991, Ageskog and Jansson 1999, Hökmark et al. 2009/.
These studies have all generated consistent results. The temperature at the buffer/canister transition
reaches a peak after about 5–15 years, depending on layout and rock thermal properties, while therock wall peak temperature is reached after 30–50 years. If, for example, the canister spacing is 7.5 m,
the tunnel spacing 40 m, the canister output at the time of deposition 1,700 W and the effective rockthermal conductivity 2.6 W/(m·K), the maximum temperature increase at the wall of the deposition
hole will be 50°C and about 8°C higher if the canister spacing is 6.0 m. One of the objectives ofthese studies has to been to find out how the canister spacing influences the maximum rock wall
temperature and to establish layout guidelines that ensure that the peak buffer temperature does notexceed 100°C for any canister. The duration of the temperature pulse is a few thousand years. After
2,000 years, the maximum excess temperature in the repository is 15–20°C, again depending on
the repository layout and on the rock thermal properties. After 5,000 years, the maximum elevationin temperature from the initial ambient is about 10°C. The thermal gradients within the buffer arealmost zero already after 2,000 years.
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49

/Hökmark and Fälth 2003/ and /­Hedin 2004/ used results from different near-field thermal analyses asboundary conditions in numerical and analytical calculations of the heat transfer across the bentonite
buffer and across the different gaps. The results indicated that the effects of open canister-buffer gaps
are more important than possible temporal and spatial variations of the heat transport properties within
the buffer.
Experimental studies: /­Börgesson et al. 1994/ measured the thermal conductivity of samples ofcompacted MX-80 bentonite by use of a heat pulse technique, Figure 3-1. The results match well withcorresponding results obtained for MX-80 by /­Kahr and Müller-Vonmoos 1982/ and with results obtained
for similar materials, for instance the bentonite tested in the FEBEX experiment /­Huertas et al. 2000/.
The Prototype Repository at Äspö HRL includes six full-scale deposition holes in two tunnel sections
separated by a concrete plug /­Goudarzi et al. 2003a/. There are two deposition holes in the outer section
and four in the inner one. Inflow measurements performed prior to bentonite emplacement and start of
the actual test showed that there are very significant differences between individual holes. The supplyof water to the two holes in the outer section is sufficiently low that the inner parts of the bentonite
buffer were practically in its initial state of saturation after about 6 months of test operation. The
temperature drop across the still open canister/bentonite space in the dry holes was about 19°C 50 days
after test start. In at least one of the holes in the inner section, there are indications, e.g. no measurable
temperature drop between canister and bentonite, that the annular space between canister and rock was
almost saturated (i.e. the initial gap was closed) about two years after test start.
The effective thermal conductivity λeff of the buffer in the annular space between canister and rockwall at canister mid-height can be estimated by use of the following expression:

λeff =

qc
R⋅ Rc ⋅ ln 2 								(3-2)∆T
R1

Here ∆T is the temperature difference between two points at distances R1 and R2, respectively, fromthe heater axis. Rc is the canister radius. The canister surface heat flux qc at canister mid-height mustbe estimated from the total power and corrected to account for the non-uniform distribution of the heat
output over the canister surface /­Hökmark et al. 2009/. Applying the expression to the dry hole in theouter tunnel section described above using data reported by /­Goudarzi et al. 2003a/ gives bentonite
conductivity values ranging between 1.2 and 1.3 W/(m·K) /­Hökmark et al. 2009/.
The TBT (Temperature Buffer Test) experiment in Äspö HRL is run under high thermal gradients
and with maximum buffer temperatures well above 130°C /­Goudarzi et al. 2008/. The buffer material
is MX80 bentonite with the same properties (e.g. density and initial saturation) as the Prototype
1,4

1,2

λ, [W/(mK)]

1

0,8

0,6

e=0.44e=0,65e=0.8
e=1.35e=1.45

0,4

0,2

0
0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

Sr

Figure 3-1. Heat conductivity of laboratory–scale bentonite samples as function of saturation for a few
values of the void ratio, e /­Börgesson et al. 1994/.

50

TR-10-47

Repository bentonite blocks. The temperatures along a radial scan-line from the cylindrical, 0.6 m
diameter heater to the rock wall are monitored by use of numerous, densely positioned, thermocouples.
Using Equation 3-2 above, the thermal conductivity, or rather the change in thermal conductivity,
along the scan-line is evaluated and used as an indicator of wetting/drying. The conductivity was
found to drop by about 50% in a 0.10–0.15 m wide zone around the heater soon after test start
because of drying. Some five years after test start the conductivity has increased and seems to bealmost, but not completely, uniformly distributed along the scan-line. This is an effect of water
being supplied from the surrounding pressurised sand filter boundary.
Natural analogues/observations in nature

Not applicable.
Time perspective

There are two relevant time perspectives: the water saturation perspective and the heat production
perspective. The time it takes to achieve full water saturation is dependent on, inter alia, the pressure
conditions in the groundwater in the near field. The process is estimated to take a number of years,
see Section 3.3.1. After that, all heat transport takes place by conduction and under well-definedconditions with known thermal conductivities. Before then, heat transport can be influenced by
the presence of gaps and joints.
After a few thousand years, the heat production, and thereby the heat transport through the buffer,
will have been reduced to a few percent of their original values.
Handling in the safety assessment SR-Site

The integrated thermal evolution of the buffer and rock is modelled with the set of models describedin /­Hökmark et al. 2009/ and /­Hökmark et al. 2010/. Aspects of heat transport relevant to the THM
evolution of the buffer are treated in /­Åkesson et al. 2010a/.
An important purpose of the modelling is to evaluate the peak buffer temperature that must notexceed 100°C, taking all relevant uncertainties into consideration. These include the possible presence of gaps between canister and buffer and between buffer and rock and data for describing heattransport across these.
The peak buffer temperature will depend not only on the properties of the buffer and the gaps, butalso on the repository layout and the rock thermal properties. Therefore the critical temperature
calculations were conducted prior to establishing the layout. These calculations are reported in the
Site Engineering Report /­SKB 2009a/. In SR-Site there are references to the Site Engineering Reportand additional evaluations of the results and the relevance of the margins applied in the layout work.
Time periods: The modelling will encompass at least 10,000 years, but the critical timescale is that
period during which the peak canister temperature is reached, i.e. the first tens of years. For timescales
beyond a few thousand years, it is important to determine if permafrost conditions could lead tofreezing of the buffer, see further Section 2.2.2. This is done by a comparison of the rock thermal
evolution during permafrost conditions to the buffer freezing temperature, taking the residual powerof the canisters into account if relevant.
Boundary conditions: The treatment of heat transfer over the buffer boundaries is described in/­Hökmark et al. 2009/.
Handling of variables influencing the process: The process is coupled to the water saturation process(Section 3.3.1) which is difficult to model in detail, especially the evolution of gaps. Also, the water
saturation process is highly dependent on the uncertain hydraulic conditions in the rock around thedeposition hole. The peak canister temperature will therefore be determined under the assumptionthat no additional water is taken up by the buffer after deposition, i.e. a time-independent, pessimistically chosen heat conductivity is used. Hereby, the influences of varying hydraulic conditions inthe saturating buffer, including porosity variations, are conservatively neglected. The possibilityof moisture being redistributed as a result of vaporisation and vapour diffusion is accounted for byadditional margins. The thermal conductivity of the buffer will also be calculated after taking into
consideration the possible influence of impurities and presence of stray materials in the system.
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51

The gap between the canister and the buffer will conservatively be assumed to be open until the peaktemperature has been reached.
Handling of variables influenced by the process: The temperature is explicitly calculated. The watercontent is handled in process 3.3.1.
The special cases of failed canister and of earthquakes: Canister failures and earthquakes of amagnitude that could affect the thermal evolution are not expected during the roughly 1,000 year
time period in which the buffer temperature evolves significantly. The process is thus not handled
for these cases. Should the assumptions regarding significant, early canister failures or earthquakes
not be confirmed by the results of the safety assessment, handling of the special cases will bereconsidered.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The methods used for calculation of the peak buffer temperature are described in detail in the reporton strategy for dimensioning of the repository /­Hökmark et al. 2009/. The process of selecting buffer
parameter values and parameter values for gap properties is described in that same report, which will
undergo a documented factual- and quality review. Thermal properties for the buffer used in the calculations of the THM-evolution (Section 3.3.1) are qualified in the THM-data report /­Åkesson et al.
2010b/. The principles of rock wall temperature calculations are, in addition to being described in thethermal strategy report /­Hökmark et al. 2009/, also described in peer reviewed papers, e.g. /­Hökmarkand Claesson 2005/. The basis for selection of rock thermal data is found in the thermal site reports/­Back et al. 2007, Sundberg et al. 2008/. Both thermal site reports have undergone documented
factual- and quality review.
Uncertainties

Uncertainties in mechanistic understanding
Heat transfer from the hot canisters via the buffer to the near-field rock and backfill is in principle
a simple process that can be described with reference to basic laws of physics. The character of the
sub-processes that participate in the heat transport is also known and the theoretical modelling of
their function is based on well-known physical principles.
Model simplification uncertainties in SR-Site
Several modelling simplifications regarding coupling to the hydraulic evolution are described under“Handling in the safety assessment” above. The conservative simplifications used in the models are
justified since the prime purpose of the modelling is to obtain upper limits on canister and buffertemperatures. Sensitivity analyses will in some cases shed light on the effects of the simplifications.
Input data and data uncertainties in SR-Site
Critical input data for the modelling of this process will be given in the SR-Site data report /­SKB
2010l/. Critical uncertain input data for use in determining the peak canister temperature include
the heat power of the fuel, the thermal conductivity of the partly saturated buffer material and of thehost rock. Also uncertainties of the emissivities of the inner buffer and in particular the outer copper
surfaces are decisive in determining the peak canister temperature. All uncertainties associated withpeak buffer temperature calculations are presented in /­Hökmark et al. 2009/. Sensitivity calculations
will be done to clarify the influence of the various data uncertainties.

3.2.2

Freezing

Overview/general description

The freezing point of a soil is defined as the temperature where ice start to form in the material.
The phase transition between ice and liquid state of the soil porewater is affected by pressure andthe presence of solutes in the same manner as bulk water. However, the freezing point is generally

52

TR-10-47

depressed in a soil compared to the corresponding bulk solution due to the geometrical constraints of
the water in a porous medium. The freezing point of a soil is thus dependent on the stress state, theporewater chemistry, the detailed distribution of the pore volume, the chemical and physical propertiesof the surfaces of the constituting minerals, and the degree of water saturation.
A further consequence of ice formation taking place in a porous material is that all the porewater
will not freeze at one specific temperature. There exists a temperature range below the freezing
point where equilibrium exists between ice and water in liquid form. The evidence establishing theexistence of a continuous, unfrozen water phase that separates ice from the mineral matrix in porous
media such as soils is widely accepted /­Anderson 1963, Miller 1963, Nersesova and Tsytovich 1965,
Williams 1964/. For a frozen soil, it is therefore relevant to partition the total water content into
unfrozen water content and ice content.

w = wu + wi 									(3-3)
The maximum amount of unfrozen water wu, max naturally decreases with temperature (keeping allother factors constant). wu, max depends on various other factors among which particle mineral composition, specific surface area of particles, solute concentrations and pressure are the most significant/­Anderson and Tice 1972/. The influence of pressure and solutes is not unique for soil porewater andis known to be such that the freezing point lowers with increasing pressure and solute concentration.
Numerous studies of the relationship between the (maximum) unfrozen water content and temperature,for different types of soils, have been performed; see e.g. /­Anderson and Tice 1973, Nersesova andTsytovich 1965, Andersland and Ladanyi 1994/.
It has been concluded /­Anderson and Tice 1972/ that several of the variables describing a specificsoil in turn can be related to the specific surface area S, and that a relation between only wu, max, S,
and temperature usually gives a satisfactory description (for constant pressure and solute concentration). By fitting a mathematical expression to empirical data for 10 representative soils (isothermal
calorimeter measurements), /­Anderson and Tice 1972/ achieved the following equation relating
maximum unfrozen water content to surface area (m2/g) and θ, the temperature below freezing (°C):
ln wu = 0.2618 + 0.5519 ln S –1.449 S–0,264 ln θ					(3-4)
Equation 3-4 can be graphically presented as in Figure 3-2.
Ice will not be present in a soil unless the total water content exceeds wu, max. Hence, Equation 3-4
also gives the freezing point of a soil whose total water amount equals wu, max. Specifically,
Equation 3-4 can be used to determine the freezing point of the buffer material.

Figure 3-2. Maximum unfrozen water content as function of θ for various values of specific surface areas.
θ denotes the number of degrees (°C) below the freezing point. (After /­Anderson and Tice 1972/).

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53

From Equation 3-4 it is found that a material with a specific surface area of the magnitude 800 m2/g
at a temperature of –5°C gives a wu, max of ca 35%. This value is larger than the total water content
of the saturated buffer and from the above it can be concluded that the freezing point of the watersaturated buffer is below –5°C.
The other factors not taken into account in the present estimation (pressure, solutes) of the buffer
freezing point will contribute to an even lower value.
If ice is not present in the system no increase in volume and no stress increase will take place andtherefore questions related to ice lens formation will be of no relevance in the buffer.
Despite the significant freezing point depression in the buffer, the effect of freezing will influenceswelling pressure at temperatures above the actual freezing point. Swelling and swelling pressure
are phenomena which depend not only on properties of the bentonite itself but also on properties ofthe external aqueous reservoir. Hence the bentonite swelling pressure will be affected by freezing assoon as the external reservoir freezes.
It is possible to derive the temperature dependence on swelling pressure of a water saturated swellingclay /­Birgersson et al. 2008/.

Ps (w, T ) ≈ Ps ( w,0 C) −∆s ( w)⋅ T 					vc.w. (w)

(3-5)

where T still denotes temperature below 0ºC, vc.w, (w) is the partial molar volume of porewater, ∆s(w)is the partial molar entropy difference between water in the clay and water in an external ice phase.
For values of water content relevant to buffer conditions, ∆s(w) is of the same order as the molar
entropy difference between ice and liquid bulk water (22 J/ºC /­mol), and a swelling pressure dropwith temperature is expected, of the order of 1 MPa/ºC. Note, at the temperature given byTc ≈vc.w. (w)Ps ( w,0 C) 						∆s ( w)(3-6)

swelling pressure is completely lost. This temperature corresponds to the situation when the possibleamount of unfrozen water equals the total water content, i.e. expresses the freezing point depression.
Assuming a typical buffer swelling pressure above 0ºC of 7 MPa and a partial molar volume equal tothe value for bulk water (18 cm3/mol) gives Tc ≈–6 ºC, in good agreement with the Anderson & Tice
expression presented in Equation 3-4 and Figure 3-2.
Dependencies berween process and buffer variables

Table 3-3 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Boundary conditions

For the buffer to freeze, the water in the surrounding rock must be present as ice. The ability for(water) mass transport across this boundary is therefore very limited.
Model studies/experimental studies

The swelling pressure response of water-saturated samples of the SR-Site reference material MX-80
at various water contents has been measured down to temperatures of around –10ºC /­Birgersson et al.
2008, 2010/. A typical result is presented in Figure 3-3. The pressure drop as described by Equation 3-5
was shown to agree quantitatively with measurements. Also the existence of a water content dependent
temperature where swelling pressure is completely lost (Equation 3-6) was demonstrated. The processes
were shown to be completely reversible during several freezing/thawing cycles. No pressure peaks
around 0ºC were observed.
Furthermore, the experiments were successfully explained by thermodynamics and the assumptionof a homogeneous bentonite with only one type of nm-wide slit like pores. The behaviour wasconcluded to be completely analogous to freezing point depression in electrolyte solutions.
54

TR-10-47

Table 3-3. Direct dependencies between the process “Freezing” and the defined buffer variables
and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Buffer geometryYes. If extended toregions with very low
temperatures

Neglected. Large
scale freezing is notexpected to occur

Yes. Geometry could
Neglected. Large scalechange throughfreezing is not expected
expansion towards the to occur
backfill

Pore geometry

Yes

See Equation 3-7,
Yeswhich is the mainargument why freezing
can be neglected

Radiation intensity

No

No

Temperature

Yes

Water content

Yes via freezing point Included in theand transport capacity assessment

Yes, If freezing takes
place redistribution ofwater

Gas content(Yes)

Indirect through watercontent

No

Hydrovariables
Yes, freezing induces(pressure and flows) water transport

Included in theassessment

Yes

Stress state

Yes, influences thefreezing point to asmall degree

Neglected, effectis smallYes. Swelling pressure Calculated/estimatedaffected also before
ice formation

Bentonitecomposition

Yes, via The specificsurface area

Included in theassessment

No

Montmorillonite
composition

Yes, via the specificsurface area andtypes of counter ions

Included in theassessment

No

Porewater
composition

Yes, influences thefreezing point

Neglected, sinceNovariations will be small

Structural and straymaterials

Yes (potentially)

Neglected, since nostray materials are
expected.

TR-10-47

Included in theassessment

Neglected, No increasein pressure is expected

No
Included in theassessment

Included in theassessment

No

55

7

Freezing01

6

Pressur e (MPa)5Cycle 3
Cycle 44Cycle 5
Cycle 6

3

Cycle 7
Theory2

Cycle 3+

1

0
10

-5

0

5

10

15

20

Temperature (°C)

Figure 3-3. Measured swelling pressures below and above 0ºC of a sample of MX-80 bentonite which
exerts a swelling pressure of approximately 6 MPa at 0ºC /­Birgersson et al. 2010/.Different colors refers todifferent freezing/thawing cycles. The theoretical prediction below 0°C (Equation 3-8) is labeled “Theory”.

Natural analogues/observations in nature

No specific studies of natural analogues of frozen clays for the purpose of radioactive waste disposal
have been made. However, knowledge can probably be achieved by studying permafrost areas.
Time perspective

The process of freezing is relevant during the entire repository lifetime.
Handling in the safety assessment SR-Site

The freezing process is studied in /­Birgersson et al. 2010/. As the water content at full saturation will
be lower than the unfrozen water content for temperatures higher than -5°C, no ice will be formed.
Furthermore, the swelling pressure drop only occurs when the transport paths in the surroundingrock are frozen. Since the buffer is completely recovered at thawing, any deterioration of the sealing
properties due to the pressure drop is of no concern. Consequently, the process of freezing of thebuffer can be neglected.
Handling of variables influencing the process: The temperature evolution is estimated in /­SKB
2010h/.
Uncertainties

Uncertainties in mechanistic understanding
The freezing of soils is a well known process. The description of swelling pressure response due totemperature changes is based on robust, fundamental thermodynamic arguments.
Model simplification uncertainties for the above handling in SR-Site
The reproducibility of the pressure response due to freezing is good. The process can be welldescribed with the equations presented in this section.

56

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Input data and data uncertainties for the above handling in SR-Site
The bentonite freezing point is strongly dependent on density. If a significant loss of material occurs,e.g. due to chemical erosion, a corresponding change in the freezing behaviour is expected.
The precise value of (minimum) temperature is uncertain. If it stays above 0°C, no phase change at
all will occur.
Adequacy of references supporting the suggested handling in the safetyassessment SR-site

The supporting references /­Birgersson et al. 2008/ and /­Birgersson et al. 2010/ are a peer reviewed
article and a reviewed report according the SR-Site quality system that are available in the open
literature. In addition, these particular studies are aimed directly at dealing with the freezing process
in swelling clays.

3.3

3.3.1

Hydraulic processes

Water transport under unsaturated conditions

Overview/general description

Water transport in the buffer under unsaturated conditions is a complex process that is dependent on,inter alia, temperature, smectite content, degree of water saturation and water content in the different
parts of the buffer. The most important driving force for water saturation under deep repository conditions is a gradient in negative water pressure or relative humidity in the pores of the buffer that leads
to water uptake from the surrounding rock. The supply of water from the rock by water transport infractures and in the rock matrix is also a decisive factor for the temporal evolution of the process.
The following is a detailed description of the process. The degree of detail is appropriate to indicate
a general understanding, but is not necessary for the needs of the safety assessment to, for example,
model the main features of the hydraulic evolution of the buffer.
The following hydraulic sub-processes can be distinguished:• Transport of water in liquid phase, which is controlled and driven by
A1) a pressure gradient in the water,
A2) a temperature gradientA3) an osmotic gradient,
A4) gravity (coupled to A1),
A5) a density gradient (coupled to A1).
• Transport of water in vapour form, where transport is controlled and driven by
B1) a pressure gradient in the water,
B2) a temperature gradient,B3) an osmotic gradient,B4) gravity,
B5) a density gradient.
• Phase transitions between water and vapour by:
C1) evaporation,C2) condensation.
• Thermal expansion of
D1) water,
D2) air.
• Compression of
E1) water,
E2) air.
• Transport in water ofF1) dissolved air.

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57

These processes are coupled, and are in some cases different sides of the same process. They can be
described and modelled in different ways. An important variable is the porewater pressure, which
can be described according to Equation 3-7.
ψ =( (ua–uw)+π 									(3-7)

where ψ is the free energy of the soil water (usually named total suction), uw is the porewater pressure,ua is the pore gas pressure and π is the osmotic suction of the external groundwater. (ua–uw) is called
the matric suction.
The total suction can thus be described from a geotechnical viewpoint as the sum of the matric suction,
which can be said in simplified terms to stem from capillary effects, and the osmotic suction, which
is dependent on differences in ionic concentration.
The main driving force for water flow in unsaturated clay is usually considered to be the gradientin porewater pressure, which means that the matric suction and the pore gas pressure are importantvariables in the modelling of water flow in unsaturated materials. The total suction is a function of
the water content of the clay (called retention curve).
The total suction can also be expressed in terms of relative humidity of the clay. Using a thermodynamic
approach, total suction is the free energy state of soil water, which can be determined from the partial
vapour pressure of soil water /­Fredlund and Rahardjo 1993/. Equation 3-8 shows the relation between
the partial vapour pressure p and the suction, ψ. The ratio p/ps corresponds to the relative humidity
and Equation 3-8 thus states a strict relation between relative humidity and suction.

ψ =−

R ⋅T
p
ln( ) 								(3-8)ν w0 ⋅ ων
ps

where
ψ
=
T
=
R
=
vw0 =
=
ρw
=
ωv
p
=
=
ps
		

suction (kPa)absolute temperature (K)universal gas constant (8.31432 J/(mol K))
specific volume of water (1/ρw m3/kg)
density of water (kg/m3)
molecular mass of water vapour (18 kg/kmol)
partial pressure of porewater vapour (kPa)saturation pressure of water vapour over a flat surface of pure water at
the same temperature (kPa).

The understanding of the processes that drive water transport in unsaturated buffer and backfill
materials and the knowledge of how the processes are influenced by different factors are not complete.
The following description (A1–A5, B1–B5, C1–C2, D1–D2, E1–E2 and F2) pertains chiefly to themodels that are used for non-swelling soil materials /­Fredlund and Rahardjo 1993/. It has not yet
been fully clarified whether the approach is sufficiently all-inclusive to fully describe the processesin swelling clay minerals. However, the understanding is sufficiently good to carry out model
calculations in the safety assessment.
A1. Transport of water in the liquid phase, which is driven by a water pressure gradient, can be
described by a refined version of Darcy’s Law. The parameters that control the process are thematerial’s hydraulic conductivity K and the difference in porewater pressure uw. Both of thesecan be described as functions of the void ratio e, the degree of saturation Sr, the ion concentrationin the porewater ic and the temperature T. The porewater pressure of unsaturated clays is also
a function of the wetting history, i.e. whether the material undergoes wetting or drying, andthe total average external stress and (in the saturated state) the hydraulic head.
The primary driving force for the liquid-phase water flow is the porewater pressure gradient.
The hydraulic conductivity varies strongly with the degree of saturation.
A2. The influence of a temperature gradient on the transport of water in the liquid phase is small
and mainly indirect via change in water retention properties and water density with temperature
except for temperatures below 0ºC (see Section 3.2.2 about freezing).

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A3. Transport of water in the liquid phase, which is driven by an ion concentration gradient, osmosis.
Significant osmotic effects can arise at high salinities in the groundwater and in conjunction
with salt enrichment in the buffer. The driving force should be visualised as a diffusion process
driven by an osmotic gradient.
A4. Transport of water in the liquid phase, which is driven by gravity. The influence of gravity isreflected in the hydraulic head and thus taken care of by process A1.
A5. Transport of water in the liquid phase, which is driven by a density gradient in the water.
Differences in the density of the water can be caused by e.g. differences in temperature or
salinity. Density-generated flow is probably negligible in buffer material, cf process A1.
B1. Transport of water in the vapour phase, which is driven by a vapour pressure gradient. The
vapour pressure uv in the fraction of the pores that is air-filled or the relative humidity (RH= uv/uvo where uvo is the saturation pressure in the air) is in equilibrium with the total suction
potential in the liquid phase. If RH is higher than the equilibrium relationship, water vapour
condenses and increases the quantity of liquid, which in turn reduces the total suction until
equilibrium is re-established. If the total suction differs in different parts of a soil, for example
due to different water ratios, this leads to different vapour pressures, and transport of water
vapour in the unfilled portion of the pores takes place with the vapour pressure difference as adriving force. At similar temperature and other circumstances, water vapour is thus transported
in the gradient of the relative humidity and (due to the direct correlation) in the gradient of total
suction. However, this transport is parallel to process A1, which takes place in the liquid phase.
The relationship between these transport mechanisms has not been fully clarified.
B2. Transport of water in the vapour phase, which is driven by a temperature gradient, appears
to be the most important process for vapour transport. This process can also be regarded as apart of process B1. A temperature difference brings about a vapour pressure difference whichtransports vapour from a warmer part to a colder part. It can also be modelled as a diffusion
process driven by a temperature difference /­Philip and de Vries 1957/.
The vapour transport causes a water ratio difference and thereby a difference in suction potential,
which in turn leads to a liquid-phase transport in the opposite direction. In a closed system, with atemperature gradient and constant mass of water, an equilibrium situation arises after some time
when the vapour transport is equal to the liquid transport in all parts. A steady-state condition withwater ratio differences that reflect the temperature differences in the system will then prevail.
B3. Transport of water in the vapour phase, which is driven by an osmotic gradient. This is anindirect process since the vapour is driven by a difference in vapour pressure that can be settled
by a difference in osmotic pressure.
B4 and B5.
Transport of water in the vapour phase, which is driven by gravity or a density gradient. These
processes are presumably insignificant in the buffer’s pore system, but may be of great importancein gaps, particularly in the vertical gap between canister and bentonite.
C1 and C2.
Phase transitions via evaporation and condensation are constantly occurring in a temperature
gradient where evaporation in the warmer part is accompanied by condensation in the colder
part. Aside from the fact that it can lead to heat transport, it is only significant as evaporation if
the buffer is not isolated from the atmosphere, which may be the case if a very dry deposition
hole is allowed to stand open towards the tunnel for a very long time or if the backfill is dry.
D1 and D2.
Thermal expansion of water in the liquid and vapour phases can indirectly affect the flow bychanging either the water pressure or the density (convection). Other consequences are dealt
with under mechanical processes.
E1. Compression of water in the liquid phase is of little importance for the liquid transport
processes other than for a water-saturated system, where the size of the pressure build-up
associated with a temperature increase is dependent on the compressibility of the water.
E2. Compression of air can be an important factor in the water saturation process (Boyle’s Law).
If the air is trapped, it must be compressed and go into solution in the water in order for water
saturation to be achieved. In highly compacted bentonite, the suction potential appears to be
TR-10-47

59

sufficiently high for this to occur and full water saturation is achieved without any external waterpressure. Highly-compacted pure bentonite is thus able to achieve water saturation by itself. Parts ofthe backfill, however, may have such low suction that a pressurised air volume will remain if the air
cannot seep out. A large part will disappear when full water pressure has developed.
F1. Transport of dissolved air in water. When trapped air is compressed during the water saturation phase,a portion of the volume is dissolved in the water. This solubility (Henry’s Law) is approximately 2
percent at room temperature but decreases rapidly at increasing temperature. In order for additional
air to be dissolved, the dissolved air must be transported away, which takes place by diffusion.
Dependencies between process and buffer variables

Table 3-4 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Influence of porewater composition on the wetting rate: High salinity in the groundwater that isabsorbed from the near-field rock limits the swelling of the bentonite grains and initially leaves wider
passages for penetration of water. The hydraulic conductivity increases with increasing salinity especiallyfor low densities.
The salinity of the groundwater influences the vapour pressure relation and thereby the water saturation process. For typical Swedish groundwater conditions, the effect in the buffer is small, for buffer
densities (> 1,900 kg/m3saturated density) the influence is small also at high salinity.
Influence of hydrovariables: At unsaturated state, the driving force is the matric suction (negativeporewater pressure (ua–uw)). The relation between the total suction (or relative humidity according toEquation 3-8) and the water ratio is called the retention curve. The retention curve under free swellingof MX-80 is shown in Figure 3-4.
The retention curve describes an important relationship for modelling the wetting of buffer materials.
It is influenced by a number of factors such as (see /­Dueck and Nilsson 2010/):
The wetting path. There is a significant hysteresis effect, which means that wetting yields lower watercontent than drying at the same RH.
The initial conditions. After mixing with water the measured RH lies between the wetting and drying curve.
The temperature. An increase in temperature yields an increased RH for the same water content.
The confining conditions. If the bentonite is confined and thus not allowed to swell RH will be higher
than if it is not confined.
The external stress. An increased external stress increases RH at constant water content.
Influence of temperature: The viscosity of water is affected by the temperature, which means thatthe hydraulic conductivity increases with increasing temperature. This means that an increased overalltemperature in the buffer increases the wetting rate. In addition, the temperature influences the retention curve (see above) and the vapour diffusivity.
Influence of water and gas content: The hydraulic conductivity is strongly affected by the degreeof saturation. A commonly used relationship between hydraulic conductivity and the degree of water
saturation is provided in Equation 3-9.
Kp = SrδK									(3-9)
where
Kp
Sr
K
δ

=
=
=
=

hydraulic conductivity of partly saturated soil (m/s)
degree of water saturation
hydraulic conductivity of completely saturated soil (m/s)
parameter ≈ 3 for MX-80 at buffer densities

The vapour diffusivity is also affected by the degree of saturation.
The porewater pressure in an unsaturated buffer is strongly affected by the water content (retention
curve). See Figure 3-4.
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Table 3-4. Direct dependencies between the process “Water uptake and transport under unsaturated conditions” and the defined buffer variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Buffer geometry

Yes, mainly throughdistance to wet rock

Given as input data insaturation calculation

Yes. Influencing
through the swellingof blocks

Pore geometry

Yes, the poregeometry (density)
influences the waterand vapour transport
rate

Included in material
data

Yes, through the
Degree of saturation
degree of waterand void ratio aresaturation that
calculated
together with the void
ratio determines
the pore geometry

Radiation intensity

No

No

Temperature

Yes, through water
Included in the models
viscosity and throughthe hydrovariables ina temperature gradientYes, through the
influence of degreeof saturation and void
ratio on the thermal
properties

Water content

Yes, decisive forthe retention curve

Initial water content is Yes, changes thegiven as input data in water content
saturation calculation.
The degree of saturation and the void ratio,
which determines thewater content, arevariables in the model

Degree of saturationand void ratio arecalculated

Gas content

Yes, included in the
degree of saturation

Initial gas content isYes, changes
given as input data in the gas content
saturation calculation.
The degree of saturation and the void ratio,
which determines thegas content, are variables in the model

Degree of saturationand void ratio arecalculated

Hydrovariables(pressure and flows)

Yes, main variables in Included variables
the saturation process in the saturation
calculation

Yes, changes thehydrovariables

Included variablesin the saturation
calculation

Stress state

Yes, influences theretention curve.

Yes, changes theswelling pressure

The pressure andsuction are calculated

Bentonitecomposition

Yes, influence through Included through input No, unless via
the retention curve
data in the saturation mineral conversion
and other properties
calculation

Not relevant for thesaturation period

Montmorillonite
composition

Yes, influence through Included through input No, unless via
the retention curve
data in the saturation mineral conversion
and other properties
calculation

Not relevant for
saturation period

Porewater
composition

Yes, influences thehydraulic properties

Included through input Yes, through ion
See 3.5.6
data in the saturation transport in a
calculation
temperature gradientand from absorbed
groundwater

Structural and straymaterials

Insignificant

Excluded, no strayNomaterials are expectedin the buffer

TR-10-47

Included variablesin the saturation
calculation

See 3.4.1

Temperature as afunction of time andspace is calculated forthe resaturation phase.
See also 3.2.1

61

wini = 0% and T = 20 °C
40

Sorption balance Wadsö et al. (2004)35

SB_0_20 (2006)
Sorption balance Kahr et al. (1990)

w (%)

30
25
20
15
10
5
0

0

10

20

30

40

50

60

70

80

90

100

RH (%)

Figure 3-4. Retention curve of MX-80 /­Dueck 2004/. The lower curve is absorption and the upper is desorption.

Influence of the stress state

Under volume constraints, the negative porewater pressure is reduced during water uptake and goesto zero (or equals the applied water pressure) at complete water saturation. At the same time, thetotal pressure from the bentonite towards the confinement increases and reaches the final swellingpressure when the porewater pressure is zero. There is thus interplay between the suction and theexternal pressure (see also Section 3.4.1).
Boundary conditions

Interaction with the rock: A key issue for the saturation process is the interaction between rock andbuffer. If water is only conducted to the buffer in the water-bearing fractures and the rest of the water
has to go through the rock matrix, water saturation can be both uneven and take a long time. If there
is a permeable excavation-disturbed zone (EDZ) at the wall of the deposition hole, which distributes
water from the fractures along the rock wall, the wetting will be faster and more even. However, tests
performed to quantify this feature indicate that if diamond drilling is used for borehole excavation,
there is only a small disturbance of the rock, which reaches a few cm into the rock but also that the
disturbance is not of significant importance for the wetting. See e.g. /­Autio et al. 2003/.
Interaction with the backfill: Recently the reference backfill has been changed from a mixture ofbentonite and crushed rock compacted in situ to centrally placed clay blocks and pellets in the slot
between the blocks and the rock surface. If suction is higher in the buffer blocks than in the backfill
blocks, water may be transported from the backfill to the buffer and thus wet the buffer and dry the
backfill if the water inflow is very low. If the wetting of the backfill from the rock is faster than thewetting of the buffer, this water will also be redistributed to the buffer and thus help the wetting of
the buffer. See e.g. /­Börgesson et al. 2006/.
Model studies/experimental studies

A material model containing unsaturated water flow has been devised and is being developed. The
wetting calculations with this model have been performed using the finite element method with theprogram Abaqus. Certain general parts of the model are described in the manual /­Abaqus manuals/,
while other parts are tailor-made for SKB. See e.g. /­Börgesson and Hernelind 1999, Börgesson et al.
2005/. The model includes the processes A1, A4, A5, B2, D1 and E1, which are the most importantones for the buffer. It is a completely coupled thermo-hydro-mechanical model and thus also includes
thermal and mechanical processes in the buffer. It does not, however, include the processes that handle
air (except for B2), which can be important for the saturation process in the backfill.

62

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The model has been both calibrated and partially verified by laboratory experiments. Models
for unsaturated buffer have been evaluated within several international projects (Äspö TF EBS,DECOVALEX, CATSIUS CLAY and VALUCLAY). These projects include calculation examples
and comparisons with measurement results for both laboratory experiments and field tests. See e.g.
/­Börgesson and Hernelind 1995, Alonso and Alcoverro 2000, Börgesson et al. 2005/.
In recent years, another finite element program has been tested and used for thermal and hydraulicprocesses, namely Code_Bright /­Cimne 2002/. This code was developed by UPC (Technical
University of Catalonia, Barcelona). Most processes are similar to the processes in Abaqus but there
are some important differences. The main difference is that it can handle the gas phase and thus alsomodel processes B1, B4, B5, D2, E2 and F1. It can also handle phase transitions (C1 and C2). This
code is used for modelling e.g. the Äspö HRL large scale tests CRT, Prototype Repository and TBT.
The influence of external pressure and of temperature etc has been investigated in several series oflaboratory tests. See e.g. /­Dueck 2004, Dueck and Börgesson 2007, Dueck and Nilsson 2010/.
Several large-scale field tests have been installed with e.g. the purpose to study the wetting processof the buffer material in a repository.
The big field test BMT (Buffer Mass Test) /­Pusch et al. 1985/ was performed from 1981 to 1985 in
Stripa, Sweden. In this test, highly-compacted bentonite and heaters were installed in six simulated,
half-scale deposition holes. The tunnel above two of these holes was backfilled with sand-bentonite
mixtures containing 10 and 20 percent bentonite. The tests were interrupted after a few years, whereupon
buffer and backfill were excavated with detailed sampling and water ratio determination. The results
showed that the wetting had gone to near water saturation in three of the holes, while the others had
not increased their water content more than marginally. The water inflow in the three first-mentioned
holes was greater than in the others. On the other hand, the gap towards the rock was left open inthese three holes, while the gap in the three “dry” holes was filled with bentonite powder. There
was no tendency towards uneven water uptake from the rock. The three wet holes with empty gaps
had a high, uniform water ratio with a degree of saturation higher than about 80 percent. The three
dry holes with powder-filled gaps were dry nearest the heater and wet nearest the rock, in a waythat could be expected to result from the redistribution process that occurs in the presence of atemperature gradient.
In the Äspö HRL, several deposition hole tests with heated canister and buffer material are runningand two tests have been finished and excavated. These tests are the Canister Retrieval Test (CRT),
the Prototype Repository and the Temperature Buffer Test (TBT) /­SKB 2005/. All tests simulate
heating and wetting of the buffer. The CRT and TBT have artificial wetting of the buffer throughfilters covering all or parts of the rock surface, while the Prototype Repository has natural wettingfrom the rock. All six deposition holes in the Prototype Repository also have an upper boundaryof backfill material that contributes to the wetting. CRT was excavated in 2006 about 5 years after
installation and a large number of samples taken for measurement of e.g. density and water ratio
so that the wetting phase can be studied not only from the results of the sensors measurements but
also from the measured state at the end of the test. During 2008, CRT was modelled and the resultscompared with measurements within the Äspö EBS Task Force. The findings from the Task Forceare not reported yet. However, the results are used in the modelling and published within the SR-Sitemodelling /­Åkesson et al. 2010a/.
The wetting of the buffer in these tests have been modelled and, for the Prototype Repository, theinfluence of the rock structure and properties have been studied in detail /­Börgesson and Hernelind
1999/. This study was originally done for the safety assessment SR-97 but has also been used for
the Prototype Repository. It is applicable for deposition holes in general. In the study the permeabilityof the rock matrix, the water supply from the backfill, the water pressure in the surrounding rock,the permeability of the disturbed zone around the deposition hole, the water retention properties of
the rock, and the transmissivity of two fractures intersecting the deposition hole have been varied.
Additional modelling of the wetting process has been performed and reported by /­Börgesson et al.
2006/ and by /­Åkesson et al. 2010a/. In the former report special emphasis was given to the influenceof the backfill on the wetting of the buffer. However, the backfill was assumed to be compacted insitu. Updating the calculations with the block and pellets backfill is part of the SR-Site modelling
programme /­Åkesson et al. 2010a/.

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63

The results indicate that wetting takes 5–10 years if the permeability of the rock is so high that theproperties of the bentonite determine the wetting rate. However, it may take considerably more than
30 years if the rock is very tight and the water pressure in the rock is low. The calculations also showthat the influence of the rock structure is large except for the influence of the transmissivity (T) of
the fractures, which turned out to be insignificant for the values used in the calculations.
The results of the calculations show that the influence of most changes in rock properties and boundary
conditions are rather large. There is a lack of knowledge with regard to rock properties and buffer/rock interaction and a need for improved material models of the buffer. These matters are a partof the ongoing SKB R&D programme and are included in the Äspö TF EBS (Äspö Task Force on
Engineered Barrier Systems).
An extreme case investigated by /­Börgesson et al. 2006/ is one where no water is supplied by the rockto the deposition hole but that a fracture zone intersecting the deposition tunnel results in rapid water
saturation and a full water pressure of 5 MPa (in a repository located at 500 m depth) in the backfill.
This scenario yields a time to saturation of 250–500 years.
The other extreme case with no wetting from either rock or backfill can be divided into two cases:
1. No water can leave the deposition hole.
2. Water can leave the deposition hole and enter the backfill.
If water is supplied to the backfill or if the backfill has a naturally high RH (e.g. 30/70 with the water
ratio 12%, which has an initial RH of 97%) the backfill and the swelling buffer in contact with the backfill
will create a natural lid that does not allow any water to escape. Alternatively, if the backfill is composedof bentonite blocks with low RH, which are piled in the deposition tunnel, and if the deposition tunnel aswell as the deposition hole are very dry, there is a possibility that the hot environment around the canisterin combination with slots between the blocks may dry the buffer and lead the moist air out into thebackfill by the vapour flux in the buffer and backfill block pore system and by vapour convection in
the pellets pore system.
Additional calculations with sensitivity analyses of e.g. the distance to hydraulic boundaries andsome model parameters will be done for SR-Site and reported in the buffer THM modelling report/­Åkesson et al. 2010a/. They will also include the new backfill material and further study the drying
scenario case 2.
Natural analogues/observations in nature

Wetting of bentonite in nature has not been studied and would probably not contribute to the knowledge
necessary for the assessment of water uptake in a repository.
Time perspective

The above analyses show that the time to full water saturation of the buffer can vary a great deal,
depending above all on the interaction with surrounding rock. For wet conditions in the host rock,saturation may be reached within 10 years. If there are no fractures and no supply of water from thebackfill, all water has to go via the rock matrix, which may take 100 years in the granitic rock types
occurring in Sweden.
The influence of the backfill material on the wetting of the buffer has been investigated in a numberof calculations /­Börgesson et al. 2006/. If wetting is achieved from the backfill (30/70) the time tofull saturation of the buffer may be reduced by 50% at normal rock conditions. The calculations is
updated for SR-Site.
A very long time until saturation is probably not a problem but the consequences have not been fully
investigated (see Model studies).

64

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Handling in the safety assessment SR-Site

A number of calculations of the wetting phase of the buffer with respect to the influence of fracture
distribution, distance to hydraulic boundary, backfill conditions and critical parameters of the material
models etc are performed for SR-Site and are reported in the buffer THM modelling report /­Åkesson
et al. 2010a/.
The initial THM evolution covering the saturation phase of the buffer was modelled for a numberof hydraulic boundary conditions in SR-Can since the supply of water at the outer buffer boundary
strongly influences the wetting rate. The calculations have mainly been made with the old THMmodel used for SR-Can /­Börgesson and Hernelind 1999, Börgesson et al. 2006/ but updated models,e.g. with Code Bright, are also used in the new calculations. The old calculations have assumed
backfill compacted in situ but the revised concept with blocks and pellets is used in the new calculations. The purpose of these calculations is to study the sensitivity of rock and backfill properties tothe wetting time of the backfill and the indirect influence on the wetting of the buffer. The outcome
is increased knowledge on how much different processes and properties influence the saturation time.
Timescales: The THM modelling encompasses the wetting period for the backfill and buffer.
Consequently, the detailed timescale will be determined by the hydraulic conditions in the bedrock,which control the wetting. Typical timescales range from less than ten to more than a hundred years.
Boundary conditions: The THM modelling is performed for a number of different hydraulic conditions in the bedrock.
Handling of variables influencing this process: The process is coupled to a number of other processes in the buffer THM model. The modelling is done for a realistic thermal gradient over the buffer
and includes thermal expansion. All influencing variables in Table 3-4 are included in the modelling
except effects of varying porewater composition. It is recognised that the salinity of the porewater
could influence the final swelling of the buffer. This is, however, included in the near-field evolution
model where the effect of salinity on swelling is assessed; see Section 3.5.8 (osmosis).
Handling of variables influenced by the process: As shown in Table 3-4 many of the variables are
influenced by the wetting process. These effects are included in the coupled THM-model and are
addressed in the THM calculations for SR-Site. The buffer saturation as a function of time as wellas the swelling of the buffer at the end of the saturation process (considering friction between buffer
and the walls of the deposition hole), the associated compaction of the backfill and possible movements of the canister in the deposition hole as a consequence of these phenomena are calculated.
The special cases of failed canister and of earthquakes: Canister failures and earthquakes of amagnitude that could affect the wetting process are not expected during the time period in which
the buffer saturates. The process is thus not handled for these cases.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The codes used for modelling the initial THM evolution of the buffer are mainly Abaqus and CodeBright. Abaqus is one of the world’s leading general finite element codes that undergoes documented
quality controls and has been used in numerous applications reported in peer reviewed articles.
The material models used for modelling unsaturated buffer materials have been assessed in several
international projects (see section Model studies/experimental studies).
Code Bright is a university code that has been designed especially for THM modelling of unsaturated
soils. It has recently been made available and the code and the material models are updated regularly
and assessed in peer reviewed articles as well as in international projects (e.g. Äspö EBS TF).
The two main reports referred to in the description of the water saturation phase /­Börgesson andHernelind 1999, Börgesson et al. 2006/ are SKB Technical Reports that have not undergone a documented factual- and quality review. However, they are widely referred to by other scientific groups.
The revised model of the wetting phase is included in the buffer THM modelling report /­Åkesson
et al. 2010a/, which will undergo a documented factual- and quality review.

TR-10-47

65

Uncertainties

Knowledge of the hydraulic behaviour of unsaturated buffer material is gained from investigations
carried out on MX-80 whereas very few tests have been performed on the other buffer candidate(Deponit CA-N bentonite). At the density considered for the buffer material, their behaviour is
expected to be similar but this needs to be confirmed.
Uncertainties in mechanistic understanding
Understanding of the process is deemed sufficient for the needs of the safety assessment. Somehydraulic parameters and processes in the buffer are uncertain e.g. the suction close to water saturationand the influence of the swelling pressure (currently being studied), the influence of entrapped air
and the magnitude of vapour flux and the influence of gaps, which imply very inhomogeneous initialconditions. Although detailed knowledge of these processes might not be needed for the safetyassessment, they are studied in order to gain further confidence in the wetting modelling.
Model simplification uncertainties in SR-Site
The water uptake modelling done so far has assumed that the bentonite blocks and the gaps (filled or
unfilled) are homogeneous so that the buffer has uniform density from the start. This simplification
is not correct but the error is likely to be smaller than that caused by the uncertainty in the rockproperties. Attempts to take these inhomogeneities into account will be made in the CRT modelling
and the ongoing development of the codes.
The detailed description of the wetting process is complicated, but it can be modelled with sufficientaccuracy for the needs of the safety assessment where it is not necessary for all details of the process
to be incorporated. It is, however, important to show that full water saturation is eventually reachedin order to provide a reliable description of the long-term evolution following saturation and to deter­mine the distribution of buffer and backfill mass at the end of the saturation phase. It is further important
to assess possible chemical alterations of the buffer during the saturation process. Such alterations
are, however, not expected to influence the saturation process and hence need not be considered forthis process.
Input data and data uncertainties in SR-Site
The greatest uncertainty and variability concerns the hydraulic conditions in the near-field rock andthe hydraulic interaction between the rock and the buffer. These are addressed by analysing a numberof cases with differing assumptions regarding rock and buffer properties.

3.3.2

Water transport under saturated conditions

Overview/general description

Water transport in the saturated buffer is a complex interplay between several sub-processes ona microscopic scale. At a macroscopic level, the result is that the permeability of a saturated buffer
is very low, and this is also the essential result for the safety assessment.
Water flow in the saturated buffer is a special case of unsaturated flow. The number of processes
to consider is fewer (vapour flux, phase transitions and air phases do not exist). Those processes
that are involved are essentially the same as for unsaturated conditions, but can vary widely inimportance when compared with unsaturated conditions.
The most important mechanism under saturated conditions is transport of water in the liquid phase,
which is driven by a water pressure gradient. This transport process can be described by Darcy’s
Law. Any deviations from Darcy’s Law, which can occur at low gradients, are favourable in thatthey lead to a reduced flow-through rate (e.g. /­Dixon et al. 1999/ and /­Hansbo 1960/. The hydraulicconductivity, K is a function of the composition of the buffer, the void ratio, e, the ion concentrationin the porewater, ic, and the temperature, T.
The hydraulic conductivity of the reference buffer (MX-80 and Deponit CA-N) to low-salinity water
after complete saturation is approximately 10–13 m/s at a density of 2,000 kg/m3 (see below).
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An approximate condition for transport by flow to be more important than diffusive transport isgiven by: De/∆L < Ki, where De = the effective diffusivity, ∆L = the transport length (one-dimensional), K = the hydraulic conductivity and i = the hydraulic gradient. For the stage after restorationof the hydraulic situation in the repository, i can be set equal to 10–2 and, for anions De = 10–12 m2/s,
which for the buffer thickness ∆L = 0.35 m requires that K exceeds 3·10–10 m/s. This value of K is
at least two orders of magnitude higher than the actual conductivity of the buffer. The margin for
cations, which have a higher diffusivity, is considerably greater. The conclusion can therefore be
made that the only important transport mechanism for both water and dissolved species throughthe buffer is diffusion. Diffusive transport is dealt with in Section 3.5.3.
The requirement to have a hydraulic conductivity that is so low that advection (flow) can be neglected
has led to a demand on the hydraulic conductivity of the buffer of
K < 10–12 m/s,
which is about 2 orders of magnitude lower than theoretically required for the mass transport process
to be diffusion-dominated. Since this demand can only be fulfilled if all initially-present slots or gaps
are healed and there is a perfect contact between the buffer and the rock, a requirement for a minimumswelling pressure, σs, in the system also exists and is:σs > 1 MPa
This value is chosen since the hydraulic conductivity is clearly lower than 10–12 m/s at that swelling pressure. 1 MPa is also high enough to efficiently fill irregularities on the rock surface in a deposition hole.
Dependencies between process and buffer variables

Table 3-5 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Influence of density, temperature and salinity
The hydraulic conductivity is primarily dependent on the geometry and composition of the buffer,the density (expressed in Table 3-5 as water content or pore geometry), the ion concentration in theporewater and the temperature.
In the buffer, K = 5·10–14–5·10–13 m/s at the densities in question both for MX-80 and Deponit CaN
bentonite. The correlation with density is strong. Figure 3-5 and Figure 3-6 show the hydraulicconductivity of MX-80 and Deponit CaN bentonites measured at different densities and withdifferent ion concentrations in the saturating solutions /­Karnland et al. 2006/. Blue markers indicate
pure water conditions, and these results consequently show the effect of the original clay mineralogy
including original cations. The other markers show the results with chloride solutions of the dominant cation in the two clays, respectively. Additional tests show that ion exchange of MX-80 clay
to a calcium end member leads to a hydraulic conductivity similar to those in the original Deponit
CaN material, and ion exchange of Deponit CaN clay to the sodium end member leads to hydraulicconductivities similar to those in the original MX-80 clay.
Transformations to other minerals with reduced montmorillonite content generally lead to a higher
hydraulic conductivity. Temperature influences the conductivity by changing the viscosity of thewater, whereby a temperature increase from 20°C to 90°C entails an approximate increase in conductivity by a factor of 3. The influence of salinity on the hydraulic conductivity is not pronounced
at buffer densities (> 1,900 kg/m3). Results from measurements by /­Karnland et al. 2006/ are shownin Figure 3-5 and Figure 3-6.
Influence of stress state
Water is transported into or out from a piece of buffer material when it expands or is compressed.
This process is coupled to the mechanical process of swelling and compression, which is treated in
Section 3.4.1. The hydraulic part of this process determines the rate of volume change by the time
it takes for the water to flow in or out.

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67

Table 3-5. Direct dependencies between the process “Water transport under saturated conditions” andthe defined buffer variables and a short note on the handling in SR-Site.
Variable

Variable influence on process

Process influence on variable

Influence present?
(Yes/No)
Description

Handling of influence(How/Why not)Influence present?
(Yes/No)
Description

Handling of influence(How/Why not)Buffer geometry

Yes, mainly by
the flow path

Given as input data. The
effect of the geometry,
meaning how the buffer
expands into othervolumes (the backfill) is
described in
Section 3.4.1

Yes, during swellingsince water flowing
into the voids makes
the buffer swell if
there is space
available forswelling

Included in the mechanicalmodel

Pore geometry

Yes, determines thehydraulic conductivity(see text)

Included in the value ofthe hydraulic conductivity
as a function of thedensity. The poregeometry can be affectedby mineral transformation
or a change in porewatercomposition. The effects
are discussed in Sections
3.5.9 and 3.5.8

Yes, during swelling Included in the mechanicalsince water flow into model
the voids makes thebuffer swell if there isspace available

Radiation intensity

No

Temperature

Yes, affects waterviscosity. The increase of
hydraulic conductivity as
a function of temperature
is about a factor of 3
between 20°C and 90°C

Water content

Indirectly throughdensity (pore geometry)Gas content

Insignificantly(dissolved gases)

No
This is neglected in
SR-Site because thethermal pulse will be overbefore any significant
radionuclide release may
have entered the buffer

Insignificantly. The
heat transport due
to water transport isvery limitedNo

Yes, during swelling Included in the mechanicalsince water flow into model
the voids makes thebuffer swell if there isspace available
Excluded since itwould reduce hydraulicconductivity

No

Included in the flow
model

Yes

Diffusion only for intact buffer.
If the hydraulic conductivity
once the process is taken into
account is below 10–12 m/s,
and the swelling pressure
above 1 MPa, advection will
be neglected. If the hydraulicconductivity and/or theswelling pressure do notmeet the assigned function
indicators, it has to be evaluated if advection could be ofimportance to the system

Stress state

Yes. If the effective stress Included in theon the buffer deviates
mechanical model.
from the swelling pressure
the bentonite will change
volume and water will flow
into or out from it

Yes

Included in the mechanicalmodel

Bentonitecomposition

Yes, determines thehydraulic conductivity(see text)

Included in the model

No

Montmorillonite
composition

Yes, determines thehydraulic conductivity(see text)

Included in the model

No

Porewater
composition

Yes, determines thehydraulic conductivity(see text)

Included in the model

Yes

Structural and straymaterials(Insignificant)Neglected, since no stray No
materials are expected

Hydrovariables
Yes, main variables(pressure and flows)

68

Replacement of porewater
is discussed in 3.5.6

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Hydraulic conductivity, m/s

10-8
10-9

WyR1

10-10

0.0 M

10-11

0.1 M
0.3 M

10-12

1.0 M

10-13

3.0 M

10-14
10-15
0

500

1000

1500

2000

Dry density, kg/m3

Figure 3-5. Hydraulic conductivity of MX-80(WyR1) bentonite measured at different densities and molar
concentration of NaCl in the saturating solution /­Karnland et al. 2006/.

Hydraulic conductivity, m/s

10-8
10-9

MiR1

10-10

H2O

10-11

0.1 M

10-12

0.3 M

10-13

1.0 M
3.0 M

10-14
10-15
0

500

1000

1500

2000

Dry density, kg/m3

Figure 3-6. Hydraulic conductivity of Deponit CaN(MiR1) bentonite measured at different densities andmolar concentration of CaCl2 in the saturating solution solution /­Karnland et al. 2006/.

Boundary conditions

The outer boundary conditions are determined by the hydraulic properties of the backfill on top of
the deposition hole and the rock surrounding the deposition hole. In particular, the properties of possible fractures intersecting the deposition hole and of the excavation damaged zones that may exist
in the peripheries of both deposition holes and deposition tunnel floor are important.
Model studies/experimental studies

Flow in water-saturated buffer has been studied on the laboratory scale in the form of permeability
tests and swelling/compression tests in oedometers /­Börgesson et al. 1995, Karnland et al. 2006/.
Natural analogues/observations in nature

No studies of water flow in natural buffer-like formations have been undertaken for several reasons;the main being that the measured values from the laboratory are very low compared to the requirements
and that changes in hydraulic conductivity with time will be a consequence of chemical alterations
that are treated in other chapters. However, such studies could be considered because they may yieldvaluable information on long term behaviour.
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69

Time perspective

The hydraulic gradient that exists after water saturation and pore pressure equilibrium have beenreached around a repository is expected to be low. A simple estimate can be made of the time required
for all the water in the buffer to be exchanged. If the gradient is set to 0.01, the hydraulic conductivity
K to 10–13 m/s and the cross-sectional area to 10 m2, according to Darcy’s Law it takes 15 million years
for the approximately 5 m3 of water present in the buffer to be exchanged. Transport and exchange ofions via advection is thus a slow process compared to ion diffusion (see Sections 3.5.2 and 3.5.3).
The process also affects the rate of swelling into and out from a fracture in contact with the buffer
and the rate of homogenisation after full water saturation has been reached.
Handling in the safety assessment SR-Site

The primary handling of this process is to consider how the hydraulic conductivity could vary for the
evolving conditions in the buffer throughout the assessment period and to compare the values withthe function indicators.
Time periods: After saturation and throughout the assessment period.
Handling of variables influencing this process: The evolution of buffer density, temperature andionic composition is studied and the hydraulic conductivity and the swelling pressure evaluated forthe changing conditions caused by the chemical evolution and alterations of density:• The effect of the geometry, meaning how the buffer expands into other volumes (the backfill) is
described in Section 3.4.1.
• The pore geometry can be affected by mineral transformation or a change in porewater composition.
The effects are discussed in Sections 3.5.9 and 3.5.8.
• The increase of hydraulic conductivity as function of temperature is about a factor 3 between
20°C and 90°C. This is neglected in SR-Site because the thermal pulse will be over before any
significant release of radionuclides to the buffer.
• The hydrovariables are used to calculate the flow in the buffer• The amount and composition of the swelling component in the bentonite are evaluated according
to Section 3.5.9.
• The effect of a change in groundwater/porewater composition are evaluated according to
Section 3.5.8.
A combination of several factors will also be discussed.
The hydraulic conductivity is evaluated by application of the empirical relationships describedabove.
Handling of variables influenced by the process: If the hydraulic conductivity is below 10–12 m/s, andthe swelling pressure above 1 MPa, advection will be neglected.
If the hydraulic conductivity and/or the swelling pressure do not meet the assigned function indicators,
it has to be evaluated whether advection could be of importance to the system. In this situation, advective transport is included in the calculations of canister corrosion and radionuclide transport.
Boundary conditions: The hydraulic gradient for altered conditions during permafrost and glaciation
is estimated.
The special cases of failed canister and of earthquakes: These are handled via possible mass
redistributions, see Section 3.4.1.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The main references for results of the hydraulic conductivity measurements on the buffer material
and the influence of different factors noted above are /­Börgesson et al. 1995/ and /­Karnland et al.
2006/. /­Börgesson et al. 1995/ is a SKB Technical Report that has not undergone a documented70

TR-10-47

factual- and quality review. However, it is widely referred to by other scientific groups and can beconsidered peer reviewed. /­Karnland et al. 2006/ has undergone a documented factual- and quality
review.
Uncertainties

So far, few tests have been performed on Deponit CA-N bentonite, but the results available indicatethat the properties of this material are very similar to the properties of MX-80 at the densities considered
for the buffer material (see Figure 3-6).
Uncertainties in mechanistic understanding
An uncertainty exists as to whether Darcy’s Law applies at the low hydraulic gradients that are expected
to arise, since all laboratory tests are done at very high gradients. All measured deviations show, however,that the flow is lower than is expected by application of Darcy’s Law to results obtained at highhydraulic gradient (see e.g. /­Dixon et al. 1999, Hansbo 1960/).
Another uncertainty is the effect of transformations, which for some cases are expected to givehigher hydraulic conductivity. This is discussed in Section 3.5.9. The effects of very high salinities
are also inadequately understood, but this is not relevant for the repository conditions anticipated.
Model simplification uncertainties in SR-Site
The model used is empirical, connecting hydraulic conductivity to temperature, density and ionicstrength of the porewater. The uncertainties in this description are primarily given by the uncertainties
of the empirical relationships on which the model is based. The primary aim is to establish thattransport by diffusion is considerably more important than advective transport, meaning that theexact value of the hydraulic conductivity is a secondary concern.
Input data and data uncertainties in SR-Site
Input data for the study of hydraulic conductivity are primarily the empirical relationships between
hydraulic conductivity and density, temperature and porewater composition described above. The
primary aim is to establish that transport through diffusion is considerably more important thanadvective transport, meaning that the exact value of the hydraulic conductivity is a secondary concern.

3.3.3

Gas transport/dissolution

Overview/general description

Transport of gas in the buffer can occur in two phases of the repository’s evolution:• When the repository is sealed, air will be trapped in the buffer. As the buffer becomes saturated
with water, the air must escape.
• If a canister should be defective such that water could penetrate through the copper shell, the castiron insert is expected to corrode, resulting in hydrogen gas formation. If more hydrogen is produced
than can be dissolved in the water resident in the canister and surrounding buffer, a separate gas
phase will form.
Gas which is trapped in or by the buffer can escape by two principal mechanisms:• If the production rate is low or the gas quantity small, the gas can be dissolved in the porewaterand be removed by diffusion.
• If the production rate is higher or the gas quantity is larger than can be removed via dissolution
and diffusive processes, a gas phase will form, the pressure will rise, and a flow path is expected
to be formed through the buffer at a critical pressure.
Water saturation phase: Gas flux in conjunction with wetting of the buffer is described in Section 3.3.1.
Under these conditions, all gas originally present in the buffer is expected to be dissolved in the buffer’s
porewater and transported by diffusion.
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71

Gas transport from failed canister before saturation: Before the buffer is saturated, water is only
expected to be able to get into a damaged canister via vapour-phase diffusion. Hydrogen production
from corrosion will therefore be limited and the gas that is formed is expected to be able to leave thebuffer via diffusive transport. The gas transport resistance in an unsaturated buffer is also considerably lower than in a saturated one. Canister damages at this early stage are very unlikely and so thisprocess is considered to be very unlikely.
Gas from defective canister after saturation: After saturation, no gas is expected to be present in
the buffer unless there is a penetration defect in the canister and water can get into contact with thecast iron insert. The following section only describes the situation with a breached canister.
Diffusion: Hydrogen gas formed by corrosion of the cast iron insert can dissolve in the porewaterand migrate from the canister by diffusion. The maximum transport capacity for different conditions
has been estimated by /­Wikramaratna et al. 1993/.
The results of the calculations show that the diffusive transport capacity is considerably lower thanthe hydrogen gas production from corrosion, if it is assumed that the entire surface of the cast ironinsert is accessible for corrosion and the water supply does not limit corrosion. Under such conditions
it is probable that a gas phase will be formed inside the canister and that the gas must escape by
gas-phase flow.
Gas flow: The following picture of the subsequent course of events is an interpretation of the resultsof theoretical and experimental studies: At a critical pressure (entry pressure), the buffer is expected
to allow the gas to pass through. When the pressure reaches this value, a transport pathway is formed
through the buffer and gas is released. After the pathway is formed, two different evolutions are possible:
1) If gas production is maintained, the pressure will fall to a steady-state pressure. When the production
ceases the pressure falls, and if the gas production is low enough, the transport pathway is expected
to close. This takes place at a so-called “shut-in pressure”, which could be dependent on the swellingpressure. At pressures lower than the shut-in pressure, gas migrates solely by diffusion.
2) If the gas production rate is small compared to the permeability of the initial flow path(s), asgas pressure declines, the permeability of the pathways will decline. If gas production is unable
to produce sufficient gas to maintain an adequate supply then the aperture of the pathways will
decrease (and may well close), potentially resulting in an episodic type response.
Gas entry, breakthrough, peak and steady-state gas pressures in the buffer are dependent on theproperties of the buffer and the nature of the boundary conditions. The most important parameters
seem to be the swelling pressure, the mechanical properties of the buffer including the stress-strain
properties of the clay and host rock and the number, location and geometry of sinks (i.e. host rockfractures) which can accommodate the gas once it has moved through the clay.
Based on the experimental interpretation by /­Harrington and Horseman 2003/ for gas tests performedon volumetrically constrained bentonite samples, the peak gas pressure can be expressed as:
pg (peak) = T + 2σeff + pw

(3-10)

based on the ‘classic theory’ of hydrofracture in rock /­Haimson and Fairhurst 1967/. Applying thisprocess to the clay buffer, T is the tensile strength of the clay, σeff (see also Section 3.4.1) is theisotropic effective stress outside the region of stress concentration and pw is the porewater pressurein the clay.
In the post-peak region, the bentonite will have lost its tensile strength normal to the plane of thefracture, while a gas phase is present. The gas pressure is now equal to the total stress, σ. On thisbasis, the gas pressure at any point on the post-peak transient can be written as:
pg (post-peak) = σ = σeff + pw

(3-11)At the (near) steady-state condition, the gas permeability of the fracture network must remain moreor less constant with time, with no tendency for crack dilation or closure. Conceptually, the fracture
must be propped open at any point along its length by the local gas pressure.
When gas production ceases, gas pressure drops and then gradually falls away along a very protracted
pressure transient. The gas pressure and total stress are approximately equal at all points along thetransient.
72

TR-10-47

As gas pressure decreases, individual flow pathways should collapse leading to declining overall gas
permeability. Under constant volume conditions, the gas pressure cannot fall below the sum of thecapillary pressure, pc, and the externally-applied water pressure (backpressure), pwe, or:pg (shut-in) = pc + pwe(3-12)

where pc is the capillary pressure, which is equal to the swelling pressure. This provides a lower
bound value for the gas pressure at shut-in.
The energy that drives gas transport comes from compression of the gas inside the canister and is
proportional to the available gas volume. If it is assumed that the volume is a cubic metre and the gaspressure drops from 12 to 9 MPa, 30 MJ will be released when the transport pathway is formed. Ina system with confined clay (deposition holes), the gas will presumably be released in a controlled
fashion, i.e. it will not entrain the buffer material along with it, but experiments (performed in low
density clays) under unconfined conditions, show that gas breakthrough can be a violent process/­Donohew et al. 1998/.
Available experimental results show that gas can migrate through a highly-compacted buffer without
jeopardising the continued function of the engineered barriers. However, no experiment has been
conducted with a gas volume equivalent to the volume in the canister cavity (approximately 1 m3),and it is therefore not completely clear what effect the release of large quantities of gas will have on
the performance of the repository. However, this aspect of gas migration can be explored through theongoing Lasgit project /­Cuss et al. 2010/ which is a full-scale demonstration test.
Dependencies between process and buffer variables

Table 3-6 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
The gas transport resistance in an unsaturated buffer is considerably lower than in a saturated one.
Tests on unsaturated Avonlea bentonite have shown breakthrough pressure to be negligible below
a degree of saturation of around 80 to 90% /­Hume 1999/. This range marks the saturation at which
there are no continuous gas channels through the clay. In Kunigel VI bentonite, the critical saturation
for interconnected gas voids was found to be 72% /­Tanai et al. 1997/.
Boundary conditions

The essential boundary conditions for this process are the geometries of the conducting features atthe buffer interfaces to the canister, the backfill and the rock around the deposition hole, the pressureand volume of gas trapped inside the inner interface (the rate/evolution of these components will
also influence the gas migration process), and the material properties of the buffer and host rock.
The geometric aspects of these boundaries include the nature of the opening in a defective canister,
the number, location and geometry of fractures intersecting the deposition hole and the excavation
damaged zones around the deposition hole and in the floor of the deposition tunnel.
Once the gas has reached the excavation-disturbed zone (EDZ) in the near-field rock, the pressure
required for it to migrate further is much lower than in the buffer. This will result in an expansion of thegas volume and the displacement of porewater around the deposition hole and gallery opening. It must
exceed the sum of the water pressure and the capillary tensions in the fine fractures in the EDZ or inchannels in fractures that intersect the deposition holes, which together gives a pressure of 5–10 MPa.
Model studies/experimental studies

Gas dissolution: No specific studies have been conducted for the purpose of studying how gas
dissolves in the porewater in the bentonite. However, experience from water saturation tests showsthat highly-compacted bentonite normally achieves complete water-saturation and that no trappedgas remains. The mechanistic interpretation is that the suction of the bentonite compresses trappedgas (significantly reducing its volume), which is then dissolved in the porewater and this gas isultimately transported away by diffusion. Gas dissolution essentially follows Henri’s law.

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73

Table 3-6. Direct dependencies between the process “Gas transport/dissolution” and the definedbuffer variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Buffer geometry

The importance of thebuffer volume on gas
transport is currently
unknown

Assumed to be
unimportant

It is important todemonstrate thatthe gas does NOT
influence the buffer
geometry

Qualitative assessment

Pore geometry

Is assumed to
See stress state
determine breakthrough pressure –coupled to stress state

Fracturing from gas
transport

No effect on watertransport is assumed

Radiation intensity

No

No

Temperature

Gas pressure andbuffer properties
are temperaturedependent

Included in the THM
No
modelling of thesaturation.
For the case of abreached canister thiswill be neglected sincegas transport will takeplace after the thermal
periodWater content

The gas permeabilityof the bentonite isstrongly dependent
on the water content

This is considered in
The gas may
the calculation of thepotentiallysaturation process
desaturate the clay
See Section 3.3.1 This
will be neglected fora defective canister
since in that case gastransport will takeplace after thesaturation phase

Included in the modellingof saturation.
This is found to beof limited extent andcan be neglected forcorrosion gases. A casewhere contaminatedwater is pushed out fromthe canister is assessed
within the radionuclilde
transport calculations

Gas content

See water content

See water content

The gas may
potentiallydesaturate the clay

The de-saturation effectis assumed to be small
based on experimental
evidence. Diffusivetransport of hydrogenis considered

Hydrovariables(pressure and flows)

Gas pressure (gasgeneration rate) andhydrostatic pressure
determine how andwhen gas can betransported throughthe buffer

Gas pressure (gasgeneration rate)is included in thedescription of theprocess

The gas pressure
is a coupled component in the total
near-field pressure

Maximum pressure isestimated

Stress state

The swelling assumed
to determine breakthrough pressure ina saturated clay

The swelling pressure
is included in
the description of
the process inthe assessment

The gas pressure
is a component in
the total near-fieldpressure

Maximum pressure
and pressure evolution
estimated

Bentonitecomposition

Indirect influence
through stress stateNo

Montmorillonite
composition

Indirect influence
through stress state

No

Porewater
composition

Indirect influence
through stress state

No

Structural and straymaterials

No

No

74

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Gas transport: A number of gas migration experiments in compacted clays, with different materials,
geometries and boundary conditions, have been performed over the last 20 years. Several of theseare summarised in /­Harrington and Horseman 2003/.
/Harrington and Horseman 2003/ presents the most recent gas migration experiments at laboratory
scale within the SKB programme. In the test geometry, the specimen was volumetrically constrained,
preventing dilation of the clay in any direction. Figure 3-7 shows the actual test set up. All bentonite
specimens were fully hydrated before gas testing by back-pressuring with de-aired and distilledwater. This procedure gave initial degrees of saturation that were demonstrably close to 100%.
Figure 3-8 shows a gas injection history from /­Harrington and Horseman 2003/ for a laboratory-scale
experiment performed on compact bentonite. The test clearly shows the interaction between gas
pressure, total stress and porewater pressure during a gas injection history. It should be noted thatthe measured peak pressures in these constant volume test are substantially higher than the ones
measured in the earlier constant stress experiments.
There is strong evidence that gas flows through a network of pressure-induced pathways. Very little,
if any, displacement of water occurs during gas movement. The crack-like pathways are propped open
by the elevated gas pressure. When gas pressure falls, individual pathways can snap shut leading to a
reduction in overall gas permeability. What is absolutely clear from these gas injection experiments isthat it is possible to pass up to 60 litres (STP) of helium through specimens of buffer clay (at gas pressures of over 8.0 MP for times in excess of 5 months) without any measurable desaturation occurring.
This strongly reinforces the observations made by others on the lack of desaturation occurring during
gas flow through fully hydrated bentonite /­Pusch and Forsberg 1983/.
Viton ‘O’ ring

Bentonite

Stainless steel filters

Remote porewaterpressure sensor
Cap screw

Push-rod

End closure

Injectionassembly
Annular tube
Axial stresssensorPressure vessel

1

2

Radial sink arrays

Radial stress sensor

3

PT5

PT4

PT6
PT2

PT1

PT3

Sink configuration

Stress sensor locations

Figure 3-7. Cut-away diagram of the constant volume and radial flow (CVRF) gas migration apparatus/­Harrington and Horseman 2003/. [1], [2] and [3] indicates the radial sink arrays. Sensors are as follows:[PT1] – axial total stress on the backpressure end-closure, [PT2] – axial total stress on the injection endclosure, [PT3] – radial total stress close to the injection end-closure, [PT4] – porewater pressure close to
the injection end-closure, [PT5] – radial total stress at the mid-plane, and [PT6] – radial total stress close
to the backpressure end-closure.
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75

Sink array [1]Sink array [2]Sink array [3]Injection flow rate
Injection pressure

6.0 X 10-8

5.0 X 10-8

24
22

4.0 X 10-8

18
16

3.0 X 10-8
14
2.0 X 10-812

Gas pressure (MPa)

Flow rate (m3.s-1) at STP

20

10
1.0 X 10-8
8
0

6
135

145

155

165

175

185

195

Elapsed time (days)

Figure 3-8. Gas injection pressure versus time for the first breakthrough and shut-in stages of test Mx80-10,
showing axial and radial total stresses, porewater pressure and flow rate from different sink arrays/­Harrington and Horseman 2003/.

There is no evidence from these tests that the development of pressure-induced gas pathways in any
way compromises the sealing capacity of the bentonite barrier. Gas pathways are ephemeral featuresof the buffer that tend to close up when gas pressure falls. Breaks in slope of monitored pressuretransients can be interpreted as discrete pathway ‘sealing’ events that coincide with decreases in therate of discharge of gas to the sinks. More importantly, a history of gas movement through the buffer
has been shown to have no detectable effect on permeability to water.
A consortium of radioactive waste disposal agencies, the GAMBIT Club, has been supporting efforts
to develop an appropriate model of gas migration through bentonite, with the objectives that the model(a) should adequately represent the principal features observed in experiments of gas migration throughcompacted bentonite, (b) can be used to analyse and interpret experimental results, and (c) will provide
the basis of a model that can be used to assess pressure build up and escape of hydrogen gas throughbentonite around waste canisters in a repository /­Rodwell et al. 2003/. From that study a number ofconclusions can be drawn: A model in which gas migration in highly compacted bentonite occurs by
fissure propagation is able to account qualitatively for the behaviour seen, although heuristic arguments
are required to explain the dependence of the gas permeability on gas pressure after breakthrough,and the model is limited in the range of phenomena it portrays.
a) In order to explain the observed experimental results based on the assumption that gas displaces
water from capillary-like pathways, with the threshold for gas entry controlled by the capillary entry
pressure, requires that the capillary radii (or pore throats) be very small. This gives rise to the large
threshold entry pressures reported for highly compacted bentonite. However, a large number ofpathways must also then become gas filled to give rise to the observed gas permeability. While suchcircumstances may be possible, it is difficult to conceive how they would develop in practice.
b) The continuum model of gas pathway propagation with deformation of the clay describes a
range of observed phenomena, including pathway resealing and hysteresis through changes in
the water content of the clay, although the agreement obtained with experimental data is onlyof comparable quality to that obtained with the other models.
c) A model that extends a formulation analogous to that referred to in the previous paragraph (b), to
include a proper treatment of the interaction of stresses and strains in the clay with gas migration
gives comparable agreement in simulations of the same experiment. Work to simulate gas propagation in samples of different geometrical configurations is ongoing within the Äspö Task Force.
76

TR-10-47

Experimental data on the mechanism of gas migration in bentonite that allows confirmation of whichof the preceding choices of modelling approach is/are most appropriate is currently lacking.
A summary of a major part of the experimental evidence around gas transport in bentonite buffers
can be found in /­Rodwell 2005/. Results from experiments and modelling from several groups arepresented and some conclusions are drawn:• A threshold pressure for gas entry is observed in many experiments (exception: long term
Canadian experiments). The gas entry pressure could be related to the swelling pressure.
• Only small volumes of water (if any) are displaced, though a case can be made for some degreeof dewatering by internal consolidation of the clay at elevated gas pressures.
• After breakthrough, gas flows at pressures below the threshold, but flow ceases at pressures
above back pressure.
• Porewater pressure changes in response to applied gas pressure.
• Evidence for macroscopic fracturing in radial flow experiments. The lab results also indicate thatgas migration processes are sensitive to the nature of the boundary conditions (i.e. test geometry).
The reasons for this remain unclear.
Although there remains uncertainty in the mechanisms of gas migration in bentonite, it is believedthat the research into the subject undertaken in a number of laboratories has satisfactorily addressed
the key issues with relevance to the effect of gas generation on the safety performance of bentonite
buffers in radioactive waste repositories. However, while significant improvements in our understandingof the mechanisms governing gas migration in buffer bentonite have taken place, the paucity of laboratory data available for further model development and calibration (both conceptual and numerical)
is a major issue in relation to the quantitative treatment of gas in performance assessment. As an aid toprocess understanding and model development, additional laboratory tests are required. Uncertainty
associated with up-scaling is addressed through the Lasgit international collaborative project.
In the Lasgit, two series of gas injection tests have so far been performed. The first is presented in/­Cuss et al. 2010/. In the test a sharp rise in flow rate occurs when the gas pressure is marginallygreater (approximately 0.2 MPa) than the local total stress measured on the rock wall, but is around0.25 MPa lower than the radial stress monitored on the canister surface. The measured axial stress
was also marginally higher than the gas pressure (around 0.3 MPa). Gas pressure continues toincrease reaching a peak pressure of 5.66 MPa at day 972.3. This is followed by a small spontaneous
negative transient leading to a quasi steady state illustrated in detail in Figure 3-9 at a gas pressure ofaround 5.5 MPa. This behaviour is qualitatively similar to results from laboratory scale tests reportedby /­Harrington and Horseman 2003/. However, the peak pressure in the field scale test is much lower
than in the laboratory tests.
To examine the effect, if any, on the hydraulic properties of the buffer following gas injection, hydraulic tests were performed before and after the gas injection test. Figure 3-10 shows the evolution inflow rate from both hydraulic tests (pre- and post gas injection) plotted against the duration of eachtest. The tests clearly indicate that little, if any, significant change in permeability has occurredbecause of the injection of gas. The slight offset in the red line is indicative of a small change inhydraulic storage. Based on the data available, the nascent gas pathways would appear to have nosignificant effect on the hydraulic performance of the buffer.
Natural analogues/observations in nature/Tissot and Pelet 1971/ discuss oil and gas movements in clay-shale: “The extraction of oil or gas
from a finely structured clay matrix runs contrary to the capillary laws and is in principle impossible.
However, the barrier can be broken in one way. The pressure in the fluids that is formed in the poresin the clay increases when kerogen is formed. When this pressure exceeds the mechanical strengthof the clay, microchannels will form which are orders of magnitude larger than the natural pores andwill thereby allow an oil or gas phase to pass, until the pressure falls below a threshold value and thechannels fill again and a new cycle begins.” These observations are consistent with the preceding
discussions on mechanisms from the gas transport experiments in bentonite.

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77

5.0×10-8

5,700

Flow rate (m3/s) at STP

5,500
3.0×10-8

5,400
5,300

2.0×10-8

Flow into system
Flow into clay

5,200

Gas pressure (kPa)

5,600

4.0×10-8

Injection pressure

1.0×10-8

5,100
0
971.0

5,000
971.5

972.0

972.5

973.0

973.5

974.0

Elapsed time (days)

Figure 3-9. Unsmoothed flow rate and pressure responses around peak gas pressure. Gas flow into the clay
rapidly increases following the peak which is followed by a small spontaneous negative transient /­Cuss
et al. 2010/.

Figure 3-10. Evolution in flow rate for hydraulic tests performed before and after gas injection /­Cuss et al.
2010/.

In the context of gas migration in shallow marine sediments, /­Judd and Sim 1998/ go so far as todeclare “Preliminary modelling suggests that, at shallow subsea depths, gas migration will be initiated by fracture failure rather than capillary migration for all sediment types except coarse sands”.
/­Clayton and Hay 1992/ extend the debate to argillaceous rocks, suggesting that shaly cap-rocks
at current depths of burial less than 500 m are always breached by fracturing, the fracture network
providing the main route for leakage of liquid and gaseous hydrocarbons across the cap-rock.

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Time perspective

Gas transport can occur when water is in contact with the cast iron insert in a damaged or breached(corroded) canister. Unless damaged prior to installation or as the result of some other post-emplacement
mechanism, it is unlikely that gas will be generated during the first 10,000 years of the repository
lifetime.
Handling in the safety assessment SR-Site

Unsaturated conditions: The gas/air quantity in a deposition hole is approximately six percent of thewhole volume at atmospheric pressure. When the hydrostatic pressure has built up, this gas volume
will have declined by a factor of 50, at an assumed 5 MPa hydrostatic head. The fate of trapped gas
is treated in the modelling of the THM-evolution described in Section 3.3.1.
Saturated conditions: The bentonite buffer is an important barrier in the KBS-3 system. The key
purpose of the buffer is to serve as a diffusional barrier between the canister and the groundwater inthe rock. An important performance requirement on the buffer material is not to cause any harm to
the other barriers. Gas build-up from corrosion of the iron insert could potentially affect the buffer
performance in five ways:
1. Permanent pathways in the buffer could form at gas break-through. This could potentially lead toa loss of the diffusional barrier.
2. If the buffer does not let the gas through, the pressure could lead to mechanical damage of theother barriers. The main concern is damage to the near field rock.
3. The gas could de-water the buffer.
4. A gas phase could push water containing radionuclides through the buffer along gas-generated
pathways.
5. Some radionuclides may be transported in a gaseous phase.
The process of gas transport within the buffer is neglected except for deposition holes containing
a defective canister. For that case, results of model and experimental studies of gas build-up andtransport are considered in an integrated assessment of the internal evolution of a damaged canister,
including corrosion of the cast iron insert and water transport in the canister. The coupling between
gas, porewater pressure and stress within the deposition hole and near field are considered.
Model: No quantitative model describing the consequences of gas generation is available, hence theconsequences are estimated based on the experimental evidence mentioned above.
Boundary conditions: Gas is generated inside the canister and will be released to the rock throughthe buffer. Increased total pressure (and porewater pressure) may occur anywhere in the system.
Handling of variables influencing this process: The global rate of corrosion of the insert determines
if the hydrogen gas produced can escape by diffusion or if a separate gas phase will be formed.
Therefore, the corrosion rate primarily determines the rate of pressure increase. This pressure may betransferred to the near-field rock and the backfill. It is assumed that the buffer will open and let thegas through at a certain gas pressure. This maximum pressure is (probably) dependent on the swellingpressure of the buffer. However, while the swelling pressure (at constant volume) is a very importantparameter which should be approximately equal to the minimum pressure at which gas remains mobile
in the buffer, the maximum gas pressure is strongly associated with the ease at which gas can escapefrom the deposition hole, i.e. the number, location and geometry of available sinks. If the gas fails tofind a sink, the pressure continues to rise. It is also influenced by the hydrostatic pressure and therefore
the effect of a glacial overburden, leading to increased pressures, needs special consideration. The gas
pathway in the buffer will stay open as long as the gas production rate is sufficiently high.
The integrated assessment of the internal evolution of a damaged canister will be carried out in the
SR-Site Main Report.
After repository closure, the only direct flammability hazard to man that could arise is from therelease of hydrogen at the surface. This has been assessed and found to be extremely unlikely to present any hazard (the calculation was for release into a building /­Baker et al. 1997/). The flammability
hazard is not analysed in SR-Site.

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79

Adequacy of references supporting the suggested handling in the safetyassessment SR-site

The handling in SR-Site is based on the experimental results by /­Harrington and Horseman 2003/.
This is a SKB Technical Report which has not undergone a formal peer review. However, the reporthas been cited by a number of external organisations. The handling of the gas migration in SR-Siteis based only on empirical data from the experimental observations by /­Harrington and Horseman
2003/. The suggested model in the report, which is described above, is not used in the treatment of
the process.
Uncertainties

Uncertainties in mechanistic understanding
Gas transport: A remaining uncertainty in the understanding of gas transport in the buffer material
concerns the number, size and spatial arrangement of the gas-bearing features and the volume (stressstrain) behaviour of the clay during gas injection. As mentioned in the Experiments/Models section
above, the gas transport observed through bentonite can be interpreted in a number of different ways.
One critical uncertainty is the break-through pressure, i.e. the pressure when the buffer opens andlets the gas through. This determines the maximum pressure that can be created within the near fieldof the repository. Another uncertainty is the closure pressure, the pressure at which the pathways inthe bentonite close. A further uncertainly relates to the volume of water displaced during gas flow.
Potential de-watering of the clay may affect the engineering performance of the buffer.
These uncertainties are considered in the integrated assessment mentioned above.
Model simplification uncertainties in SR-Site
The evolution of the gas pressure and the gas transport in bentonite in SR-Site is still based entirely on
values obtained from experimental results. The complexity of the flow processes and the uncertainties
regarding the interpretation of the experiments make predictive modelling difficult at this stage(see above). The paucity of laboratory data available for further model development and calibration(both conceptual and numerical) is a major issue in relation to the quantitative treatment of gas inperformance assessment.
Input data and data uncertainties in SR-Site
The maximum gas pressure in the near-field is determined by the break-through pressure in the
bentonite. In some experiments, this pressure has been found to be above 20 MPa for a bentonite
with a swelling pressure of ~ 6 MPa. The high pressure may be an effect of the small experimental
specimen, or possibly of the rate with which the gas pressure was increased, but since This aspectof the gas migration process is explored through the Lasgit project and there the maximum pressure
was substantially lower /­Cuss et al. 2010/ Even with an early failure it would take a very long timeto generate a gas pressure. The effect of elevated temperatures on gas pressures can therefore beneglected.
The gas production (corrosion) rate, which determines the timescale of the gas evolution history, is
uncertain, as well as the effect of the rate of pressure increase on the process itself.
These data uncertainties are considered in the integrated assessment mentioned above.

3.3.4

Piping/Erosion

Overview/general description

Water inflow into the deposition hole will take place mainly through fractures and will contributeto wetting of the buffer. However, if the inflow is localised to fractures that carry more water than
the swelling bentonite can absorb, there will be a water pressure in the fracture acting on the buffer.
Since the swelling bentonite is initially a gel, which increases its density with time as the water goes
deeper into the bentonite; the gel may be too soft to stop the water inflow. The results may be piping

80

TR-10-47

in the bentonite, formation of a channel and a continuing water flow and erosion of soft bentonite
gel. There will be competition between the swelling rate of the bentonite and the flow and erosion
rate of the buffer.
Piping will take place and the pipes remain open if the following three conditions are fulfilled:
1. The water pressure pwf in the fracture, when water flow is prevented, must be higher than the sumof the counteracting confining pressure from the clay and the shear resistance of the clay.
2. The hydraulic conductivity of the clay must be so low that water flow into the clay is sufficiently
retarded to keep the water pressure at pwf .
3. There is a downstream location available for the flowing water and the removal of eroded materials
in order for the pipe to stay open.
Erosion will take place if the drag force on a clay particle from water movement is higher than thesum of the frictional and attractive forces between the particle and the clay structure.
Piping probably only occurs before complete water saturation and homogenisation since the swellingpressure of the buffer material is very high. Erosion can occur both as a consequence of channels
caused by piping and, over the long-term, at the interface between the clay and the fractures in
the rock. Since the water flow rate in the latter case is very low, erosion will only be important forcolloids leaving the clay gel that has penetrated into the fractures, see further Section 3.5.11.
The consequence of piping will be a channel and outflow of water to dry or unfilled parts of therepository. Since the clay swells the channel will reduce in size with time but, on the other hand, erosion will counteract and abrade bentonite particles and thus increase the size of the channel. There isthus a competition between swelling clay and eroding clay. If the inflow is low and the increase inwater pressure slow the pipe may seal before water pressure equilibrium has been reached.
After complete water saturation and homogenisation of the buffer and backfill and re-establishmentof the hydrostatic water pressure the water pressure will be separated from the swelling pressure
according to the effective stress theory. The pipes or openings caused by the erosion will thus be
sealed and a swelling pressure established if the density and resulting swelling pressure are highenough to overcome internal friction. Later on, there is very little risk of piping since piping requiresa strong and fast increase in water pressure gradient locally in the rock at the contact with the buffer
or backfill.
The status of knowledge in 2006 is summarised in a report /­Börgesson and Sandén 2006/ but
additional investigations were performed and reported in 2007 (see below).
Dependencies between process and buffer variables

Table 3-7 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Boundary conditions

The piping and erosion that are treated here mainly takes place in the pellet-filled gap between thebuffer material and the rock.
Model studies/experimental studies

The processes are studied in the following projects• KBS-3H1 project.
• Baclo project.
• Buffer installation project.
1
In the KBS-3H concept, canisters are deposited horizontally in long tunnels, rather than vertically inindividual deposition holes.

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81

Table 3-7. Direct dependencies between the process “Piping/erosion” and the defined buffer variables
and a short note on the handling in SR-Site.
Variable

Variable influence on process

Process influence on variable

Influence present?
(Yes/No)
Description

Handling of influence(How/Why not)Influence present?
(Yes/No)
Description

Handling of influence(How/Why not)Buffer geometry

Yes, the distance
between the bentonite
blocks and the rock surface strongly influences
the susceptibility to
piping, since it affects
the time to reach a highswelling pressure

The geometry of thebuffer is simplified –only a mass balanceis usedYes, through lost buffermaterial

Amount of lost buffer
is calculated

Pore geometry

Yes, by influence of void Included in the mass
size distribution in thebalance estimations
pellet gap and indirectly
through the stress stateNo, but indirect throughstress state

Radiation intensity

No

No

Temperature

No

Water content

Yes, a change in watercontent changes
the swelling pressure,which influences thepiping risk. It also
influences the hydraulicconductivity, which influences the swelling rate

Piping will only occur
before the buffer isfully saturated andhomogenised

Yes. Piping may
Amount of lost buffer
increase the saturation
is calculated
rate and thus affect thewater ratio and degreeof saturation since it
may distribute the waterin a more homogeneous
manner via a net ofpipes inside the pellets.
Erosion will change thefinal water content

Gas content

Yes, the degree ofsaturation and theporewater pressure in
the backfill influences
the risk of piping andthe erosion rate

Piping will only occur
before the buffer isfully saturated andhomogenisedNo

The hydraulic gradient,
the water flow and theduration are included in
the estimate of piping
and erosion

Yes

Hydrovariables (pres- Yes, basic variables
sure and flows)No

An “after piping”
hydraulic conductivity
based on the loss ofmass is estimatedStress state

Yes, determines if piping Included inoccursthe consequence
estimation

Yes

Bentonitecomposition

Yes, the bentonite
and montmorillonite
composition affects
important variables

Included in
the consequence
estimation madeindirectly throughstress stateNo

Montmorillonite
composition

Yes, the bentonite
and montmorillonite
composition affects
important variables

Included in
the consequence
estimation madeindirectly throughstress state

No

Porewater
composition

Yes, the salinity of
the water affects many
ivariables that governsusceptibility to piping
and erosion, i.e. theswelling pressure, theswelling rate (throughhydraulic conductivity)
and the erodability

Included inNo
the consequence
estimation since
the porewatercomposition will affectthe amount of eroded
buffer. However, aconservative upper limit
is used in SR-Site

Structural and straymaterials

No82

The pipes are assumed
to seal when thehydraulic gradients arerestored.

An “after piping”
swelling pressure
distribution is estimatedNo

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KBS-3H
KBS-3H is particularly vulnerable to erosion of buffer material since a channel can pass close to alarge number of canisters. Tests have been done in three different scales. The results show that the
bentonite is very sensitive to piping and erosion and that it may take a considerable time until it heals
if the inflow in one spot from a fracture is strong and the build-up of water pressure in the fracture isfast. See /­Börgesson et al. 2005/ and /­Sandén et al. 2010/.
The tests show that the bentonite cannot prevent piping and subsequent erosion unless the slot
between the bentonite blocks and the rock is very small (a few mm). The tests also show that theprocess that stops piping is clogging with valve affects rather than derived from swelling pressure.
The expected high water pressure in the fractures also tends to deform the bentonite blocks so muchthat new slots and channels will be formed. The conclusions from the KBS-3H tests and modelling
are that it is impossible to prevent piping without building a plug that can withstand the full waterpressure. The present design includes water tight end plugs in the deposition tunnels, which limits
the effect of erosion to acceptable levels.
Baclo
Tests have been done within the Baclo project in order to investigate the erosion properties of different bentonite materials that may be used both as backfilling blocks and as pellets for filling the slot
between the buffer or backfill blocks and the rock /­Sandén and Börgesson 2008/.
The results show that the erosion rate depends mainly on the composition of the material, the saltcontent in the eroding water and the total volume of eroded water, but not so much on the flow rate
and the length of the flow path. A reduction in eroding rate with time (or water volume) was also
observed. A preliminary model (Equation 3-13) that takes into account the decrease in erosion ratewith increasing total volume of eroding water has also been derived /­Sandén et al. 2008/.
ms = β × (mw)α 									(3-13)
where
ms

=

accumulated mass of eroded bentonite (g)mw =accumulated mass of eroding water (g)β=0.02–2

α

=

0.65.

The model is illustrated in Figure 3-11 together with a large number of measurements on different
materials under different conditions.
The model may be used for estimating the total amount of eroded buffer material in various situations. The model needs to be checked with additional tests, especially long time tests.
Buffer installation project
The behaviour of the buffer material during the installation phase is investigated in this project.
Examples of tests performed include scale tests simulating water inflow into a deposition hole justafter filling the slot between the buffer blocks and the rock with pellets. The mechanical effect on theblocks before and after backfilling above the deposition hole and the hydraulic processes occurring
when water seeps in from a rock fracture at different inflow rates have been studied. Results to dateshow that piping and subsequent erosion occurs as expected and that the material eroding out fromthe hole into the backfill follows Equation 3-13 with low values of the parameter b /­Sandén and
Börgesson 2010/.
The phenomenon has been observed in two field tests in Äspö HRL (LOT /­Karnland et al. 2000/
and in the full scale test Lasgit /­Cuss et al. 2010/). One of the deposition holes in the Prototype
Repository has a rather large water inflow and may be used as a check of one scenario for piping
erosion. However, this section of the test is not planned to be excavated for at least another 10 years.

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83

Black: Tube tests
Red: Large slot tests
Blue: Block tests

Accumulated eroded material, g

100000

α=1.0

10000

Model

α=0.65

1000

100

10

1

0.1
0.1

1

10

100

1000

Accumulated water flow, lFigure 3-11. Illustration of Equation 3-13 together with results from different erosion measurements on
various bentonite materials. α corresponds to the inclination of the straight line in the logarithmic diagram.
/­Sandén et al. 2008/.

Natural analogues/observations in nature

Piping and erosion may occur in natural deposits but has so far not been studied. The very special
circumstances with the high water pressure at repository depth make such observations doubtful.
Time perspective

The piping and erosion (excluding colloid erosion) takes place only before complete saturation andhomogenisation. The high swelling pressure of the buffer will ultimately seal the piping channels,
since the sum of the swelling pressure and the porewater pressure in the buffer will always be higher
than the external water pressure. The water inflow must though not be so strong that the buffer has
lost too much bentonite through erosion.
Handling in the safety assessment SR-Site

The consequences if piping and subsequent erosion may be severe since the process cannot be
stopped by the swelling bentonite in the deposition hole until either a large part of the buffer issaturated and piping is stopped by the swelling pressure or until the voids in the buffer and backfill
have been filled with water and the water pressure gradient is taken up by the plug in the deposition
tunnel (see the general description). The consequences of erosion are estimated for different inflow
scenarios. If the consequences are unacceptable, the deposition hole will be abandoned.
Model: The maximum consequences of this process are estimated by model calculations. This isbased on experience from the studies reported and the model according to Equation 3-13.
Time periods: The piping/erosion process is only relevant during the installation and saturation
period and is neglected for all other time periods.
Boundary conditions: The hydraulic boundary conditions at the buffer/deposition hole interface aredecisive for this process.

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Handling of variables influencing this process: The water flow and the water pressure together withthe rate of increase in water pressure are the key variables for this process. Erosion is also affectedby the geometry of the initial gaps in the system. The effect of the salinity of the groundwater is also
included in the description. All of these variables are included in the preliminary model.
Handling of variables influenced by the process: The consequence of extensive piping together witherosion would be a loss of buffer density, which is calculated for different cases.
The special cases of failed canister and of earthquakes: The process only occurs at very early stages
when these special cases are not relevant.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Equation 3-13 is used to estimate the effect of erosion of bentonite from a deposition hole. The basic
reference for this model is /­Sandén et al. 2008/ although this report refers to backfill material. Tests
more relevant for the conditions in a deposition hole are ongoing and will be reported and evaluatedin the same way. /­Sandén and Börgesson 2010/. This report and the modelling report /­Åkesson et al.
2010a/ will undergo a documented factual- and quality review.
Uncertainties

Uncertainties in mechanistic understanding
Piping, erosion and subsequent sealing is a complicated process with many components, muchdepending on the hydraulic behaviour of the rock. The uncertainties are considerable regarding both
the demands on the rock and the ability of the buffer to resist these processes. Furthermore, the timerequired for the buffer to seal such an inflow from a fracture is not known. Until a limiting inflow
that can be handled by the buffer without erosion of bentonite out from the deposition hole is found;the consequences of erosion must be considered.
Knowledge of when piping and erosion might occur and the consequences are not fully understoodat present. Further tests are ongoing and planned.
Model simplification uncertainties for the above handling in SR-Site
The model is based entirely on empirical data.
Input data and data uncertainties for the above handling in SR-Site
The constants used in the model are derived from a set of measured data. The spread of the measurements is large.
The boundary condition for this process is the inflow to the deposition hole, which is expected tovary widely.

3.4

Mechanical processes

3.4.1 Swelling/mass redistribution
The swelling process has been combined with other stress-strain-related processes that can cause
mass redistribution within the buffer, namely thermal expansion, creep and a number of interactions
with the canister, near field rock and the backfill.
Overview/general description

Water is absorbed by both unsaturated and saturated bentonite (that is able to physically expand)and causes swelling. If the bentonite is unable to expand freely, a swelling pressure develops, whichreaches its peak at full water saturation.

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85

In a deposition hole, the buffer is initially inhomogeneous due to the gaps between the buffer blocks
and the rock and canister surfaces. When water from the rock fills the outer pellet–filled slot andenters the bentonite blocks there will be swelling of the blocks and compression of the pellet fillingin the outer slot. At first the swelling will be pronounced because of the overall low bulk density ofthe pellet-filled slot. The resistance to compression is thus small relative to that of the buffer. Thismeans that the outer part of the blocks will swell to a lower density than the average density expected
after complete homogenisation. Ultimately, the water will be drawn so deeply into the blocks that theswelling pressure compresses both the pellet-filled gap and the swollen outer part of the blocks. With
time, saturation is achieved and the compression of the outer part and the expansion of the inner partwill come to some kind of equilibrium. This will not be a completely homogenous material due toinner friction in the bentonite and hysteresis effects. A small density gradient is expected to persist
with higher density close to the canister and lower close to the rock.
Swelling also leads to compression of the backfill above the deposition holes, a mechanical pressure
on the canister and rock surface of the deposition hole, and clay intrusion in fractures around thedeposition hole. Depending on the conditions in the near-field rock, the clay in the fractures could be
carried away by groundwater (under conditions of high local groundwater flow through the fracture(s)),
allowing new clay to penetrate. This could lead to gradual erosion of the buffer (see Section 3.5.11).
Besides mechanical effects, the buffer’s hydraulic conductivity and diffusion properties are also alteredby swelling.
Other phenomena that could lead to mass redistribution, expansion or contraction of the buffer
include creep, shear movements and convergence of the deposition hole, canister movements,pressure exerted by canister corrosion products and thermal expansion of the buffer porewater.
The following description illustrates the principles adopted for modelling of swelling from ageotechnical perspective. This approach does not address the underlying mechanisms that control
the behaviour.
The swelling can be conceived as being caused by a force of repulsion between the montmorillonite
layers. If there is a limited supply of water in a free specimen, the swelling is counteracted by anegative pressure in the porewater. If a specimen is water-saturated, i.e. all pores are filled withwater; the swelling is counteracted by the formation of a negative pressure in the porewater in thewater menisci on the surface of the specimen. The negative pore pressure is equal to the swellingpressure if no external pressure is applied. If the specimen is unsaturated, the water menisci develop
inside the specimen as well. The negative pressure in the porewater is chiefly a function of thewater ratio in the specimen, i.e. the quantity of water per unit weight of dry material. This negative
pressure is called suction potential (see also Section 2.3.1). When water is added to an unconfined
specimen, the water ratio increases and the repulsion forces and the suction potential decrease. This
causes the specimen to swell until a new equilibrium is established with a lower internal swellingpressure. If the volume is kept constant, a portion of the internal swelling pressure is insteadtransferred to an external swelling pressure, which can be measured. When a specimen with constantvolume is completely water-saturated and the porewater pressure is kept positive, the entire swellingpressure becomes an external pressure. At water saturation, the swelling pressure and the porewaterpressure are independent quantities and give a total pressure that is the sum of the pressures (effective stress theory) according to Equation 3-14.
σ = σ’+Au					(3-14)
where
σ =

total stressσ’ =effective stress = swelling pressureu =porewater pressureA =1.0 for normal soils.

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Dependencies between process and buffer variables

Table 3-8 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
The data and discussion of influences in the following text are, in principle, valid for MX-80 but
the mechanical properties of Deponit CA-N bentonite is similar to the properties of MX-80, whichmeans that they can also be applied to Deponit CA-N bentonite.
Table 3-8. Direct dependencies between the process “Swelling/mass redistribution” and thedefined buffer variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Buffer geometryYes. Any phenomenon
leading to alterationsof the buffer bulk
geometry influencesswelling

The initial positionof the buffer is thestarting point of theevaluation

Yes

The buffer geometry
after swelling is includedin the model

Pore geometryYes. Through void
ratio and indirectly
through stress state

Void ratio is includedin the modelYes. Through void
ratio and indirectly
through stress state

Void ratio is included in
the model

Radiation intensity

No

Temperature

Yes, by thermal
expansion andindirectly via thehydro-variables

Excluded fromassessment (seebelow)(Insignificantly)

Water content

Yes, decisive variable

Included in THM
modelling of
unsaturated buffer

Yes

Included in THM
modelling of
unsaturated buffer

Gas content

Yes, degree ofsaturation is a model
variable

Included in THM
modelling of
unsaturated buffer

Yes, degree ofsaturation is a model
variable

Included in THM
modelling of
unsaturated buffer

Hydrovariables
Yes, model variables(pressure and flows)

Included in THM
modelling of
unsaturated buffer

Yes, model variables

Included in THM
modelling of
unsaturated buffer

Stress state

Yes, model variable

Included in THM
Yes, model variable
modelling of
unsaturated buffer
and included in
calculation of themechanical interaction
with the canister aswell as backfill

Bentonitecomposition

Yes, influences the
suction potential andthe swelling pressure

The initial composition No
of the buffer isthe starting point of
the evaluation

Montmorillonite
composition

Yes, influences the
suction potential andthe swelling pressure

The initial composition No
of the buffer isthe starting point of
the evaluation

Porewater
composition

Yes, influences the
suction potential andthe swelling pressure

Included in the model
via the suction potential and the swellingpressure etc

Structural and straymaterials

UnclearNot considered withNo
the SR-Site assessment, since no straymaterials are expectedin the buffer

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No

Included in THM
modelling of
unsaturated bufferIs
and included in
calculation of
the mechanicalinteraction with backfillNo87

Measured swelling pressure at constant volume after water saturation is shown in Figure 3-12 andFigure 3-13 for both MX-80 and Deponit CA-N bentonites /­Karnland et al. 2006/. The influence ofdensity and ion concentration in the saturating water is measured and included in the figures. Similar
investigations have been undertaken by /­Börgesson et al. 1995/.
102

Swelling Pressure, MPa

MX-80
101

0.0 M
0.1 M
1

0.3 M
1.0 M
3.0 M

10-1

0.3 M

10-2
0

500

1000

1500

2000

Dry density, kg/m3

Figure 3-12. Swelling pressure of MX-80 bentonite measured at different densities and molar concentrationof NaCl in the saturating solution. All tests have been performed with increasing salt concentration except
for the tests denoted +, which have been performed after reduction in salt concentration from 3 molar/­Karnland et al. 2006/.

102

Swelling Pressure, MPa

Deponit CA-N
101

0.0 M
0.1 M
0.3 M

1

1.0 M
3.0 M

10-1

0.3 M

10-2
0

500

1000

1500

2000

Dry density, kg/m3

Figure 3-13. Swelling pressure of Deponit CA-N bentonite measured at different densities and molar
concentration of CaCl2 in the saturating solution. All tests have been performed with increasing salt concentration except for the tests denoted +, which have been performed after reduction in salt concentrationfrom 3 molar /­Karnland et al. 2006/.

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Influence of density: The density at water saturation is decisive for the water ratio, which determines
the swelling pressure. Figure 3-12 and Figure 3-13 illustrate the difference in swelling pressure atdifferent densities and ion concentrations in the saturating water /­Karnland et al. 2006/.
Influence of montmorillonite content and species of adsorbed cation: According to Figure 3-12
and Figure 3-13 the swelling pressure is relatively independent if the adsorbed ion is sodium or
calcium, if the density at saturation exceeds approximately 1,800 kg/m3 and the montmorillonite
contents are the same. At lower densities the swelling pressure is lower if the adsorbed ion species is
Ca. Reduced montmorillonite content can be conceived of as being equivalent to the addition of an
inactive aggregate and the influence is best expressed with the effective montmorillonite dry density(dry mass of montmorillonite divided with the net volume (the total volume minus the volume takenby the non-montmorillonite particles)).
Influence of chemical composition of porewater: Increased ion concentration in the groundwater
generally leads to a reduced swelling and thereby to effects on the pore geometry that are similar to
those of ion exchange to divalent ions. The increase in ion concentration further leads to a reductionof the swelling pressure due to osmotic effects, and to changes in the pore geometry that affect thehydraulic conductivity of the material at low saturated densities (< 1,800 kg/m3) (see Section 3.5.8).
Influence of degree of saturation: In an unsaturated state, the bentonite/water system shrinks on
drying and expands on wetting. The volume change on water uptake in e.g. a block may differ between
the unsaturated and the saturated state, since the swelling in unsaturated state takes place partiallyin previously unfilled pores. Shrinking due to drying is expected to take place near the canister,which can lead to cracking of the buffer. After full saturation, the cracks will be healed and the buffer
homogenised in the same way as all initial slots and joints between blocks in a deposition hole.
The mechanical properties of unsaturated bentonites are complicated. Different models are usedin Abaqus and Code Bright. A new model that takes into account the thermo-mechanical interplay
between suction and swelling pressure has been suggested /­Dueck and Börgesson 2007/.
If volume is constrained during water uptake, which is the case for the buffer in a deposition hole,the expansion will be replaced by a swelling pressure that gradually increases as the degree ofsaturation increases. If there is a difference or gradient in degree of saturation, the pressure in a lesssaturated part can be higher than measured under ideally confined conditions due to transmission ofpressure from more saturated parts.
Influence of temperature: A change in the temperature of the buffer will cause water, particles andpore gas in the buffer to change volume, which may influence the mechanical properties of the buffer.
The influence of temperature on the mechanical properties of the buffer in the water-saturated stateis small /­Börgesson et al. 1995/. In the unsaturated state, this influence is also rather small /­Villar
2002/ and /­Dueck and Nilsson 2010/ and mainly concerns the retention curve. At temperatures below
zero the effect is very strong; see Section 3.2.2 on freezing.
The expansion of the particles and the pore gas is relatively small. The expansion of the water, which
is about 100 times greater than the expansion of the particles, completely dominates this process.
The influence of water expansion is completely dependent on whether the buffer is water-saturated
or not. Before water saturation, the volume of the water can expand with virtually no resistanceother than that offered by the compression properties of the pore gas. At constant volume this leads(to simplify slightly) to an increase in the degree of saturation when the temperature increases. The
consequences of this are that the negative pressure in the porewater (suction potential) decreases andthat the total pressure and the pore gas pressure increase. This causes the swelling pressure against an
external restraint to increase slightly. At constant pressure, a slight increase of the volume is obtainedin the same way. However, these changes are small and apparently without practical significance.
The increase in degree of saturation is about 2 percent at a temperature increase of 50 degrees. Thereis also an additional influence of temperature on the retention curve with similar consequences.

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The consequences can be more serious in water-saturated buffer, since there are no gas-filled poresto accommodate the volume increases of the solid and liquid components. At constant volume, the porepressure increase is great since the water is unable to expand, its volume expansion is large and its
compressibility low. Theoretically, the resultant pressure increase against the surroundings can amountto several tens of MPa for the temperature changes in question. However, the pressure increase iscounteracted by drainage of the water through the rock and backfill. This causes a moderation in
the magnitude of the pore pressure increase that would otherwise occur in parallel with the temperature
increase. This process of excess pore pressure dissipation is relatively fast and will greatly reduce
the pore pressure increase if the temperature increase is slow and a drainage path is available.
Influence of high water pressure
The effective stress (Equation 3-14) has been shown to be valid for most soils at relatively low
stresses and water pressure. However, for bentonites such as MX-80, high water pressure seems toinvalidate this equation in the sense that A≠1. However, the deviation is not strong since the value of
A is between 0.9 and 1.0 depending on density and water pressure /­Harrington and Birchall 2007/.
Boundary conditions

The boundary conditions of interest mainly concern the interaction between the buffer’s interfaces
with the near field rock, the canister and the deposition tunnel backfill.
Interaction buffer/backfill
At the interface between the buffer and the backfill, the buffer exerts a swelling pressure against the
backfill and vice versa. Since the difference in swelling pressure may be great, a net pressure arises
against the backfill whereby the buffer swells and the backfill is compressed. In this process, theswelling pressure from the buffer decreases as the density decreases. At the same time, the counterpressure from the backfill increases as it is compressed and its density increases. The swelling of thebuffer and compression of the backfill are counteracted to some extent by friction against the rock.
When the force of the swelling pressure in the buffer is equal to the sum of the force of the counterpressure in the backfill and the friction against the rock, the process ceases since equilibrium has been
established.
The size of the swelling pressures generated depends on the original densities of the buffer andthe backfill, the type of backfill and associated expansion and compression properties.
Owing to friction against the rock and friction in the buffer, homogenisation of the buffer is notcomplete. When the buffer swells upwards due to compression of the backfill, the friction in the buffer
leads to an axial gradient in swelling pressure and density. The density increases with increasing
distance and ultimately reaches the average density at some distance from the interface.
Figure 3-14 illustrates the displacement of the interface between the buffer and backfill and thedisplacement of the canister as a result of a finite element calculation of the wetting of the buffer
for a rock with one fracture intersecting the centre of the deposition hole /­Börgesson and Hernelind
1999/. The results show that the buffer/backfill interface heaves about 8 cm. The upwards swellingalso affects the canister before full saturation of the buffer. The canisters at first heave 5–10 mm dueto the uneven wetting of the buffer but settle at completion of the wetting and ultimately reach theirinitial position. The results also show that after full saturation and complete consolidation the density
and swelling pressure are reduced above the canister but not along the sides of the canister.
Upwards swelling for different backfill types and backfill densities have been investigated withanalytical calculations /­Johannesson and Nilsson 2006/. They were made in order to find the required
density of different backfill materials in order not to lose too much in buffer density on top of thecanister (> 1,950 kg/m3). Additional modelling is ongoing and will be presented as a part of SR-Site/­Åkesson et al. 2010a/.

90TR-10-47

Figure 3-14. Heave (m) of the buffer/backfill interface (red) and the canister (green) as a function of time(s) elapsed from deposition. Example from a FE-calculation /­Börgesson and Hernelind 1999/.

Interaction buffer/near field rock
The following mechanical interactions between buffer and near-field rock have been identified.
1. Swelling pressure from the buffer is transferred to the rock but is not expected to lead to significant rock movements. The elastic deformation of the rock at the expected swelling pressures
is small due to the rock’s high modulus of elasticity. Plastic deformations caused by movements
along a fracture plane can occur along rock wedges that go up to the tunnel floor. The swellingpressure also reduces the ongoing rock stress release (from the rock excavation) and the extension of the damaged rock zone adjacent to the deposition hole (see the geosphere process report).
2. Pressure from the canister can be propagated via the buffer. Such pressures can arise fromcanister corrosion products and gas pressure build-up.
3. Forces of friction arise against the rock walls in the deposition hole due to swelling of the buffer
against the backfill.
4. Thermal expansion of the buffer can be serious for the rock if the buffer is water-saturated
when the temperature increase occurs. This process is described above. A slow temperature
increase in combination with drainage through the rock implies that this is not a problem but itshould be checked by calculation. If the rock is very tight, drainage could be hindered, but such arock will not provide enough water to yield saturation before the peak of the thermal pulse.
5. Convergence of the deposition hole occurs when the rock creeps due to the high rock stresses
after rock excavation. Assuming an anisotropic primary state of stress with reasonable components, calculations have shown that the diameter can decrease by approximately a centimetre
after 10,000 years /­Pusch 1995/. The resulting density increase in the buffer leads to an increasein the swelling pressure by roughly a few MPa. Recent conservative estimates of creep-induced
deformations of deposition tunnels have given convergence values of about 1% /­Glamheden et al.
2004/. For a deposition hole this would give between 1 or 2 cm of diameter reduction. The recent
estimates are based upon the notion that creep will relax fracture shear stresses over time until
a low threshold stress has been reached. The effect can be even greater at higher rock stresses,
caused by e.g. glaciation.
6. Shear movements due to sliding along a fracture in the rock is a process that can be serious
for the canister if the displacement is rapid and large. The process is partially dealt with below(Interaction buffer/canister). Rock movements are dealt with in the process descriptions for thegeosphere.
7. Intrusion of bentonite into fractures in the rock is very limited due to the small aperture of thefractures and the shear resistance caused by the friction between the bentonite and the fracturesurface. If there is no loss in bentonite due to erosion the effect is positive since it implies asealing of the fracture. Problems may occur if the gel that forms at the outer part of the penetrated
bentonite is eroded and carried away by groundwater.
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The depth of intrusion is a function of the fracture width, the friction between the bentonite and therock surface and the swelling pressure in the deposition hole. The penetration depth, z, into an ideal
fracture can be estimated according to the Equation 3-15 (derived from simple integration of force
equilibrium):z=δ
2 tan φ(ln σ 0 − ln σ z )								(3-15)

where
δ

=

fracture widthφ=friction angle (10–30 degrees)

σ0=swelling pressure on the rock (7–13 MPa)σz=swelling pressure in the fracture at the distance z from the rock surface.

The penetration depth will thus approach infinity when the swelling pressure at the gel limit
approaches zero. If a swelling pressure of 1 kPa is set as a limit, the penetration from a buffer withswelling pressure 10 MPa into a fracture with width 0.1 mm will be 1.3 mm if a friction angle of 20
degrees is assumed (an average). If a limiting pressure of 1 Pa is set, the penetration will be 2.2 mm.
According to Equation 3-15, the penetration depth increases by about 20% for a 10 fold decrease
in applied swelling pressure. Since the minimum swelling pressure in a stable gel is 0.1–1 Pa, themaximum penetration for MX-80 can be estimated to be 2.2–2.6 mm. According to Equation 3-15,
the penetration depth is also proportional to the fracture width, which means that as a rule of thumb
the penetration is limited to about 25 times the fracture width.
At present, a large research project is running with the purpose of investigating the penetration andloss of bentonite to fractures in deposition holes. This project is expected to yield more information.
The process is also dealt with in Section 3.5.11.
The influence of the buffer on the rock is best investigated experimentally by measuring the swellingpressure on the rock at full scale. This has been done in BMT in Stripa /­Pusch et al. 1985/ and inseveral of the full-scale tests in Äspö HRL (see e.g. /­Goudarzi et al. 2003b/).
Calculations of the effect on the rock of corrosion of the cast iron insert show that the influence issmall /­Bond et al. 1997/. The effect on the rock of the buffer’s upward swelling in the deposition
hole has been modelled /­Pusch and Börgesson 1992/. These studies show that a rock wedge can, if
the backfill is poorly compacted, move upward 0.2–0.3 m and reduce the density of the buffer inthe upper part of the deposition hole. This scenario presumes that a triangular wedge reaches fromthe floor in the tunnel more than a metre down to the buffer in the deposition hole. It also presumes
that the wedge is attached to the rock during installation of the buffer and backfill, but detachedeither during compaction of the backfill or due to thermal stresses (or similar) after deposition. The
swelling pressure and the axial upwards swelling of the buffer can then move the wedge upwards
and create an increased volume of the deposition hole. This combination of events has thus been
considered but is very unlikely.
The influence of the rock on the buffer in connection with shear movements along fracture planes
has been studied in both experimental and model studies. This process is not harmful for the buffer(if its rheological properties have not been changed) but only for the canister (see below).
Processes 1, 3, 4 and 7 listed above are most likely to occur during the first 100 years after closure of
the repository. The other processes occur or can occur throughout the lifetime of the repository.
Uncertainties concerning processes 1–4 and 6 stem mainly from uncertainties in other processes andare dealt with elsewhere (canister corrosion, gas formation, canister/buffer interaction, etc).
Convergence of the deposition holes is normally negligible.
The penetration of buffer into fractures is of no importance for the buffer unless followed by erosion.
This process is treated in Section 3.5.11.

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Interaction buffer/canister
Mechanical interactions between the buffer and the canister arise from the buffer through the clay
matrix. Both compressive stresses and shear stresses are generated in the clay. The porewater and gas
within the buffer will generate only compressive stresses. Changes in these three factors take place
during the water saturation process and can also occur in response to external forces. The weight ofthe canister influences the buffer, whereas the influence of the weight of the buffer on the canister isnegligible.
The mechanical interaction between buffer and canister is summarised in the following 7 processes:
1. Swelling pressure and water pressure exerted on the canister.
The magnitude of the swelling pressure is described above. During the saturation process, theswelling pressure may be uneven mainly due to uneven wetting, but after full saturation the swellingpressure is rather homogeneous. The water pressure is added to the swelling pressure to give thetotal pressure the surface of an intact canister will experience and is also uniform with the expectedmagnitude given by the depth of the repository. During glaciation, the water pressure may besubstantially higher.
Recent tests have revealed that a high water pressure can affect the validity of the effective stresstheory in the sense that the hydraulic pressure cannot be added to the swelling pressure. The sum
is instead slightly lower (only about 90% of the porewater pressure is added). This small deviationfrom the effective stress theory is favourable for the canister.
Another finding is that a highly compacted bentonite sample that has been exposed to very highwater pressure (~30 MPa) does not resume its original swelling pressure after removal of the waterpressure /­Harrington and Birchall 2007/. This process and its consequences are being investigated.
2. Thermally induced porewater pressure exerted on the canister.
This process is described above. The process can be detrimental for the canister if the following two
criteria are fulfilled:
1. The buffer is water-saturated during the period of increasing temperature
2. The rate at which water volume decreases in response to the pressure reduction during drainage
of water through the buffer to the rock and backfill is slower than the water volume increasecaused by thermal expansion of the water during the period of increasing temperature.
Preliminary calculations have shown that the latter criterion is not probable /­Pusch and Börgesson
1992/. The first criterion above may be fulfilled but is not enough on its own to cause high porepressure and requires the second in order to become an issue.
3. Trapped gas in the buffer exerts pressure on the canister.
Gas generation is treated in Section 3.3.3. High gas pressures are only expected as a possible
consequence of corrosion of the insert of a failed canister.
4. Canister corrosion products exert pressure on the canister.
External corrosion of the copper canister and corrosion of the steel insert in the case of a defective
copper canister will increase the volume of the canister since the corrosion products have lower
density than the original material. This material expansion will result in compression and consolidation of the buffer and thereby increase the pressure from the buffer on the canister. This process is
discussed further in the Canister Process Report /­SKB 2010k/.
5. Vertical movement of canister in buffer.
The average density of the canister is much higher than that of the buffer, which means that there
is a net vertical stress on the bottom part of the buffer in the deposition hole. This stress difference
results in a density that is slightly higher below the canister than in the other parts of the buffer. Thismeans that there is a net vertical sinking of the canister in relation to the original location. On theother hand, there is an upward swelling of the buffer above the canister (due to the compression of
the backfill; see above), resulting in a decreased density and decreased swelling pressure above thecanister. This may lead to a swelling pressure below the canister that is higher than the sum of theweight of the canister divided by the cross sectional area and the swelling pressure on the top of thecanister. Since the upward-directed force on the canister will therefore be higher than the downwarddirected force, the canister will heave instead of sink.
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There is thus a complicated combination of pressure (and forces) on the canister. The result canbe calculated and leads to equilibrium after complete water saturation and porewater pressurestabilisation, which includes stress differences and shear stresses in the buffer. This equilibrium istypically established within 100 years after closure of the repository whereupon creep phenomena
will dominate the displacements in the buffer.
There are three types of time-dependent displacements of the buffer resulting in movements of thecanister. One is related to what in soil mechanics is named “primary consolidation”, the second one
to so called “secondary consolidation” (volumetric creep) and the third to time-dependent shear
deformations (deviatoric creep).
During primary consolidation after loading, the porewater pressure is the controlling variable andthe time perspective is determined by the hydraulic conductivity and the compression properties.
Before equilibrium in effective stresses in the clay skeleton is established, the porewater carries thetotal pressure difference. With time, as the porewater pressure difference is reduced due to outflow
of water, the pressure induced by the canister mass is transferred from the porewater to the clay
skeleton and the clay is thus deformed. This process dominates as long as there are porewater pressure differences in the buffer.
After pore pressure equilibrium, the creep processes dominate. Volumetric creep is caused byinternal microstructural reorganisation of the clay particles and is related to the change in swellingpressure with time. This creep is not particularly large in buffer and backfill materials and notproblematic as far as vertical movements of the canister are concerned, since it will not remove any
bentonite but (in the case of constant stress) only compress it or (in the case of constant volume)
relax the stresses.
The deviatoric creep is potentially problematic since the existence of deviatory stresses (or shear
stresses) is dependent on internal friction or cohesion in the clay. These parameters are slightly
reduced with time, which causes a deviatoric creep and thus a movement of the canister. If the friction
and cohesion are reduced to zero, the clay would behave as a viscous liquid and there would be noresistance against the canister sinking to the floor of the deposition hole.
The deviatoric creep has been investigated with a number of creep tests at different stresses and acreep model has been derived /­Börgesson et al. 1995/. The model has been used in preliminary finite
element calculations. The calculations were actually made on the concept with two canisters in adeposition hole but the results are also applicable for the reference concept with one canister. Theyshow that the creep settlement after water saturation is a few mm in 300 years and less than a cm if
extrapolated to 100,000 years. Similar creep settlement has been found by use of a quite different
approach based on creep theory of stochastic mechanical origin /­Pusch and Adey 1999/.
The latest studies of deviatoric creep /­Börgesson and Hernelind 2006a/ has aimed at establishing a
critical buffer density, such that if the density exceeds the critical value, canister sinking can be ruledout (see below).
6. Transfer of shear stresses from rock via buffer to canister, especially during an earthquake
An earthquake may induce a shear displacement along a fracture that intersects a deposition hole.
The rock displacement will pressurise the bentonite in the deposition hole on the active side and cause
high shear stresses that lead to plasticisation in the area between the fracture and the canister. The
pressure on the active side will be transferred to the canister and make the canister partly tilt and bend.
The effect on the canister depends on the stiffness or shear strength of the bentonite in relation to
the stiffness of the canister. The shear strength of bentonite depends on the density and the rate of
deformation. This scenario has been thoroughly investigated recently and is described under “Model
studies” below.
7. Intrusion of buffer in a damaged canister
Bentonite may penetrate into a damaged canister. The influence on the buffer is thought to be small
due to the small void available inside the canister.

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8. Liquefaction of the buffer material
A process that could affect the canister significantly is liquefaction of the buffer. Liquefaction is aprocess implying that a stiff material (soil) turns into liquid due to an effect with short duration, seee.g. /­Lambe and Whitman 1969/. It may take place in a loose sand when the porewater pressure isincreased either due to a vibration that makes the sand particles float in the porewater (since theytend to go into a higher degree of compaction, but the water temporarily prevents it) or due to astrong upward water flow that releases the effective stresses between the particles (quicksand). It
may also take place in clay that has been settled in salty water (forming an open structure with a highwater ratio). If the salt is partly washed out by fresh water, the clay structure cannot hold the highamount of water at moulding or vibrations, meaning that the structure collapses when exposed tovibrations.
These two types of liquefaction cannot take place in a bentonite with high density, since the effective
stress that holds the clay together is high due to the swelling pressure. This conclusion was also
made by /­Pusch 2000/. However, a similar phenomenon has been observed during compaction of
bentonite blocks at very high water ratios. If the bentonite is compacted at a very high stress to astate where the bentonite is completely water saturated, all further increases in stress will be taken up
by the water and the bentonite will behave like a liquid. This phenomenon has been observed during
uniaxial compaction when liquid bentonite has squirted from the mould.
The process requires a very strong impact of pressure and is rather unlikely to occur in a deposition
hole. It can probably only result from an earthquake and requires a reduction in the volume of thedeposition hole due to an increase in rock stress. A combination of factors may lead to an increasein rock stress of about 15 MPa /­Bäckblom et al. 2004/. Estimation according to Kirsch /­Brady and
Brown 1994/ of the convergence of a deposition hole at such an increase in rock stress may yield aconvergence of the hole of about 1 mm.
Another phenomenon that could also lead to similar loss in effective stress would be the very highwater pressure that may be derived from the melt water after a glaciation. However, as shown by/­Harrington and Birchall 2007/ the decrease in effective stress is not high enough for the effective
stress to be completely lost.
The consequences of liquefaction and subsequent complete loss in effective stress could be a settlement of the canister since the buffer would not have any shear resistance. For the reasons given, thisprocess has been ruled out in the case of the buffer.
Model studies/experimental studies

Saturated bentonite
The swelling capacity of bentonite and swelling pressure under water-saturated conditions has beenstudied in detail by means of laboratory experiments /­Börgesson et al. 1995/. The results have beenused to devise a material model that can be used for FEM calculations /­Börgesson et al. 1995/. It isbased on the effective stress theory and coincides largely with models for other soils, although themagnitude of the swelling and pressure are considerably greater for bentonite.
The material models have been used to calculate different evolutionary scenarios that are associatedwith volume changes in a repository. Examples of the modelling exercises include the following:• Swelling of buffer against the backfill in deposition holes /­Pusch and Börgesson 1992/. A calculation of an extreme case was performed for PASS /­Pusch and Börgesson 1992/ where the buffer
was assumed to have a density of 2,100 kg/m3 after water saturation and the density of the backfill
was assumed to be 1,900 kg/m3 after water saturation. These values correspond to higher density
in the buffer and lower in the backfill than is the case in SR-Site, (Section 2.2.1 and 2.3.1). Theygave an upward displacement of about 30 cm.
• Thermo-hydro-mechanical calculations of the water saturation phase in a deposition hole withdifferent hydraulic rock properties, done for SR 97, included the displacement of the buffer/
backfill interface and movements of the canister /­Börgesson and Hernelind 1999/. An averagedensity of the buffer after saturation of 2,000 kg/m3 and an E-modulus of the backfill of 30 MPa
resulted in a heave of the backfill interface of 7–8 cm and a temporary heave of the canister ofabout 1 mm.
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• Settlement experiment in Stripa. Comparisons of calculated and measured movements and densities when canisters are loaded in deposition holes.
• Modelling of canister movements for a design with two canisters in a deposition hole. In
conjunction with studies of the function of deposition holes with two canisters, a calculation wasmade of the movements of the canisters which included upswelling towards the backfill. In thiscalculation the buffer was assumed to have a high density (2,100 kg/m3 at saturation) and thebackfill was assumed to be rather stiff, with a density after water saturation of 2,200 kg/m3. The
calculated upswelling of the buffer was about 7 cm and the total heave of the canister (including
both consolidation and creep) after 32,000 years was 3 mm.
• For a rock shear, the influence of the rock density and the rate of rock displacement has been
investigated for very fast shearing /­Börgesson et al. 2003/. The influence is illustrated with thefollowing examples: At a density of 2,000 kg/m3 at water saturation, the shear strength is 1 MPa
at a shear rate of 5·10–8 m/s (4 mm/day), whereas the shear strength is 3 times higher (3 MPa) at ashear rate of 1 m/s. At a density of 2,100 kg/m3 at water saturation, the shear strength is 2.6 MPa ata shear rate of 5·10–8 m/s (4 mm/day). The shear strength is also about 3 times higher (7.6 MPa)at this density at a shear rate of 1 m/s.
• A material model has been established to calculate the effect of rock shear on the canister andbuffer. The influence of density, shear rate, magnitude of shear displacement and location of theshear plane has been investigated with finite element modelling /­Börgesson et al. 2003/. The
results show that there may be strong plastication both of the copper canister and the steel insert
but that the plastic strain is limited to a few percent for the reference case (density 2,000 kg/m3,
shear rate 0.1 m/s, shear displacement 10 cm).
The maximum plastic strain in the steel insert, after a shear displacement of 10 cm at a density
2,100 kg/m3, is about 9%, which does not yield rupture of the steel but should anyhow be considered an upper limit. The buffer material should thus not have a density higher than 2,100 kg/m3 inorder to avoid damage to the canister in case of an earthquake.
Recently a number of additional calculations have been made in order to study further the influenceof mechanical disturbances on the buffer and canister.
Canister settlement
The vertical displacement of the canister in the KBS-3V concept has been studied in a numberof consolidation and creep calculations using the FE-program Abaqus /­Börgesson and Hernelind
2006a/. The creep model used for the calculations is based on Singh-Mitchell’s creep theory, which
has been adapted to and verified for the buffer material in earlier tests. A porous elastic model with
Drucker-Prager plasticity has been used for the consolidation calculations. For simplicity the buffer
has been assumed to be water saturated from start.
In one set of calculations, the consolidation and creep in the buffer was studied without considering
the interaction with the backfill. In the other set of calculations, the interaction with the backfill was
included for a backfill with the same properties as an in situ compacted mixture of 30% bentonite
and 70% crushed rock. The motivation for studying the behaviour of the buffer alone was that the
final choice of backfill material and backfilling technique had not yet been made so that the set of
calculations simulates a backfill that has identical properties to the buffer. The cases thus represent
two extremes, one with a backfill that has the lowest allowable stiffness and one that has the highest
possible stiffness.
The base cases in the calculations correspond to the expected swelling pressure of 7 MPa in a buffer
with the final average density at saturation of 2,000 kg/m3. In order to study the sensitivity of thesystem to loss in bentonite mass and swelling pressure, seven additional calculations were done with
reduced swelling pressure down to 80 kPa, corresponding to a density at water saturation of about
1,500 kg/m3.
The calculations included two stages, where the first stage models the swelling and consolidationthat takes place in order for the buffer to reach full equilibrium. This stage takes place during thesaturation phase and the subsequent consolidation/swelling phase. The second stage models thedeviatoric creep in the buffer over 100,000 years.
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Volumetric creep is not modelled, which may cause a slight underestimation of canister displacement. The reason for excluding volumetric creep is that a canister settlement caused by volumetriccreep will not change the total mass of bentonite under the canister but will only increase its density
and is thus not judged to be a problem. Moreover, the volumetric creep is of the same order ofmagnitude as the deviatoric, which means that canister displacement caused by this creep will be as
insignificant as the deviatoric creep.
The calculations show that canister settlement is very small even at low swelling pressure anddensity. The base case corresponding to the expected final swelling pressure of the buffer of 7 MPayields a total settlement of the canister of only 0.35 mm for the fixed boundary case, while there isa heave of the canister of about 4.5 mm for the other case with 30/70 backfill due to the upwards
swelling of the buffer. At reduced swelling pressure, settlement increases but is not more than about
23 mm at the very low swelling pressure of 80 kPa for both cases.
Figure 3-15 shows the consolidation and creep settlement as a function of applied swelling pressure
for the two sets of calculations.
Another phenomenon that may affect the buffer is a reduction in strength or friction angle with time.
This affects the degree of mobilised shear strength, which is a critical creep parameter. A reduced
friction angle will reduce the strength and thus affect the degree of mobilised strength in a similar
way as a reduced swelling pressure. So the reduction in shear strength can, for the creep, also
correspond to a reduction in friction angle with the same initial density and swelling pressure. At aswelling pressure of 80 kPa, the deviatoric stress at failure is only 70 kPa, which for the swellingpressure 7 MPa corresponds to a friction angle of only 0.29 degrees. This shows that not even astrong reduction in friction angle is a threat to the canister integrity.
The conclusion is thus that the expected displacement of the canister in a deposition hole fromconsolidation and creep over 100,000 years is very small and for the case of 30/70 backfill will
actually result in a heave of the canister. The sensitivity analyses with reduced swelling pressure
corresponding to reduced density or reduced friction angle also show that canister displacement isvery insensitive to such phenomena; the total settlement will be less than a few cm even at a buffer
density at saturation of 1,500 kg/m3 or at a friction angle of 0.3°.

Only consolidation
Consolidation + creep

Figure 3-15. Vertical displacement of the canister as a function of swelling pressure for the two cases with
fixed boundary buffer/backfill (left) and free boundary with 30/70 backfill (right). Negative displacement
implies sinking /­Börgesson and Hernelind 2006a/.

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Rock shear
Additional calculations of the influence of an earthquake-induced 20 cm rock shear with the shear
rate 1 m/s along a fracture intersecting a deposition hole in a KBS-3V repository has been done fora number of different shear cases and for different properties of the buffer material /­Börgesson andHernelind 2006b/. The influence of the following factors has been investigated:
1. Inclination of the intersecting fracture.
2. Shear direction when the fracture is not horizontal (inclination deviates from 90˚).
3. Location of the shear plane when the inclination is 90˚.
4. Magnitude of the shear displacement.
5. Bentonite type.
6. Bentonite density.
7. Transformation of the buffer to illite or cemented bentonite.
The results from the calculations show that all of these factors have an important influence on thedamage to the canister. For most factors the influence is not easily described since there are mutualinterferences. The results are further discussed below.
Plastic strain larger than 1% was reached in the copper at 10 cm shear in all cases with Na- andCa- bentonite. However, for several cases of Na-bentonite and one case of Ca-bentonite, such plasticstrain was only reached in the lid.
The plastic strain in the steel was generally smaller than in the copper mainly due to the higher yieldstress in the steel.
A shear inclination of 45˚ was more harmful for the copper tube than a shear inclination of 90˚ whentension shear was considered. The highest value of the plastic strain in the copper tube was 19% andwas reached after 20 cm shear in Ca-bentonite with a density 2,050 kg/m3.
At a shear angle of 90˚, higher plastic strain in the copper tube was reached at eccentric shear than at
centric shear for all cases except the high density Ca-bentonite case.
The maximum plastic strain in the copper tube and in the iron insert was for all cases larger for
Ca-bentonite than for Na-bentonite.
Conversion to illite implies that the effect on the canister of a rock shear is insignificant compared tothe case when no transformation has taken place since the stiffness and strength of the illitic clay is
only a tenth of the strength of MX-80, as almost the entire swelling pressure is lost.
Cementation of bentonite to a thickness of 8.75 cm around the canister results in the effect of a rockshear being more severe than in the original bentonite due to the increased stiffness of the buffer.
However, the properties of cemented bentonite are not well known so the calculation must be
regarded as an example rather than as a prediction.
Additional modelling is ongoing and will be reported for SR-Site. See also the process report of thecanister.
Consequences of loss of bentonite
The self healing of the bentonite buffer after loss of bentonite by e.g. erosion has been investigatedin a number of FEM calculations using the basic swelling properties of the bentonite developed for
Abaqus. The calculations have been performed so that part of the buffer is initially missing in anotherwise homogeneous water saturated buffer with a buffer density at saturation of 2,000 kg/m3.
The results are reported in /­Börgesson and Hernelind 2006c/.
The results show that the self healing properties of the bentonite are good and heal even large holes.
However, friction in the bentonite prevents complete homogenisation and leaves a rather large
density and swelling pressure gradient. E.g. if 121 kg is lost, the lowest resulting swelling pressure
will still be higher than 1 MPa.
The extreme case of loss of all buffer material for 1 metre along the canister (corresponding to
2,400 kg dry mass of bentonite) results in the lowest estimated swelling pressure of a few hundred kPa.
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Upwards swelling into a dry tunnel
If the backfill material in the deposition tunnel consists of highly compacted blocks and pellets ofswelling clay instead of field compacted mixtures, the rather high swelling pressure from the block
backfill prevents swelling of the buffer so that this process becomes acceptable with a high safetymargin for all block types /­Gunnarsson et al. 2006/. However, if the deposition hole is wet and thetunnel so dry that the buffer material becomes water saturated when the blocks are still close to theinitial state there may be large displacements since there is still no swelling pressure from the backfill. This case has been investigated with some simplified calculations /­Börgesson and Johannesson
2006/. The calculations yielded an estimated upwards swelling of about 8 cm. The results must be
considered preliminary since the assumptions and calculation methods are uncertain and call for
supplementary tests and calculations, which will be done for SR-Site and reported in the generalbuffer THM modelling report.
Field and laboratory tests
Experience related to swelling and homogenisation of the buffer in an emplacement borehole isavailable from the results of field tests in Stripa. In BMT (Buffer Mass Test), six deposition holes
were simulated at half scale. The movement of the interface between the buffer and the backfill
was measured with a coin that was placed in the interface and levelled before and after the tests.
The measured upwards swelling varied between 4 and 7 cm, depending on test time and degree ofsaturation in the buffer.
Further information related to the homogenisation of the buffer and the mechanical interaction
between buffer and backfill is also being collected from several of the full-scale tests in Äspö HRL.
One of these, the Canister Retrieval Test had achieved full water saturation in the majority of its
buffer volume at the time of its excavation, which took place in the beginning of 2006. The results
show that almost full homogenisation had occurred between the blocks and the pellets in the slot
between the blocks and the rock surface but also that there is (as expected) a residual density difference due the friction in the bentonite.
Several scale tests have been run in the laboratory (mostly at the scale 1:10) to complete water
saturation. Two examples of the types of tests conducted are:• Rock shear was simulated at the scale 1:10 in three laboratory tests performed in 1986 /­Börgesson
1986/. Before the shear was simulated, the bentonite was brought to complete water saturation
through filters at the periphery of the buffer and the swelling pressure measured. The three tests wereidentical with the exception of the shear rate that was varied between 0.03 mm/min to 160 mm/min.
The bentonite was carefully sampled and the water ratio and density of the samples were determined
after the test was completed. The tests gave valuable information about both the effect of shear atdifferent rates and the homogenisation of the bentonite in a deposition hole. These tests have recently
been used for validating the material models and the modelling technique.
• The KBS-3H concept has also been simulated in the laboratory at the scale 1:10. Two canisters
with buffer material perforated steel containers and distance plugs were installed in a tube andthe swelling pressure and relative humidity RH measured. By measuring RH the total suctionof the clay and the resulting degree of saturation can be evaluated. The bentonite was brought
to complete water saturation through filters with an increased water pressure of 1 MPa. The test
arrangement was left for one year and the axial hydraulic function of the buffer was tested byapplying hydraulic gradients in the filters. Finally the test was terminated and water ratio anddensity measured after comprehensive sampling. The test was used to evaluate the swellingand homogenisation of the distance blocks and the buffer inside the perforated container and to
study the resulting axial water flow properties. The results showed satisfactory agreement within
expectations.
No heating was applied to the two tests described above.
Unsaturated bentonite
Many experiments have been conducted to study the volume change as a function of degree ofsaturation and void ratio under different conditions. These experiments have been used to calibrate
the existing material model for unsaturated clay and to improve them.

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The interaction between external stress, porewater pressure (suction) and degree of saturation in an
unsaturated bentonite has been investigated in a PhD project /­Dueck 2004/. This work sheds further
light on the development of swelling pressure in an unsaturated buffer. A laboratory programme was
run with the ultimate purpose of investigating the influence of confinement on the water retention
properties, the development of swelling pressure during decrease in suction and the effect of externalload and swelling pressure on suction.
The results from the laboratory tests were used to find a relationship between water content, void
ratio, swelling pressure and suction. Two equations for swelling pressure represent the outline of
the model. In the first of the equations, the swelling pressure developed during water uptake isnormalised by a pressure corresponding to the swelling pressure at saturation. This is done in order
to be independent of void ratio. A relationship between the normalised swelling pressure and thedegree of saturation is suggested. The second equation describes a relationship between the swellingpressure, the water content and the actual suction (or relative humidity). The equation is based ona thermodynamic relationship and includes the retention curve (i.e. water content vs. suction underfree swelling conditions). The model can be used for a state where two of the four parameters; watercontent, void ratio, swelling pressure and suction are known and can thus be useful to e.g. evaluate
field measurements and model the late stage of the wetting process. The equations are mainly basedon results from tests with increasing degrees of saturation under constant void ratio but are also suggested for use with increasing void ratio. See also /­Dueck 2007/ and /­Dueck and Börgesson 2007/.
Thermal expansion has been simulated under water-saturated conditions in laboratory tests, whichshow that expected stress increases occur, followed by falling stresses due to drainage of water.
A numerical modelling of the process /­Börgesson 1992/ shows that the maximum pore pressure
increase amounts to 8 MPa and that this is due to the fact that the rock drains the buffer. However,this calculation assumes that the rock acts as a filter for the porewater meaning that the porewater isallowed to enter the rock matrix freely.
Modelling of the Canister Retrieval Test is currently ongoing within the international project ÄspöTask Force on Engineered Barrier Systems. The results from the Task Force work will be used andreported as a part of the mechanical assessment of the buffer in SR-Site. The modelling includes
fully coupled thermo-hydro-mechanical simulation of the wetting and homogenisation of the buffermaterial. Since the test includes both unsaturated bentonite blocks with high density, a 5 cm slot
filled with pellets and a 1 cm empty slot, the available mechanical models can be evaluated.
Additional experiments need to be done to improve the understanding of the volume change
behaviour of initially unsaturated buffer and to better model its evolution.
Time perspective

The water saturation phase is expected to take between 10 and 100 years, depending on theproperties of the rock. During this time span, the proportion of the expansion that is limited by the
consolidation rate (which is controlled by the hydraulic conductivity) probably also takes place.
The subsequent volume creep can proceed for thousands of years, but is judged not to be of such amagnitude that it appreciably influences buffer performance. The deviatoric creep will also continue
during the lifetime of the repository and its main effect will be the canister sinking (see above)
but will also influence the homogenisation of the buffer. As mentioned earlier, the friction in the
bentonite counteracts homogenisation and is responsible for e.g. the following effects:
1. The decrease in buffer density at the buffer/backfill interface is not spread to the entire buffer.
2. The penetration of bentonite into the fractures in the rock is limited to about 25 times the fracture
aperture unless lost by colloid release.
3. There will be a small remaining radial difference in density originating from the swelling of theblocks and compression of the pellets.
The deviatoric creep can be regarded as a decrease in friction with time. This process, which has
been investigated in relation to the canister sinking, is very slow and not expected to have a dramatic
influence on the buffer in the lifetime of a repository.

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Thermal expansion: Temperature calculations (see e.g. /­Thunvik and Braester 1991/ and /­Hökmarkand Fälth 2003/) show that the maximum temperature in the bentonite at the canister wall is reachedafter about 10 years, at the rock wall after about 30 years and furthest away from the canister after
another ten years or so. In the most favourable case, water saturation can be reached between thecanister and the rock in a few years and in the whole buffer after 10–15 years. In other words, there
is a risk that a pressure increase will arise in the interval between 10 and 50 years after deposition.
However, the temperature increase is very moderate and slow during this time interval. Furthermore,
a temperature decrease takes place simultaneously nearest the canister after about 10 years, whichcounteracts the effect. As stated above the judgement is that this process is not a problem.
Natural analogues/observations in nature

No natural analogues concerning mechanical behaviour have been studied. Manmade bentonite
seals are made in completely different ways (mixtures with low density) and natural bentonites haveunknown histories. The existence of relevant natural analogues is probably very limited and difficult
to evaluate.
Handling in the safety assessment SR-Site

The process is relevant in all timescales, is influenced by a range of boundary conditions/effects andcoupled to a number of other processes.
Early effects
The initial swelling is modelled as part of the integrated initial THM evolution of the buffer, see
further Section 3.3.1. Also thermal expansion and interaction buffer/backfill is included in thismodelling.
Renewed calculations of the interaction between the buffer material and the new backfill consistingof blocks and pellets are done. Some results are reported in /­Börgesson and Hernelind 2009/ and/­Åkesson et al. 2010/.
Long-term effects
Long-term effects of ion exchange and saline groundwater are evaluated using empirical relationships described above. The effects of salinity can also be modelled by the Donnan model referred
to in Section 3.5.8.
Canister sinking: The process is provisionally neglected, based on the above modelling results. A moredeveloped basis for this assumption, based on a buffer density criterion to justify neglecting of thephenomenon might need to be established. The consequences of canister sinking may be assessed ina residual scenario.
Effects of convergence of deposition hole: These effects will be estimated based on results fromongoing studies as part of discussion of integrated, long-term mechanical effects.
Effects of intrusion into fractures and subsequent colloid formation and dispersion: Neglected aslong as the concentration of divalent cations in the groundwater is high enough to prevent the formation of an expansive gel, otherwise the process has to be quantified. See further Section 3.5.11.
Earthquakes: The mechanical effects of earthquakes on the buffer will be handled in separate,
integrated assessments in the SR-Site main report, encompassing also earthquakes that do not cause
canister failure. Generally, it is noted that a shear movement does not alter the volume of a deposition hole significantly.
Swelling and homogenisation after loss of buffer material are modelled for SR-Site. This is a very
important sub-process that extensively studied in order to improve the knowledge and modellingcapability.

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Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The relationship between swelling pressure and density for MX-80 and Deponit CA-N and theinfluence of different factors are reported by e.g. /­Karnland el al. 2006/. This report will undergo adocumented factual- and quality review. In addition tests have been done on other bentonite types
in other countries as Spain, France and Japan. An example of such a material is the Febex bentonite,which is a calcium dominated bentonite that has been used in the large scale tests in the Febex
project. Results from these tests are reported in the series published by ENRESA (Publicationes
tecnicas).
The effect of canister settlement has been found to be insignificant according to /­Börgesson andHernelind 2006a/. The essential contents of this report will be transferred to the general THM report/­Åkesson et al. 2010a/.
Buffer homogenisation (both natural during the saturation phase and caused by the sealing of erosion
damage) and buffer upwards swelling are reported in old SKB Technical Reports but will be updated
and reported in a general buffer THM modelling report that will undergo a documented factual- andquality review /­Åkesson et al. 2010a/.
Uncertainties

Uncertainties in mechanistic understanding
After complete water-saturation, the swelling process is deemed to be sufficiently well-known andthe final density distribution in the buffer can be calculated with sufficient accuracy. Self-healing
after loss of bentonite is important and the calculations need to be checked with scale tests. Suchtests are ongoing.
In the long term, swelling properties can change due to mineral transformations and cementation; see
Section 3.5.9 “Montmorillonite transformation”. The mechanical consequences of transformations
can be assumed to lead to a higher hydraulic conductivity and a decrease in swelling pressure.
Cementation may cause significant changes in the physical properties of the buffer, since they can
produce a material that is brittle and without swelling capacity and thus can crack and open channels
and gaps in the buffer. Since this is a chemical process it is treated elsewhere, see Sections 3.5.9 and3.5.6.
The swelling process and the swelling/compression properties at water saturation are well known
and can be modelled with sufficient accuracy /­Börgesson et al. 1995/.
The mechanical function of water-unsaturated bentonite is difficult to model, and the models thatexist today are incomplete, especially for modelling of volume change and swelling pressure. Workis ongoing and progress have recently been reached and are included in the THM report /­Åkesson ate
al. 2010a/. The mechanical processes before water saturation are not considered important for safety,
but have been investigated in order to improve the general understanding of the saturation phase.
The mechanisms of the different interactions (mechanical interaction buffer/backfill, buffer/rock andbuffer/canister) are well known regarding the final state after saturation and stress equilibrium.
Thermal expansion: The process is well understood for water-saturated bentonite. For waterunsaturated bentonite the thermo-mechanical theory formation is not complete, but the consequencesof this process are deemed in this case to be unimportant for safety.
The largest uncertainty besides the behaviour in the unsaturated state concerns creep. Although acreep model exists and has been verified (for deviatoric stress), the validity for long-term effects isdifficult to prove.
Model simplification uncertainties in SR-Site
See the above heading.

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Input data and data uncertainties in SR-Site
Water saturated state: Some uncertainty arises in calculating the magnitude of the swelling pressure
after swelling, where the possible deviation is judged to be ± 30 percent, depending on the scatterin swelling pressure measurements and the uncertainty in certain material parameters. Some validation has been done by means of laboratory /­Pusch and Adey 1986/ and field tests, and continued
validation is taking place via the field tests in the Äspö HRL and laboratory tests /­Börgesson et al.
1995, Karnland et al. 2005/ some of which are ongoing . Other parameters required are fairly wellknown, e.g. the friction angle (which is a function of density) and the compressibility, which havean uncertainty of 10–20%. However, the friction angle at the interface between the bentonite andthe rock or the canister has a special uncertainty since it depends on the roughness of the surface.
Additional investigations of these interfaces will be performed.
The quality of the input data is further discussed in the THM data report /­Åkesson et al. 2010b/.
Unsaturated state: Since the mechanical processes during the unsaturated period are rather poorly
known, the uncertainties are large. These processes are thought to mainly influence the path thesystem will take to reach the saturated state and only to an insignificant degree influence the final,
saturated state.

3.4.2

Liquefaction

Overview/general description

This process was included in the Process Report for SR-Can. However, it has been concluded thatthe phenomena cannot occur in a KBS-3 type buffer material. The reasoning justifying this conclusion is discussed in Section 3.4.1. Liquefaction is no longer regarded as a process in the buffer.
Handling in the safety assessment SR-Site

Since the process cannot occur, it will not be handled in SR-Site.

3.5

3.5.1

Chemical processes

Introduction

Bentonite minerals

High quality commercial bentonites normally contain over 80% montmorillonite, which means thatdifferent bentonite products have similar properties in many respects. The distribution of accessory
minerals may vary within, and especially between different quarries. Typical accessory minerals arefeldspars, quartz, cristobalite, gypsum, calcite and pyrite.
The montmorillonite mineral belongs to the smectite group, in which all the minerals have an articulated layer structure. The thickness of an individual lattice layer is around 1 nm (Figure 3-16) andoften extends several hundred nanometres. Each layer is composed of a central sheet of octahedrally
coordinated cations, which on both sides is linked through shared oxygen to sheets of tetrahedrally
coordinated cations. Clay minerals of this type are often referred to as 2:1 layer structures.
By definition, in montmorillonite the octahedral sheet has aluminium as central ion, which is partly
substituted, principally by magnesium and iron. The tetrahedral sheet has silicon as central ion,
which may be partly substituted, principally by aluminium. The substitutions results in a net negative
charge of the montmorillonite layer in the range of 0.4 to 1.2 unit charges per O20(OH)4–unit, andthe octahedral charge is larger than the tetrahedral. The induced negative layer charge is balanced
by cations (c) located between the individual layers (interlayer space). The structure leads to a water
affinity in the interlayer space and a variable number (n) of water molecules may be incorporated
between the individual mineral layers.
The ideal montmorillonite formula may be written:
Si 8–x Al x		 Al 4–y (Mg Fe)y O20 (OH)4

cx+y n(H2O)(x < y) andtetrahedral		interlayer

0.4 < x+y > 1.2

octahedral			and the structure may schematically be illustrated as in Figure 3-16.
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Si (Al)
Al (MgFe)
Si (Al)

O
O OH
O OH
O

Cations H2O
Si (Al)Al (MgFe)
Si (Al)

O
O OH
O OH
O

montmorillonite
layer ~ 1 nm
interlayer
spacetetrahedral sheet
octahedral sheet
tetrahedral sheet

Figure 3-16. Edge view schematic of two montmorillonite layers with interlayer cations and watermolecules /­Karnland et al. 2006/.

Bentonite porewaterIons in aqueous solution reduce the chemical potential and a concentration difference between
solutions normally leads to water transport from the high potential (low concentration) to the low
potential (concentrated) volume. In parallel, ions will diffuse from the high concentration to low
concentration volume, and the equilibrium condition, i.e. equal chemical potentials, leads to uniform
ion concentration.
The total suction of a soil, as described in Section 3.3.1, includes osmotic suction and matrix suction.
By definition, the osmotic suction is due to lowering of the chemical potential in porewater by ions,and the matrix suction refers to the lowering of the chemical potential of the porewater by mineralsurface interaction.
In bentonite, the conditions are extraordinary due to the negatively charged montmorillonite layers
and the charge compensating cations in the interlayer space. Water molecules can be incorporated
between the individual montmorillonite layers to create an interlayer ionic solution. The cations
cannot diffuse freely away from the mineral surface because of the demand for electrical neutrality.
Water will consequently be transported into the inter-layer space, if water with a higher chemical
potential is available, and the interlayer distance will increase, which is synonymous with bentonite
swelling. This water uptake will continue until the chemical potentials are equal, which in the caseof a pure water source theoretically leads to infinite swelling in e.g. sodium bentonite. The extent ofwater uptake in bentonite may thereby be orders of magnitude larger and have a different character
than in other soil materials.
In a fixed total volume, water uptake into the interlayer space will reduce the volume of initiallylarger pores. Uptake is forced to stop when the total available pore volume is completely filledwith introduced water. At this full water saturation, water will continue to move in order to balance
the interlayer ion concentration and thereby the chemical potential in the system. The interlayer
distances will thereby increase in interlayer space with high concentrations at the expense ofinterlayer space with low concentration, and the final distances will be a function of the local
montmorillonite layer charge. The remaining difference in ion concentration, between the highconcentration interlayer solution and the water supplying solution, leads to an osmotic pressurebuild-up in the clay (swelling pressure), which equals the chemical potential of water in the system.
The homogenisation of the pore size, due to the levelling of the chemical potential, may be partly
counteracted by mechanically stable structures, e.g. in volumes with complicated stacking of themontmorillonite layers, or in clusters of accessory minerals. In extreme cases the porewater in suchclusters may eventually be in equilibrium with the water supplying solution, without influence fromthe surrounding montmorillonite charge compensating cations.
The degree of final pore size homogeneity has been analysed and discussed extensively in theliterature, and quite different conceptual and quantitative models have been proposed. The conditions
are of major importance since the pore-size distribution governs inter alia the possible transport ofcolloids (Section 3.5.4).

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The concentration of the charge compensating cations in the interlayer space is highest close to
the negatively charged montmorillonite surfaces and will decrease drastically with distance fromthe surfaces. The distribution, which may ideally be calculated analytically or numerically, is ofmajor importance for equilibrium with other ions in the system, which may originate from anexternal solution (Section 3.5.8 ) or from accessory minerals in the buffer (Section 3.5.6).
The volume of a deposition hole and the corresponding buffer mass are pre-determined, andthe mean interlayer cation concentration can consequently be calculated. The total pore volume in
the buffer in one deposition hole is around 5,250 litres, and the bentonite mass is around 19,250 kg.
The montmorillonite layer charge may be experimentally determined and is usually given as cationexchange capacity (CEC). The reference bentonites have a CEC of around 0.75 equivalents per kilogram clay material, which give a total cation content of around 14,430 equivalents in each borehole.
The mean charge compensating cation concentration in this system is consequently almost 3 M formonovalent ions if it is in equilibrium with pure water.

3.5.2

Advective transport of species

Overview/general description

In this context advection refers to transport of any forms of additional matter, e.g. ions, molecules or
colloids, with pore-water flow. The transport direction is thereby principally from volumes of highwater pressure to volumes of lower pressure. The process leads to redistribution of solutes in the porewater and thus affects the pore-water composition. There are several possible causes of pressure
gradients in the buffer material, e.g. external water pressure, affinity for water in the bentonite andtemperature induced volume change of the water. The advection is thereby closely related to waterflow in the buffer, which is comprehensively described in Section 3.3.2. The advection may though
be different compared to pure water flow due to ion equilibrium effects in the case of ions, and due
to filtering by nano-sized pores in the case of colloids.
Advection is of special importance in the buffer during the water saturation phase when a net flow ofwater takes place in the buffer. The principal flow direction during this phase is towards the canister, pro­vided that the rock supplies the buffer with groundwater. After placement of a canister, a minor counterflow may initially also take place in the direction from the canister due to the temperature gradient.
Under saturated conditions, the transport of solutes in the porewater is expected to be dominated bydiffusion, see Section 3.5.3. Exceptions may be sudden events such as gas pulses or earthquakes,
which can cause local pressure changes in the porewater. The improbable condition with a failure ofa canister will lead to water flow into the canister due to the groundwater pressure. Corrosion of thesteel insert will lead to hydrogen gas production, and the successively increasing gas pressure will
reduce the inflow. The flow will stop if the water level in the canister is below the canister damage
region at the time when the gas pressure exceeds the groundwater pressure. Alternatively, the flow
direction will change towards the rock if the water level is above the damage. Eventually the gaspressure may reach the sum of the swelling pressure and the groundwater pressure, which would
lead to a significant water flow in the latter case (See gas Section 3.3.3).
Dependencies between process and buffer variables

Table 3-9 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Boundary conditions

The external boundary condition for this process is the hydraulic gradient across the buffer.
Model studies/experimental studies

Several studies by SKB and others have examined water flow in bentonite material, both during
water uptake and after complete water saturation, and a wealth of quantitative data, which describewater transport, is available under different physico-chemical conditions, e.g. densities, salt solutions/­Karnland et al. 2006/ temperatures /­Cho et al. 1999/, preparation techniques /­Karnland et al. 2009/(see also Section 3.3.2.
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Table 3-9. Direct dependencies between the process “Advective transport of species” andthe defined buffer variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Buffer geometry

Yes, major importance Included in geochemical No
before full watermodelling. This variable
saturation
may be considered
constant after water
saturation

Pore geometry

Yes, interlayer distance Included in geochemical No, but indirectly, throughand density is amodelling via thedissolution/precipitation
decisive parameterparameter hydraulicconductivity

Radiation intensity

No

Temperature

Yes, through thermal
expansion andviscosity

Viscosity effectNo
is included ingeochemical modelling

Water content

Yes, major
importance

Included ingeochemical
modelling

Yes, through dissolution/
precipitation

Included ingeochemical
modellingGas content

Yes, reduced flow inunsaturated clay

Neglected since
the presence of gas
generally will reduce
advection

Yes, through transport/
precipitation ofdissolved gas

Neglected, since
it always reduce
advection

Included ingeochemical
modelling

Yes, minor osmotic andviscosity effects possible

Neglected

Hydrovariables
Yes, by definition(pressure and flows)

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Included ingeochemical
modellingNo

Stress state

No, Indirectly through Neglected
the coupling to poregeometry

Yes, minor effect possible

Neglected

Bentonitecomposition

Yes, throughmontmorillonite
content

Included ingeochemical
modelling

Yes, through dissolution/
precipitation

Neglected

Montmorillonite
composition

Yes, minor at buffer
density

Included ingeochemical
modelling

Yes, through ion
exchange

Included ingeochemical
modelling

Porewater
composition

Yes, by definition

Included ingeochemical
modelling

Yes, by definition

Included ingeochemical
modelling

Structural and straymaterials

Potentially, as
alternative transport
paths

Neglected, since noNostray materials are
assumed to be present
in the buffer

The Long Term Tests of Buffer material (LOT) field test at Äspö HRL shows a total increase in ion
concentration in the porewater compared to that originally present in the bentonite as a consequenceof the salinity of the saturating groundwater /­Karnland et al. 2000, 2009/. Precipitation of calcite andgypsum close to the central heater indicate ion enrichment and cementation (Section 3.5.6).
Natural analogues/observations in nature

Comment: Sediments rich in smectite may maintain large chemical gradients over geologicaltimescales /­Neuzil 2000/.
Time perspective

Advective flow transport in conjunction with water saturation takes place on a timescale of up to
hundreds of years. Flow transport after water saturation is expected to be negligible in relation todiffusive transport.

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Handling in the safety assessment SR-Site

Before saturation: No detailed modelling of advection is required. It should be noted that thecomposition of the porewater will be influenced by the composition of the intruding water during
saturation. The groundwater composition should be considered when defining initial conditions formodelling of the chemical evolution after saturation, see further Section 3.5.6.
After saturation: Based on the scenario-specific evaluation of water transport under saturated conditions, process 3.3.2, advection can normally be neglected compared to diffusion. If the expected low
hydraulic conductivity is for some reason not maintained, advection is included in the modelling of
the canister corrosion and of radionuclide transport.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The handling is based on the low hydraulic conductivity of the buffer material, since the resulting
water flow by definition will govern advective transport. There are a large number of publications
supporting the low conductivity of highly compacted sodium and calcium bentonites. The supporting
reference /­Karnland et al. 2000/ is not peer- or factual reviewed, but it is scientifically consistent
with other references that are peer reviewed articles or papers. The references /­Karnland et al. 2006/
and /­Karnland el al. 2009/ are SKB reports that presently undergo factual- and quality reviews. The
references /­Cho et al. 1999/ and /­Karnland et al. 2008/ and /­Neuzil 2000/are peer reviewed articles
which are available in the open literature (Applied clay science, Physics and chemistry of the Earth,and Nature).
Handling of variables influencing this process: See process 3.3.2
Failed canister: Advective flow from the buffer due to hydrogen gas pressure will be calculated as a
special case in the radionuclide transport calculations.
Earthquakes: Pressure transients may arise, but will not prevail in the buffer over timescales that
could lead to significant water flow.
Uncertainties

Uncertainties in mechanistic understanding
See Section 3.3.2 for uncertainties concerning water advective transport. The magnitude of geometrical and ion equilibrium effects are uncertain but both effects will reduce the advective transport ofspecies.
Model simplification uncertainties in SR-Site
Unsaturated conditions: see Section 3.3.1.
Saturated conditions: The process can be modelled with a standard mass-transfer approach. The main
uncertainty lies in the coupling between the fractured rock and the porous buffer.
Input data and data uncertainties in SR-Site
Unsaturated conditions: See Section 3.3.1.

3.5.3

Diffusive transport of species

Overview/general description

Solutes can be transported in stagnant porewater by diffusion, following concentration gradients.
Solutes move from areas of higher concentration to areas of lower concentration. The process leads
to a redistribution of dissolved porewater constituents as well as radionuclides in the porewater.
Therefore, diffusion is a critical process for radionuclide migration in the buffer, but also affects
the actual porewater composition.

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The diffusion process is strongly coupled to nearly all chemical processes in the buffer, since it
accounts for transport of reactants to and reaction products from the canisters. Diffusion of Na+ andCa2+ ions is, for example, of crucial importance to ion exchange processes, diffusion of K+ is an important limiting factor for illitisation etc. Diffusion is central to the entire chemical evolution in the buffer.
Diffusion in bentonite has been studied thoroughly in conjunction with radionuclide transport.
Diffusion equations for radionuclides (as well as some experimental measurement techniques relevant for determining the respective parameters) are described in detail in /­Yu and Neretnieks 1997/.
Diffusion in a porous medium takes place according to Fick’s first law (Equation 3-16):
J = −Dpε dCp/dx									(3-16)
Where: J is the diffusive flux, Dp the diffusion coefficient in the pore, ε is the (physical) porosity andCp the concentration of the diffusing species in the pore.
The pore diffusion coefficient Dp in a solution-filled pore is lower than the diffusion coefficient in an
unconfined volume of the same solution, Dw. This is mainly due to the tortuosity of the pores, which
increases the length of the diffusion path. The relationship between Dp and Dw is:
Dp = Dw δ/τ2									(3-17)
Where: δ is the physical constrictivity and τ the tortuosity of the pores. The constrictivity takes into
account any narrowing or widening of pores along the flow path. Since bentonite is viewed as ahomogeneous porous medium on a macroscopic level, δ is usually set equal to unity.
Fick’s second law, taking into account conservation of mass, describes changes in concentration of adiffusing species in time and space. For diffusion of a sorbing species in a porous material, it can beformulated as follows (Equation 3-18):
ε ∂Cp/∂t + ρ ∂q/∂t = ε Dp ∂2Cp/∂x2 						(3-18)
Where: ρ is the bulk density of the material and q is the tracer concentration in the solid phase. For
conditions where sorption is linear (i.e., not dependent on the concentration of the diffusing tracer),
Equation (3-19) can be written:∂Cp/∂t = De/(ε + Kd ρ) ∂2Cp/∂x2 = Da ∂2Cp/∂x2					(3-19)
Where: Kd is the distribution coefficient (see Section 3.5.5), De is the effective and Da the apparent
diffusion constant, which are defined as follows:
De = εDp 									(3-20)
Da = De/(ε + Kdρ) 								(3-21)The term (ε + Kdr) is also known as the capacity factor a of a porous medium. When sorbing solutes
are present at trace levels, sorption is independent of their concentration (i.e. the concept of a linear
sorption isotherm applies), and Cp can be replaced by the total concentration in the porous material,
which gives:∂C/∂t = Da ∂2C/∂x2								(3-22)
where C is the total concentration of the diffusing ion. Equation (3-22) is the relationship that is normally used for diffusion studies of radionuclides in bentonite. For diffusion of inert, non-interacting
species (implying Kd = 0 in Equation (3-22)), Da is equal to De/ε.
Note that this relationship does not apply to inert anionic tracers, even though a formal Kd = 0 is
typically assigned to these species. This is because the diffusion of anionic tracers through clay is
influenced by the permanent negative charge of clay surfaces, which gives rise to the anion deficit/
Donnan potential in clay pores (see below and Section 3.5.8). In terms of using Equation (3-21) inthe context of diffusive transport of trace contaminants, the effect of anion deficit is typically handled
by assigning a value to ε which is lower than the physical porosity (see section on uncertainties).

108

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Enhanced apparent cation diffusion: It has been observed in several cases (cf. /­Yu and Neretnieks
1997/) that the apparent diffusion coefficient (Da) for cations and their respective distribution
coefficient (Kd) do not correspond as expected. Da has been higher than expected from Kd values andtraditional porewater diffusion-sorption theory. While several explanations of this phenomenon havebeen proposed (see below), it is of foremost importance to recognise that an agreement of Da and Kd
is only to be expected for self-consistent datasets. i.e. both Da and Kd need to correspond to the same
set of conditions, in particular to the same porewater composition. Thus, several of the discrepancies
between Da and Kd observed in the past can be traced to the fact that incompatible conditions havebeen compared. In particular, Da values that invariably correspond to the porewater composition incompacted bentonite have been compared directly with Kd values from batch experiments involving
typically much more dilute solutions. Given that Kd values are derived for relevant porewater conditions, sorption and apparent diffusion coefficients for most elements agree within the overall data
uncertainty /­Ochs et al. 2003, Bradbury and Baeyens 2003/.
Nevertheless, there are cases where it seems clear that certain cations may have apparently enhanced
diffusivities, which must be taken into consideration in safety assessments. The presently available
information suggests that these cases are largely restricted to mobile and cationic elements: Cs, andsometimes Sr and Ra /­Ochs et al. 2004/ and the SR-Site Data Report /­SKB 2010l/.
Enhanced cation diffusion can be explained by a cation excess in the electrical double layer (EDL)
extending from negatively charged clay surfaces into the pore space /­Kato et al. 1995, Sato et al.
1995, Ochs et al. 2001/, see also Section 3.5.8 Note that cation excess (as well as anion deficit, see
below) have been well established in clay chemistry (see e.g. /­van Olphen 1991/) and that the meancation excess/anion deficit in the EDL corresponds to the differences in cation/anion concentrations
in compacted clay pores as calculated through Donnan equilibrium (see Section 3.5.8).
Because these electrostatic effects increase the dissolved concentration of cations in the pore spacein comparison to the corresponding (external) bulk solution and in comparison to neutral and anionicspecies, diffusing cationic tracers have higher concentration gradients and therefore higher diffusion
rates than would be expected in the absence of the electric charge effects in the clay pores. Note that
this holds for cations that accumulate in the diffuse layer, thereby retaining their mobility parallelto (but not perpendicular to) a mineral surface; but does not hold for specifically (i.e. chemically)sorbed surface species formed at surface complexation or ion exchange sites. i.e., for an evaluationof diffusive transport in compact clays, the simple accumulation in the counter-ion swarm of anelectrical field near a surface and actual sorption (represented by Kd) need to be distinguished.
While presumably based on the same underlying process, the so-called surface diffusion mechanism/­Muurinen 1994, Eriksen and Jansson 1996, Yu and Neretnieks 1997/ makes a direct link withactual sorption by introducing an additional surface diffusion coefficient that is multiplied by Kd.
As pointed out above, however, there is no reason to assume that (specifically) sorbed species retain
their mobility. A reduction of Kd in highly-compacted bentonite due to a decreased accessibility ofsurface sites has also been considered as a possible explanation for the disagreement of diffusion
data with batch Kd values /­Wanner et al. 1996/, but it was shown by /­Kato et al. 1995/ that the entire
physical porosity in compacted bentonite is accessible.
Anion exclusion: With increasing density, the pore width in clays decreases. In compacted bentonite,
the pore width is small enough to cause a superimposition of the electrical double layers between
two negatively charged pore walls. i.e., the EDL’s in compacted clay are truncated and no free (bulk)
porewater exists /­Ochs et al. 2001, 2004/. Under such conditions, the anion deficit in the diffuse
layer applies to the entire pore space (see above and Section 3.5.8) and causes, for anions, the opposite effect as was explained above for cations: Diffusing anionic tracers have lower concentration
gradients, and therefore lower diffusion rates than would be expected in the absence of the electriccharge effects in the clay pores. An alternative view of the same phenomenon is that the anion deficit
in the clay pores decreases the available diffusion pathway for an anionic tracer. Therefore, thisphenomenon is often termed anion exclusion. This terminology is accepted for the present report, but
it needs to be made clear that it refers to exclusion in terms of concentration (anion deficit or Donnan
exclusion, see Section 3.5.8) and does not refer to exclusion due to restrictions in physical pore
width. This means that the underlying concept considers a uniform physical porosity and only one
type of pores in compacted bentonite /­Ochs et al. 2001, 2004/, which is consistent with the concept
described in Section 3.5.8.

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109

Evidence from activation energy measurements suggests that the overlap of EDL’s may occur at adry density of about 1,200–1,500 kg/m3 /­Kozaki et al. 1998/. The effect of anion exclusion becomesless at high salinities, because high ionic strength leads to a depression of the EDL /­Stumm andMorgan 1996/, and in sand-bentonite mixtures it is negligible. The safety-related importance ofanion exclusion is considerable – it is one of the few phenomena that limit the transport of nonsorbing anions from a damaged canister.
Dependencies between process and buffer variables

Table 3-10 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Influence of pore geometry: The influence of pore geometry is included by considering tortuosityand constrictivity in the equations for diffusion coefficients.
Influence of temperature: Based on the influence of temperature on the diffusivity of ions in freewater /­Robinson and Stokes 1959/, it can be expected that De will increase about twofold whentemperature is increased from ambient conditions to about 50–60°C.

Table 3-10. Direct dependencies between the process “Diffusive transport of species” andthe defined buffer variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Buffer geometry

No

No

Pore geometry

Yes

Radiation intensity

No

No

Temperature

Yes

Neglected for radionuclide No
transport – the temperature
will be low when theprocess occurs

Water content

Through pore geometry and porewatercomposition(saturated conditions
assumed)No

Gas content

No(saturated conditions
assumed)In case of dissolved
gases such as CO2
or CH4

Hydrovariables(pressure and flows)

No

No

Stress state

Indirectly through poregeometry

Possibly indirectly
through influence onporewater composition and swelling

Bentonitecomposition

Indirectly throughporewater composition

Indirectly throughinfluence on pore­water composition

Montmorillonite
composition

Extent of EDL
depends on magnitude of charge andphysical pore width

Included in data selection

Composition of
exchangeable ions:indirectly throughinfluence on pore­water composition

Porewater
composition

Extent of EDL
depends on ionicstrength

Included in data selection

Through diffusivetransport of main
constituents

Structural and straymaterials

Through influence on
physical porewidth

No stray materials are
assumed to be present
within the buffer

Possibly indirectly
through influence onporewater composition

110

Porosity is includedin the model

Possibly indirectly
through influence onporewater compositionand swellingSee 3.3.1

Included in model,
see Section 3.5.6

TR-10-47

Influence of bentonite/montmorillonite composition and structural/stray materials: The diffusion properties are dependent on the interlamellar space in the bentonite which is in turn dependent
on the permanent charge characteristics and the composition of the exchangeable cations. Soluble
stray materials may exert an effect through influencing the porewater composition. Structural andstray materials may also influence pore geometry.
Influence of porewater composition and speciation: Different species of the same element canhave radically different diffusivities. This holds for free water diffusivities, but in particular for
diffusion in bentonite pores. Due to their interaction with the diffuse layer extending from negative
clay surfaces, diffusivities decrease in the order anionic species < neutral species < cationic species.
For example, dissolved strontium has lower diffusivity at high concentration of sulphate, which canbe explained by the formation of the neutral aqueous complex SrSO4 /­Ochs et al. 2001/. Changes inporewater composition that cause changes in ionic strength also influence diffusion by affecting theextent of the EDL.
Influence of sorption: The influence of sorption is taken into account in the equation for Da.
Influence of buffer density: The transport of radionuclides in bentonite is influenced by the densityof the material (the degree of compaction). This is a direct consequence of the corresponding
change in porosity. In addition, the presence of soluble impurities will lead to changes in porewatercomposition and, in particular, to an increase of ionic strength as a function of density. The influenceof porewater composition is discussed above. In most experiments, the apparent diffusivity decreases
with increasing density.
Influence on buffer variables: As pointed out above, the diffusion process is coupled to nearly
all chemical processes in the buffer through the diffusive transport of reactants. How chemical
processes, such as exchange reactions of major ions, may further influence other buffer variables isdiscussed in Section 3.5.6.
Boundary conditions

Diffusive transport in the buffer is relevant regarding two aspects:• Any ions or dissolved gases involved in geochemical alterations of the buffer material (as well as
any corresponding reaction products) have to be transported through the buffer by diffusion.
• In case of canister failure, the dissolved concentration of most radionuclides will be limited by
the formation of radionuclide-bearing solid phases, including pure solids (such as oxides, carbonates) and solid solutions. Migration of the traces remaining in solution towards the geosphere islimited by the slow diffusive transport through the buffer.
The transfer of species from the bentonite to the rock can be visualised as the diffusion of dissolved
species from the bentonite into flowing water in the near-field fracture network (equivalent flow rateor Qeq). The equivalent flow rate can be derived by solving the boundary layer theory equations for diffusive transport into flowing water. The Qeq depends on the geometry of the contact area, the water
flux, the flow porosity and the water diffusivity.
Model studies/experimental studies

For experimental reasons, most diffusion studies are performed as transient in-diffusion experiments.
These experiments yield Da, which is a lump-sum representation of all processes relevant for radionuclide migration, including diffusion and sorption. This holds, in particular, for moderately andstrongly sorbing radionuclides. Steady-state through-diffusion studies, which can yield directly therequired input parameters De and porosity, are largely restricted to mobile tracers (Cs, HTO, anions).
A large number of investigations are described in the literature concerning diffusion experiments
in bentonite and similar clays; see /­Yu and Neretnieks 1997/ for a good overview and compilationof diffusion coefficients. Further results are discussed in /­Ochs and Talerico 2004/. More recently,
results of through-diffusion experiments have become available especially for clay rocks (argillites)
as well as some bentonites. These new data are considered in /­SKB 2010l/ and are used to update
the selection of diffusion parameters.

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111

/Yu and Neretnieks 1997/ also give a detailed discussion of different experimental methods andhow the results can be interpreted. For comparison of data from different studies, it is important
to observe which technique has been used and how the diffusivities have been determined. If, forexample, metal filters have been used, they can greatly influence the results. It also has to be realisedin this respect that the extraction of diffusion coefficients invariably involves fitting of experimental
data to the diffusion equations described above.
Few attempts have been made so far to derive diffusion coefficients through semi-mechanistic models.
Some examples are discussed above in the context of anion exclusion and enhanced cation diffusion.
Natural analogues/observations in nature

The usefulness of natural analogues for evaluating diffusion behaviour and for determining constants
is limited due to the difficulty of determining the conditions that existed in the past. Natural analogues can illustrate that diffusive transport is greatly slowed down for many sorbing radionuclides(e.g. /­Smellie and Karlsson 1996/, see also Section 3.5.5).
Time perspective

Diffusion processes in the buffer material are of the greatest importance on all timescales. If thecanister is intact, the process is of importance for the stability of the canister and the buffer. In theevent of a defective canister, slow diffusion is very important in delaying, reducing and in manycases completely preventing releases of radionuclides.
Handling in the safety assessment SR-Site

Before saturation: The process is neglected since advection dominates.
After saturation: The process is included in the scenario-specific modelling of the buffer chemical
evolution for the thermal phase and the post-thermal long-term phase, see heading “Handling”
for process alteration of impurities (Section 3.5.6). The process is treated in a simplified way withidentical diffusivity for all elements in the buffer chemical evolution.
Failed canister: Included in the modelling of radionuclide transport for the long-term phase, seeheading “Handling” for process 3.6.1. For the treatment of radionuclide diffusion, element-specificeffective diffusivities are used together with corresponding porosities (see /­Ochs and Talerico 2004/
and the SR-Site Data Report /­SKB 2010l/) for the definition of data and associated uncertainties).
Boundary conditions: See Sections 3.5.6 and 3.6.1 referred to above.
Influences and couplings: Dependence on buffer density and bentonite type, expected ionic chargeof diffusing species, ionic strength and temperature is considered in selecting diffusion constants fortransport of radionuclides. For the treatment of diffusion within the buffer chemical evolution, see
Section 3.5.6.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Most general issues relevant for handling diffusion are discussed in the Section 3.5.6 and 3.6.1. Allradionuclide-specific data are taken from the SR-Site Data Report /­SKB 2010l/, which will undergoa documented factual review. Most of the data are based on a SKB report /­Ochs and Talerico 2004/
specifically dedicated to the derivation of porosity data as well as diffusion and sorption coefficientsthat are consistent with the specified buffer density and composition considered in SR-Site. Setsof diffusion-available porosities and effective diffusion coefficients were selected by /­Ochs andTalerico 2004/ for anions, cations (essentially Cs) and elements existing largely as neutral species(on average) under the relevant porewater conditions. To define selected parameter sets, a large
amount of original experimental data, published for the most part in peer-reviewed journal articles
and proceedings, was evaluated. In a follow-up peer-reviewed paper /­Ochs et al. 2006/, the approach
used by /­Ochs and Talerico 2004/ was presented, together with a few selected datasets. /­Ochs andTalerico 2004/ was also peer-reviewed according to the instructions for the SR-Can assessment.

112

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Uncertainties

Uncertainties in mechanistic understanding
Because of the experimental inaccessibility of bentonite pores, diffusion data are of macroscopic
nature and are not well suited for any mechanistic interpretation. On a mechanistic level, the main
shortcomings probably lie in the understanding of EDL properties as well as porewater structure andtheir influence on diffusion (and sorption) processes in compacted bentonite. This is particularly truefor high densities that lead to an overlap of opposing EDL’s.
The basic processes of enhanced cation diffusion and anion exclusion have been discussed above.
The processes giving rise to these phenomena are the establishment of a cation excess and an anion
deficit in the diffuse layer extending from a negatively charged mineral surface. Thus, these phenomena
are caused by effects of enhanced or diminished ion concentrations in the volume of the pore space,rather than actual available porosity or different diffusivities. However, several approaches can beused to include the effects of this process in the diffusion equation that is used to define Da: /­Sato
et al. 1995, Ochs et al. 2001/ expressed both cation excess and anion deficit through a concentrationcorrection factor for constrictivity (which leads to a decrease of De for anions and increase of De forcations), while assuming that the entire physical porosity is available for diffusion. On the other hand,
it may be more practical to express this correction factor directly through the diffusion-available
porosity, as the anion deficit in the diffuse layer leads to a decrease of the capacity factor α (which
includes porosity, but not constrictivity). In clay chemistry, anion exclusion is also expressed by actual
negative sorption /­van Olphen 1991/, which would also decrease α. Similarly, cation excess andsorption will increase α.
For the selection of radionuclide diffusion data and uncertainties for SR-Site (SR-Site Data Report/­SKB 2010l/ and /­Ochs and Talerico 2004/), anion exclusion is taken into account through a decreasein De and a reduced diffusion-available porosity. Enhanced cation diffusion is accounted for by anincrease in De in combination with the physical porosity.
Model simplification uncertainties in SR-Site
As pointed out above, diffusion is represented in a simplified way, through the use of selected constanteffective diffusion coefficients and available porosities in the relevant transport codes (see Sections 3.5.6
and 3.6.1). In that sense, the diffusion input parameters have to be viewed as conditional; i.e. theirapplication will only be valid under the conditions considered in data derivation. Therefore, uncertainties will be related to the selection of consistent conditions. If, for example, the THM processes occurring during buffer evolution lead to a significant alteration of bentonite properties, it may be necessary
to use input parameters that differ from those for standard MX-80 bentonite.
In principle, this situation could be resolved by directly including diffusion processes in the model used
for consequence calculations, which would require a THMC model approach. However, in addition
to the great difficulties still associated with coupling THM processes on the one hand and C processeson the other, the present state of the art for quantifying radionuclide diffusion in compacted bentonite
does not allow this. To date, semi-mechanistic models have been proposed and tested for only a few
elements (Cs, Sr, anions; see above). At the same time, in the case of many relevant radionuclides there
is a clear lack of systematic data from through-diffusion experiments that would be needed to further
develop diffusion models in a meaningful way. Except in the case of fairly mobile elements, throughdiffusion experiments often cannot be performed within a reasonable timeframe.
Whilst the abovementioned models are clearly still simplified, they appear to be quite robust as
shown by the results of blind model predictions that are consistent with experimental data forseveral bentonites and over a range of densities /­Ochs et al. 2001/. The respective model outputs
are also consistent with the selections made for SR-Site. In addition, /­Ochs and Talerico 2004/ used
the selected De values and corresponding Kd values for each element (and relevant oxidation state)
to calculate apparent diffusivities, which were then compared to independent experimental data toprovide an additional evaluation of the consistency of selected sorption and diffusion parameters.
Based on the present evidence, it is expected that the direct use of diffusion parameters will not
introduce significant additional uncertainties, given that important boundary conditions in SR-Sitewill be similar to the conditions considered in the derivation of input data.

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113

Input data and data uncertainties in SR-Site
Input data to SR-Site are effective diffusivities and available porosities to be directly used in consequence calculations. Data and uncertainties as a function of bentonite density are given in /­Ochs andTalerico 2004/, from which data for consequence calculations are chosen in the SR-Site data report.

3.5.4

Colloid transport

Overview/general description

Particles with sizes of the order of 10−9 to 10−6 m (colloids) could form on dissolution of the fuel.
The diffusive transport of fuel colloids through highly-compacted bentonite is, however, assumed tobe negligible, due to the tortuosity and small size of the bentonite pores. The fundamental (dispersed
in water) particle size of the bentonite is < 2 μm. At emplacement, the bentonite has a water ratioof 17% (weight of water divided by weight of solid), which corresponds to a water film with amean thickness of 6·10–10 m between the individual montmorillonite flakes (interlayer distance).
Larger pores, between the original bentonite grains from which the blocks are produced, make up
the remaining pore space. The water film thickness increases during water saturation, and the final
theoretical mean interlayer distance is around 10–9 m, which obviously would dramatically reduce
colloid transport. However, a complete homogenisation of the bentonite cannot be expected, and acertain variation in the final pore size will remain also at full saturation. The disjoining force (swelling pressure), and internal friction that counteract the homogenisation may be seen as governing
variables.
Experiments with 15 nm gold colloids show that the microstructure of a bentonite with a dry densityof 1,000 kg/m3 effectively filters gold colloids /­Kurosawa et al. 1997/, /­Holmboe and Wold 2010/.
This corresponds to a saturated clay density of about 1,640 kg/m3. However, organic colloids will
diffuse through bentonite, but the transport capacity is limited (see below).
Dependencies between process and buffer variables

Table 3-11 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Boundary conditions

The process only considers the transport of colloids from the canister interior to the rock contact.
Model studies/experimental studies/Kurosawa et al. 1997/ reports that 15 nm gold colloids are effectively filtered out at a dry density of
1,000 kg/m3 for bentonite/sand mixtures with a sand content of up to 40%. At 50% sand there is anindication of colloid breakthrough, but even there the retention is strong. No colloid breakthrough isobserved for a 50/50 mixture when the dry density is increased to 1,800 kg/m3.
/Wold 2003/ reports on diffusion experiments with organic colloids through MX-80 bentonite witha range of dry densities from 600 to 1,800 kg/m3. The lignosulfonate (LS) and humic acid (HA)
colloids used had an average size of 80 and < 10 nm, respectively, but the size distribution was
broad. The LS and HA colloids showed diffusivities in the same range as negatively charged ionslike Cl− and I−, which is surprising, especially for the large LS, since the mean interlayer spacing ina saturated bentonite is between ~ 5 and 10 Å. One possible explanation for lack of colloid filtration
could be the presence of larger pores, the broad size distribution of the LS colloids, or transformationof the molecules during the course of the experiment. The issue of organic colloid-related transportof radionuclides is one that warrants further examination.
/Holmboe and Wold 2010/ have performed diffusion experiments with colloids in compacted bentonite. Gold colloids are effectively filtered by compacted bentonite. Some transport of gold colloids
takes place in bentonite of lower density (1.0 g/cm3), but very slowly, see Figure 3-17.
Time perspective

The process is important for all repository timescales, but only if canisters have failed.
114

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Table 3-11. Direct dependencies between the process “Colloid transport” and the defined buffer
variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Buffer geometry

No
Colloid filtration
is assumed to be
on or offNo

Pore geometry

Decisive importanceRadiation intensity

No

Temperature

Very minor

Water content

No (if saturated
conditions assumed)No

Gas content

No (if saturated
conditions assumed)

No

Hydrovariables(pressure and flows)

No (if saturated
conditions assumed)

No

Stress state

Indirectly through poregeometry

No

Bentonitecomposition

Indirectly through poregeometry

No

Montmorillonite
composition

Through layer charge

The lowest buffer
density to facilitate
colloid transport isdefinedNo

Porewater
composition

Possibly a very
minor effect throughviscosity

Excluded

No

Structural and straymaterials

No

TR-10-47

The lowest buffer
density to facilitate
colloid transport isdefinedNo

No
Excluded – Nodata available andtemperature range
will be narrow

No

No

115

400

ppb Au/g bentonite

ppb Au/g bentonite

400
300

0.8 g/cm3

200
100

300

100

0
1

2

3

Distance (mm)

4

0

5

0

1

2

3

Distance (mm)

4

5

400

400

ppb Au/g bentonite

ppb Au/g bentonite

0

1.2 g/cm3

200

300

1.6 g/cm3

200
100

300

100

0
0

1

2

3

Distance (mm)42.0 g/cm3

200

5

0
0

1

2

3

Distance (mm)45Figure 3-17. Concentration profiles of gold in the compacted bentonite pellets that were used in the diffusion
experiment. The high value for the first data point in the graph, 2.0 g/cm3, is presumably the result ofcontamination, since the swelling pressure at the high dry density caused deformation of the metal filter/­Holmboe and Wold 2010/.

Natural analogues/observations in nature

No applicable observations of the colloid filtration effect are available from nature.
Handling in the safety assessment SR-Site

The concern in the safety assessment is the transport of radionuclides with colloids from a potentially failed canister, in particular fuel colloids. This transport mechanism is neglected, provided thebuffer completely envelops the canister and has a dry density of at least 1,000 kg/m3, i.e. a saturated
buffer density of at least 1,650 kg/m3. This is based on the experimental results from /­Kurosawa
et al. 1997/ and /­Holmboe and Wold 2010/.
The consequence of a loss of the colloid filtration effect is treated as a separate case with transport ofradionuclides in/sorbed on a colloid phase. This would mean that the concept of solubility limits is
not valid. Radionuclide solubilties as well as sorption coefficients are adjusted for that case.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site/Kurosawa et al. 1997/ and /­Holmboe and Wold 2010/ are peer reviewed publications. Their findings
are also consistent with each other.
Uncertainties

Uncertainties in mechanistic understanding
As seen in the Model/Experiment section above, different types of colloids behave very differently.
It is not clear why there is such a large difference between inorganic and organic colloids.
Model simplification uncertainties in SR-Site
If the buffer density is high enough to exclude colloid transport, no model is needed in the assessment. Bounding calculations can be done by adjusting input data.

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Input data and data uncertainties in SR-Site
No data are used for the same reasons as are outlined above.

3.5.5

Sorption (including ion-exchange of major ions)

Overview/general description

Radionuclides and major ions in the buffer porewater can be bound to the surfaces of the bentonite
material in several ways (see below). Together, these processes are termed sorption and are ofessential importance for the function of the buffer, since they drastically affect the mobility of
most radionuclides and also of major ions, which is, in turn, related to the evolution of the buffer.
The principal mineral in bentonite is montmorillonite, which consists of octahedral alumina sheets
sandwiched between tetrahedral silica sheets (2:1 clay). Such clay minerals feature two distinctly
different types of surface, where two main types of sorption take place /­e.g. Sposito 1984, Stummand Morgan 1996/.
• The siloxane (‘layer’) surfaces of clay minerals are permanently charged surfaces. These charges
derive from isomorphous substitutions, which result in a constant negative surface charge. Thischarge is partly compensated for by cations located at (and bound to) the clay surface (e.g. at
the di-trigonal cavity /­Sposito 1984/), and partly by cations simply accumulated in a counter-ion
swarm. The latter is equivalent to an electrical double layer extending from the surface (seeSection 3.5.1). Macroscopically, sorption takes place when compensating ions are exchanged. Ion
exchange models of the site-binding type have been used in soil science since the first quarter ofthe 20th century. Several formalisms have been developed to correct for activity changes of theexchanger as a function of the composition of the charge-compensating ions. The most common
are the equivalent fraction (Gaines-Thomas, GT) and mole fraction (Vanselow) models.
• The edge surfaces of clay minerals are variably charged surfaces. They carry a net positive or
negative surface charge depending on the species sorbed to their surfaces (potential-determiningions), often involving surface-bound OH– groups. Surface complexation and ligand exchange
models were established by Stumm, Schindler and co-workers in the 1970s by extending protonbinding and metal coordination chemistry in a rigorous fashion to surface chemistry. To accountfor the electrostatic field, the mass laws for surface equilibria often include an electrostatic
correction term. The scientific basis for these corrections is derived from Electrical Double Layer(EDL) theory.
Ion exchange is the typical sorption mechanism for alkali, and alkaline-earth elements, as well astransition metals at low pH values where positive species are predominant. Surface complexation is
generally the more relevant process for all reactive elements (transition metals, actinides, lanthanides,
reactive anions such as carbonate). Note that both ion exchange and surface complexation take place
simultaneously, but at different surfaces.
The processes can be roughly divided into ion exchange and alteration of the total quantity of
cations. The most important factor for both processes is the ion content of the porewater, which isprincipally determined by the original cations in the exchange positions of the montmorillonite,
readily soluble minerals in the original bentonite material, and by the surrounding groundwater.
H+ ions are bound to the edge sites of clays (as well as to ion exchange positions at very low pH),which means that pH changes can occur due to surface chemical reactions. Owing to its amphotericproperties, i.e. ability to both receive and donate hydrogen ions, the montmorillonite can counteract
external changes in pH /­Wanner et al. 1992/.
Reversibility of sorption is a fundamental paradigm with regard to• quantitatively expressing solid/liquid partitioning of solutes through Kd values;• the use of Kd values in transport models (for dissolved radionuclides and any other solutes).
Therefore, the available evidence for the processes relevant for radionuclide sorption to clays– surface complexation and ion exchange – is examined in the following sections.
In the absence of any evidence to the contrary, a fundamental and widely accepted principle isthat cation exchange and surface complexation on clay and oxide-type minerals is reversible,
which follows directly from analogies with corresponding exchange and complexation reactions

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117

in homogeneous aqueous solutions; see e.g. /­Sposito 1981, 1984, Stumm and Morgan 1996/.
Accordingly, most studies concerning surface reactions on these minerals focus on sorption, whereasmuch fewer studies have been published that systematically investigate the degree of reversibility
in sorption experiments. On the other hand, the pertinent literature offers several examples wheresome type of irreversibility was observed. Below, in the Model studies/experimental studies section,
several examples are examined and their relevance with respect to radionuclide sorption on clays
is evaluated. While this discussion is not based on an exhaustive review of the literature, it showsthat sorption on clays and clay-like substrates is typically found to be reversible if the experimental
boundary conditions are carefully evaluated.
Dependencies between process and buffer variables

Table 3-12 shows how the process influences and is influenced by all buffer variables.
Table 3-12. Direct dependencies between the process “Sorption” and the defined buffer variables
and a short note on the handling in SR-Site.
Variable

Variable influence on process

Process influence on variable

Influence present?
(Yes/No)
Description

Handling of influence(How/Why not)Influence present?
(Yes/No)
Description

Buffer geometry

No(Total buffer mass isa parameter in the
RN-transport model)

No

Pore geometry

Possibly indirectly
through influence
on EDL properties

Possibly indirectly,
through influence on
EDL properties/swelling

Radiation intensity

Possibly indirectly,
through influence on
mineral properties

No

Temperature

Influence of temperature
on sorption must be
acknowledged, buteffect is not clear

No

Water content

Indirectly throughporewater compositionNo

Gas content

Indirectly throughporewater composition(CO2)

Possibly indirectly,
through influence of
Ca-exchange on
carbonate equilibria

Hydrovariables
Indirectly through(pressure and flows) porewater composition

Possibly indirectly,
through porewatercomposition/swelling

Stress state

Indirectly throughinfluence on porewatercomposition and EDL
properties

Indirectly throughinfluence on porewatercomposition and EDL
properties/swelling

Bentonitecomposition

Yes

Used for derivation
of Kd

Yes

Included in geochemicaltransport modelMontmorillonite
composition

Yes

Used for derivation
of Kd

Yes

Included in geochemicaltransport model

Porewater
composition

Yes

Used for derivation
of Kd

Yes

Included in RN-transportmodel (Section 3.5.6)
and geochemicaltransport model(Section 3.6.2)

Structural and straymaterials

No stray materials are
assumed to be present
within the buffer

118

Handling of influence(How/Why not)Indirectly throughinfluence on porewatercomposition and related
mineral equilibria

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Influence on buffer variables: Exchange reactions of major cations directly influence the surface
composition of the clay minerals as well as the porewater composition. In turn, changes in the porewater chemistry may induce further changes in the bentonite composition through mineral dissolution
and precipitation. On the other hand, the pore geometry, and thereby also the hydraulic conductivity,
are not expected to be significantly altered by ion exchange processes, because the volume of the bufferat reference density is so constrained that a maximum swelling of the bentonite cannot be achieved
even after complete conversion to the Ca-form (or in the presence of high ion concentrations in the pore­water). At saturated densities below 1,800 kg/m3, these processes may lead to a significant reductionin the swelling pressure due to osmotic effects and a suppression of the EDL, and to an increase in
the hydraulic conductivity (see Sections 3.4.1 and 3.5.8).
Influence of bentonite and porewater composition: The two most important variables with regard
to radionuclide sorption and exchange of major ions are the bentonite and porewater composition.
The mineralogical composition of bentonite is directly related to the number of sorption sites available
and their specific properties. The porewater composition is the result of groundwater-bentonite interaction (ion exchange, mineral equilibria) and, in turn, exerts a major influence on radionuclide sorption.
On the other hand, Kd is clearly independent of the solid/water ratio and, therefore, of the buffer
density (given that the solution composition remains constant). Under a given set of conditions,
sorption increases if the amount of bentonite (i.e. the solid/water ratio) is increased. By expressing
sorption through Kd (see below), which includes the solid/water ratio, this effect is already taken
into account. It appears from diffusion experiments that the apparent Kd is influenced by density, forexample the Kd for caesium is halved when the density of the water-saturated system changes from1,300 to 1,950 kg/m3. However, this can be entirely attributed to an increase of ionic strength causedby the dissolution of impurities from an increasing mass of bentonite, and is easily reproduced bysimple thermodynamic sorption models /­Ochs et al. 2001/. This effect is mostly relevant in the caseof dilute groundwaters.
Influence of temperature: Most experiments have been carried out at room temperature, and notenough sorption data as a function of temperature are available to clearly evaluate temperatureeffects. In general, a decrease of sorption with temperature would be expected, but some isolated
studies /­e.g. Lu et al. 2003/ indicate the opposite. Thus, an increase or decrease of the temperature
is potentially of importance for sorption, but the effects within the temperature range expected in
the repository are deemed to be covered with good margin by other uncertainties.
Boundary conditions

There are no particular boundary conditions to discuss for this process. The relevant boundary
conditions in order to treat the process quantitatively are those of the transport processes that control
the exchange of solutes between the buffer porewater and the water in its adjacent components, i.e.
the boundary conditions of the processes diffusion and advection.
Model studies/experimental studies

Ion exchange and surface complexation are well-established processes and quantitative thermodynamic models exist for calculation of equilibrium states. The relationship of these sorption processes
to the different surfaces available in montmorillonite has been discussed above.
Effect of sorption/exchange processes on buffer properties: Of primary relevance for the propertiesof the buffer is the exchange of major cations between groundwater and bentonite pores, as well as
pH-buffering by edge site protolysis equilibria. See also the process 3.5.6.

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119Effect of sorption on radionuclide transport: In assessing radionuclide transport from a repository,
sorption is normally described as a linear, equilibrium relationship between the sorbed concentrationand the concentration in solution, characterised by a distribution coefficient Kd , which is defined as: Cinit − Cequil  v 						(3-23)
Cequil m

Kd = q /­Cw = Kd = 
where:
Cinit

is the initial aqueous concentration of a key element [mol/m3] as measured in the reference
solutions without solids

Cequil is the final equilibrium aqueous concentration of this element [mol/m3]
Cw

is the concentration in solutionv

is the volume of the solution used for a batch experiment [m3]mis the mass of the solid phase used for a batch experiment [kg]qis the sorbed concentration as mass per weight unit of solid phase.

When the concentration of the species in solution is low (more specifically, at a state sufficiently far
from surface saturation), which is normally the case for radionuclides, the linear approximation isjustified. Kd can also be calculated in a straightforward fashion from the output of thermodynamic
sorption models:Kd [m3/kg] =stoichiometric sum of surface species of element X [mol / kg ]stoichiometric sum of solution species of element X mol / m3[]

(3-24)

Surface complexation models with parameters from well-controlled experiments that include
sufficient variation of key geochemical parameters can often be used to describe the dependenceof sorption on external parameters such as pH, etc, see e.g. examples in /­NEA 2001, 2005/.
Alkai and alkali earth element sorption to clays
With regard to the reversibility of cation exchange, /­Verburg and Baveye 1994/ published a criticalreview of a number of studies where hysteresis (i.e., irreversibility) had been observed in cationexchange experiments. In their analysis, /­Verburg and Baveye 1994/ come to the conclusion thatthe observed effects are related to hysteresis in clay swelling and aggregation in disperse laboratory
systems (formation of quasi-crystals when the supporting electrolyte cation is changed from e.g. Na
to Ca), rather than to the actual exchange process. Aggregation processes influence the exchange
behaviour of clays, and their kinetics are much slower than the kinetics of exchange reactions(roughly, weeks instead of seconds).
At this point, it is important to distinguish the conditions in classical exchange experiments fromthose relevant for the adsorption of traces:• The classical exchange experiments typical involve i) a complete change of the exchangeable
ions (e.g., conversion of Na-clay to Ca-clay) and, concomitantly, a change of the supporting
electrolyte solution (e.g. from NaCl to CaCl2).
• On the other hand, under PA-relevant conditions, traces of a radionuclide (Sr, Cs…) are exchanged
against other cations in a electrolyte solution of constant composition. Also, the exchanged trace
cations do not influence the surface composition of the exchanger. In this case, no change inthe clay’s aggregation state has to be expected.
A somewhat special case where reversibility has been questioned is the sorption of Cs to clays.
This is largely caused by a failure to distinguish between sorption on low.charge, expandable 2:1
clays (e. g. smectites) and high-charge (e.g. vermiculite) or non-expandable clays (e. g. chlorite or
illite). While it is well accepted that micaceous clays, such as illite, possess so-called ‘frayed edge
sites’ (FES) that are only accessible to few ions (Cs+, K+, protons) for steric reasons /­Sposito 1984,
Baeyens and Bradbury 2004/, the effect of changes in the structural properties of such clays areoften not taken into account. For example, /­Grütter et al. 1990/ demonstrated that sorption of Cs ions
may lead (at high surface loadings) to a collapse of the interlayer of vermiculite-like clays, trapping
sorbed Cs in the mineral structure and rendering desorption impossible. In the same study reversible
sorption was observed at low surface loadings (i.e., where no collapse of the interlayer occurs).
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There is also clear evidence that the above issues are specific to a few cations that easily loose their
hydration shell and are not relevant for other ions. In an interesting study, /­Cui and Eriksen 1995/
showed the reversibility of Sr(II) sorption as well as isotope exchange on several fractions of infill
material(feldspar, quartz, muscovite, Fe-chlorite, probably also other Fe-minerals) from the Stripa
mine, Sweden. Using reaction times between ca. 30–100 d they observed complete reversibility of Sr
sorption with regard to:• Sr concentration (no hysteresis for sorption and desorption isotherms covering 5 orders ofmagnitude in terms of Sr concentration);• pH change (from pH 4 to ca. 12.5 and back);• Sr isotope exchange.
Ni and Co sorption to clays/Scheidegger and Sparks 1996/ examined the sorption/surface precipitation and detachment of
Ni by pyrophyllite (a low-charge 2:1 clay mineral) as a function of time. Surface precipitation
was presumably caused by high dissolved Ni concentrations (speciation calculations based on thethermodynamic data by /­Hummel et al. 2002/ indicate that Ni was oversaturated by a factor of ca.
10 with respect to Ni(OH)2(s), /­Scheidegger and Sparks 1996/ assumed that their system was notoversaturated).
It was observed that Ni uptake was initially fast, followed by a slow process; overall equilibrium wasreached after ca. 200 h (pH 7.5). The fast and slow processes were attributed to surface complexationand surface precipitation, respectively. Desorption experiments were initiated by lowering the pHfrom 7.5 to 6 after equilibrated samples had been stored for an additional 1–8 weeks and showedthat.
• a few percent of Ni were removed within the first 2–3 h, presumably by desorbing from thesurface complexation sites;• the remaining Ni was removed much slower (based on the observed constant rate, complete
removal would need about 250 days), which was ascribed to the dissolution of a Ni-surface
precipitate (hypothesized as mixed Ni-Al-hydroxide);• in comparison to experiments where the reaction products were removed every 24 h, Ni detachment was significantly slower when the reaction products were not removed, probably because of
re-adsorption.
Overall, this shows that Ni adsorption onto clay surfaces is fast and reversible within similar
timeframes. Similarly, /­Bradbury and Baeyens 1997/ studied the sorption of trace concentrations of
Ni onto Na-montmorillonite at pH 4.5–10. They report that no kinetic effects were observed for reaction times between 1 and 18 days and further state that reversibility of Ni sorption was confirmed(without giving specific data).
For Co, /­O’Day et al. 1994/ observed (by EXAFS) multinuclear surface complexes of Co on kaolinite at high surface loading (just below complete monolayer coverage), whereas only mononuclear
surface complexes were observed at low coverage.
Time perspective

An ion exchange process is fast, so the dissolution rate of other minerals and the transport rate ingroundwater and bentonite pores will control ion exchange from Na+ to Ca2+ end member. Relevantchanges in buffer properties are expected to occur on the hundred-thousand-year scale /­Bruno et al.
1999/. It has to be pointed out further that modelling the conversion of Na- to Ca-bentonite by
implementing ion exchange in a mixing-tank type of model, as in /­Bruno et al. 1999/, is in all likelihood a pessimistic approach, in the sense that it overestimates the rate of the exchange process: A
certain cation concentration is required to compensate for the diffuse layer charge in the pore space/­Ochs et al. 2004/. While the maximum concentration of Ca is limited by the solubility of calcite
or gypsum, the concentration of Na is not limited, and it is doubtful that a full conversion to theCa-form is possible under these conditions.

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121

Sorption in the buffer is an important process for the retention of many radionuclides. The timescales
that are of interest are dependent on the half-life of the individual nuclide. If the travel time throughthe buffer is of the same order of magnitude or longer, sorption is of great importance for theamount released. If the travel time through the buffer, including retardation from sorption, is short incomparison to the half-life of the radionuclide, the sorption process is less important.
Natural analogues/observations in nature

Many bentonite deposits contain porewater with high salinities. Investigations of the effects of highsalinity in natural smectites exist but have not been utilised specifically for repository-related questions. Natural analogues also offer evidence that migration of sorbing radionuclides occurred only
over limited distances away from a source in clay-rich substrates /­Smellie and Karlsson 1996/.
Handling in the safety assessment SR-Site

Before saturation: The process is neglected.
After saturation: Ion exchange is included in the scenario-specific modelling of the buffer chemical
evolution for the thermal phase and the post-thermal long-term phase, see heading “Handling” forprocess 3.5.6.
Failed canister: Sorption is included in the modelling of radionuclide transport for the long-term
phase, see heading “Handling” for process 3.6.2. Sorption is treated through element-specific Kdvalues derived for specific conditions (see the SR-Site Data Report /­SKB 2010l/ and /­Ochs andTalerico 2004/ for the definition of data and associated uncertainties).
Concerning the treatment of the fundamental assumption of reversibility, brought up at the beginningof this section, can be confirmed if:• at least similar timeframes are considered for sorption and for desorption; but it seems that inmany cases, desorption will be somewhat slower than sorption;• there is a significant gradient or driving force for desorption (e.g. dilution, change in chemical
conditions, presence of a competing sorbent), with removal of desorption products being a
prerequisite for maintaining such gradients.
Exceptions to this are mainly reactions that lead to incorporation instead of sorption. For the case ofclays, the main mechanism would be the collapse of certain expandable 2:1 layer structures at highsurface loading. However, this is not relevant for smectite clays, due to their relatively small permanent layer charge. Based on the present information, there is no reason to assume irreversibility ofsorption reactions of radionuclides on clays under relevant conditions.
Boundary conditions: See sections referred to above.
Influences and couplings: Dependence on buffer properties as well as composition and pH of the porewater in the buffer, resulting from bentonite-groundwater interaction, is considered in selecting sorption
constants for radionuclide transport. Because of the conditional nature of sorption constants, the data
selected are derived for several sets of relevant safety assessment conditions. Temperature effects are
not considered explicitly, but the effects within the temperature range expected in the repository are
deemed to be covered with good margin by the uncertainties associated with the selected Kd values.
For the influence of ion exchange and surface acid-base equilibria on buffer chemical evolution, see
Section 3.5.6; these processes are also taken into account in the derivation of radionuclide Kd values.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Most general issues relevant for handling ion exchange of major ions and radionuclide sorption are
discussed in Sections 3.5.1 and 3.5.6.
All radionuclide-specific sorption data are qualified in the SR-Site Data Report /­SKB 2010l/ andmost data are taken from an SKB report /­Ochs and Talerico 2004/ specifically dedicated to thederivation of diffusion and sorption coefficients that are consistent with the specified buffer density

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and composition, as well as with the range of groundwater compositions considered in SR-Site.
Distribution coefficients were calculated by /­Ochs and Talerico 2004/ on the basis of porewatercompositions calculated for specific bentonite-groundwater interaction scenarios. As source data forthe derivation of scenario-specific distribution coefficients, a large amount of original experimental
data, published for the most part in peer-reviewed journal articles and proceedings, was evaluated.
For some radionuclides, calculations with thermodynamic sorption models were carried out forcomparison. In a follow-up peer-reviewed paper /­Ochs et al. 2006/, the approach used by /­Ochs andTalerico 2004/ was presented, together with a few selected datasets. /­Ochs and Talerico 2004/ wasalso peer-reviewed according to the instructions for the SR-Can assessment.
Uncertainties

Uncertainties in mechanistic understanding
Both conceptual understanding and a large quantity of measurement data exist for simplified
systems. Detailed reviews of sorption processes, underlying experimental data, model results, as wellas the relationship to performance assessment can be found in e.g. /­NEA 2001, 2005, Davis and Kent
1990/. In particular, it has been shown that thermodynamic sorption models developed for purified
Na-montmorillonite can be applied to Ca-montmorillonite /­Bradbury and Baeyens 1999/, and thatsuch models give results for compacted bentonite that are compatible with diffusion data /­Ochs
et al. 2003/. To date, there is no doubt that sorption processes are well understood in a quantitative
way and in terms of thermodynamic description, and that this understanding can be used to makeeducated predictions of Kd values to be used in safety assessment calculations. However, there is aclear lack of data for understanding and predicting the influence of some important variables (suchas dissolved carbonate concentration) on radionuclide sorption, and this has to be taken into accountin evaluating uncertainties of Kd values (SR-Site Data Report /­SKB 2010l/ and /­Ochs and Talerico
2004/). In the case of some radionuclides, the underlying thermodynamic database is insufficient toquantitatively evaluate their behaviour for different conditions.
Model simplification uncertainties in SR-Site
As pointed out above, the Kd concept for radionuclide transport modelling is basically justified underconditions far from surface saturation, which can be expected in the case of diffusion-limited radionuclide migration in a bentonite buffer. At the same time, it is clear that a Kd is a highly conditionalparameter in terms of chemical conditions (pH, ionic strength, etc) and has to be derived for each setof conditions. This was done in the case of data selection for the SR-Can assessment using a semiquantitative, traceable procedure to transfer Kd data from experimental to safety assessment-specificconditions /­Ochs and Talerico 2004/. The same approach will be used in SR-Site. For several
elements, it was possible to use state of the art thermodynamic sorption models to derive Kd values
for the specified conditions. The model-derived data were in all cases within the uncertainty rangeof the values that had been derived using the semi-quantitative conversion procedure. This strongly
suggests that for the specified conditions, use of a sorption model and direct use of recommended Kdvalues would lead to consistent results (i.e., use of Kd would not introduce additional uncertainties,
provided the conditions remain constant). To avoid potential problems related to the direct use of Kd
in cases where conditions may vary to some degree (or to avoid re-derivation for each condition),thermodynamic sorption models could be directly coupled with transport codes (see Section 3.5.6).
Again, for many radionuclides, not enough systematic sorption data for the relevant conditions areavailable to date to develop or sufficiently constrain thermodynamic models.
As long as the bentonite buffer can be expected to represent a homogeneous geochemical compartment, no spatial variability needs to be taken into account.
Input data and data uncertainties in SR-Site
Kd values are input directly to Site for use in consequence calculations. Kd values for radionuclides
for various conditions, as well as the associated uncertainties are given in /­Ochs and Talerico 2004/,
from which data for consequence calculations are chosen in the SR-Site Data Report /­SKB 2010l/.

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3.5.6

Alterations of impurities

Overview/general description

The buffer material consists not only of montmorillonite, but also accessory minerals as wellas impurities. The mineral composition of MX-80 and Deponit CA-N bentonites can be foundin Section 2.2.1. In the repository environment, these accessory minerals can be dissolved andsometimes re-precipitated depending on the prevailing conditions.
The behaviour of these bentonite accessory minerals and their effect on the overall performanceof the barrier depends on the interaction of groundwater with the bentonite buffer. The accessory
minerals in bentonite are either stable or metastable in the environment where they were originallymined. In the repository, however, they will be exposed to a fluid that differs in some respects fromthat at the original site, in terms of both composition and temperature. Most accessory minerals
in the bentonite are nevertheless stable in contact with the granitic groundwaters prevailing in the
Fennoscandian shield. The main potential effects on the bentonite buffer are cementation of the barrier (i.e. precipitation of large amounts of accessory minerals in specific parts of the buffer), changes
in the montmorillonite composition, especially the exchangeable ions that can affect the swelling
capacity of the bentonite and changes in the chemical composition of porewater, which can affect theretention capacity of the near-field system.
Moreover, evaporation of incoming groundwater during the saturation phase in the warmer parts
may lead to different concentrations of dissolved elements at different positions in the buffer and theconsequent redistribution of accessory minerals and changes in the montmorillonite composition,
resulting in an overall heterogeneous composition of the bentonite.
Another aspect of interest is the behaviour of the buffer in contact with the potential corrosion
products of the cast iron insert (see Section 3.5.10 Iron-bentonite interaction). Anoxic corrosion of
the cast iron insert will generate iron in solution and hydrogen, both diffusing into the bentonite.
Dissolved iron entering the bentonite can precipitate as new accessory minerals (.e.g. magnetite),
replace other major cations in the montmorillonite interlayer sites (cation exchange), or replace octahedral cations in the montmorillonite, which can result in montmorillonite transformation towards
iron-rich clay minerals (see Section 3.5.9 Montmorillonite transformation).
Dependencies between process and buffer variables

Table 3-13 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Buffer geometry: The geometry of the buffer can affect the alteration of accessory minerals
indirectly through transport of solutes in the groundwater and bentonite porewater, thus affecting
geochemical reactions during its interaction. Moreover, buffer geometry determines the total
proportion of accessory minerals able to react in the system due to the interaction of groundwaterand bentonite.
Pore geometry: The pore geometry of the bentonite buffer affects the diffusive transport of solutes,
which in turn affects the chemical composition of porewater at different points within the buffer and,
therefore, the geochemical reactions taking place in the buffer. In addition, precipitation-dissolutionof accessory minerals can modify the porosity distribution in the buffer as well as the pore geometry.
Radiation intensity: Only in the case of very high radiation fields; the chemical composition of
buffer porewater can be modified and thus affects geochemical reactions involving accessory minerals.
Temperature: Thermodynamic constants of chemical reactions are dependent on temperature. The
temperature gradient exerted by the waste canister will have a large impact on the dissolutionprecipitation behaviour of accessory minerals, especially in those areas of the bentonite close to thecanister. The effect of temperature on diffusion has not been considered.
Water content: The water content has an indirect effect on the alteration of accessory minerals.
As the saturation process affects the transport of solutes, this in turn affects the chemical reactions.
In addition, the coupled effect of temperature and saturation can result in significant changes in
the redistribution of accessory minerals and on the chemical composition of buffer porewater.
Precipitation-dissolution of accessory minerals can result in minor changes of porosity, and therefore
the water-solid ratio or available porosity for water uptake. However, this change in porosity is
expected to be very small.
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Table 3-13. Direct dependencies between the process “Alteration of impurities” and the definedbuffer variables and a short note on the handling in SR-Site.
Variable

Variable influence on process

Process influence on variable

Influence present?
(Yes/No)
Description

Handling of influence(How/Why not)Influence present?
(Yes/No)
Description

Buffer geometry

Yes, the total amountof minerals in the buffer
and the bentonite/porewater ratio affects
the process

The geometry and theNobentonite mass, aswell as the bentonite/porewater ratio is
considered in the model

Pore geometryYes. the ratio between Included in the initialaccessory minerals and conditions
porewater

Yes.

Radiation intensity

No

Only very high radiation
fields could influence
the processesNo

Temperature

Temperature strongly
influences the extent
of mineral – porewater
reactionsTemperature andNotemperature gradient isincluded in the chemical
model

Water content

Yes, could affectconcentrations

The saturation phasewill be considered

Gas content

An indirect effect fromwater content variations

Hydrovariables(pressure andflows)

Yes

Darcy flow is included in No
the model

Stress state

Yes

Partial pressures ofgases are included in
model

Yes

The results from themodel are evaluatedto consider effects on
swelling pressure

Bentonite composi- Yestion

Compositional data areconsidered in modelYes

The evolution of thecomposition is calculatedwith the modelMontmorillonite
composition

Yes

Composition al data areconsidered in modelYes

The evolution of thecomposition is calculatedby the model

Porewater composition

Yes

Compositional data areconsidered in modelYes

The evolution of thecomposition is calculatedby the model

Structural and stray Potentially, The
materials
degradation of thesematerials can affectthe chemical evolutionof the system

No stray materials are
considered in the buffer.
However, effects of othersystem components are
considered

Potentially, Secondary
Secondary mineralmineral formation/formation/dissolution
dissolution may influence is considered
the alteration of structuraland stray materials

Precipitation-dissolutionof minerals

Handling of influence(How/Why not)Variation in porosity is
considered through theeffect of “cementation”,
which is included in
the model

Precipitation-dissolutionof accessory minerals
modifies the porosity

The effect on the overall
porosity is small and notaccounted for

Yes

Dissolved gases areincluded in the modelGas content: The formation of a gas phase (i.e. water vapour due to high temperature close to thecanister) can modify the concentration of solutes in the porewater, and therefore the stability ofaccessory minerals. However, this process is only expected for the water saturation stage. Other
gases are not expected to affect the behaviour of accessory minerals significantly. The formation of
H2(g) can affect the chemical evolution of the system only in the case of anoxic corrosion of the castiron insert or other steel components in the system.
Hydrovariables (pressure and flows): Transport of dissolved chemical species in the system can
modify the dissolution-precipitation rates of accessory minerals by maintaining the saturation stateof these phases far from equilibrium (i.e. fast water flows around the buffer).
Stress state: Increase in the partial pressure of gases can affect the stress in the system. The build up
of such partial pressures could be a direct consequence of the alteration of buffer impurities.

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Bentonite composition: This is one of the most relevant variables influencing the alteration of
impurities. The type and amount of accessory minerals (in addition to the montmorillonite content)
determines how their dissolution-precipitation behaviour will affect the chemical evolution of thesystem. Moreover, the dissolution-precipitation of these mineral phases will modify the bentonite
composition through their interaction with porewater.
Montmorillonite composition: The composition of montmorillonite, especially the cationexchangeable (type of cations and occupancy) is very relevant, as interaction with buffer porewater
will modify the chemical composition of this porewater and, hence the dissolution-precipitation ofaccessory minerals in the bentonite. In addition, the alteration of bentonite impurities will modify
the chemical composition of bentonite porewater and hence the montmorillonite composition.
Porewater composition: The porewater composition will determine which of the accessory minerals
in the bentonite will dissolve or precipitate.
Structural and stray materials: The interaction of bentonite porewater with structural and straymaterials can result in modification of the chemical composition of porewater and, therefore, inthe behaviour of accessory minerals and vice versa.
Boundary conditions

There are no particular boundary conditions to discuss for these processes. The relevant boundary
conditions in order to treat the process quantitatively are those of the transport processes that control
the exchange of solutes between the buffer porewater and the water in its adjacent components, i.e.
the boundary conditions of the processes diffusion and advection.
The exchange of a number of solutes such as sodium and calcium ions, carbonate and oxygen isrelevant to the alteration of impurities.
Model studies/experimental studies

Experimental studies regarding cementation
Hydrothermal tests with purified standard bentonite (SWY-1) heated to 150–200°C have shown thatcooling leads to precipitation of silicon compounds in various forms. The precipitation is assumed
to cause cementation effects, including a strength increase, which has been demonstrated in several
laboratory investigations /­Pusch and Karnland 1988/.
According to these investigations, the extent of precipitation is dependent on the temperature. At thehighest temperature in the buffer (90°C), cementation was not of such a great extent that it could be
regarded as problematic. The tests were carried out without a temperature gradient, which cannot be
regarded as pessimistic conditions.
Precipitation of sulphate and carbonate could be observed in one-year experiments with hydrothermal
treatment of MX-80. XRD analyses indicated that sulphates and calcite had gone into solution andhad been transported to the hot iron surface, where they had precipitated. Based on a geochemical
model /­Pusch et al. 1993a/, it is likely that feldspars dissolved and that quartz was enriched at the colder
boundary. In the Buffer Mass Test in Stripa (max 90°C), the buffer was analysed with respect tothe distribution of silica, but no definite conclusion could be drawn regarding possible enrichment in
the coldest part /­Pusch 1985/. However, the study of cooling-related precipitation of silica predictedby the Grindrod/Takase/Pusch model was confirmed by the Kinnekulle case /­Pusch et al. 1998/.
In the LOT tests at Äspö HRL, buffer material is exposed to high temperature gradients (maximum
130 to 80°C over a 10 cm distance). Material from the one year tests was analysed at 25 positions
with respect to elemental distribution and rheological properties. No significant gradients were
found for silica content. The tensile strength was slightly reduced and the tensile strain was slightly
increased, which shows that no cementation had taken place in the major part of the buffer material.
However, precipitation of (mainly) anhydrite was found in a 1–2 mm thick rim around the copper
tube. The ongoing evaluation of the five-year tests /­Karnland et al. 2009/ is expected to provide
information on whether the precipitation is an effect of evaporation of the inflowing water as partof the saturation process, or if the process also continues after full water saturation.

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Several laboratory investigations have been carried out for the purpose of studying the distributionof readily soluble ions in the pore space of the buffer after water uptake against a temperature
gradient. Effects of clay density, degree of saturation, testing time, open or closed conditions, ion
content and pressure in the ambient solution have been investigated. The experimental results showthat the enrichment of solutes is insignificant in specimens with high clay density, and that a highoriginal water content and high ambient water pressure reduce the enrichment in specimens withlower density. Similar studies are currently being conducted in field tests at Äspö (the LOT project)/­Karnland et al. 2000/ and /­Karnland et al. 2009/, and in laboratory experiments under the auspices
of ENRESA.
Geochemical models
There are several types of models describing bentonite – porewater interaction:
1) Donnan equilibrium (osmosis) models, considering the entire layer charge (CEC) to be available
for influencing solution chemistry; i.e. the CEC is balanced entirely by cations in solution. Claypores are assumed to contain no bulk water at all. See Section 3.5.8 on osmosis.
2) Diffuse layer (EDL) models, where the CEC is assumed to be partly compensated by cations
immobilised at ion exchange sites (giving rise to sorption of alkali and alkali earth elements) andpartly by ions in a diffuse layer extending from the layer surface. Bulk solution in the pores can
exist at low density and/or high ionic strength /­Wersin et al. 2004, Ochs and Talerico 2004/.
3) The ion exchange models used in the context of calculating clay alterations assume that the CEC
is completely balanced by exchangeable cations. As a result, the layer surface is electricallyneutral to the contacting solution, and bulk water can exist between clay platelets.
Models considering the latter case (ion exchange models) have been reviewed in this section.
Several geochemical models have been developed in order to predict the long-term behaviour of thebuffer under repository conditions. Some of the models are based on a mixing tank concept /­Wanneret al. 1992, Bruno et al. 1999/ where the thermal stage has not been considered. Other models arereactive transport models where the thermal stage is considered /­Arcos et al. 2000/. All these models
suggest that cation exchange is the dominant process in the buffer resulting in direct control on
calcite precipitation-dissolution, thus buffering the pH of the system. Also, the effect of protonationdeprotonation of montmorillonite edge sites can play a role in pH buffering. However, earlier models/­Bruno et al. 1999, Arcos et al. 2000/ indicate that calcite equilibrium is the main process buffering pH,
and only if calcite is exhausted will edge sites be able to buffer pH in the system. This is in agreementwith experimental data obtained in the frame of the EC integrated project NF-PRO /­Muurinen andCarlsson 2007, Arcos et al. 2008/, where compacted bentonite was put in contact with an external
solution at high-pH and pH was measured in the bentonite at different distances from the bentonite –external water interface. The data showed that high pH values in porewater were only recorded near
the interface, but pH in this zone rapidly decreased over 100 days, indicating that a chemical reactionin the bentonite is buffering the pH. Unfortunately, no information was provided on the type ofreaction buffering the pH; the most likely reactions are either precipitation of calcite or protonationdeprotonation reactions on the montmorillonite surface.
The redox buffering capacity of the bentonite is more controversial. Some of the models predict thatredox buffering is due to pyrite oxidation and iron (III) hydroxide precipitation; whereas other models
point to iron carbonate (siderite) dissolution and iron (III) oxy-hydroxides as controlling the redox
buffering capacity of the system. Experimental data obtained in the frame of the integrated projectNF-PRO /­Muurinen and Carlsson 2007, Arcos et al. 2008/ indicate that there is a fast and reversible
redox buffering reaction occurring in the bentonite. When compacted bentonite was placed in contact
with an external solution under aerobic conditions, higher Eh values were only recorded in bentonite
near the contact with the external solution. Moreover, when changing the aerobic external water
for another under anaerobic conditions, Eh values in the most external part of the bentonite rapidly
decrease to initial values, equivalent to those in the rest of the bentonite block. These results do notallow the chemical reaction responsible for such a redox buffering effect to be identified, although
the fast response of the system to changes in redox external conditions indicates that pyrite-involved
reactions can be disregarded, whereas an additional process related to the oxidation-reduction of ironin the octahedral sites of montmorillonite has to be considered and investigated /­Anastácio et al. 2008/.

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127

An additional geochemical model for use with the LOT experiment at Äspö /­Arcos et al. 2003/ hasbeen developed and has generated results similar to those obtained in previous models. Unfortunately,
there are still no experimental data to compare with. It should also be noted that when comparing the
new modelling results with experimental data /­Domènech et al. 2004/ some disagreements have been
observed. It is likely that these differences are due to the uncertainty associated with the initial conditions selected for the model. The initial conditions for the model are being revised in the frameworkof the LOT project.
The integrated geochemical evolution of the buffer was modelled in the SR-Can assessment /­Arcoset al. 2006/. An example of the results can be found in Figure 3-18.
Natural analogues/observations in nature

Development of salt crusts in arid regions resembles in some respects certain conditions during thewater saturation phase. Specific SKB-related studies have not been conducted, but a large body ofliterature exists on the subject. The information from these natural systems could be of interest to
support information on the processes occurring during the saturation phase, as indicators of the typeof minerals that can potentially precipitate in the bentonite.
Silica precipitation occurs at a large number of locations where hydrothermal transformation of
bentonite has taken place. Numerous scientific articles have been published, some directly linked torepository questions, see for example /­Pusch et al. 1998/.
SKB was involved in the study of a bentonite natural analogue in Spain, the BARRA Project, under
the coordination of ENRESA. Two processes were studied in this natural system: 1) changes in the
bentonite induced by long time exposure to a very high salinity environment, and 2) the thermalpH

Gypsum (moles/Lw)

0.38

7.15
7.14

0.25

7.13
0.13

7.11

0.00

7.10

Calcite (moles/Lw)

CaX2 occupancy (moles/Lw)

2.50

0.52

2.25

0.49

2.00

0.46

1.75

0.43

1.50

0.40

Figure 3-18. Graphics from SR-Can showing the predicted evolution in the simulated domain (Deponit
CA-N bentonite) for pH, calcite and gypsum content and calcium occupancy in the cation exchanger ofmontmorillonite after 10,000 years of simulation. Groundwater is simulated to contact the buffer through a
conductive fracture perpendicular to the deposition hole (units in moles per litre of porewater). Note that as
gypsum dissolves and calcium occupancy in the exchanger is depleted calcite precipitates and pH slightly
decreases /­Arcos et al. 2006/

128

TR-10-47

effect induced on bentonite properties by a sub-volcanic intrusion and its interaction with iron-richfluids. The initial results from this study indicate that exposure to very high salinities does not alter
the properties of the bentonite. The interaction of bentonite with iron-rich fluids at a relatively hightemperature (~100 ºC) resulted in changes to the smectite composition, which shifted to iron-richmembers due to octahedral substitution /­Pérez del Villar et al. 2005/. These studies of natural systems
can give very valuable information on the long-term effect on the bentonite-porewater system ofseveral processes that are likely to occur in the repository system, thus validating the geochemical
models based on laboratory experiments.
Time perspective

Chemical precipitation/dissolution processes are likely to be most important at an early stage ofrepository evolution when temperature and temperature gradients are high and oxygen remains in therepository tunnels. At later stages, most of the processes approach equilibrium. The accessory minerals in the bentonite are, however, also important over timescales of hundreds of thousands of years
if the chemistry in the repository should be disturbed for some reason; both pH and Eh are bufferedeffectively by the accessory minerals that are common in different bentonite materials.
Handling in the safety assessment SR-Site

Model
Before saturation: Geochemical processes will be the same both before and after saturation phases.
This is just a matter of water flow and solute transport, although the extent of the reactions (dissolution/precipitation) will be different.
After saturation: The processes are included in the scenario-specific modelling of the buffer chemical evolution for the thermal phase and the post-thermal long-term phase. This modelling includes,
in addition to alterations of impurities, the processes of diffusion (Section 3.5.3), ion exchange(Section 3.5.5), aqueous speciation and reactions (Section 3.5.7). The initial intrusion of groundwater during buffer saturation should be considered in formulating initial conditions.. The main model
will be a 3D reactive transport model based on the geochemical models of the LOT experiment/­Arcos et al. 2003, Domènech et al. 2004/. The model handles advective flow in a fracture in granite
intersecting the deposition hole, diffusive transport in the bentonite, dissolution-precipitation of themain bentonite accessory minerals, cation exchange in the bentonite, protonation-deprotonation
reactions in the smectite fraction and precipitation of potential secondary phases in both bentonite
and the granitic fracture. Prior to application in SR-Site, the reactive transport model was validatedby comparing its results with experimental data from the LOT test.
The following is a description of the main geochemical processes considered in the alteration of
impurities and how they are handled within the safety assessment SR-Site.
Precipitation of carbonates and sulphates in the areas nearest to the canisters may lead to theformation of a porous contact zone between the buffer and the canister surface and an increase in
the strength of the buffer. When the thermal period is over and temperature gradients no longer
exist to any substantial extent in the buffer, it is likely that the previously precipitated solids will be
re-dissolved to some degree and then diffuse in ionic form through the buffer. Precipitation of silica
may reduce the swelling potential, as well as decrease bentonite porosity.
The carbonate and pyrite contents are of decisive importance for determination of pH, Eh andalkalinity in the near field. Bentonite also contains small quantities of gypsum (CaSO4·2H2O),
which may be of importance at an early phase during the high temperature transient.
Calcite dissolution: Calcite is stable in groundwater. In the bentonite, however, the ion exchange
process will compete for free calcium ions (see also Section 3.5.5). The reaction
2XNa + Ca2+ ↔ X2Ca + 2Na+competes with
Ca2+ + HCO3− ↔ CaCO3(s) + H+

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The carbonate content of the groundwaters that may be encountered is sufficiently low for the ion
exchange process to dominate and calcite will be dissolved. This will entail a net proton consumption, i.e. an increase in pH. The water flux in the buffer is very low, and the reactions are close toequilibrium.
Siderite dissolution and iron hydroxide precipitation: The presence of pure siderite or ankerite, amixed calcium-iron carbonate, is of paramount importance for redox control of the system throughthe following reaction
FeCO3(s) + ¼O2(aq) + 2½H2O ↔ Fe(OH)3(s) + HCO3– + H+
Siderite oxidation leads to an increase of iron in solution allowing the precipitation of iron (III)hydroxides and thus controlling the redox of the system. However, during the early stage, when hightemperatures are expected in the buffer, hematite would be the stable iron (III) phase instead of iron(III) hydroxide, leading to the precipitation of hematite close to the canister and increasing the strength/
stiffness of the buffer.
Pyrite oxidation/dissolution: Pyrite is also stable in groundwater. However, penetrating oxygenatedwater can oxidise pyrite in accordance with
2 FeS2(s) + 7½O2(aq) + 7H2O ↔ 2Fe(OH)3(s) + 4SO42– + 8H+
Pyrite oxidation releases protons and can thereby lower the pH, but this is buffered by the dissolutionof calcite previously discussed. There is enough pyrite in the buffer to consume all initial oxygen left
after closure of the repository. The air content in the bentonite buffer is 12% by volume /­SKB 2004/,
corresponding to 1.9 moles of oxygen gas per cubic metre of bentonite, thus 0.51 moles of pyrite in
1 m3 of bentonite are needed to consume all this oxygen, which is equivalent to a pyrite content of
0.004% by weight, which is 20 times lower than the pyrite content of MX-80 bentonite or 125 times
lower than the pyrite content of Deponit-CaN bentonite. The quantity is also sufficient to prevent any
penetrating oxygenated water from coming into contact with the canister for hundreds of thousandsof years. However, as this process is kinetically driven, it is likely that pyrite oxidation exerts aminor effect on the redox of the system, which will be controlled by the iron carbonate dissolution
process.
Pyrite is a source of sulphide that could act as a corrodant on the copper canister. Under reducing
conditions pyrite dissolves as:
FeS2(s)+H+ ↔Fe2+ + HS– + S0
The concentration of HS– is an important parameter for canister corrosion.
Dissolution of calcium sulphates: Calcium sulphates (gypsum and anhydrite) and calcite have lower
solubility at high temperatures. At an early stage, when the canister temperature is high, it is possiblethat they will be dissolved in the colder portion of the buffer and precipitate near the canister surface.
Precipitation/dissolution of silica: The main impurities in the bentonite are quartz and feldspars.
Feldspars are not stable in the repository environment, although the kinetically driven dissolution isa relatively slow process that is accompanied by the precipitation (or replacement) of clay minerals,
i.e. kaolinite. The low water flux in the buffer is likely to minimise the kinetic dissolution of feldspars.
Quartz is normally stable in the natural repository environment, but its solubility increases withincreasing temperature. Silica will dissolve due to high temperature close to the canister and will betransported by diffusion outwards into the colder parts where precipitation may take place. In the
Buffer Mass Test in Stripa, the buffer was analysed with respect to the distribution of silicon, but nodefinite conclusion could be drawn regarding possible enrichment in the coldest part /­Pusch 1985/.
Failed canister: In the case of a failed canister, vast amounts of metallic iron, originating from thecorroding cast iron canister insert, will be added to the geochemical system. This was not includedin the model mentioned above, although it is envisaged to be included in the modelling in SR-Site.
The reaction between iron and bentonite is also discussed in Section 3.5.10.

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Boundary conditions: Main model: The diffusive transfer of species between the flowing water andthe bentonite is modelled explicitly. The near-field evolution model uses the Qeq concept to handle theboundary conditions. This is sufficient to estimate a general timescale on which the evolution occurs.
Handling of variables influencing this process: The modelling is in itself a coupling of the four
(groups of) processes mentioned above and is thus a coupled modelling of most of the significant
long-term chemical processes. A large number of the variable influences related to the chemical
conditions (porewater composition, montmorillonite composition, bentonite composition) are also
included.
Handling of variables influenced by the process: The geochemical conditions in the near-field asa function of time and space will be calculated.
Changes in the accessory mineral content of the buffer as a consequence of the thermal stage has
been modelled, but not validated through a comparison with experimental and/or natural analogue
data. This makes it difficult to ensure that changes due to the thermal stage are minor or that theseresults should be assumed to be the same as the initial conditions for the low temperature stage.
In addition to the processes discussed above, the behaviour of silica and clay minerals and theireffect on cementation will be addressed by performing some scoping calculations considering thepresent knowledge of kinetic dissolution-precipitation of smectite, silica-phases and the precipitationof alternative clay minerals (kaolinite and/or illite). These scoping calculations will give somequalitative information on the relative importance of these processes and the effect on bentonite
properties, e.g. porosity changes. See also Section 3.5.9.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Most references cited are peer reviewed papers. /­SKB 2004/ is a Quality Assured report. The restof the SKB reports cited describe previous experimental studies /­Karnland et al. 2000, Pusch 1985,
Pusch and Karnland 1988, Pusch et al.1992, 1998/ or models /­Wanner et al. 1992, Bruno et al. 1999,
Arcos et al. 2000/. Only the reference to the redox buffering process in the bentonite /­Muurinen
and Carlsson 2007/ has been considered for the handling in the safety assessment SR-Site. This is adeliverable from the EU integrated project NF-PRO. However, the results from this report have been
published in another peer reviewed paper cited here /­Arcos et al. 2008/, containing references toother peer reviewed papers on the experimental methodology.
Uncertainties

Uncertainties in mechanistic understanding
Some critical uncertainties remain concerning the mechanistic understanding of the processes that
control the redox state of the bentonite system. It is not clear yet to what extent pyrite and/or siderite
are the main redox controlling phases, although either process, or their combination, will scavenge
the remaining oxygen in the near field. This should be discerned by comparative modelling of pyriteand siderite kinetic availability.
In addition, most of the secondary precipitation/dissolution processes that are temperature driven
are thermodynamically controlled. Hence, there is no need for a detailed mechanistic understanding.
However, some of the silicate transformations are kinetically controlled and their mechanistic
understanding in compacted bentonite conditions is poor. This is mainly related to clay mineraltransformations (dissolution of smectite and precipitation of illite and/or kaolinite). Although thesereactions occur at a very slow rate, their effect on the overall geochemical evolution is not yet clear.
Most of the conceivable processes are in themselves well known and can be modelled for lesscomplex systems. However, conditions in the buffer, as far as transport and reaction kinetics areconcerned, are not fully understood for all processes. Transport of solutes can take place in different
forms and cannot be fully described at present. In particular, modelling problems remain during thewater saturation phase, when water is transported both in vapour and liquid form. Modelling of thesystem must therefore be coupled to the THM processes that proceed in parallel with transport andreaction kinetics.

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Silicon is the most common element in the buffer and occurs in a number of different minerals andphases. The following processes are not fully understood:• release of silica by dissolution from different minerals,• transport of dissolved silica driven by the prevailing temperature gradient,• precipitation of silica minerals.
Precipitation of silica is particularly complex, since it is dependent on several interacting factors andsince several conceivable forms occur, both crystalline and amorphous.
The greatest uncertainty concerns the scope for cementation as a consequence of dissolution,transport and precipitation of silica or aluminosilicate minerals. The scope and consequences of
cementation cannot be predicted with reasonable certainty at present.
Finally, it is assumed that pH is buffered by equilibrium with calcite. However, the amount (or eventhe presence) of this mineral phase is somewhat uncertain, depending on the bentonite type selected
for the buffer, thus the pH buffering capacity of the clay barrier could be affected.
Model simplification uncertainties in SR-Site
One of the major uncertainties concerns the conceptual model of the behaviour of porewater in theinterlayer space of the montmorillonite. As stated previously, there are several conceptual models
regarding the way the bentonite – porewater is considered (Donnan equilibrium models, diffuse layer
models and ion exchange models). The effect of such different models on the geochemical evolutionof the system will be evaluated.
Secondary dissolution and precipitation processes have not been fully implemented in a coupledmodel of buffer evolution. The processes are only considered when required for the definition ofcritical master variables in the system (pH and pe). One possibility could be to couple the THM
processes occurring with the key chemical processes that will control the geochemical evolution of
the buffer system.
There are a number of accessory minerals, such as silicates, which are not included in the models, as
most tend to dissolve at a very slow kinetic rate. Other silicates (usually clay minerals) precipitate
at similarly slow rates. However, the dissolution-precipitation processes involving these two groupsof minerals could affect the stability of montmorillonite as well as the chemical compositionof the buffer porewater, thus affecting the chemical long-term evolution of the buffer. See also
Section 3.5.9.
There is no model to account for the dissolution/precipitation of accessory minerals during the water
saturation stage of the buffer. As water flux will be faster than when the buffer is water saturated
and the buffer composition will be far from equilibrium with incoming water; some changes will
occur faster. These changes will account for step compositional gradients in the buffer and changes
in porosity as accessory minerals dissolve and/or precipitate, leading to changes in the hydraulicproperties of the buffer. Thus, modelling of the system must be coupled to the THM processes thatproceed in parallel with transport and chemical reactions. A submodel dealing with the saturation
stage of the bentonite will, at least, give an indication of the magnitude of these changes.
Input data and data uncertainties in SR-Site
Key thermodynamic data are well established and thought to be reliable. The main uncertainties
remain in the definition of the kinetically controlled processes. These are linked to the mechanistic
uncertainties mentioned previously.
Reliable data for surface reactions (i.e. cation exchange and edge site reactions) are of paramountimportance for the correct assessment of bentonite behaviour. Two types of information are needed
for geochemical modelling: i) Cation occupancy of the CEC and the protonation state of the smectite
surface, and ii) Exchange and protonation-deprotonation constants. The main uncertainty associatedwith these data is related to the experimental conditions used for their determination (i.e. low bentonite density and high solid/liquid ratios). Recent experiments have addressed this problem, but there is132

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still a large uncertainty related to the determination of cation exchange constants and their variation
as a function of bentonite density. These data and the associated uncertainties will be handled tosome extent in the SR-Site data report.

3.5.7

Aqueous speciation and reactions

Overview/general description

When the buffer is saturated, the water content is about 40 vol%. The composition of this water will
be determined by the composition of the porewater present at deposition, the composition of groundwater at the site, reactions with the buffer and canister material and the impurities in the bentonite.
Within the aqueous phase, chemical reactions will take place and the final composition of the water
can be determined by thermodynamic and kinetic calculations. Usually aqueous speciation reactions
are fast, i.e. controlled by thermodynamics. Examples are reactions such as :HCO3− ↔ CO32–+ H+ and HS–+H+ ↔ H2S(aq)Dependencies between process and buffer variables

Table 3-14 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Boundary conditions

By definition, this process takes place in the porewater. The boundary conditions will be the canister,
the groundwater, the bentonite and the impurities.
Model studies/experimental studies

Direct measurements of the porewater composition in compacted bentonite are virtually impossible.
Limited amounts of water can be squeezed out under high pressure, but the composition of that water
may not be representative of the porewater in situ. The concentration of non-sorbing elements withhigh solubility can be measured by total dispersion of the bentonite sample in water. Still, this only
gives the total amount and reveals nothing about the speciation in the bentonite.
Modelling of the chemical composition/speciation of the porewater is done with an integrated modelfor the chemical evolution in the near field. This is described in Section 3.5.6. Speciation of radionuclides is discussed in the process report for the Fuel and canister /­SKB 2010k/ (process Speciation ofradionuclides).
Natural analogues/observations in nature

Not applicable, since natural analogues are usually studied for water rock interactions and not for thewater phase alone.
Time perspective

The process is important for all repository timescales.
Handling in the safety assessment SR-Site

Before saturation: Geochemical processes will be the same before and after saturation. This is justa matter of water flow and solute transport, something to be considered in other sections.
After saturation: The process is included in the scenario-specific modelling of the buffer chemical
evolution for the thermal phase and the post-thermal long-term phase. This is described in Section 3.5.6.
Failed canister: In the case of a failed canister, vast amounts of metallic iron, originating from the castiron canister insert, will be added to the geochemical system. This will be included in the model
mentioned above, as a dedicated calculation case.

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Table 3-14. Direct dependencies between the process “Aqueous speciation and reactions” andthe defined buffer variables and a short note on the handling in SR-Site.
Variable

Variable influence on process

Process influence on variable

Influence present?
(Yes/No)
Description

Handling of influence(How/Why not)Influence present?
(Yes/No)
Description

Buffer geometry

Yes, the total amountof minerals in the buffer
and the bentonite/porewater ratio affects
the process

The geometry and theNobentonite mass, aswell as the bentonite/porewater ratio is
considered in the model

Pore geometryYes. the ratio between Included in the initialaccessory minerals and conditions
porewater

Radiation intensity

No

Only very high radiation No
fields could influence
the processes

Temperature

Temperature strongly
influences the extent
of mineral – porewater
reactionsTemperature andtemperature gradientis included in thechemical model

No

Water content

Yes, could affectconcentrations

The saturation phasewill be considered

Precipitation-dissolution of accessory
minerals modifies theporosity

The effect on the overall
porosity is small andnot accounted for

Gas content

An indirect effect fromwater content variations

Yes

Dissolved gases areincluded in the model

Yes.
Precipitation-dissolution of minerals

Handling of influence(How/Why not)Variation in porosity
is considered throughthe effect of “cementation”, which is includedin the model

Hydrovariables
Yes(pressure and flows)

Darcy flow is includedin the modelNo

Stress state

Yes

Partial pressures ofgases are included in
model

Yes

The results from themodel are evaluatedto consider effects on
swelling pressure

Bentonitecomposition

Yes

Compositional data areconsidered in modelYes

The evolution of
the compositionis calculated withthe model

Montmorillonite
composition

Yes

Composition al data are Yes
considered in model

The evolution of
the composition is
calculated by the model

Porewater
composition

Yes

Compositional data areconsidered in modelYes

The evolution of
the composition is
calculated by the model

Structural and straymaterials

Potentially, Thedegradation of thesematerials can affectthe chemical evolutionof the system

No stray materials
are considered in
the buffer. However,effects of other systemcomponents are
considered

Potentially, Secondary Secondary mineralmineral formation/formation/dissolution
dissolution may
is considered
influence the alterationof structural and straymaterials

134

TR-10-47

Boundary conditions: See Sections 3.5.6 and 3.5.10.
Handling of variables influencing this process: The modelling is in itself a coupling of the four
(groups of) processes mentioned above and thus a coupled modelling of most of the significant
long-term chemical processes. A large number of the variable influences related to the chemical
conditions (porewater composition, montmorillonite composition, and bentonite composition)
are also included.
Handling of variables influenced by the process: The chemical composition of the porewater iscalculated. The result may have influence on the processes osmosis (3.5.8) and montmorillonite
alteration (Section 3.5.9).
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

See Section 3.5.6.
Uncertainties

Uncertainties in mechanistic understanding
The model assumes that the chemistry of the porewater is identical to the chemistry in free water.
The effect of the bentonite surface and structure is not included (except ion-exchange). It is not clear
if or how this will affect the chemistry. See also Section 3.5.8.
Model simplification uncertainties in SR-Site
See Section 3.5.8.
Input data and data uncertainties in SR-Site
See Section 3.5.8.

3.5.8

OsmosisOverview/general description

A classical view of osmosis, as shown in Figure 3-19, may be used in order to illustrate the conditions in a bentonite buffer. The two sides of the water system are divided by a semi-permeable
membrane permeable only to water molecules. If salt is added to the left-hand side, the water activity
is reduced and transport of water from right to left will take place. The water table on the left willrise and a hydrostatic pressure develops (a). If salt is now added to the right-hand side, there will bea levelling of the water tables on both sides (b). If the initial volume change on the left-hand side isprohibited by an external force, pressure will rise in the salt solution (c), corresponding to the previous hydrostatic pressure. Introduction of salt into the right hand side of the membrane will lead to adrop in pressure. A complete loss of pressure will evidently take place when the two concentrations
become equal (d).
By analogy, assumes that the left-hand side of the system represents the bentonite buffer and theright-hand side an external groundwater solution (Figure 3-20). An external solution would reduce
the pressure produced by the clay, to an extent equal to the osmotic pressure of the solution, if noions could pass into the clay. A sodium chloride concentration of around 1.7 M (~10% by weight)
would then result in a complete loss of swelling pressure in a KBS-3 buffer. On the other hand, if
ions could pass freely into the clay, without restriction, then the final conditions would be equal concentrations on both sides and no effect on swelling pressure would be found. Laboratory experiments
show that neither of these conditions prevail and that there is a systematic reduction of swellingpressure, which indicates that ions from the external solution do pass into the clay to a certain extent/­Karnland et al. 2005/.
The conditions in the bentonite/groundwater system are typical of what is often referred to as a
Donnan equilibrium system /­Donnan 1911/. The equilibrium is governed by the fact that some, but
not all, ions can pass between the system parts, usually because of their large size. The condition

TR-10-47

135

F

a

b

F=0

c

d

Figure 3-19. The basic principles of osmosis as an analogy of bentonite swelling pressure /­Karnland et al.
2005/.

F
clay

electrolyte

Figure 3-20. Principle drawing of the clay-water-ion system. Clay mineral flakes and charge compensating
cations on the left side, and sodium and chloride ions on both sides of a semi-permeable membrane (bufferrock interface), which is permeable to dissolved ions and water, but not to the individual montmorillonite
layers /­Karnland et al. 2005/.

leads to prevailing concentration differences between the two parts, including the ions which canmove freely in the system. In a repository, the individual negatively charged montmorillonite minerallayers have a restricted mobility because of their relatively large size compared to the mean poresize; see Section 3.5.3 while the charge compensating cations and excess electrolyte ions may movein the bentonite/groundwater system (Figure 3-20).
The condition for equilibrium is that the electrochemical potential is equal in all system parts for allions that can move. Since the negatively charged macro-sized mineral layers are immobile, equilibrium will be established between ions in the external solution and charge compensating cations in
the montmorillonite. Ideally, for a sodium chloride solution and sodium montmorillonite these will
be only Na+ and Cl–. The condition of electrical neutrality in the entire system is:
0 = z · Cm + CCl– – CNa+								(3-25)
where z is the valence of the macromolecule, Cm, CNa+ and CCl– are the concentration of the macromolecules, sodium ions and chloride ions, respectively. In terms of ion activity, the equilibrium
condition for the diffusible ions is:{Na+c}{Cl–c} = {Na+e}{Cl–e} 							(3-26)
where the indices c and e refer to the clay-water system and to the surrounding electrolyte, respectively. The {Na+c} factor is the sum of the activities of original charge compensating ions {Na+cc}
and introduced electrolyte ions {Na+ie}. The constraint of local electrical neutrality gives {Na+e} ={Cl–e} and {Na+ie} = {Cl–ie} which in turn gives:({Na+cc}+{Na+ie}) {Na+ie} = {Na+e} {Na+e})					(3-27)

136

TR-10-47

Rearranging gives the introduced sodium ion activity:{} {Na } + 4 ⋅ {Na }  						(3-28)−  Na cc+ ±Na + = 

{

ie

}

+ 2
cc

+ 2
e

2



Consequently, it is possible to calculate the activity of sodium ions introduced in the clay-water
system if the activity of the original cations {Na+cc} in the clay and the activity of the ions in theexternal solution {Na+e} are known. The resulting drop in swelling pressure can be calculated
approximately if the activity of both the groundwater and the interlayer cations can be determined/­Karnland et al. 2005/. Figure 3-21 shows measured and calculated results for the above conditions
assuming an activity coefficient of unity. The same principle may be applied to divalent ions, whichleads to a more complicated expression, but both measured and calculated values for calcium show
similar osmotic effects to sodium.
Dependencies between process and buffer variables

Table 3-15 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Boundary conditions

The process occurs at the interface between the bentonite and an external solution, mainly the rock/bentonite interface, but could also occur at the bentonite/canister interface in the case of a defective
canister.
Model studies/experimental studies

The first preliminary study concerning influence of external groundwater salinity on swellingpressure was reported in /­Karnland et al. 1997/. A comprehensive model and experimental study of
MX-80 bentonite converted to a pure sodium state has been made by SKB /­Karnland et al. 2005/.
An additional experimental study of the two reference bentonites MX-80 and Deponit-CaN in
their natural state and after conversion to pure sodium and calcium forms has been made by SKB/­Karnland et al. 2006/.
105

Swelling Pressure, kPa

0.0
0.1

104

0.3
1.0

103

3.0
0.0

102

0.1
0.3

101

1.0
3.0

1

0

500

1000

1500

2000

Dry density, kg/m3

Figure 3-21. Measured (symbols) and calculated (lines) swelling pressure versus dry density of clay for
different concentrations of a NaCl solution in equilibrium with Na-montmorillonite. Legends show external
solution concentration in mol/dm3/Karnland et al. 2005/.

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137

Table 3-15. Direct dependencies between the process “Osmosis” and the defined buffer variables
and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Buffer geometryNo

No, but indirectly
through stress state

Pore geometry

Yes, interlayer
distance is a decisive
parameter

Radiation intensity

No

No

Temperature

Yes, linear dependence Neglected due tosmall variations

No

Water content

Yes, equivalent topore geometry insaturated buffer

Gas content

No, the process is
not well defined inunsaturated bufferNo

Hydrovariables(pressure and flows)No, but indirectly
through porewatercomposition

Yes, hydraulicconductivity effects

The effect of salinity on
hydraulic conductivity
is evaluatedStress stateNo, but indirectly
through charge
compensating
cation concentration(montmorillonite
composition)

Yes, by definition

The effect of salinity
on swelling pressure
is evaluatedBentonitecomposition

Yes, throughmontmorillonite
content

Included in data

No

Montmorillonite
composition

Yes, through type andnumber of charge
compensating cations

Included in data

Yes, counter-ion
distribution

Ion-exchange isconsidered

Porewater
composition

Yes, by definition

Included in data

Yes, by definition

Included in data

Structural and straymaterials

No

Included as a functionof buffer density

No, but indirectly
through stress state

The buffer is assumed No, not at high buffer
to be saturated for this densities
process

No

Time perspective

The process is relevant for all timescales over the lifetime of the repository. The swelling pressure
response of a change in groundwater salinity is expected to be instantaneous.
Natural analogues/observations in nature

Osmotic effects related to clay are discussed in several articles, but they are only peripherally
relevant for repository conditions. Osmotic induced pressure effects are discussed in /­Neuzil 2000/.
Handling in the safety assessment SR-Site

The osmotic swelling pressure effects are handled by direct application of empirical data forrelevant, scenario-specific conditions.
Model: The relationship presented above may be used to estimate swelling pressure and hydraulicconductivity for the buffer.

138

TR-10-47

Boundary conditions: The important boundary condition for this process is the composition of thegroundwater.
Handling of variables influencing this process: The montmorillonite composition and the groundwater/porewater composition are directly included in the model.
Handling of variables influenced by the process: The buffer swelling pressure and hydraulic conductivity are measured for all relevant conditions.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The principles for osmosis, ion equilibrium (Donnan equilibrium) and the resulting pressure effects
are basic knowledge in physical chemistry, and are consequently supported by numerous peer
reviewed articles available in the open literature. The application to bentonite buffer systems isshown in /­Karnland et al. 1997/. Both are SKB reports and the latter undergo documented factualand quality review at present. /­Karnland et al. 2005/ is not an SKB report, but is developed within
the framework of a quality assurance system that requires a documented factual- and quality review
before approval and printing and is available in the open literature. The natural analogue support forosmosis /­Neuzil 2000/ is a peer reviewed article (Nature) that is available in the open literature.
Uncertainties

Uncertainties in mechanistic understanding
The process is based on well-established theories and generally well understood. However, detailed
treatment of ion activity in the buffer is not clear.
Model simplification uncertainties in SR-Site
The ion equilibrium model for predicting swelling pressure is based on the assumption that thesystem is relatively homogeneous with respect to pore geometry. This assumption is strongly
supported by comparison of calculated and experimental data for swelling pressure, freezing(Section 3.2.2.) and tracer diffusion (Section 3.5.3).
Input data and data uncertainties in SR-Site
The input data to the model are the activity of ions in the groundwater and the activity of theinterlayer cations in montmorillonite. The model is relatively insensitive to the difference in activity
and concentration of groundwater and standard methods may be used for calculating this activity.
The relevance of calculating the activity of interlayer ions by standard methods (Debye-Hückel,
Davis, Pitzer etc) is not obvious. Uncertainties still remain as to how this activity should be treatedin a relevant way.

3.5.9

Montmorillonite transformation

Overview/general description

The advantageous physical properties of the buffer, principally swelling pressure and low hydraulicconductivity, are determined by the capacity for water uptake between the montmorillonite layers(swelling) in the bentonite, see Sections 3.3.2 and 3.4.1. Montmorillonite can transform into otherminerals (Figure 3-22) of the same principal atomic structure but with less or no ability to swell incontact with groundwater. The transformation processes usually consist of several basic mechanisms.
At the physico-chemical conditions expected in a repository, the following possible mechanisms
have been identified.

TR-10-47

139

Tetrahedral charge

Octahedral charge
Celadonite
K2(Fe23+Mg2)Si8

Glauconite
PhengiteMuscovite
K2(Al4)Si6Al2

2.0

Illite1.5

Illite/Smectites
Fe-Pyrophyllite(Fe43+)Si8

Smectites

M

B

0.5

Fe3+ + Fe2+

Al-Celadonite
K2(Al2Mg2)Si8

1.0
Total
layer
charge0.0
Pyrophylite(Al4)Si8
Figure 3-22. Ideal end-member minerals in the pyrophyllite – mica series with potassium as charge compensating cation, and approximate compositional ranges for illite and smectite (modified from /­Newman
and Brown 1987/). B denotes beidellite, M denotes montmorillonite. The compositional positions of themontmorillonite minerals in the reference bentonite materials are indicated by a triangle (MX-80) and bya square (IBECO RWC). All formulae contain the basic O20(OH)4 unit.

Congruent dissolution

Montmorillonites will not necessarily be in chemical equilibrium with repository groundwater(Figure 3-23). As mineral solubility is low, no significant mass loss is expected from this mechanism. However, solubility is temperature and pH dependent /­Aagard and Helgeson 1983/.
Reduction/oxidation of iron in the mineral structure.
This process alters the layer charge and may destabilize the mineral structure /­Stucki et al. 1984/.
Corrosion of metallic iron or bacterial activity /­Kim et al. 2004/ could promote the process (seeSection 3.5.10).
Atomic substitutions in the mineral structure.
This process alters the layer charge by e.g. Al replacement of Si in the tetrahedral sheets, or Al
replacement by Mg.
Octahedral layer charge elimination by small cations.
At high temperatures, e.g. Li+ may penetrate into the octahedral sheet, which reduces the layer
charge /­Greene-Kelley 1953/.
Replacement of charge compensating cations in the interlayer.
Substitution of counter ions is referred to as exchange and is treated elsewhere (see ion exchange
process 3.5.5). Due to charge balance, any mechanism which alters the layer charge must becompensated by addition or removal of counter ions in the interlayer. Such compensation is fundamentally coupled to processes governing transport in the porewater, see diffusion, 3.5.3.
Transformation from montmorillonite to illite, which is the most common alteration observed innatural sediments, is well documented in different geological environments, and has been reproduced
under laboratory conditions. The illitization process may be seen as the first step in a transition from140

TR-10-47

8.0

quartz saturation
amorphus silica saturationlog (aK+/aH+)

6.0

Muscovite
Microcline
4.0

mixed layer illite/
montmorillonie

Illite2.0

Gibbsite
Kaolinite
Montmorillonite

0.0
-5.0

-4.0

-3.0

-2.0

-1.0

0.0

log (a SiO2 (aq))

Figure 3-23. Silicate mineral equilibria with the partial component potassium at 25°C (Simplified from/­Aagard and Helgeson 1983/).

montmorillonite to mica (muscovite/celadonite) minerals (Figure 3-22), and all intermediate stages
from swelling to non-swelling material may be found (mixed layer smectite-illite). The conversion
always involves a layer charge increase, mainly due to a decrease in silica content, and an uptakeof charge compensating potassium ions. However, there is no general consensus concerning thedetailed mechanisms in the conversion, and several models have been proposed.
Advanced illite replacement may be illustrated by the mineralogical composition of a natural illite
from Montana, US, with less than 10% swelling layers /­Hower and Mowatt 1966/:(Si 6.81 Al 1.19)(Al 3.25 Fe0.14 Mg 0.59)

O20 (OH) 4

Ca 0.1 Na 0.02 K 1.53

tetrahedral

octahedral				interlayerThis illite mean structural formula can be compared with the mean structural formula for themontmorillonite in the MX-80 SR-Site reference bentonite /­Karnland et al. 2006/:(Si7.94 Al 0.06)(Al 3.10 Fe 0.37 Mg 0.50)

O20 (OH) 4

Ca 0.05 Mg 0.02 K 0.01 Na 0.46

tetrahedral

octahedral				interlayer

The main mineralogical differences are that illites have approximately one unit higher tetrahedral
charge, and potassium as the main charge compensating cation. Simplified, the total illitization
reaction may be expressed:
Ca2+/Na+-montmorillonite + K+ + (Al3+) → Illite + Silica + Ca2+/Na+
In massive natural sediments the physico-chemical conditions are similar over huge volumes andconcentration gradients are generally insignificant. The total reaction therefore involves a seriesof sub-reactions; release of silica from montmorillonite, release of potassium from e.g. feldspars,potassium fixation and precipitation of new silicate minerals. The total reaction rate is governed by
the slowest of the sub-reactions.
According to the equilibrium conditions for 25°C (Figure 3-23), montmorillonite may be thermo­
dynamically stable for silica activities above 6·10–4. Amorphous silica, with an equilibrium activityof around 2·10–3 may consequently be in equilibrium with montmorillonite. However, quartz withan activity of 1·10–4 will not be in equilibrium with montmorillonite. Precipitation of quartz may
therefore destabilize the montmorillonite, and the precipitation rate may be the kinetically controllingmechanism in illitization of montmorillonite /­Abercrombie et al. 1994/. Figure 3-24 shows thedecrease in silica activity in sediment porewater, as a function of temperature (depth in the sediments).

TR-10-47141

1

Site 794

SiO2 activity

10-1

Amorphous Silica

Site 795

Smectite-to-Illite reaction

10-2

smectite
illite10-3

Quartz

10-4

10-5
0

20

40

60

80

Temperature, °C

Figure 3-24. Silica activities for waters from Ocean Drilling Program sites 794 and 795 and saturation
curves for amorphous silica, smectite-to-illite reaction, and quartz. Modified from /­Abercrombie et al. 1994/.

At temperatures below 40°C the porewater is in equilibrium with amorphous silica, and at temperatureshigher than 60°C the porewater is in equilibrium with quartz. The onset temperature of quartz
precipitation in this system should consequently be in the range 40 to 60°C.
However, at low temperatures the precipitation of quartz is very slow and the porewater may staystrongly supersaturated with respect to quartz. Consequently, a high smectite content is commonly
found in old formations exposed to repository temperatures. For example, /­Velde and Vasseur 1992/
studied the time-temperature distribution of illitization in seven deep wells in four sedimentary basinsin the US, Japan and France. In all wells there was a typical reduction of smectite content with depth,
which represents an increase in both age and temperature (Figure 3-25). The prerequisites fortransformation are obviously present in the sediments with time and temperature the governingparameters. A decrease to around 60% smectite was observed in the Californian Norwal formation
after 4.5 million years at a depth of 5 km, representing a final temperature elevation of over 100°C.
The same transformation took around 60 million years at a depth of 2 km and a temperature elevation of around 70°C in the Texan Peelan sediments. The reaction rate at these repository relevanttemperatures is consequently very slow in relation to the timescale of a repository.
Norwal (California)

Peeler (Texas)

00T.G. = 27.5C/km
B.R.= 1.1 km/Ma

2
3

2
3
4

4
5
100

T.G. = 36C/km
B.R.= 0.03km/Ma

1

Depth, km

Depth, km

1

80

60

40

20

Smectite content in I/S, %

0

5
100

80

60

40

20

0

Smectite content in I/S, %Figure 3-25. Smectite content versus depth in smectite-illite mixed layer material in two sediments representing relatively fast burial rate (left) and slow burial rate (right). T:G: indicate the present temperature
gradient and B.R. indicates the burial rate. Redrawn from /­Velde and Vasseur 1992/.

142

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Brammallite/rectorite may be seen as a sodium equivalent to illite. The mean layer charge is higher
than in illite, normally close to 2 unit charges per O20(OH)4. It is expected that the transformationfrom montmorillonite into brammallite/rectorite is governed by the same basic mechanisms as
in illitization. The release of silica from the montmorillonite may theoretically continue until therequired tetrahedral charge is reached in the absence of potassium. In consequence, brammallite/
rectorite are less commonly found as alteration products in natural sediments compared to illites,
and there are no indications that the basic mechanisms should be faster than in illitization.
Chlorites have the same basic structure as montmorillonite, but the layer charge is normally close
to 2 unit charges per O20(OH)4. The layer charge is balanced by positively charged, octahedrally
coordinated, hydroxide sheets, where the central ion may be any di- or tri-valent metal ion, normally
Mg, Al or Fe. Transformation from montmorillonite to chlorite is occasionally found in parallel with
illitization, and it is expected that both transformations are governed by similar basic mechanisms.
It cannot be excluded that chlorite may be formed from montmorillonite without illitization in theabsence of potassium, but there are no indications that this process would be faster than the illitization process under repository conditions.
Silica solubility increases significantly above pH 9 due to the H4SiO4 -species losing one or two of
its protons. The total equilibrium SiO2 concentration ([H4SiO4] + [H3SiO4 –] + [H2SiO4 2–]) is givenby the law of mass action:[SiO2]TOT ≈ {H4SiO4} (1+K1/{H+}+K1K2/{H+}2)
where K1 = 10–9.9 and K2 = 10–11.7 denote the equilibrium constants for the dissociation reactions. The
total SiO2-concentration as a function of pH is shown in Figure 3-26, assuming an H4SiO4activity forsaturated amorphous silica.
The tetrahedral silica in the montmorillonite consequently equilibrates at higher concentrations above pH
9. Diffusive removal of silica or precipitation of new silica minerals thereby lead to a faster increase of thetetrahedral layer charge compared to near neutral conditions. The corresponding increase in concentrationof charge compensating cation leads to a change in the interaction with water and thereby to a change
in sealing properties. The layer charge may reach the critical value for collapse, which results in total lossof expandability and in principle, to the same consequences as for illitization. Eventually, the process
may lead to a general dissolution of the montmorillonite /­ECOCLAY II/, /­Karnland et al. 2005/.
At pH 11, the divalent anion H2SiO4 2– starts to play an important role leading to a dramatic increasein silica solubility. At pH 11 the total silica concentration is calculated to be approximately 16 times
greater than at neutral pH conditions, and at pH 12.4, representing matured Portland cement, thetheoretical increase in total silica solubility is more than 3 orders of magnitude higher than at near
neutral conditions. The total difference in silica concentration between the bentonite porewater

Concentration (mol/l)

1{H4SiO4}
0{H3SiO4–}{H2SiO42–}

0.1

Total [SiO2]

0.01

0.001
7

8

9

pH

10

11

12

Figure 3-26. Total concentration of silica as a function of pH assuming H4SiO4activity for saturated
amorphous silica at 25°C. At pH=9.9 H3SiO4 – starts to dominate and above pH=11.7 H2SiO4 2– dominates.
The upper limit for the KBS-3 design criterion, pH=11, is indicated /­Karnland et al. 2006/.

TR-10-47

143

and the groundwater increases approximately by the same factor, assuming the groundwater is inequilibrium with quartz. This leads to diffusional transport of aqueous silica from the bentonite into
the groundwater. Besides the difference in equilibrium concentration, the limiting factors for thismass loss will likely be the contact area between the buffer and the groundwater and the groundwater
flow. /­Huertas et al. 2005/ studied the dissolution rate of smectite as a function of pH. Figure 3-27
plots log dissolution rate vs. pH at 25, 50, and 70°C. In alkaline solutions, the smectite dissolution
rate increases as pH increases, showing a steeper slope for pH values higher than 11, which seems
to be a critical value for smectite dissolution and stability. The conclusion from /­Huertas et al. 2005/
is that the results indicate that dissolution rates are strongly affected by both pH and temperature.
This effect is particularly important for pH values above 11. The effect of the alkaline plume shouldbe especially strong for young cement waters and for a repository concept in clay rocks, where thevousoirs surrounding the bentonite are closer to the canister that the concrete plug in the granite
concept, and therefore subjected to higher temperatures. In cooler regions and slightly alkaline
solutions, the bentonite components should be much more stable.
Effects of montmorillonite transformation
If montmorillonite transformation occurs the buffer functions will alter. Layer charge changes in themontmorillonite lead to changes in the interplay with water and thereby affects the swelling pressure.
During the early stages of illitization and similar transformations, montmorillonite/water interaction
may become stronger due to moderate layer charge increase. However, as the transformation
progresses, fixation of K+ between sufficient highly charged layers leads to loss of interaction withwater. The pore geometry is thereby changed towards fewer and larger pores, which in turn leads to a
successive general deterioration of the buffer properties. A total transformation of all the montmorillonite in the buffer into illite or similar minerals would lead to complete loss of buffer function.
Many transformation processes and also congruent dissolution of the montmorillonite mineralreleases solutes which could precipitate as new minerals which may act as cementing agents.
Cementation could be enhanced due to transport of material from high solubility to low solubility
regions, in situations where a solubility gradient prevails across the buffer, e.g. due to temperatureand pH differences, /­Karnland and Birgersson 2006/. For instance, in the case of silica, solubility increases dramatically at pH above 11 due to deprotonisation. Cementation may lead to a
deterioration of the buffer’s rheological properties as well as decreased capacity to swell , but theseeffect are expected to be small since montmorillonite transformation is expected to be minor (seeSection 3.5.15).
-11

-12

Si 25°C

Al 25°C

Si 50°C

Al 50°C

Si 70°C

Al 70°C

70°C

50°C

-13

25°C
-14

-15

7

8

9

10

11

12

13

14

pH

Figure 3-27. Experimental (symbols ) and estimated (lines) dissolution rates /­Huertas et al. 2005/.

144

TR-10-47

Table 3-16 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Boundary conditions

The relevant boundary conditions for the buffer are temperature and solute concentrations in theadjacent components, i.e. the same boundary conditions as for the processes heat transport, diffusion
and advection.
Model studies/experimental studies

The illitization process is of great commercial importance in connection with prospecting and extraction of oil deposits which has led to very extensive research since the 1960s.
Table 3-16. Direct dependencies between the process “Montmorillonite transformation” and thedefined buffer variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Buffer geometry

Yes, the size affects thetransport of dissolved
species and total
buffer mass

Total mass considered. Yes, for extreme
Transport resistances (unexpected)
are pessimisticallytransformationneglected

Separate scenario

Pore geometry

Yes, the density willaffect the transport ofdissolved species andion equilibrium

A kinetic expression
is used. Transport
resistances are pessimistically neglected

Yes, by definition

Separate scenario

Radiation intensity

Possible minor effect

Neglected, see 3.5.12

No

Temperature

Yes, major impact

A kinetic expression
is used.

No

Water content

Yes, direct effect on
hydraulic conductivity

Irrelevant for the timeframes considered

Yes, for extreme(unexpected)
transformation

Gas content

Yes, direct effect on
hydraulic conductivity

Irrelevant for the timeframes considered

No

Hydrovariables
Yes, minor through(pressure and flows) transport of dissolved
species

Transport resistances
are pessimisticallyneglected

Yes, major impact

Separate scenario

Yes, major impact

Separate scenario

Separate scenario

Stress stateNo, pressures are too
small to be significant

Bentonite composition

Yes, through available
species in accessory
minerals

Minor importance,
included in theassessment of effectsof cementation

Yes, by definition

Separate scenarioMontmorillonite
composition

Yes, by definition

Included inassessment

Yes, by definition

Separate scenario

Porewater composi- Yes, by definition
tion

Included in theassessment of effectsof cementation

Yes, by definition

Minor change thathas little impact on
performance

Structural and straymaterials

Neglected in this case, No
since no stray materials
are assumed to bepresent

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Yes, indirectly throughdissolved species.
Especially pH effects
from cement

145

Kinetic models have been developed for the smectite-to-illite transformation. Relationships andconstants have been established by means of laboratory batch experiments and comparisons withnatural systems /­e.g. Eberl and Hower 1976, Pytte 1982, Velde and Vasseur 1992, Huang et al. 1993/.
Most models take the form of an Arrhenius expression and the Huang et al. model may be shown as
an example:
The overall kinetics of the smectite-to-illite reaction can be described by Equation 3-30:–dS/dt = A·[K+]·S2 · exp(–Ea/RT),							(3-30)
where S is the smectite fraction in the illite/smectite material, A is the frequency factor, Ea is theactivation energy, R is the universal gas constant, and T is the temperature. After integration of
Equation 3-30 the smectite content at a certain time can be calculated if the temperature and potassium concentration in the porewater are known. The potassium concentrations in the Äspö groundwater have been measured and found to be in the range of a few ppm up to 45 ppm /­Laaksoharju
et al. 1995/. According to the model, practically no clay conversion is possible in a KBS-3-type
repository under these conditions as shown in Figure 3-28.
The mineralogical stability of montmorillonite is being studied by SKB in the Lot tests at Äspö HRL/­Karnland et al. 2009/. The test series represents both anticipated repository and adverse conditions.
The latter are used to accelerate possible degradation processes by use of:••••Higher temperature.
Higher temperature gradient.
Higher content of accessory minerals (calcite, gypsum, K-feldspar).
New substances (Portland cement).

At the highest temperature (~130°C), a test period of 10 years corresponds to the entire period of
elevated temperature in a KBS-3 repository with respect to illitization according to the Huang model.
The introduction of additional substances and increased temperature gradients is made in order toaccelerate mass transport processes. . The effect of the temperature gradient is, however, not possible
to describe in a simple and generalised way by means of increased temperature and concentrationsof reactants. No mineral transformation to illite has been detected in the tests performed at standard
or adverse conditions by use of standard clay mineralogical analyses, nor has any K+-fixation been
observed by the performed mineralogical and chemical analyses.

Montmorillonite part '

1.0

T, °C
10
50

0.5

90
130
170
0.0

0

10

100

1,000

10,000100,000

1,000,000

Time, years

Figure 3-28. Remaining smectite fraction for different temperatures in a hydrothermal system with
[K+] = 0.0012 mol/liter (45 ppm) according to the Huang et al. kinetic model and laboratory determined
constants (Ea = 28 kcal·mol–1 and A = 8.08E4 L·s–1·mol–1) /­Karnland and Birgersson 2006/. Legend shows
temperature in °C.

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Natural analogues/observations in nature

Apart from SKB-related investigations, the transformation of smectite to illite is well-documented
in a large number of geological formations. Natural sediments have been studied by, among others,/­Burst 1969, Perry and Hower 1970, Hower et al. 1976, Colten-Bradley 1987, Velde and Vasseur
1992, Lynch 1997/. The studies show that increased sediment depth (increasing temperature) leads
to increased illite content and that low availability of potassium can be linked to reduced transformation. Silica activity as a kinetically controlling factor has been studied in sediments /­Abercrombie
et al. 1994/. Formations where temperature effects have been obtained from e.g. volcanic activity
have revealed a similar transformation /­e.g. Pytte 1982, Lynch and Reynolds 1985, Brusewitz 1986/.
Cementation is a consequence of montmorillonite transformation (see Section 3.5.15).
Time perspective

Reduction/oxidation reactions of iron may take place on a short time scale. Montmorillonite layer
charge changes and associated precipitations may take place during the initial elevated temperaturephase. Changes in montmorillonite content are relevant for the whole life-time of the repository.
Handling in the safety assessment SR-Site

The montmorillonite transformation in a KBS-3 repository is assumed to be small based on thefollowing observations and arguments:
1. The time scale for significant montmorillonite transformation at repository temperatures innatural sediments is orders of magnitude longer than the period of elevated temperature in aKBS-3 repository /­e.g. Velde and Vasseur 1992/.
2. The bentonite material is close to mineralogical equilibrium to start with /­e.g. Fritz et al. 1984/.
3. Transformation is limited by low transport capacity, principally regarding potassium /­Hökmark
et al. 1997/ and silica /­Karnland and Birgersson 2006/.
4. All published kinetic models, based both on natural analogues and laboratory experiments indicatethat the transformation rate is very slow under repository conditions /­e.g. Huang et al. 1993/.
The montmorillonite transformation process can also be quantified by modelling, as exemplified
above. Based on the above description, no mineral transformation is expected to be faster thanillitization as a result of elevated temperature. Consequently, the maximum temperature effect ismodelled by the kinetic expression for illitization proposed by Huang et al. and by use of different
but realistic potassium concentrations. Adverse pH conditions could lead to increased dissolution of
the montmorillonite and possible mass loss.
Two safety function indicator criteria have been defined. As long as the maximum temperature is
below 100°C and the pH of the water in the rock is below 11 the montmorillonite in the buffer isassumed to be stable for the timescale of the repository.
In the reference evolution, alteration is not expected to proceed to a level where it will affect theproperties of the buffer. Alteration is therefore treated as a separate scenario.
Boundary conditions: The temperature and the composition of the groundwater.
Handling of variables influencing this process: See Model above
Handling of variables influenced by the process: The amount of remaining montmorillonite as afunction of time will be calculated. The content of neoformed silica impurities will also be calculated.

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Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Montmorillonite alteration is one of the most studied processes in clay mineralogy and the above
argumentation is supported by numerous peer reviewed articles and conference papers available in
the open literature. Those cited above represent only a minor fraction of what is available and the
key issues are rather the relevance and relative quality of the peer reviewed material. A literature
study with reference to specific KBS-3 conditions, including validation of alteration models wasmade by SKB /­Karnland and Birgersson 2006/ in order to address these issues. This report may beseen as a comprehensive evaluation of the adequacy of references.
Uncertainties

Uncertainties in mechanistic understanding
There is a generally accepted explanation of the digenetic processes by which smectite is converted
into illite accompanied by a release of silica, water and cations. There is, however, no general consensus
concerning the details in the smectite-to-illite transformation mechanisms in a repository environment and a number of explanations for the character and mechanisms of the smectite-to-illite
conversion have been proposed /­Moore and Reynolds 1989/:
Model simplification uncertainties for the above handling in SR-Site

Laboratory experiments have shown transformation rates that are faster than predicted by the kinetic
models under the following extreme conditions. Consequently, these are not covered by the present
models:•••Influence of low water content /­Couture 1985/.
Extreme potassium ion concentration /­Lee et al. 2010/
pH effects:– high pH: /­Velde and Vasseur 1992, Savage et al. 2002, Gaucher et al. 2004, Karnland et al.
2007, Savage et al. 2010/.
– Low pH: /­Zysset 1992/.

Input data and data uncertainties for the above handling in SR-Site
Thermodynamic and kinetic data for modelling silica dissolution/precipitation are not readily
available especially for high pH and high temperatures. In transport calculations, the diffusion
constants in rock and bentonite constitute an uncertainty. For obvious reasons, the kinetic constants
are determined in laboratory experiments performed over relatively short durations and at higher
temperatures and potassium concentrations when compared to the conditions they aim to simulate.
The various illitization models currently available show a large variety in constant values and model
formulae, which indicates conceptual problems. However, the set of determined constants andformulae generally results in similar long-term predictions /­Karnland and Birgersson 2006/.

3.5.10 Iron – bentonite interaction
Overview/general description

Metallic iron will be present in several places in the repository; the iron insert in the canister, in the
supercontainer in the KBS-3H concept and in rock reinforcements. The amount of iron is considerable, especially in the KBS-3H concept. The fact that metallic iron is not thermodynamically stablein the repository (E0 < 0 V) raises the questions: where does it go and what does it do? The answer iscomplex and implies that it depends on several factors such as the redox potential, the temperature,
the pH, solute concentrations and the composition of the buffer. Other less obvious factors may also
be important such as the buffer density, temperature gradients, transport rates and microbes.

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Standard iron corrosion in the absence of bentonite
The initial step in anaerobic corrosion takes place as follows:
Fe(s) + 2 H2O

Fe2+ + 2 OH– +H2(g)If any oxygen is present, the reaction that follows is:
Fe(OH)2(s) + ½ O2(g)

Fe(OH)3 (s)If carbonate concentrations are sufficiently high, carbonates will precipitate:
HCO3– + OH–

CO32– + H2O(l)CO32– + M2+

MCO3(s)If carbonic acid is absent, Fe(OH)2 will precipitate, which is stable in the absence of oxygen at low
temperatures. At higher temperatures (> 373 K), it will convert to the thermodynamically morefavoured magnetite phase. The reaction is also possible at lower temperatures but is then muchslower.
3 Fe(OH)2(s)Fe3O4(s) + H2(g) + 2 H2O(l)This possible build-up of magnetite and other corrosion products can passivate the iron surface anddecrease the corrosion rate.
Iron corrosion in the presence of bentonite
The anaerobic corrosion of iron does not seem to be passivated in the same way in bentonite. The
corrosion rate seems to be increased by the presence of bentonite. This is possibly due to the cationexchange capacity of the clay minerals which allows them to adsorb iron ions thereby minimisingprecipitation. In addition, corrosion and/or its products seem to react with the clay minerals creating
new non-swelling minerals /­Lantenois et al. 2005, Perronnet et al. 2008/.
The most probable scenarios are:
• Formation of ion exchanged iron-montmorillonite.
• Reduction of the iron in the octahedral sheet of the montmorillonite structure, possibly coupled todehydroxylation (reversible or irreversible).
• Dissolution and/or transformation of the montmorillonite.
• Formation of corrosion products with possible cementation of the buffer.
All scenarios will probably affect the buffer performance to some extent. The rate and degree ofdestabilisation of the smectite seems to depend on the nature of the smectite (dioctahedral/trioctahedral, counter ion and FeIII- content) /­Lantenois 2005/. The effect of iron reduction may havea negative impact on the hydraulic conductivity and the swelling properties. The cation fixation
capacity has been observed to increase /­Khaled and Stucki 1991/.
Dependencies between process and buffer variables

Table 3-17 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Boundary conditions

The relevant boundary condition for this process is the presence of iron in the adjacent systemcomponents. As long as the canister is intact, the only source of iron is in the construction materials
within the repository. In the case of a failed canister, there will be large amounts of iron in close
contact with the buffer.

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Table 3-17. Direct dependencies between the process “Iron – bentonite interaction” andthe defined buffer variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Buffer geometry

Yes, through transportof dissolved species
and total buffer mass

Yes, for extreme unex- Separate scenario
pected transformation

Pore geometry

Yes, through transportof dissolved species
and ion equilibrium

Yes, by definition

Radiation intensity

No

No

Temperature

Yes

Neglected, since thereaction will occur
late in the repository
evolution

No

Water content

Yes, through transportof dissolved species

Full saturation
assumedYes, for extreme unex- Neglected, importancepected transformation is minor

Gas content

Yes, through transportof dissolved species

Neglected, since thereaction will occur
late in the repository
evolution

No

Hydrovariables
Yes, minor through(pressure and flows) transport of dissolved
species

Neglected since
the major source ofiron is present inside
the canister

Yes, for extreme unex- Separate scenario
pected transformation

Stress stateNo, pressures are too
small to be significant

Bentonitecomposition

Yes, through available
species in accessory
mineralsMontmorillonite
composition

Yes, by definition

Porewater
composition

Yes, by definition

Structural and straymaterials

Yes, determines
availability of iron

Included in massbalance

Separate scenario

Yes, for extreme unex- Separate scenario
pected transformation
Neglected, due tosmall effect comapred
to other sources

Yes

Precipitation ofiron phases canbe calculatedYes, at extreme unexpected transformation

Separate scenario

Included in model/
expression used

Yes, by definition

Included in model/
expression used

No iron is assumed
to be present as straymaterial in buffer(No)

Model studies/ experimental studies

Most of the published results are based on experiments were the kinetic limitations have been
reduced in some way. The most common ways to speed things up is to increase the temperature,
increase the liquid/solid ratio or to increase the reactive surface area by intimately mixing iron andclay powders. However the experimental results are very sensitive to the conditions and hence theresults from the studies are rather scattered /­Mosser-Ruck et al. 2010/.
Typical experimental results are the following examples:(i) Iron powder mixed with different bentonites in deionised water was heated at 80ºC for 45 days.
Ion exchange was noticed, and also magnetite and 7 Å phyllosilicate formation /­Lantenois et al.
2005/.
(ii) Carbon steel (coupons and wires) in compacted MX80 bentonite was heated (30 and 50ºC)
with artificial groundwater for up to 900 days. No transformation of the montmorillonite wasobserved. Magnetite, hematite and goethite were identified as corrosion products. The results
were consistent with sodium to iron ion exchange of the montmorillonite /­Carlson et al. 2007/.
(iii) FoCa7 bentonite was mixed with various ratios of powdered metallic iron in Evian water andheated for 45 days at 80ºC. The ratio of clay and iron seems to have an impact on the newphases formed. SiAlFe gels were created, which matured into Fe-rich phyllosilicates (identified
as odinite or greenalite) /­Perronet et al. 2008/.
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Iron bentonite interaction is studied in the Alternative Buffer Materials (ABM) project at Äspö HRL.
Eleven different compacted clays are placed in contact with an iron heater at 130ºC in three different
packages. The experimental time is from two to more than five years. The clays have been selected
to give a broad variation in properties, such as smectite content, iron content, counter ion composition and accessory minerals.
Natural analogues/observations in nature

Possible analogues could be naturally occurring native iron (meteoritic or non-meteoritic) or perhaps
old archaeological artifacts in contact with bentonites or other clays. However because of the rare
nature and the uncertain history of the objects they are probably of limited value. Other natural analogues could be soils in anaerobic environments in rice fields where Fe-rich chlorites or 1:1 minerals
have been identified /­Favre et al. 2002/.
Time perspective

The process may occur as long as iron is present in the repository.
Handling in the safety assessment in SR-Site

Since there will be very small amounts of iron present in the vicinity of the deposition hole, the process
is assumed to be of insignificant importance to the buffer properties as long as the canister is intact.
For the case of a defective canister, alteration of some of the buffer caused by the interaction withmetal iron cannot be ruled out based on current knowledge.
Iron-clay interaction was not assumed to change the major properties of the buffer in the SR-Can assessment. This will still be the assumption in SR-Site. However, since there are uncertainties regarding thisprocess a sensitivity discussion will be included in the montmorillonite transformation scenario.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

All references included are peer reviewed scientific publications. General corrosion of iron in ananaerobic environment at the level presented here is rather well known. The probable iron-bentonite
interaction scenarios listed are based on those indicated by experiments, both specific to the topic
and in montmorillonite transformation in general.
Uncertainties

Uncertainties in mechanistic understanding
There are uncertainties in the mechanistic understanding and the total possible impact from the metallic
iron on buffer properties.
Model simplification uncertainties in SR-Site
No quantitative model is available. The possible consequences will be discussed with simple
sensitivity studies.
Input data and data uncertainties in SR-Site
Not really applicable in this case.

3.5.11 Montmorillonite colloid release
Overview/general description

The uptake of water and resulting swelling of the bentonite buffer is counteracted by the wallsof the deposition hole, and a swelling pressure is developed in the bentonite (see Section 3.4.1
Swelling/mass redistribution). Fractures intersecting the deposition hole mean that rigid swelling
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restrictions are not present everywhere, and that localized swelling continues into the fractures until
an equilibrium or steady state is reached. This free swelling may lead to separation of individual
montmorillonite layers (dispersion) and part of the buffer could be transported away by groundwater.
The maximum free swelling of the bentonite is strongly dependent on the valence and concentrationof the ions in the interlayer space. At low groundwater concentrations, the interlayer distance between
the individual montmorillonite layers may increase enough to give the clay/water system a sol
character, i.e. single or small groups of montmorillonite layers act as individual colloidal particles.
The maximum interlayer distance, when only monovalent ions are present in the solution, may be
discussed in terms of the Debye-Hückel screening length, which is inversely proportional to thesquare root of solution concentration. The same dependence on interlayer separation with solution
concentration has been experimentally determined /­Norrish 1954a, b/. The forces between parallel
montmorillonite layers with only monovalent ions present, can be calculated for an ideal systemfrom DLVO theory /­Derjaguin and Landau 1941, Verwey and Overbeek 1948/, in which the entropic
repulsive effects from ions and the attractive van der Waals forces are taken into account. In the casethat only divalent ions are present in the system, additional attractive forces become significant,which restrict the swelling to relatively short interlayer distances and prevent release of individual
colloid particles /­Guldbrand et al. 1984, Kjellander et al. 1988/. However, a relatively small content
of monovalent ions in a calcium-dominated system may still lead to general colloidal sol formationin deionised water or at very low ion concentrations /­Birgersson et al. 2010, Hedström et al. 2011/.
For simple model systems such as charged spheres or parallel extended flat charged surfaces, theconcept of a critical coagulation concentration (CCC) is readily defined within the DLVO theory
as the salt/electrolyte concentration where the energy barrier for particle-particle association
approached zero /­e.g. Evans and Wennerström 1999/. At the CCC and higher salt concentrations
the attractive van der Waals forces dominate the system and colloidal particles will be held together
and not disperse spontaneously. If one considers montmorillonite as extended negatively charged
surfaces ordered in parallel, the calculated CCC for a 1:1 salt e.g. NaCl within DLVO theory turns
out to be in the range 1–2 M. However, experimentally the transition from sol to attractive gel isobserved to take place at much lower concentrations /­van Olphen 1963, Le Bell 1978, Hetzel andDoner 1993, Abend and Lagaly 2000, Wold 2003, Lagaly and Ziesmer 2003, Tombács and Szekeres
2004/. This strongly suggests interactions other than face-to-face DLVO forces, in real montmorillonite systems. It has been proposed /­van Olphen 1963/ that flocculation of clay could be a result of
the interactions between positive charges present on the edges of clay particles and negative charges
on the faces of the clay particles. /­Missana and Adell 2000/ have shown that the small fraction of the
pH-dependent charges on the edge greatly influence the stability behaviour of the clay colloids andthat the standard DLVO theory for parallel clay layers is not particularly suitable for predicting thestability, mainly because it is not able to account for this charge contribution. However the conceptof a CCC need not be defined within the framework of a particular theory but from observations
alone as the concentration where an attractive gel can be formed /­e.g. Abend and Lagaly 2000/.
A CCC may be determined for monovalent systems and used as a conservative concentration limit
for spontaneous colloid particle release. Governing variables are the concentration and the layer
charge of the montmorillonite. Laboratory results have shown that montmorillonite extracted from
MX-80 bentonite and ion-exchanged to Na+-state has a CCC of around 25 mM in NaCl solution, i.e.
an attractive gel is formed /­Birgersson et al. 2010/. Two other Na- montmorillonites analyzed withhigher layer charge than in MX-80 were found to have lower CCC, in contrast to predictions from
DLVO theory for parallel layers /­Evans and Wennerström 1999/. For systems with only divalentcounterions the CCC concept is not really valid, and the CCC may be considered zero, i.e. no excess
ions are needed in order to prevent colloidal sol formation of colloidal particles as demonstrated in(e.g. Ch 4 in /­Birgersson et al. 2010/) or from theoretical considerations /­Kjellander et al. 1988/. Fora mixed system with both mono and divalent counterions one also needs to consider ion equilibrium/­Kahn 1958/. Thus one cannot speak in terms of a CCC for Ca2+ for Na-montmorillonite as such avalue cannot exist. The limiting condition for colloidal sol formation (sol formation) is thereby given
by combinations of ion concentrations and monovalent/divalent ion ratios in the clay/water system
illustrated in Figure 3-29.

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Wyoming type montmorillonite

25

X=0.9
I = 25 mM[Na+] (mM)

20

15

Possible Sol Formation Zone

10

5

0
0

1

2

3

4

[Ca2+] (mM)

Figure 3-29. Limitations of a possible sol formation zone for Wyoming type montmorillonite in equilibriumwith an external Ca/Na-solution and monovalent anions (e.g. Cl-). The lower curve (X=0.9) represents theexperimental finding that when the Ca fraction is 0.9 or higher in the clay no sol is formed irrespective of
the ionic strength in the external solution. On the upper curve the ionic strength equals 25 mM, which is
the CCC value for Wyoming-type montmorillonite in NaCl solution /­Birgersson et al. 2010, 2011/.

The central conditions for possible colloid formation in a repository are consequently the local
porewater concentration and the ratio between mono and divalent ions in the montmorillonite at
the bentonite/groundwater interface. The governing processes are ion exchange of the original montmorillonite counterions by ions originating from the accessory minerals and from the surrounding
groundwater.
Loss of bentonite material may be significantly reduced, or even eventually stopped, by space
limitations under mineralogical/chemical conditions which lead to sol formation. Laboratory results
have shown that pore sizes smaller than 0.5 microns completely prevent the montmorillonite particles
penetrating filters, but pore-sizes of 2 micron or larger do not restrict the loss significantly, as illustrated
in Figure 3-30. This figure shows the swelling pressure evolution of a compacted 5 mm thick disc of
Na-montmorillonite confined by filters of pore size 2 or 10 µm according to the set-up in Figure 3-31.
Behind the filters deionised water is circulated at a pumping rate of 1 mL/min. The drop in swellingpressure seen in Figure 3-30 is caused by montmorillonite loss through the filters. The fact that filterswith pore size of 2 µm do not prevent montmorillonite loss is in accordance with measured particle size
distributions /­Michot et al. 2004, Karnland et al. 2006/. In a repository an auto healing effect may bepresent as a result of clogging fractures by accessory minerals of different grains sizes.
Based on present knowledge, significant formation of colloids and subsequent buffer mass loss
cannot be excluded over the time scale of several glaciations.
Table 3-18 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site. Note though that the process occurs at the boundary between
buffer and rock, within rock fractures making the boundary conditions described below of prime
importance for the understanding and quantification of the process.
Boundary conditions

The boundary conditions are the cation concentration and cation valence of the groundwater in directcontact with the outermost part of the bentonite buffer and also the cation pool in the bentonite itself.

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Figure 3-30. Swelling pressure for Wy-Na montmorillonite as a function of time using filter pore sizes of
2 and 10 mm respectively. The drop in swelling pressure is caused by loss of clay through the filters (2 mmthick). /­Birgersson et al. 2010, Hedström et al. 2011/.
100 mL circulating solution
Forcetransducer

Clay sample

Filters (titanium or stainless steel)Figure 3-31. Schematic drawing of the swelling pressure test cell /­Birgersson et al. 2010/.

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Table 3-18. Direct dependencies between the process “Montmorillonite colloid release” andthe defined buffer variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Buffer geometry

Yes, the amount of
buffer present

Included in modelling

Yes, at extreme
process magnitude

Included in modelling

Pore geometry

Yes, governs thedispersing forces

Included in modelling

Yes, by definition

Included in modelling

Radiation intensity

No

No

Temperature

No, negligible

No

Water content

Yes, by definition

Full saturation
assumedYes, by definition

Gas content

Yes, no dispersion
possible fromunsaturated clay

Full saturation
assumedNo

Hydrovariables
Yes, by definition(pressure and flows)

Included in modelling

Yes, indirect throughpore geometry,central impact

Evaluated frommodelling results

Stress state

Yes, by definition

Included in modelling

Yes, major impact

Included in modelling

Bentonitecomposition

Yes, impurities may
govern the ion typeand concentrations

Included in modelling

Yes, selective loss ofthe clay component

Included in modelling

Montmorillonite
composition

Yes, important variables are magnitude
and distribution oflayer charge, andtype of cations

Included in modelling

Yes, possible throughselective formationof colloids

Neglected

Porewater
composition

Yes, major importance Included in modellingboth with respect
to cation type andconcentration

Yes, through poregeometry

Included in modelling

Structural and straymaterials

Yes, concrete,especially, may
provide calcium ions

Yes, minor effects
possible throughcalcium uptake

Neglected

Neglected, noconcrete assumed
in buffer

Full saturation assumed

Water flow in the fractures will not affect the stability of the sol phase. However, the interlayer
distance between the montmorillonite layers is continuously increasing with decreasing electrolyte
concentration and layers at large separation are more sensitive to erosion than layers at close separation. Water flow may tear loose montmorillonite even before the clay formally turns into a sol. The
extent of such particle release by flow is dependent on the flow velocity. Without flow the particles
will sediment. The treatment of erosion is presented in Section 3.3.4.
Model studies/experimental studies

The principles for swelling of montmorillonite in 1:1 electrolyte solutions have been reported in alarge number of publications. The basic theoretical principles are described by the DLVO theory/­Derjaguin and Landau 1941, Verwey and Overbeek 1948/. For divalent cations additional attractive
correlation forces come in to play which necessitates a theoretical treatment beyond the DLVO
theory /­Guldbrand et al. 1984, Kjellander and Marčelja 1984, Kjellander et al. 1988/. Interlayer distances were experimentally determined by Norrish /1954a, b/, and several subsequent studies havein principle confirmed these results, e.g. those by /­Zhang and Low 1989/. Several laboratory studies
have been made in order to determine the critical coagulation concentration for montmorillonite
in sodium chloride solutions /­van Olphen 1963, Le Bell 1978, Hetzel and Doner 1993, Abend andLagaly 2000, Wold 2003, Lagaly and Ziesmer 2003, Tombács and Szekeres 2004, Laaksoharju andTR-10-47

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Wold 2005, Birgersson et al. 2010/. A large body of work suggests the mechanism for montmorillonite coagulation in NaCl is edge-face interactions /­e.g. Lagaly and Ziesmer 2003, Tombács andSzekeres 2004, Birgersson et al. 2010/ although opinions differ concerning the pH range where theedge charges may be positive.
Ion exchange must be considered for mixed mono- and divalent systems. Such systems may bedescribed in terms of a sol formation zone (SFZ) as shown in Figure 3-29. Experiments have established that the upper boundary of the SFZ is significantly lower than predicted by the ionic strength
assumption; thus the zone is much smaller than depicted in Figure 3-29 and limited to a very thinregion close to the [Na+]-axis /­Birgersson et al. 2010, 2011/. Thus, in order to prevent colloidal
sol formation a much lower ionic strength than 25 mM is needed in mixed Ca/Na montmorillonite
systems. The possible colloid release at low ionic strength has further been investigated usingswelling pressure cells where the region near the [Na+]-axis has been explored by circulating initiallypure NaCl solutions contacted with mixed Ca/Na montmorillonite. At low ionic strength, the NaClsolution is close to the equilibrium solution because very little Ca/Na exchange will occur because
under such conditions nearly all of the calcium remains in the montmorillonite. The outcome fromsuch experiments /­Birgersson et al. 2010/ is shown in Figure 3-32 Swelling pressure evolution oferoding test CaNaErosion02 /­Birgersson et al. 2010/. The montmorillonite is of Milos type and theion population is 50/50 Ca/Na. The circulating solution is 2 mM NaCl at the beginning of the time
period displayed. The pumping rate has been lowered by a factor of 4 or 16 compared to the original
rate throughout the test period. The effects of changing pumping rate are indicated in the diagram.
The points at which the circulating solution has been replaced by a solution of lower concentration
are also indicated. Note that cessation of erosion around day 15 occurs spontaneously, withoutchanging pumping rate or circulating solution. At day 57 the solution is changed to deionised water
12.0

Pumping rate/4
CaNaErosion02

Swelling pressure (MPa)

11.5
Pumping rate/4

NaCl 1 mM

NaCl 0,5 mM

Deionized
water

Normalpumping rate
11.0

Pumping rate/16
10.5

Pumping rate/4
10.0
0

10

20

30

40

50

60

70

Time (Days)

Figure 3-32. Swelling pressure evolution of eroding test CaNaErosion02 /­Birgersson et al. 2010/. The
montmorillonite is of Milos type and the ion population is 50/50 Ca/Na. The circulating solution is 2 mM
NaCl at the beginning of the time period displayed. The pumping rate has been lowered by a factor of 4
or 16 compared to the original rate throughout the test period. The effects of changing pumping rate areindicated in the diagram. The points at which the circulating solution has been replaced by a solution oflower concentration are also indicated. Note that cessation of erosion around day 15 occurs spontaneously,
without changing pumping rate or circulating solution. At day 57 the solution is changed to deionised waterand loss of montmorillonite starts again, which demonstrates the connection between sol formation andionic strength: erosion can be controlled physicochemically.

156

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and loss of montmorillonite starts again, which demonstrates the connection between sol formationand ionic strength: erosion can be controlled physicochemically and Table 3-19. Since charge isconserved one may define the concentration where sol formation ceases in terms of positive charge
equivalents. This definition also encompasses the scenario where the circulating solution initiallycontains calcium that will be almost totally exchanged for sodium at equilibrium.
The experimental SFZ shows the behaviour of montmorillonite in thermodynamic equilibriumwith an external solute having a given composition of Na+ and Ca2+ (monovalent/divalent cations).
The composition of the glacial Grimsel water /­Auqué et al. 2006/ is such that it is below the SFZ,
thus montmorillonite in equilibrium with Grimsel water will be calcium-dominated and not form
colloidal sols. One key component in understanding the sol formation process and whether the lossof bentonite will be critical is the approach to equilibrium. Modelling attempts within the Bentonite
Erosion Project have been carried out, using various counterion compositions in the initial bentonite
and various ionic compositions of the seeping groundwater. The evolution of the buffer is then calculated based on diffusion of ions and ion exchange at the buffer/groundwater interface /­Neretnieks
et al. 2009a, 2009b/. A pessimistic approach has been taken assuming that buffer will be lost as colloidal sol if the Ca2+ fraction in the buffer at the interface falls below 0.9 (see SFZ Figure 3-29). The
modelling suggests that for some combinations of the initial and boundary conditions, the fraction of
Ca2+ in the clay may fall below 0.9. Modelling taking the upper limit of the SFZ into consideration
has not yet been carried out.
Within the Bentonite Erosion Project the DLVO based force-balance model for spherical colloids/­Petsev et al. 1993/ has been adapted to parallel clay layers /­Liu et al. 2009, Neretnieks et al. 2009b/
and used to calculate the swelling of Na-montmorillonite into fractures filled with water of low
ionic strength (1:1 salt). The force-balance model uses DLVO to describe swelling pressure andKozeny-Karman-like expression fitted to experiments to describe hydraulic conductivity. The modelis adjusted and tested towards free-swelling experiments obtained through magnetic resonanceimaging (MRI) /­Dvinskikh et al. 2009/. Advective loss of montmorillonite is modelled by combining
the force-balance model for swelling with a viscosity model for the repulsive montmorillonite gel
and the Darcy equation for two-dimensional flow in a fracture intersecting the deposition hole. The
outcome of the modelling for different water velocities in a 1 mm aperture fracture is shown in
Table 3-20. The montmorillonite release rate is found to be proportional to the water velocity to thepower 0.41 and directly proportional to the aperture.
Table 3-19. Summary of the erosion experiments conducted with Ca/Na-montmorillonite in/­Birgersson et al. 2010/. X denotes the charge fraction of calcium in the clay. The column “Observed
stability” lists the lowest NaCl concentration for which no (or undetectable) erosion occurred.
SampleClay TypeX

Observed stability

CaNaErosion01

Milos

0.5

2 mM

CaNaErosion02

Milos

0.5

0.5 mM

CaNaErosion03

Milos

0.25

1 mM

CaNaErosion04

Milos

0.75

3 mM

CaNaErosion05

Wyoming

0.5

4 mM

CaNaErosion06

Kutch

0.5

1 mM

Table 3-20. Loss of smectite by advective flow /­Neretnieks et al. 2009b/.
Water Velocity, m/yr

Smectite release for 1 mm Penetration into the fracture
fracture aperture, g/yr
at the centre, m

0.10

11

34.6

0.32

16

18.5

0.95

26

11.5

3.15

43

7.0

31.50

117

2.1

315.00292

0.5

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157

Natural analogues/observations in nature

In principle, a large number of analyses of Swedish groundwater may serve as natural analogues
since they show low contents of clay colloids despite the fact that swelling clay is often found in
fractures. Specific studies have been made at the Äspö HRL /­Laaksoharju 2003/ and at Grimsel HRL/­Degueldre et al. 1996/.
A literature review of bentonite deposits with the objective of finding natural analogues was done asa part of the Bentonite Erosion Project /­Puura and Kirsimäe 2010/. However, the conclusion was that
relevance to the Bentonite Erosion Project is still questionable because of the uncertainties related tothe changes during tens and hundreds of millions of years.
Time perspective

Colloid formation is only relevant at low ion concentrations and likely to be relevant only in
conjunction with a glaciation. The effects may be significant only in the long-term perspective.
Handling in the safety assessment SR-Site

Water with cation content higher than 2–4 mM charge equivalents is considered to prevent colloidal
sol formation provided that the calcium content in the montmorillonite is above 20%, irrespective ofwhether the montmorillonite is of Wyoming, Milos or Kutch type. This criterion is also in agreementwith reported amounts of calcium salts needed to coagulate initially homoionic Na-montmorillonites/­Swartzen-Allen and Matijević 1976, Hetzel and Doner 1994, Lagaly and Ziesmer 2003/. Ion
exchange processes during the operation of the repository may alter the counterion content relative
to the initial state. However the content of calcium is not expected to fall below 20%. Thus theprocess of colloidal sol formation can be neglected during normal operating conditions. Present
day ground water at both the Forsmark and the Laxemar sites fulfil this condition as does Baltic
Sea water. During glacial conditions the cation content may possibly fall below 2–4 mM charge
equivalents, and colloidal sol formation can therefore not be excluded.
To estimate possible losses of buffer mass for glacial conditions, three stylised cases have been simulated. One concerns flow conditions that are similar to those at present and the other two analyse
conditions of increased flow. The model is based on the relationship that the montmorillonite loss
rate is proportional to the water velocity to the power 0.41 and directly proportional to the aperture
as proposed in /­Neretnieks et al. 2009b/. The underlying force-balance model in /­Neretnieks et al.
2009b, Liu et al. 2009/ is based on DLVO for parallel surfaces and therefore not valid for divalentcounterions and does not include edge-face interactions. The model thus assumes that the bentonite
consists of only montmorillonite which is converted to a pure Na-form and the buffer porewater
is assumed to be depleted of Ca2+ ions. This means that Ca2+ ions for the prevention of colloid
formation will not be supplied by the buffer. Since the groundwater is also depleted in Ca2+ ions,
erosion is assumed to occur at a rate that is determined by the product of the equivalent flow rate at
the deposition hole. This model maximises the loss rate assuming that the mass exchange between
the bentonite suspension and the flowing water is driven by diffusion. Correlation effects present fordivalent counterions as well as edge-face interactions contribute attractive forces. The omission ofthese is also considered to be conservative.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The presence of attractive correlation forces beyond the DLVO theory for systems with divalentcounterions is well established in peer reviewed journals, e.g. /­Guldbrand et al. 1984, Kjellander andMarčelja 1984, Kjellander et al. 1988/ as well as in advanced textbooks e.g. /­Evans and Wennerström
1999/.

158

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Gel-sol transition in sodium montmorillonites around the CCC value for NaCl and other sodium salts
has been studied using various methods, such as e.g. visual inspection, turbidity measurements rheological measurements and been reported in peer reviewed journals /­Hetzel and Doner 1993, Abendand Lagaly 2000, Lagaly and Ziesmer 2003, Tombács and Szekeres 2004/. /­Laaksoharju and Wold
2005/ use visual inspection and their results are in agreement with those in peer reviewed journals.
/­Birgersson et al. 2010/ (part of the Bentonite Erosion Project) also finds CCC values for NaCl inaccordance with published data. Furthermore they stress the importance of ion-equilibrium in mixed
Ca/Na systems which lead to the concept of a sol formation zone in Na+–Ca2+ concentration space.
The criterion that sol formation is prevented when the cation content in the groundwater is higher
than 2–4 mM charge equivalents and calcium content in the clay is larger than 20% /­Birgersson
et al. 2010/ is also supported by findings reported in peer reviewed journals /­Swartzen-Allen andMatijević 1976, Hetzel and Doner 1994, Lagaly and Ziesmer 2003/.
The handling of colloidal sol formation in SR-Site will be based on the documentation produced in
the Bentonite Erosion Project, e.g., /­Birgersson et al. 2010, Neretnieks et al. 2009b/. These documents as well as other supporting work have undergone formal internal SKB review. The intention is
also to publish relevant parts in peer reviewed journals and two such articles, /­Birgersson et al. 2011,
Hedström et al. 2011/ regarding the influence of Ca-ions and sol formation at low ionic strength,
have been accepted for publication. The derivation of the force-balance model is published in peer
reviewed journals /­Petsev et al. 1993, Liu et al. 2009/. The development of the MRI technique forstudying free swelling of bentonite/montmorillonite has also been published in a peer reviewed
journal /­Dvinskikh et al. 2009/.
Uncertainties

Uncertainties in mechanistic understanding
Knowledge concerning colloidal sol formation and colloid stability is good concerning the effects
of mono- and divalent ions. However, modelling of the correlation effects caused by divalent ionsis demanding. The prevention of sol formation at low ionic strength when the calcium content in
the montmorillonite is above 20% has been verified experimentally. Detailed understanding of thiseffect is less well developed and handling edge-face interactions in modelling is unsolved. The
force-balance model is based on parallel layers and furthermore only tested versus vertical swelling
experiments and for short swelling times, up to 5 weeks. The possible transport of colloids bydiffusive and flow transport is less well known. The effects of mechanical erosion of loose gels close
to sol forming conditions have not been fully examined. Experiments show that montmorillonite
does not penetrate filters with pore-sizes of 0.5 mm. However, it has not been shown that accessory
minerals could actually form an effective filter in the fractures. The understanding of the effect pH
on colloidal sol formation is incomplete.
Ca/Na ion exchange also influences the swelling properties of montmorillonite / Kjellander et al.
1988, Birgersson et al. 2010, 2011, Hedström et al. 2011/, which means that a more realistic modelling of bentonite swelling into fractures should include ion correlation effects /­Guldbrand et al. 1984,
Kjellander and Marčelja 1984, Kjellander et al. 1988/ as well as edge-face interactions that cause
coagulation at low ionic strength /­Lagaly and Ziesmer 2003, Tombács and Szekeres 2004, Birgersson
et al. 2010, 2011/.
Model simplification uncertainties for the above handling in SR-Site
The treatment for quantifying montmorillonite loss during glacial conditions in SR-Site is based on
the model in /­Neretnieks et al. 2009b/. It should be noted that a number of simplifying assumptions
have been made.
The following assumptions must be revisited and checked for reasonableness:
1. The assumption that the force-balance model is conservative for the swelling of
Na-montmorillonite. Predicted CCC is claimed to be about 50 mM whereas the underlying
DLVO model gives 1–2 M.
2. Assumption of Darcy flow.
3. Disregarding wall friction is assumed to be conservative.

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159

The following could act to decrease the dispersion rate:
1. The change of composition in the diffuse layer is not accounted for.
2. The presence of 20% or more Ca in the clay.
3. A model beyond DLVO for parallel clay layers. Correlation as well as edge-face interactions.
4. Physical hindrance of clay particle dispersion in the fracture.
5. Clogging of pathways by the clay being pressed out into the fracture.
6. Sedimentation of clay particles in fracture.
Input data and data uncertainties for the above handling in SR-Site
The evolution of groundwater composition, and groundwater flow conditions are the important
uncertainties. These will be evaluated in integrated assessment modelling, see further the Interim
report of the safety assessment SR-Site. The state of the buffer with respect to monovalent/divalention ratio and content of accessory minerals at the time of contact with glacial water of low ionicstrength is uncertain. Diffusion coefficients for Ca and Na and their variation with water ratio arealso uncertain.

3.5.12 Radiation-induced transformations
Overview/general description

This process covers the direct effect of radiation on the stability of the montmorillonite componentof the buffer material. Indirect effects through the radiolysis of porewater are covered in that process(see Section 3.5.13).
Montmorillonite in the buffer can potentially be broken down by γ-radiation. The result is a decreasein the montmorillonite component of the buffer.
In the case of a failed canister, radionuclides sorbed in the buffer could possibly affect the propertiesby α-radiation.
Dependencies between process and buffer variables

Table 3-21 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Boundary conditions

A quantitative treatment of the process requires knowledge of the radiation field in the buffer, which
is obtained from the process Radiation attenuation, see Section 3.1.1.
Model studies/experimental studies

Experimental studies have shown that the accumulated radiation doses to which the bentonite
will be exposed in a deep repository do not cause any measurable changes in the montmorillonite
abundance. MX-80 bentonite saturated with weakly brackish water to a density of 2,050 kg/m3 wasirradiated for one year with a total radiation dose of 3·107 Gy, which is orders of magnitude morethan will be the case in the repository. The specimen was confined in a cylinder with one closed end
consisting of iron and irradiated with a 60Co source, while the other end consisted of a porous steel
filter through which a water pressure of 1.5 MPa was imposed /­Pusch et al. 1993b/. The irradiatedend, which had an absorbed dose rate of 3,972 Gy/h, was kept at 130°C, and the opposite end, which
had a dose rate of 456 Gy/h, at 90°C. Mineral assays (XRD, IR, CEC) of this specimen and of aparallel specimen not exposed to radiation but to the same hydrothermal environment, showed nosignificant change in the montmorillonite quantity in either of the specimens.

160

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Table 3-21. Direct dependencies between the process “Radiation-induced transformations” andthe defined buffer variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Buffer geometry

Total buffer massavailable

Pore geometry

No

No

Radiation intensity

Yes

Dose rates are tooNo
low to cause any
significant transformations – the process is
excluded from SR-Site

Temperature

No

No

Water content

NoNo

Gas content

No

No

Hydrovariables
No(pressure and flows)

No

Stress stateNo

No

Bentonitecomposition

Not detected(Yes)

Dose rates are too low
to cause any significant
transformations – theprocess is excluded
from SR-Site

Montmorillonite
composition

Not detected(Yes)

Dose rates are too low
to cause any significant
transformations – theprocess is excluded
from SR-Site

Porewater
composition

No

No

Structural and straymaterials

No

No

Dose rates are tooNo
low to cause any
significant transformations – the process is
excluded from SR-Site

The buffer will not be exposed to α- and β-radiation as long as the canister is intact. In the case ofa failed canister, radionuclides that migrate out will be sorbed in the buffer and may expose it to aradiation dose.
In the radionuclide calculations for SR 97 /­SKB 1999/, it was found that the total concentration of
α-emitters in the buffer closest to the canister would give a total dose of 8·1015 alphas/g of bentonite
for the first million years with the assumption of an early canister failure. In most parts of the buffer,the α-dose will be much less since the α-emitters are strongly sorbed and will stay in the vicinity of
the canister. 8·1015 alphas/g corresponds to an absorbed dose of 8 MGy. /­Gu et al. 2001/ show thatthe amorphisation dose for montmorillonite is 30 GGy. Since the total dose received is more than
three orders magnitude lower than the amorphisation dose, the impact on the properties of the buffer
can be expected to very small.
Natural analogues/observations in nature

No studies of systematic studies of clay mineralogy under the influence of alpha radiation are available.
Time perspective

See Section 3.1.1.

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161

Handling in the safety assessment SR-Site

Based on the experimental and modelling studies mentioned above it is shown that the effect ofradiation on buffer properties is insignificant and the process is therefore neglected.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site/Gu et al. 2001/ is a peer reviewed publication in the open literature. The same reference was also
used by ANDRA in /­ANDRA 2005/. /­Pusch et al. 1993b/ is an old SKB report, but there is no
documentation available that would indicate that the conclusions would be invalid.
Uncertainties

There are no uncertainties of relevance to the safety assessment.

3.5.13 Radiolysis of porewater
Overview/general description

Gamma radiation from the fuel that penetrates through the canister can decompose porewater byradiolysis, forming OH radicals, H2, O2 and several other components. The oxygen is consumed rapidlyby oxidation processes that affect the redox potential, while the hydrogen may be transported away. The
canister wall thickness is, however, sufficient so that the effect of γ-radiolysis on the outside is negligible/­Werme 1998/. This process is also described in the Fuel and Canister Process Report /­SKB 2010k/.
Dependencies between process and buffer variables

Table 3-22 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Boundary conditions

A quantitative treatment of the process requires knowledge of the radiation field in the buffer, which
is obtained from the process Radiation attenuation.
Model studies/experimental studies

Corrosion of copper in water in the presence of γ-radiation has been studied in a few experiments.
These are summarised in /­King et al. 2001/. The results show no effect on the corrosion rate evenwith higher doses than those expected in the repository.
Natural analogues/observations in nature

Not applicable.
Time perspective

γ radiation is significant during the period approximately 1,000 years following canister installation.
Handling in the safety assessment SR-Site

After water saturation, radiolysis of water near the canister will occur. This will lead to the formationof oxidants and hydrogen. If these radiolysis products are not removed, equilibrium will soon bereached whereby further production of hydrogen and oxidants ceases. If the radiolysis products are
removed, however, for example by the diffusion of hydrogen away from the canister or by reactionof the oxidants with copper or other oxidisable species in the system, radiolysis can continue.
Calculations of radiolysis outside a canister have been made by /­Christensen and Pettersson 1997/.
An estimate of the maximum possible amount of oxidised copper can be made if one assumes that162

TR-10-47

Table 3-22. Direct dependencies between the process “Radiolysis of porewater” and the definedbuffer variables and a short note on the handling in SR-Site.
Variable

Variable influence on process

Process influence on variable

Influence present?
(Yes/No)
Description

Influence present?
(Yes/No)
Description

Handling of influence(How/Why not)Buffer geometry(Determines the total
amount of water in theradiation field)

No

Pore geometry

No

No

Radiation intensity

Yes

Temperature

No

Water content

Yes, determines theamount of water
available

Gas content

No, but indirectly
through water content

Dose rates are too
low to cause any
significant radiolysis
outside canister – theprocess is excluded
from SR-Site

Handling of influence(How/Why not)No

No
Dose rates are too
low to cause any
significant radiolysis
outside canister – theprocess is excluded
from SR-Site

No

No

Hydrovariables
No(pressure and flows)

No

Stress stateNo

No

Bentonitecomposition

No

No

Montmorillonite
composition

No

No

Porewater
composition

Determines thereactions

Structural and straymaterials

No

Dose rates are too
low to cause any
significant radiolysis
outside canister – theprocess is excluded
from SR-Site

Yes

Dose rates are too low
to cause any significant
radiolysis outside
canister – the process is
excluded from SR-SiteNo

the oxidation of copper will be as efficient as the oxidation of dissolved Fe(II) in Christensen’s calculations. After about 1010 s (~ 327 years) the amount of precipitated Fe(III) is 1 mol/dm3. Assumingthat the oxidants present in a 5 mm water layer surrounding the canister reach and react with the copper
surface, the total volume of irradiated porewater for a porosity of 0.44 /­SKB 2006c/ will, (for canister
dimensions L = 5.85 m, D = 1.05 m, and neglecting the details of the lid and bottom designs), be:[4.85∙10.5∙π + 2∙(10.5/2)2∙π]∙0.05∙0.44 dm3 = 39 dm3
This gives a total amount of precipitated Fe(III) of 39 moles and, consequently, 39 moles of corroded
copper. For a canister area of [4.85∙10.5∙π + 2∙(10.5/2)2∙π] dm3 and a copper density of 8,930 g/dm3,
this gives a corrosion depth of approximately 16 μm. Conversely, the experimental observations of/­King et al. 2001/ indicate that the overall effect of radiation on copper corrosion under oxidisingconditions will be negligible or even result in a lowering of the corrosion rate.

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163

Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The calculations in /­Christensen and Pettersson 1997/ are directly applicable to the repository situation. The report has been produced within Studsvik’s Quality Assurance System according to ISO
9001, which requires review and approval by two different individuals.
Uncertainties

There are no uncertainties currently identified that are of relevance to the safety assessment.

3.5.14 Microbial processes
Overview/general description

Under certain conditions, microbial processes can result in the formation of gas and sulphide/­Pedersen 2002/. Gas formation can give rise to pore pressure and mechanical effects in the bentonite
buffer and sulphide from sulphate reducing bacteria (SRB) can corrode the copper canister. Sulphideformation must take place in the buffer, near the canister and be of considerable extent for corrosion to be a threat to canister integrity. This is mainly due to the fact that sulphide is commonly
precipitated with ferrous iron. Therefore, sulphide concentrations do not occur at high concentrations
in groundwater and its diffusive transport capacity is very low. In order for the processes describedabove to take place, the bacteria must be active and have access to water, nutrients and space/­Pedersen 2002/.
Sulphate-reducing bacteria are very common in deep Fennoscandian groundwater. They are reported
to occur abundantly down to depths of at least 600 to 700 m /­Haveman and Pedersen 2002, Pedersen
2001, Pedersen et al. 2008/. Microbes are generally very robust and can withstand harsh conditions.
The potential for sulphide production is significant at repository depth and possible effects on therepository function must be considered in the safety assessment /­Hallbeck and Pedersen 2008/.
Dependencies between process and buffer variables

Table 3-23 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Table 3-23. Direct dependencies between the process “Microbial processes” and the defined buffer
variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Buffer geometryNo, indirectly
through pore geometry

No

Pore geometry

Yes.
Microbial activity
decreases exponentiallywith increasing buffer
densities and approach
nil at densities above
2,000 kg/m3

Maximal possible
production of sulphideand gas is calculated
for all possible poregeometries (densities)
and the amounts
produced are comparedwith the amount needed
to corrode a copper
canisterNo

Radiation intensity

Yes

Lethal effects from theradiation field are notconsidered

No

164

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Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Temperature

Yes.
Microbial activity is
positively correlated withtemperature and will behigher during the thermal
period compared tothe ambient
temperature period

Maximal possible
No
production of sulphideand gas is calculated forhigh and low temperatures and the amounts
produced are comparedwith the amount needed
to corrode a copper
canister

Water content

Yes.
Decreasing water
availability reduces
the diversity and thepossibilities for microbialactivity.
At the same time,increasing water content
increase the stress state(swelling pressure)

Maximal possible
No
production of sulphideand gas is calculated forall possible water contents and the amounts
produced are comparedwith the amount needed
to corrode a copper
canisterGas content

Yes
Microbial activity ispossible with hydrogenand methane as sourcesof energy

Maximal possible
production of sulphideis calculated for all possible gas contents andthe amounts produced
are compared with theamount needed to corrode a copper canister

Hydrovariables(No)(pressure and flows)

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Yes.
Only effects on theMicrobial activity may buffer/canister are
result in the production consideredof carbon dioxide andmethaneYes
induced gas pressure
induces water flow

Neglected since theeffect is very small.

Stress state

Yes.
Microbial activity
decreases exponentiallywith increasing buffer
densities and approachzero at densities above
2,000 kg/m3

Maximal production ofNosulphide is calculated forall possible pore geometries and the amounts
produced are comparedwith the amount needed
to corrode a copper
canister

Bentonitecomposition

Yes.
Organic carbon canbe utilised and several
types of commercial
bentonites contain
viable bacteria including
thermophilic sulphate
reducing bacteria

Microbial activity canbe possible.Maximal
production of sulphideis calculated for the
bentonite in use andthe amounts produced
are compared with theamount needed to corrode a copper canister

Yes.

Montmorillonite
composition

Yes.
Iron reducing bacteria
can use Fe(III) in smectite as an electron acceptor in their respiration

The risk maximum
layer charge change
is evaluated forthe bentonite used

Yes.
Iron reducing bacteria
may degrade montmorillonite which will
decrease the swelling
properties of thebentonite

The risk for illitisation
is evaluated for different combinations ofbentonite process variables. Discussed in the
bentonite transformation
scenario

Porewater
composition

Yes

Maximal production ofsulphide and gas is calculated for all possibleporewater compositions
and the amounts
produced are comparedwith the amount needed
to corrode a copper
canisterYes

Only the indirect effects
on the buffer/canisterare considered

Structural and straymaterials

Yes

No stray materials areYesassumed to be present
within the buffer material

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No stray materials
are assumed to bepresent within thebuffer material

165

Buffer geometry: The overall dimension of the buffer will have no direct impact on the process. An
expanding buffer – into fractures or into the tunnel could however affect the pore size and impact thepore geometry.
Pore geometry: The density of the buffer affects the pore size. Pores with sizes smaller than about
200 nm cannot be penetrated or inhabited by microbes. Microbes vary in size from about 200 nm up
to 600 µm.
Radiation intensity: Microbes are generally radiation resistant, but it varies from species to species.
Increasing dose will decrease the number of surviving microbes.
Temperature: All microbes have a temperature range within which they are active. Increasing thetemperature will increase microbial activity. Temperatures above the maximum tolerable temperature
are lethal. The highest demonstrated temperature for active microbial life is 113°C. /­Stetter 1996/.
Water content: Decreasing water availability reduces microbial diversity. Many microbes cancompensate for low water content using metabolic, energy consuming processes but some water isneeded for active life. A combination of low energy availability and low water content is detrimental
to many microbes /­Potts 1994/.
Gas content: Hydrogen and methane can be consumed by microbes; a large effect is obtained if
oxygen is present. Active microbes can produce gas if organic carbon is available for metabolism.
Methanogens generate methane from hydrogen and carbon dioxide. Anaerobic methane oxidation(ANME) has been identified as a crucial microbial process. This process outputs sulphide whenmethane is oxidised with sulphate in the ANME process. Therefore, methane may imply sulphideproduction where microbial life is possible in the buffer. As judged from results from Olkiluoto,
Finland /­Pedersen et al. 2008/, and also from the Äspö hard rock laboratory (HRL) , the rate canbe high. Similarly, hydrogen from depth and/or corroding iron may result in sulphide via microbial
processes. When oxygen is present, methane is oxidised with concomitant reduction of oxygen/­Eriksson 2007/.
Hydrovariables (pressure and flows): Most microbes tolerate large variations in hydrostatic pressure and flow.
Stress state: Microbes tolerate limited mechanical pressure. Swelling clay will disrupt microbes
when the pore size becomes smaller than the size of the microbe.
Bentonite composition: There is a positive correlation between microbial activity and the concentration of organic carbon. Previous investigations and ongoing investigations of materials from theAlternative Buffer Materials project at Äspö show that sulphate reducing bacteria are common in
many clays. Several of the alternative materials appear to have thermophilic sulphate reducers thatgrow much better at temperature above 50°C, than at room temperature. It is obvious that manyof the buffer materials come with sulphate reducers of which several withstand high temperatures/­Masurat et al. 2010a/.
Montmorillonite composition: Iron-reducing bacteria have been demonstrated to dissolve iron-richsmectite through reduction of structural Fe(III) at room temperature and 1 atmosphere within 14
days under good growth conditions /­Kim et al. 2004/. Microbial processes may consequently have an
influence on the variable if the montmorillonite is rich in Fe(III).
Porewater composition: There is a positive correlation between microbial activity and the concentration of organic carbon. There is a limited effect on the porewater chemistry if the microbes are
metabolically active. Gas and sulphide production can occur.
Structural and stray materials: There is a positive correlation between microbial activity andorganic carbon present in stray material. Microbial degradation of organic stray material is possible.
See previous discussion under Bentonite Composition. No stray materials are assumed to be present
in the buffer.

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Boundary conditions

The relevant boundary conditions in order to treat the process quantitatively are those of the transport processes that control the exchange of solutes (microbe nutrients) and dissolved gas (sourcesof energy) between the buffer porewater and the water in its adjacent components, i.e. the boundary
conditions of the processes diffusion and advection.
Model studies/experimental studies

A Large scale Canadian buffer experiment
A full-scale experiment (Buffer-Container Experiment, BCE), with buffer material consisting of
50/50% bentonite/sand was performed at Atomic Energy of Canada Limited´s (AECL) undergroundlaboratory in Canada. The results showed that microbes, with a few exceptions, could only be
cultured from buffer samples with a gravimetric water content of 15% or more, which is approximately equivalent to the water content in a 100% bentonite having a saturated density of 2,000 kg/m3/­Stroes-Gascoyne et al. 1997/. Elevated temperatures had no effect on the microbes. These results
were interpreted as an effect of limited availability of water. The result of the BCE raised questions
about the survival of microbes, and especially SRB, in buffer materials with 100% bentonite and has
subsequently led to detailed laboratory experiments.
Laboratory experiments exploring the survival of bacteria in buffer
The Canadian field experiment was followed up by laboratory experiments. Two species of sulphatereducing bacteria were mixed with MX-80 bentonite at varying saturated bentonite densities, from1,500 kg/m3 to 2,000 kg/m3 /­Motamedi et al. 1996/. The species were Desulfovibrio aespoeensis andDesulfomicrobium baculatum, both isolated from deep groundwater at the Äspö HRL. None of thespecies survived 60 days at densities above 1,800 kg/m3. D. baculatum survived the better of thetwo, remaining culturable for 60 days at 1,500 kg/m3. Although microbial activity diminished rapidly, it was argued that this was due to the laboratory conditions during this experiment, which may
have added some extra constraints to the ones found in the field situation. The laboratory experiment
represents a closed situation with an artificial microbial population, while field conditions would
be of the open system type with a natural microbial population. A long-term field experiment wastherefore initiated to assess microbial issues.
Survival of microbes in long term buffer tests with in situ conditions
The long-term test (LOT) of buffer performance aims to study models and hypotheses of the physical properties of a bentonite buffer. Several species of bacteria with different relevant characteristics
were introduced into the LOT bentonite. Mesophilic, thermophilic and spore-forming sulphate
reducing bacteria, together with desiccation resistant, chemo-organotrophic or chemolithotrophic
spore- and non-spore forming bacteria were mixed with bentonite clay to give approximately
100 million bacteria per gram (dry weight) of clay /­Pedersen et al. 2000a/. The clay with bacteria
was subsequently formed into cylindrical plugs of 20 mm length and diameter, and installed in bentonite blocks exposed to low (20–30°C) and high (50–70°C) temperatures. The blocks were installedin the LOT boreholes immediately after the bacterial plugs were introduced /­Pedersen et al. 2000a/.
The experiment was terminated after 15 months. The major outcome was the effective elimination,
to below detection limits, of all bacteria except the spore-forming populations. All of the three spore
formers survived at the lower temperature. The numbers remaining were, however, much lower
than those initially introduced. The approximately 100 million spore-forming bacteria per gram (dryweight) of clay were reduced 100- to 10,000-fold. This can be interpreted as showing that the cell
death rate was higher than the growth rate, which may have been zero, or close to zero. The sporeforming sulphate-reducing bacterium was the only one of the three that survived at high temperature.
It was concluded that the spore formers most probably survived as spores, and spores do not produce
sulphide. Survival is not equivalent to activity. Since the methods used in this experiment did notreveal activity, additional experiments were set up to include measurement of sulphide production insimulated repository conditions.

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Laboratory experiments exploring microbial sulphide production in buffer
The worst-case scenario for copper canister corrosion would be if SRB formed biofilms on the canisters or grew intensively in the buffer close to the canister. To mimic this situation, several experiments have been executed and evaluated. In a first series, swelling pressure oedometers were loadedwith bentonite at different densities, corresponding to different water activity values /­Pedersen
et al. 2000b/. A copper disc was placed between the bottom lid and compacted bentonite. Different
SRB were added to the clay and the discs, together with 35SO42–. The species used were laboratory
cultures. Finally, oxidised silver foil was placed between the disc and the clay. The oedometers werereassembled and incubated for 4 weeks at the respective optimum temperatures and three different
saturated bentonite densities: 1,500, 1,800 and 2,000 kg/m3, corresponding to water activities at theunpressurised condition of 0.999, 0.994, and 0.964, respectively. After incubation, 35S-Ag2S was
localised on the silver foils and quantified by electronic autoradiographic imaging (Packard instantimager electronic autoradiography system, Meriden, U.S.A.). The amount of Ag2S formed was used
as a measure of the sulphate-reducing activity.
The SRB used were active at a saturated bentonite density of 1,500 kg/m3, but sulphide production
was virtually absent at higher densities. This experiment indicated that SRB probably cannot be
active at the canister surface at a repository bentonite density of 2,000 kg/m3 at full water saturation.
However, the experiment was run with two laboratory species and it may be possible that otherspecies that were not tested could survive better. Therefore, additional experiments were carried out
using natural groundwater that commonly contains many hundreds of different microbial species,
including several naturally occurring species of SRB.
Field experiments exploring microbial copper sulphide production in buffer
The activity of SRB was again investigated in compacted bentonite with densities of 1,500, 1,800
and 2,000 kg/m3 at full water saturation using stainless steel oedometers with 35SO42− as an oxidisedsulphur source for the SRB respiratory processes /­Masurat and Pedersen 2003, Masurat et al. 2010b/.
This time, the investigations were performed under in situ conditions, including a hydrostaticpressure of 25 bars at a depth of 450 m at the Äspö Hard Rock Laboratory. The sources of SRB were
the groundwater and the bentonite. Cultured SRB were not added. Canister type, copper plates wereplaced in the oedometers, with the top either in contact with the added groundwater (exposed) orwith 3–4 mm bentonite between the plate and the added groundwater (embedded) (experiment B).
The initial slot between bentonite and rock in a repository was simulated by a gap between the water
inlet filter and the compacted bentonite. The bentonite density was thereby initially low at this endof the sample. The water uptake into the sample led to a density homogenisation and a successive
increase of density to the final test density. Radioactive hydrogen sulphide formed by the SRB
reacted with the copper and formed radioactive copper sulphide, which was measured by electronic
radiography as described for the lab experiments above. A similar set-up (experiment G) with filtersterilised groundwater (0.2 µm) was used as control. Finally, a third bentonite set-up was heat treatedat 120°C for 12 h and sterile groundwater added. The results showed that SRB were active andproduced hydrogen sulphide during the initial phase of bentonite swelling, which was monitored forabout 70 days. The copper sulphide production by the SRB was inversely related to the final densityof the bentonite, as the highest density had the lowest overall SRB activity (Figure 3-33).
The experiments with sterile groundwater on the one hand, and sterile groundwater and heat
treatment of the bentonite on the other, showed that SRB were present in a dormant state in thecommercial MX-80 bentonite. By addition of water, these dormant SRB became active and started
to produce hydrogen sulphide. It is obvious that there are two sources of SRB to buffer material; thegroundwater and the bentonite itself.
Natural analogues/observations in nature

There is a natural analogue in Dunarobba, Italy, where a bentonite slide buried a forest approximately
one and a half million years ago. Trees that have not been decomposed by microorganisms can still
be found. This indicates that conditions for microbial activity in bentonite clay are unfavourable.

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Figure 3-33. Graphic representation of the mean copper sulphide production rates on copper plates thatwere exposed (ex) and embedded (em) at different saturated bentonite densities (1,500, 1,800 and 2,000 kg m–3).
The treatments were un-filtered (uf) filtered (f) in experiment G and exposure to 25°C (25) and 12°C (120)
during 15 h. See text for details. Figure is from /­Masurat et al. 2010b/.

Time perspective

The time perspective concerning survival and activity of microbes in compacted bentonite has beensummarised in a conceptual model as follows /­confer Figure 3-4 in Pedersen 2002/. At the time ofdeposition, there will be a canister, bentonite blocks and a hole in the rock. The next step will be toallow water to fill all the void volume. This water can be groundwater from the rock or, alternatively,
groundwater or process water added from above at the time of container deposition. Irrespectiveof the source, microbes will be present in the water and these microbes will mix with the buffer, as
described above.
Swelling of the clay will introduce groundwater microbes into the clay, to a depth that is dependent
on the initial slot, on both the rock and the canister side of the bentonite. Additionally, the microbes
indigenous to the bentonite will be present in the entire bentonite buffer. The experimental dataon survival and activity of microbes in bentonite suggest that the number of viable microbes will
decrease rapidly during swelling and that very few viable cells will be present at full compaction.
Sulphate-reducing activity will also approach zero when full compaction is achieved and the only
survivors will be microbes that have formed spores. Our results indicate that viable cell activity will
be impossible at full compaction, as spores are inactive. Although spores generally are very resistant
to difficult environmental conditions, they do still die. All our experiments so far indicate a decreasein the number of viable spores at full compaction so that a slow, but significant, death rate of spores
would eventually lead to the complete eradication of life in the buffer. The timing for such sterilisation has not yet been clarified. Once the bentonite becomes sterile, it will most probably not be
re-infected. The theoretical pore size of the clay is 100–1,000 times smaller than the average-sized
microbe, meaning that no new microbes can enter into the buffer.
Handling in the safety assessment SR-Site

There is a limit to the rate of microbial sulphide corrosion that must not be exceeded. This rateis defined by the thickness of the canister and is based on the assumption that sulphide corrosionof copper is of a non-pitting type. A production rate of about 25,000 fmol Cu2S/mm2 a day will

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corrode 50 mm depth of a canister in 1 million years /­Masurat et al. 2010b/. If the safety limit is set
to 25 fmol Cu2S/mm2 a day, the corrosion will correspond to 0.5 mm over 1 million years, leaving
49.5 mm of the canister intact. This safety limit corresponds to the maximum measured initial corrosion rate at a swelling pressure of about 8 MPa (2,000 kg/m3) under field conditions (Figure 3-33).
Decreasing swelling pressure will result in an increasing microbial copper sulphide production
rate. The microbial copper sulphide production rate also depends on the diffusion rate of nutrientsand energy from groundwater, radiation intensity, the water content and the level of heat stress(temperature). For additional information see the process “copper corrosion” in the Fuel and Canister
process report /­SKB 2010k/.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

All the references in this section are from peer reviewed scientific journals. However, the mostrecent may be in the progress of being published.
Uncertainties

Uncertainties in mechanistic understanding
The rate of microbial copper sulphide production (if any) at the final swelling pressure remains to be
determined. Rate limiting factors such as diffusion of nutrients, energy and sulphide, radiation, watercontent and heat stress (temperature) must be clarified and defined.
Migration of microorganisms through compacted bentonite is not well studied. The restrictions due
to a low pore size may need to be defined experimentally.
Formation of biofilms on the canister surface has been suggested to occur. This may be the caseduring the initial swelling phase. However, those microorganims will obey the same restrictions as
any microbe in other parts of the buffer. The stress from heat and radiation will be more severe at thecanister surface than anywhere else in the buffer.
The mechanisms for survival of microbes in bentonite and buffer materials are not fully understood.
Evidence for the presence of viable microbes in commercial MX-80 bentonite suggests that SRB andalso other species can withstand the very dry conditions (10%) in commercial bentonite. The present
hypothesis is that microbes are introduced to the clay during mining and processing. The clay causes
dehydration, which will gently remove water from the cell. The cells will be inactive, but viable.
This process is similar to freeze drying of cells, which also results in inactive but viable cells, e.g.
freeze dried baker’s yeast. Adding water revives the microbes to an active life.
The observed disappearance of microbial activity and viability in highly compacted bentonite underrepository conditions is hypothesised to be a combined effect from desiccation, mechanical pressureand other stress factors. The sum of stress factors, radiation, heat, low initial water availability anda high mechanical pressure after saturation of the buffer results in death of the microbes. However,
the exact mechanisms for the observed disappearance of viability of microorganisms remain to beclarified. At full water saturation, water activity is fully restored, but a mechanical pressure develops
which corresponds to the water activity in the unpressurised clay according to the Kelvin equation.
The low concentration of sulphide observed in many groundwater samples depends on the availability of ferrous iron, if the availability of iron decreases, the aqueous concentration of sulphide will
increase, as it does at some depths in Olkiluoto and recently found also in monitored boreholes at
Forsmark and Laxemar, where concentrations approach 10–20 mg sulphide/L. pH must be taken into
consideration as dissociation of hydrogen sulphide (gas) depends on pH.
Model simplification uncertainties in SR-Site
Not relevant since a quantitative model for microbial processes in the buffer is lacking.
Input data and data uncertainties in SR-Site
Not relevant since a quantitative model for microbial processes in the buffer is lacking.

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3.5.15 Cementation
Overview/general description

The term “cementation” has often been used in a broad sense to describe processes, which leadto specific changes in rheology and swelling properties of the buffer material. A number of quite
different chemical/mineralogical and mechanical processes could conceivably cause cementation, and these are treated here. . The accompanying sections dealing with related processes, i.e.
montmorillonite stability, ion exchange, accessory mineral alteration, diffusive transport etc areconsequently of most relevance for the cementation process. The main concerns regarding the effectsof cementation of the bentonite buffer are increase in hydraulic conductivity reduction of swelling
ability and increase of shear strength.
Cementation due to precipitation of minerals and amorphous compounds commonly leads to lower
porosity, which in turn usually leads to lower hydraulic conductivity in soil and rock materials.
However, the porosity in the bentonite buffer is as high as 43%, and so the low hydraulic conductivity is obviously not due to low porosity but to montmorillonite/water interaction. Consequently,
a large reduction in porosity caused by cementation may still lead to an increase in hydraulicconductivity if the montmorillonite/water interaction is reduced or counteracted by cementation.
Bentonite buffer rheology is characterized by low shear strength, i.e. the material deforms at
relatively low shear forces. Precipitates may be expected to increase the shear stress and decrease
the strain at failure, i.e. the material becomes more brittle. A rock displacement may thereby transmit
higher shear stresses to the canister.
The most obvious cause of cementation is precipitation of minerals and amorphous solids in the bentonite. Potential mineral phases include gypsum, or anhydrite at high temperatures, calcite and silica
minerals/compounds and iron oxide/hydroxide minerals/compounds. The underlying source of ions
may be the molecular structure of the montmorillonite mineral layer, the interlayer space, accessory
minerals and the groundwater. The resulting cementation does not necessarily have to appear in thesame volume, since released species can be transported in the buffer owing to the prevailing thermaland concentration gradients.
In addition to the mineralogical causes of cementation, there is a (principally) mechanical effectwhich may be of importance. The swelling pressure at a certain clay density is significantly
dependent on the stress history of the bentonite material /­Börgesson et al. 1995/. A similar hysteresis
may also be apparent in the rheological properties and the mechanical strength in consolidated clay
would therefore have higher shear strength than a swollen sample at the same density. Such a purely
mechanical cause of rheological changes is expected to have a limited and relatively small effecton buffer performance. At present, SKB is running laboratory tests in order to quantify and better
describe such mechanically-induced rheological changes /­Dueck 2010/.
The mineralogical processes that lead to cementation may take place at relatively low temperatures,
but all experimental data indicate that the processes are only of importance at relatively hightemperatures, in response to temperature gradients or after relatively large temperature changes.
Dependencies between process and buffer variables

Table 3-24 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Boundary conditions

The boundary conditions for the cementation process are the porewater composition, groundwatercomposition, temperature in different parts of the buffer and the buffer composition.
Model studies/experimental studies

The principal mineralogical processes that may cause cementation are treated separately in thedescriptions of underlying processes; mainly Montmorillonite Transformation (Section 3.5.9),

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Table 3-24. Direct dependencies between the process “Cementation” and the defined buffer
variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Buffer geometry

No

No

Pore geometry

Yes, through ionic
equilibria

Radiation intensity

No

Temperature

Yes, major impact

See Sections 3.5.6
Yes, minor effects
and 3.5.9 , Alteration of through changes in
impurities and montmo- thermal conductivity.
rillonite transformation

Neglected unless modeling shows significant
amounts of precipitation

Water content

Yes, Major impact

See Section 3.5.6,
Alteration of impurities

Yes, throughchanges in porosity

See Section 3.5.6,
Alteration of impurities

Gas content

Yes, throughconcentration

See Section 3.5.6,
Alteration of impurities

No

Hydrovariables
Yes, through(pressure and flows) transport of species

See Section 3.5.6,
Alteration of impurities

Yes, indirect throughpore geometry,central impact

Stress state

Yes, through ionic
equilibria

See Section 3.5.6,
Alteration of impurities

Yes, major possible
See Section 3.5.6,
impact by changed
Alteration of impurities
rheological properties

Bentonitecomposition

Yes, accessory
See Section 3.5.6,
minerals are of major Alteration of impurities
importanceYes, by definition incase of precipitation

Montmorillonite
composition

Yes, through mineralcomposition andionic equilibria withcharge compensating
ions

See Sections 3.5.6,
3.5.5 and 3.5.9,
Alteration of impurities,
sorption and montmorillonite transformation

Yes, by definition
See Sections 3.5.5 andif the underlying
3.5.9, sorption and montprocess is montmoril- morillonite transformation
lonite alteration

Porewater
composition

Yes, by definition

See Sections 3.5.6,
Yes, by definition in
3.5.5 and 3.5.9 ,case of precipitation
Alteration of impurities,
sorption and montmorillonite transformation

Structural and straymaterials

Yes, especially theconcrete and iron

See Sections 3.5.6,
No
3.5.5, and 3.5.9 ,Alteration of impurities,
sorption and montmorillonite transformation

See Sections 3.5.6
Yes, main effectand 3.5.9, Alteration of
impurities and montmorillonite transformation

See Section 3.5.6,
Alteration of impuritiesNo

See Sections 3.5.6, 3.5.5
and 3.5.9, Alterationof impurities, sorption
and montmorillonitetransformation

See Sections 3.5.6 and3.5.9 , Alteration of impurities and montmorillonitetransformation

See Sections 3.5.6, 3.5.5
and 3.5.9 , Alterationof impurities, sorption
and montmorillonitetransformation

Alteration of Impurities (see Section 3.5.6), and Sorption (Section 3.5.5). In general, there isextensive knowledge concerning precipitation in soil material and a vast number of publications
with regard to natural systems, experimental work and modelling. There are also a number ofpublications dealing more specifically with cementation in radioactive waste repositories and several
are related to SKB projects e.g. /­Pusch 1977/, /­Pusch et al. 1998/. The CEA/SKB field test at hightemperatures (170ºC) relative to KBS-3 conditions, which were performed in the Stripa mine,
showed extensive precipitation of especially anhydrite but also of silica compounds in the warmest
part. The ductility of the innermost centimetre was completely lost and the hydraulic conductivity
dramatically increased /­Pusch et al. 1993/. Material from the one year LOT S1 test, performed at
a maximum temperature of 90ºC at the Äspö HRL, did not show any increase in shear strength
or in hydraulic conductivity /­Karnland et al. 2000/. Material from the LOT A2 test, exposed to a
maximum temperature of 130ºC for 6 years, had slightly altered rheological properties in the form of
reduced strain at failure, but no changes in hydraulic conductivity were found /­Karnland et al. 2009/.
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Minor mineralogical changes were found in the warm sections in this material; calcium sulphate, inparticular, was redistributed and precipitated in the form of anhydrite in the warm sections. No clear
correlation was found, however, between the calcium sulphate content and reduced strain. In subsequent laboratory experiments, it was found that a short heating pulse also produced similar changes
in the strain at failure. SKB is presently running two laboratory projects concerning cementationin order to quantify the cementation effects of short-term heating, and to identify the mineralogicalchanges in the LOT A2 parcel in detail. /­Arthur and Zhou 2005/ have performed geochemical modelling and concluded that the total extent of mineral mass transfer, represented by spatial and temporalvariations in porosity, may be small in a KBS-3 repository throughout the non-isothermal period.
Assessing the effects of such changes on buffer performance is problematic, however, because it
is unclear how mineralogical changes affect the physical, mechanical and rheological properties ofthese materials. Further relevant geochemical modelling is presented in the section on Alteration of
Impurities 3.5.6.
Natural analogues/observations in nature

There is a vast literature on soil cementation due to mineral precipitation, and it concerns a variety ofmineralogical alterations. The challenge is therefore to discriminate processes relevant for repository
conditions and to determine the prevailing geological conditions under which the cementation took
place; attempts have been made by several groups, e.g. /­Pusch and Madsen 1995, Metcalfe andMoore 1998, Pellegrini et al. 1999, Henry et al. 2007/. Cementation effects as a consequence of
illitization have been studied by, among others, /­Hower et al. 1976, Boles and Franks 1979/.
Time perspective

Cementation is expected to take place mainly during the water saturation phase and within the
non-isothermal period. The effects may, at least to some extent, remain during the whole life-time of
the repository.
Handling in the safety assessment SR-Site

The mineralogical evolution in the buffer is presently being modelled, and cementation effects
will be judged based on the extent of precipitation. Presently, the process is neglected based on themodelling performed previously and experimental results for rheology.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The modelling part is discussed under Section Alteration of Impurities. The experimental part ispublished in peer review journals or in SKB internally reviewed reports.
Uncertainties

Uncertainties in mechanistic understanding
The role of bentonite pore structure and the handling of pore space in geochemical modelling are
subject to debate. There is a general problem of how to assess changes in mineralogy on buffer
performance, because it is not clear how all possible changes affect the physical, mechanical andrheological properties of the bentonite buffer.
Model simplification uncertainties for the above handling in SR-Site
See Section 3.5.6 (Alteration of impurities) concerning the geochemical modelling.
Input data and data uncertainties for the above handling in SR-Site
See Section 3.5.6 (Alteration of impurities) concerning the geochemical modelling.

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3.6

Radionuclide transport processes

3.6.1

Speciation of radionuclides

Overview/general description

This process discusses the speciation of radionuclides in the aqueous phase. Speciation of radionuclides is of importance for sorption and diffusion processes in the buffer. These processes dependon the aqueous radionuclide species that are kinetically and thermodynamically stable under theconditions of interest at the boundary of the buffer, i.e. inside the canister and also on the chemical
conditions in the buffer.
Dependencies between process and buffer variables

Table 3-25 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Boundary conditions

The boundary conditions for the process are the source term from the canister/fuel, i.e. the speciation
inside the canister.
Table 3-25. Direct dependencies between the process “Speciation of radionuclides” andthe defined buffer variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Buffer geometry

No

No

Pore geometry

No

No

Radiation intensity(The effect fromradiolysis in the buffer
will be small)No

Temperature

Yes. Chemical
reactions aretemperaturedependent

Water content

No, but indirectly,
because porewatercomposition changes
with water content

No

Gas content

No, but indirectly
through porewatercompositionNo

The uncertainty withNo
respect to temperature
is considered. However,
the temperature range
in the repository is
expected to be narrow

Hydrovariables
No(pressure and flows)

No

Stress stateNo

No

Bentonitecomposition

Indirectly throughporewater composition

No

Montmorillonite
composition

Indirectly throughporewater composition(Sorbed radionuclides
will change the smectite
composition, but theeffect is limited)

Porewater
composition

Yes. Direct
dependence

Structural and straymaterials

Indirectly throughporewater composition

174

Used in calculation

Yes

Results from
calculations

No

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Model studies/experimental studies

If the water chemistry in the buffer is known, the speciation of radionuclides can be calculated witha thermodynamic equilibrium model, such as PHREEQC or EQ3/6 together with a self-consistentthermodynamic database.
Time perspective

This process is relevant for the whole assessment period, but only if there is a canister failure.
Natural analogues/observations in nature

Natural analogues are usually studied for water rock interactions – not for the water phase alone.
Handling in the safety assessment SR-Site

The solubility and speciation of radionuclides is calculated and used as a source term for the modelling of
release from a defective canister (See /­SKB 2010k/). This is done for a wide range of water compositions.
In the buffer, the speciation is used to determine the transport properties of the radionuclides. However,the parameters for the transport properties used in SR-Site are selected based on experiments done underrelevant conditions or from thermodynamic sorption models; see Sections 3.5.5 and 3.5.7. The consistency of the predicted speciation with the speciation calculated in the source term will be checked.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

See Sections 3.5.5 and 3.5.7 and Fuel and Canister Process Report (Speciation of radionuclides)/­SKB 2010k/.
Uncertainties

Uncertainties in mechanistic understanding
See processes Sections 3.5.5 and 3.5.7.
Model simplification uncertainties in SR-Site

See processes Sections 3.5.5 and 3.5.7.
Input data and data uncertainties in SR-Site

See processes Sections 3.5.5 and 3.5.7. There may also be uncertainties in thermodynamic data. This isdiscussed in Sections 3.5.5 and 3.5.7.

3.6.2

Transport of radionuclides in the water phase

Overview/general description

A number of processes will or could influence the transport of radionuclides in the water phase:• advection, if the hydraulic conductivity of the buffer is high,• diffusion, will generally be the main transport mechanism through the buffer,• sorption, will have a strong effect on retarding the transport of most radionuclides,• speciation, will determine the form (including oxidation state) in which the radioelements exist andtherefore also its maximum solubility, as well as how the radioelements are sorbed and transported(sorption can thus be viewed as a part of the speciation),• colloid transport, could enhance the transport of radionuclides,• radioactive decay will determine how much of the radionuclide inventory will eventually leave the buffer.
All these have been discussed above among the processes occurring in the buffer, or, in the case of decay,
under fuel processes.
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Dependencies between process and buffer variables

Table 3-26 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Boundary conditions

The inner boundary for this process is the concentration of radionuclides within a defective canister.
The outer boundary is the fractures in the wall of the deposition hole or the deposition tunnel.
Model studies/experimental studies

Not done for relevant conditions. However, the individual processes are studied; see for example
Sections 3.5.3 and 3.5.5.
Table 3-26. Direct dependencies between the process “Transport of radionuclides in the waterphase” and the defined buffer variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Buffer geometryYes. Gives the volumes and geometries
available for transport

Geometry is includedin RN-transport
calculations

No

Pore geometryYes. Determines
diffusivity.

Included in data

No

Radiation intensity

No

Temperature

Yes, transport andchemical processes
are generally temperature dependent

Neglected. RNtransport occurs afterthermal pulse. The
temperature range inthe repository will benarrow

Water content

Yes.

Radionuclide transport No
is assumed to takeplace when the buffer
is saturated

Gas content

Yes.

Radionuclide transport No
is assumed to takeplace when the buffer
is saturated

Hydrovariables(pressure and flows)

Yes. Determines
the exchange withgroundwater

Near field flow is
considered in
calculations

Stress stateNo

Bentonitecomposition

Yes. Determines
sorption and transport
properties

Included in data See
3.5.3 and 3.5.5

No

Montmorillonite
composition

Yes. Determines
sorption and transport
properties

Included in data See
3.5.3 and 3.5.5

Yes.By cationexchange

Neglected since effectfrom radionuclides
is smallPorewater
composition

Yes. Determines sorp- I Included in data Seetion/speciation
3.5.3 and 3.5.5

Yes. Contamination.

Output from RNcalculations

Structural and straymaterials

Indirectly throughporewater composition

No

176

No
No

No

No

–

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Natural analogues/observations in nature

By definition, this process cover the integrated treatment of radionuclide transport in the near field
and it is therefore difficult to find applicable natural analogues. Natural analogues could however byused to study the individual processes, e.g. sorption or solubility.
Time perspective

The process is active at all timescales after canister failure.
Handling in the safety assessment SR-Site

Radionuclide transport in the buffer is handled in the integrated modelling of radionuclide transport
for the repository.
Model: The near field code COMPULINK/COMP23 /­Romero et al. 1999/ and model summary
report /­SKB 2010n/ will be used for the calculations. The code calculates the non-stationary nuclidetransport in the near field of a repository. The system is divided into compartments, where the only
restriction is that a compartment is formed of the same material. The model, which is basically acoarsely discretisised Integrated Finite Difference Model, embeds analytical solutions at locations
which otherwise would require a very fine discretisation, such as entrances and exits from small
holes and fractures. In the repository, radionuclides leaking out through a small hole in the canister
wall diffuse into the buffer and then may migrate through various pathways into the flowing water inthe rock fractures.
The buffer processes that need to be considered are treated in the following way:
• Advection is neglected if the buffer conductivity is < 10–12 m/s, see Section 3.5.2.
• Diffusion is treated with element-specific diffusivities and porosities (considering specific oxidation states), see Section 3.5.3.
• Sorption is treated with linear sorption coefficients (Kd) for all elements (considering speciation
as well as bentonite and porewater composition), see Section 3.5.5.
• Colloid transport is neglected in the buffer, provided the buffer completely envelops the canisterand has a dry density of at least 1,000 kg/m3, i.e. a clay density of at least 1,650 kg/m3, see
Section 3.5.4.
• Speciation of radionuclides is directly included in the calculation of the source term from theinner boundary condition. The speciation is also included in the determination of the sorption
coefficient.
• Radioactive decay and chain-decay are considered in the calculations.
In SR-Can, a simplified, analytic model /­Hedin 2002/ was used for scoping calculations ofradionuclide transport in the near field. This model is built on the same conceptual understanding ofradionuclide transport in the buffer as COMP23. It may be possible to use this model for comparison
in SR-Site as well.
Boundary condition: Radionuclide transport in the buffer is strongly dependent on the conditions at
the inner and outer boundaries. The geometry of defects and fractures will determine the rate of masstransport.
Handling of variables influencing this process: The buffer geometry is included as compartments inthe model. The buffer variables, montmorillonite composition, bentonite composition and porewatercomposition will together determine the transport parameters. These include: porosity, diffusivity
and sorption coefficients.
Handling of variables influenced by the process: The concentrations and mass flows of radionuclides
are calculated.
An earthquake of large magnitude may potentially change the boundary conditions for the process,while the radionuclide transport within the buffer will remain the same.

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Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The two models used for radionuclide transport in the near field /­Romero et al. 1999/ and /­Hedin
2002/ has been verified against each other. /­Hedin 2002/ is a peer reviewed publication in a scientificjournal.
Uncertainties

Uncertainties in mechanistic understanding
This process is an integration of several other processes all of which have uncertainties (see above).
Model simplification uncertainties in SR-Site

The COMPULINK/COMP23 code uses rather a coarse discretisation of the buffer and analyticalsolutions for the boundary conditions at the canister and the rock.
Input data and data uncertainties in SR-Site

The data uncertainties for the transport parameters in the buffer are presented in Sections 3.5.3 and3.5.5. All input data to the radionuclide transport calculations will be handled in the SR-Site data report.

3.6.3

Transport of radionuclides in a gas phase

Overview/general description

A pulse of hydrogen gas can be released from a defective canister see Section 3.3.3. Some radionuclides could potentially enter the gas phase and thereby be transported to the surface much more
rapidly than would be the case for the aqueous pathway. In principle, only C-14 and Rn-222 canenter the gas phase. Theoretically, I-129 could also be present as elemental iodine, but the amountwill be negligible.
Dependencies between process and buffer variables

Table 3-27 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Boundary conditions

The inner boundary for this process is the gas pressure within a defective canister. The outer boundary is the fractures in the wall of the deposition hole.
Model studies/experimental studies

Not done for relevant conditions.
Natural analogues/observations in nature

Not relevant.
Time perspective

The process will only occur if there is a defect in a canister and if a sufficient amount of water is
supplied to the iron insert to generate, over time, a gas pressure that is higher than the openingpressure of the bentonite.

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Table 3-27. Direct dependencies between the process “Transport of radionuclides in the gas
phase” and the defined buffer variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Buffer geometryYes. The buffer
See 1.3.3
volume determines thetransport

No

Pore geometry

No. Only for dissolved
gases. Indirectly
through stress stateNo

Radiation intensity

No

No

Temperature

Yes, but the thermal
phase will be overbefore high enoughgas pressures canbe reached

Neglected

No

Water content

Yes.

The process is
assumed to occur
only when the buffer
is saturatedNo

Gas content

Yes.

The process is
assumed to occur
only when the buffer
is saturated

Yes

Hydrovariables(pressure and flows)

Yes. The gas pressure
will be dependent
on the hydrostaticpressure

The pressure build up
determines the time
for gaseous releases

No

Stress state

Yes. The openingpressure is dependent
on the swellingpressure

The pressure build up
determines the time
for gaseous releases

No

Bentonitecomposition

Indirectly via stress
state

No

Montmorillonite
composition

Indirectly via stress
state

No

Porewater
composition

Yes and also indirectly
via stress state that
affects solubility/
speciation of gases,
especially CO2

Structural and straymaterials

No

Pessimisticallyneglected

Output from transportestimate

No

No

Handling in the safety assessment SR-Site

Radionuclide transport in a gas phase is treated as a separate case in SR-Site. The handling is basedon the experiment by /­Harrington and Horseman 2003/ presented in Section 3.3.3. It is assumed that
if gas production exceeds the ability of the surrounding groundwater to take it into solution andtransport it away from the container, a pressure will build up within and adjacent to the container.
The bentonite is assumed to ultimately open and release gas when the internal pressure exceeds
20 MPa. A rapid outflow will occur until the pressure falls to 10 MPa. This means that half of the gas
inside the canister will be released instantaneously together with the radionuclides contained in thatamount of gas. The only important radionuclides that can be transported in a gas phase are C-14 andRn-222. Since its half-life is of C-14 is 5,568 years, neither the buffer nor the geosphere is expected
to significantly delay the transport to the biosphere. Isotope dilution in a gas phase is not considered.
Rn-222 has a very short half-life, but is always present inside the canister. Hence, delay in the geosphere
could affect the releases, but is pessimistically neglected in SR-Site.
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The pathway may either stay open as long a there is a gas production in the canister or open andclose as the buffer reseals leading to repeated pulses of gas (see Section 3.3.3). In the first case, thegas is expected to be released at the same rate as it is produced by corrosion after the breakthroughpulse. The second case will lead to repeated pulses of about equal magnitude.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

See Section 3.3.3.
Uncertainties

Uncertainties in mechanistic understanding
The mechanisms for gas release through bentonite are still not fully understood, see Section 3.3.3.
Theoretically, I-129 could also be present in a gas phase as elemental iodine, but the amount will
be negligible. Some metals and metalloids form volatile methylated and/or permethylated species,
whether these could appear in the fuel at low temperature has not been discussed.
Model simplification uncertainties in SR-Site
The handling of the gas transport pathway in SR-Site is based on a few experiments only. The values
for opening and steady-state pressures have been selected from these experiments.
Input data and data uncertainties in SR-Site
As mentioned above, the values for gas opening and steady-state pressures are based on experimental evidence. However, under any conditions, the gas pressure differential should be in the range of
5–15 MPa. Another uncertainty is the fraction of C-14 that can enter a gas phase. For the time being,
this fraction is pessimistically assumed to be 100%. For Rn-222, it is probably fair to assume that allof the inventory will be in the gas phase.

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4

Processes in the tunnel backfill

4.1

Thermal processes

4.1.1

Heat transport

Overview/general description

Heat is transported from the canister surface to the buffer, through the buffer and finally from thebuffer to the rock, both directly and via the tunnel backfill. The thermal evolution within the backfill
is determined by the canister power, the canister spacing and the rock thermal properties while thethermal processes conditions in the deposition holes are unimportant.
The heat conductivity of the repository host rock is in the range of 2.0–4.0 W/(mK) /Back et al. 2007,
Sundberg et al. 2008/. The backfill conductivity is usually set at 1.5 W/(mK) /Börgesson and Hernelind
2000/, but may be lower depending on the degree of saturation. This means that the conductivity of thesurrounding rock is about twice that of the backfill. In addition, the volume fraction of the heated nearfield that is occupied by the backfill is small. These facts together mean that the thermal conditions in
the backfill are dominated and controlled by the temperature of the surrounding rock and that details
in the backfill heat transport properties are unimportant to the overall thermal development in the nearfield. The disturbance induced by the presence of the low conductivity tunnel backfill will be small.
For a typical case the temperature increase may be about 40°C in the tunnel floor region and about
35°C in the roof region. The floor-roof gradient may be 2°C/m at maximum. After 100 years, thegradient is less than 1°C/m (Figure 4-1). In central parts, thermal gradients along the tunnel axes aremuch smaller and disappear after a couple of years. Close to tunnel faces and to sealing plugs, these
gradients will be larger. Similar to the conditions in the buffer, the majority of the heat transport
within the backfill will take place by conduction with thermal conductivities that depend on void
ratio and saturation. While the buffer thermal conductivity and variations in buffer conductivity areimportant to the buffer temperatures, corresponding conditions in the backfill do not have muchinfluence on the backfill temperatures.
Dependencies between process and tunnel backfill variables

Table 4-1 summarises how the process influences and is influenced by all backfill variables and how
these effects are treated in SR-Site.

Temperature increase (degr C)

45

floor, 0.2 m

40roof, 0.2 m
floor, 3 m

35roof, 3 m

30
25
20

Initial power 1700 W,
Canister spacing 6 m
Tunnel spacing 40 m,
Rock heat conductivity 3.0 W/(mK)Rock heat capacity = 2.08 MJ/m3

15
10
5
1

10

100

1000

10000

Time (years)

Figure 4-1. Temperatures in tunnel floor and tunnel roof for two positions: 0.2 m and 3 m, respectively,from the axis of the nearest canister. Temperatures are calculated with an analytical solution given by/Claesson and Probert 1996/ without regard to the properties of the backfill.
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Table 4-1. Direct dependencies between the process “Heat transport” and the defined tunnel
backfill variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No) Description(How/Why not)Process influence on variable
Influence present?
Handling of influence(Yes/No) Description(How/Why not)Backfill geometry

Yes

Ignored. The volume
occupied by the
backfill is sufficiently
small that geometry
variations are without
importance.

No, but indirectly via
temperature

Backfill pore geometry Yes

Neglected, since thethermal expansion of
the solid phase willaffect the void ratio
very insignificantly.

No, but indirectly via
temperature (andthermal strain)

Temperature

Yes

Conservatively
ignoredYes

Water content

Yes

Insignificant incomparison to effects
of other processesNo

Gas content

No, but indirectly
through water content

No

Hydrovariables(pressure and flows)NoNo, but indirectly via
temperature

Stress stateNoNo, but indirectly via
temperature (andthermal strain)

Backfill composition

Yes

Backfill porewatercomposition

No

No

Structural and straymaterials

Yes.

Ignored since theNoamounts are too small
to have any impact.

Backfill thermal
conductivity given as
input data

Ignored, since general
near-field temperaturemodels confirm thattemperatures andtemperature gradients
within backfill are small.

No

Backfill geometry: The geometry of the backfill is defined by the shape of the excavated tunnels
and possibly by formation of small gaps below the roof. None of these conditions are expected tochange in a way that can affect the heat transport in the backfill. There will always be sufficient
backfill/rock contact to ensure that the rock and the backfill are in thermal equilibrium, meaning thatthe backfill temperature will be controlled by the temperature of the surrounding rock, irrespectiveof geometry variations.
Backfill pore geometry: The thermal conductivity varies with the void ratio. The thermal expansion
will affect the void ratio very insignificantly.
Temperature: The influence of heat transport on the backfill temperature is obvious. Different models
have been proposed for estimating the influence of the temperature on the thermal conductivity ofbentonite /Knutsson 1983/. Similar models could be applied to the backfill. The effect is a small but
uncertain increase, which can be conservatively ignored.
Water content: Conductivity-saturation relations of the type that apply for the buffer are in principle
valid also for the backfill, i.e. there is a significant dependence on the water content. Backfill thermal
conductivity variations are however not important to the thermal evolution of the near-field, cf.
/Kristensson and Hökmark 2007 Hökmark et al. 2009/.

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The thermal gradients within the backfill are much smaller than in the buffer. The temperatures are
low in comparison to those found in the buffer. Therefore any thermally driven redistribution of waterfrom the floor region to the roof region during the time period of incomplete saturation is judged to be
insignificant in comparison to effects of other processes that control the state of saturation in different
parts of the backfill.
Gas content: The heat transport will not affect the gas content. The gas content in itself does nothave any influence on the heat transport. If there is a gas phase, the effects of this will be just thoseof the incomplete saturation.
Hydro variables: Temperature variations will cause changes to the porewater pressure and initiate
some convective flow.
Stress state: There may be some increase of the backfill thermal conductivity when stresses increase,but the effect is not sufficiently verified to be credited /Börgesson et al. 1994/. The increased temperature will induce some thermal stresses.
Backfill composition: The composition of the backfill will influence the thermal conductivity. Someminerals have higher conductivities than others.
Backfill porewater composition: No influences.
Impurity content: The presence of impurities may affect the thermal conductivity. To be of any
importance, the impurity content must be high and the thermal properties of the impurities significantly different from those of the backfill.
Structural and stray materials: To be of any importance to the heat transport, there must be verysignificant amounts of non-backfill materials.
Boundary conditions

The tunnel peripheries, the tunnel faces and sealing tunnel plugs are the boundaries of the backfill.
These boundaries are all approximately in thermal equilibrium with the host rock which changes its
temperature slowly as a result of the decreasing heat output from the fuel, buried a couple of metres
below the tunnel floor, and the transport of heat from the near field to the geosphere boundaries. The
temporal and spatial variation of the rock temperature is conceptually well understood.
Model studies/experimental studies

Model studies. There are no studies that explicitly address the thermal conditions in the backfill.
Because of its small importance and its weak coupling to other processes, modelling results from thisprocess are available only indirectly from studies that address thermal issues in general. Predictionsof the temperature evolution in the near field have, for instance, been done analytically /Claesson
and Probert 1996, Hökmark and Fälth 2003, Hedin 2004, Hökmark et al. 2009/ and numerically/Thunvik and Braester 1991, Ageskog and Jansson 1999/.
Natural analogues/observations in nature

Not applicable.
Time perspective

The duration of the thermal pulse is a few thousand years. The general statements made for thethermal pulse hold also during following phases, i.e. heat transport in the backfill is completely
controlled by the thermal development of the host rock. Effects of climate changes and variations
in geothermal heat flow will be imposed on the backfill by the surrounding rock.

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183

Handling in the safety assessment SR-Site

There is no specific handling of heat transport in the backfill during the thermal phase since thebackfill temperature is not directly related to safety. Results from thermal analyses, obtained using
either rock mass values or generic values of the backfill heat transport parameters, give sufficient
information on the backfill temperature range during the thermal phase.
For permafrost and glacial conditions, the minimum temperature of the backfill is a concern, see
further Section 4.1.2. For these conditions, the backfill temperature can be taken to be equal to thatof the host rock. See further the SR-Site Geosphere process report /SKB 2010i/ for the treatment ofrock temperatures for these conditions.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Because of the small thermal gradients and the relatively low temperatures in the backfill there is no
specific handling of the heat transport process. The gradient and temperature statements are supported
by results obtained using calculation schemes described in /Claesson and Probert 1996/. While thatparticular report is not peer or factual reviewed, the calculation scheme has been repeatedly applied inpeer reviewed papers /Hökmark and Claesson 2005/, and in reports that have undergone (/Hökmarkand Fälth 2003/) or will undergo (/Hökmark et al. 2009/) a documented factual and quality review.
Uncertainties

Uncertainties in mechanistic understanding
There are no major uncertainties in mechanistic understanding. In the unsaturated state, the backfill isa system with solid, liquid and gas phases and obeys physical laws for such systems, but for practical
purposes, the heat transport is expediently described as pure conduction with a saturation-dependentor fixed conductivity.
Model simplification uncertainties in SR-Site
Not relevant since the process is not modelled.
Input data and data uncertainties in SR-Site
Not relevant since the process is not modelled.

4.1.2

Freezing

Overview/general description

The overview and general description of the corresponding buffer process, Section 3.2.2, are also
valid for the backfill. Since also the backfill material exerts a swelling pressure, freezing (ice formation) is expected to occur at a temperature lower than 0°C.
However, because backfill materials might have a lower content of swelling mineral (montmorillonite), it is crucial to empirically verify that the swelling pressure dependence on temperature still
follows the thermodynamic description as described by /Birgersson et al. 2008/.
The swelling pressure of the backfill can be significantly lower than that in the buffer, and it cannot
be excluded that it will be frozen in certain time intervals during periods of permafrost. Furthermore,
the backfill is extended vertically in ramp and shafts and therefore parts of it will experience temperatures lower than those at repository level. Freezing of the backfill is not a problem per se, as theprocess has been demonstrated to be reversible. However, it could be imagined that freezing occurs
in the backfill in a position closer to surface level while the rock surrounding the repository is frozen.
This could lead to increasing pressures in the repository as liquid water is “trapped” in a frozen rockmatrix. Such a scenario can only occur, however, when the temperature at repository level is below
0°C and maximum pressure increase can be estimated to 26 MPa /Birgersson et al. 2010/. Note that
this is a hydrostatic pressure and will not be harmful to the canisters.

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Dependencies between process and tunnel backfill variables

Table 4-2 summarises how the process influences and is influenced by all backfill variables and how
these effects are treated in SR-Site.
Boundary conditions

If the backfill freezes, the water in the surrounding rock will be in ice form. This means that theavailability of extra water during ice-lens formation within the backfill probably is very limited.
The effect of freezing on the interaction with the buffer is not clear. There may be a competition
for water.
Model studies/experimental studies

A study of the pressure response due to temperature changes of the IBECO-RWC-BF material has
been conducted /Birgersson et al. 2010/. The results show a behaviour very similar to what havebeen measured for the SR-site reference buffer materials (see Section 3.2.2), i.e. a drop in swellingpressure at temperatures below 0°C and no pressure peaks around 0°C has been observed.
Natural analogues/observations in nature

No studies have been conducted. However, knowledge can probably be achieved by studying
permafrost areas.
Table 4-2. Direct dependencies between the process “Freezing” and the defined tunnel backfill
variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Backfill geometry

Yes, if extended toregions with very low
temperatures

Yes, geometry couldchange throughexpansion

Neglected, since
only local freezing is
discussed

Neglected, since nolasting effect on thegeometry is expected

Backfill pore geometry Yes

Yes, see Section 3.2.2 Yes

Included in assessment
discussion

Temperature

Yes

Included in assessment No
discussion

Water content

Yes, via freezing point Full saturation isand transport capacity assumed whenfreezing may occur.

Yes, If freezing takes
place redistribution ofwater

Gas content

(Yes)

1)

No

Hydrovariables(pressure and flows)

Yes, freezing involves
water transport

Included in assessment Yes
discussion

Stress state

Yes, Influences thefreezing point to asmall degree

Neglected since effect Yes. Swelling pressure Included in assessment
is smallaffected also before
discussion
ice formation

Backfill materials– composition andcontent

Yes, via the specificsurface area andtypes of counter ionsin montmorillonite

Included in assessment No
discussion

Backfill porewatercomposition

Yes, influences thefreezing point

Neglected, assessment No
is based on dilute
water

Structural and straymaterials

No

Included in assessment
discussion

Included in assessment
discussion

No1) A gas content would affect the process but the backfill will be completely saturated when freezing

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Time perspective

The process of freezing is relevant during the entire repository lifetime.
Handling in the safety assessment SR-Site

The current backfill will have a swelling pressure of ~3 MPa, using the description in
Section 3.2.2 the freezing point of the backfill would be around –2°C.
As it has been demonstrated to be reversible, the process of freezing of the backfill is neglected.
Handling of variables influencing the process: The temperature evolution is estimated in /SKB 2010h/.
Handling of uncertainties in SR-Site

Uncertainties in mechanistic understanding
The freezing of soils is a well known process. The description of swelling pressure response due totemperature changes is based on robust, fundamental thermodynamic arguments.
Model simplification uncertainties for the above handling in SR-Site
The reproducibility of the pressure response due to freezing is good. The process can be welldescribed with the equations presented in this section.
Input data and data uncertainties for the above handling in SR-Site
The bentonite freezing point is strongly dependent on density. If a significant loss of material occurs,e.g. due to chemical erosion, a corresponding change in the freezing behaviour is expected.
Adequacy of references supporting the suggested handling in the safety assessment SR-site
The supporting references /Birgersson et al. 2008/ and /Birgersson et al. 2010/ are a peer reviewed
article and a reviewed report according the SR-Site quality system that are available in the open
literature. In addition, these particular studies are aimed directly at dealing with the freezing process
in swelling clays.

4.2

Hydraulic processes

4.2.1

Water uptake and transport under unsaturated conditions

Overview/general description

Water transport in the backfill under unsaturated conditions is a complex process that is dependent on
temperature, smectite content, degree of water saturation and water content in the different parts of thebackfill. The most important driving force for water saturation under deep repository conditions is theexternal water pressure from the water in the rock fractures and the negative capillary pressure in the
pores of the backfill that lead to water uptake from the surrounding rock. The hydraulic sub-processes
are identical to those of the buffer (Section 3.3.1) and will not be reproduced in this discussion.
There are some differences in behaviour and parameter importance between the buffer and thebackfill that is valuable to point out.
Sub-process B2, transport of water in vapour form controlled and driven by a temperature gradient, isprobably insignificant for a major part of the backfill due to the low temperature gradient. However, it
can be of importance for the backfill just above the buffer in the deposition hole and lead to drying if
the supply of water from the rock is small.
Sub-process E2, compression of air, may be more important for the water saturation process (Boyle’slaw) of the backfill than the buffer if the backfill is composed of clay with lower smectite content
like IBECO-RWC-BF. If the air is trapped, it must be compressed and go into solution in the waterin order for water saturation to be achieved. In highly compacted smectite rich bentonite, the suction
potential appears to be sufficiently high for this to occur and full water saturation is achieved without

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any external water pressure. IBECO-RWC-BF has lower suction, which means that a pressurised
air volume may remain if the air cannot seep out. Due to compression of the air, a large part ofthe volume will disappear when full water pressure has developed, but the remaining part of theentrapped air must be dissolved and diffuse out through the backfill and the rock.
Sub-process F1, transport of dissolved air in water. When trapped air is compressed during the water
saturation phase, a portion of the volume is dissolved in the water. This solubility (Henry’s law) isapproximately 2 percent at room temperature but decreases rapidly at increasing temperature. In order
for additional air to be dissolved, the dissolved air must be transported away, which takes place bydiffusion. The rate of diffusion in the backfill is partly limited by the transport capacity (diffusion andflow) in the rock. If gases are dissolved in the groundwater in the rock, the diffusion rate may be very
low and the time to saturate the backfill very long.
Dependencies between process and tunnel backfill variables

Table 4-3 summarises how the process influences and is influenced by all backfill variables and how
these effects are treated in SR-Site. The table illustrates the complexity of this process since all variableinfluence the process and the process influence all variables but two.
A wide range of materials as backfill candidates has been investigated. They are exemplified byFriedland clay representing the lowest quality (a smectite rich mixed layer clay) and MX-80 representing the highest quality (a high grade Na-bentonite). New tests on IBECO-RWC-BF are recently
published /Johannesson et al. 2010/
Influence of salinity and temperature on wetting rate: These influences are practically the same as
for water-saturated conditions and are described under that process. The retention curve is influenced
by the temperature (see the buffer).
Influence of water and gas content: See the buffer; Section 3.3.1.
The hydro-variables expressed as suction (negative porewater pressure) in an unsaturated backfill
is strongly affected by the water content (retention curve) and the backfill composition. The retention
curve of MX-80, IBECO-RWC-BF, Asha and Friedland clay are shown in Figure 4-2 /Johannesson
et al. 2010/. It is dependent on several factors as described for the buffer.
The backfill geometry is of course affecting the wetting since the hydraulic gradient depends on the
distance to the boundary. Usually the geometry is not affected by the wetting since the boundary of
the backfill is the rock and the buffer, but if the backfill consists of different types of backfill (e.g.
blocks and pellets) the geometry of the different types may change due to swelling or compression.
Boundary conditions

Interaction with the rock: A key issue for the saturation process is the interaction between rockand backfill. Water is conducted to the backfill in the water-bearing fractures and the rock matrix,
which means that the water saturation process can be both uneven and take a long time for a sparsely
fractured rock. If there is a highly permeable excavation-disturbed zone (EDZ) at the wall of thedeposition tunnel, which distributes water from the fractures along the rock wall, the wetting will befaster and more even.
However, there is another process that may strongly increase the wetting rate of the backfill. When
the water inflow from a rock fracture is stopped there will be an increase in the water pressure in thefracture. Since the pellets filling between the rock and the backfill blocks cannot withstand a highwater pressure there will be piping and the water may flow in channels mainly along the rock surface
and fill up the voids between the separate pellets. The most likely scenario is that water successively
fills the pellets-filled slot and when it reaches the tunnel plug it will not continue past the plug since theplug will be built to resist high water pressure. Instead, water will continue to fill up the pellets-filled
slot until all the large voids between the pellets in the entire tunnel are filled with water. There will,of course, be a parallel water uptake and swelling of the backfill blocks in contact with the pellets but
if the inflow is not very low it is likely that the entire pellets-filled slot will be water filled before theblocks have swelled and sealed a part of the tunnel. This process is also treated in Section 4.2.4.
Interaction with the buffer: See Section 3.3.1.

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Table 4-3. Direct dependencies between the process “Water uptake and transport under unsaturated conditions” and the defined backfill variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence?
(Yes/No)(How/Why not)Description

Process influence onvariable
Influence present?
Handling of influence?
(Yes/No)(How/Why not)Description

Temperature

Yes, through waterviscosity and temperature gradientIncluded in the models

Yes, wetting anddrying affects thermal
conductivity

Neglected since theeffects are small in the
backfill

Water content

Yes, decisive for theretention curve

Initial water content isgiven as input data insaturation calculation.
The degree of saturation and the void ratio,
which determines thewater content, are variables in the model.

Yes, changes thewater content

Degree of saturationand void ratio arecalculated

Gas content

Yes, included in the
degree of saturation

Initial gas content isgiven as input data insaturation calculation.
The degree of saturation and the void ratio,
which determines thewater content, are variables in the model.

Yes, changes the gas
content

Degree of saturationand void ratio arecalculated

Hydro-variables
Yes, main variables(pressure and flows) in the saturation
process

Included variables in the Yes, changes thesaturation calculationhydrovariables

Included variables in thesaturation calculation

Backfill geometry

Yes, mainly throughdistance to wet rock

Given as input data insaturation calculation

Yes. Influencing
through the swellingof blocks

See Section 4.3.1

Backfill poregeometry

Yes, the poregeometry influences
the water and vapour
transport rate.

Included in material
data

Yes, through thedegree of water saturation that together
with the void ratio
determines the poregeometry

Degree of saturationand void ratio arecalculated

Stress state

Yes, influences theretention curve.

Included variables in the Yes, changes thesaturation calculation
swelling pressure

Backfill composition

Yes, influenced
through the retention
curve and otherproperties

Given as input data insaturation calculationNo, unless via mineral Not relevant for saturaconversion
tion period

Backfill porewatercomposition

Yes, influences thehydraulic properties

Included through input
data in the saturation
calculation

Yes, the composition See Section 4.4.4of the wetting water
will influence thebackfill porewatercomposition by definition. Also throughiron transport in atemperature gradient

Structural and straymaterials

Insignificant

ExcludedNo

The pressure and suction are calculated

Model studies/experimental studies

A preliminary material model containing unsaturated water flow has been derived (see /Börgessonand Hernelind 1999/) and is being further developed. The models were originally developed forbuffer material and can also be used for the backfill materials (see /Börgesson and Hernelind 1998/
and /Börgesson et al. 2006/). The calculations are performed using the finite element method withthe program Abaqus. See also process 3.3.1. The model does not include the processes that handle
air (except for B2 described above), which can influence the saturation process in the backfill if itconsists of Friedland type of clay.
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50
45

RWC-BF_18.4%

40

w (%)

35

Asha_1.5%
Johannesson et al. (2008)

30
25

Friedland _7.8%
Johannesson et al. (2008)20
15

MX-80_9.8%
Dueck&Nilsson(2008)

10
5
0
0

10

20

30

40

50

60

70

80

90

100

RH (%)

Figure 4-2. Specific retention curve for the RWC-BF material obtained at 25°C shown together withprevious results for Asha and Friedland at 25˚C and MX-80 at 20oC. The labels show material and initialwater content (%). The arrows show the moisture paths /Johannesson et al. 2010/.

The model has been both calibrated and partially verified by laboratory experiments as shown in
Section 3.3.1.
In recent years another finite element program has been tested and used for thermal and hydraulicprocesses, namely Code Bright /Cimne 2002/. This code is developed by UPC, Barcelona. Most
processes are similar to the processes in Abaqus but there are some important differences. The maindifference is that it can handle the gas phase and thus also model processes B1, B4, B5, D2, E2 andF1. It can also handle phase transitions (C1 and C2).
Several large-scale field tests have been installed with i.e. the purpose to study the wetting processof the backfill material in a repository. However, these tests are made with in situ compacted
30/70 mixtures of bentonite and crushed rock and are thus not relevant for the current backfill
design. The full-scale tests of deposition holes with canister and buffer are more relevant for studying the behaviour of backfill with high grade bentonite especially the upper part of the deposition
holes.
Model studies

Besides the modelling of the Backfill and Plug Test and the Prototype Repository, model studies ofthe saturation time of the backfill in a deep repository have been done for SRCan /Börgesson et al.
2006/. The studies concerned the two backfill types 30/70 and Friedland clay. A large number of calculations of the wetting of the tunnel backfill with primary variations of fracture frequency, fracturetransmissivity and backfill type and secondary variations of the rock matrix hydraulic conductivity,
the distance to the water supplying boundary and the existence of a highly permeable zone at the rocksurface have been done. These calculations will be updated for the new backfill concept /Åkesson
et al. 2010a/.
The conclusions drawn from the calculations made for SR-Can are provided below.
The time to complete saturation varies from a few years with 1 m between fractures to more than
150 years with 24 m between the fractures.
Examples of results from the calculations with Friedland clay as backfill are shown in Figure 4-3.
However, the calculations assume a homogeneous backfill with a rather low dry density(1,590 kg/m3) and will be updated.

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Saturation of backfill (Friedton)

Time until saturation (years)

1000

F: T=5 x 10-11100

F: T=5 x 10-10
F: T=5 x 10-9
F: Kr=10-12
F: Perm. Zone

10

1
1

10

100

Fracture distance (m)Figure 4-3. Summary of results from calculations regarding Friedland clay (Friedton) backfill with
K=0.7·10–11 m/s in water saturated state. Influence of time until saturation as a function of the fracture
distance at different fracture transmissivities T, rock matrix hydraulic conductivity (Kr=10–13 m/s in all
calculations except two). The influence of a highly permeable zone between the backfill and rock (orextensive piping) is also shown. /Börgesson et al. 2006/

The influence of fracture frequency is strong since very little water is transported in the rock matrix
at the matrix hydraulic conductivity 10–13 m/s. The time to full saturation is almost proportional to
the fracture distance.
The influence of transmissivity is insignificant since the transmissivity is high enough, comparedto the hydraulic conductivity of the backfill, to yield a high water pressure in the fracture/backfill
interface and the water inflow is thus hindered by the backfill and not by the fracture.
Another conclusion is that the effect of piping and water filling of the pellets-filled slot can be verystrong on the rate of wetting since the slot will function as a filter on the rock surface at least in the
beginning of the wetting phase.
Natural analogues/observations in nature

Wetting of bentonite in nature has not been studied and can probably not contribute to the knowledge.
Manmade bentonite seals are made in completely different ways (mixtures with low density). Natural
bentonites have unknown histories
Time perspective

The above analyses show that the time to full water saturation of the backfill can vary a great deal,
depending above all on the interaction with surrounding rock. If conditions in the rock are wet,saturation may be reached within 10 years for a backfill of Friedland clay with low density. If there
are no fractures, all water has to go via the rock matrix, which may take several hundred years in the
granitic rock types occurring in Sweden.
Very long time until saturation of the backfill is not a problem for the backfill but the consequences
in a very dry rock may be a delay of the water saturation of the buffer (see buffer processes).

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Handling in the safety assessment SR-Site

As mentioned above, the initial THM evolution covering the saturation phase of the backfill wasmodelled for a number of hydraulic boundary conditions in SR-Can since the supply of water at theouter buffer boundary is decisive for the wetting rate. However, they are updated in order to take into
account the new backfill concept.
Model: The following sub-investigations have been made:1. Investigation of the influence of the backfill properties and wetting conditions on the water
saturation phase of the buffer with the old FEM-model used in earlier wetting calculations for
SR-97 /Börgesson and Hernelind 1999/. The old calculations have been updated regarding theinfluence of the backfill. /Börgesson et al. 2006/. 2D rotational symmetric and 2D plane models
have been used.
2. Influence of the rock conditions on the wetting phase of the backfill in the deposition tunnels
for three different backfill types /Börgesson et al. 2006/. 2D rotational symmetric and 2D planemodels have been used.
3. Influence of having entrapped air on the wetting phase of the backfill in the deposition tunnels.
These calculations have been done with Code Bright with a material model that includes the airphase. 2D rotational symmetry /Börgesson et al. 2006/.
In addition to this, examples of complete interaction between buffer and backfill with a 3D model
have been done /Börgesson and Hernelind 2009/.
Timescales: The THM modelling will encompass the wetting period for the backfill. The detailed
timescale will consequently be determined by the hydraulic conditions in the bedrock, which to someextent control the wetting. Typical timescales range from less than ten to several hundred years.
Boundary conditions: The THM modelling is performed for a number of different hydraulic conditions in the bedrock.
The swelling of the backfill and the bentonite seal following the disintegration of the plug will beanalysed with mechanical axis-symmetric 2D Abaqus models. The issue of penetration of bentonite
into the central area will also be studied.
Handling of variables influencing this process: All the influences, except temperature, in the table
above are included in the model.
Handling of variables influenced by the process: The degree of saturation (water content) in the
backfill as function of time and space is calculated.
The special cases of failed canister and of earthquakes: Canister failures and earth quakes of a magnitude that could affect the wetting process are not expected during the time period in which the backfill
saturates. The process is thus not handled for these cases. Should the assumptions regarding significant,early canister failures or earth quakes not be confirmed by the results of the safety assessment, thehandling of the special cases will be reconsidered.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The codes used for modelling the initial THM evolution of the backfill are mainly Abaqus and CodeBright. Abaqus is one of the world’s leading general finite element codes that undergoes documented
quality controls and has been used in numerous applications reported in peer reviewed articles. The
material models used for modelling of unsaturated backfill materials have been assessed in several
international projects (see the buffer).
Code Bright is a university code that has been especially designed for THM modelling of unsaturated soils. It has recently been officially available and the code and the material models are steadily
updated and assessed in peer reviewed articles.

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191

The main reference for modelling of the wetting scenarios /Börgesson et al. 2006/ is a SKB Technical
Report that has not undergone a documented factual- and quality review. However, these calculations
will be updated considering the new reference design and the results reported in a general THM
modelling report that will undergo a documented factual- and quality review /Åkesson et al. 2010a/.
This report will also include modelling of the disintegration phase of the plug.
The main report for the geotechnical properties of IBECO-RWC-BF backfill is /Johannesson et al.
2010/. This report will undergo a documented factual- and quality review.
Uncertainties

The basic hydraulic properties of the backfill materials are rather well known. The main uncertainties of these properties concern the influence of piping and early wetting of the pellets-filled slot.
Uncertainties in mechanistic understanding
Just as mentioned under uncertainties of the buffer processes, there is a lack of understanding in how
the high water pressure cause piping and distributes the water into the backfill from these channels.
This process exists also in the buffer. See also Section 4.2.4 (Piping/erosion).
Model simplification uncertainties in SR-Site
The same model simplifications have been done for the backfill as for the buffer. Two of those are ofhigher concern for the backfill than for the buffer namely piping/erosion and inhomogeneities, since thehomogenisation and self sealing processes may not be as good in the backfill as in the buffer. These processes are dealt with in Section 4.2.4 (Piping/erosion) and Section 4.3.1 (Swelling/mass redistribution).
Input data and data uncertainties in SR-Site
The greatest uncertainty/variability concerns the hydraulic conditions in the near-field rock andthe hydraulic interaction between the rock and the backfill. The data needed for the modelling are
discussed in /Åkesson et al. 2010b/

4.2.2

Water transport under saturated conditions

Overview/general description

Water transport under saturated conditions is mainly driven by a water pressure gradient. The
process can be described by Darcy’s law. Any deviations from Darcy’s law, which can occur at low
gradients, are favourable in that they lead to a reduced through flow rate. The hydraulic conductivity
K depends on the composition of the buffer, the void ratio e, the ion concentration in the porewater ic
and the temperature T.
Water transport in saturated backfill is a complex interplay between several sub-processes on a
microscopic scale. On a macroscopic level, the result is that the permeability of a saturated backfill
is low, and this is also the essential result for the safety assessment.
Water flow in saturated backfill is a special case of unsaturated flow. The processes involved are thesame as for unsaturated conditions.
The process is in general identical to the corresponding process in the buffer and the behaviour isexpressed by the hydraulic conductivity, K. The difference in hydraulic conductivity between the
backfill and the buffer is mainly the magnitude. The influence of groundwater salinity is stronger forthe backfill than for the buffer, since the clay density is lower (see Section 3.5.8).
The hydraulic conductivity of Friedland clay, as an example, in non-saline water after complete saturation is approximately 2.0·10–12 m/s at the dry density 1,700 kg/m3 (2,070 kg/m3 at water saturation).
The influence of water salinity is rather strong. The corresponding hydraulic conductivity is about
5 times higher at a salinity of 3.5%.
In order to prevent the deposition tunnels from being conductive pathways that influence the water
movement in the repository, the backfill shall, over the entire length and cross-section of the tunnel,
have a hydraulic conductivity that is in the same order of magnitude as that of the surrounding rock,or so low that diffusion is the dominating transport mechanism.
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To meet this requirement, the demand on the average hydraulic conductivity of the backfill in the deposition tunnel has been set to K < 10–10 m/s and the swelling pressure to > 100 kPa to ensure a good contact
with the rock wall /Gunnarsson et al. 2004/. See also the Design Premises Report /SKB 2009c/.
Dependencies between process and tunnel backfill variables

Table 4-4 summarises how the process influences and is influenced by all backfill variables and how
these effects are treated in SR-Site.
Table 4-4. Direct dependencies between the process “Water transport under saturated conditions” and the defined tunnel backfill variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Temperature

Yes, the increase of
hydraulic conductivity
as function oftemperature is abouta factor 3 between
20°C and 90°C.

This will be neglected
in SR-Site because
the thermal pulse will
be over when any
significant amountof radionuclides may
have entered thebackfill

No

Water content

Influenced via density(pore geometry)

Determines thehydraulic conductivity,
which is included in
the model.

Yes, during swellingsince water flow into
the voids makes the
backfll swell if it can.

Included in themechanical modelGas content

Insignificantly(dissolved gases)

Excluded since it(Advective transport
would reduce hydraulic of dissolved gases)
conductivity

See Section 4.4.1

Hydrovariables(pressure and flows)

Yes, main variables

Included in the flow
model

Yes

Sensitivity analyses
will be performed
where the objective
is to identify theparameter space
where changes in
conductivity result
in large changes in
performance.

Backfill geometry

Yes, mainly by theflow path

Given as input data.

Yes, during swellingsince water flow into
the voids makes thebuffer swell if there isspace available.

Included in themechanical model

Backfill poregeometry

Yes, determines thehydraulic conductivity

The pore geometry
can be affected by amineral transformation or a change inporewater composition. The effects are
discussed in Sections
4.4.7 and 4.4.6

Yes, during swellingsince water flow into
the voids makes thebuffer swell if there isspace available.

Included in themechanical modelStress state

Yes. If the effective
Included in the
stress on the backfill
mechanical model.
deviates from theswelling pressure thebentonite will change
volume and water willflow into or out from it.

Yes

Included in themechanical model

Backfill composition

Yes, determines thehydraulic conductivity(see text)

Included in the modelNo

Backfill porewatercomposition

Yes, determines thehydraulic conductivity(see text)

Included in the model

Yes

Structural and straymaterials

Insignificant

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Replacement ofporewater is discussed
in Section 4.4.4

No

193

A wide range of materials as backfill candidates has been investigated. They are exemplified byFriedland clay, representing the lowest quality (a smectite rich mixed layer clay), and MX-80,
representing the highest quality (a high grade Na-bentonite).
Influence of density, temperature and water salinity: The hydraulic conductivity is primarily
dependent on the geometry and composition of the backfill, the density, the ion concentration in theporewater and the temperature.
For Friedland clay, the influence of density and salt concentration in the porewater has been investigated /Pusch 2001, Johannesson and Börgesson 2002/.
For MX-80: see the buffer Section 3.3.2.
For other backfill candidates the measured hydraulic conductivity as a function of the dry density
is shown in Figure 4-4 /Johannesson and Nilsson 2006/. Data for IBECO RWC-BF is available in/Johannesson et al. 2010/ and shown in Figure 4-5.
DPJ
Friedland II
Asha IIDPJ Darcy
Milos Darcy

Hydraulic cond. (m/s)

10-8
10-9

Friedland
Asha
Milos Backfill
Asha Darcy
Friedland Darcy

10-10
10-11
10-12
10-13

1000

1100

1200

1300

1400

1500

1600

1700

1800

Dry density (kg/m3)

Figure 4-4. Measured hydraulic conductivity as a function of dry density of different backfill candidates.
Measurements have been done by applying a hydraulic gradient (termed Darcy) or are evaluated fromoedometer tests /Johannesson and Nilsson 2006/.

10-9
IBECO-RWC-BF 1% salt
IBECO-RWC-BF 1% salt grinded

Hydraulic Cond. (m/s)

10-10

IBECO-RWC-BF 3.5% salt
IBECO-RWC-BF 3.5% salt grinded

10-11

10-12

10-13

10-14
1100

1200

1300

1400

1500

1600

1700

Dry Density (kg/m3)

Figure 4-5. The hydraulic conductivity as function of the dry density for the reference backfill material(IBECO-RWC-BF) /Johannesson et al. 2010/

194

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Transformations to other minerals resulting in reduced montmorillonite content generally lead to ahigher hydraulic conductivity.
The temperature influences the conductivity by changing the viscosity of the water, whereby a temperature increase from 20°C to 60°C entails an approximate increase in conductivity by a factor of 2.
Boundary conditions

Only outer boundaries exist. There are three types of outer boundaries of the backfill in a deposition tunnel:•••

The rock surface of the deposition tunnel and the upper metre of the deposition holes,
The interfaces between the backfill and the buffer in the deposition holes,
The inner surface of the concrete plug placed at the entrance of the deposition tunnels.

The hydraulic conditions of these boundaries are very different. The hydraulic interaction with therock after full saturation of the backfill is dominated by the hydraulic properties of the fractures in
the rock. The hydraulic interaction with the buffer is governed by the hydraulic conductivity of thebuffer. The interaction with the outer plugs is also complicated and depends on the tightness of theplug, the existence of a leakage through the rock/plug interface and the design of the plug. It may
also change with time when the concrete degrades.
Model studies/experimental studies

Flow in water-saturated backfill has been studied in laboratory scale in the form of permeability tests(se references in Figures 4-4 and 4-5) and in full scale in the Backfill and Plug Test /Gunnarsson
el al. 2001/. However, the results from this latter test are of limited value since the backfill wascomposed of a mixture of 30/70 bentonite/crushed rock.
In addition to these studies there is a project “Backfilling and closure of the deep repository” running. In this project the hydraulic conductivity of different backfill materials has been investigated.
The results relevant for IBECO-RWC-BF are reported in /Johannesson et al. 2010/.
Natural analogues/observations in nature

No studies of water flow in natural backfill-like formations have been undertaken for several
reasons; the main being that the measured values from the laboratory are very low compared to therequirements and that changes in hydraulic conductivity with time will be a consequence of chemicalalterations that are treated in other chapters. However, such studies could be considered because they
may yield valuable information on long term behaviour.
Time perspective

The hydraulic gradient that exists after closure of the repository when water saturation and porepressure equilibrium in the backfill and surrounding rock have been reached is expected to be low.
Since the backfill is designed to have a similar hydraulic conductivity to the rock, the water flowin the backfill will be small. However, there is a time after closure of the deposition tunnel, but
before closure of the entire repository, when there may be a rather high axial hydraulic gradient in
the backfill. The axial water transport will during that time be largely depending on the tightness ofthe plug, the properties and geometry of the fractures in the rock around the plug and the possible
existence of unhealed flow paths in the backfill caused by earlier piping and erosion. The latter flow
paths are expected to be healed either during this period or just after pore pressure equilibrium has
been reached.
Handling in the safety assessment SR-Site

The change from in situ compacted mixtures to blocks of natural swelling clay or bentonite as
backfill material has resulted in a very low hydraulic conductivity that fulfils the conductivity
demands with high margins. The base case with an unaltered backfill that has not lost any material

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195

will thus not be investigated in any special way. However, sensitivity analyses are performed where
the objective is to identify the parameter space where changes in conductivity result in changes in
performance. Thus, rather than assessing the influence of a few selected conductivity values, it is
attempted to identify sensitive parameter transition regions.
Time periods: After saturation and throughout the assessment period.
Handling of variables influencing this process: The evolution of backfill density and composition
together with the composition of the groundwater/porewater will be studied. The hydraulic conductivity and the swelling pressure will be evaluated for the changing conditions caused by the chemical
evolution and alterations of density, Section 4.4.6.
Handling of variables influenced by the process: The conductivity will be evaluated by applicationof the empirical relationships described above. The swelling pressure will be evaluated according to
Section 4.3.1.
Boundary conditions: The hydraulic gradient for altered conditions during permafrost and glaciation
will be estimated.
The special cases of failed canister and of earthquakes: Handled via possible mass redistributions,
see Section 4.3.1. Consideration will also be given to pressure transients induced by earthquakes.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The main references of results of hydraulic conductivity measurements on the backfill materials andthe influence of different factors are /Johannesson et al. 2010/ and /Karnland et al. 2006/. These
reports will undergo a documented factual- and quality review.
Uncertainties

Uncertainties in mechanistic understanding
The uncertainties in mechanistic understanding are the same as those for the buffer (Section 3.3.2).
Model simplification uncertainty in SR-Site
The model used is empirical, connecting hydraulic conductivity to temperature, density and ionicstrength of the porewater. The uncertainties in this description are primarily given by the uncertainties
of the empirical relationships on which the model is based.
In addition there are two uncertainties, which are more important for the backfill than for the buffer.
One is the above-mentioned effect of piping and erosion that may take place during installation of
the backfill and before final closure. However, the created channels are expected to heal when thewater flow is stopped and the backfill composition is designed for this. This ability of the backfill tobe healed is checked in the tests in the project “Backfilling and closure of the deep repository”.
Another uncertainty is the inhomogeneity of the backfill in macro scale. The concerns are if blocks
that are friction-fit together may experience incomplete swelling and homogenisation of the slots, if
they can close and heal pipes that have eroded and if the pellets filled parts will be sufficiently compressed during homogenisation and swelling of the blocks. All these uncertainties are investigatedin the project: “Backfilling and closure of the deep repository” /Gunnarsson et al. 2004/. The mostrecent findings from the project is presented in /Keto et al. 2009/.
Input data and data uncertainty in SR-Site
Input data for the study of hydraulic conductivity are primarily the empirical relationships between
hydraulic conductivity and density, temperature and porewater composition. In SR-Site, this is handled
by sensitivity studies with different hydraulic conductivities in the tunnels.

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4.2.3

Gas transport/dissolution

Overview/general description

Transport of gas in the backfill can occur in two phases of the repository evolution.
Water saturation phase
The backfill will be emplaced with a water saturation of 40–80%, which means that there is a
substantial amount of air that needs to be dissolved and transported away. Also the groundwater inthe rock may contain dissolved gas. This was discussed in Section 4.2.1.
Gas transport from a defective canister after saturation
Hydrogen gas from the corrosion of the cast iron insert could potentially escape from the repository
by transport through the tunnel backfill. /Wikramaratna et al. 1993/ has estimated the gas transportand gas storage capacity of the backfill. The backfill could act as a passageway for gas under certain
conditions. The backfill could then act as a sink and decrease the gas pressure in the near field of
the repository. However, it is not evident that gas from the canister will ever reach the tunnel. Gas
release through the backfill is pessimistically neglected in SR-Site.
Dependencies between process and tunnel backfill variables

Table 4-5 summarises how the process influences and is influenced by all backfill variables and how
these effects are treated in SR-Site.
Boundary conditions

The rock may either be water unsaturated or completely water saturated. It may be unsaturated either
by drying caused by low relative humidity in the tunnel or from gas dissolved in the groundwater(due to the pressure drop). If the rock is saturated it may include different amounts of dissolved gas.
The buffer and the tunnel plugs are also geometrical boundaries for this process, but they will be oflimited importance, since the transport capacity for gas in the rock will be much higher.
Model studies/experimental studies

The influence of trapped air on the saturation time was studied in the SR-Can calculations (see/Börgesson et al. 2006/). Five 1D, axially symmetric models have been run with Code_Bright.
Three different rock permeabilities were tried and in two of the models, a constant gas pressure wasassumed (no trapped air). The results yielded the following conclusions.
•

If there are no escape routes for the air hosted in the initially unsaturated backfill other than
through the host rock, the trapped air will have an impact on the saturation process. The trapped
air forms a “bubble”, which holds back the inflowing water and delays the saturation. This effectis more important the more permeable the rock is, since in a permeable rock, water enters fasterthan the gas can dissolve/diffuse away. When the water supply from the rock is high, the gas
diffusion rate will control the water saturation process.

•Since the only way gas can escape is by being dissolved into the porewater and then diffuse
away, the diffusion rate has a great importance for the saturation process. If the porewater in theundisturbed host rock contains considerable quantities of dissolved gas, the diffusion rate will be
low with a correspondingly low saturation rate. If the rock is saturated with gas, the backfill will
not be water saturated.

Natural analoges/observations in nature

Not applicable for the backfill.

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Table 4-5. Direct dependencies between the process “Gas transport/dissolution” and the definedtunnel backfill variables and a short note on the handling in SR-Site.
Variable

Variable influence on process?
Influence present?
Handling of influence?
(Yes/No)(How/Why not)Description

Process influence on variable?
Influence present?
Handling of influence?
(Yes/No)(How/Why not)Description

Temperature

Yes, but insignificant, Neglected
since the temperaturein the backfill willremain low

Yes, wetting anddrying affects thermal
conductivity

Neglected since theeffects are small in the
backfill

Water content

Yes, the water content See 4.2.1
decides the gas content and is decisive
for the retention curve

Yes, affects the watercontent

See 4.2.1

Gas content

Yes by definition

Yes, changes the gas
content

See 4.2.1

Hydro-variables(pressure and flows)

Yes, main variables in See 4.2.1
the saturation process

Yes, changes thehydrovariables

See 4.2.1

Backfill geometry

Yes, mainly throughdistance to wet rockSee 4.2.1

Yes. Influencing
through the swelling ofblocksSee 4.2.1

Backfill poregeometry

Yes, the poregeometry influences
the water and vapour
transport rate.

See 4.2.1

Yes, through theSee 4.2.1
degree of water
saturation that together
with the void ratio
determines the poregeometry

Stress state

Yes, influences theretention curve.

See 4.2.1

Yes, affects theswelling pressure

Backfill compositionNo, but indirectly
through water content

No

Backfill porewatercompositionNo

Yes, gases can dissolve in the porewater

Structural and straymaterials

Insignificant

See 4.2.1

Excluded

See 4.2.1

Neglected, of noconcern

No

Time perspective

The delay of the water saturation of the backfill is relatively greater when the permeability of therock is high rather than when it is low. However, since the saturation rate is only critical when
the permeability of the rock is low the preliminary conclusion was that trapped air is not a verysignificant factor unless the rock is almost saturated with gas.
Handling in the safety assessment SR-Site

The influence of trapped air on the saturation time of the backfill is investigated in SR-Site. The
process involving trapped air is only relevant during the saturation phase (see Section 4.2.1).
The backfill could potentially act as a sink for the escape of corrosion gas from the canister.
However, the beneficial effect is rather limited /Wikramaratna 1993/ and the process is not considered in SR-Site. The desaturation of the backfill from gas transport is expected to be very minor withthe same reasoning as for the buffer (see Section 3.3.3).
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The influence of trapped air in the backfill will be investigated in the integrated THM-report which
will be formally reviewed. Gas release through the backfill is pessimistically neglected in SR-Site.

198

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Uncertainties

The parameter values for the modelling of the gas dissolution and transport are rather well defined.
The largest uncertainty is the rock properties and the partly connected question of whether the air istrapped or not.

4.2.4

Piping/erosion

Overview/general description

Water inflow into the deposition tunnel will take place mainly through fractures and will contributeto the wetting of the backfill. However, if the inflow is localised to fractures that carry more water
than the backfill can absorb, there will be a build-up of water pressure in the fracture and therefore
an increase in the hydraulic gradient across the backfill. The backfill close to the rock surface
initially consists of pellets with low density. As a result the backfill will probably not be able to stop
the water inflow due to the high water pressure that will be achieved in the fracture. The results will
be piping, formation of a channel and a continuing water flow, water filling of the space between
the pellets and erosion. The processes are described in Section 3.3.4 (buffer). The knowledge of thisprocess and its consequences for the backfill seems to be sufficient today (/Sandén and Börgesson
2010, Sandén et al. 2008/) but research is ongoing.
The plug and the sealing of leakages in the plug that may occur with time will reduce the flow rate
and move the hydraulic gradient from the backfill to the plug, which will give the backfill time toself-heal. The flow channels will be closed when the blocks have been sufficiently wetted to cause
expansion and consolidation of the pellets, thereby yielding a sufficient resistance to erosion.
Dependences between process and tunnel backfill variables

Table 4-6 summarises how the process influences and is influenced by all backfill variables and how
these effects are treated in SR-Site.
Boundary conditions

The piping and erosion that are treated here mainly take place in the pellets-filled part between theclay blocks and the rock.
Model studies/experimental studies

The processes are studied in the following projects:•

KBS-3H project,•

Baclo project,•

Buffer installation project.

The process and these projects are described in Section 3.3.4.
Natural analogues/observations in nature

Piping and erosion may occur in natural deposits but has so far not been studied. The very special
circumstances with the high water pressure at repository depth make such observations doubtful.
Time perspective

The piping and erosion only takes place before complete saturation and homogenisation. The highswelling pressure of the clay will ultimately always seal the piping channels since the swelling
ability of the reference backfill (IBECO RWC-BF) is high.

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Table 4-6. Direct dependences between the process “Piping/erosion” and the defined tunnel
backfill variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Temperature

No

No

Water content

No, but a change
Not taken into accountin water content– a conservativechanges the swelling estimate is usedpressure, which influences the piping risk.
It also influences thehydraulic conductivity,
which influences theswelling rate

Yes. Piping may
increase thesaturation rate andthus affect the water
ratio and degree ofsaturation since it may
distribute the water ina more homogeneous
manner via a netof pipes inside thebackfill. Erosion willchange the final watercontent.

Gas content

No, but the degree
Not taken into account Same as waterof saturation and the– a conservative
content
porewater pressure
estimate is usedin the backfill will
indirectly influence therisk of piping and theerosion rate

Hydro-variables(pressure and flows)

Yes, basic variables

The hydraulicYes
gradient, the waterflow and the duration
are included in theestimate of piping anderosionThe pipes are assumed
to seal when thehydraulic gradients arerestored.
An “after piping” hydraulic conductivity basedon the loss of mass isestimated

Backfill geometry

The distance tothe plug influences
the water pressure
gradient

The effect of piping isevaluated with a simplified mass-balance
approachYes, through lost
backfill material

Amount of lost backfill
material is estimated

Backfill pore geometry Yes, by influence ofvoid size distribution
in the pellets slot andIndirectly through thestress state

Included in the massbalance estimations

Indirect through stressstate

Stress state

Yes, determines if
piping occurs

Included in the conse- Yes
quence estimation

Backfill composition

Yes, the backfill
and montmorillonite
composition affects
variables that areimportant

Included in theconsequence estimation made indirectly
through stress stateNo

Backfill porewatercomposition

The salinity of
the water affects
many variables thatare important for
disposition to piping
and erosion, i.e. theswelling pressure, theswelling rate (throughhydraulic conductivity)
and the erodability

The effect of piping isevaluated with a simplified mass-balance
approach

No

Structural and straymaterials

No

200

The effect of piping on
the saturation rate of thebackfill will speed up thesaturation. The change
in final water content
due to erosion is modelled (see Section 3.3)

An “after piping” swellingpressure distribution isestimated

No

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Handling in the safety assessment SR-Site

Piping and erosion could lead to a loss or redistribution of the swelling clay component in the
backfill. The implication of this is an increased hydraulic conductivity and a decreased swellingpressure. See also Section 3.3.4.
In SR-Site, the magnitude of this process is estimated and the results will be used as an input foranalyses of the water transport under saturated conditions (Section 4.2.2).
Model: The maximum consequences of this process is estimated by model calculations. This will
be based on the experiences from the reported studies and the model according to Equation 3-13 inSection 3.3.4.
Time periods: The process is only relevant during saturation before the water pressure gradient istaken by the plug. The process is neglected for all other time periods.
Boundary conditions: The hydraulic boundary conditions at the backfill/tunnel wall interface aredecisive for this process.
Handling of variables influencing this process: The water flow and the water pressure are the key
variables for this process. The erosion is also affected by the geometry of the initial slots in thesystem. The effect of the salinity of the groundwater is also included in the description.
Handling of variables influenced by the process: The consequence of an extensive piping together
with erosion would be a localised loss of backfill density, which is calculated for different cases.
The special cases of failed canister and of earthquakes: The process only occurs at very early stages
when these special cases are not relevant.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Equation 3-13 in Section 3.3.4 will be used to estimate the effect of internal erosion of bentonite in adeposition tunnel. The main reference for this model is /Sandén et al. 2008/.
Uncertainties

Piping, erosion and subsequent sealing is a complicated process with many components, muchdepending on the hydraulic behaviour of the rock. The uncertainty is thus very large regarding both
the demands on the rock and the ability of the backfill to handle the processes and it is assumed thatthe process cannot be stopped by the backfill itself until the hydraulic gradient has been reduced. The
tests performed for the backfill project have yielded better understanding. The end plug needs to berather tight in order to stop the erosion.

4.3

Mechanical processes

4.3.1 Swelling/mass redistribution
The swelling process has been merged with other processes that cause mass redistribution within the
backfill, namely thermal expansion, creep and a number of interactions with the buffer, near-fieldrock and the plug.
Overview/general description

Water is absorbed by both unsaturated and saturated swelling backfill materials and causes swelling. If
the backfill is unable to expand freely, a swelling pressure develops, which locally reaches its peak atfull water saturation. The process is described in Section 3.4.1 for the buffer. The same basic processdescription is valid for the backfill.
Creep is not a very important factor for the backfill since it is smaller than for the buffer and does
not affect the tunnel filling function of the backfill.
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Some erosion with loss of backfill material is expected to take place during the installation andsaturation phase. The channels and possible holes caused by the lost backfill are expected to behealed by swelling and homogenisation of the backfill. Additional measurements and calculations inorder to verify this are ongoing (see Section 4.2.4).
Dependencies between process and tunnel backfill variables

Table 4-7 summarises how the process influences and is influenced by all backfill variables and how
these effects are treated in SR-Site.
A wide range of materials as backfill candidates has been investigated. They are exemplified by Friedland
clay representing the lowest quality (a smectite rich mixed layer clay) and MX-80 representing the highest
quality (a high grade Na-bentonite). New tests on IBECO RWC-BF (Milos backfill) are presented in/Johannesson et al. 2010/.
The swelling pressure of the backfill is a function of all the variables included in the Table 4-7.
Figure 4-6 shows example of results on different backfill materials /Johannesson and Nilsson 2006/.
Table 4-7. Direct dependencies between the process “Swelling/mass redistribution” and thedefined tunnel backfill variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence?
(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence?
(Yes/No)(How/Why not)Description

Temperature

Yes, affects thermal
expansion and also
indirectly via thehydro-variables

Excluded from theassessment(see below)(Insignificantly)

Backfill geometry

Yes, any phenomenon
leading to alterationsof the backfill bulk
geometry influencesswelling

The initial positionof the backfill is thestarting point of theevaluation

Yes

The backfill geometry after swellingis included in themodelling

Backfill pore geometry Yes, through stressstate and void ratio

Void ratio is included in Yes, through stressthe models
state and void ratio

Void ratio is included in
the models

Water content

Yes, decisive variable

Included in THM modelling of unsaturated
backfill

Yes

Included in THM modelling of unsaturated
backfill

Gas content

Yes, degree ofsaturation is a model
variable

Included in THM modelling of unsaturated
backfillYes, degree ofsaturation is a model
variable

Included in THM modelling of unsaturated
backfill

Hydro-variables(pressure and flows)

Yes, model variables

Included in THM modelling of unsaturated
backfillYes, model variables

Included in THM modelling of unsaturated
backfill

Stress state

Yes, model variable

Included in THM modelling of unsaturated
backfills and included
in calculation of themechanical interaction
with buffer

Yes, model variable

Included in THM modelling of unsaturated
backfills and included
in calculation of themechanical interaction
with buffer

Backfill composition

Yes, influences the
suction potential andthe swelling pressure

The initial compositionof the backfill is thestarting point of theevaluation

No

Backfill porewatercomposition

Yes, influences the
suction potential andthe swelling pressure

Included in the modelNo
via the suction potential
and the swellingpressure tec.

Structural and straymaterials

Yes

Not included inassessment

202

No

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Figure 4-7 shows the swelling pressure of IBECO-RWC-BF as a function of dry density at different
conditions. Despite the scatter there is an obvious influence of porewater salinity and composition.
The swelling pressure of MX-80 is shown in Section 3.4.1.
Influence of degree of saturation and temperature is the same as for the buffer; see Section 3.4.1.
The influence of the stress state expressed as compression properties is, together with the swellingpressure, important for the interaction with the buffer. Figure 4-8 shows, as an example, results fromoedometer tests on different backfill candidates. The evolution of the dry density as a function of theapplied stress is plotted /Johannesson and Nilsson 2006/.
The results in Figure 4-8 can be used for investigating the interaction between the backfill and thebuffer and calculate the upwards displacement of the buffer/backfill interface.
The mechanical properties of the backfill blocks and the pellets filling before wetting are of special
interest for modelling of the case with buffer swelling towards a dry tunnel backfill. Figure 4-9
shows examples of results from measurement of the strength of unwetted bentonite blocks at different
dry densities.
Boundary conditions

The backfill has three main boundaries, namely the buffer, the rock and the plug at the end of thedeposition tunnel.
800

700

Asha 3.5%

Asha 7%

Milos bf 3.5%

Milos bf 7%

DPJ 3.5%

DPJ 7%Friedland 3.5%

Friedland 7%

Swelling Pressure (kPa)

600

500

400

300

200

100

0
800

1000

1200

1400
Dry Density (kg/m3)

1600

1800

Figure 4-6. Swelling pressure of different backfill materials. The legend also shows the salinity of the water(50/50 CaCl/NaCl) /Johannesson and Nilsson 2006/.

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12000Swelling Pressure (kPa)

10000

IBECO-RWC-BF 1% salt
IBECO-RWC-BF 1% salt ground
IBECO-RWC-BF 3.5% salt
IBECO-RWC-BF 3.5% salt ground

8000

6000

4000

2000

0
1100

1200

1300

1400

1500

1600

1700

Dry Density (kg/m3)

Figure 4-7. Swelling pressure of IBECO-RWC-BF as a function of dry density for the natural material andafter grinding (legend: ground) at different porewater salinity /Johannesson et al. 2010/.

1900
DPJ
Friedland II
Asha II

Dry density (kg/m3)

1800

Friedland
Asha
Milos backfill

1700
1600
1500
1400
1300
1200
1100
0

500

1000

1500

2000

2500

3000

3500

4000

Vertical stress (kPa)

Figure 4-8. Evolution of the dry density as a function of the applied stress for different backfill candidates
at different initial dry densities. Tests performed at full saturation /Johannesson and Nilsson 2006/.

Interaction buffer/backfill
At the interface between the buffer and the backfill, the buffer exerts a swelling pressure against the
backfill and vice versa. Since the swelling pressure is higher for the buffer than the backfill there
is a positive net vertical pressure against the backfill whereby the buffer swells and the backfill iscompressed. In this process, the swelling pressure from the buffer decreases as the density decreases.
At the same time, the counter-pressure from the backfill increases as it is compressed and its density
increases. The swelling of the buffer and compression of the backfill are counteracted to someextent by friction against the rock. When the force of the swelling pressure in the buffer is equal
to the sum of the force of the counter-pressure in the backfill and the friction against the rock, theprocess ceases since equilibrium has been established. An evaluation of the swelling of the buffer
and corresponding compression of the backfill is provided in the buffer chapter (Section 3.4.1).
The influence of the properties of the backfill has been investigated by /Johannesson 2008/ and by

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Maximum Deviator Stress (kPa)

8000
7000
6000
5000
4000
3000
2000
1000
0
1500

1550

1600

1650

1700

1750

Dry Density (kg/m3)

Figure 4-9. The deviator stress at failure as function of the dry density at unconfined compression tests madeon IBECO-RWC-BF material. The water content is about 17% for all specimens /Johannesson et al. 2010/.

/Johannesson and Nilsson 2006/ with analytical calculations. In the former report also the effect of
having a completely dry backfill when the buffer swells is analysed. Finite element calculations ofthese processes will be presented as a part of the SR-Site THM modelling /Börgesson and Hernelind
2009, Åkesson et al. 2010a/.
Interaction backfill/near-field rock
The following mechanical interactions between backfill and near-field rock have been identified:
1. Swelling pressure from the backfill is transferred to the rock. The pressure is not expected tolead to significant rock movements. A positive effect is that the backfill supports the rock andhelps keeping the rock surface unaffected.
2. Swelling pressure from the buffer is transferred through the backfill to the roof of the tunnel.
The pressure is reduced by friction and load spreading and is not expected to harm the rock but
rather to support it in addition to the support according to item 1.
3. Forces of friction are developed between the backfill and the rock at the upper metre of the rockwalls in the deposition hole due to swelling of the buffer against the backfill.
4. Thermal expansion of the backfill could be serious for the rock if the backfill is water-saturated
when the temperature increase occurs. However, the slow temperature increase in combination
with the drainage through the rock means that this is not a problem.
5. Convergence of the deposition tunnel occurs when the rock creeps due to the high rock stresses
after rock excavation. However, such convergence cannot affect the backfill negatively since itwill only compress the backfill and increase its density. The concern arising from this process isthe effect the creep has on the properties of the rock itself.
Intrusion of bentonite into fractures in the rock is very limited due to the small aperture of thefractures and the shear resistance caused by the friction between the backfill and the fracture surface.
See the buffer Section 3.4.1.
Interaction backfill/plug
All deposition tunnels are likely to be ended with a water-tight plug. The mechanical influence fromthe swelling pressure is rather small compared to the influence from the water pressure that will
prevail before closure of the repository. After closure, the water pressure will be equal on both sides.

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Model studies/experimental studies

The swelling pressure and the compression properties of different backfill candidates have been
investigated in the project “Backfilling and closure of the deep repository”. The results have yielded
valuable information for estimating the required density of the backfill in the tunnels; see e.g.
/Gunnarsson et al. 2006/ and /Keto et al. 2009/.
Laboratory tests on the mechanical properties of different backfill materials have been done (see e.g.
/Sandén et al. 2008/ and /Johannesson et al. 2010/)
The only model studies done concerning mechanical processes in the backfill are the interaction
buffer/backfill (see above and Section 3.4.1). Additional model studies will be done for SR-Site.
The mechanical properties of some of the backfill materials have been investigated /are not enoughknown today in order to make proper model calculations. Additional mechanical tests are planned.
Three large-scale tests with backfill material have been installed (BMT in Stripa and Backfill andPlug Test and the Prototype Repository in Äspö HRL). However, these tests are of limited value
since the backfill was composed of bentonite/sand mixtures and was in situ compacted. In thesetests, the total stress from the swelling pressure of the backfill has been or is being measured. Also
the displacement of the interface between the buffer and the backfill was measured in BMT and canbe measured after excavation of the Prototype Repository.
Self healing of backfill material lost by erosion needs to be confirmed. See also Sections 4.2.4 and 4.4.8.
Time perspective

Full saturation of the backfill and buffer is expected to be achieved within a couple of hundred years.
This means that the stress equilibrium with full swelling pressure and fully developed swelling of thebuffer against the backfill is expected to be achieved within that time. The creep of the backfill is small
and can be neglected.
Natural analogues/observations in nature

No natural analogues concerning mechanical behaviour have been studied. Manmade bentonite seals are
made in completely different ways (mixtures with low density) and natural bentonites have unknown
histories. The existence of relevant natural analogues is probably very limited and difficult to evaluate.
Handling in the safety assessment SR-Site

One purpose of the backfill is to keep the buffer down in the deposition hole. The handling of themechanical interaction between the buffer and the backfill in SR-Site is described in Section 3.4.1.
It is important to ensure that there is a tight connection between the backfill and the tunnel wall. Thisis achieved by the swelling pressure of the backfill material.
The evolution of backfill density and composition together with the composition of the groundwater/porewater will be studied. The hydraulic conductivity and the swelling pressure will be evaluated forthe changing conditions caused by the chemical evolution, Section 4.4.6 and alterations of density,
Sections 4.2.4, 4.4.8 and 4.4.7. The swelling pressure will be evaluated according to Section 3.4.1.
The initial swelling is modelled as part of the integrated initial THM evolution of the backfill /Åkesson
et al. 2010a/. Also thermal expansion and interaction buffer/backfill is included in this modelling.
Homogenisation after loss of large masses of backfill after colloid erosion or plug disintegration ismodelled for SR-Site /Åkesson et al. 2010a/.
The possible loss of backfill material expected during the installation and saturation phase isestimated as described in Sections 3.3.4 and 4.2.4. The channels and possible holes caused by thelost backfill are expected to be healed by swelling and homogenisation of the backfill (see e.g.
/Johannesson et al. 2010/). Additional calculations in order to verify this will is done in the THMmodelling in SR-Site /Åkesson et al. 2010a/.
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Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The relation between swelling pressure and density for various bentonites and the influence of different factors as well as other mechanical properties are reported by e.g. /Karnland el al. 2006/, and/Johannesson et al. 2010/. These reports will undergo a documented factual- and quality review.
Modelling of backfill homogenisation (both the natural during the saturation phase and the sealingof erosion damages), buffer/backfill interaction and other mechanical effects on the backfill will bereported in a general THM modelling report that has been subjected to a documented factual- andquality review /Åkesson et al. 2010a/.
Uncertainties

Uncertainties in mechanistic understanding
After complete water-saturation, the swelling process is deemed to be sufficiently well-known andthe final density distribution in the backfill can be calculated with sufficient accuracy. In the longterm perspective, the swelling properties can change due to transformations and cementation. These
are chemical processes that are treated in Section 4.4.7.
Due to lack in good mechanical models of the lower range backfill material (as Friedland clay), thehomogenisation and healing properties of these materials are uncertain.
Model simplification uncertainties in SR-Site
See corresponding process for the buffer (Section 3.4.1)
Input data and data uncertainties in SR-Site
See above. The relevant data uncertainties are discussed in the THM-data report /Åkesson et al. 2010b/.

4.3.2

Liquefaction

Overview/general description

Liquefaction is a process implying that a stiff material (soil) turns into liquid due to a disturbanceof short duration.. Just as for the buffer it is highly unlikely that it occurs in the backfill. If for any
reason it should occur, the consequences for the backfill would be negligible. The only conceivableeffect is that a loose rock block in the roof may sink but the effect is local and since the total volume
is constant, the liquid backfill will fill the hole in the roof after the block moves.
Since the process is unlikely to occur and the consequences are negligible, the process will not be
further dealt with.
Handling in the safety assessment SR-Site

The process is neglected.

4.4

Chemical Processes

4.4.1

Advective transport of species

Overview/general description

In this context, advection refers to transport of any forms of additional matter, e.g. ions, molecules
or colloids, with porewater flow. The transport direction is thereby principally from volumes of highwater pressure to volumes of lower pressure. The process leads to redistribution of solutes in theporewater and thus affects the porewater composition. There are several possible causes of pressure

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gradients in the tunnel backfill material, e.g. external water pressure, affinity for water in the bentonite
and temperature induced volume change of the water. Advective transport is closely related to waterflow in the tunnel backfill, which is described in Section 4.2.2. Advection may though be different
from pure water flow due to ion equilibrium effects in the case of ions, and due to filtering by nanosized pores in the case of colloids.
In principle, advective transport in the tunnel backfill material is governed by the same variables asin the buffer material (se Section 3.5.2). However, advective transport in the tunnel backfill material
is expected to be higher than in the buffer since the backfill bentonite is of lower quality and thatrock volumes with high water flow may be excluded for deposition holes. Still, the transport ofsolutes in the backfill is expected to be dominated by diffusion (see Section 4.4.2).
Dependencies between process and tunnel backfill variables

Table 4-8 summarises how the process influences and is influenced by all backfill variables and how
these effects are treated in SR-Site.
Boundary conditions

The external boundary condition for this process is the gradient caused by the change in hydrostaticpressure across the backfill.
Model studies/experimental studies

See the description of the corresponding process in the buffer, Section 3.5.2 for general information.
Specific tunnel material conditions may occur close to the roof and floor, mainly due to lower density
at the roof interface and the interaction with the highly permeable disturbed zone in the floor causedby the rock excavation technique.
Natural analogues/observations in nature

See the description for the corresponding process in the buffer, Section 3.5.2.
Time perspective

Advective transport of solutes in the backfill may occur at all timescales.
Handling in the safety assessment SR-Site

Advective transport of species in the tunnel backfill is included in the integrated modelling of thechemical evolution of the near field (Section 3.5.6).
Model: Radionuclide transport in the backfilled tunnels will be modelled by assuming transport controlled by advection-dispersion with sorption. The sorption coefficient will be based on the assumptionof equilibrium sorption on the backfill material. As an alternative, transport dominated by diffusion into
the backfill material with subsequent sorption on backfill material can be assumed.
Boundary conditions: The process is controlled by the hydraulic boundary conditions at the location
where the tunnel in question is situated in the repository.
Handling of variables influencing this process: All backfill properties and the hydraulic boundary
conditions are included in the model. The effect of temperature is neglected.
Handling of variables influenced by the process: The mass transfer of species in the porewater iscalculated.

208

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Table 4-8. Direct dependencies between the process Advective transport of species” and thedefined tunnel backfill variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Backfill geometry

Yes, major importance Included in transportmodelling throughhydraulic conductivity

Yes, minor effects
Neglected
possible throughdissolution/precipitation

Backfill pore geometry Yes, major importance Included in transportmodelling throughhydraulic conductivity

Yes, through dissolution/precipitation

Neglected

Temperature

Yes, through thermal
expansion andviscosity

Effect is small andnot considered intransport calculations

No, low flow rates

Water content

Yes

Neglected since low
water content will
reduce the process

Yes, minor effects
through dissolution/
precipitation

Neglected

Gas content

Yes, reduced flow inunsaturated clay

Neglected sincehigh gas content will
reduce the process

Yes, through transport/precipitation ofdissolved gas

Neglected

Hydro-variables(pressure and flows)

Yes, by definition

Included in transportmodelling throughhydraulic conductivity

Yes, minor osmotic and Included in transport
viscosity effects
modelling

Stress state

Yes

Included in transportmodelling throughhydraulic conductivity

Yes, minor osmotic
effects possible

Neglected

Backfill composition

Yes, density throughmontmorillonite
content.

Included in transportmodelling throughhydraulic conductivity

Yes, through dissolution/precipitation

Neglected

Backfill porewatercomposition

Yes, by definition

Included in transportmodelling throughhydraulic conductivity

Yes, by definition

Included in transportmodelling

Structural and straymaterials

Yes, as alternativetransport paths

Neglected

Yes, throughdissolution

Neglected

Uncertainties

Uncertainties in mechanistic understanding
The uncertainties in mechanistic understanding are the same as those for water transport undersaturated condition for the buffer (Section 3.3.2).Model simplification uncertainties in SR-Site
Input data and data uncertainties in SR-Site
Input data for the study of hydraulic conductivity are primarily the empirical relationships between
hydraulic conductivity and density, temperature and porewater composition. In SR-Site, this ishandled by sensitivity studies with different hydraulic conductivities in the tunnels.

4.4.2

Diffusive transport of species

Overview/general description

An overview of diffusion processes and phenomena as relevant for the buffer is given in Section 3.5.3;
these considerations also apply to a backfill consisting of low-grade bentonite (IBECO RWC-BF).

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The importance of these processes for the backfill material considered here is difficult to quantify,
because diffusion processes have not been studied in detail so far. It can generally be assumed that
the EDL-effects on enhanced cation diffusion and anion exclusion will decrease with decreasing
content of expandable 2:1 minerals and decreasing compaction/density (increasing pore size). In
sand-bentonite mixtures EDL-effects are expected to be negligible (except at sufficiently highbentonite/sand ratios). In the proposed backfill of low-grade bentonite, the content of smectite isabout 70%, (plus < 10% smectite/illite mixed layer) and a dry density of about 1,500 kg/m3 (clayfraction) is envisaged.
•

Evidence from several different studies suggests that in case of material with a smectite content of
about 95% to 45%, the overlap of EDL’s leading to the so-called anion exclusion (see descriptionof diffusion in the chapter on [processes in the buffer]) occurs at a dry density of about 1,200–
1,500 kg/m3 /Kozaki et al. 1998/. At a dry density of 1,500 kg/m3, the porosity of Kunigel-V1
with a montmorillonite content < 50% is about 0.44; i.e. nearly identical to the foreseen porosityof the backfill. Apparent anion exclusion (expressed by an apparently smaller diffusion-available
porosity for anions in comparison to HTO) has been found in natural clay formations with similaror even higher porosity (data for London and Boom clay quoted in /Nagra 2002/). Thus, it canbe expected that this process will also be operative in IBECO RWC-BF clay under the foreseen
conditions (porosity » 0.46).

•/Sato 1998/ gives a comparison of effective diffusivities for HTO and anions (Cl, I) for Kunigel-V1
Na-bentonite (dry density: 1,600 kg/m3) as a function of wt% sand added. The De values for both
HTO as well as the anions increase by a factor of about 2–3 when the sand content is increased
from 0 wt% to 70 wt%. This implies that under these conditions, anion exclusion was still operativein the bentonite/sand mixture.

•/Ochs et al. 2001/ found a similar effect of enhanced cation diffusion on De for Cs in the density
range of about 1,800 – 800 kg/m3 for MX-80 as well as Kunigel-V1 bentonite, which has asmectite content of about 45%. Therefore, it can be assumed that this process may also play a role
for the diffusive transport in the backfill. Note that enhanced cation diffusion is typically only
relevant for Cs, because Cs is nearly always present in purely cationic form.

•

Both enhanced cation diffusion and anion exclusion are further dependent on the ionic strengthof the pore solution. High salinity depresses the EDL, thereby decreasing the importance of bothprocesses.

Dependencies between process and tunnel backfill variables

Table 4-9 summarises how the process influences and is influenced by all backfill variables and how
these effects are treated in SR-Site.
Influence of temperature, porewater composition and speciation, pore geometry, sorption:
Analogous to the relations discussed for the buffer in Section 3.5.3.
Influence of backfill composition/density and structural/stray materials: To a first approximation, the additional non-expandable minerals present in the backfill are viewed as a dilution of the
expandable clay fraction. This is consistent with observations on materials with different contents ofexpandable clay minerals (e.g. MX-80, Kunigel-V1, and Opalinus Clay, see above). The influence ofsoluble impurities on porewater composition is analogous to the buffer.
Influence on backfill variables: As pointed out in Section 3.5.6, the diffusion process is coupled tonearly all chemical processes through the diffusive transport of reactants. How chemical processes,
such as exchange reactions of major ions, may further influence other backfill variables is discussed
in Section 4.4.4.
Boundary conditions

Limitation of the migration of major dissolved ions and radionuclide traces (remaining dissolved
concentrations below the respective solubility limits) is analogous to the buffer. The transfer of
species from the buffer to the backfill can be visualised as the diffusion of dissolved species from thebentonite into a volume of similar clay with a lower density. The transfer of species from the backfill
to the host rock can be visualised as the diffusion of dissolved species from the clay into flowing
water in the near-field fracture network (equivalent flow rate or Qeq, see Section 3.6.2).
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Table 4-9. Direct dependencies between the process “Diffusive transport of species” and thedefined tunnel backfill variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Temperature

Yes

The temperaturedependence ofthe diffusivity is
not included in thetransport calculations.
The importance isregarded as small

No

Water content

Yes.

Saturated conditions
assumed.

No

Gas content

Yes, by affecting porespace available for
diffusion

Saturated conditions
assumed.

No, but indirectly
through porewater composition and dissolved
gases such as CO2 or
CH4, which may comeout of solution

Hydro-variables(pressure and flows)

Yes.

Saturated conditions
assumedNo

Backfill geometry

Yes

Backfill dimensions
are included in
calculations

No

Backfill poregeometry

Yes

Defines the magnitude No, but possibly indiof the diffusivities
rectly through influence
on porewater composition and swelling

Stress stateNo, but indirectly
through pore geometry

Backfill composition

Through porewatercomposition

Defines the magnitude No, but indirectlyof the diffusivities
through influence onporewater composition

Backfill porewatercomposition

Extent of EDL
depends on ionicstrengthDefines the magnitude Through diffusive trans- See 3.4.7of the diffusivities
port of main constituents

Structural and straymaterials

Indirect on thegeometry,,throughinfluence on physicalpore width)

No, but possibly indirectly through influence
on porewater composition and swelling

Possibly indirectly
through influence onporewater composition

Model studies/experimental studies

While a large number of investigations are described in the literature concerning diffusion experiments in bentonite and similar smectite-rich clays (see /Yu and Neretnieks 1997/ for an overview),
much less work has been done for natural clays poor in smectite and bentonite/ballast mixtures.
Some data relevant for such materials are compiled in /Yu and Neretnieks 1997, Nagra 2002/, but nodata are available specifically for IBECO RWC-BF.
Most diffusion studies are performed as transient in-diffusion experiments. These experiments yield
Da, which is a lump-sum representation of all processes relevant for radionuclide migration, including
diffusion and sorption. This holds in particular for moderately and strongly sorbing radionuclides.
Steady-state (or transient) through-diffusion studies, which can directly yield De and porosity, arelargely restricted to relatively mobile tracers (alkaline ions, HTO, anions) or low degrees of compaction. Few attempts have been made so far to derive diffusion coefficients through semi-mechanistic
models. Some examples are discussed in Section 3.5.3 in the context of anion exclusion and enhanced
cation diffusion.

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Natural analogues/observations in nature

The usefulness of natural analogues for determining diffusion constants is limited due to the difficultyof determining what conditions existed in the past.
Time perspective

The process is active in all timescales, but is of less concern during the saturation phase.
Handling in the safety assessment SR-Site

Before saturation: The process is neglected since advection dominates.
After saturation: The process is included as a boundary condition in the scenario specific modellingof the backfill chemical evolution for the post-thermal long-term phase, see heading “Handling” forprocess 4.4.4. The process is treated in a simplified way with identical diffusivity for all elements.
Failed canister: Included in the modelling of radionuclide transport for the long-term phase, seeheading “Handling” for process 3.5.3. For the treatment of radionuclide diffusion, element-specificeffective diffusivities are used together with corresponding porosities (see the SR-Site Data Report/SKB 2010l/).
Boundary conditions: See sections referred to above.
Handling of variables influencing this process: Dependence on density, expected ionic charge ofdiffusing species, porewater composition and temperature is considered in the selection of diffusion
constants for transport of radionuclides. For the treatment of diffusion within the backfill chemical
evolution, see Section 4.4.4.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Most general issues relevant for handling diffusion in the backfill are discussed in the sections 4.4.4
and 3.5.3. All radionuclide-specific data (effective diffusivities and porosities) are taken from SKB
reports /Ochs and Talerico 2004/ and SR-Site Data Report /SKB 2010l/ dedicated to the derivationof system-specific data that are consistent with the specified properties of IBECO RWC-BF Sets ofdiffusion-available porosities and effective diffusion coefficients are selected / for anions, cations(essentially Cs) and elements existing largely as neutral species (on average) under the relevant porewater conditions. To define selected parameter sets, a large amount of original experimental data,
published in peer-reviewed journal articles and proceedings and in reviewed reports, was evaluated.
Uncertainties

Uncertainties in mechanistic understanding
Uncertainties in mechanistic understanding of diffusion in compacted, expandable clay materials are
discussed in Section 3.5.3 this discussion applies also to the case of backfill.
Model simplification uncertainties for the above handling in SR-Site
As in the case of the buffer, diffusion is represented in a simplified way, through the use of selected
constant effective diffusion coefficients and available porosities in the relevant transport codes (seeSections 4.4.4 and 3.6.2). In that sense, the diffusion input parameters have to be viewed as conditional; i.e. their application will only be valid under the conditions (in particular backfill density)
considered in data derivation. Therefore, uncertainties will be related to the selection of consistentconditions.
Input data and data uncertainties for the above handling in SR-Site
Input data to SR-Site are effective diffusivities and diffusion-available porosities to be directly usedin consequence calculations. Data and uncertainties for IBECO RWC-BF are accepted from /Ochs
and Talerico 2004/; after scaling to the specified reference density.
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4.4.3

Sorption (including exchange of major ions)

Overview/general description

An overview of sorption and ion exchange processes relevant for the buffer is given in Section 3.5.5.
Since the current backfill material is similar to the buffer in most respects, the process can be treatedin the same fashion.
Dependencies between process and tunnel backfill variables

Table 4-10 summarises how the process influences and is influenced by all buffer variables and how
these effects are treated in SR-Site.
Influence of backfill and porewater composition, and of temperature: Analogous to the situation of
the buffer (see Section 3.5.5).
Influence on backfill variables: The influence of ion exchange on the surface composition of layer
silicates and on porewater composition is analogous to the situation of the buffer.
Table 4-10. Direct dependencies between the process “Sorption (including exchange of majorions) and the defined tunnel backfill variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No) Description(How/Why not)Backfill geometry

No

No(Total backfill massconsidered)Backfill pore geometry No, but possibly
indirectly throughinfluence on EDL
properties

No, but possibly
indirectly throughinfluence on EDL
properties/swelling
Influence of temperature on sorption mustbe acknowledged, buteffect is not clear

See 3.5.5

Temperature

Yes

No

Water content

No, but indirectly
through porewatercompositionNo

Gas content

No, but indirectly
through porewatercomposition (CO2)No, but possibly
See 3.5.5
indirectly through influence of Ca-exchange
on carbonate equilibria

Hydro-variables (pres- No, but indirectly
sure and flows)
through porewatercompositionNo, but possibly
indirectly, throughporewater composition/swelling

Stress stateNo, but indirectly
through influence onporewater composition
and EDL properties

Indirectly throughSee 3.5.5
influence on porewatercomposition and EDL
properties

Backfill composition

Yes

Chemical data used
for sorption coefficientestimates(Surface composition)

See 4.4.4

Backfill porewatercomposition

Yes

Chemical data used
for sorption coefficientestimates

Yes

See 4.4.4

Structural and straymaterials

Yes

Sorption on straymaterials is conservatively excluded in
SR-SiteIndirectly throughinfluence on porewatercomposition andrelated mineralequilibria

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See 3.5.5

213

Boundary conditions

Analogous to the situation of the buffer (see Section 3.5.5).
Model studies/experimental studies
Analogous to the situation of the buffer (see Section 3.5.5).
Time perspective

Analogous to the situation of the buffer (see Section 3.5.5).
Natural analogues/observations in nature

Analogous to the situation of the buffer (see Section 3.5.5).
Handling in the safety assessment SR-Site

Before saturation: Sorption of radionuclides is neglected before saturation since there is no continuous
water path available. The geochemical evolution of the backfill during saturation is not considered in
SR-Site.
After saturation: Ion exchange is included in the scenario-specific modelling of the backfill chemical
evolution for the thermal phase and the post-thermal long-term phase, see heading “Handling” in thedescription of process Alterations of backfill impurities, Section 4.4.4.
Failed canister: Sorption is included in the modelling of radionuclide transport for the long-term phase,
see heading “Handling” for process 3.6.1 As for the buffer, sorption is treated through element-specific
Kd values derived for specific conditions (see /Ochs and Talerico 2004/ and the SR-Site Data Report/SKB 2010l/ for the definition of data and associated uncertainties).
Boundary conditions: See sections referred to above.
Handling of variables influencing this process: Dependence on the composition of the backfill and thecorresponding porewater is considered in selecting Kd values for radionuclide transport calculations.
Because of the conditional nature of Kd, the data selected are derived for conditions expected to be
relevant for safety assessment calculations. Temperature effects are not considered explicitly, but theeffects within the temperature range expected in the backfill are deemed to be covered with good
margin by the uncertainties associated with the selected Kd values. For the influence of ion exchange
and surface acid-base equilibria on the chemical evolution of the backfill, see Section 4.4.4; theseprocesses are also taken into account in the derivation of radionuclide Kd values.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Most general issues relevant for handling ion exchange of major ions and radionuclide sorption
are discussed in Sections 4.4.4 and 3.6.2. As in the case of radionuclide sorption in the buffer, allradionuclide-specific data are taken from a SKB report /Ochs and Talerico 2004/ specifically dedicated to the derivation of diffusion and sorption coefficients. That report was subjected to a factual
review according to the standard for the SR-Can project. Distribution coefficients were calculated
by /Ochs and Talerico 2004/ on the basis of a porewater composition calculated for a specified
clay-groundwater interaction scenario. As source data for the derivation of scenario-specific distribution coefficients, a large amount of original experimental data, published in peer-reviewed journal
articles, proceedings and reports, was evaluated. In a few cases, calculations with thermodynamic
sorption models were carried out for comparison. The sorption data to be used in the radionuclidetransport calculations are qualified in the SR-Site Data report /SKB 2010l/.

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Uncertainties

Uncertainties in mechanistic understanding
The discussion given for the corresponding process in the buffer (Section 3.5.5) is also applicable tothe backfill.
Model simplification uncertainties in SR-Site
The data selection for the considered backfill materials is a direct extension of the approach taken
for the buffer. The Kd concept for radionuclide transport modelling is basically justified under conditions far from surface saturation, which can be expected in the case of diffusion-limited radionuclide
migration in a clay-based backfill. At the same time, Kd is a highly conditional parameter and has tobe derived for each set of conditions. As long as the backfill material can be expected to represent a
homogeneous geochemical compartment, no spatial variability needs to be taken into account.
Input data and data uncertainties in SR-Site
Input data to SR-Site are Kd values to be directly used in consequence calculations. In case of abentonite/ballast mixture, Kd values for radionuclide migration for various conditions, as well as theassociated uncertainties are directly based on /Ochs and Talerico 2004/ by scaling for the smectite
content. The sorption data to be used in the radionuclide transport calculations are qualified in the
SR-Site Data report /SKB 2010l/.

4.4.4

Alterations of backfill impurities

Overview/general description

The backfill material consists not only of montmorillonite, but also other accessory minerals as wellas impurities. In the repository environment, these can be dissolved and sometimes re-precipitateddepending on the prevailing conditions.
Basically, the same chemical processes that occur in the buffer may also act in the backfill(Section 3.5.6).
The main differences are:
1. The temperature in the backfill is much lower and the thermal gradients are small.
Dissolution/precipitation of minerals due to temperature effects is of minor importance in the backfill.
2. The initial composition is different. However, the IBECO-RWC-BF is geochemically rather similar
to the Deponit CA-N (IBECO-RWC) buffer material, since they originate from the same deposit.
3. Both advective and diffusive transport of species may be active in the backfill.
Dependencies between process and tunnel backfill variables

Table 4-11 summarises how the process influences and is influenced by all backfill variables andhow these effects are treated in SR-Site.
Temperature: Thermodynamic constants of chemical reactions are highly dependent of the temperature. However, the maximum expected temperatures in the backfill are around 40ºC during the first
100 to 200 years (see Section 4.1.1). Although even at these temperatures some changes can occur,
the overall effect is likely to be less critical here than in the buffer.

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Table 4-11. Direct dependences between the process “Alterations of backfill impurities” and thedefined tunnel backfill variables and a short note on the handling in SR-Site.
Variable

Variable influence on process

Process influence on variable

Influence present?
(Yes/No)
Description

Handling of influence(How/Why not)Influence present?
(Yes/No)
Description

Backfill geometry

Yes, the total amount ofminerals in the backfill andthe bentonite/porewater
ratio affects the process

The geometry and theNobentonite mass, as wellas the bentonite/porewater ratio is considered in the model

Backfill poregeometryNo, but indirectly throughporewater composition

Determines the diffusiv- Precipitation-dissolution Included in model but
ity see 4.4.2, whichof mineralsof minor concern inis considered in the
backfill
model.

Temperature

Yes. Temperature highly
influences the extent ofprecipitation/dissolution of
secondary minerals.

Minor effect since theNotemperature increase in
the backfill is small

Water content

No, but indirectly throughHowever, the chemical
effects on solute transportmodel assumes
and porewater composition. saturated conditionsNo

Gas content

No, but indirectly throughdissolution of gas andporewater composition.

Yes

Hydro-variables(pressure andflows)

No, but indirectly throughporewater composition

Darcy flow is includedin the model, see 4.4.1

No

Stress state

Yes

Partial pressures ofgases are included in
model

Yes

The results from themodel is evaluated toconsider effects on
swelling pressure

Backfill composition

Yes

Composition data is
considered in modelYes

The evolution of
the composition is
calculated with themodel

Backfill porewater Yes
composition

Composition data is
considered in modelYes

The evolution of
the composition is
calculated with themodel

Structural andstray materials

See 4.4.7

Secondary mineralformation/dissolution
may influence thealteration of structuralcomponents. This isparticularly true forsulphates.

The fate of structural
material is not evaluated in SR-Site

No, but indirectly by affecting porewater composition.

Handling of influence(How/Why not)Dissolved gases areincluded in the model

Water content: The effect of water content on the alteration of accessory minerals is indirect. As thesaturation process affects the transport of solutes, this in turn affects the chemical reactions. Therefore
the water saturation process can result in significant changes in the redistribution of accessory minerals and on the chemical composition of porewater. Precipitation-dissolution of accessory minerals
can result in minor changes of porosity, and therefore to the water-solid ratio or available porosity forwater uptake. However, this change in porosity is expected to be very small.
Gas content: The formation of a gas phase can modify the concentration of solutes in the porewater,
and therefore to the stability of accessory minerals. However, this process is expected to be very
limited in the backfill.

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Hydrovariables (pressure and flows): Transport of dissolved chemical species in the system can
modify the dissolution-precipitation rates of accessory minerals by maintaining the saturation stateof these phases far from equilibrium.
Backfill geometry: The geometry of the backfill can affect indirectly the alteration of accessory
minerals through the transport processes associated to the solutes in the groundwater and bentonite
porewater, thus affecting the behaviour of geochemical reactions during its interaction. Moreover,the backfill geometry determines the total pool of accessory minerals able to react in the system dueto the interaction of groundwater and bentonite.
Backfill pore geometry: The pore geometry of the backfill affects the transport of solutes modifying
the chemical composition of porewater within the backfill and hence, affecting the geochemical
reactions taking place in the backfill. In addition, precipitation-dissolution of accessory minerals canmodify the porosity distribution in the backfill as well as the pore geometry.
Stress state: The increase in the partial pressure of gases can affect the stress in the system. The
build up of such partial pressures could be a direct consequence of the alteration of impurities.
However, as gas phases are not expected to form in the backfill, the influence of this variable isexpected to be negligible.
Backfill composition: This is one of the most relevant variables influencing the alteration of impurities. The type and amount of accessory minerals and montmorillonite is essential to determine how
their dissolution-precipitation behaviour will affect the chemical evolution of the system. Moreover,the dissolution-precipitation of these mineral phases will modify the bentonite composition during
the interaction with porewater. The composition of montmorillonite, especially the cation exchanger(type of cations and occupancy) is very relevant, as the interaction with backfill porewater will
modify the chemical composition of this porewater and, hence, the dissolution-precipitation ofaccessory minerals in the bentonite. In addition, the alteration of bentonite impurities will modify thechemical composition of bentonite porewater and hence the montmorillonite composition.
Backfill porewater composition: The porewater composition will determine which of the accessory
minerals in the bentonite will dissolve or precipitate, and thus the dissolution-precipitation of theseminerals will modify the chemical composition of porewater.
Structural and stray materials: The interaction of bentonite porewater with structural and straymaterials can result in the modification of the chemical composition of porewater and, therefore, inthe behaviour of accessory minerals and vice versa.
Boundary conditions

There are no particular boundary conditions to discuss for these processes. The relevant boundary
conditions in order to treat the process quantitatively are those of the transport processes that control
the exchange of solutes between the tunnel backfill water and the water in its adjacent components,
i.e. the boundary conditions of the processes diffusion and advection.
For alterations of impurities, the exchange of a number of solutes like sodium and calcium ions,carbonate and molecular oxygen is relevant.
Model studies/experimental studies

No specific studies of chemical processes in the backfill have been done prior to SR-Site. Some of
the work done with buffer materials is of relevance for the backfill as well (Section 3.5.6).
Natural analogues/observations in nature

Some of the work done for buffer materials may be of relevance for the backfill as well (Section 3.5.6).
Time perspective

The process is relevant for the whole assessment period.

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Handling in the safety assessment SR-Site

Before saturation: Geochemical processes will be the same both before and after saturation phases.
This is just a matter of water flow and solutes transport. No specific modelling of the geochemical
evolution in the backfill for the saturation phase will be done.
Thermal period: The thermal effects on the chemical evolution of the backfill are not included in
SR-Site, since the temperature elevation and the thermal gradients are small.
After saturation: The process is included in the scenario-specific modelling of the backfill chemical
evolution for the post-thermal long-term phase. The model considers advection, diffusion, ionexchange and mineral dissolution/precipitation in the backfill compartment. Figure 4-10 shows anexample from the modelling of the pH evolution in the backfill together with calcite and gypsum
content and calcium occupancy in the exchanger of the backfill. pH in the backfill is buffered bycalcite precipitation-dissolution, which in turn is affected by calcium availability due to cationexchange processes. As exchange reactions reach equilibrium with incoming water more easily inthe backfill than in the buffer (where the CEC is larger), the excess of calcium is used to precipitate
calcite until equilibrium is obtained, decreasing the pH of porewater in the backfill.
Failed canister: In the case of a failed canister, vast amounts of metallic iron, originating from thecorroding cast iron canister insert, will be added to the geochemical system. This was not included in
the model mentioned above, although it is envisaged to be included in future modelling exercises.
Boundary conditions: Main model: The diffusive transfer of species between the flowing water andthe bentonite is explicitly modelled. The near-field evolution model uses the Qeq concept to handle
the boundary conditions. This is sufficient to estimate a general timescale on which the evolution
occurs.
Handling of variables influencing this process: The modelling is in itself a coupling of the four
(groups of) processes mentioned above and is thus a coupled modelling of most of the significant
long-term chemical processes. A large number of the variable influences related to the chemical
conditions (porewater composition, montmorillonite composition, bentonite composition) are also
included.
Handling of variables influenced by the process: The geochemical conditions in the near field as afunction of time and space will be calculated.
The changes in the buffer accessory mineral content as a consequence of the thermal stage has beenmodelled, but not validated through a comparison with experimental and/or natural analogue data.
This makes it difficult to ensure that changes due to the thermal stage are minor or that these results
should be assumed to be the initial conditions for the low temperature stage.
In addition to the processes discussed above, the behaviour of silica and clay minerals and theireffect on cementation could be addressed by performing some scoping calculations considering thepresent knowledge on kinetic dissolution-precipitation of smectite, silica-phases and the precipitationof alternative clay minerals (kaolinite and/or illite). These scoping calculations would give somequalitative information on the relative importance of these processes and the effect on bentonite
properties, i.e. porosity changes. See also Section 3.5.9.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

No separate references are used for the backfill. However, the geochemical processes are the same as
for the corresponding process in the buffer. Therefore, the same references apply (see Section 3.5.6).
Uncertainties

Uncertainties in mechanistic understanding
Some critical uncertainties remain concerning the mechanistic understanding of the processes that
control the redox state of the bentonite system. It is not clear yet to which extent pyrite and/or
siderite are the main redox controlling phases, although either process, or their combination, will
scavenge the remaining oxygen in the near field. This should be discerned by comparative modellingof pyrite and siderite kinetic availability.
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pH

Gypsum (moles/Lw)

Calcite (moles/Lw)

CaX2 occupancy (moles/Lw)

Figure 4-10. Graphics showing the predicted evolution in the simulated domain (Deponit CA-N bentonite
buffer and Friedland backfill) for pH, calcite and gypsum content and calcium occupancy in the cationexchanger of montmorillonite after 60,000 years of simulation. Groundwater is simulated to enter thesystem through a conductive fracture perpendicular to the deposition gallery at the left boundary of thedomain (units in moles per litre of porewater). Note that as gypsum dissolves both calcium occupancy inthe exchanger and calcite content increase and pH slightly decreases in the backfill /Arcos et al. 2006/.

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219

In addition, most of the secondary precipitation/dissolution processes that are temperature driven
are thermodynamically controlled. Hence, there is no need for a detailed mechanistic understanding.
However, some of the silicate transformations are kinetically controlled and their mechanistic
understanding in compacted bentonite conditions is poor. This is mainly related to clay mineraltransformations (dissolution of smectite and precipitation of illite and/or kaolinite). Although thesereactions occur at a very slow rate, their effect on the overall geochemical evolution is not yet clear.
Finally, it is assumed that pH is buffered by the equilibrium with calcite. However, the amount (oreven the presence) of this mineral phase is somewhat uncertain, depending on the bentonite typeselected as backfill component. Thus, the pH buffering capacity of the backfill could be affected.
Model simplification uncertainties in SR-Site
Secondary dissolution and precipitation processes have not been fully implemented in a coupledmodel of backfill evolution. The processes are only considered when required for the definitionof critical master variables in the system (pH and pe). A possibility could be to couple the THM
processes occurring with the key chemical processes that will control the geochemical evolution of
the backfill system.
There are a number of accessory minerals, such as silicates other than silica, which are not includedin the models, as most of them tend to dissolve at a very slow kinetic rate. Other silicates (usuallyclay minerals) precipitate at similarly slow rates. However, the dissolution-precipitation processes
involving these two groups of minerals could affect the stability of montmorillonite as well as thechemical composition of the backfill porewater, thus affecting the chemical long-term evolution of
the bentonite. See also Section 3.5.9.
There is no model accounting for the dissolution/precipitation of accessory minerals during the water
saturation stage of the backfill. As water flux will be faster than when the backfill is water saturated
and the bentonite composition will be far from equilibrium with incoming water, some changes
will occur faster. These changes will account for step compositional gradients in the backfill andchanges in porosity as accessory minerals dissolve and/or precipitate, leading to changes in hydraulicproperties of the backfill. Thus, modelling of the system must be coupled to the THM processes thatproceed in parallel with transport and chemical reactions. A submodel dealing with the saturation
stage of the backfill will, at least, give an idea on the magnitude of these changes.
As there is some uncertainty associated to the accessory mineral content of the bentonite component,
especially for those minerals that can play a significant role on the pH and/or redox buffering capacity of the backfill, the model will consider variable amounts of these minerals, even their absencefrom the initial accessory mineral pool. Thus the effect of dissolution/precipitation of these minerals
on the master variables will be assessed.
Input data and data uncertainties in SR-Site
Key thermodynamic data are well established and therefore reliable. The main uncertainties
remain in the definition of the kinetically controlled processes. These are linked to the mechanistic
uncertainties previously mentioned.
Proper data for surface reactions (i.e. cation exchange and edge site reactions) is of paramountimportance for the correct assessment of the bentonite behaviour. Two types of information areneeded for geochemical modelling: i) Cation occupancy of the CEC and protonation state of thesmectite surface, and ii) Exchange and protonation-deprotonation constants. The main uncertainty
associated with these data is related to the experimental conditions used for their determination (i.e.
low bentonite density and high solid/liquid ratios). Recent experiments have addressed this problem,
however, there is still a large uncertainty related to the determination of cation exchange constants
and their variation as a function of bentonite density. These data and the associated uncertainties will
tentatively be handled in the SR-Site data report.

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4.4.5

Aqueous speciation and reactions

Overview/general description

This process is in general identical with the corresponding process in the buffer (Section 3.5.7)
Handling in the safety assessment SR-Site

Before saturation: Geochemical processes will be the same before and after saturation. This is just a
matter of water flow and solutes transport, something to be considered in other sections.
After saturation: The process is included in the scenario-specific modelling of the backfill chemical evolution for the thermal phase and the post-thermal long-term phase. This is described in
Section 4.4.4.
Failed canister: The backfill is assumed to be unaffected geochemically from the canister insert.
However, the properties of the backfill are considered in the radionuclide transport calculations.

4.4.6

OsmosisOverview/general description

The general principle for this process is the same as for the corresponding buffer process(Section 3.5.8). The absolute effects of osmosis increases with increasing montmorillonite/water
ratio, but swelling pressure increases more, and the relative pressure drop is consequently more
pronounced at low montmorillonite/water ratios. The tunnel backfill material has to balance theswelling pressure from the buffer.
Dependencies between process and tunnel backfill variables

Table 4-12 summarises how the process influences and is influenced by all backfill variables andhow these effects are treated in SR-Site.
Boundary conditions

The boundary conditions are the cation concentration and cation valence of the groundwater incontact with the tunnel backfill.
Model studies/experimental studies

The studies reported in the buffer section are relevant also for the tunnel backfill (Section 3.5.8).
Natural analogues/observations in nature

The study reported in the buffer section is relevant also for the tunnel backfill (Section 3.5.8).
Time perspective

The process is relevant for all time perspective in the lifetime of a repository. The effect of a change
in groundwater salinity is expected to result in an almost instantaneous response in swelling pressure.
Handling in the safety assessment SR-Site

The effects of the ionic strength of the groundwater on the hydraulic properties of the backfill are
handled by direct application of empirical data mainly given in /Karnland et al. 2006/.

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221

Table 4-12. Direct dependencies between the process “Osmosis” and the defined tunnel backfill
variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence?
(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence?
(Yes/No)(How/Why not)Description

Backfill geometryNo

Yes, possible at low
backfill density

Empirical assessment

Backfill pore geometry Yes, interlayer
distance is a decisive
parameter

The density is given
as initial condition

Yes, rearrangement of Empirical assessment
pore sizes at extreme
conditions

Temperature

Yes, linear
dependence

Neglected due tosmall differences intemperature

No

Water content

Yes, equivalent topore geometry insaturated backfill

Saturated conditions
assumedYes, possible at low
backfill density

Gas content

No, the process is
not well defined inunsaturated backfill

No

Hydrovariables (pressure and flows)

No, only indirectly
since flow may modify
ion equilibrium

Yes, hydraulicconductivity effects

Empirical assessment

Stress stateNo, only indirectly
through charge
compensating cation
concentration

Yes, by definition

Empirical assessment

Backfill composition

Yes, through montEmpirical assessment
morillonite content.
Number and type of
charge compensating
cations are of decisive
importanceNo

Backfill porewatercomposition

Yes, by definition

Yes, by definition.
Of no concern
Governs the interlayer
ion concentration

Structural and straymaterials

No

Empirical assessment

Neglected

No

Time periods:
Resaturation/thermal period and period up to first glaciation: The hydraulic conductivity and swelling pressure of the backfill are estimated based on the groundwater composition at the site.
Glacial cycle: The hydraulic conductivity and swelling pressure of the backfill are estimated basedon the evolution of the groundwater composition.
Handling of variables influencing this process: The montmorillonite composition and the ground­water/porewater composition are directly included in the model.
Handling of variables influenced by the process: The backfill swelling pressure and hydraulicconductivity will be calculated.
Boundary conditions: The composition of the groundwater in the repository.
The special cases of failed canister and of earthquakes: Will not affect this process.

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Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The principles for osmosis, ion equilibrium (Donnan equilibrium) and the resulting pressure effects arebasic knowledge in physical chemistry, and are consequently supported by numerous peer reviewed
articles available in the open literature. The coupling to bentonite buffer systems is shown in /Karnland
et al. 1997/ and /Karnland et al. 2006/. Both are SKB reports and the latter undergo documented factualand quality review at present. /Karnland et al. 2005/ is not an SKB report, but is developed within
the framework of a quality assurance system that requires a documented factual- and quality review
before approval and printing and is available in the open literature. The natural analogue support forosmosis /Neuzil 2000/ is a peer reviewed article (Nature) that is available in the open literature.
Uncertainties

The uncertainties are the same as those for the buffer (see Section 3.5.8) except for a larger uncertainty
in the theoretical calculations of pressure effects due to the larger amount of accessory minerals in the
low grade bentonite.

4.4.7

Montmorillonite transformation

Overview/general description

The general principle for this process is the same as for the corresponding buffer process (Section 3.5.9).
In principle all possible transformation processes are very temperature dependent. The temperature issignificantly lower in the tunnel backfill compared to the buffer, and transformation processes in the
backfill is consequently even less probable than in the buffer. One possible exception is pH dependantlocal alteration in volumes that are in direct contact with concrete, e.g. tunnel plugs or wall reinforcement.
Dependencies between process and tunnel backfill variables

Table 4-13 summarises how the process influences and is influenced by all backfill variables andhow these effects are treated in SR-Site.
Boundary conditions

See the corresponding buffer process; Section 3.5.9).
Model studies/experimental studies

The studies reported in the buffer section are relevant also for the tunnel backfill (see Section 3.5.9).
Natural analogues/observations in nature

The studies reported in the buffer section are relevant also for the tunnel backfill (see Section 3.5.9).
Time perspective

Significant transformation is not expected during the lifetime of the repository according to thekinetic model for illitisation.
Handling in the safety assessment SR-Site

The alteration of the swelling component in the backfill could be treated in the same way as for the
buffer (Section 3.5.9). The only difference is the maximum temperature and the amount of material.
Since the temperature is low under all conditions, montmorillonite alteration, in the form of illitization, can be neglected in the backfill. It is also assumed that the cement in the repository will be of
the “low-pH” type and will have insignificant impact on the properties of the backfill.

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Table 4-13 . Direct dependencies between the process “Montmorillonite transformation” and thedefined tunnel backfill variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Backfill geometry

Yes, determines
total mass available,
indirectly throughtransport of dissolved
species

Yes at extreme
transformation

Included in evaluation

Backfill pore geometry No, indirectly throughtransport of dissolved
species and ion
equilibrium–

Yes, directconsequence

Temperature

Yes, major impact

Included in transformation expression

No

Water content

No, indirectly throughtransport of dissolved
species and ion
equilibrium

No credit taken

Yes, possible but notnecessary

Gas content

No, indirectly throughwater content

Hydrovariables (pressure and flows)

Yes, controls supply
of dissolved species(mass balance)Stress stateNo, pressures are tosmall

Backfill composition

Yes, through available
species in accessory
minerals

Backfill porewatercomposition

Yes, major impact

Structural and straymaterials

No, but indirectly
Discussed in thethrough porewater
buffer alterationand dissolved species. sceario.
Especially pH effects
from cement

No
Included in transformation evaluation

Yes, change in
Effect on hydraulichydraulic conductivity conductivity can beevaluated
Yes, direct consequence

Loss of swelling canbe evaluatedIncluded in transformation expression

Yes, by definition

Included in transformation expression

Included in transformation expression

Yes, major impact

Only effects on the
backfill itself areconsideredNo

Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Montmorillonite alteration is one of the most studied processes in clay mineralogy and the above argumentation is supported by numerous peer reviewed articles and conference papers that are available in
the open literature. The above cited ones represent only a minor fraction of what is available and the key
issues are rather the relevance and relative quality of the peer reviewed material. A literature study withspecial respect to KBS-3 conditions including a validation of alteration models, especially quantitative,
was made by SKB /Karnland and Birgersson 2006/ in order to address these issues. This report may beseen as a comprehensive exposition of the adequacy of references.
Uncertainties

The uncertainties are the same as for the corresponding buffer process (Section 3.5.9).

4.4.8

Backfill colloid release

Overview/general description

Water uptake by the tunnel backfill material and its resultant swelling is limited by the tunnel walls,
and a swelling pressure is developed in the backfill material. Fractures intersecting the tunnel imply
that no swelling restrictions are present, and that swelling continues until a thermodynamic equilibrium
224

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is reached without the development of swelling pressure. This free swelling may lead to separation ofindividual montmorillonite layers, or small groups of mineral layers (dispersion). The same principles
are valid for the backfill system as for the buffer (see section 3.5.11) with the following exceptions.
Deposition holes will not be placed in rock volumes with larger fractures or severe groundwater
flow. The option to fully avoid such zones may not be present in the placement of deposition tunnels.
The tunnel backfill material may thereby be exposed to more adverse conditions than the buffermaterial both with respect to fracture apertures and flow rates.
The tunnel backfill material will have lower content of montmorillonite or other swelling minerals
compared to the buffer material. Colloid release leads to replacement of lost montmorillonite byswelling of the remaining montmorillonite, which leads to a successive deterioration of the sealing
properties. In principle, high quality bentonite materials will thereby stay relatively homogeneous andthe sealing properties will decrease generally but slowly. In contrast, accessory minerals in low quality
bentonite materials may serve as a filter and prevent replacement of lost material. This potential filter
effect may reduce the loss of montmorillonite, but it may also lead to high local hydraulic conductivity
in the depleted part of the backfill. The content and the grain size distribution of the accessory material will determine the extent of such a filter function.
Dependencies between process and tunnel backfill variables

Table 4-14summarises how the process influences and is influenced by all backfill variables and how
these effects are treated in SR-Site.
Table 4-14. Direct dependencies between the process “Colloid release” and the defined tunnel
backfill variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence?
(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence?
(Yes/No)(How/Why not)Description

Backfill geometry

Amount of backfill
present

included in modelling

Yes, at extreme
colloid release

included in modelling

Backfill pore geometry

Yes, governs thedispersing forces

included in modelling

Yes, by definition

included in modelling

Temperature

No, negligible

Water content

Yes, by definition

included in modelling

Yes, by definition

Gas content

Yes, no dispersion
possible fromunsaturated clay

Boundary condition

NoNo
included in modelling

Hydrovariables (pressure Yes, groundwater
included in modellingand flows)
flow may determines
the mass loss

Yes, indirect troughpore geometry,central impact

Evaluated from modelling results

Stress state

Yes, by definition

Yes, major impact

included in modelling

Backfill composition

Yes, impurities may
govern the ion typeand concentrations

Backfill porewatercomposition

Yes, major
included in modelling
importance both withrespect to cation
type and concentrationNo, but indirectly
through pore geometry which affects
the counterion
concentration

Structural and straymaterials

No, but indirectly
through porewatercomposition.
Especially concrete
may support withcalcium ionsIndirectly, a minor
effect of calcium
uptake from cement
structures could be
imagined

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included in modelling

Yes, selective loss of included in modellingthe montmorillonite
content
included in modelling225

Boundary conditions

The boundary conditions are the same as for the corresponding buffer process; see Section 3.5.11
Model studies/experimental studies

All studies reported in the buffer section on colloid release are relevant also for the tunnel backfill(Section 3.5.11).
Natural analogues/observations in nature

All studies reported in the buffer section are relevant also for the tunnel backfill (Section 3.5.11).
Time perspective

Colloid release is only possible at low ion concentrations and likely only relevant in conjunction
with glaciations. The effects will be significant only in the long-term perspective.
Handling in the safety assessment SR-Site

The handling of the process is the same as for the corresponding process in the buffer (Section 3.5.11).
The main difference is that the total mass of backfill is much larger than of the buffer.
Uncertainties

The uncertainties are the same as for the corresponding buffer process (Section 3.5.11).

4.4.9

Radiation-induced transformations

Overview/general description

Montmorillonite in the backfill can be broken down by γ radiation. The result is a decrease in themontmorillonite concentration. The process can be neglected in the backfill based on the same
reasoning as for the buffer (see Section 3.5.12).
Handling in the safety assessment SR-Site

The process is neglected since the dose rate in the tunnel backfill is too low to have any effects(see Section 3.5.12).

4.4.10 Microbial processes
Overview/general description
Micro-organisms interact with their surroundings, and they commonly have a significant effect on
the geochemical record of their environment. From a microbiological perspective, the backfill will
constitute an environment with mixed characters from two other well defined environments of therepository concept; the geosphere and the buffer. This backfill environment will, however, havesome characteristics that significantly distinguish it from the two other mentioned environments.
One is the possible presence of organic material in structural and stray material. Another is thegas/water interface that will develop during the saturation process. Finally, there will be oxygenin the backfill until it has been consumed and reduced by microbes and/or by inorganic oxygen
scavengers such as sulphides.
The effects of microbial processes on the chemical stability of the geosphere are thoroughly discussed in
the Geosphere process report /SKB 2010i /. Similar effects can be obtained in the backfill, but the differences in the variables “structural and stray materials, the temperature, and the gas content” compared to
the geosphere may increase the rates of microbial processes during an initial phase that is relatively shortcompared to the expected lifetime of the repository.
Microbial processes in the buffer have been discussed elsewhere in this report (Section 3.5.14). Those
processes are expected to be limited to an initial phase, after which temperature, radiation, desiccationand the swelling pressure are assumed to sterilise the buffer /Pedersen 2002/. The radiation, desiccation and the swelling pressure limiting effects on microbial processes in the backfill will be less226

TR-10-47

pronounced or as good as absent with respect to radiation, and continued microbial activity will bepossible. On the other hand, the consequences of microbial activity is rather positive than negative inthis part of the repository because of the oxygen reducing and redox-lowering effects caused by mostactive microorganisms.
Dependencies between process and tunnel backfill variables

Table 4-15 summarises how the process influences and is influenced by all backfill variables andhow these effects are treated in SR-Site.
Table 4-15 . Direct dependencies between the process “Microbial processes” and the definedtunnel backfill variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
(Yes/No)
Description

Process influence on variable

Handling of influence Influence present?
(How/Why not)(Yes/No)
Description

Backfill geometry

No

No

Backfill poregeometryNo

Growth of microbes generate biomass that may clog
pores

Temperature

Yes
Most microbes havean ideal temperature
range in which they
thrive. Maximum
temperature for life is
113°C.

The microbial effects
are treated with amass-balance.

No

Water content

Yes
Water is needed for
active life.

Full saturation isassumedNo

Gas content

Yes
The microbial effects
Hydrogen and methane are treated with amass-balance.
can be consumed
by microbes that willgrow and produce
biomass; largest effectis obtained if oxygenis present. Somemicrobes produce
acetate with hydrogenand carbon dioxide

Handling of influence(How/Why not)Microbes both produce andconsume gases

Hydro-variables
No(pressure and flows)Yes
Gas pressure may lead toadvective flow

Stress stateNo

No

Backfill composition

Organic impurities will
have an effect

The microbial effects
are treated with amass-balance.

No

Backfill porewatercomposition

Microbes are sensitive
to the geochemical
situation, and may
utilise porewatercomponents for growth

The microbial effects
are treated with amass-balance.

Microbial activity may
change the composition, e.g.
by consumption of methane,sulphate and organic carbon
and production of acetate,sulphide and carbon dioxide

The possibility of
microbial reactions
will be considered.
See 3.5.14

Structural and straymaterials

Organic parts of thismaterial and hydrogenfrom corroding metal
will be a source ofcarbon and energy for
microbes with growth
as the result.

The microbial effects
are treated with amass-balance.

Microbes may oxidiseorganic components in thismaterial and hydrogen fromanaerobic corrosion.

The microbial effects
are treated with amass-balance. However, these materials
serve no long termfunction

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227

Boundary conditions

The access to metals and organic material, e.g. stray materials, including hydrogen from anaerobic
corrosion of iron, e.g. rock bolts and armouring and methane and electron acceptors /SKB 2010i/
will set the limits for microbial process rates in the backfill.
Model studies/experimental studies

The ongoing backfill experiment at Äspö hard rock laboratory includes microbial investigations. Largeamounts of different micro-organisms were added locally to positions in the backfill. At decommissionof the Backfill & Plug Test, the survival and activity of added as well as naturally occurring microbes
in the backfill will be analysed. However, this material is no longer considered as tunnel backfill. The
studies on pure bentonite discussed for the buffer processes will represent the backfill quite well, as it
will consist of bentonite only.
In addition, analyses of gas in the gas phase in the prototype repository at Äspö HRL and microorganisms in the groundwater around the prototype repository have been performed. The results
showed presence of micro-organisms in the porewater and demonstrated that porewater gases suchas methane and hydrogen have increased significantly in concentration in the gas phase since closure/Eriksson 2007/.
Natural analogues/observations in nature

There is a natural analogue in Dunarobba, Italy, where a bentonite slide buried a forest approximately
one and a half million years ago. Trees that have not been decomposed by microorganisms can still befound. This indicates that conditions for microbial activity in bentonite clay are unfavourable.
Time perspective

The time perspective concerning survival and activity of microbes in backfill have been summarised
in a conceptual model as follows /confer Figure 3-5 in Pedersen 2000/. At the time of deposition
there will be backfill, possibly structural and other stray materials and an oxygenated gas phase in
the tunnels. The first event will be filling of the void gas volume with groundwater. Microbes will
be present in the water and these microbes will mix with the backfill. There will also be spores andmicroorganisms in the backfill that can grow when water becomes available. As the backfill will havesome organic material from the bentonite and possibly structural and stray material at local spots
along the tunnel walls and floor, microbial activity will start soon. Methane will also intrude. As longas oxygen is present, rapid microbial oxidation of methane and organic material will turn the backfill
environment to a reduced and anaerobic environment, which is beneficial for the repository. Thisis a process that will commence as soon as groundwater enters the backfill. Oxygen will probably
disappear within weeks in the groundwater. Pockets with oxygen in a gas phase may stay oxygenised
for longer times. But as soon as oxygen diffuses into the water, where microbes are active, it will be
consumed and reduced. The risk for oxygenic copper corrosion will consequently rapidly diminish asa result of microbial processes.
Handling in the safety assessment SR-Site

Mass balance calculations can be made to show how different kinds of residual materials remaining
in the repository will contribute to microbial oxygen reduction. It can be assumed that all organic
matter will be able to serve as nutrients for microbes. Furthermore, the availability of hydrogen will
be decisive for microbial activity in the long term. This means that an increased effect of microbialactivity must be expected until all organic matter has been consumed, and subsequently an activity
that is controlled by the supply of hydrogen will proceed. This means that oxygen, ferric iron, sulphate and carbon dioxide will be reduced to water, ferrous iron, sulphide and methane, respectively.
The scope of these reactions is dependent on mass flows.

228

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Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

All the references in this section are from peer reviewed scientific journals.
Uncertainties

Uncertainties in mechanistic understanding
The knowledge about microbial processes in the backfill is not yet well supported by experimental
data. Rather, the processes are inferred from research on microbial processes in the geosphere and in
buffer material.
Model simplification uncertainties in SR-Site
The effects of microbial reactions are maximised since a mass-balance expression is used.
Input data and data uncertainties in SR-Site
The amount and availability of organic materials and metals in the backfill and its vicinity will determine the extent of microbial reactions. The range of data can be found in /the Backfill Production
Report /SKB 2010e/.

4.5Radionuclide transport processes

4.5.1

Speciation of radionuclides

Overview/general description

See the corresponding buffer process; Section 3.6.1.
Handling in the safety assessment SR-Site

The handling of the process is the same as for the corresponding process in the buffer(see Section 3.6.1.). Data and boundary conditions for the backfill will be used.

4.5.2

Transport of radionuclides in the water phase

Overview/general description

See the corresponding buffer process; (Section 3.6.2).
Handling in the safety assessment SR-Site

The handling of the process is the same as for the corresponding process in the buffer(see Section 3.6.2). Data and boundary conditions for the backfill will be used.

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5

Processes in the tunnel plugs

5.1

Thermal processes

5.1.1

Heat transport

Overview/general description

Tunnel plugs are typically located some 6 m from the outermost canister in each tunnel /SKB 2009b/.
The plug volumes are sufficiently small that the temperatures are controlled by the temperature of thesurrounding rock, meaning that the plug temperatures can be calculated without account of the heattransport properties of the plug material. Therefore, the process is without significance to the thermal
evolution in the plug region. Figure 5-1 shows the temperature at four points in the tunnel plugregion as function of time, calculated using the analytical method described in /Hökmark et al. 2009/.
Temperatures are at most about 28°C above the initial conditions. The peak occurs some 300 years
after deposition. The temperature gradients are small vertically as well as horizontally. The maximum,at early times, is about 0.7°C/m.
Dependencies between process and tunnel plug variables

Table 5-1 summarises how the process influences and is influenced by all tunnel plug variables andhow these effects are treated in SR-Site.
30

3 m (F)
3 m (R)
10 m (F)10 m (R)28
26Rock temperature increase(C)

24
22
20
18
16
14
12
10
8
6

Canister spacing 6 m
Tunnel spacing 40 m
Rock thermal conductivity 3.57 W/(m·K)
Rock heat capacity 2.06 MJ(m3)

4
2
0
1

10

Time (years)

100

1000

Figure 5-1. Temperature increase at 3 and 10 m distances from the outermost canister in a tunnel. Results
shown are for two elevations: Tunnel floor (solid lines) and tunnel roof (dashed lines). Results obtainedusing calculation scheme described in /Hökmark et al. 2009/.

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231

Table 5-1. Direct dependencies between the process “Heat transport” and the defined tunnelplug variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Plug geometry

No

No

Pore geometry

Yes

Process neglected.
Of no significans forthe thermal evolution
compared to otherprocesses.

Indirectly via
tempearture

Process neglected.
Of no significans forthe thermal evolution
compared to otherprocesses.

Temperature

Yes
Heat transport isslightly temperaturedependent.

Process neglected

Yes. But insignificant .
(Controlled by rocktemperature).

Process neglected

Water content

Yes.
Thermal conductivityof the plug

Process neglectedNo

Gas content

No, but indirectly via
water content

No

Hydrovariables(pressure and flows)NoNo, but indirectly via
temperature

Process neglected

Stress stateNoNo, but indirectly via
temperature

Process neglected

Plug materials –composition andcontent

Yes
Composition affects
thermal conductivity

Porewater
composition

No

Process neglected

No

No

Plug geometry: The plug geometry is not important to the heat transport within the plug or to theplug temperature. The plug temperature is controlled by the slowly varying boundary conditions, i.e.
the temperature of the rock surrounding the plug. Heat transport does not influence the geometry.
Plug pore geometry: The pore geometry may influence the transport properties but since plug heattransport does not control the plug temperatures, this is of no importance. The temperature may
influence the pore geometry, but this is not controlled by the plug heat transport.
Temperature: Heat transport influences the temperature by definition. For the plug, however, it is
the heat transport in the surrounding rock that controls the temperature. Any temperature dependences of the plug heat transport properties are unimportant.
Water content: The heat transport will not affect the water content, other than indirectly via thetemperature. However, the temperature is determined by heat transport in the rock, not by the plugheat transport. Similar to the backfill, the plug thermal conductivity depends on the water content,
but since plug heat transport does not impact on anything related to the safety assessment, these is no
reason to quantify the content-conductivity relation.
Gas content: Same as above for the water content.
Hydro variables: None of the hydrovariables depend on the heat transport.
Stress state: The temperature increase will reduce or decrease the stresses in the plug, dependingon whether the volume expansion of the plug material is smaller or larger than that of the rock. The
temperature is controlled by the rock temperature, not by the heat transport within the plug.
Plug composition: The composition of the plug will influence the thermal conductivity. Someminerals have higher conductivities than others.

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Plug porewater composition: The composition cannot influence the heat transport. The temperature,
but not the heat transport within the plug, may promote chemical reactions.
Boundary conditions

The rock walls and the tunnel backfill are boundaries with slowly varying temperatures which arecompletely determined by the overall thermal evolution of the rock mass.
Model studies/experimental studies

There are no model studies that explicitly address the thermal conditions in tunnel plugs.
Natural analogues/observations in nature

Not applicable.
Time perspective

The duration of the thermal pulse generated by the decaying fuel is a few thousand years.
Handling in the safety assessment SR-Site

There is no specific handling of heat transport in the tunnel plugs.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Because of the small thermal gradients and the relatively low temperatures in the tunnel plugs there isno specific handling of the heat transport process. The gradient and temperature statements are supported by results obtained using calculation schemes described in /Claesson and Probert 1996/. While
this particular report is not peer or factual reviewed, the calculation scheme has been repeatedly appliedin peer reviewed papers /Hökmark and Claesson 2005/ and in reports that have undergone (/Hökmarkand Fälth 2003/) or will undergo (/Hökmark et al. 2009/) a documented factual and quality review.
Uncertainties

Uncertainties in mechanistic understanding
There are no major uncertainties in the mechanistic understanding of the heat transport in tunnel plugs.
Model simplification uncertainties in SR-Site
Not relevant since the process is not modelled.
Input data and data uncertainties in SR-Site
Not relevant since the process is not modelled.

5.1.2

Freezing

Overview/general description

Freezing of the tunnel plugs is of no significance. When freezing due to permafrost growth starts, therepository is closed and saturated and the tunnel plugs have no performance requirement at this stage.
Handling in the safety assessment SR-Site

Process is neglected since there are no long-term performance requirements on the plugs.

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233

5.2

Hydraulic processes

5.2.1

Water uptake and transport under unsaturated conditions

Overview/General description

The hydro-mechanical evolution of the plugs is as follows:
After installation, water will seek its way to the drainage material, mostly from the backfill that is
not able to seal inflowing water, until the hydraulic gradients inside the tunnel has ceased. The waterthat flows into the drainage material will be drained from the top after filling up the drainage section.
This drainage will continue until the plug functions mechanically, which takes a few months. During
this time, water will not come in contact with the bentonite seal unless directly from inflow fromthe rock. There will thus be no or insignificant wetting of and swelling pressure in the bentonite seal
until the drainage is closed. It should be noted that the water that drains out may contain eroding
backfill material.
When the drainage has been closed, water will enter the bentonite seal and a successive wetting of
the bentonite takes place and eventually a water pressure will build up and act on the plug. Although
the plug is grouted, water may leak past the plug. This leakage will stop rather soon, since water
has to pass either the bentonite seal or the backfill pellets before it enters the leaking fractures, andinvestigations have shown that bentonite in contact with leaking fractures will seal if the fracturesare smaller than 100 μm /Sandén et al. 2008/.
With time, the bentonite seal will be saturated and exert full swelling pressure and water pressure on
the plug. Due to the swelling pressure from both the backfill and the bentonite seal, the sand filter(drainage material) will be compressed and some of the swelling pressure on the concrete plug islost.
The plug has fulfilled its purpose to prevent water flow and eroding bentonite out of the tunnel whenfull water pressure is reached outside the plug. When there is full water pressure on both sides ofthe plug, the plug will start its slow degrading process. When the mechanical function is lost, theswelling pressure inside the plug will affect the plug as listed below.
• A small axial displacement of the plug and compression of the crushed rock outside the plug willtake place for the outer plugs, which are located between the transport tunnels and the crushedrock in the central area.
• The inner plugs between deposition tunnels and transport tunnels will have the same swellingpressure on each side after full water saturation and the resulting displacements will thus be small
and only include compression of the plug parts, mainly the filter.
• The bentonite seal will swell into parts of the degraded plug and also into the open slot that exists
in the roof of the crushed rock filling
Due to the friction in the bentonite and crushed rock there will be a remaining small axial density
gradient in the backfill inside the plug, in the bentonite seal and in the crushed rock outside the plug.
A special case that may need some attention is if the crushed rock outside the plug is water saturated
and exerts a hydraulic pressure on the plug before the backfill and bentonite seal become water saturated. Since the plug is not designed to withstand pressure from outside it may fail and water may thus
flow into the backfill inside the tunnel. This case may not be a problem for the repository performance,
but will be investigated.
The process “water uptake and transport under unsaturated conditions” treated in this section is only
relevant for the bentonite seal, and the following description as well as the influence table refers tothis part of the plug.
The process is described in detail in both the buffer and the backfill Sections (3.3.1 and 4.2.1) andwill not be described here.
Dependencies between process and tunnel plug variables

Table 5-2 summarises how the process influences and is influenced by all tunnel plug and how theseeffects are treated in SR-Site. The table is relevant only for the bentonite seal in the plug.
234

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Table 5-2 . Direct dependencies between the process “Water uptake and transport underunsaturated conditions” in the bentonite seal in the plug and the defined tunnel plug variables
and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Plug geometry

Yes

Geometry included in
the models

No

Pore geometryYes. Via void ratio(density) and void
geometry

Variable in the model(Yes) If erosion takes
place (secondary
effect)See erosion

Temperature

Yes. Indirect throughwater viscosity andhydrovariables in atemperature gradient

Negligible

Yes Wetting anddrying affects thermal
conductivity

Negligible

Water content

Yes. Via degree ofsaturation and retention properties

Variable in the model

Yes

Variable in the modelGas content

Yes. Via the gaspressure and degreeof saturation

Variable in the model

Yes

Variable in the model

Hydrovariables(pressure and flows)

Yes. Basic variables.

Variable in the model

Yes

Variable in the modelStress state

Yes

Variable in the model

Yes

Variable in the model

Plug materials –composition andcontent

Yes. Hydraulic conductivity and retention
curve etc.

Sensitivity analyses(Yes). If erosion takes
place (secondary
effect)See erosion

Porewater
composition

Yes

Controls some param- Yes
eters in the model

See Section 5.6

Boundary conditions

Interaction with the rock: A key issue for the saturation process is the interaction between rockand backfill. Water is conducted to the backfill in the water-bearing fractures and the rock matrix,
which means that water saturation can be both uneven and take a long time for a sparsely fractured
rock. If there is a highly permeable excavation-disturbed zone (EDZ) at the wall of the tunnel, which
distributes water from the fractures along the rock wall, the wetting will be faster and more even.
This interaction is identical to the interaction with the backfill in the tunnels.
Interaction with the other parts within the plug: The bentonite seal in the plug has a wall ofconcrete beams on each side, which in turn are in contact with the drainage material and the concrete
plug. The grain size distribution and compaction in the drainage material is designed such that the filter
can collect and drain water so that the concrete plug will not be exposed to high water pressures until it
has gained sufficient strength. It is designed and verified in accordance with conventional geotechnicalprocedures. When the drainage of the filter is closed a water pressure may build up and axial wetting
may also take place.
Since the plug is demanded to be water tight also before the bentonite seal is water saturated, waterflow and water pressure derived from the backfill inside the plug will be stopped and the leaking
fractures sealed by the bentonite in the seal.
Model studies/experimental studies

All studies performed on wetting are done for the backfill and the buffer (see Sections 3.3.1
and 4.2.1).

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Some tests have been performed to study the sealing ability of a pellets filling of unsaturated bentonite
in contact with fractures of different widths. The results indicate that fractures with a width of
100 mm will be sealed /Sandén et al. 2008/.
Additional studies of the consequences on saturation of a high water flow and water pressure on the
bentonite seal inside the plug before full swelling will be done /Åkesson et al. 2010a/.
Natural analogues/observations in nature

Wetting of bentonite in nature has not been studied and can probably not contribute to the knowledge.
Time perspective

Analyses made for the backfill and the buffer show that the time to full water saturation can vary agreat deal, depending above all on the interaction with surrounding rock and the axial water transport
in the tunnel. If conditions are wet with respect to Swedish rock, saturation may be reached within
10 years. If there are no fractures all water has to go via the rock matrix, which may take 100 years
or more in the granitic rock types occurring in Sweden. Artificial wetting through the filter has beenstudied and show that the time to saturation may be reduced /Åkesson et al. 2010/.
Handling in the safety assessment SR-Site

All calculations done so far have concerned the buffer and backfill and the process is very similar forthe bentonite seal in the plug. However, a hydro-mechanical sensitivity analyses is done in order toevaluate the concrete plug is affected by the swelling pressure.
The water saturation evolution of the bentonite seal as well as the sealing properties of the plug will
be modelled and reported in the general THM modelling report /Åkesson et al. 2010a/.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The general THM modelling report /Åkesson et al. 2010a/ will undergo a documented factual- andquality review.
Uncertainties

The uncertainties are the same as those reported for the corresponding buffer and backfill processes(see Sections 3.3.1 and 4.2.1).

5.2.2

Water transport under saturated conditions

Overview/General description

As described in section 5.2.1, the plug is required to seal for a short period only. The hydro-mechanicalevolution of the plug is described in that section. After full water saturation and when the hydrostatic
water pressure is reached, the plug has no sealing function and very little water will flow past the plugdue to the low hydraulic gradient. The plug will thus be an integrated part of the backfill and will
have one very tight section with the bentonite seal and two highly permeable sections (the filter andthe concrete plug). However, the permeable sections are only local and will not influence the overall
hydraulic behaviour of the tunnel backfill.
On these reasons, water transport under saturated conditions in the plugs will be handled very
briefly. The process is already described for the buffer and tunnel backfill in Sections 3.3.2 and4.2.2, respectively.
Dependencies between process and tunnel plug variables

Table 5-3 summarises how the process influences and is influenced by all variables of the plugs andhow these effects are treated in SR-Site.

236

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Table 5-3 . Direct dependencies between the process “Water transport under saturated conditions”
and the defined tunnel plug variables and a short note on the handling in SR-Site.
Variable

Variable influence on process

Process influence on variable

Influence present?
(Yes/No)
Description

Handling of influence(How/Why not)Influence present?
(Yes/No)
Description

Handling of influence(How/Why not)Plug geometryYes

The process isneglected based on
the discussion in thissectionNo

The process isneglected based on
the discussion in thissectionPore geometryYes. void ratio(density) and void
geometry

Negligible(Yes) If erosion takes
place (secondary
effect)See erosion

Temperature

Yes. affects waterviscosity

Negligible

No

Water content

Indirect through poregeometry (density)No

Gas content

No, there is no gas
phase in a saturated
plug.

No

Hydrovariables(pressure and flows)

Yes. Basic variables.

Negligible since very
local influence

Yes

Assumed to be a local
part of the tunnel
with high hydraulicconductivity

Stress state

Yes

Negligible

Yes

Dimensioning. The
plug must be able towithstand hydraulicwater pressure.

Plug materials –composition andcontent

Yes. Hydraulicconductivity

Negligible since very
local influence(Yes). If erosion takes
place (secondary
effect)Porewater
composition

Yes through viscosity

Negligible

Yes, from degrading
substances

Negligible

Boundary conditions

Only outer boundaries exist. There are three types of outer boundaries of the plug:•••

The rock surface of the tunnel,
The interface between the backfill and the drainage section,
The interface between the outer backfill of crushed rock (or bentonite in some parts) andthe concrete plug.

Model studies/experimental studies

Only studies of buffer and backfill material have been done and special studies of the plug function
after saturation is not required.
Natural analogues/observations in nature

Not applicable
Time perspective

When full water saturation and hydrostatic pressure has been reached, which may take up to 100
years, the plug has no function. There may be a period after which full saturation in the plug and thebentonite seal has been reached when there is still a high hydraulic gradient across the plug due topossible very slow wetting inside the plug at dry tunnels. However, this situation is more favourable
than if the plug and the bentonite seal are not saturated and will not be further dealt with.

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237

Handling in the safety assessment SR-Site

Due to that there is no demand of the plug after saturation this process will not be handled in SR-Sitein any other way than a discussion around a local part of the tunnel with higher hydraulic conductivity.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

No references required for this process.
Uncertainties

The average hydraulic conductivity of the plug after degradation is not known but a high pessimistic
value may be used.

5.2.3

Gas transport/dissolution

Overview/General description

Gas processes in the plugs are only relevant for the saturation period and only for the bentonite partof the plug – this is described in Section 5.2.1
Handling in the safety assessment SR-Site

The process is treated as a part of the THM description of the plug, see Section 5.2.1.

5.2.4

Piping/erosion

Overview/General description

The process of erosion is described in the buffer chapter, Section 3.3.4. Since erosion is mainly
a concern for the bentonite seal in the plug, the same description is valid here.
After full saturation and reestablishment of hydrostatic water pressure, the plug is not required tohave any sealing function. Any consequences of erosion of the plug itself are thus not important.
The plug and bentonite seal are instead required to stop eroding water from the tunnel to pass theplug during the saturation phase. The bentonite seal is expected to prevent leakage. If there are open
fractures between the concrete and the rock, and water flows through these fractures due to malfunction of the bentonite seal when the plug is expected to seal, the function of the plug is not fulfilled.
However, erosion will help to seal these fractures since the eroding material will get stuck and clog
the fractures as shown in tests.
Erosion of the bentonite seal is thus not a problem but will instead stop flow past the plug.
Dependencies between process and tunnel plug variables

Table 5-4 summarises how the process influences and is influenced by all tunnel plug variables andhow these effects are treated in SR-Site. The table is relevant only for the bentonite seal in the plug.
Boundary conditions

The piping and erosion that are mainly treated here only take place in the pellets-filled part between
the clay blocks in the clay seal and the rock.

238

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Table 5-4. Direct dependencies between the process “Piping/erosion” in the bentonite seal in theplug and the defined tunnel plug variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Plug geometryYes

No specific assessment of piping in theplug will be done

No

Pore geometry

Yes, via density

Unimportant(Yes)

Temperature

No

Water content

Yes, via swellingpressure etc.

Unimportant

Yes

Unimportant

Gas content

Yes

Unimportant

Yes

Unimportant

Hydrovariables(pressure and flows)

Yes

No specific assessment of piping in theplug will be done

Yes

Sensitivity assessments for the hydraulicproperies of the entire
tunnel will be doneStress stateYes

No specific assessment of piping in theplug will be doneYes

Via saturation rate
(water content)

Plug materials –composition andcontent

Yes

No specific assessment of piping in theplug will be done

No

Porewater
composition

Yes

No specific assessment of piping in theplug will be doneNo

Unimportant

No

Model studies/experimental studies

The processes have been studied in conjunction with studies for KBS-3H, which is especially vulnerable for erosion of buffer material, since a channel can pass a large number of canisters. Tests havebeen done in three different scales. The results show that the bentonite is very sensitive to piping anderosion and that it may take a considerable time until it heals if the inflow in one spot from a fractureis strong and the built-up of water pressure in the fracture is fast.
Although the plug is grouted, it may leak water past the plug. This leakage will stop rather soon
since water has to pass either the bentonite seal or the backfill pellets before it enters the leaking
fractures, and investigations have shown that bentonite in contact with leaking fractures will seal if
the fractures are smaller than 100 μm. The experiments are described in the chapter about unsaturated flow (Section 5.2.1).
At present, the processes are studied in the project Backfilling and closure of the deep repository.
Natural analogues/observations in nature

Not applicable for this process.
Time perspective

The piping and erosion only takes place before complete saturation and homogenisation. The highswelling pressure of the clay will ultimately always seal the piping channels, since the sum of theswelling pressure and the porewater pressure in the clay part of the plug will always be higher than
the water pressure if the final average density of the clay is high enough.
The sealing expected to take place by the eroded bentonite will take place before full saturation isreached and thus only relevant for less than 100 years.

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239

Handling in the safety assessment SR-Site

The process for the plug will not be separately handled in SR-Site. Possible erosion from thebentonite seal does not influence the overall safety since the seal has no function after full saturationof the surrounding tunnels and rooms. The consequences of an increased hydraulic conductivity inthe entire tunnel are discussed in Sections 4.2.2 and 4.4.1.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

See the corresponding process in the buffer chapter, Section 3.3.4.
Uncertainties

Piping, erosion and subsequent sealing is a complicated process with many components, muchdepending on the hydraulic behaviour of the rock. The uncertainty is thus very large regarding both
the demands on the rock and the ability of the plug to handle the processes. The tests performed forthe backfill project are expected to yield better understanding. Only the sealing ability of the erodedmaterial is of interest. See also buffer Section 3.3.4.

5.3

Mechanical processes

5.3.1 Swelling/mass redistribution
The swelling process has been merged with other processes that cause mass redistribution within theplug, namely thermal expansion, creep and a number of interactions with the buffer, near field rockand the neighbouring backfill.
Overview/General description

The plug is a complex composite of different materials. A general description of the plug, its required
function and the hydro-mechanical evolution are given in Section 2.4.1. The description shows that
there are some mechanical processes that are important for the function. These are described in
Section 5.2.1.
The mechanical processes associated with the bentonite seal in the plug are identical to those of thebuffer and the backfill and are described in Sections 3.4.1 and 4.3.1, respectively.
Dependencies between process and tunnel plug variables

Table 5-5 summarises how the process influences and is influenced by all variables of the tunnelplug and how these effects are treated in SR-Site.
Boundary conditions

The plug has the rock, the inner backfill and the outer backfill material as boundaries. On the other
hand the plug itself contains at least five parts that interact mechanically. Three walls of concretebeams are intended to separate the different parts of the plug from each other. The beams only havea mechanical function required for the installation period and will be design to come loose from therock at very small axial pressure. This means that they will mechanically move as a passive boundary between the different parts of the plug.
Interaction backfill / drainage material
The backfill will during saturation increase its swelling pressure and compress the drainage material.
Interaction drainage material / bentonite seal
The bentonite seal will also increase its swelling pressure during saturation and compress
the drainage material.

240

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Table 5-5. Direct dependencies between the process “Swelling/mass redistribution” and thedefined tunnel plug variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Plug geometry

Yes

Geometry included in
the models

No

Pore geometryYes. Via void ratio(density) and stress

Included in the models Yes

Temperature

Yes. Thermal
expansionNo, negligible

Water content

Yes, the water content Variable in the models Yesof the bentonite seal
and in the sand filter.

Variable in the models

Gas content

Yes. Degree of satura- Included in the models Yestion and gas pressure

Included in the models

Hydrovariables(pressure and flows)

Yes. Basic variables.

Variable in the models Yes

Variable in the models

Stress state

Yes

Variable in the models Yes

Variable in the models

Plug materials –composition andcontent

Yes. Determines many Sensitivity analyses
parameters in themodelsNo

Porewater
composition

Yes. Influences many
parameters in themodelsNo

Sensitivity analyses

Included in the models

NoInteraction bentonite seal / concrete plug
The bentonite seal will increase its swelling pressure during saturation and stress the plug. In
addition, there will be a hydraulic pressure on the concrete plug. The concrete plug is designed towithstand this pressure so it will only move slightly and improve the contact with the rock in the slot.
Interaction concrete plug / backfill outside the plug
If the backfill outside the plug consists of crushed rock, which will be the case for the plug facing
the central area, there is no pressure on the plug from that side until a water pressure is built up.
However, this interaction has two implications that must be considered:
1. If the water pressure outside the plug comes before swelling, (i.e. the central area is water filled
before the backfill in the tunnel) and water pressure is derived inside the concrete plug there may
be consequences for the plug since it is not designed to take such pressure. This implies that theconcrete plug may break and that water may flow backwards through the plug. However, there
are probably no negative consequences of this case, but this may need to be investigated.
2. After long time the concrete plug is degraded the bentonite from the bentonite seal and thebackfill inside the plug may expand and penetrate into the possible open spaces in the plug andfurther on into the slot between the roof and the crushed rock in the backfill of the central area.
In stagnant water, this is not expected to yield problems and can be estimated but if there is water
movements in the slot there may be substantial erosion. This case may need to be investigated or
prevented with an engineering solution.
Model studies/experimental studies

The mechanical processes relevant to the bentonite seal are the same as those reported for the buffer
and backfill; see Sections 3.4.1 and 4.3.1, respectively. No other studies have been done so far but
are planned.
The mechanical effect on the plug is handled by the design and related calculations provided in the
Backfill production Report /SKB 2010e/.
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241

Time perspective

The mechanical interaction is relevant during the entire lifetime of the repository. Although the plug
has no function after about 100 years, the long-time effect of erosion of bentonite penetrating into
the slot can continue if no other solution is invented.
Natural analogues/observations in nature

Not applicable.
Handling in the safety assessment SR-Site

The mechanical performance of the plug is discussed in the Backfill Production Report /SKB 2010e/
The water saturation evolution and the evolution of the stresses and displacements of the bentonite
seal are modelled and reported in the general THM modelling report /Åkesson et al. 2010a/ as wellas the swelling of the backfill and the bentonite seal following the disintegration of the plug.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The general THM modelling report /Åkesson et al. 2010a/ and the Backfill Production Report /SKB
2010e/ will undergo a documented factual- and quality review.
Uncertainties

Uncertainties in mechanistic understanding
The mechanistic understanding of the mechanical processes is sufficient for the safety case.
Model simplification uncertainties in SR-Site
See above.
Input data and data uncertainties in SR-Site
The largest uncertainty is probably the fact that the design of the plug still is preliminary. A goodmodelling of the process can be done when a final design has been decided.

5.4

Chemical processes

5.4.1

Advection

Overview/general description

In this context advection refers to transport of any forms of additional matter, e.g. ions, molecules
or colloids, with porewater flow. The transport direction is thereby principally from volumes of highwater pressure to volumes of lower pressure. The advection is closely related to water flow in thetunnel plug, which is described in Section 5.2.2.
The maximum pressure gradients over the tunnel plug will occur during the installation period when
one side of the plug will be pressurised by the hydrostatic pressure and the other side is kept drained.
After closure, different inflow rates and volumes in the compartments may lead to a significant
gradient. After full water saturation of the compartments, the pressure gradient will be induced by
the regional pressure gradients, and comparatively much smaller. Additional causes of small pressure
gradients may also be present by e.g. differences in salinity and temperature on the two sides of theplug.
Dependencies between process and tunnel plug variables

Table 5-6 summarises how the process influences and is influenced by all tunnel plug variables andhow these effects are treated in SR-Site.
242

TR-10-47

Table 5-6. Direct dependencies between the process “Advective transport of solutes” and thedefined tunnel plug variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Concrete geometry

Yes, greatly by cavities The process isand fractureneglected based on
the discussion in section Water transport,
Section 5.2.2

No, but indirectly
through porewatercomposition and dissolution/precipitation

Concrete poregeometry

Yes, dominating effectthrough hydraulicconductivity

Yes, through dissolution/precipitation

Radiation intensity

Not relevant

Temperature

Yes, indirect throughwater viscosity

Neglected

No

Water content

Yes, by definition

Neglected

No

Gas content

Yes, through watercontent

Neglected, will always
reduce the process

Yes, dissolution/release of gas

Irrelevant since no performance is expected

Hydrovariables(pressure and flow)

Yes, by definition

Neglected

Yes, by definition

Irrelevant since no performance is expected

Stress state

Yes, hydraulic
Neglected
conductivity is related
to swelling pressure in
the bentonite. Throughfracture aperture in theconcreteYes, minor throughosmotic effects andprecitation/dissolution

Irrelevant since no performance is expected

Concretecomposition

Yes, throughporosity/hydraulicconductivity

Neglected

Yes, through dissolution/precipitation

Irrelevant since no performance is expected

Concrete porewatercomposition

Yes, by definition

Neglected

Yes, by definitions

Irrelevant since no performance is expected

Neglected

Irrelevant since no performance is expected

Not relevant

Boundary conditions

The external boundary condition for this process is the hydraulic pressure gradient and the concentration gradients for species across the tunnel plug system component.
Model studies/experimental studies

Similar types of plugs are present in the Äspö HRL, and have been examined with respect to water
flow. A vast knowledge concerning concrete construction in general is present and may to a large
extent be used for the specific tunnel plug repository compartment.
Natural analogues/observations in nature

Nothing relevant.
Time perspective

Advective flow transport in conjunction with water saturation takes place on a timescale of up to
hundreds of years. After full saturation of the tunnel system, the advective transport is expected to
be low due to the minor pressure gradients. However, the function of the plug will eventually be lost
due to degradation of the concrete.

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243

Handling in the safety assessment SR-Site

Since there are no requirements on stability of the plug after saturation of the repository, this process
will not be handled in SR-Site in any other way than as a discussion of the implications of a local
part of the tunnel with higher hydraulic conductivity.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

No references required for this process.
Uncertainties

The average hydraulic conductivity of the plug after degradation is not known but a high pessimistic
value may be used

5.4.2

Diffusive transport of species

Overview/general description

An overview of diffusion processes and phenomena relevant for bentonite and concrete are given in
Sections 3.5.3 and 8.4.2, respectively. These considerations also apply to the concrete- and bentoniteparts of the tunnel plug. /Luna et al. 2006/ conclude that the main diffusion parameters (including
porosity) will not be significantly affected by an alkaline plume.
Handling in the safety assessment SR-Site

These system components are not included in the radionuclide transport calculations. Diffusion isneglected, since advection will dominate in a degraded plug.

5.4.3

Sorption (including exchange of major ions)

Overview/general description

Sorption processes relevant for the concrete part of the plug are the same as those for the concrete
bottom plate described in Section 8.4.3, and for the bentonite seal, the processes are the same as thosefor the buffer described in Section 3.5.5. Concrete-bentonite interaction occurring at the interface ofthe two parts of the tunnel plugs may lead to changes in the mineralogy of the respective materials,
which will also influence sorption.
Handling in the safety assessment SR-Site

Sorption of radionuclides in tunnel plugs is neglected in SR-Site.
Uncertainties

Uncertainties in mechanistic understanding
Uncertainties in mechanistic understanding are discussed in Section 3.5.5 for sorption on bentonite,and briefly in Section 8.4.3 for sorption/uptake on concrete.
Model simplification uncertainty in SR-Site
Not applicable since sorption in tunnel plugs is neglected.
Input data and data uncertainties in SR-Site
Not applicable since sorption in tunnel plugs is neglected.

244

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5.4.4

Alteration of concrete

Overview/general description

The cement-water mixture is transformed into hardened cement paste, with main binding phases
comprising calcium silicate hydrates (CSH) and calcium hydroxide (CH). In addition, two othermajor hydration products are produced from reactions involving the aluminate, alumino-ferrite, andcalcium sulphate, known as AFt and AFm, which are complex calcium ferri-aluminates.
Hardened cement paste is a porous material, due to the large water/cement ratio of the original
cement grouting. The interstitial solution of an ordinary Portland-cement-based paste will reachchemical equilibrium with the cement constituents, resulting in a hyperalkaline porewater (pHaround 13) with large concentrations of alkaline Na+, K+, OH− and Ca2+ ions. In the future Swedish
spent fuel repository, it is planned to use a “low-pH” cement, although the actual recipe, Backfill
Production Report /SKB 2010e/, will probably be optimised before the repository construction andduring the relatively long period of repository operation. The concept behind the elaboration ofsuch low-pH grout lies in the substitution of cement by considerably large amounts of silica fume.
One of the main advantages of the addition of silica fume into the mixture of grout consists of aremarkable decrease of the volume fraction of the highly soluble portlandite (Ca(OH)2). It also
contributes to reduce the content of aluminum compounds and to lower the pH of the pore solution.
Indeed, the porewater pH in equilibrium with low-pH cement constituents is estimated to be around
11 or lower. This particular feature has beneficial effects on long-term safety because the porewaterof the material is much less reactive with the bentonite clay that surrounds the copper canisters in
the repository. Yet, the contact of this alkaline cement porewater with dilute groundwater (pH lower
than 9) creates large concentration gradients which induce diffusive mass transport (of mainly Ca2+and OH− ions) outwards from the porous cement paste. Renewal of the external groundwater by
advection will drive away the cement porewater components and accelerate the diffusion of ions andthe degradation of cement component. The alkaline cement porewater will react with fracture filling
and rock minerals through a complex set of reactions that finally neutralise the alkalinity and produce
calcium silicate hydrates, see for example /Hoch et al. 2004, Wan et al. 2004/.
The durability of cement paste is affected by the decalcification process, since calcium is the main
component of cement hydrates. The continuous outwards diffusion of Ca2+ alters the original chemical
equilibrium, which induces dissolution of calcium compounds as the natural tendency is to restore
chemical equilibrium. The dissolution of highly soluble calcium hydroxide (portlandite) and CSH
phases leads to an increase in the porosity of the cement paste, which at the same time enhances the
diffusion rate. Low-pH cements have negligible quantities of portlandite and larger amounts of CSH
phases than ordinary Portland cements. CSH solubility is complex due to its amorphous to semicrystalline structure, which might be described in terms of a mixture of tobermorite-like phases withothers of jennite-like structure /Taylor 1986/. Successive investigations have shown that CSH have alarge variability in their composition, nanostructure and morphology /Richardson 2004/. Perhaps themost relevant features of CSH solubility concern their conditions of formation /Flint and Wells 1934,
Roller and Ervin 1940, Taylor 1950, Kalousek 1952, Greenberg and Chang 1965, Fujii and Kondo
1981, Barret et al. 1983, Brown et al. 1984, Grutzeck et al. 1989/. Recent studies /Chen et al.
2004/ suggest that CSH solids are present under different metastable phases which are susceptible totransform (recrystallise) into one another in the long term.
Simultaneously with the dissolution of calcium silicate hydrates, the rest of the mineral phases arealso involved in the cement leaching phenomena. AFm dissolution releases Al(OH)4 – which will also
out-diffuse and could induce precipitation of either secondary AFm or ettringite (in the presence ofsulphates), whose large molar volume and crystalline morphology may close off the porous structure
and reduce the mechanical strength of the cement by producing micro-cracks /Lagerblad 2001/. In thecase of an external groundwater containing carbon dioxide, precipitation of calcite might be expected,
which was found to reduce the degradation rate by sealing the pore network on the exposed surfaceof cement paste /Pfingsten 2001/. Brucite may also be prone to precipitate at the cement-groundwaterinterface, due to the lower pH environment /Lagerblad 2001/.
The chemical composition of groundwater or backfill porewater affects the rate of concrete plugdegradation. Laboratory experiments have shown that the degradation depth is highly dependent on
the carbonate concentration of the external solution. A study by /Moranville et al. 2004/ showed thatleaching depth decreases by a factor of 5 when the external water is in equilibrium with atmospheric
TR-10-47245

CO2, due to the sealing produced by calcite precipitation at the cement-water interface. An external
groundwater containing dissolved sulphate could also have a large impact on the longevity of concretematerial /Höglund 2001/ due to ettringite precipitation. The reason is that ettringite has the ability
to bind crystal (“adsorbed”) water, resulting in a large molar volume of this mineral phase, which
results in cracking and mechanical deterioration of the cement paste. Chloride concentration of thegroundwater can also affect the long-term behaviour of the cement paste due to the precipitation ofFriedel salts (i.e. aluminium and calcium chlorides).
Dependencies between process and tunnel plug variables

Table 5-7 shows how the process influences and is influenced by all tunnel plug variables.
Plug geometry: The geometry of the plug affects the process by considering the total mass ofconcrete and the reactive phases it contains. Therefore, the total pool of these mineral phases willaffect the geochemical evolution of the system.
Pore geometry: The solid-to-liquid ratio will affect the degradation of concrete, at higher ratios
the degradation will be slower, as a minor mass of minerals dissolved is needed to attain saturation.
On the other hand, the dissolution – precipitation of cement mineral phases can modify the poregeometry and the total porosity of the system, and thus, modifying the degradation rate of concrete.
Temperature: Thermodynamic constants of chemical reactions are highly dependent of the temperature. However, temperatures in concrete plug are not expected to vary significantly, and therefore,
minimal effects on the geochemical processes are foreseen.
Water content: This variable will affect the dissolution of cement phases as reactions can only takeplace in wetted surfaces. However, the process only considers full water saturation, as minimalchanges are expected to occur until that time.
Table 5-7. Direct dependencies between the process “Alteration of concrete” and the definedtunnel plug variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Plug geometry

Yes

Plug mass considered The degradation will
reduce the mass

Plug mass considered

Pore geometryYes

The solid-to-liquidratio is considered.

Porosity changes
due to dissolution –precipitation reactions
is considered.

Temperature

Yes, temperatureeffect on thermodynamic constants.

Constant temperature No
assumedWater content

Yes, Could affectconcentrations

Saturated conditions
assumedGas content

No

No

Hydrovariables(pressure and flows)

Yes. Effect on reaction Included in mass
rates and transport of balancechemical components

No

Stress state

No

No

Plug materials –composition andcontent

Yes, reactivity withminerals. Cement
mineralogy

Included in modelling Yes, modification ofmineral composition.
Concrete degradation

Porewater
composition

Yes, Affecting
reactivity of systemcomponents

Included in modelling

246

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Yes. Dissolution –precipitation of solid
phases can modify theporosity of the system.

No

Included in modelling

Yes, reaction withminerals results inchanges in porewatercomposition

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Gas content: Gases can form or dissolve through chemical reactions, although their partial pressures
are considered in the models, it is not expected that gases can form in the concrete plug.
Hydrovariables (pressure and flows): Transport of water and solutes is a very important variable
on the degradation rate of concrete. The faster the solutes are transported away from the system, thefaster the dissolution rates occurs, as the saturation state of those minerals being dissolved are maintained at low values. For the same reason, the precipitation of secondary phases will be inhibited,as the concentrations in porewater won’t increase enough to reach saturation. However, if transportof solutes occurs at a slow rate, then dissolution of cement phases will occur at a slower rate, but
precipitation of secondary phases is more likely to occur.
Stress state: This variable has no effect on the alteration of concrete process.
Plug materials – composition and content: This is one of the most relevant variables influencing
this process. The type and amount of minerals is essential to determine how their dissolution – precipitation behaviour will affect the chemical evolution of the system, modifying both the porewatercomposition and the minerals present in the system (i.e. newly precipitated minerals and amount ofexisting minerals).
Porewater composition: The porewater composition will determine which of the minerals in thesystem will dissolve or precipitate, and thus the dissolution – precipitation of these minerals will
modify the chemical composition of porewater.
Boundary conditions

There are no particular boundary conditions to discuss for these processes. The relevant boundary
conditions in order to treat the process quantitatively are those of the transport processes that control
the solutes transport between the concrete plug, the backfill and the groundwater as adjacent systemcomponents, i.e. the boundary conditions of the processes diffusion and advection.
Model studies/experimental studies

A large amount of research has been reported in the scientific literature involving accelerated
laboratory leaching tests, among the most recent papers are those of /Faucon et al. 1998, Carde et al.
1996, Saito and Deguchi 2000, Catinaud et al. 2000, Moranville et al. 2004/. Experimental evidence
has also been reported on long-term leaching through the analysis of old, man-made, cementitious
structures /Álvarez et al. 2000, Lagerblad 2001/.
From the modelling standpoint, works focused on the evaluation of the long-term deterioration of
low-pH cement are still on an early stage of development since, as noted by /Savage and Benbow
2007/ “not all cement models are applicable to modelling low-pH systems”.
As /Benbow et al. 2007/ point out, modelling of cement degradation is a complex task in view ofthe long-term pH buffer is controlled by the incongruent dissolution behaviour of the CSH gel. In
their report, an intercomparison is performed between three different models of CSH, namely theones developed by /Börjesson et al. 1997/, /Walker 2003/ and /Sugiyama and Fujita 2005/. Both
/Börjesson et al. 1997/ and /Walker 2003/ focus their approach to modelling solid solutions byproviding a method of determining the solid phase activity coefficients based on the calculation of
the excess energy of the solid solution as a function of its composition. The expression that relates
the excess energy and the composition of the solid solution is of Margules type (i.e. the excessenergy is expressed as a power series of mol fractions), under the form of the so-called Guggenheim
mixing model. Their approaches, however, differ in the method applied calculating the Guggenheim
parameters. On the other hand, /Sugiyama and Fujita 2005/ base their approach on the “conditional
equilibrium constant”, but it can be related back to determining the solid phase activity coefficients.
One additional difference between the models of /Börjesson et al. 1997/ and /Walker 2003/ withrespect to the one developed by /Sugiyama and Fujita 2005/ consists of the selection of the endmembers of the binary solid solution: the former consider portlandite (Ca(OH)2) and a CSH gel, it
being expressed by an empirical formula (CaH2SiO4) representing the simplest stoichiometry of a
CSH gel in terms of its component oxides; in the latter’s model, the end-members are taken to be
Ca(OH)2 and SiO2(s).

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Whereas results of the intercomparison have shown that the three models are generally in agreementwith the behaviour of the cement, the different choice of end-members may make the Sugiyama andFujita model more advantageous regarding the modelling of low-pH cements, since the lower valuesof the spectrum of Ca/Si ratios can then be contemplated.
Recent models of cement degradation have also addressed the task of considering the effects ofevolution of physical properties (i.e. porosity, diffusivity) on the behaviour of the leaching process.
Some of them on the basis of fitting curves derived from experimental data /Yokozeki et al. 2004/
and some others on either “pure” random models /Bentz and Garboczi 1992/ or “hybrid” models,
where randomness is constrained by appropriate user-defined probability distributions /Marchandet al. 2001/. A pure mechanistic approach for cement degradation modelling requires the coupling ofmulti-component solute diffusion and chemical reactions taking place in the system. Dimensionlessanalyses of diffusion and reaction rates demonstrate the adequacy of a local equilibrium assumption
for coupled reactive transport modelling of cement leaching /Barbarulo et al. 2000/. By thisapproach, a mixed non-linear set of equations can be solved numerically. The domain of the problem
could be represented either as a homogeneous medium, obtained by averaging the variables of interest over a representative elementary volume, or a pore structure modelled on the basis of microstructural information, such as that provided by mercury intrusion porosimetry, or on the reconstructionof the microstructure by means of random models /Bentz 1999/. The latter was recently addressed in/Moranville et al. 2004/.
A relevant decision to be taken in the reactive transport-based models of cement degradation arises
from the selection of the geochemical processes to be included. The inclusion of K+, Na+, Mg2+, Ca2+,
SO42−, OH−, Al(OH)4− and Cl−, is claimed in /Maltais et al. 2004/ to be sufficient to describe mostdegradation processes, although other sets of chemical components have also been proposed, e.g. in/Höglund 2001/.
Natural analogues/observations in nature

Silicate minerals in granitic bedrock and backfill material react with hydroxide ions, which may
derive from cement. The silicate minerals dissolve as a consequence of hydrolysis reactions, after
which secondary reaction products are precipitated. The course of the reaction has been studied at
the hyperalkaline springs in Maqarin, Jordan /Smellie 1998/. The reaction rate is unknown, but it isestimated that the reaction can reduce the pH from about 13 to the range 10–11 and that it is presumably an extremely thin layer of minerals that reacts.
Time perspective

Since cement leaching is mainly a coupled diffusion-reaction phenomenon, the time needed forcomplete degradation of concrete plugs will be highly dependent on the relative surface area of theconcrete where diffusion exchange with backfill porewater can take place. This parameter will be
related to the thickness of the plug. However, the porosity and permeability increase due to concretedegradation is not always taken into account which could lead to an overestimate of the calculateddegradation times.
Handling in the safety assessment SR-Site

The cement plug durability is important as its degradation can result in an increased pH in the near fieldin the repository. The rate of degradation will impact the geochemical conditions in the contact areawith the backfill and potentially also further away from the plug. Therefore, the cement plug durability
has to be evaluated by long term reactive transport modelling, accounting for the coupling between
dissolution/precipitation of minerals and porosity/diffusivity/hydraulic conductivity changes.
The models evaluates quantitatively the long term durability of cementitious components of therepository, including the time evolution of the main physical and chemical properties, the time
evolution of the hydrochemical conditions close to the cementitious components as a result of thealkaline perturbation produced by cement leaching, and the hydrogeochemical effects produced by
the alkaline perturbation on the adjacent barriers.

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Handling boundary conditions:
The degradation of cementitious components will be highly dependent on the external boundary
conditions. On one hand, the rate of external groundwater renewal and the chemistry composition of
the external groundwater have a strong influence on the out diffusion of ions from the cement paste:
a renewal of the external groundwater by advection would accelerate the degradation of cement.
On the other hand, the chemistry of the external groundwater will also influence the processes. For
example, high carbonate concentrations in the external groundwater would probably produce calcite
precipitation and/or the precipitation of other secondary minerals, which may reduce the porosity
and therefore the cement degradation rate.
Handling influences between variables and process:Cement degradation phenomena have a strong coupling between physical and chemical processes.
Then, solute transport and geochemical reactions should be coupled in the models to account for
relevant processes such as dissolution/precipitation of CSH gels and porosity changes due to mineraldissolution and precipitation. At the same time, changes in porosity are coupled with solute diffusion
and water flow, so numerical models should account for these fundamental relations.
Handling coupling to other processes:
Cement degradation will produce an alkaline perturbation and a leachate plume that could reach thebuffer, backfill and geosphere. The performance requirement for the buffer is currently a pH<11. The
purpose of the handling of this process is to demonstrate that this requirement is met. The interactionof the hyperalkaline plume with the external groundwater could trigger mineral dissolution/precipitation processes that could influence the groundwater flow and solute transport properties of the buffer,
backfill and geosphere. Coupled reactive transport models should account for these couplings inorder to evaluate the magnitude these effects.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The main part of the references cited is peer reviewed papers.
Uncertainties

Uncertainties in mechanistic understanding
A main uncertainty is related to the complex solubility behaviour of CSH phases, which has been
oversimplified in previous models of cement degradation. Other uncertainties are associated withthe long-term evolution of the pore network and the diffusivity in the cement paste. The diffusivity
depends strongly on the porosity values, and the coupling between the dissolution processes and theresulting microstructural changes are not fully understood.
Model simplification uncertainty in SR-Site
The calculations to evaluate the degradation of the concrete plug can be made using a simplified 2D
geometry, where the plug is sandwiched between two sections of bentonite backfill. Furthermore, thesolid phases considered is a simplification of the CSH phases that really precipitate. The implicationsof this will be discussed in the modelling report /Arcos et al. 2010/.
Input data and data uncertainty in SR-Site
The main uncertainties concern the composition of the concrete plug and the thermodynamic stability of calcium-silicate-hydrates. The implications of this will be discussed in the modelling report/Arcos et al. 2010/.

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5.4.5

Aqueous speciation and reactions

Overview/general description

This process is in general identical with the corresponding process in the buffer (Section 3.5.7).
Handling in the safety assessment SR-Site

The process is included in a separate modelling task of the plug (concrete part) degradation. This isfurther described in Section 5.4.4.

5.4.6

OsmosisOverview/general description

Flow of a solvent into a solution through a semipermeable membrane is termed osmosis if the driving
force is differences in concentration. The concrete may serve as a semipermeable membrane andosmotic flow may take place in the system component.
Handling in the safety assessment SR-Site

The magnitude and especially the lack of significant osmotic flow in the tunnel plug make thisprocess less important and will not be considered in SR-Site.

5.4.7

Montmorillonite transformation

Overview/general description

This process is in general identical with the corresponding process in the buffer (3.5.9) for the
bentonite part of the plug.
Handling in the safety assessment SR-Site

The process is neglected since no long term performance is expected from the bentonite part in thetunnel plug component.

5.4.8

Montmorillonite colloid release

Overview/general description

This process is in general identical with the corresponding process in the buffer (3.5.11) for the
bentonite part of the plug.
Handling in the safety assessment SR-Site

The process is neglected since no long term performance is expected from the bentonite part in thetunnel plug component.

5.4.9

Microbial processes

Overview/general description

Micro-organisms interact with their surroundings, and they commonly have a significant effect on
the geochemical record of their environment /Madigan et al. 2008/.
Plastisisers that will be added to the concrete will contain organic polymers of varying compositions,e.g. polycarboxylate polymers. The amounts to be used will be significant. The amount of plastisisers will be 6-7 kg /m3 concrete (Table 2-10). Plastisisers introduce a significant uncertainty, because
their composition and solubility, and thereby their availability to microbes, are not well known. In
cases where a grouting activity fails and the grout do not solidify properly; large amount of organiccarbon will be available for microbial growth. Such growth can occur in the grouted aquifers and in
the central area, ramp and shaft on places where groundwater from grouted sites seep through thetunnel walls.

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Dependencies between the process and the tunnel plug variables

Table 5-8 shows how the process influences and is influenced by all tunnel plug variables.
There are two main influences from the structural material variable on microbial processes.
1. Microbial sulphate reduction during microbial growth on plastisisers will result in sulphide. The
sulphide may accumulate as iron sulphide inside the plugs. During the open phase, oxygen willoxidise the sulphide and sulphuric acid can form. Local acid attacks on the concrete may occur.
This effect will diminish after backfilling on both sides of the plug, when oxygen is consumed,
but it may introduce a problem during the open phase.
2. Excess amounts of organic material in aquifers will trigger growth of opportunistic microorganisms that will increase in numbers. Several groundwater microorganisms produce complexing
agents that very efficiently mobilise trace elements, including many radionuclides /Kalinowski
et al. 2004, 2006, Johnsson et al. 2006, Essén et al. 2007, Moll et al. 2008/. Given the large
amount of added organic carbon where grouting have been performed, it is possible that the
microbial production of complexing agents will be significant which may notably reduce theretardation effects accounted for by the rock and the rock matrix.
Table 5-8. Direct dependencies between the process “Microbial processes” and the definedtunnel plug variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Plug geometry

No

No

Pore geometry

No

This effect is not
Growth of microbes
generate biomass that considered in SR-Site
may clog pores

Temperature

Yes, microbialactivity increases withtemperatureThis effect is notconsidered in SR-Site

No

Water content
Gas content
Hydrovariables(pressure and flows)

Yes, water is neededThis effect is not
for growth of microbes considered in SR-Siteand growth on hydrogen and methane ispossible. Pressure is
not importantYes, microbes may
form methane andhydrogenStress state

No

No

Plug materials –composition andcontent

Yes, the content oforganic carbon andmicrobes in different
backfill clays varies

Porewater
composition

Yes, microbes areThis effect is notsensitive to theconsidered in SR-Site
geochemical situation,and may utilise porewater components for
growthTR-10-47

This effect is notconsidered in SR-Site

This effect is notconsidered in SR-Site

No

Yes, microbial activity
may change thecomposition, e.g.
by consumption of
methane, sulphateand organic carbon
and production ofsulphide and carbon
dioxide.

This effect is notconsidered in SR-Site

251

Boundary conditions

The access to organic material, as organic additions to concrete, hydrogen from anaerobic corrosionof iron, organic material from other material that cannot be stripped or cleaned out and methaneand electron acceptors from the geosphere will set the limits for the microbial processes in the plug
Given the large amount of plastisiser that will be added, access to organic carbon can be assumed fora long time.
Model studies/experimental studies

None.
Natural analogues/observations in nature

No relevant natural analogues or observations have been identified.
Time perspective

Microbial processes will start when water becomes available. The process rates will be largest inareas where organic carbon and hydrogen are available. Elevated microbial process rates, comparedto the geosphere, will continue until all sources for growth have been consumed.
Handling in the safety assessment SR-Site

Mass balance calculations are made to show how different kinds of residual materials remaining
in the repository will contribute to microbial oxygen reduction. It can be assumed that all organic
matter will be able to serve as nutrients for microbes. Furthermore, the availability of hydrogen will
be decisive for microbial activity in the long term. This means that an increased effect of microbialactivity must be expected until all organic matter has been consumed, and subsequently an activity
that is controlled by the supply of hydrogen will proceed. This means that oxygen, ferric iron, sulphate and carbon dioxide will be reduced to water, ferrous iron, sulphide and methane, respectively.
The scope of these reactions is dependent on mass flows.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

All the references in this section are from peer reviewed scientific journals. However, the mostrecent ones may be in the progress of being published. See also the same topic in Section 4.4.10.
Uncertainties

Uncertainties in mechanistic understanding
The knowledge about microbial processes in the plugs is not yet well supported by experimental
data. Rather, the processes are inferred from research on microbial processes in the geosphere and in
buffer material.
Model simplification uncertainties in SR-Site
The effects of microbial reactions are maximised since a mass-balance expression is used.
Input data and data uncertainties in SR-Site
The amount and availability of organic materials and metals in the plugs will determine the extent of
microbial reactions. The data can be found in the Backfill Production Report /SKB 2010e/.

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5.5

Radionuclide transport processes

5.5.1

Speciation of radionuclides

Overview/general description

See corresponding process description for the buffer, Section 3.6.1.
Handling in the safety assessment SR-Site

This process is indirectly handled through the selection of parameters for radionuclide transport in
the backfill. In general, the handling of the process is the same as for the corresponding process inthe buffer (see Section 3.6.1).

5.5.2

Transport of radionuclides in the water phase

Overview/general description

The plug covers a limited volume with tunnel backfill on each side (except the plugs facing the centralarea). The hydraulic conductivity of a degraded plug may be high and transport of dissolved species
may be rapid, but since the geometry is restricted, the plug will not affect the overall transport patternin the repository.
Handling in the safety assessment SR-Site

The plug is considered to be a part of the tunnel backfill in the radionuclide transport calculations.
No specific considerations are done for the plug.

5.5.3

Transport of radionuclides in a gas phase

Overview/general description

The permeability for gas in the degraded tunnel plug may be high and they could potentially act assink for a gas phase that is transported away from a defect canister. This could delay the release ofradioactive gases from the repository.
Handling in the safety assessment SR-Site

The delay of a gaseous release of radionuclides is neglected in SR-Site. The overall process is treatedaccording to Section 3.6.3.

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253

6

Processes in the central area

6.1

Thermal processes

6.1.1

Heat transport

Overview/general description

The thermal conductivity of the repository host rock is in the range 2.0 – 4.0 W/(m·K) /Back et al.
2007, Sundberg et al. 2008/. The thermal conductivity of the crushed rock filling out shafts andramps in the central area is significantly lower, although there are no parameter values available. In
addition, the volume fraction of the central area or, more generally, of the rock between the deposition areas that is occupied by backfilled excavations is very small. These facts together mean that thethermal conditions in backfilled shafts and ramps are controlled and dominated by the temperature
evolution of the surrounding rock, and that details in the description of the heat transport within the
crushed rock backfill are unimportant.
The temperature increase in rock volumes between the deposition areas will be modest, at most c.
20-25°C a few hundred years after closure. Thermal gradients will be modest too: 5-10°C/100 m
vertically in rock between deposition areas separated by 70 m, which would be representative for thelocation of the central area (Figure 6-1) /Hökmark et al. 2009/. The low temperatures and the small
gradients mean that evaporation and vapour diffusion cannot contribute to the heat transport and thatthe influence of the heat transport on all variables other than the temperature are vanishingly small.
Dependencies between process and central area variables

Table 6-1 summarises how the process influences and is influenced by all crushed rock variables andhow these effects are treated in SR-Site.
25

Temperature increase (°C)

Five panels separated by 70 m
Canister spacing: 6 m
Tunnel spacing: 40 m
In situ temperature: 0°C
λ = 3.55 W/(m·K)
C = 2.18 MJ/(m3·K)

z=0mz = 50 m
z = 100 m
z = 150 m
z = 200 m
z = 250 m

20

15

10

5

0
1

10

100

1000

10000

1000001000000

Time (years)

Scan-linete mperatures

Temperature contours, 600 years

200

2.0

200

300

400

500

x

600

700

2.0

100

800

900

1000

4.

0

8.

0

0

.0

10

12.0

0

-100

28.0
26.0

26.0
24.0
22.0
20.0
18.0
16.0
14.0

6.

28.0

0

.0
24

z

100

Figure 6-1. Example of contours of temperature increase (lower) and temperature increases on vertical scanlines between deposition areas (upper). Results obtained using the method described in /Hökmark et al. 2009/.
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255

Table 6-1. Direct dependencies between the process “Heat transport” and the defined central
area variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Central area geometry No

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

No

Central area poregeometryYes. Affects thermal
conductivity

Not specificallyaddressed sinceconsequenses are
insignificant

Temperature

Yes
Rock heat transport
is slightly temperaturedependent.

Not specificallyYes.
addressed Effect is very By definition
small for the modest
temperature increase in
the central area.

Water content

Yes

Not specificallyaddressed sinceconsequenses are
insignificant

No

Gas content

No, but indirectly
through water content-

No

Hydrovariables(pressure and flows)NoNo, but indirectly via
temperature

Stress stateNoNo, but indirectly
via temperature andthermal strain

Central area materials Yes– composition andcontent

Not specificallyaddressed sinceconsequenses are
insignificant

No, but indirectly
via temperature andthermal strain
Ignored, sincetemperature increasein the central area
is small and of noconsequenceNo

Central area porewater composition

No

No

Structural and straymaterials

No

No

Central area geometry: The geometry of the crushed rock filling out shafts and ramps is given by
the shape of the excavations. This cannot be expected to change in a way that can affect the heattransport in the crushed rock. There will always be sufficient backfill/rock contact to ensure that therock and the crushed rock are in thermal equilibrium.
Central area pore geometry: The crushed rock thermal conductivity varies with the void ratio,
as for all porous materials and similar to the way it varies with the void ratio of the backfill in thedeposition tunnels. The small thermal expansion of the solid particles will affect the void ratio very
insignificantly.
Temperature: Heat transport influences the temperature by definition. There is a small temperaturedependence in the heat transport properties of rock and crushed rock. For the modest temperature
increase in the central area this is of no importance.
Water content: The heat transport will not affect the water content. The conductivity-saturation
relations that apply for the buffer and the deposition tunnel backfill are in principle valid also for thecrushed rock in the excavations in the central area. Since the thermal evolution in the central area ispractically independent of the heat transport in the excavations, this is of no importance.
Gas content: The heat transport will not affect the gas content. The gas content in itself does nothave any influence on the heat transport. If there is a gas phase, the effects of this will be just thoseof the incomplete saturation, i.e. they are of no importance.

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Hydro variables: The heat transport in the excavations does not control the temperature andconsequently none of the hydro variables. Water flow can contribute to the heat transport, but highflow rates are required.
Stress state: Heat transport does not change the stresses. The stress state does not have any practically influence on the heat transport, although the crushed rock thermal conductivity may increase
marginally when the stress increases. The effect is, however, to small and uncertain to be credited/Börgesson et al. 1994/.
Crushed rock composition: The composition of the backfill will influence the thermal conductivity.
Some minerals have higher conductivities than others.
Crushed rock porewater composition: No influences.
Structural and stray materials: To be of any importance to the heat transport, there must be verysignificant amounts of non-backfill materials.
Boundary conditions

The peripheries of the excavated tunnels, intersections with other tunnels, tunnel faces and sealing
tunnel plugs are the boundaries of the crushed rock. The boundary conditions of the process are thetemperatures at these boundaries. These boundaries are all approximately in thermal equilibriumwith the host rock which changes its temperature slowly as a result of the decreasing heat output
from the deposition areas. The temporal and spatial variation of the rock temperature is conceptuallywell understood.
Model studies/experimental studies

Model studies. There are no studies that explicitly address the thermal conditions in the central area.
Because of its small importance and its weak coupling to other processes, modelling results from thisprocess are available only indirectly from studies that address the thermal evolution of the repositoryrock mass in general. Predictions of the temperature evolution have, for instance, been done analytically /Claesson and Probert 1996, Hökmark and Fälth 2003, Hedin 2004/ and numerically /Thunvik
and Braester 1991, Ageskog and Jansson 1999, Hökmark et al. 2007/.
Natural analogues/observations in nature

Not applicable.
Time perspective

The duration of the thermal pulse generated by the decaying fuel is a few thousand years. The
general statements made for the thermal pulse hold true also during following phases, i.e. heattransport in the excavations in the central area is completely controlled by the thermal developmentof the host rock. Effects of climate changes and variations in geothermal heat flow will be imposedon the backfill by the surrounding rock.
Handling in the safety assessment SR-Site

There is no specific handling of heat transport in the central area during the thermal phase since thetemperature in the central area is not directly related to safety. Results from thermal analyses of therepository rock mass give sufficient information on the temperature in central area.
For permafrost and glacial conditions, the minimum temperature of the crushed rock may be of concern, see further Section 6.1.2. For these conditions, as well as for the conditions during the temperate
period, the backfill temperature can be taken to be equal to that of the host rock. See further the SR-Site
Geosphere process report /SKB 2010i/ for the treatment of rock temperatures for these conditions.

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257

Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Because of the small thermal gradients and the relatively low temperatures in the central area, there isno specific handling of the heat transport process. These statements are supported by results obtainedusing calculation schemes described in /Claesson and Probert 1996/. While this particular report is not
peer- or factual reviewed, the calculation scheme has been repeatedly applied in peer reviewed papers/Hökmark and Claessson 2005/ and in reports that have undergone (/Hökmark and Fälth 2003/) or
will undergo (/Hökmark et al. 2009/) a documented factual and quality review.
Uncertainties

Uncertainties in mechanistic understanding
There are no major uncertainties in mechanistic understanding. In the unsaturated state, the backfill isa system with solid, liquid and gas phases and obeys physical laws for such systems, but for practical
purposes (should the process be modelled) the heat transport is expediently described as pure conduction with a saturation-dependent or fixed conductivity.
Model simplification uncertainties in SR-Site
Not relevant since the process is not modelled.
Input data and data uncertainties in SR-Site
Not relevant since the process is not modelled.

6.1.2

Freezing

Overview/general description

During periods of cold climate it cannot be excluded that the temperature in the central area will
drop to values below 0°C /SKB 2010h/. This means that the water in the crushed rock backfill may
freeze. This will affect the hydraulic and mechanical properties of the material.
The hydraulic conductivity of the crushed rock will be very much dependent on the grain size
distribution, but is estimated to be in the range 10-6 – 10-7 m/s (see Section 6.2.1).
It is known that changes in hydraulic conductivity may take place in granular material due to freezing
and subsequent thaw.
Largest changes in hydraulic conductivity are found if clay particles are present and fill the voids
between the bigger particles. During the freeze/thaw process the fine grained fractions, filling thevoids, will consolidate due to the freezing process. The coarser particles will create a stable structure
controlling the compressibility and therefore no change in overall void ratio occur, but the clay
particles will consolidate to a denser structure and consequently, the void space is filled with less,but denser, clay aggregates. The hydraulic conductivity will therefore increase due to freeze/thaw.
This process only takes place if the amount of clay is in the order of approximately 10% or more. If
the clay content is less, the changes in hydraulic conductivity will be very small, if any. In a coarse
grained material, like the material used in the ramp and shafts, the created voids and cracks will beclosed and sealed by relocation of small particles. /Viklander 1997/ states that a type of self-healing
takes place during thaw. The conclusion and observations are supported by results presented by/White and Williams 1994/.
The overburden, or mean stress level, has impact on the changes in hydraulic conductivity due to freezing and thawing. For high stress levels, changes in hydraulic conductivity are relatively smaller thanfor lower, see e.g. / Benson and Othman 1993/. The stress level which is needed to keep the changes ofhydraulic conductivity unchanged after freeze/thaw varies with the grain size distribution of the material. Highest stress levels are needed for the finest soils. For mean stress levels higher than 200 kPa, nochanges in hydraulic conductivity have been reported due to freeze/thaw. For coarse grained material,
50-100 kPa is enough to close openings and fissures created by freeze/thaw /Viklander 1997/.

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Particle migration and particle sorting are processes taking place in soils subjected to freezing andthawing. According to /Corte 1963/, coarse particles move towards colder parts where they are
accumulated. Coarse particles (> 1 mm) need a low freezing rate to be moved. In laboratory tests,
these phenomena are not found very often and /Corte 1963/ concludes that as the phenomena areeasily observed in permafrost areas it indicates that a relatively great number of freeze/thaw cycles
are needed to initiate the phenomena. /Corte 1963/ indicates that several hundreds of cycles might beneeded in order to have the migration and sorting phenomena in a well graded moraine.
The phenomena with stone heave are related to the migration and sorting phenomena in a moraine
subjected to cycles of freeze/thaw. Coarse particles and stones are lifted when embedded in a frost
heaving soil. /Viklander 1997/ concludes that stone heave is restricted to approximately 5 mm/cycle.
If the material in the central area volume is subjected to freezing conditions more than 1,000 times,
this will imply that coarse material will be relocated to the top of the volume while more fine grainedmaterial will be found at lower levels of the tunnel. The mass of fill material in the volume stays
constant, and therefore this will not have a great impact of the overall hydraulic conductivity eventhough there will be a redistribution and sorting of the particles.
Dependencies between process and central area variables

Table 6-2 summarises how the process influences and is influenced by all crushed rock variables andhow these effects are treated in SR-Site.
Table 6-2. Direct dependencies between the process “Freezing” and the defined central area
variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Central area geometry Yes. Determines thetotal volume of water
which can be frozen

Process influence on variable
Influence present? Handling of influence(Yes/No)(How/Why not)Description

The effects of freezingNoin the central area,
ramp and shaft can beneglected based on thediscussion in this section

Central area poregeometry

Yes.

Yes, redistributionof material

Temperature

Yes. Obvious

No

Water content

Yes, but freezing will
always occur after fullsaturationNo

Gas content

No. Not relevantNo

Hydrovariables(pressure and flows)

Yes. The freezing
point is affected bypressure

No flow in frozen
water

Stress state

No

Yes

Central area materials Indirectly through– composition andpore geometry
content

No

Central area
porewater
composition

Yes. The freezing
point of water isaffected by salinity

Yes. To a limited
extentStructural andstray materials

No

No

TR-10-47

The effects of freezingin the central area,
ramp and shaft can beneglected based on
the discussion in thissection259

Boundary conditions

The only boundary condition for the process is the vertical temperature profile in the surrounding rock.
Model studies/experimental studies

No studies on materials to be used in the central area have been done.
Natural analogues/observations in nature

The described phenomena are well known from observations in nature.
Studies of changes in hydraulic conductivity due to freeze/thaw of coarse-grained materials, likemoraine, show that hydraulic conductivity of a fine-grained glacial moraine, exposed to freezing
and thawing, increased in the order of two orders of magnitude, e.g. /Johnston and Haug 1992/. It
was concluded that the increase was mostly due to the amount of fines in the moraine. /White andWilliams 1994/ has reported changes in hydraulic conductivity of 2-5 times for a silty moraine and/Viklander 1997/ increases in the order of 1-10 times for Swedish well-compacted moraines. The
hydraulic conductivities reported are all measured in a direction parallel to the heat flow direction.
Changes in hydraulic conductivity in a direction perpendicular to the heat flow are reported by/Chamberlain 1980/ and are in the same range as those parallel to the heat flow direction.
Time perspective

The process is relevant during the entire lifespan of the repository, although it only takes place
during cycles of permafrost.
Handling in the safety assessment SR-Site

Due to the small amount of ice lens formation and the limited impact on the hydraulic conductivity
on the material in central area, the process can be neglected.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The references in this section are peer-reviewed publications available in the open literature.
Uncertainties

The uncertainty is related to the amount of clay and corresponding grain size distribution of the fill material. With higher clay content, the impact of freeze/thaw on the hydraulic conductivity will be greater.

6.2

Hydraulic processes

6.2.1

Water uptake and transport under unsaturated conditions

Overview/General description

Water transport in the crushed rock filled in the central area before saturation is mainly governed by
the inflow rate and inflow distribution in the rock. Since the expected hydraulic conductivity of thecrushed rock is rather high, and since there will be an open slot at the roof, the resistance to water
inflow into the filling material will be very low.
In principle the hydraulic subprocesses are identical to those (A-E) described for the buffer (seeSection 3.3.1) and will not be reproduced here. However, most processes connected with unsaturated
water flow in the buffer and backfill are not relevant for the crushed rock. E.g. the suction in the
crushed rock is insignificant and the temperature gradient is so low that there is no temperaturedriven vapour flow.

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The only relevant processes for the crushed rock are the following:•Transport of water in liquid phase, which is controlled and driven by
A1) a pressure gradient in the water,
A4) gravity (coupled to A1),
A5) a density gradient (coupled to A1).

•

Compression ofE2) air.

•

Transport in water ofF1) dissolved air.

A1. Transport of water in liquid phase, which is driven by a water pressure gradient, can be
described by a refined version of Darcy’s law. The parameters that control the process are thematerial’s hydraulic conductivity K and the difference in porewater pressure uw. Both of thesecan be described as functions of the void ratio e, the degree of saturation Sr and the temperature
T.
A4. Transport of water in liquid phase, which is driven by gravity. The influence of gravity isreflected in the hydraulic head and thus taken care of by process A1. Gravimetrically generated
flow can be an important process, especially in the ramp due to the high hydraulic conductivity.
A5. Transport of water in liquid phase, which is driven by a density gradient in the water.
Differences in the density of the water can be caused by e.g. differences in temperature or salinity. Density-generated flow is probably negligible in the crushed rock, cf. process A1.
E2. Compression of air can be an important factor in the water saturation process (Boyle’s law).
If the air is trapped, it must be compressed and go into solution in the water in order for water
saturation to be achieved. The crushed rock has such low suction that a pressurised air volume
will remain if the air cannot seep out. A large part will disappear when full water pressure hasdeveloped.
F1. Transport of dissolved air in water. When trapped air is compressed during the water saturation
phase, a portion of the volume is dissolved in the water. This solubility (Henry’s law) isapproximately 2 percent at room temperature but decreases rapidly at increasing temperature. In
order for additional air to be dissolved, the dissolved air must be transported away, which takes
place by diffusion.
Dependencies between process and central area variables

Table 6-3 summarises how the process influences and is influenced by all central area variables andhow these effects are treated in SR-Site.
Influence of pore geometry: The hydraulic behaviour depends very much on the grain size distribution of the crushed rock. If the percentage of fines (<74 μm) is low (<10%), the hydraulic conductivity will be high (K>10-6 m/s) and there will be no relevant suction even at very low water content.
If the rock is crushed and milled to a large portion of fines (>30%), the hydraulic conductivity will
be rather low (K<10-7 m/s) and there may be a relevant suction at low water content. However thedensity of the crushed rock and grain size sorting factor (well-graded or equigranular) also influencethese properties significantly.
There are a number of factors influencing the hydraulic properties of crushed rock. The mostimportant ones are• Representative grain size (e.g. effective grain size d10 = 10 µm meaning 10% weight of particles
smaller than 10 μm)• Sorting coefficient (well-graded or equigranular)• Dry density
Numerous suggestions of equations relating the hydraulic conductivity and the capillary rise tothese and other parameters have been made. Figure 6-2 shows, as example, the relation between thehydraulic conductivity and the effective grain size d10 that can be used for rough calculations.

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261

Table 6-3. Direct dependencies between the process “Water uptake and transport underunsaturated conditions” and the defined central area variables and a short note on the handlingin SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Central area geometry Yes

Modelling

No

Central area poregeometryYes. Via void ratio(density) and void
geometry

Model parametersNo, but indirectly
through hydro variables and erosion, if ittakes place

See erosion,
Section 6.2.4

Temperature

Yes. Through water
Negligible
viscosity and temperature gradientNo, but indirectly
through water content
which affects thermal
conductivity

Negligible

Water content

Yes. Via degree ofsaturation

Modelling

Yes

ModellingGas content

Yes. Via the gaspressure and degreeof saturation

Modelling

Yes

Modelling

Hydrovariables(pressure and flows)

Yes. Basic.

Modelling

Yes

Modelling

Stress stateNo

No

Central area materials Yes. Hydraulic– composition andconductivity
content

Model parametersNo, but indirectly
through hydro variables and erosion, if ittakes place

See erosion

Central area
Yes, small influence
porewater composition on viscosity anddensity

Negligible

Yes, settled by the
inflowing water

No, the influenceon the behaviour isnegligible

Structural and straymaterials

No

No

0
10–1

Hydraulic cond., m/s

10–2
10–3

Loose filling
Dense filling

10–4
10–5
10–6
10–7
10–8
10–9
10–10
0,001

0,01

0,1

1

10

100

Effective grain size (d10), mm

Figure 6-2. Relation between the hydraulic conductivity and the effective grain size d10 that can be used
for rough calculations for rather even-grained soils /Fagerström and Wiesel 1972/.

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The percentage of fines in the crushed rock is thus very important for the hydraulic conductivity.
When TBM (Tunnel Boring Machine)-muck from TBM drilling is crushed, the percentage fines
is rather high and (as shown below) the hydraulic conductivity can be very low. When rock fromblasting is crushed, there will be no fines and the hydraulic conductivity is not expected to be lower
than 10-5 m/s, unless grinding is performed.
If the percentage of fines is low and thus the hydraulic conductivity of the crushed rock is high andthe suction negligible, the water saturation phase is expected to take place in a front like pattern, inthe sense that the pores in the crushed rock are filled with water and will not fill the following pores
until they are almost completely saturated. The influence of the degree of saturation on the flow rateis negligible.
If the percentage of fines is high and thus the hydraulic conductivity of the crushed rock is low, thesuction may not be negligible if the initial water content is low. The water saturation rate may in thatcase be aided by the suction but, on the other hand, the hydraulic conductivity is for such a material
lower and a function of the degree of saturation, which will delay the saturation.
A process that may strongly delay the saturation is when air has been trapped in the crushed rock.
Since the water inflow is unevenly distributed, there may be many air pockets, especially at the roof
in the central area, where there are no escape ways upwards.
Boundary conditions

Interaction with the rock: A key issue for the saturation process is the interaction between rock andcentral area. Water is conducted to the central area in the water-bearing fractures and the rock matrix,
which means that water saturation can be both uneven and take a long time for a sparsely fractured
rock. The resistance to water inflow is low due to the high hydraulic conductivity of the crushedrock, so the inflow rate and saturation rate of the crushed rock is expected to be mainly governed by
the rock hydrology and the natural inflow rate to the empty central area, ramp and shaft.
However, from highly water bearing fractures, the inflow rate may be so high that the crushed rock
will prevent some inflow, which leads to an increased water pressure in the fracture. This could
cause piping and erosion effects but should be possible to avoid by rock grouting.
Interaction with the tunnel plug: The crushed rock in the central area is in direct contact with theend plugs of the transport tunnels. Since the plugs are designed to seal off all water transport fromthe trunk and transport tunnels to the central area during the entire water filling and resaturation
time, there will only be hydraulic interaction when full hydrostatic pressure has been reached.
However, since there is very little resistance against water filling from the rock into the central area
filled with crushed rock, these parts may be water filled before the transport tunnels and full waterpressure may act on the tunnel plugs before there is any counter pressure inside the plug. Since theplugs are not designed to withstand an elevated water pressure on the outside they may fail, in whichcase the tunnels will be filled by water from the central area.
Model studies/experimental studies

In Äspö HRL, there is a full scale test ongoing (Backfill and Plug Test) where the hydraulic propertiesof crushed rock as backfill material are investigated.
The Backfill and Plug Test /Gunnarsson et al. 2001/ includes tests of backfill materials and emplacement
methods and a test of a full-scale plug. It is a test of the integrated function of the backfill material andthe near field rock in a deposition tunnel excavated by blasting. It is also a test of the hydraulic andmechanical functions of a plug. Two different backfill materials are tested, namely a mixture of 30%
bentonite and 70% crushed rock and 100% crushed rock. Rock debris from TBM drilling (TBM-muck)
was used. Since TBM-drilling yields a large portion of fine material, due to the grinding process during
drilling, the hydraulic conductivity of this material is very low.

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263

The main objectives of the Backfill and Plug Test are•

to develop and test different materials and compaction techniques for backfilling tunnels
excavated by blasting,•to test the function of the backfill and its interaction with the surrounding rock in full scale in atunnel excavated by blasting,•to develop technique for building tunnel plugs and test the function.

The entire test set-up with backfilling and casting of the final part of the plug was finished in autumn
1999 and the water saturation, with water filling of permeable mats, started in late 1999. In the years
up to 2001, the water saturation has continued and data from transducers have been collected andreported. The plug has been grouted and tested and in 2002 the water pressure in the permeable mats
was increased to 500 kPa in order to shorten the time to reach full saturation. The backfill was water
saturated in 2003 so the flow testing started in 2004. Hydraulic in situ tests of the section with crushedrock are still ongoing.
The hydraulic conductivity of the crushed rock has been investigated, but only after full water
saturation /Johannesson et al. 1999/. The grain size distribution of the crushed rock material is shownin Figure 6-3 and the measured hydraulic conductivity as a function of the dry density is shownin Figure 6-4. The measured hydraulic conductivity is rather low due to the high content of fines(d10 ~ 0.03 mm), the sorting coefficient and the high dry density.
The retention curve that describes the suction of the material as a function of the water content has
been measured. As shown in Figure 6-5, the suction is high at very low water content (<1%) but
decreases fast with increasing water ratios and is negligible at the water ratio optimal (Proctor) forcompaction (5-12%).
No model studies of the saturation phase of crushed rock have so far been done but hydraulic andhydrogeological modelling of the water filling will be done for SR-Site /Åkesson et al. 2010/.
Natural analogues/observations in nature

The grain size distribution of crushed rock reminds that of natural moraine soils. Many laboratory
and field tests of such materials have been performed over the years and the results confirm theoverall behaviour presented here. A general description of hydraulic behaviour of different coarse
grained soils is given in /Fagerström and Wiesel 1972/.
100
90

Percent Finer by Weight80

TBM-muckCrushed twice

70

Field test mtrl

60

Wanted

50
40
30
20
10
0
0.01

0.1

1

Grain Size (mm)

10

100

Figure 6-3. Grain size distribution of the natural uncrushed TBM muck, the ballast material used in thelab tests (Crushed twice) and the ballast material used in preparatory field tests /Johannesson et al. 1999/.

264

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Hydraulic Conductivity, m/s

10–6
Ø 250 mm - 2

10–7

Ø 101 mm

10–8

10–9

10–10

10–11

10–12

1.8

1.9

2

2.1

2.2

2.3

2.4

Dry Density, g/cm3

Figure 6-4. Measured hydraulic conductivity as a function of the dry density for crushed TBM muck
(0/100) /Johannesson et al. 1999/.
30000

25000

Suction (kPa)

Crushed TBM muck
20000

15000

10000

5000

0
0.30

0.35

0.40

0.45

0.50

0.55

0.60

Water ratio (%)

Figure 6-5. Measured total suction of crushed TBM muck as function of water content /Johannesson
et al. 1999/.

Time perspective

Rough estimates show that the time to full water saturation of the central area can vary a great deal,
depending, above all, on the hydraulic properties of the surrounding rock. In areas with rather high water
inflow rates, saturation may be reached fast. If there are no fractures, all water has to go via the rockmatrix, which may take considerable time in the granitic rock types occurring in Sweden. However, thetime to reach full saturation will probably be shorter than that, since water may be transported along
the rock surface from fractures located at other positions. In this case, full saturation is expected tobe established in a few years, except where there are air pockets, which may remain for many years.

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265

Handling in the safety assessment SR-Site

The inflow and successive water filling into these parts is modelled and the influence of different
rock conditions is investigated with sensitivity analyses. This is done as a part of the THM modellingof the buffer, backfill and other system components /Åkesson et al. 2010a/.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The inflow modelling will be reported in a general THM modelling report that will undergo a documented factual- and quality review /Åkesson et al. 2010a/.
The report describing the hydraulic conductivity of crushed rock by /Johannesson et al. 1999/ is
not peer- or factual reviewed, but it is scientifically consistent with other references that are peer
reviewed articles or papers.
Uncertainties

The basic hydraulic properties of the crushed rock in the central area are rather well known.
The main uncertainties of these properties concern the risk and effect of piping.
Uncertainties in mechanistic understanding
The uncertainties are mainly related to the piping. The hydraulic model is well known.
Model simplification uncertainties in SR-Site
The model assumes that there is no piping.
Input data and data uncertainties in SR-Site
The model parameters need to be determined in detail when the material has been produced. The
data used are discussed in the THM data report /Åkesson et al. 2010b/.

6.2.2

Water transport under saturated conditions

Overview/General description

Water transport under saturated conditions is mainly driven by a water pressure gradient. The process
can be described by Darcy’s law. The general description of the water transport processes given in theprevious section for unsaturated crushed rock is also valid under saturated conditions. In addition, thetemperature gradient and salt content gradient may, after full pore pressure equilibrium is established,
be an important factor for driving water flow due to the low natural pressure gradient.
After full water saturation and porewater equalisation, the main water transport in the central area
will take place in the slot between the crushed rock and the roof. Such a slot will occur because of
the difficulties in compacting crushed rock at the roof and because of the subsequent settlement of
the material, unless the slot is filled with swelling material. The crushed rock in the shaft can, on theother hand, probably be compacted without leaving any open pathways, except for possible channels
caused by piping.
Dependencies between process and central area variables

Table 6-4 summarises how the process influences and is influenced by all crushed rock variables andhow these effects are treated in SR-Site.
The basic hydraulic properties of water saturated crushed rock are presented in the previous section.

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Table 6-4. Direct dependencies between the process “Water transport under saturated conditions” and the defined central area variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Central area geometry Yes

Modelling

No

Central area poregeometry

Yes, void ratio(density) and void
geometry

Model parameters(Yes) If erosion takes
place

See erosion
Section 6.2.4

Temperature

Yes. Through water
Neglected
viscosity and temperature gradientYes, by convection

Neglected

Water content

Yes. Hydraulicconductivity

Saturation assumed

No-

Gas content

No

-

Yes

Saturation assumed

Hydrovariables(pressure and flows)

Yes. Basic variables.

Modelling

Yes

Modelling

Stress state

No

-

No

-

Central area materials Yes. Hydraulic– composition andconductivity
content

Model parameters(Yes). If erosion takes
place (secondary
effect)See erosion,
Section 6.2.4

Central area porewater composition

Yes, via viscosity ofwater

Neglected

Yes

Neglected

Structural and straymaterials

No

-

No

-

Influence of density, temperature and water salinity

The hydraulic conductivity is primarily dependent on the pore geometry, the composition of thecrushed rock and the density. The ion concentration in the porewater and the temperature affectthe hydraulic conductivity via the viscosity of the water but are of minor importance. They may,
however, be important as pressure gradient generators.
Influence of other factors

After full water saturation and porewater equalisation, the open slot in the roof in the central area
and the possible channels caused by piping are expected to dominate the behaviour unless thehydraulic conductivity of the crushed rock is very high. The very small pressure gradients will befurther reduced since there is almost no flow resistance in these channels. This fact means that thepressure gradients caused by temperature differences and salt content differences may dominate thewater transport.
Boundary conditions

Interaction with the rock: The water transport caused by the water pressure gradients in the rockfractures is dominated by the large fractures and fracture zones.
Interaction with the tunnel plug: The crushed rock in the central area is in direct contact with theend plugs of the transport tunnels. After long time when the plugs have been more or less disintegrated, there may be a direct hydraulic interaction also with the backfill material in the deposition
tunnels, but the low hydraulic conductivity of the backfill strongly limits the hydraulic effect on the
crushed rock.

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267

Model studies/experimental studies

See Section 6.2.1.
Natural analogues/observations in nature

See Section 6.2.1.
Time perspective

The hydraulic gradient that exists after closure of the repository when water saturation and porepressure equilibrium in the central area and surrounding rock have been reached is expected to bevery low. Since the hydraulic behaviour will be dominated by open paths at the roof and along therock wall from piping damages, it is expected that hydrostatic pressure will prevail in these parts.
The hydraulic situation will be very sensitive to changes in hydrology on the ground surface.
Handling in the safety assessment SR-Site

The central area is an isolated part of the repository. However, the hydraulic conductivity can be high andthere may be a need to include this feature in the hydrogeological model, at least as a sensitivity study.
Uncertainties

Uncertainties in mechanistic understanding
The hydraulic model for a high conductivity porous material is well known.
Model simplification uncertainty in SR-Site

The central area will be abstracted in to a section of the rock with very high hydraulic conductivity.
Input data and data uncertainty in SR-Site
All data used for this process will be pessimistic assumptions.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The handling itself is a simplification to show sensitivity. The background reference for the overall
understanding can be found in Section 6.2.1.

6.2.3

Gas transport/ dissolution

Overview/general description

Transport of gas in the central area can occur in two phases of the repository evolution:
1. Water saturation phase
The backfill consisting of crushed rock will be emplaced dry, which means that there is a substantial
amount of air that needs to be dissolved and transported away. Also the rock may contain dissolvedgas. This was discussed in Section 6.2.1
2. Gas transport from a defective canister after saturation
Hydrogen gas from the corrosion of the cast iron insert could potentially escape from the repository
by transport through the central area, ramp and shaft. The crushed rock backfill could act as apassageway for gas under certain conditions. However, it is not evident that gas from the canister
will ever reach these areas. Gas release through the central area as well as the ramp and shaft ispessimistically neglected in SR-Site.

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Dependencies between process and central area variables

The influence table for this process is in all important aspects identical to the table for the process“water transport under unsaturated conditions” (see Table 6-3)
Boundary conditions

The rock may either be water unsaturated or completely water saturated. It may be unsaturated either
by drying caused by low relative humidity in the tunnel or from gas dissolved in the groundwater(due to the pressure drop). If the rock is saturated it may include different amounts of dissolved gas.
Model studies/experimental studies

The Backfill and Plug Test in the Äspö HRL /Gunnarsson et al. 2001/ includes a full scale test where
the hydraulic properties of crushed rock as backfill material are investigated (see also Section 6.2.1).
However, the information on trapped gas from that experiment is limited.
Natural analoges/observations in nature

Not applicable.
Time perspective

The delay of the water saturation of the crushed rock backfill is relatively greater when the permeability of the rock is high rather than when it is low. However, since the saturation rate is only criticalwhen the permeability is low the preliminary conclusion is that trapped air is not a very significant
factor unless the rock is almost saturated with gas.
Handling in the safety assessment SR-Site

The influence of trapped air on the saturation time of the crushed rock backfill is investigatedin SR-Site. The process involving trapped air is only relevant during the saturation phase (seeSection 6.2.1).
The escape of corrosion gas from the canister through the crushed backfill is not consideredin SR-Site.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

See 6.2.1
Uncertainties

The parameter values for the modelling of the gas dissolution and transport are rather well defined.
The largest uncertainty is the rock properties and the partly connected question of whether the air istrapped or not.

6.2.4

Piping/erosion

Overview/General description

Water inflow into the central area will take place mainly through fractures and will contribute to thewetting of the crushed rock. However, if the inflow is localised to fractures that carry more water thanthe crushed rock can adsorb, there will be a build-up of water pressure in the fracture and therefore
an increase in the hydraulic gradient through the crushed rock. If the total pressure from the waterpressure in the fracture is higher than the resisting total pressure, the crushed rock will be deformed
and an open channel or gap will be formed, where water can penetrate and continue to penetrate along
the rock surface. This flow may also result in erosion of fine particles along the pipes. The processes
are similar to the corresponding processes for the buffer described in Section 3.3.4.

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269

The risk of piping depends on a number of factors the main ones being the following:•

The hydraulic conductivity of the crushed rock.

•

The water inflow rate.

•

The water pressure increase rate when water flow is stopped.

•

The water inflow pattern (point wise or along a fracture).

The magnitude of erosion depends mainly on the grain size distribution and the percentage of fines
in the crushed rock but of course also on the flow rate. Well graded crushed rock is less sensitive toerosion than equigranular. Piping and erosion will only take place during the water saturation phase.
After porewater pressure equilibrium is established in the repository, the flow rate and water pressure
gradients are too low. Erosion and how to avoid erosion is described e.g. in /Handboken BYGG /.
No self sealing of the crushed rock by swelling will take place.
Dependencies between process and central area variables

Table 6-5 summarises how the process influences and is influenced by all central area variables andhow these effects are treated in SR-Site.
Boundary conditions

Piping and erosion in the crushed rock will take place at the interface between the rock and the back­filling. It will start at rock fractures and probably seek its way to the open slot at the roof if there issuch a slot.
Table 6-5. Direct dependencies between the process “Piping/erosion” and the defined variables
and a short note on the handling in SR-Site.
Variable

Variable influence on process
Handling of influence
Influence present?
(How/Why not)(Yes/No)
Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Central area geometry Yes, limits the amountof eroded material.

Simplified calculations

No-

Central area poregeometryNo, but indirectly
through hydro variables.

Simplified calculations

Yes if erosion takes
place

Simplified calculations

Temperature

No

-

No

-

Water content

Insignificantly

-

No

-

Gas content

No

-

No

-

Hydrovariables(pressure and flows)

Yes. Basic variables.

Simplified calculations

Yes

Simplified calculations

Stress state

Yes. The stresscaused by the own
weight of the crushedrock effects the piping

Simplified calculations

Insignificantly-

Central area materials Yes both the risk of
Hydraulic conductivity– composition andpiping and the erosion
content(Yes). If erosion
takes place

Simplified calculations

Central area pore­water composition

No

-

No

-

Structural and straymaterials

No

-

No

-

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Model studies/experimental studies

The processes are studied in conjunction with studies of the bentonite buffer and the bentonite backfill materials. No model studies or experimental studies concerning crushed rock have been done
so far. However, the problem of erosion is important for other engineering constructions (e.g. large
dams for water power production) and a lot of information is available.
Natural analogues/observations in nature

Erosion has been observed and is a problem for many engineering constructions, especially largedams. One example is the failure of the Teton Dam in Idaho, USA, which was caused by piping anderosion of filling material under the dam /Seed and Duncan 1981/.
Time perspective

Piping and erosion in the crushed rock only takes place before complete saturation and porewater
equilibration. The hydraulic gradient and resulting water flow rate is too low after equilibrium.
Handling in the safety assessment SR-Site

Piping and erosion could lead to a loss or redistribution of fine particles of the crushed rock in thecentral area, ramp and shaft. However, the consequence in the central area and ramp is considered
not to affect the behaviour since the open slot at the roof will dominate. In the shaft, where no open
slot is expected after emplacement and compaction, piping and erosion could significantly change
the properties.
For the safety assessment, the central area will be considered to have direct hydraulic contact with its
boundaries.
Uncertainties

Piping and subsequent erosion are complicated processes that probably are difficult to avoid in the
crushed rock although a proper choice of grain size distribution could minimise the effect. However,since the backfill in the central area is assumed to have a high hydraulic conductivity under all circumstances in the safety assessment the uncertainties can be disregarded.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

No references are used.

6.3

Mechanical processes

6.3.1 Swelling/mass redistribution
This Section includes descriptions of all mechanical processes in the crushed rock in the central area
except liquefaction. Thermal expansion, creep and a number of interactions with the end plug, thenear-field rock and the top seal will be treated.
Overview/General description

The crushed rock has no swelling potential so mechanically it will only have a passive role. The interaction with the other parts will be governed by the stiffness and friction angle of the crushed rock.
The stresses in the crushed rock are rather low since they are only caused by the own weight of thematerial. In the central area the effective vertical stress (= total stress minus porewater pressure)varies from zero at the roof to 50-100 kPa in the floor, depending on the height of the rooms. The
horizontal stress is about half the vertical one.

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The compressibility (or stiffness) of the material can be expressed according to Equation 5-1.
M = Δσ/Δε									(5-1)
where
M = compression modulus
σ = uniaxial stress with no lateral strain
ε = uniaxial strain with no lateral strain
The compression modulus can be determined with oedometer tests on the actual crushed rock. A test
series have been performed on the crushed TBM-muck used in the Backfill and Plug Test at different
densities (see Section 6.2.1).
If the compression modulus is evaluated between σ=100 kPa and σ = 400 kPa the compression
modulus will be
M = 15–50 MPa
These values are probably typical for crushed rock but depend on the grain size distribution, theshape of the grains and, especially, the dry density.
The friction angle is rather high. It depends on the same parameters as the compressibility. It wasmeasured for the crushed TBM muck and was found to be f = 55° for the dry density 2,200 kg/m3 at
the normal stress 1 MPa.
The influence of the degree of saturation can be neglected since the capillary pressure (suction) isvery low.
Some settlement during water saturation and from long-term creep may also occur in the crushedrock although these effects are small due to the good compaction properties and high density that
may be achieved after compaction.
Dependencies between process and central area variables

Table 6-6 summarises how the process influences and is influenced by all crushed rock variables andhow these effects are treated in SR-Site.
Boundary conditions

The central area has three main mechanical boundaries, namely the plugs in the transport tunnels,
the rock and the backfill at the ramp and shafts.
Interaction crushed rock / plug
There may be a mechanical effect on the crushed rock when the mechanical function of the concrete
plug has ceased due to disintegration of the concrete. Then the swelling pressure from the backfill
will be transferred to the crushed rock, yielding compression of a small part of the crushed rockand swelling of a small part of the backfill. The consequences can be estimated and are positive forthe crushed rock, since the density will be further increased but also a loss in density of the tunnel
backfill material will occur close to the plug.

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Table 6-6. Direct dependencies between the process “Swelling/Mass redistribution” and thedefined central area variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Central area geometry No, influences indirectly the stressesNo-

Yes affects thedensity

A hydraulic conductivity of the material is
assmumed in thehydrological model

Central area poregeometryYes. Determines
the density, which
is important for thecompressibility

Temperature

Insignificantly

No-

Water content

No, via density (poregeometry)

InsignificantlyNo

Gas content

InsignificantlyNo-

Hydrovariables (pressure and flows)

Insignificantly since
they can only slightly
delay the process

Insignificantly porepressure will beequilibrated fast

A hydraulic conductivity of the material is
assmumed in thehydrological modelStress state

Yes. Basic variable for Neglected, sincemechanical processes mechanical effects are
small and will have
limited impacted

Yes.

Neglected since
it will have limited
impact on the overall
performance

Central area materials Yes. Determines– composition andstiffness and friction
content
angle
Central area porewater composition

No

Structural and straymaterials

Yes, corrosion ofreinforcements may
cause compression of
the crushed rock bothby volume expansion(negligible) and byloss of rock stability

Neglected, sincemechanical effects are
small and will have
limited impacted

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Neglected, sincemechanical effects are
small and will have
limited impacted

Neglected, sincemechanical effects are
small and will have
limited impacted

No

No

-

No

-

Interaction Crushed rock / Near field rock
The following mechanical interactions between the near-field rock and the backfill of crushed rockhave been identified.
1. The lateral stress from the own weight of the crushed rock on the rock surface. It is small but may
keep loosened rock pieces in place.
2. Stresses from detached rock pieces. Such pieces can affect the backfill of crushed rock, especially
if they fall from the roof.
3. Convergence of the central rooms, ramp or shaft. This process compresses the backfilling andimproves the properties.
4. Thermo-mechanical effects from heating and cooling the rock. The cooling phase will causea small expansion of the open spaces. This will decrease the lateral stress from the backfill on
the rock walls and will widen the gap at the roof, but the effect is small since the temperaturechanges are small in these parts.
5. Rock displacements from earth quakes or rock creep can locally affect the crushed rock.

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Model studies/experimental studies

Backfill material of crushed rock has been used as one of two alternative backfill materials in the
Backfill and Plug Test (see Section 6.2.1). The mechanical properties of this material have beentested in the field, 5-6 years after installation and water saturation with a 0.5 m diameter pressure
cylinder that was installed in the floor of the tunnel. An oil pressure inside the cylinder was appliedand stepwise increased up to 5 MPa during long time and the applied pressure and resulting
deformation were measured. The results are shown in Figure 6-6. The test confirmed the very goodmechanical properties of this material and showed that the achieved dry density after compaction
was high. The total displacement at 5 MPa pressure was only about 2 cm.
Time perspective

Full saturation of the central area is expected to be achieved unevenly in a few years after closure of
the repository and the hydraulic conductivity of the rock is the controlling factor. The degradation of
the plugs, which may be fast if they collapse from high water pressure on the outside of the plug ortake thousands of years due to dissolution of the concrete, creates a swelling pressure from the tunnel
backfill on the inside of the plug. When porewater equilibrium has been reached, a small mechanicalimpact will come from the cooling phase.
In the long-time perspective, there is a (negligible) creep settlement of the crushed rock backfill
and there may be local breakage of the near-field rock that is caused by high stresses, excavation
damages and creep in the rock.
Natural analogues/observations in nature

No natural analogues concerning the mechanical behaviour of crushed rock backfill have so far beenstudied. The existence of relevant natural examples is probably very limited and would be difficult toevaluate. There are though some experiences from backfilling of mines.
Handling in the safety assessment SR-Site

Most of the mechanical effects related to mass redistribution are so small that they can be ignoredin the safety assessment since the demands on the crushed rock backfill are low. The interaction
between the crushed rock and the tunnel plugs is handled by a simplified model of both the effectof a sudden breakage of the plug and the swelling of the bentonite blocks in the plug after slow
disintegration of the plug (see also Section 4.3.1.
Effective stress (kPa)
0,0

100

300

500

700

900

1100

1300

1500

1700

0,5
1,0

Strain (%)

1,5
2,0
2,5
3,0
3,5
4,0
4,5

crushed 85% proctor
crushed 87% proctor
crushed 89% proctor
crushed 94% proctor salt.

Figure 6-6. Results of the pressure cylinder test in the Backfill and Plug Test showing the displacementof the cylinder (red line) as a function of applied pressure. The stepped blue line is caused by delayedcompression of the backfill due to consolidation effects (work in progress).

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Modelling of the effect of tunnel plug disintegration (Section 5.3.1) is done for SR-Site and reported
in the THM model report /Åkesson et al. 2010a/.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Not applicable – the process is neglected in SR-Site.

6.3.2

Liquefaction

Overview/General description

Liquefaction is a process implying that a stiff material (soil) turns into liquid due to an affect withshort duration, see e.g. /Lambe and Whitman 1969/. It may take place in a loose sand when theporewater pressure is increased either due to a vibration that makes the sand particles float in theporewater (since they tend to go into a higher degree of compaction, but the water temporarily
prevents it) or due to a strong upward water flow that releases the effective stresses between theparticles (quicksand). It may also take place in clay that has been settled in salty water (forming
an open structure with a high water ratio). If the salt is partly washed out by fresh water, the clay
structure cannot hold the high amount of water at moulding or vibrations, meaning that the structure
collapses when exposed to vibrations.
These two types of liquefaction cannot take place in a backfill of crushed rock that has been
compacted, since the density is too high.
The consequences of liquefaction would anyway be negligible. The only conceivable effect is thata loose rock block in the roof may sink but the effect is local and does not affect the safety.
Handling in the safety assessment SR-Site

Since the process cannot occur and the consequences are negligible if it should occur, the process
is neglected.

6.4

Chemical Processes

6.4.1

Advective transport of species

Overview/general description

In this context, advection refers to transport of any forms of additional matter, e.g. ions, molecules
or colloids, with porewater flow. The transport direction is thereby principally from volumes of highwater pressure to volumes of lower pressure. The process leads to redistribution of solutes in theporewater and thus affects the porewater composition. The advection is closely related to water flowin the central area, which is described in Section 6.2.2.
The backfill material in the central area consists of only crushed rock with a fuller distribution. The
hydraulic conductivity is thereby relatively high (>2·108 m/s). Advective flow will thereby be thedominant transport process in these repository components.
Table 6-7 summarises how the process influences and is influenced by all central area variables andhow these effects are treated in SR-Site.
Boundary conditions

The external boundary condition for this process is the hydraulic and concentration gradient acrossthe various repository components.

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Table 6-7. Direct dependencies between the process “Advective transport of species” and thedefined central area variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Central area geometry Yes, major impact

Included in hydrogeological model

Process influence on variable
Influence present?
Handling of influence(Yes/No) Description(How/Why not)No

Central area poregeometry

Yes, the density will
Included in hydrogeoinfluence the hydraulic logical model
conductivity

Temperature

Affects viscosity,
Indirectly throughtemperature induced
gradients andhydraulic conductivity.
Freezing is of major
importance

Neglected

No

Water content

Yes, by definition

Saturated conditions
assumed for transport
calculationsNo

Gas content

Yes, dissolved gas istransported with flow.
Indirectly through flow
properties due to gas
pressure, gas dissolution and precipitation.

Saturated conditions
assumed for transport
calculations

Yes, dissolved gas
transported away

Saturated conditions
assumed for transport
calculations

Hydrovariables(pressure and flows)

Yes, by definition

Included in hydrogeological model

Yes, density effects
may induce flow

Neglected

Stress stateNo, or insignificant

No, insignificant

Central area materials Yes, indirectly through– composition andsorption, precipitation
content
and dissolution reactions

Included in hydrogeological model

No , indirectly throughsorption, precipitationand dissolution reactions

Central area porewater composition

Yes, by definition

Only transport of water Yes, by definition
and radionuclides is
considred

Structural and straymaterials

No ,indirectly throughdissolution of concrete, reinforcement
etc.

Only transport of waterand radionuclides is
considred

No ,indirectly throughdeterioration of thematerials

Model studies/experimental studies

See Section 6.2.2
Natural analogues/observations in nature

See Section 6.2.2
Time perspective

Advective flow transport takes place on all timescales.
Handling in the safety assessment SR-Site

See Section 6.2.2
Handling of variables influencing this process:See Section 6.2.2
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Uncertainties

See Section 6.2.2

6.4.2

Diffusive transport of species

Overview/general description

A basic description of this process is given in Section 3.5.3. Since advection is expected to dominate
in the crushed rock of the central area, see Section 6.4.1, diffusion is neglected.
Influence on variables of the central area: No influence on these variables is expected.
Boundary conditions

There are no particular boundary conditions to discuss for this process.
Model studies/experimental studies

Not relevant.
Time perspective

Not relevant.
Natural analogues/observations in nature

Through observations in nature (as well as in laboratory systems) it is well established that diffusion
can be neglected in comparison to advective transport under the conditions (hydraulic conductivity
and gradient) relevant for the central area; see e.g. /Grathwohl 1998/.
Handling in the safety assessment SR-Site

The process is neglected, since advective transport will dominate.

6.4.3

Sorption

Overview/general description

Radionuclides and major ions in percolating or porewater of the central area can be bound to thesurfaces of the crushed rock material in several ways (see below). These processes are henceforthsummarised under the term sorption and can affect the mobility of most radionuclides. The transportof major ions in the crushed rock is also influenced by these processes, which may influence the rateof weathering, see Section 6.4.3
In fresh crushed crystalline rock, the principal (primary or rock-forming) minerals offering surfaces relevant for sorption are micas. In comparison, the other major primary minerals in typical crystalline rock(quartz and feldspars) have a significantly smaller specific surface area and/or a lower concentrationof relevant surface sites per unit mass. Upon exposure of the fresh surfaces to water, further surfacesof secondary minerals may be created as a result of weathering, such as clay minerals from feldspars
or, under non-reducing conditions, Fe(III)-hydroxides from Fe in the crystal lattice of different primaryminerals. Under reducing conditions, Fe(II/III) minerals (magnetite) or Fe(II) phases, such as siderite,
may form. In weathered material, sorption will probably be dominated by the secondary minerals.
The principle type of surfaces offered by these minerals and the respective possible sorption reactions(surface complexation, ion exchange) are described in Section 3.5.5. Both mica as well as clay minerals
are 2:1 layer silicates that consist of octahedral alumina sheets sandwiched between tetrahedral silica
sheets, 1:1 clay minerals (kaolinite) may also occur. They differ in terms of layer charge and type ofinterlayer cation: Micas possess a high layer charge compensated by K+. The interlayer distance issmall and the layers are not expandable. The surface area and CEC are therefore smaller than in caseTR-10-47277

of clay minerals. Clay minerals have a lower layer charge, with large hydrated cations (especially Na)
occupying the interlayer space. Typically, this renders clay minerals expandable (exceptions are the 1:1
clays and 2:1 clays with a collapsed structure, such as illite), with a large interlayer surface area andhigher CEC. Both minerals feature the same two types of distinctly different types of surfaces where
two main types of sorption take place /e.g. Sposito 1984, Stumm and Morgan 1996/: i) planer siloxane
or “layer” surface carrying a permanent charge and ii) variable charge, oxide-type edge surfaces with
exposed AlOH and SiOH functional groups. The surfaces of Fe(III)-hydroxides and -oxides are similar
to the edge surfaces of clays, but the exposed FeOH-groups typically show a higher affinity for many
metal ions and reactive anions than clay edge surface groups /Dzombak and Morel 1990/. Further,hydrous forms of these minerals may (partly) re-crystallise over time, which can lead to the (irreversible) incorporation of adsorbed species. Because Fe is a redox-sensitive element, adsorbed species may
also undergo redox reactions on these surfaces.
Dependencies between process and central area variables

Table 6-8 shows how the process influences and is influenced by all central area variables.
Influence on variables of central area, ramp and shaft: Exchange reactions of major cations directly
influence the composition of the exposed surfaces of the crushed rock material as well as the porewater composition. It is also possible that changes in the porewater chemistry may induce changes inthe surface composition of the crushed rock material through mineral dissolution and precipitation.
However, bulk properties, including hydraulic conductivity, are not expected to be significantly
altered by sorption and ion-exchange processes.
Influence of the composition of crushed rock and porewater: The two most important variables withregard to radionuclide sorption and exchange of major ions are the composition of porewater and of
the crushed rock material. In case of the latter, sorption will be determined by the surface composition of the crushed rocks rather than by their bulk composition. I.e. sorption will be closely linked toweathering reactions. Based on the specific surface areas and sorption properties that are known fora range of relevant primary and secondary minerals, it appears to be likely that sorption will increase
as weathering of the crushed rock material progresses. The porewater composition is the result ofgroundwater-rock interaction (ion exchange, mineral equilibria) and, in turn, exerts a major influence
on radionuclide sorption. Of particular importance are pH, pe, and the concentration of important
ligands and competing cations. It can be expected that the mineralisation of percolating porewater
will vary as a function of contact time with the rock surface (distance infiltrated) and as a function of
the rock/water ratio or infiltration rate.
Kd is generally independent of the rock/water ratio per se. On the other hand, it can be expected that thesorption of radionuclides that mainly take part in ion-exchange reactions (Cs, Sr, Ra) will decrease withincreasing mineralisation of the porewater. The sorption of other radionuclides (actinides, lanthanides,
transition metals) is likely to increase with increasing pH (within the range of natural waters) and to
decrease with an increase in the concentration of complexing ligands, such as dissolved carbonate.
Influence of temperature: As pointed out in Section 3.5.5, not enough sorption data as a function oftemperature are available to clearly evaluate temperature effects. While an increase or decrease of
the temperature (in comparison to room temperature) is potentially of importance for sorption, theeffects within the temperature range expected in the repository are deemed to be covered with good
margin by other uncertainties.
Boundary conditions

There are no particular boundary conditions to discuss for this process. The relevant boundary conditions in order to treat the process quantitatively are those of the transport processes that control theexchange of solutes between the porewater in the crushed rock material and the water in its adjacentcomponents, i.e. mainly the boundary conditions of the process advection.

278

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Table 6-8. Direct dependencies between the process “Sorption (including exchange of majorions)” and the defined central area variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Central area geometry No

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description(total mass of crushed No
rock considered)Central area poregeometry

No (not on macroscopic scale)No

Temperature

Yes

Temperature effects
neglected. Variations
within expected
temperature interval
are covered by otheruncertainties insorption data.

No

Water content

Yes (quantitative effectof solid/water ratio).
Qualitatively possibly
also indirectly throughporewater composition

Sorption is expressed
as Kd, whereby
solid/water effect iscancelled out.

No

Gas content

No, but in case ofreactive gases:
indirectly throughporewater composition

No

Hydrovariables(pressure and flows)No, but possibly
indirectly throughporewater composition

No

Stress stateNo

No

Central area materials Yes– composition andcontent

Composition data
Yes, affects surface
used for estimate ofcomposition
sorption (see Geosphere process report,
Sorption process /SKB
2010i/)

Effect neglected (seeGeosphere processreport, Sorption process/SKB 2010i/)and section
on alteration)

Central area
porewater
composition

Yes

Composition data
Yesused for estimate ofsorption (see Geosphere process report,
Sorption process /SKB
2010i/))

See Section 6.4.4, radionuclide concentration
and release are calculated (see Geosphereprocess report, Sorption
process /SKB 2010i/)

Structural and straymaterials

Yes

Sorption on straymaterials is conservatively excluded in
SR-SiteNo, but indirectly
through porewatercomposition/mineralequilibria

Model studies/experimental studies

Ion exchange as well as surface complexation are well-established processes and quantitative thermodynamic models exist for calculation of equilibrium states. The relation of these sorption processes tothe different surfaces available on fresh and weathered crushed rock has been discussed above.
The effect of sorption on radionuclide transport has been discussed for clay systems in Section 3.5.5;
this discussion is equally valid for the crushed rock material of the central area. On the other hand,
effect of sorption on the properties of the crushed rock material can be neglected.

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279

Time perspective

An ion exchange process in itself is normally very fast, so with respect to the distribution of major
cations, the rates of other processes (dissolution/formation of minerals, transport of dissolved species) will control any apparent time dependency.
Surface complexation, which is the main sorption process for most radionuclides (see Section 3.5.5
and e.g. /NEA 2005, Dzombak and Morel 1990/), is a slower process than ion exchange; it may need
up to several tens of days to reach equilibrium. The timescales that are of interest are thus dependent
on the travel time of percolating water through the crushed rock material.
Natural analogues/observations in nature

Various well-researched examples exist regarding the interaction of radionuclides and major ionswith unconsolidated, weathered rock /Davis et al. 2002, Davis and Curtis 2003/. These examples
clearly show that sorption is dominated by and strongly dependent on the nature of the particularweathering products and on the composition of the contacting groundwater.
Handling in the safety assessment SR-Site

Sorption in the central area is pessimistically neglected in the transport modelling for the geosphere,
see Geosphere process report, Sorption process /SKB 2010i/ for the justification.
Uncertainties

Uncertainties in terms of handling the process in SR-site are discussed in Geosphere process report,
Sorption process /SKB 2010i/, data uncertainties in /Crawford et al. 2006/ and in the SR-Site Data
report /SKB 2010l/. Uncertainties in mechanistic understanding are also discussed in Section 3.5.5.

6.4.4

Alteration of the central area backfill

Overview/general description

The geochemical evolution due to the interaction of groundwater with the crushed rock fillingthe central area is not expected to be very different than what it is expected from the groundwaterinteraction with the rock matrix or fracture-filling minerals Geosphere process report, processes:
Reactions groundwater/rock matrix and Dissolution/ precipitation of fracture-filling minerals, /SKB
2010i/. The processes driving the geochemical evolution of these systems are mainly related to thedissolution – precipitation of minerals. Some of these reactions are relatively fast, as dissolution– precipitation of carbonate minerals and other fracture-filling minerals; whereas other reactions
occur very slowly, limited kinetic dissolution rates (i.e. dissolution of most granite-forming silicates)
and irreversible. All these reactions will control the composition of porewater that potentially canreach the near field of the repository system, especially considering that thr central area may be apreferential pathway for groundwater (see Section 6.2.2).
Dependencies between process and central area variables

Table 6-9 shows how the process influences and is influenced by all central area variables.
Central area geometry: The central area geometry will determine the total mass of minerals able toreact with porewater. In addition, these system parts will connect with water conducting fractures aswell as with other parts of the repository, thus allowing that evolved porewater from the central area
and the ramp and shaft can interact with these other system parts.
Central area pore geometry: The pore geometry in the central area will affect the geochemical
behaviour of the system in several ways. First it will determine the solid/water ratio, affecting thereactivity of the system, i.e. more water is able to dissolve more minerals. The pore geometry will
also affect the hydraulic conductivity, and therefore, changes in groundwater flow velocity will affectthe dissolution – precipitation reactions, especially those reactions kinetically driven. Finally, thepore geometry affects the reactive surface of minerals, as rock fragments will have a larger reactive
surface than surrounding rock matrix.

280

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Table 6-9. Direct dependencies between the process “Alteration of the central area backfill” andthe defined central area variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Central area geometry Yes, important frommass-balance pointof view, connects
to water conducting
fractures as well as toother repository parts

Included in modelling/evaluation(mass-balance)

No

Central area poregeometryYes. Gives solid/liquid
ratio
Indirectly throughhydraulic conductivity

Included in modelling

May be affected bydissolution precipitation

Temperature

Yes, temperatureeffect on thermodynamic constants andkinetic rates.

Neglected since thetemperature variationsin the central area will beminorNo

Water content

Yes, reactions only
take place in thesaturated domain.

Neglected, onlyNosaturated conditions are
considered

Gas content

No, but indirectly
through water composition by affecting content
of dissolved gases

Hydrovariables(pressure and flows)

Yes, determines theturn-around rate ofwater

Stress stateNo

Potentially, if a gas
phase would form

Included in modelling

Neglected, sincehydraulic conductivity
always will remain
high

Neglected, since
the formation of agas phase is of noconcern

No

No

Central area materials Yes– composition andcontent

Included in modelling

Yes

Included in modelling

Central areaYesporewater composition

Included in modelling

Yes

Included in modelling

Structural and straymaterials

Included in modelling

yes

Yes

The increase in reactive surface area of minerals and porosity will mainly affect the dissolution kinetic
rates of both rock-forming minerals and fracture-filling minerals. As the present-day groundwater at
repository depth has been interacting with all these minerals for a long period of time, increasing thesurface area of such minerals will have a minor effect on the chemical composition of the groundwater. On the other hand, it is expected that initially, after filling the tunnels and shaft with crushedrock, all the void spaces will be filled with air, leading to an increase in the dissolution rate of thoseminerals able to oxidise (i.e. pyrite and iron-bearing silicates). Then, the increased mineral surface
areas and the larger water-to-solid ratios will help to consume the oxygen faster, thus neutralising thepossible effect of having oxidant waters at the repository depth.
Although the dissolution – precipitation of minerals can modify the pore geometry, the high initial
total porosity is not expected to change significantly due to this process.
Temperature: Thermodynamic constants of chemical reactions are highly dependent of the temperature. However, temperatures in the central area are not expected to vary significantly, and therefore,
minimal effects on the geochemical processes are foreseen.

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Water content: Geochemical reactions are only expected to occur once this system part is saturated.
Once the system is water saturated the effect of this variable will take place as described for thecentral area pore geometry variable.
Gas content: Gases can form or dissolve through chemical reactions, although their partial pressures
are considered in the models, it is not expected that gases can form in these system parts. If gases
from other system parts reach the central area, then they can dissolve in the relatively large volume
of water, thus affecting the dissolution – precipitation of minerals.
Hydrovariables (pressure and flows): The high groundwater flow velocities expected in the central
area and shaft will clearly affect the dissolution – precipitation reactions in the system, as porewater
will be replaced relatively fast and dissolution can continue as equilibrium will be hardly achieved.
Stress state: This variable has no effect on the geochemical evolution process.
Central area materials – composition and content: This is one of the most relevant variables
influencing this process. The type and amount of minerals is essential to determine how their dissolution – precipitation behaviour will affect the chemical evolution of the system, modifying boththe porewater composition and the minerals present in the system (i.e. newly precipitated minerals
and amount of existing minerals).
Central area porewater composition: The porewater composition will determine which of theminerals in the system will dissolve or precipitate, and thus the dissolution – precipitation of theseminerals will modify the chemical composition of porewater. Moreover, changes in surface waters,
as infiltration of ice-melting derived water, will very rapidly affect the chemical composition of theporewater, and hence the chemical evolution of the system.
Structural and stray materials: An additional aspect that can affect the geochemical behaviourof the crushed rock backfill is the presence of structural and stray materials. The time-length fromthe excavation of the repository until the use of excavated material as backfill will be relatively
large. In this time frame it is expected that some biological activity take place in the depots where
excavated material will be stored. Therefore, a certain amount of organic matter (i.e. dead animals
and plants) will be present in the backfill material. Also the presence of other stray material (i.e.
rejected metallic components) cannot be disregarded. The presence of such materials in the backfill
can contribute to the geochemical evolution of the system, especially considering the likely activityof bacteria. Thus, the presence of organic matter can contribute to maintain the system under reducing
conditions via the activity of sulphate-reducing bacteria and/or iron-reducing bacteria, or even,in the case of oxygen intrusion (i.e. ice-melting waters) due to the activity of aerobic bacteria see
also Section 6.4.9. In this later case, it is worth noting that due to the larger hydraulic conductivity
and porosity (compared to the surrounding rock) this will be a preferential path for surface waters,
including ice-melting water during glaciation periods.
Boundary conditions

There are no particular boundary conditions to discuss for these processes. The relevant boundary
conditions in order to treat the process quantitatively are those of the transport processes that control
the exchange of solutes between the backfill water and the water in the adjacent geosphere and surface
waters through the top seal above, i.e. the boundary conditions of the processes diffusion and advection.
Model studies/experimental studies

No specific studies of chemical processes in the crushed rock backfill of central area, ramp and shaft
have been done. However, see also the description of the processes: Reactions groundwater/rock matrix
and Dissolution/precipitation of fracture-filling minerals in the Geosphere process report /SKB 2010i/.

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Natural analogues/observations in nature

No specific natural analogues have been investigated concerning the geochemical processes expected
in excavated tunnels and backfilled with crushed rock. However, the processes will be equivalent tothose occurring in organic matter-bearing aquifers, as gravel aquifers or backfilled abandoned mines
where organic matter is present /Postma and Jakobsen 1996, Jakobsen and Postma 1999, Banwart
1999, Hartog et al. 2004, Massmann et al. 2004, Park et al. 2006, Jakobsen and Cold 2007/. In thesesystems, reducing conditions are maintained through different types of bacterial mediated organic
matter degradation reactions. The specific type of organic matter degradation depends on the type andamount of organic matter as well as on the dominant electron acceptor in groundwater.
Time perspective

The processes considered are of relevant importance at different timescales. In early stages after
repository closure (once the crushed rock backfill is emplaced) the processes involving oxygen
consumption from trapped air will be the most relevant. Once the reducing conditions are restored,
the water – rock interaction processes will continue for the whole repository life-span. A special
case is related to the intrusion of ice-melting derived water and penetration of oxygen to repository
depths. In this case, the processes involving oxygen consumption will be constrained to the glaciation – deglaciation timescales.
Handling in the safety assessment SR-Site

The geochemical evolution in the crushed rock backfill of the central area is evaluated through areactive transport equilibrium or mass-balance model, where the processes in this system component
are implemented together with transport processes (advection). Two different time periods have tobe evaluated, one for the initial times (during the water saturation after backfilling until a chemical
stationary state is reached) and the other considering the response of the system to the intrusion ofice-melting water (glaciation – deglaciation timescales) from the top and assuming the system isfully saturated.
Initially, the system is assumed to be unsaturated and the pore space filled with air and thereafter thesystem gradually saturates with present-day groundwater from the tunnel walls and surface waterfrom the surface. The evolution of the system has to consider the consumption of dissolved gases
in equilibrium with atmosphere conditions (i.e. pCO2= 10-3.5 bar and pO2= 10-0.7 bar) and how thesedissolved gases affect the dissolution – precipitation of rock-forming and fracture-filling minerals
present in the system. Moreover, the effect of organic matter degradation has to be considered,first through oxidation reactions involving aerobic bacteria and, once oxygen has been consumed,
through reactions involving sulphate-reduction or iron-reduction bacteria. The evolution of thesystem has to be assessed in order to evaluate the oxygen consumption and to determine the stationary redox state of the system.
The second part of the system evaluation has to consider a glaciation – deglaciation time period,
where oxygen-rich ice-melting water can intrude the system from the surface. In this case, fullsaturation can be assumed (i.e. the output conditions from the previous case) and the water-rockreactions consuming oxygen, including organic matter degradation, implemented in the model. This
will allow assessing the penetration depth of oxygen and the potential impact on the repository, as
well as the redox buffering capacity of the system.
Handling boundary conditions:Two main boundary conditions have to be considered. One is the transport conditions from tunnel
walls into the backfilled system and the other is the composition and flow conditions of surface
water entering into the system from the surface.
Transport conditions have to be handled according to the process advection. Initial chemical compositions of both groundwater and surface water (including ice-melting derived water) will be based on
the hydrogeological modelling, see Section 6.2.2.

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Handling influences between variables and process:
Groundwater flow and composition are defined as part of the boundary condition as stated above.
The volume and porosity of the central area is considered and fracture and matrix minerals are fully
included in the modelled system, as well as stray materials. The saturation phase is neglected in thegeochemical model, since it is short in duration and will have limited impact.
Handling coupling to other processes:
The model here considered only includes the coupling with transport processes as well as with themicrobial activity.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

All the references cited are peer reviewed papers except the Geosphere Process Report /SKB 2010i/,
which is a SKB Quality Assured report.
Uncertainties

Uncertainties in mechanistic understanding
Some critical uncertainties remain concerning the mechanistic understanding of the processes that control
the redox state of the system. These are mainly related to the degradation of organic matter. Although ageneric organic matter and mass-balance can be used (CH2O), the lack of knowledge on the mechanistic
degradation of the specific organic matter could introduce a large uncertainty to the system.
In addition, most of the secondary precipitation/dissolution processes that are temperature driven
are thermodynamically controlled. Hence, there is no need for a detailed mechanistic understanding.
However, some of the silicate transformations are kinetically controlled and their mechanistic
understanding in the considered conditions is poor. This is mainly related to the dissolution ofrock-forming silicates and their transformation into clay minerals. Although these reactions occur at
a very slow rate, their effect on the overall geochemical evolution is not yet clear.
Model simplification uncertainty in SR-Site
There are several rock-forming minerals, such as silicates other than silica, which tend to dissolve
at a very slow kinetic rate and irreversibly. Other silicates (usually clay minerals) precipitate at
similarly slow rates. However, the dissolution-precipitation processes involving these two groupsof minerals could affect the pH evolution of the system as well as the chemical composition of thegroundwater, thus affecting the chemical long-term evolution of the system.
Modelling of chemical reactions driven by microbial activity generally makes use of kinetic rates fororganic matter degradation. The computed rates are generally fast as compared with the timescale ofreactions with rock-forming minerals. The results of the modelling are strongly coupled to the microbial processes considered and to the values of the parameters used. The introduced simplifications,such as considering a generic organic matter (i.e. CH2O), could result in differences with respect toactual rates of oxygen consumption.
Input data and data uncertainty in SR-Site
Another source of uncertainty relates to the quantitative estimation of sensitive parameters
associated with microbial kinetics, such as growth rate and half saturation constants in the kinetic
expressions. Such parameters are both microbe dependent and environment dependent. Usually,parameter values are obtained in laboratory conditions by means of physiological (i.e. growth) in
vitro experiments which cannot be fully representative of repository conditions. Results of current insitu microbe investigations, as well as model calibration based on field experimental data could be
helpful for the up-scaling of key microbial parameters. Nevertheless, model calculations involving
microbial processes, even if they are calibrated against experimental field data, are necessarily of
qualitative value.

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6.4.5

Alteration of concrete components

Overview/general description

In the central area, cement-bearing materials can be present as construction concrete and grout canremain in the system after backfilling this system part. The alteration of such materials can affectthe system in two different ways: 1) as a consequence of its degradation the porosity can increase,thus creating favourable pathways for groundwater to enter into the system, and 2) the increase ofalkalinity due to the interaction of groundwater with these materials can enhance the dissolution rateof smectite (Section 4.4.7 in Backfill chapter and Section 3.5.9 in Buffer chapter), weakening theeffectiveness of the bentonite backfill in the deposition tunnels as a hydraulic barrier.
The processes associated to the alteration of concrete components are equivalent to those considered
for concrete in the tunnel plug (Section 5.4.4).
Handling in the safety assesment SR-Site

The degradation of the concrete in the central area is assumed to be equivalent with the degradationof the concrete part of the tunnel plug (Section 5.4.4). No separate modeling will be done.

6.4.6

Aqueous speciation and reactions

Overview/general description

This process is in general the same as the corresponding process in the buffer (Section 3.5.7.)
Handling in the safety assessment SR-Site

The process is included in a separate modelling task of the central area geochemical evolution. Thisis further described in Section 6.4.4.

6.4.7

OsmosisOverview/general description

The general process is described for the buffer system in Section 3.5.8. In the crushed rock the effectwill be insignificant since the material itself will not have a semipermable behaviour.
Handling in the safety assessment SR-Site

The process is neglected since osmotic effects will be insignificant in the crushed rock material.

6.4.8

Corrosion of steel components

Overview/general description

Steel in the reinforcements in the central area will not be stable under repository conditions.
Corrosion of steel is described in the Canister Process Report /SKB 2010k/. The main resulting
products will be some type of iron-oxide and hydrogen gas. Neither of the corrosion products is
assumed to have any negative impact on the repository performance.
Handling in the safety assessment SR-Site

The total amount of steel is included in all mass-balance calculations of effects of microbial activity,
see Section 6.4.9. The corrosion rate of steel reinforcements will pessimistically be neglected.

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6.4.9Microbial processes

Overview/general description

Micro-organisms interact with their surroundings, and they commonly have a significant effect on thegeochemical record of their environment /Madigan et al. 2008/. From a microbiological perspective,
the central area environment will constitute an environment with mixed characters from two otherwell defined environments of the repository concept; the geosphere and the buffer. The central area
environment will, however, have some characteristics that significantly distinguish them from the twoother mentioned environments. One is the presence of organic material in structural and stray material. Another is the gas/water interface that will develop during the saturation process and hydrogenformation from anaerobic corrosion of rock bolts and nets. The effects of microbial processes on thechemical stability of the geosphere have been thoroughly discussed /SKB 2010i-microbial processes/.
Similar effects can be obtained in the backfill, but the differences in the variables “structural and straymaterials and the gas content” compared to the geosphere may introduce some additional effects thatmust be considered.
Plasticisers that will be added to grout and shotcrete commonly contain organic polymers of varying
compositions, e.g. polycarboxylate polymers. The amounts to be used will be significant, in the rangeof 100 tonnes or more as exemplified in /SKB 2010g/. Plasticisers introduce a significant uncertainty,
because their composition and solubility, and thereby their availability to microbes, are not well known.
In cases where a grouting activity fails and the grout do not solidify properly; large amount of organiccarbon will be available for microbial growth. Such growth can occur in the grouted aquifers and in thecentral area, ramp and shaft on places where groundwater from grouted sites seep through the tunnel
walls. There are two main influences from the structural material variable on microbial processes.
1. Microbial sulphate reduction during microbial growth on plasticisers will result in sulphide. The
sulphide may accumulate as iron sulphide in slime formations on the shotcrete and on rock tunnel
walls. During the open phase, oxygen will oxidize the sulphide and sulphuric acid can form. Localacid attacks on the shotcrete will occur. This effect will diminish after backfilling, when oxygen isconsumed, but it may introduce a safety concern during the open phase.
2. Excess amounts of organic material in aquifers will trigger growth of opportunistic microorganismsthat will increase in numbers. Several groundwater microorganisms produce complexing agentsthat very efficiently mobilize trace elements, including many radionuclides /Kalinowski et al. 2004,
2006, Johnsson et al. 2006, Essén et al. 2007, Moll et al. 2008/. Given the large amount of addedorganic carbon where grouting have been performed, it is possible that the microbial production ofcomplexing agents will be significant which may notably reduce the retardation effects accounted
for by the rock and the rock matrix.
Dependencies between the process and the tunnel plug variables

Table 6-10 shows how the process influences and is influenced by all central area variables.
Boundary conditions

The access to organic material, e.g. stray materials and construction material such as organic additions
to the grout and shotcrete, hydrogen from anaerobic corrosion of iron, organic material from othermaterial that cannot be stripped or cleaned out and methane and electron acceptors from the geosphere
will set the limits for the microbial processes in the central area, ramp and shaft. Given the large
amount of plasticizer that will be added, access to organic carbon can be assumed for a very long time.
Model studies/experimental studies

None.
Natural analogues/observations in nature

No directly relevant natural analogues or observations are known.

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Table 6-10 Direct dependencies between the process “Microbial processes” and the definedCentral area variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Central area geometry No

No

Central area poregeometryNo

Growth of microbes
This effect is not
generate biomass that considered in SR-Site
may clog pores

Temperature

Yes, microbialactivity increases withtemperature

Water content

Yes, availbility of water Saurated conditions
assumedd

Gas content

Indirectly throughwater content

Hydrovariables(pressure and flows)

Yes, water is neededThis effect is not
for growth of microbes considered in SR-Siteand growth on hydrogen and methane ispossible. Pressure is
not importantYes, microbes may
form methane andhydrogenStress stateNo

No

Central area materials Yes, the content of– composition andorganic carbon andcontent
microbes in different
backfill clays varies

This effect is notconsidered in SR-Site

This effect is notconsidered in SR-Site

No

This effect is notconsidered in SR-Site

No

Central area
porewater
composition

Yes, microbes areThis effect is notsensitive to theconsidered in SR-Site
geochemical situation,and may utilise porewater components for
growthYes, microbial activity
may change thecomposition, e.g.
by consumption of
methane, sulphateand organic carbon
and production ofsulphide and carbon
dioxide.

This effect is notconsidered in SR-Site

Structural andstray materials

Yes, organic
components in grout
and hydrogen fromanaerobic iron corrosion will constitutesources for microbial
growth.

Yes, microbes may
oxidise organic
components in thismaterial.

This effect is notconsidered in SR-Site

Production of
microbial biofilms,
acid and complexing
agents from organiccarbon in grout isconsidered in SR-Site

Time perspective

Microbial processes will start when water becomes available. The process rates will be largest inareas where organic carbon and hydrogen are available. Elevated microbial process rates, comparedto the geosphere, will continue until all sources for growth have been consumed.
Handling in the safety assessment SR-Site

Microbial reactions will be considered in the description of the geochemical evolution in the central
area. This is done with mass-balance considerations.
Handling of uncertainties in SR-Site
Microbial degradation of organic carbon and consumption of hydrogen from metal corrosion will
be considered in the modeling of the geochemical evolution repository. This will be done through a
combination of thermodynamic, kinetic and mass balance models/assumption.
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287

Uncertainties in mechanistic understanding
The amount of hydrogen from anaerobic iron corrosion and the amount of organic material fromgrouting that becomes available for microbial growth and activity is unknown.
Model simplification uncertainty
Not relevant since no quantitative model for microbial processes in the backfill is used.
Input data and data uncertainty
Not relevant since no quantitative model for microbial processes in the backfill is used.

6.4.10 Colloid release
Overview/general description

Fractures intersecting the central area mean that rigid volumes restrictions are not present everywhere,
and that small-sized material may enter the fractures. Separation of individual particles from theintroduced material may then take place (dispersion).
The behaviour of the particles will be dominated by thermal motion if the particles are small enoughi.e. they have colloidal properties. The individual colloidal particles may then be transported awayfrom the tunnels into fractures by diffusion or by groundwater flow in the fractures.
The magnitude of the process is governed by the size distribution of the crushed rock. The material
can be chosen in such a way that colloid formation may be neglected.
Handling in the safety assessment SR-Site

The process is neglected due to the very small possible effect, also for the unlikely case that all
particles of colloidal size are transported away. No credit is taken for the fine particles under any
circumstances and the hydraulic conductivity is assumed to be high under all circumstances.

6.4.11 Radiation induced transformation
Overview/general description

The central area is too far away from the canisters to be affected by the g-radiation. The effect ofa-radiation from radionuclides from damaged canisters can also be neglected, since the levels will be
extremely small (see Section 3.5.12).
Handling in the safety assessment SR-Site

The process is neglected.

6.5Radionuclide transport processes

6.5.1

Speciation of radionuclides

Overview/general description

See corresponding process description for the buffer, Section 3.6.1
Handling in the safety assessment SR-Site

This process is indirectly handled through the selection of parameters for radionuclide transport in
the rock. In general, the handling of the process is the same as for the corresponding process in thegeosphere, see process transport of radionuclides in the water phase in the Geosphere Process Report/SKB 2010i/.

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6.5.2

Transport of radionuclides in the water phase

Overview/general description

The central area is represented as a volume with high hydraulic conductivity and high porosity in thegeosphere transport model.
Handling in the safety assessment SR-Site

Delay of radionuclides in the central area is pessimistically neglected. See process transport ofradionuclides in the water phase in the Geosphere Process Report /SKB 2010i/.

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7

Processes in the top seal

7.1

Thermal processes

7.1.1

Heat transport

Overview/general description

At the bottom boundary of the top seal, the rock peak temperature occurs about 1,000 years after
deposition, and will be between 5 and 7°C above the initial temperature at 100 m below groundsurface, and correspondingly smaller at shallower depths /Hökmark et al. 2009/. The volume of the
top seal is small enough for its thermal evolution to be completely controlled by the slow and modestchange in the temperature of the surrounding rock mass, regardless of the heat transport properties of
the top seal materials. The modest temperature variations cannot influence the variables in any way
that impacts on the performance of the top seal. Therefore, the process heat transport in the top seal
is not addressed further.
Handling in the safety assessment SR-Site

The process is neglected.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The reference /Hökmark et al. 2009/ will undergo a documented factual and quality review.

7.1.2

Freezing

Overview/general description

The top seal of the shafts from ground surface down to approx. 100 m depth will be filled with big
boulders. The size of the boulders will be in the range of metres. During freezing periods, the water
between the boulders will be frozen and the system will have temperatures well below 0°C. Whenwater undergoes phase change to ice, energy is released (333 J/g) and volume expansion takes place(approximately 9%). In a porous medium containing water in the pores, water will turn into ice if
the temperature is low enough. Not all the water in the pores will freeze at a given temperature. The
amount of water in liquid phase is known as unfrozen water. The evidence establishing the existenceof a continuous, unfrozen water phase that separates ice from the mineral matrix in porous media
like soils is widely accepted /Anderson 1966, Miller 1963, Nersesova and Tsytovich 1965, Williams
1964/. The amount of unfrozen water depends on various factors /Anderson and Tice 1972/ among
which particle mineral composition, specific surface area of particles, temperature, presence ofsolutes and pressure are the most significant.
In a very coarse-grained material, like boulders, the amount of unfrozen water is negligible due tothe very small specific surface in the system and corresponding big voids between particles. In relation to the salt content of the water, the porewater turns into ice at temperatures close to 0°C. As theunfrozen water content is negligible, this means that all water freezes to ice at the same temperature,i.e. close to 0°C.
During freezing, water expands. In a system with high hydraulic conductivity, the expansion duringfreezing will expel water out of the pore system at the same rate as water expands during the ice
forming process. Neither volume increase nor stress increase of the system will take place as longas water can be expelled in an open system. No ice lensing with accompanying frost heave will takeplace, as the unfrozen water content will be negligible and thus no suction will be developed.
Frost penetration rates in ground, relevant for frost penetration due to low temperatures at the groundsurface, are at most in the range 2-3 m/year.

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A freezing ground can be regarded as an open system if the hydraulic conductivity is greater than
10-5 m/s (conservative value) for normal freezing rates. A very coarse-grained material like boulders
will have a hydraulic conductivity of about 10-1 m/s and therefore no frost phenomena will be present.
Particles of rock materials will be affected by freezing if fractures are present in the particles before
freezing and thawing and the particles stay in a water-saturated environment. Water having penetrated
into open fractures and voids in the particles will expand, and if the fractures are small, or if freezing
takes place in a direction that closes the entry of the fractures, an open system will not be present in theparticle itself. High pressures can build up thus causing cracking of the particle. The phenomenon is
known from nature as frost cracking. The process goes on until solid parts of rocks, without fractures,
remain. Particles without cracks will not be affected by freezing. This is also well known from nature.
The boulders used as top seal will thus be affected as they to some extent will have cracks. The
material will be successively finer and the end result of the particle size will be dependent on theamount of cracks in the material used. Boulders of granite or similar hard rock will not end up in asmaller material than gravel and stones. If the boulders initially are free of fractures, freezing will
not affect the material.
Dependencies between process and top seal variables

Table 7-1 shows how the freezing process influences and is influenced by all top seal variables.
Table 7-1. Direct dependencies between the process “Freezing” and the defined top seal
variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Top seal geometry

Determines the total
The effects of freezing No
volume of water which in the top seal can be
can be frozen
neglected based on
the discussion in thissectionTop seal poregeometry

Yes

Yes, redistribution ofmaterial

Temperature

Obvious

No

Water content

Yes, but freezing will
always occur after fullsaturationNo

Gas content

Not relevantNo

Hydrovariables(pressure and flows)

The freezing point isaffected by pressure

No flow in frozen
water

Stress state

No

Yes

Top seal materials– composition andcontent

Yes, the clay content
of the backfill affects
the freezingNo

Top seal porewatercomposition

The freezing point ofwater is affected bysalinity

To a limited extent

Structural and straymaterials

No

No

292

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

The effects of freezingin the top seal can beneglected based on
the discussion in thissectionTR-10-47

Boundary conditions

When the top seal is frozen, no stress increase or expansion of the system will be present as long as
the frost penetration in the material is less than 100 m per year. In natural freezing, frost penetrationis less than in the order of 2-3 m/year.
The particles will be affected by freezing and particle size of the boulders, after hundreds of freezing
cycles, will be in the range of stones.
Model studies/experimental studies

No specific studies on freezing of top seal materials have been done.
Natural analogues/observations in nature

The described phenomena are well known from observations in nature.
Time perspective

The process takes place during cycles of permafrost.
Handling in the safety assessment SR-Site

Due to the high hydraulic conductivity of the top seal, no stress increase or expansion will takeplace. The process can therefore be neglected.
Particles will be affected and broken into smaller pieces. The final condition will be particles
of stone size. This can have an effect on the accessibility to the repository.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The references in this section are peer-reviewed articles from the open literature or engineering
handbooks.
Handling of uncertainties in SR-Site

The uncertainty is related to the size of the boulders after a large number of freezing and thawing
cycles. The end product can be bigger than stones but not very much smaller. How the size of theparticles in the top seal affect the accessibility to the repository has to be evaluated.

7.2

Hydraulic processes

7.2.1

Water uptake and transport under unsaturated conditions

Overview/General description

Water transport in the crushed rock filled in the top before saturation is mainly governed by the
inflow rate and inflow distribution in the rock. Since the expected hydraulic conductivity of thecrushed rock is very high and since there may be an open slot at the roof in the ramp, since thebackfill material has no swelling properties, the resistance to water inflow into the filling material
will be very low.
In principle the hydraulic subprocesses are identical to those (A-E) described for the buffer andwill not be reproduced here. However, most processes connected with unsaturated water flow inthe buffer and backfill are not relevant for the crushed rock. E.g. the suction in the crushed rock is
insignificant and the temperature gradient is so low that there is no temperature driven vapour flow.

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The only relevant processes for the crushed rock are the following:• Transport of water in liquid phase, which is controlled and driven by
A1) a pressure gradient in the water,
A4) gravity (coupled to A1),
A5) a density gradient (coupled to A1).
• Compression ofE2) air.
• Transport in water ofF1) dissolved air.
A1. Transport of water in liquid phase, which is driven by a water pressure gradient, can be describedby a refined version of Darcy’s law. The parameters that control the process are the material’s
hydraulic conductivity K and the difference in porewater pressure uw. Both of these can be
described as functions of the void ratio e, the degree of saturation Sr and the temperature T.
A4. Transport of water in liquid phase, which is driven by gravity. The influence of gravity isreflected in the hydraulic head and thus taken care of by process A1.
A5. Transport of water in liquid phase, which is driven by a density gradient in the water.
Differences in the density of the water can be caused by e.g. differences in temperature or salinity. Density-generated flow is probably negligible in the crushed rock, cf. process A1.
E2. Compression of air can be an important factor in the water saturation process (Boyle’s law).
If the air is trapped, it must be compressed and go into solution in the water in order for water
saturation to be achieved. The crushed rock has such low suction that a pressurised air volume
will remain if the air cannot seep out. A large part will disappear when full water pressure hasdeveloped.
F1. Transport of dissolved air in water. When trapped air is compressed during the water saturation
phase, a portion of the volume is dissolved in the water. This solubility (Henry’s law) isapproximately 2 percent at air temperature but decreases rapidly at increasing temperature. In
order for additional air to be dissolved, the dissolved air must be transported away, which takes
place by diffusion.
Dependencies between process and top seal variables

Table 7-2 summarises how the process influences and is influenced by all top seal variables and how
these effects are treated in SR-Site.
The hydraulic behaviour depends very much on the grain size distribution and the density of thecrushed rock. Since the exact values of hydraulic conductivity are not determined and since there
are no demands on the hydraulic conductivity it is assumed that it is so high that the top seal fillingmaterial is filled with water at the same rate as it enters from the fractures in the rock and flows
downwards until the water comes into contact with the underlying less permeable backfill. Part of
the water will then flow into this backfill and part will stay and fill up the top seal. The blasted andcrushed rock used for the top seal is expected to have a hydraulic conductivity of 10-1 m/s or higher.
A process that could delay the saturation is when air has been trapped in the top seal. However, thetrapped air will disappear upwards since there is virtually no capillary forces that entrap the air andsince there is a free way upwards towards the atmosphere.
Boundary conditions

Interaction with the rock: The high hydraulic conductivity of the top seal material makes the
inflowing water flowing downwards and filling up the ramp or shaft.
Interaction with the atmosphere: The upper part of the top seal will connect and adapt to thegroundwater system in the biosphere.
Interaction with the backfill underlying the top seal: There will be a hydraulic contact between
these parts and some of the inflowing water may be lost downwards in the beginning.

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Table 7-2. Direct dependencies between the process “Water uptake and transport under unsaturated conditions” and the defined top seal variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present? Handling of influence(Yes/No)(How/Why not)Description

Top seal geometry

Yes

Considered in inflow
estimates.

No

Top seal poregeometryYes. Via void ratio(density) and void
geometry

Considered in inflow
estimates.

No

Temperature

Yes.

Neglected. Insignificant

Yes.

Neglected. Insignificant

Water content

Yes. Via degree ofsaturation

Neglected. Insignificant.

Yes

No. Insignificant.

Gas content

Yes. Via the gaspressure and degreeof saturation

Neglected. Insignificant.

Yes

No. Insignificant.

Hydro variables(pressure and flows)

Yes. Basic.

Considered in inflow
estimates.

Yes

No. Insignificant.

Stress stateNo

Top seal materials– composition andcontent

No, but indirectly
through hydrovariables (Hydraulicconductivity)

Top seal porewatercomposition

No

No

Structural and straymaterials

No

No

No
See row hydrovariables

No

Model studies/experimental studies

No studies of these kinds have been performed, but there is a solid knowledge of the hydraulicbehaviour of fillings with crushed or blasted rock.
No model studies of the saturation phase of crushed rock have so far been done but the rate of water
filling and the interaction with the underlying crushed rock should be calculated.
Natural analogues/observations in nature

Not required since the process is well known.
Time perspective

The time it takes to fill up the top seal with water depends on the inflow rate from the rock and theloss of water to the underlying backfill. However, it is probably a matter of years and not decades.
Handling in the safety assessment SR-Can

This process is only handled by a rough estimate of the time it may take to fill the top seal with water.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Water flow in sand and crushed rock fillings are well known and demonstrated in numerous examples.
See e.g. /Handboken Bygg/ and /Cederwall and Larsen 1976/ as well as /Pusch 2008/.

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Uncertainties

The hydraulic properties of the blasted and crushed rock in the top seal will be known well enoughwhen the laboratory tests on the actual material can be done.
Uncertainties in mechanistic understanding
None.
Model simplification uncertainties in SR-Can
Not relevant.
Input data and data uncertainties in SR-Can
Not relevant.

7.2.2

Water transport under saturated conditions

Overview/General description

Water transport under saturated conditions is mainly driven by a water pressure gradient. The
process can be described by Darcy’s law. The general description of the water transport processes
given in the previous section for unsaturated crushed rock is also valid under saturated conditions. In
addition, the temperature gradient and salt content gradient may after full pore pressure equilibrium
be an important factor for driving water flow due to the low natural pressure gradient.
Dependencies between process and top seal variables

Table 7-3 summarises how the process influences and is influenced by all top seal variables and how
these effects are treated in SR-Site.
Influence of density, temperature and water salinity: The hydraulic conductivity is primarily
dependent on the pore geometry, the composition of the crushed rock and the density. The ion
concentration in the porewater and the temperature affect the hydraulic conductivity via the viscosityof the water, but are probably of minor importance. They may however be important as pressure
gradient generators.
Influence of other factors
After full water saturation and porewater equalisation, the natural pressure gradient is small. Thismeans that the pressure gradients caused by temperature differences and salt content differences may
dominate the water transport.
Boundary conditions

Interaction with the rock: The water transport caused by the water pressure gradients in the rockfractures is dominated by the large fractures and fracture zones.
Interaction with the atmosphere: The upper part of the top seal will connect and adapt to thegroundwater system in the biosphere.
Interaction with the backfill underlying the top seal: There will be a hydraulic contact between
these parts.
Model studies/experimental studies

No studies of these kinds have been performed but there is a solid knowledge of the hydraulicbehaviour of fillings with crushed or blasted rock.

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Table 7-3. Direct dependencies between the process “Water transport under saturated
conditions” and the defined top seal variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influ(Yes/No)
ence
Description(How/Why not)Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Top seal geometry

Yes

Included in thehydrogeologicallarge-scale modelNo

Top seal poregeometryYes. Via void ratio(density) and void
geometry

Model parameters

No

Temperature

Yes. Through water
Insignificant
viscosity and temperature gradientYes, by convection

Insignificant

Water content

Yes. Hydraulicconductivity

Insignificant, saturation will be rapidNo-

Gas content

No

-

Yes

Insignificant

Hydrovariables(pressure and flows)

Yes. Basic variables.

Included in thehydro-geologicallarge scale model

Yes

Included in the hydrogeological large scale
model

Stress state

No

-

No

-

Top seal materials– composition andcontent

Yes. Hydraulicconductivity

Model parameters

No

Top seal porewatercomposition

Yes, via viscosityof water

Included in thehydro-geologicallarge scale model

Yes

Included in the hydrogeological large scale
model

Structural and straymaterials

No

-

No

-

Natural analogues/observations in nature

Not required since the process is well known.
Time perspective

The hydraulic gradient that exists after closure of the repository when water saturation and pore pressure equilibrium have been established in the top seal will be adapted to the groundwater situation in
the area and will remain so for the lifetime of the repository.
Handling in the safety assessment SR-Site

Included in the hydrogeological large scale model as a feature with high hydraulic conductivity, see
Geosphere Process Report /SKB 2010i/.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

See Section 7.2.1 Water transport under unsaturated conditions.

7.2.3

Gas transport/dissolution

Overview/general description

Gas transport in the top seal is basically the same process as that in the central area, see
Section 6.2.3. However, since the conductivity of the material will be high, trapped gas will escapevery easily and the process can be neglected in the assessment.

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Handling in the safety assessment SR-Site

The process is neglected and the saturation of the top seal is assumed to be rapid, see also
Section 7.2.1.

7.2.4

Piping/erosion

Overview/General description

Water inflow into the top seal will take place mainly through fractures and will contribute to thewetting of the crushed rock. However, if the inflow is localised to fractures that carry more water
than the crushed rock can adsorb, there will be a build-up of water pressure in the fracture andtherefore an increase in the hydraulic gradient through the crushed rock. If the total pressure fromthe water pressure in the fracture becomes higher than the resisting total pressure the crushedrock will be deformed and an open channel or slot will be formed, where water can penetrate andcontinue to penetrate along the rock surface. This flow may also result in erosion of fine particles
along the pipes. The processes are similar to the corresponding processes in the buffer described in
Section 3.3.4.
The risk of piping depends on a number of factors the main ones being the following:•

The hydraulic conductivity of the crushed rock,•

The water inflow rate,•

The water pressure increase rate when water flow is stopped,•

The water inflow pattern (point wise or along a fracture).

The magnitude of erosion depends mainly on the grain size distribution and the percentage of fines
in the crushed rock, but of course also on the flow rate.
Since the hydraulic conductivity of the blasted and crushed rock in the top seal is so high, there will
be no resistance to water inflow, not even when the crushed rock is dry. This means that all inflowing
water is easily absorbed by the crushed rock and that piping cannot take place. Since there are nofine-grained particles in the filling, no or little erosion will take place. If erosion of any small amountof fines would occur it would not affect the properties in any significant way.
Handling in the safety assessment SR-Site

Since piping and erosion cannot take place in the top sealing, this process is neglected.

7.3

Mechanical processes

7.3.1 Swelling/mass redistribution
This section includes all mechanical processes in the blasted and crushed rock in the top seal except
liquefaction.
Overview/General description

The crushed rock has no swelling potential so it will mechanically only have passive roles. The interaction with the other parts will be governed by the stiffness, the friction angle and the own weight ofthe crushed rock.

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In the top seal of the ramp the effective vertical stress (= total stress minus porewater pressure)varies from zero at the roof to 50-100 kPa in the floor, depending on the height of the rooms. In
the top seal of the shaft, the stresses in the bottom of the top seal can be rather high due to the large
depth (200 m), but the friction against the rock walls reduces the stresses significantly (silo effect).
The horizontal stress is about half the vertical one.
In order to avoid that particles from the top seal penetrate into the pellets filling in the backfill below
the top seal during installation there has to be a separating layer between these two parts, e.g. a
concrete layer.
The backfill and the top seal of the shaft will have a complicated interaction with the clay backfill
in the lower part during the filling due to the increase in vertical stress. Additional creep settlements
may occur also afterwards especially due to influence of weathering (e.g. from freezing and thawing).
The compressibility (or stiffness) of the material can be expressed with the compression modulus
according to Equation 7-1.
M = Δσ/Δε									

(7-1)

where
M = compression modulus
σ = uniaxial stress with no lateral strain
ε = uniaxial strain with no lateral strain
The friction angle is also very high.
The compression modulus and the friction angle depend on the grain size distribution, the shape ofthe grains and especially the dry density. Larger particles, more uneven shaped particles and higher
dry density yield higher compression modulus and higher friction angle. All these three factors willprobably be larger for the blasted and crushed rock in the top seal than for the crushed rock in theunderlying parts (except maybe the density if the filling of the top seal is not compacted).
The compression modulus can be determined with oedometer tests on the actual crushed rock andthe friction angle can be determined with triaxial tests or shear tests. However, such tests have notbeen done since it requires very large apparatuses (~1 m in diameter) due to the large particle size.
It is probably not necessary to do such tests since the compression modulus and the friction angle
are expected to be higher than for the underlying crushed rock and by using the same parameters therough calculations are on the safe side.
Dependencies between process and top seal variables

Table 7-4 summarises how the process influences and is influenced by all top seal variables and how
these effects are treated in SR-Site.
Interaction crushed rock in the top seal / atmosphere
If the top seal ends without any additional seal the blasted rock blocks applied in the upper part of
the ramp and shaft will be in direct contact with the atmosphere and there will be no mechanicalinfluence but human actions and ice age effects.

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Table 7-4. Direct dependencies between the process “Mass redistribution” and the defined top
seal variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Top seal geometry

Yes influences indirectly the stresses

Simplified calculations No

Top seal poregeometryYes. Determines
the density, which
is important for thecompressibility

Simplified calculations Yes affects the density Simplified calculations

Temperature

Insignificantly

No

No

-

Water content

Yes via density

See pore geometry

InsignificantlyNo

Gas content

Insignificantly

No

No

-

Hydrovariables(pressure and flows)

Insignificantly

No

No

Stress state

Yes. Basic variable for Simplified calculations Yes.
mechanical processes

Top seal materials– composition andcontent

Yes. Determines
stiffness and friction
angle

Simplified calculations No

Top seal porewatercomposition

No

-

No

-

Structural and straymaterials

No

-

No

-

-

Simplified calculations

Interaction crushed rock in the top seal/ near field rock
The following mechanical interactions between the near field rock and the backfill of crushed rockhave been identified.
1. The lateral stress from the own weight of the crushed rock on the rock surface of the shaft andramp. This stress will increase with depth especially in the shaft. It may keep loosened rockpieces in place and stabilise the rock wall.
2. Friction between the filling material and the rock surface especially in the shaft.
3. Stresses from detached rock pieces. Such pieces can affect the filling if they fall from the roof in
the ramp.
4. Convergence of the rock walls. This process compresses the crushed rock in the top seal andimproves the properties
5. Thermo-mechanical effects from heating and cooling the rock. The cooling phase will cause asmall expansion of the open spaces and thus decrease the lateral stress from the top seal on therock walls, but the effect is very small since the temperature changes are small in these parts.
6. Rock displacements from earth quakes or rock creep will locally affect the top sealInteraction crushed rock in the top seal / underlying backfill
The pressure from the top seal on the underlying backfill will compress the underlying backfill,especially in the shaft, since the interface layer will probably not be able to carry the entire load.
The stress increase, compression of the backfill blocks in the lower part of the ramp and shaft andprobable crushing of them and resulting displacement of the interface will take place immediately
at placement and thus end when the filling is completed. This means that there will probably be
movements of the interface during filling. The movements will probably take place by jerks since
the friction against the walls must be exceeded before there is a movement. After completed water
saturation of the underlying backfill there will be a new equilibrium based on the swelling pressure
and compression properties of the backfill.
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Boundary conditions

The top seal has three main boundaries, namely the underlying clay backfill, the rock and the atmosphere.
Model studies/experimental studies

No such studies have been done or are foreseen to be required.
Time perspectiveCompression of the underlying backfill and simultaneous own compression of the top seal material
will take place immediately.
In the long-time perspective, there is a (probably negligible) creep settlement of the crushed rockbackfill in the top seal and there may be local breakage of the near field rock that is caused by highstresses, excavation damages and creep in the rock.
Natural analogues/observations in nature

No natural analogues concerning mechanical behaviour have so far been studied. The existenceof relevant natural examples is probably very limited and would be difficult to evaluate. There arethough some experiences from backfilling of mines and other types of fillings with blasted rockstones.
Handling in the safety assessment SR-Site

Most of the mechanical effects related to mass redistribution are so small that they can be ignored in
the safety assessment since the demands on the backfill of the top seal are only to preserve the seal
and obstruct human intrusion.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The mechanical properties of crushed rock fillings are well known and demonstrated in numerous
examples. See also /Pusch 2009/.
Uncertainties

Uncertainties in mechanistic understanding
The mechanisms of mechanical processes in crushed rock are known well enough.
Model simplification uncertainties in SR-Site
No model will be used.
Input data and data uncertainties in SR-Site
No model will be used.

7.3.2

Liquefaction

Overview/General description

Liquefaction is a process implying that a stiff material (soil) turns into liquid due to an impact
with short duration, see e.g. /Lambe and Whitman 1969/. It may take place in a loose sand when
the porewater pressure is increased either due to a vibration that makes the sand particles float in
the porewater (since they tend to go into a higher degree of compaction, but the water temporarily
prevents it) or due to a strong upward water flow that releases the effective stresses between theparticles (quicksand). It may also take place in clay that has been settled in salty water (forming

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an open structure with a high water ratio). If the salt is partly washed out by fresh water, the clay
structure cannot hold the high amount of water at moulding or vibrations, meaning that the structure
collapses when exposed to vibrations.
These two types of liquefaction cannot take place in a backfill with large stones of crushed rock.
Handling in the safety assessment SR-Site

Since the process cannot occur in the top seal, it is neglected.

7.4

Chemical Processes

7.4.1

Advective transport of species

Overview/general description

In this context, advection refers to transport of any forms of additional matter, e.g. ions, molecules
or colloids, with porewater flow. The transport direction is thereby principally from volumes of highwater pressure to volumes of lower pressure. The process leads to redistribution of solutes in theporewater and thus affects the porewater composition. Advection is closely related to water flow inthe top seal, which is described in Section 7.2.2.
The backfill material in the top seal consists of moraine, crushed rock and rock-like material. The
hydraulic conductivity is not known but can be assumed to be high (~0.1 m/s). Advective flow will
thereby be the dominant transport process in this component.
Dependencies between process and top seal variables

Table 7-5 summarises how the process influences and is influenced by all top seal variables and how
these effects are treated in SR-Site.
Boundary conditions

Interaction with the rock: The water transport caused by the water pressure gradients in the rockfractures is dominated by the large fractures and fracture zones.
Interaction with the atmosphere: The upper part of the top seal will connect and adapt to thegroundwater system in the biosphere.
Interaction with the backfill underlying the top seal: There will be a hydraulic contact between
these parts.
Model studies/experimental studies

No studies of these kinds have been performed but there is a solid knowledge of the hydraulicbehaviour of fillings with crushed or blasted rock.
Natural analogues/observations in nature

Not required since the process is well known.
Time perspective

The hydraulic gradient that exists after closure of the repository when water saturation and pore pressure equilibrium have been established in the top seal will be adapted to the groundwater situation in
the area and will remain so for the lifetime of the repository.

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Table 7-5. Direct dependencies between the process “Advective transport of species” and thedefined top seal variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Top seal geometry

Yes, major impact

No

Top seal pore geometry Yes, indirectly
through the hydraulicconductivity.
Unfilled tunnel withgaps in e.g. roof will
increase the transportcapacity

Indirectly, dissolution
Neglected
and precipitation reactions may increase or
reduce the porosity,
respectively.

Temperature

Indirectly throughtemperature induced
gradients andhydraulic conductivity.
Freezing is of major
importanceNo

Water content

Yes, by definitionNo

Gas content

Yes, dissolved gas istransported with flow.
Indirectly through flow
properties due to gas
pressure, gas dissolution and precipitation.

Yes, dissolved gas
transported away

Neglected

Hydrovariables(pressure and flows)

Yes, by definition

Yes, density effects
may induce flow

Neglected

Stress stateNo, or insignificant

No, insignificant

Top seal – compositionand content

Yes, indirectly throughsorption, precipitationand dissolution reactions

Yes, , indirectly
through sorption,
precipitation and dissolution reactions

Top seal porewatercomposition

Yes, by definition

Yes, by definition

Structural and straymaterials

Yes, ,indirectly
through dissolution of
concrete, reinforcement etc.

Yes, ,indirectly throughdeterioration of thematerials

Handling in the safety assessment SR-Can

The top seal is included in the hydrogeological large scale model as a feature with high hydraulicconductivity, see the Geosphere Process Report /SKB 2010i/. This model provides the evolution of
the salinity field, but also data on the transport and mixing of water types.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

See Section 7.2.1 Water transport under unsaturated conditions.

7.4.2

Diffusive transport of species

Overview/general description

A basic description of this process in the central area is given in Section 6.4.2 and in the buffer in
Section 3.5.3. Because of the hydraulic situation in the top seal, diffusion is of no importance in thissystem part.

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Handling in the safety assessment SR-Site

The process is neglected since advective transport will dominate in the top seal.

7.4.3

Sorption (including exchange of major ions)

Overview/general description

Radionuclides and major ions contained in water in the top seal can be bound to the surfaces ofthe large rocks in the same way as has already been described for the crushed rock material of thecentral area, (Section 6.4.3). In a qualitative sense, the sorption processes will be nearly identical tothose taking place in crushed rock material, although the proportion of the various types of mineralsurfaces is likely to differ somewhat between crushed vs. non-crushed material. Quantitatively,
sorption onto large rock blocks will be of comparatively little importance, due to their small specificsurface area and the comparatively short contact time of percolating water.
Dependencies between process and top seal variables

Table 7-6 shows how the process influences and is influenced by all top seal variables.
Table 7-6. Direct dependencies between the process “Sorption” and the defined top seal
variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence?
(Yes/No)(How/why not)Description

Process influence on variable
Influence present?
Handling of influence?
(Yes/No)(How/why not)Description

Top seal geometryNo

No

Top seal poregeometry

No (not on macroscopic
scale)

No

Temperature

Yes

Influence of temperature on sorption mustbe acknowledged,
but effect is not clear.
Sorption is neglected.

No

Water content

Yes (quantitative effect
Sorption is neglectedof solid/water ratio would
be included in Kd).

No

Gas content

In case of reactive
Sorption is neglectedgases: indirectly throughporewater composition

No

Hydrovariables(pressure and flows)

Possibly indirectly
through porewatercomposition

No

Stress stateNo

Top seal materials– composition andcontent

Yes

Sorption is neglected

Surface composition

Effect neglected

Top seal porewatercomposition

Yes

Sorption is neglected

Yes

See chapter on alteration (Section 7.4.4);
radionuclide concentration and release arecalculated

Structural and straymaterials

Yes

Sorption on straymaterials is conservatively excluded in
SR-SitePossibly indirectly
through porewatercomposition/mineralequilibria

See chapter on(Section 7.4.4);
radionuclide concentration and release arecalculated

304

Sorption is neglectedNo

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Influence on top seal variables: No notable influence on top seal variables is expected.
Boundary conditions

There are no particular boundary conditions to discuss for this process.
Model studies/experimental studies

There are no particular issues to discuss for this process. The basic relations are given in
Sections 3.5.5 and 6.4.3.
Time perspective

In case of relatively short contact times of percolating water, it is expected that sorption, including
ion exchange of major components, may not reach equilibrium. Thus, the contribution of thesereactions to the mineralisation of the percolating water will also be limited.
Natural analogues/observations in nature

Surface reactions/weathering of the large rock blocks in the top seal will be similar to what is observed
for large blocks of rocks in nature. I.e. their contribution of dissolved constituents to water will belimited, and the rate of weathering with respect to their overall mass will be very slow.
Handling in the safety assessment SR-Site

The process is neglected.
Uncertainties

There are no relevant uncertainties.

7.4.4

Alteration of concrete components

Overview/general description

In the top seal, cement-bearing materials can be present as construction concrete, and grout willremain in the system after backfilling this system part. The alteration of such materials can affectthe system in two different ways: 1) as a consequence of its degradation the porosity can increase,thus creating favourable pathways for groundwater to enter into the system, and 2) the increase ofalkalinity due to the interaction of groundwater with these materials could possibly enhance the dissolution rate of montmorillonite in the buffer and backfill (Section 4.4.7 in the Backfill chapter andSection 3.5.9 in the Buffer chapter), weakening the effectiveness of bentonite in buffer and the clay
backfill in the deposition tunnels and in the lower part of the ramp and shaft as a hydraulic barrier.
The processes associated to the alteration of concrete components are equivalent to those considered
for concrete in the tunnel plug (Section 5.4.4)
Handling in the safety assessment SR-Site

The process is neglected since:
1. The concrete used is supposed to be of low pH type,2. There is a long distance from concrete structures in the top seal to the clay components in thedeposition tunnel backfill and the buffer and the performance requirements on the clay backfill in
the ramp and shaft is limited.
See also Section 5.4.4.

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7.4.5

Aqueous speciation and reactions

Overview/general description

This process is in general the same as the corresponding process in the buffer (Section 3.5.7.)
Handling in the safety assessment SR-Site

No geochemical processes are considered in the top seal because of the distance to, and thus limited
impact on, essential repository barriers.

7.4.6

Colloid release

Overview/general description

Fractures intersecting the top seal mean that rigid volumes restrictions are not present everywhere,
and that small sized material may enter the fractures. Separation of individual particles from theintroduced material may then take place (dispersion).
The behaviour of the particles will be dominated by thermal motion if the particles are small enoughi.e. they have colloidal properties. The individual colloidal particles may then be transported awayfrom the deposition hole in fractures by diffusion or by groundwater flow in the fractures.
The process is considered not relevant for the upper part (100 m) of the top seal component, since
the coarse gravel material in principle has the same properties as the surrounding rock with respect
to colloid release. The lower part of the top seal component (100–200 m below surface) consists ofcrushed rock with a fuller size distribution. The smallest fractions may in principle form colloids at
the groundwater interface.
Handling in the safety assessment SR-Site

This process is neglected in the top seal due to the very small possible effect, also for the unlikely
case that all particles of colloidal size are transported away.

7.4.7

Steel corrosion

Overview/general description

Steel in the reinforcements in the top seal will not be stable under repository conditions, seeCorrosion of steel in the Fuel and Canister Process Report /SKB 2010k/. However, there are no longterm performance requirements on the reinforcements and the corrosion products and gases are notexpected to have any impact on the other parts of the repository.
Handling in the safety assessment SR-Site

The process is neglected based on the reasoning above.

7.4.8

Microbial processes

Overview/general description

Microbial processes in a crushed rock environment is described in the section about the central area
(Section 6.4.9)
Handling in the safety assessment SR-Site

It is not foreseen that microbial processes can interfere with the top seal in a way that will influence
the performance of the repository. The process is therefore neglected.

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7.4.9Radiation induced transformation

Overview/general description

The top seal is too far away from the canisters to be affected by the γ-radiation. The effect ofa-radiation from radionuclides from damaged canisters can also be neglected, since the levels will be
extremely small (see Section 3.5.12 ).
Handling in the safety assessment SR-Site

The process is neglected.

7.5Radionuclide transport processes

7.5.1

Speciation of radionuclides

Overview/general description

See corresponding process description for the buffer, Section3.6.1.
Handling in the safety assessment SR-Site

This process is indirectly handled through the selection of parameters for radionuclide transport in
the rock. In general, the handling of the process is the same as for the corresponding process in thegeosphere, see the process Transport of radionuclides in the water phase in the Geosphere Process
Report /SKB 2010i/.

7.5.2

Transport of radionuclides in the water phase

Overview/general description

The top seal can be considered as a part of the rock with high hydraulic conductivity and a highporosity. If radionuclides would enter the central area the additional transport time to the surface
would be rather limited.
Handling in the safety assessment SR-Site

The top seal is considered to be a part of the rock in the radionuclide transport calculations, see theprocess Transport of radionuclides in the water phase in the Geosphere Process Report /SKB 2010i/.

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8

Processes in the bottom plate

8.1

Thermal processes

8.1.1

Heat transport

Overview/general description

The concrete part of the bottom plate will have thermal properties similar to those of the rock andwill have a very modest influence on the temperature, whereas the copper plate will contributeto distribute the heat below the buffer horizontally. Figure 8-1 shows the temperature field in thebottom region with and without account of the copper bottom plate. The concrete part is ignored.
The effect of the 20 mm copper plate below the bentonite barrier will be to even out the temperaturesuch that the central parts and the peripheral parts both tend to approach the area average. In addition, there will be a small general increase of the vertical heat flux across the bottom bentonite block,leading to a small reduction (approximately 0.2°C, cf. Figure 8-1of the canister temperature.
Dependencies between process and bottom plate variables

Table 8-1 shows how the process influences and is influenced by all bottom plate variables.

Figure 8-1. Comparison between temperatures in deposition holes with (left) and without (right)
copper bottom plate. The results shown here include a 10°C in-situ temperature as a typical value of theundisturbed initial background temperature at repository depth, but not the temperature contribution fromother canisters. From /Hökmark et al. 2009/.

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Table 8-1. Direct dependencies between the process “Heat transport” and the defined bottom
plate variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Bottom plate geometry Yes

Copper plate geometry accounted forin thermal near-fieldmodelsIndirectly via tempera- Ignored
ture

Bottom plate poregeometry

Yes

IgnoredIndirectly via tempera- Ignored
ture

Temperature

Yes. Heat transport isslightly temperaturedependent.

IgnoredYes. By definition

Bottom plate watercontent

Yes. Thermal conductivity of the concrete
foundation depends
on water content

IgnoredIndirectly via
Ignoredtemperature. Hightemperatures and hightemperature gradients
may dehydrate
concrete foundation(or delay saturation)

Bottom plate gas
content

Indirectly via watercontent

IgnoredIndirectly via watercontent

IgnoredHydrovariables (pressure and flows)

No

Indirectly via
temperature

IgnoredStress stateNo

Indirectly via
temperature

IgnoredResult of near-fieldthermal model.

Bottom plate materials Yes. Composition– composition andaffects thermal
content
conductivity

Copper plate incudedin modelPossibly via
temperature

Ignored

Bottom plate porewater composition

IgnoredPossibly via
temperature

IgnoredPossibly via
temperature

Bottom plate geometry: The geometry of the bottom plate, i.e. the copper plate thickness and theconcrete foundation thickness control the way the bottom plate influences the heat dissipation fromthe lower parts of the deposition hole. The temperature increase will give a modest volume expansion.
If the thermally induced stresses lead to fracturing of the concrete foundation there could be some
additional, but modest, volume expansion because of dilation (see text below under “bottom plate
pore geometry”).
Bottom plate pore geometry: The copper plate does not have any pore system. The thermal conductivityof the concrete foundation varies with the void ratio, as it does for all porous materials and similar to
the way it varies with the void ratio of the backfill in the deposition tunnels. The pore geometry could
potentially be affected by thermally induced crack formation, i.e. corresponding to the way spalling
may affect the porosity in the walls of the deposition holes. The uniaxial compressive strength of
concrete ranges between 10 and 60 MPa depending on composition /Boverket 2004/. The thermallyinduced stresses in the bottom plate region are about 30 MPa at the time of the peak stress (assuming
the thermo-elastic properties to be similar to those of the rock) /Hökmark et al. 2010/. For typicalconcrete materials, the expansion coefficient is some 50-100% higher than for the rock types foundat the sites, while the E-modulus is in the range of 20-40 MPa, i.e. some 30-60% of typical rock type
moduli /Ingelstam et al. 1971/. This means that the stress estimate above is reasonably valid and thatthe stress in the concrete foundation may approach or exceed the concrete uniaxial strength. For dry
deposition holes where the vertical support pressure is limited by the weight of the canister/buffer
system, this may lead to fracture initiation and possibly fracturing with an associated change inporosity and pore geometry. The fracturing cannot be expected to be extensive; a modest amount of
inelastic deformation will release the stresses, that just marginally exceed the strength, sufficiently
that the stability is restored. The change in porosity and pore geometry will have some impact on thepermeability of the concrete plate (see text under “hydrovariables”).
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Temperature: Heat transport influences the temperature by definition. There is probably a small
temperature dependence in the heat transport properties of the concrete foundation (as there is forrock). The volume fraction of the near-field taken up by the bottom plate is too small to have any
significant influence on the thermal evolution in the interior of the deposition hole (cf. Figure 8-1 ).
The copper plate is nevertheless explicitly included in the thermal near-field models.
Water content: The heat transport will not affect the water content, other than indirectly via thetemperature. The conductivity-saturation relations that apply for the buffer and the deposition tunnel
backfill are, in principle, valid also for the concrete foundation. Because of the small volume taken
up by the bottom plate, water content-induced conductivity variations are not important to thethermal evolution in the interior of the deposition hole.
Gas content: The heat transport will not affect the gas content. The gas content in itself does nothave any influence on the heat transport. If there is a gas phase, the effects of this will be just thoseof the incomplete saturation, i.e. they are of no importance.
Hydro variables: The heat transport in the bottom plate has a very small influence on its own temperature and consequently insignificant influence on the hydro variables. The raised temperature and thefollowing thermal strain may generate some fracturing of the concrete foundation that could increase thepermeability (see text above under “pore geometry”). The small volume of the concrete foundation andthe way it is hydraulically separated by the copper plate from the buffer means that possible permeabilitychanges caused by the temperature increase can be ignored. Similar as for the rock, effects of water flowon the heat transport can be neglected.
Stress state: The elevated temperatures will affect the stresses. Depending on the thermal volume expansion of the concrete foundation, there may be different types of stress gradients or stress concentrations.
Possibly, there could be thermally induced fracturing of the concrete foundation (see text under “poregeometry” above”. The stress state does not have any practically influence on the heat transport, although
there may be some stress dependence on the thermal conductivity.
Bottom plate materials – composition and content: The composition of the bottom plate will
influence the thermal conductivity. Relevant conductivity estimates are made in the near-fieldmodels /Hökmark et al. 2009/.
Bottom plate porewater composition: The copper plate does not contain any porewater. The
composition of the small amounts of water contained in the pores of concrete foundation cannot haveany significant influence on the heat transport properties. Temperature effects on the composition are
possible, cf. handling of chemical process.
Boundary conditions

The bentonite-copper interface, the vertical rock walls and the horizontal bottom of the deposition
hole are the boundaries of the bottom plate. There is a vertical heat flux on the top boundary. That flux
decreases proportionally with the canister power. At the rock walls, the temperature varies slowly overtime at a rate given by the canister power, the rock thermal properties and the repository layout.
Model studies/experimental studies

There are no studies that specifically address the thermal conditions in the bottom plate. The bottom
plate is, however, included in near-field thermal analyses.
Natural analogues/observations in nature

Not applicable.
Time perspective

The duration of the thermal pulse generated by the decaying fuel is a few thousand years. For thebottom plate, the temporal evolution coincides with that of the buffer (cf. Section 3.2.1).

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Handling in the safety assessment SR-Site

Because of the modest impact on the temperatures in the interior of the deposition hole, there isno specific handling of heat transport in the bottom plate. The copper plate is, however, explicitly
included in the thermal near-field models.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site/Hökmark et al. 2009/ and /Hökmark et al. 2010/ will undergo a documented factual and quality review.
Uncertainties

Uncertainties in mechanistic understanding
There are no major uncertainties in mechanistic understanding of the heat transport itself. The concrete
foundation is a system with solid, liquid and gas phases and obeys physical laws for such systems.
Model simplification uncertainties in SR-Site
Not relevant since the process is not modelled (other than by inclusion of the copper plate in thethermal near-field models)
Input data and data uncertainties in SR-Site
Not relevant since the process is not modelled (other than by inclusion of the copper plate in thethermal near-field models).

8.1.2

Freezing

Overview/general description

Freezing of the bottom plate is of no significance, since no long term performance is expected fromthe concrete. The same process takes place in the material around the bottom plate as described in
the corresponding process description for the buffer (see Section 3.2.2). The bottom plate will notchange anything in respect to freezing and thawing.
Handling in the safety assessment SR-Site

Freezing of the bottom plate is not treated in SR-Site.

8.2

Hydraulic processes

8.2.1

Water uptake and transport under unsaturated conditions

Overview/General description

The hydro-mechanical evolution of the bottom plate during the repository lifetime is the following.
The copper plate is installed and concrete poured into the about 15 cm thick slot between the rocksurface and the copper plate. When the concrete has fully matured, the entire space is assumed to be
filled. In the mean time until the bentonite buffer is emplaced, the concrete will be completely water
saturated.
The buffer and canister is emplaced and, due to the bentonite protection and by draining and discharging water from the deposition hole, there is only a thermo-mechanical interaction between the bottom
plate and the buffer. When the protection is removed and the slot filled with pellets, water will enterand the water uptake starts in the buffer.

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At that time, a rather complicated interplay between the bottom plate, the buffer and the rock will
start. Since the contact zones between the concrete and both the rock and the copper plate are nottight, water will penetrate and a water pressure will act on the plate. If this water pressure is higher
than the weight of the buffer and the canister, and later also the backfill, the entire package will lift
and be moved upwards. The water pressure required for this unpleasant scenario is 120–220 kPa
depending on if the backfill has been placed. There are some processes that prevent or limit this
scenario:•

The water pressure will spread to the pellets filling and the pellets filling cannot withstand highwater pressure. Piping will instead occur, which means that at some level of the water pressure, apiping channel will occur and the water will flow out of the deposition hole•

If the backfill is placed and almost in contact with the roof, the upwards displacement is limited.

•

A displacement will in itself regulate the pressure and the inflow rate must be rather high in orderto maintain the pressure.

After full saturation, there will be full swelling pressure on the plate. With time, the copper plate
will corrode and the concrete be dissolved and the cement dissipate. Although the ballast material
is supposed to be left, there will probably be some settlement of the buffer due to the compressionof these parts. There will also be a remaining very permeable zone along the entire bottom of thedeposition hole.
The hydraulic processes are separated so that the saturation phase is assumed to take place before
emplacement of the buffer and all the complicated hydro-mechanical processes will take place after
full saturation and are handled in Section 8.2.2.
Dependencies between process and bottom plate variables

Table 8-2 summarises how the process influences and is influenced by all bottom plate variables andhow these effects are treated in SR-Site.
Table 8-2. Direct dependencies between the process “Water uptake and transport underunsaturated conditions” and the defined bottom plate variables and a short note on the handlingin SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Bottom plate geometry

Yes

Negligible

No

Bottom plate poregeometryYes. Via void ratio(density) and void
geometry

Negligible

No

Temperature

Yes.

Negligible

Yes

Negligible

Bottom plate Water
content

Yes. Via degree ofsaturation

Negligible

Yes

Negligible

Bottom plate gas
content

Yes. Via the gaspressure and degreeof saturation

Negligible

Yes

Negligible

Hydrovariables(pressure and flows)

Yes. Basic.

Negligible

Yes

Negligible

Stress stateNo

Bottom plate materials– composition andcontent

Yes. Hydraulicconductivity

Bottom plate porewater No
composition

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No
Negligible

No

No

313

Boundary conditions

Interaction with the rock: The rock supplies water to saturate the concrete. Very little water is needed.
Interaction with the buffer: The plate prevents hydraulic interaction
Interaction with the radial slot that is left after installation of the bottom plate: Supplies waterto saturate the concrete
Model studies/experimental studies

A similar design of the bottom plate has been used in the Prototype Repository but no information
about the wetting of the concrete has been available and is not considered to be of interest.
Natural analogues/observations in nature

Not applicable.
Time perspective

Water saturation of the concrete in the bottom plate is expected to take place before the buffer
is emplaced.
Handling in the safety assessment SR-Site

The water saturation of the bottom plate itself is not considered in SR-Site. However, the mechanicalinteraction between the buffer and the bottom plate is included in the THM-modelling, see
Section 8.3.1.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Not relevant since not handled in safety assessment
Uncertainties

Not relevant since not handled in safety assessment.

8.2.2

Water transport under saturated conditions

Overview/General description

Two main cases can be distinguished.
1. One is a high water pressure in the entire surface between the rock and the concrete that can bepresent rather soon after installation of the buffer.
2. The other one is the long term effect with a highly permeable zone between the rock in thebottom of the hole and the buffer material when the copper has corroded and the concrete havebeen disintegrated.
The hydro-mechanical evolution is described in Section 8.2.1.
Dependencies between process and bottom plate variables

Table 8-3 summarises how the process influences and is influenced by all bottom plate variables andhow these effects are treated in SR-Site. The table mainly concerns the long term effect.

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Table 8-3. Direct dependencies between the long term effect of the process “Water transport
under saturated conditions” and the defined bottom plate variables and a short note on thehandling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Bottom plategeometry

Yes

No

Bottom plate poregeometry

Yes, after degradation Included in the model

Yes

Temperature

Yes

No

Bottom plate Water
content

No

No

Bottom plate gas
content

No

No

Hydrovariables(pressure and flows)

Yes. Basic variables.

Calculations

Yes

Calculations

Stress state

No

-

No

-

Bottom plate materiYes, after degradation Model parameters
als – composition andcontent

Yes

Washing out of
cementBottom plate porewater composition

No

Yes

Included in models

Negligible

Negligible

Washing out of
cement-Boundary conditions

Interaction with the rock
Case 1: Since the contact zones between the concrete and both the rock and the copper plate are nottight, water will penetrate and a water pressure will act on the plate. If this water pressure is higher
than the weight of the buffer and the canister, and later also the backfill, the entire package will lift
and be moved upwards. The water pressure required for this unpleasant scenario is 120–220 kPa
depending on if the backfill has been emplaced. As described earlier, there are some processes thatprevent or limit this scenario.
Case 2: With time the copper plate will corrode and the concrete will suffer from degradation and leaching. There will thus be a remaining very permeable zone along the entire bottom of the deposition hole.
Interaction with the buffer
After long time (case 2) there will be hydraulic contact between the permeable zone in the degradedconcrete and the buffer.
Interaction with the annular slot at the rock surface that is left after installation of thebottom plate
Case 1: Water that enters from the rock surface beneath the concrete in the bottom plate will flow out
to the radial slot that is filled with bentonite pellets and will speed up the water filling of the pellets inthe slot. If the pellets filling has time to interact with the bentonite block and form a swelling pressurethat is high enough to seal, there may be a substantial water pressure. Tests done so far have shownthat the pellets filling cannot withstand high water pressure without piping, but 100-200 kPa (whichis in the same order as needed for lifting the bentonite/canister package) can probably be resisted.
Case 2: After full saturation and homogenisation, the slot will be filled with bentonite with asomewhat lower density than the rest of the buffer. This slot will thus be in direct hydraulic contact
with the permeable zone formed by the degrading concrete.

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315

Model studies/experimental studies

A similar design of the bottom plate has been used in the Prototype Repository. No severe consequences of case 1 have so far been observed.
The consequences of case 2 with a permeable concrete zone will be included in the safety case model.
Natural analogues/observations in nature

Not applicable.
Time perspective

Case 1: Takes place before full saturation is reached, that is within about 100 years.
Case 2: Long time effect that must be considered for the lifetime of the repository.
Handling in the safety assessment SR-Site

Case 1 is difficult to handle. A serious investigation of this is still done. Due to the vulnerabilityof the bentonite pellets to piping there will probably not be any high water pressure anyway not in
the beginning. With time, as the swelling pressure increases, the water pressure may be higher but
then there is also strength (friction) between the rock and buffer that prevents axial displacements.
However, it is a complicated scenario and it may be very difficult to prove that it does not occur.
Case 2 is included in the model for the safety assessment.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Modelling of the two cases is reported in a general THM modelling report that will undergoa documented factual- and quality review /Åkesson et al. 2010a/.
Uncertainties

Uncertainties in mechanistic understanding
Case 1: The mechanistic understanding of this process is good but the consequences are difficult
to evaluate.
Case 2: Can be modelled as a porous material, which is well known.
Model simplification uncertainty in SR-Site
Case 1: There is no good way to model this situation. Experiences and model tests are needed.
Case 2: No model simplification needed.
Input data and data uncertainty in SR-Site
Case 1: Large uncertainty in the data and the consequence. The data selection is discussed in thebuffer THM Data Report /Åkesson et al. 2010b/
Case 2: The hydraulic properties are not known so a high hydraulic conductivity must be assumed.
The data selection is discussed in the buffer THM Data Report /Åkesson et al. 2010b/

8.2.3

Gas transport/dissolution

Overview/general description

The bottom plate will be saturated before the deposition of the buffer and the canister. The saturation
phase is therefore of no concern in the assessment (see also Section 8.2.1).

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A degraded porous bottom plate could potentially act as a sink/pathway for gas release from a corroding
canister insert in a defective canister. This effect would be beneficial since it would make it easier for thegas to escape. However, it will be impossible to demonstrate that this actually is the case.
Handling in the safety assessment SR-Site

The process is neglected based on the reasoning above.

8.2.4

Piping/erosion

Overview/General description

This process may not take place in the bottom plate but is of great importance in the buffer material,
especially in the pellets-filled slot, for the hydraulic scenario that may occur after installation (seecase 1 for the process “Water transport under saturated conditions” Section 8.2.2).
Handling in the safety assessment SR-Site

The process is not relevant for the bottom plate and is therefore not handled.

8.3

Mechanical processes

8.3.1

Swelling/mass redistribution

Overview/General description

Just as for the hydraulic processes the mechanical can be divided into two cases.
Case 1 is the same as for the hydraulic processes. A high water pressure may occur in the surface
between the rock and the concrete and can be present rather soon after installation of the buffer.
Since the contact zones between the concrete and both the rock and the copper plate are not tightwater will penetrate and a water pressure will act on the entire plate. If this water pressure is higher
than the own weight of the buffer and the canister and later also the backfill the entire package willlift and be moved upwards. The water required for this unpleasant scenario is 120-220 kPa depending on if the backfill has been placed. There are some processes that prevent or limit this scenario:•

The water pressure will spread to the pellets filling and the pellets filling cannot withstand highwater pressure. Piping will instead occur, which means that at some water pressure a piping
channel will occur and the water will flow out of the deposition hole•

If the backfill is placed and almost in contact with the roof the upwards displacement is limited.

•

A displacement will in itself regulate the pressure and the inflow rate must be rather high in orderto maintain the pressure.

Case 2 is also similar to case 2 for the hydraulic processes. After full saturation there will be fullswelling pressure on the plate. With time the copper plate will corrode and the concrete will sufferfrom degradation and leaching. Although the ballast material is supposed to be left there will probably be some settlement of the buffer due the compression of these parts.
Table 8-4 summarises how the process influences and is influenced by all bottom plate variables andhow these effects are treated in SR-Site. The table mainly concerns both cases if not otherwise stated.
Boundary conditions

Interaction with the rock
Since the rock is almost incompressible it will act as a boundary for the displacements in the bottom plate.

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317

Table 8-4. Direct dependencies between the process “Swelling/Mass redistribution” and thedefined bottom plate variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present? Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Bottom plate geometry Yes

Included in models

Yes

Case 1: Estimation ofconsequences Case 2:
Modelling

Bottom plate poregeometry

Case 1: No.
Case 2: Yes, via
compressibility

Case 2: Included in the Case 1: No
compressibility model
Case 2: Yes

Case 2: Included in thecompressibility model

Temperature

Yes

Negligible

Case 1: Yes if
displacement takes
place.
Case 2: No

Neglected

Bottom plate Water
content

Case 1: No
Case 2: Yes via
compressibility

Case 2: Modelling ofconsequences

Yes

Case 1: NeglectedCase 2: Modelling ofconsequences

Bottom plate gas
content

No

Hydro-variables(pressure and flows)

Case 1: Yes. Basicvariables.
Case 2: No

Case 1: Estimation ofconsequences

Stress state

Case 1: Yes. Own
weight of buffer etc.
Case 2: Yes, via
compressibility

Case 1: Yes. Own
Yesweight of buffer etc.
Case 2: Included in thecompressibility model

No
Case 1: Yes
Case 2: No

Case 1: Estimation ofconsequences
Case 1: Included inmodelling
Case 2: Included in thecompressibility model

Bottom plate materials Case 1: No.
Case 2: Included in the No– composition andcompressibility model
Case 2: Yes, via
content
compressibility after
degradation
Bottom plate pore­water composition

No

No

Interaction with the buffer:
The contact between the plate and the buffer will move with the displacement of the plate in case 1
and with the compression of the degraded concrete in case 2.
Interaction with the annular slot at the rock surface that is left after installation of thebottom plate
Case 1: Water that enters from the rock surface beneath the concrete in the bottom plate will flow out
to the radial slot that is filled with bentonite pellets and will speed up the water filling of the pellets
in the slot. If the pellets filling has time to interact with the bentonite block and form a swellingpressure that is high enough to seal, there may be a substantial water pressure. Tests done so far haveshown that the pellets filling cannot withstand high water pressure without piping, but a pressure of
100-200 kPa (which is in the same order as needed for lifting the bentonite/canister package) can
probably be resisted.
Case 2: After full saturation and homogenisation the slot will be filled with bentonite with asomewhat lower density than the rest of the buffer. The bentonite in this slot will thus be in direct
mechanical contact with the porous material formed by the degrading concrete.

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Model studies/experimental studies

A similar design of the bottom plate has been used in the Prototype Repository. No severe consequences of case 1 have so far been observed.
The consequences of case 2 with a compressible porous material will be studied.
Natural analogues/observations in nature

Not applicable.
Time perspective

Case 1: Takes place before full saturation is reached, that is within about 100 years.
Case 2: Long-time effect that must be considered for the lifetime of the repository.
Handling in the safety assessment SR-Site

Case 1 is difficult to handle but an attempt to model it will be done. A serious investigation of thiscase will be done. Due to the vulnerability of the bentonite pellets to piping, there will probably
not be any high water pressure build up, anyway not in the beginning. With time, as the swellingpressure increases, the water pressure may become higher, but then there is also strength (friction)
between the rock and buffer that prevents axial displacements. However, it is a complicated scenario
and it may be very difficult to prove that it does not occur.
Case 2 will be modelled.
Both cases are analysed by /Åkesson et al. 2010a/.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Modelling of the two cases will be reported in a general THM modelling report that will undergo adocumented factual- and quality review /Åkesson et al. 2010a/.
Uncertainties

Uncertainties in mechanistic understanding
Case 1: The mechanistic understanding of this process is good but the consequences are difficult toevaluate.
Case 2: Can be modelled as a porous material, which is well known.
Model simplification uncertainty
Case 1: There is no good way to model this situation but some simplified limiting calculations will
be done. Experiments and model tests might be needed.
Case 2: No model simplification needed.
Input data and data uncertainty
Case 1: Large uncertainty in the data and the consequence. This is discussed in the Buffer THM datareport /Åkesson et al. 2010b/
Case 2: The mechanical properties are not known but knowing the composition of the concrete aworst case can be considered. This is discussed in the Buffer THM data report /Åkesson et al. 2010b/.

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8.3.2

Liquefaction

Overview/General description

Liquefaction may only occur in the porous material after degradation of the concrete. However,since there is a high swelling pressure on the porous material liquefaction is highly unrealistic. If it
anyway would occur the consequences are insignificant since the material cannot move anywhere
and will afterwards be the same as before.
Handling in the safety assessment SR-Site

Since the process is highly unlikely to occur and the consequences are negligible, the process
is neglected.

8.4

Chemical processes

8.4.1

Advective transport of species

Overview/general description

The general description provided for the buffer in Section 3.5.2 is applicable also for the bottom
plate. Specific conditions for the bottom plate are addressed below.
No advective flow may take place through the copper plate as long as it is intact, and all flow has
to take place outside the edge of the plate. The advective flow in the underlying concrete will becontrolled by the hydraulic conductivity as long as the cement is intact. This retarding effect will bedramatically reduced in case of a fracture in the concrete, which can develop at any time. Neither
the copper plate nor the concrete are protected by bentonite. The longevity of these components isthereby controlled by the exposure to groundwater from fractures in the vicinity. The resistance toadvective flow may eventually be strongly reduced, and the bottom part of the deposition hole will
be a conductive zone.
Dependencies between process and bottom plate variables

Table 8-5 summarises how the process influences and is influenced by all bottom plate variables andhow these effects are treated in SR-Site.
Boundary conditions

The external boundary condition for this process is the hydraulic and concentration gradient acrossthe bottom plate system component.
Model studies/experimental studies

No specific studies by SKB or others have examined water flow in a similar system. General studies
of concrete give information of the conditions in a non degenerated concrete.
Natural analogues/observations in nature

No relevant studies available.
Time perspective

Advective flow transport in conjunction with water saturation takes place on a timescale of up to
hundreds of years. Flow transport after degeneration of the cement and copper may be significant.

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Table 8-5. Direct dependencies between the process “Advective transport of solutes” and thedefined bottom plate variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Bottom plate geometry Yes, no material
Included in the modeltransport will takefor radionuclide
place through an intact transportcopper plate. The
transport of material
is governed by thehydraulic conductivity
if the concrete is
intact. Introduction of
fractures or chemical
deterioration may
strongly increase theadvective flow

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Yes, corrosion of copper due to transport ofcorrodants and deterioration of concretedue to interaction withflowing groundwater

Neglected since
no performance is
expected from thebottom plate

Bottom plate poregeometry

Yes, main parameterfor hydraulic conductivity. Introduction of
fractures or chemical
deterioration may
strongly increase theadvective flow

The hydraulic conduc- Yes, deterioration due
tivity is assumed to be to interaction withhigh. Included in theflowing groundwatermodel for radionuclidetransport

Neglected since
no performance is
expected from thebottom plate

Temperature

Yes, affects solubility.
Indirectly throughporewater viscosity

Neglected, since effect Yes, but small,is smallinsignificant

Neglected

Bottom plate Water
content

Yes, by definition

Assumed to besaturated

Yes, by definition

Assumed to besaturated

Bottom plate gas
content

Yes, by reduction ofavailable flow area

Neglected since bottom plate is assumed
to be saturated

Yes, dissolution ofgas, or release of
dissolved gas

Neglected since
no performance is
expected from thebottom plate

Hydrovariables(pressure and flows)

Yes, by definition

Included in the model
for radionuclidetransport

Insignificant

Stress state

Yes, induction of
fractures and regulating fracture width

Neglected, a highhydraulic conductivity
is pessimissticaly
assumed.

No

Bottom plate materials Yes, through– composition anddissolution
content

Neglected, a highhydraulic conductivity
is pessimissticaly
assumed.

Yes, by corrosion,
introduction orreactants and removalof reaction products

Neglected since
no performance is
expected from thebottom plate

Bottom plate porewater composition

Neglected, effect isassumed to be small

Yes, by definition

Neglected since
no performance is
expected from thebottom plate

Yes, throughdissolution

Handling in the safety assessment SR-Site

The bottom plate can be treated as a volume with high hydraulic conductivity in the model forradionuclide transport. In SR-Site this volume is treated the same way as “spalling” zone in thecanister wall /Neretnieks et al. 2010/. This may be of importance if there is a water conductive
fracture which intersects the bottom plate.
Handling of variables influencing this process:The geometry of the bottom plate and the fracture flow is included in the model. The material
properties are simplified into an assumption of “high hydraulic conductivity”.

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Adequacy of references supporting the suggested handling in the safetyassessment SR-Site/Neretnieks et al. 2010/ is subjected to a peer review according to the review plan for SR-Site.
Uncertainties

Uncertainties in mechanistic understanding
It is uncertain whether the bottom plate will at all have any effect on the mass transfer in the near field.
However, the treatment described in this section will always be pessimistic for the mass-transfer.
Model simplification uncertainties for the above handling in SR-Site
The model approach is further discussed in /Neretnieks et al. 2010/.
Input data and data uncertainties for the above handling in SR-Site
There is no uncertainty regarding the geometry. A pessimistic value is assumed for the hydraulicconductivity. The uncertainty regarding the flow in the rock is discussed in the Geosphere Process
Report /SKB 2010i/.

8.4.2

Diffusive transport of species

Overview/general description

A description of this process is given for the buffer in Section 3.5.3. In principle, the same processes
are operative in hardened, water-saturated cement paste or concrete as in clay. Briefly, solutes can betransported in stagnant porewater by diffusion, following concentration gradients. The process leads
to a redistribution of dissolved porewater constituents (as well as radionuclides) in the porewater.
The diffusion process is strongly coupled to nearly all chemical processes in the concrete, since it
accounts for transport of reactants to and reaction products from the processes.
Diffusion in cement-based materials has not been studied as thoroughly as in clays, for example.
However, diffusion can be described by the same diffusion equations as are used for clays and otherporous media; these equations are described in detail in e.g. /Yu and Neretnieks 1997/. In contrast to
clays, no significant electrostatic effects (and hence no diffuse double layer/Donnan potential effects
on diffusion and on porewater concentrations of ions) in the pore space need to be considered.
At least in experiments, the reactive nature of hardened cement paste presents a particular challenge. Alteration reactions of the solid material occur on roughly the same timescales as diffusion
processes. Such transient geochemical conditions make it impossible to interpret diffusion profiles/Jakob et al. 1999/. Therefore, diffusion needs to be measured in stable systems, e.g. using cementthat has been leached to reach the portlandite-dominated phase and well pre-equilibrated water.
Dependencies between process and bottom plate variables

Table 8-6 shows how the process influences and is influenced by all system variables.
Influence on variables of bottom plates: Through transport of dissolved species to and from theconcrete system, diffusion will influence the overall composition and rate of alteration.
Influence of pore geometry: The influence of pore geometry is included by the presence of parameters that account for tortuosity and constrictivity in the equations for diffusion coefficients.
Influence of temperature: Based on the influence of temperature on the diffusivity of ions in freewater /Robinson and Stokes 1959/, it can be expected that De will increase about twofold whentemperature is increased from ambient conditions to about 50–60°C.
Influence of composition: The diffusion properties of concrete are dependent on the porosity and poregeometry which may be influenced by the composition, such as the amount and size of aggregate material. Also, sources of Ca that may lead to the precipitation of calcite will have an important influence.
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Influence of porewater composition and speciation: Different species of the same element canhave different diffusivities in free water and thereby also in pores. Speciation will also influence
sorption, which is taken into account in the equation for Da.
Boundary conditions

Diffusion is only considered in the context of concrete alteration rate, not for the transfer of species
to other system components. At least over longer time frames, the parameters determining diffusivetransport will be significantly influenced by the composition of the adjacent porewater. Carbonaterich water will lead to the precipitation of calcite in the concrete pore structure, further decreasing
porosity. On the other hand, water that is nearly carbonate free may dissolve cement minerals to thepoint where advective flow would become dominating /Pfingsten and Shiotsuki 1998/.
Carbonate-rich water entering as well as alkaline solutions leaving the concrete part may lead to theprecipitation of minerals at the concrete-bentonite/rock boundary. This would affect the boundary
conditions for the processes responsible for transferring dissolved species to and from the concrete.
In the presence of an intact copper plate, diffusion in the direction of the buffer will not be relevant.
Table 8-6. Direct dependencies between the process “Diffusive transport of solutes” and thedefined component variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present? Handling of influence?
(Yes/No)(How/why not)Description

Process influence on variable
Influence present?
Handling of influence?
(Yes/No)(How/why not)Description

Bottom plategeometry

Yes

Neglected since
advection is assumed todominate

No

Concrete poregeometry

Yes

Neglected since
advection is assumed todominate

No, but indirectly
through influence onporewater composition

Temperature

Yes

Influence of temperature
on diffusion in concretemust be acknowledged,
but effect is not establishedNo

Water content

Yes

Saturated conditions are
assumedNo

Gas content

Saturated
Saturated conditions areconditions assumed assumed

For dissolved gases
such as CO2, CH4

Hydrovariables(pressure and flows)

Yes

No

Stress stateNo, but indirectly
through poregeometryNo, but indirectly
through influence
on porewatercomposition / mineralequilibria

see Section 8.4.4

Bottom plate materials – compositionand content

Yes, defines magnitude of diffusivities

No, but indirectly
through influence
on porewatercomposition / mineralequilibria

see Section 8.4.4

Porewater
composition

Through effect on
speciation. Defines
magnitude ofdiffusivities

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Neglected since
advection is assumed todominate

Neglected since
advection is assumed
to dominate

Neglected since
advection is assumed
to dominate

Through diffusive
see Section 8.4.4transport of main
dissolved constituents

323

Model studies/experimental studies

In the context of radionuclide migration, there is a very limited number of well-documented studies
on hardened cement paste available e.g. /Jakob et al. 1999, Tits et al. 2003/. Most experiments weredone with relatively inert tracers, and only a few experiments used tracers that interact significantlywith the cement matrix. These studies show that experiments can only be interpreted when transientconditions (alteration during the experiment) are avoided.
There are additional studies that have been performed in the context of concrete use for construction.
These are often of a more macroscopic character and/or rely on observations made on existing
concrete structures. Because of its implications for steel corrosion, there is a vast amount of such
literature on chloride (see e.g. /Ollivier and Nilsson 1997/ and references therein.
Time perspective

As pointed out above, the time frame of diffusion is similar to the time frame of mineral alteration.
Natural analogues/observations in nature

Observations made in recent structures and on historic concrete samples show that the compositionof the contacting water, and its effect on mineral dissolution/precipitation is the key to understandingtransport processes.
Handling in the safety assessment SR-Can

The process is neglected in the context of radionuclide transport, since advection is assumed to dominate.
The handling of diffusion of major ions is described in Section 8.4.4 Alteration of concrete.
Uncertainties

The assumption that advection will dominate is pessimistic. However, if diffusion would dominate, themass transfer in the bottom plate would be negligible compared to the mass transfer in the buffer itself.

8.4.3

Sorption (including exchange of major ions)

Overview/general description

For the actual copper plate (copper metal surface), no sorption reactions are expected to take place
and this process is neglected.
In the concrete part of the bottom plate, radionuclides and major ions may undergo a range ofinteractions with the matrix (hardened cement paste). In comparison, interaction with the aggregatematerial can often be neglected. Exceptions to this may be sorption of simple cations, especiallyCs, to layer silicate minerals contained in the aggregate material derived from crystalline rock. This
process is discussed in Section 6.4.3.
The interaction of radionuclides and major ions with the cement matrix is probably best describedwith the term “uptake”. Because the cement matrix consists of several types of mineral phaseswith different properties, a range of solid-liquid interactions may take place. At the same time, therelative proportion of mineral phases as well as their properties are known to change as a functionof cement matrix degradation. This process will to some degree depend on cement and groundwatercomposition, but is in general terms fairly well understood and the cement matrix composition canbe modelled fairly well as a function of porewater exchange cycles.
In case of the more crystalline aluminate phases (ettringite/monosulphate, hydrogarnet), solid-liquidinteractions may involve ion exchange and surface complexation reactions as well as the formationof classical solid solutions. However, the most relevant solid phases in hydrated cement systems
are calcium-silica-hydrates (CSH), which are amorphous, very reactive, and possess a high specificsurface area. Interaction of major ions, and in particular of actinides/lanthanides and transition
elements, with these mineral phases typically results in some type of incorporation, often in an
ill-defined and non-stoichiometric fashion. The latter case may be non- or only partly reversible.

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As a result of this complexity, the nature of the uptake process has not yet been established for mostradioelements. This is corroborated by the amorphous and reactive nature of cement mineral phases.
Further, nearly all cements contain a relatively high inventory of stable isotopes of many potentiallyrelevant radionuclides, which means that isotopic exchange rather than chemical interactions have to
be considered in some cases.
On the other hand, a number of systematic uptake studies has become available during the last
decade, which allows defining Kd for many relevant elements as a function of conditions withinreasonable uncertainty ranges. As indicated in Section 5.4.3 (Sorption in tunnel plug), no adverse
effects on sorption by concrete admixtures are expected. Available cement degradation models
further allow tying sorption data developed for one particular cement degradation state to otherrelevant stages.
Interaction of the solid cement matrix with groundwater influences the overall system composition (andhence sorption of trace elements), but the nature of the respective processes are closer to dissolution/precipitation phenomena than to ion exchange in the classical sense (see Section 8.4.4 for more details).
Dependencies between process and bottom plate variables

Table 8-7 shows how the process influences and is influenced by all bottom plate variables.
Table 8-7. Direct dependencies between the process “Sorption” and the defined bottom plate
variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence?
(Yes/No)(How/why not)Description

Process influence on variable
Influence present?
Handling of influence?
(Yes/No)(How/why not)Description

Bottom plategeometry

No

Only for concrete(leaching, sorptionof Ca on secondary
phases, possible
changes in mechanical properties)

Pore geometry

No (not on macroscopic scale)

Temperature

Yes, influence oftemperature on
exchange reactions
must be acknowledged, but effect is
not clear

Water content of
concrete

Possibly indirectly
through porewatercompositionNo

Gas content of
concrete

In case of reactive
gases: indirectly
through porewatercomposition

No

RN sorption by bottom
plate materials isneglected. Total massconsidered for reactions
with major ions

See Section 8.4.4

Only for concrete
See Section 8.4.4(leaching of Ca,
possible changes inmechanical properties)
Neglected

No

Hydrovariables
No(pressure and flows)

Possibly indirectly
through porewatercomposition / mineralequilibria

Stress stateNo

No

Composition of
concrete

Yes

Radionuclide sorption
is neglected

Yes

See Section 8.4.4

Concrete porewatercomposition

Yes

Radionuclide sorption
is neglected

Yes (depending on
degradation stage)See Section 8.4.4

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Influence on variables of bottom plate: As pointed out above, interaction of the solid cement matrix
with groundwater influences the overall system composition. This may result in an alteration or (partial)
loss of mechanical properties of the concrete part.
Influence of the composition of concrete part: The concrete will impose very alkaline conditions(pH > 11) in the immediate vicinity, irrespective of small variations in its composition. As pH is
one of the main parameters influencing sorption, the solid-liquid distribution of trace and major
elements in the affected areas will be affected as well. In the concrete part of the bottom plate itself,radionuclide sorption is neglected.
Influence of temperature: As pointed out in Section 3.5.5, not enough data as a function of temperature are available to clearly evaluate temperature effects. This is also true for cementitious materials.
While an increase or decrease of the temperature (in comparison to room temperature) is potentiallyof importance for uptake, it will most likely have a more pronounced effect by directly affecting themineralogy of the cement matrix.
Boundary conditions

There are no particular boundary conditions to discuss for this process. The relevant boundary conditions
in order to treat the process quantitatively are those of the transport processes that control the exchangeof solutes between the porewater in the concrete material and the water in its adjacent components, i.e.
mainly the boundary conditions of the process advection.
Model studies/experimental studies

A vast amount of experimental as well as a number of modelling studies is available regarding theuptake of various radionuclides by hardened cement paste, individual cement mineral phases (CSH,etc.), as well as whole concrete and mortar samples. According to a fairly recent analysis of thestate of the art /ANDRA 2005/, the uptake mechanism is in many cases not established yet, but theavailable database has become sufficiently systematic to define Kd for many elements, especially forthe portlandite-dominated state. Some systematic datasets as well as some model approaches can beused to relate such values to other degradation states. Since radionuclide sorption in the concrete partof the bottom plate is neglected, no further discussion of these topics is given here. For a discussionof degradation processes and stages, see Section 8.4.4.
Time perspective

Uptake processes are in many cases initially completed within weeks, followed by a much slower
process that may extent over the timescale of years. Available evidence /Ziegler 2000, Ochs et al.
2003/ suggests that the first process represents some type of (near)-surface attachment, whereas
the latter is related to further diffusion into particles, incorporation, etc. Especially after long timeframes of interaction, uptake is likely to be only partly reversible.
Natural analogues/observations in nature

Because cement is a technical product, only few (natural) analogues exist. Best studied is the Marqarin
site; the respective research activities and results are extensively documented (see /Smellie 1998/ andreferences therein). Because of the reactivity of hydrated cement mineral phases, it is generally difficult
to deduce the original uptake process on the basis of aged samples, however.
Handling in the safety assessment SR-Site

Radionuclide sorption on the bottom plate is neglected. For handling of possible effects of concrete
alteration, see Section 8.4.4.
Uncertainties

As pointed out above, the mechanisms of uptake are still not established for many cases. As the process of radionuclide sorption is neglected, no uncertainties regarding handling need to be discussed.
For a discussion of uncertainties related to concrete alteration, see Section 8.4.4.
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8.4.4

Alteration of concrete

Overview/general description

This process is in general identical with the corresponding alteration process for concrete in thetunnel plug (Section 5.4.4)
Handling in the safety assessment SR-Site

The alteration process for the bottom plate is assumed to be comparable with the same process forthe tunnel plug, Section 5.4.4. Only the effect on the geochemistry is studied. The hydraulic propertiesof the bottom plate are always assumed to be “degraded”.

8.4.5

Aqueous speciation and reactions

Overview/general description

This process is in general the same as the corresponding process in the buffer (see Section 3.5.7)
Handling in the safety assessment SR-Site

The process is included in a separate modelling task of the concrete degradation. This is further
described in the section for the plug (Section 5.4.4).

8.4.6

Corrosion of copper

Overview/general description

The copper plate will behave similar to the copper in the canister overpack, see Fuel and Canister
Process Report /SKB 2010k/, section copper corrosion. The difference is that the copper plate only
has bentonite on one side and the other side will be exposed to the concrete part of the plate.
Handling in the safety assessment SR-Site

The copper plate is neglected in SR-Site. It is assumed to have no mass-transfer resistance in theassessment.

8.4.7

Microbial processes

Overview/general description

Microbial processes are stimulated by organic compounds and the gases hydrogen and methane/Madigan et al. 2008/. The addition of organic compounds to low pH cement, such as polycarboxylate
polymers, may support microbial growth and activity in cases where the added compounds dissolve,
or never solidifies. The peripheral slot volume, 30 mm wide, where the concrete, the copper plate, thebuffer and the rock meet may be beneficial for microbial growth and activity, if water is available andthe buffer does not reach full swelling pressure in this position. The microorganisms can then obtainenergy and nutrients for growth from both the buffer material and the concrete. The higher pH of theconcrete to be used (<11), relative to groundwater, is not a conceptual problem for microorganisms
as they have been found active at pH above 12 /Pedersen et al. 2004/. Microbial growth on concrete
additions may result in production of sulphide, acid, gas and complexing agents as briefly outlined
next. Sulphide production may occur if organic concrete additions dissolve and sulphate is available.
If sulphide production occurs in the interfaces of concrete and other structures during the aerobic
swelling phase, sulphuric acid may form by the spontaneous oxidation of sulphide with oxygen.
Sulphide production may also be a problem for the copper plate as sulphide is corrosive to copper.
Production of methane may occur if low molecular carbon compounds such as formaldehyde areadded to and subsequently leached from the concrete. Bacteria form complexing agents /Johnsson
et al. 2006, Essén et al. 2007, Moll et al. 2008/ during growth. Such agents may produce if organic
concrete additions become available to microbial processes.
Dependencies between process and bottom plate variables

Table 8-8 shows how the process influences and is influenced by all bottom plate variables.
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327

Table 8-8. Direct dependencies between the process “Microbial processes” and the definedbottom plate variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Bottom plategeometry

No

No

Concrete poregeometry

No

No

Temperature

Yes, microbial activity
increases withtemperatureThis effect is notNoconsidered in SR-Site

Hydro-variables(pressures & flows)

Yes, water is needed
for growth of microorganisms. Microbes
grow on hydrogen andmethane Pressure is
not important

This effect is notYes, microbes may
considered in SR-Site form methane andhydrogenStress state

No

Materials –composition andcontent

Yes, if concrete isadded with substancesthat dissolve and support microbial activity

Limited growth isassumed due to low
availability of waterand nutrients

Yes, if acids areproduced from degradation of concrete
additions

Limited growth isassumed due to low
availability of waterand nutrients

Porewater composition and content

Yes, if dissolved
organic components
are present, microbes
can grow

Limited growth isassumed due to low
availability of waterand nutrients

Yes, decompositionof organic dissolved
components may
occur, pH may be
influenced

Limited growth isassumed due to low
availability of waterand nutrients

Limited growth isassumed due to low
availability of waterand nutrientsNo

Boundary conditions

The boundary condition for the process is the amount of dissolved organic compounds from the low
pH cement and the bentonite at the interface between the copper plate, concrete, rock and buffer.
Model studies/experimental studies

Not available.
Natural analogues/observations in nature

Growth and activity of microbes have been observed at pH above 12 /Pedersen et al. 2004/.
Time perspective

Microbial processes will start when water is available. The process rates will probably be largest in
the peripheral slot with interfaces between buffer, the copper plate, rock and concrete during a wet
oxygenic stage, before full swelling and reducing conditions occur.
Handling in the safety assessment SR-Site

In the treatment of the geochemical evolution in the bottom plate, microbial reactions need to beconsidered. Production of sulphide in the bottom plate will be considered in the assessment ofcanister corrosion.

328

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Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

All the references in this section are from peer reviewed scientific journals. However, the mostrecent ones may be in the progress of being published.
Uncertainties

The key uncertainty is the presence and availability of nutrients. This can be handled bya sensitivity study.
Uncertainties in mechanistic understanding
The amount of dissolving organic concrete additions to the porewater and the peripheral slot
environment is unknown.
Model simplification uncertainty in SR-Site
Not relevant since no model for microbial processes in contact with low pH cement is used.
Input data and data uncertainty in SR-Site
Not relevant since no model for microbial processes in contact with low pH cement is used.

8.5Radionuclide transport processes

8.5.1

Speciation of radionuclides

Overview/general description

See corresponding process description for the buffer, Section 3.6.1
Handling in the safety assessment SR-Site

This process is indirectly handled through the selection of parameters for radionuclide transport in
the buffer. In general, the handling of the process is the same as for the corresponding process in thebuffer (see Section 3.6.1).

8.5.2

Transport of radionuclides in the water phase

Overview/general description

As long as the copper plate is intact there will be no transport of radionuclides in or through thebottom plate. However, the copper is not protected by a bentonite barrier and can be exposed tocorrosion from species in the groundwater. This is especially the case when the bottom plate is incontact with a water-carrying fracture and the concrete is degraded (see also Section 3.6.2).
If the copper is corroded and the concrete is degraded the bottom plate may act as a porous media
with a rather high hydraulic conductivity. This means that the bottom plate could act as a sink forradionuclides and as a source for canister corrodants.
Dependencies between process and bottom plate variables

Table 8-9 summarises how the process influences and is influenced by all component variables andhow these effects are treated in SR-Site.

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329

Table 8-9. Direct dependencies between the process “Transport of radionuclides in the waterphase” and the defined component variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Copper plategeometry

Yes, determines if
radionuclide transport
can occur

Neglected, a limited
No, the process has
lifetime for the copper
no relevance for theplate can be assumed
copper platein radionuclide transport
calculations

Concrete geometry

Yes, gives the actual
size of the potential sinkIncluded as a volume inradionuclide transport
calculations

Concrete poregeometry

Yes, determines theporosity

Included as a porosityNo,
in radionuclide transport
calculations

Radiation intensity

No

No

Temperature

Yes

Neglected, since theNoeffect of the temperatureon the radionuclidetransport is very minor

Water content

Yes

Assumed to be satuNo
rated when radionuclidetransport occurs

Gas content

Yes

Assumed to be satuNo
rated when radionuclidetransport occurs

Hydrovariables(pressure and flow)Yes

The water flow
is included in theradionuclide transport
calculations

Stress stateNo

No

Concretecomposition

No, assumed to be
degraded when radio­
nuclide transport occurs

No

Concrete porewatercomposition

Yes

Neglected, since theeffect of the sorption in
the degraded concrete
is conservatively
neglected

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

No

No

Yes

Output from radionuclide transport
calculations

Boundary conditions

The inner boundary for this process is the concentration profile of radionuclides in the buffer. The
outer boundary is the water in rock, especially if a fracture intersects the bottom plate.
Model studies/experimental studies

In general, this process represents diffusion of radionuclides from bentonite into a porous volume.
This is described for the buffer in Section 3.5.3.
However, no studies that directly assess this particular process have been performed.
Natural analogues/observations in nature

Not relevant

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Time perspective

This process may be active all timescales after canister failure.
Handling in the safety assessment SR-Site

Radionuclide transport in the buffer is handled in the integrated modelling of radionuclide transport
for the repository as described in Section 3.6.1. The bottom plate will be included as an elementin the COMPULINK integrated near field model, see the SR-Site Model summary report /SKB
2010n/. The conceptual and mathematical model is described in /Neretnieks et al. 2010/, see also
Section 8.4.1.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The SR-Site Model summary report /SKB 2010i/ and /Neretnieks et al. 2010/ will be subjected to a
documented factual- and quality review according the SR-Site standards.
Uncertainties

See Section 8.4.1.

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9Processes in borehole seals

9.1

Thermal processes

9.1.1

Heat transport

Overview/general description

For borehole seals extending a distance l = 400 m from the repository horizon to the ground surface,
the heat flow Q would be less than 0.2 W if the temperature difference DT between the ends of the
tube is about 40°C even if the seals are made of solid 75 mm (A= 0.0044m2) diameter copper rods
with thermal conductivity λ=390 W/(m·K). (Q=λ*A*DT/l). In reality it will be much less.
Because of the small heat flow, the temperature disturbances (cooling of rock in the bottom end andheating in the top end) will be very modest and involve very small rock volumes in the immediate
vicinity of the tube. In the concrete portions and, in particular, in the bentonite filling not covered bycopper tubes, the heat flow is vanishingly small. The temperatures of the borehole seals are almost
completely determined by the thermal evolution of the surrounding rock, i.e. practically independentof the heat transport within the seals.
Handling in the safety assessment SR-Site

The evolution of the rock temperature at different positions within and around the repository is
calculated in /Hökmark et al. 2009/. Because of the small thermal gradients and the relatively low
temperatures in the borehole seals, there is no specific handling of the heat transport process in theborehole seals.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

The gradient and temperature statements are supported by results obtained using calculation schemes
described in /Claesson and Probert 1996/. While this particular report is not peer or factual reviewed,
the calculation scheme has been repeatedly applied in peer reviewed papers /Hökmark and Claesson
2005/ and in reports that have undergone (/Hökmark and Fälth 2003/) or will undergo (/Hökmark
et al. 2009/) a documented factual and quality review.

9.1.2

Freezing

Overview/general description

The overview and general description of the corresponding buffer process, Section 3.2.2, are also
valid for the bentonite part of the borehole seals. In contrast to the corresponding buffer process, two
new aspects must be considered:•Freezing will occur in the borehole seals.
The extension of the frozen uppermost section of the ground during permafrost periods will vary withtime and is predicted to have a maximum penetration of approximately 250 m /Hartikainen 2004/.

•

A temperature gradient is imposed over the bentonite, due to the vertical geometry.
This temperature gradient, in turn, corresponds to a suction gradient and water may be transported in the bentonite to form ice lenses e.g. at the interfaces between bentonite and concrete.

When bentonite freezes, the swelling pressure will fall to zero in the temperature interval 0°C < T< Tc, where Tc is a density dependent critical temperature. For a density of 2,000 kg/m3, the criticaltemperature Tc is < –5°C. A thermal gradient below 0°C will thus correspond to a swelling pressure
gradient which might cause redistribution of the bentonite.
When the pressure drop occurs, the transport paths in the surrounding rock will be frozen.

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333

Water flow from the induced suction gradient can be estimated by identifying swelling pressure withsuction:∂P∇ψ = −∇Ps = − s ∇T 								(9-1)∂Twhere ψ is suction (see Section 3.3.1). The derivative ∂Ps has an estimated value of 1 MPa/°C for∂T
temperatures below 0°C (see Section 3.2.2).
A typical geothermal temperature gradient is 0.025°C/m and using this value together with a valueof the hydraulic conductivity of 10-13 m/s, the corresponding water flux is 2.5∙10-13 m/s or approximately 8 μm/y, which is also the estimated ice lens growth.
Table 9-1 summarises how the process influences and is influenced by all borehole seal variables andhow these effects are treated in SR-Site.
Boundary conditions

When freezing of the bentonite in the borehole seals occurs, the water in the surrounding rock will
be in ice form. Hence, transfer of water between rock and bentonite at the same level will be very
limited. The slowly varying temperatures of the rock surrounding the seals also constitute a boundary condition.
Table 9-1. Direct dependencies between the process “Freezing” and the defined borehole seal
variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Borehole geometry

Yes, determines thethermal boundary
conditions

Included in theassessment

No

Pore geometry

Yes

Included in theassessment

No

Temperature

Yes

Included in theassessment

No

Water content

Yes, via freezing point Included in theand transport capacity assessment

Yes, If freezing takes
place redistributionof waterGas content(Yes)

A gas content would
affect the process
but the buffer will becompletely saturated
when freezingnoHydrovariables(pressure and flows)

Yes, freezing induces
water transport

Included in theassessment

Yes

Stress state

Yes, Influences thefreezing point to asmall degree

Included in theassessment

Yes. Swelling pressure Included in theaffected also before
assessment
ice formation

Sealing materials– composition andcontent

Yes, via the specificsurface area

Included in theassessment

No

Porewater
composition

Yes, via the specificsurface area andtypes of counter ions

Neglected, effectassumed to be small

No

Structural and straymaterialsNo

334

Neglected

Included in theassessment

No

TR-10-47

Model studies/experimental studies

The general temperature response in compacted bentonite has been worked out both theoreticallyand experimentally in /Birgersson et al. 2010/. The same report also estimates the water transportcapacity in a vertically extended bore hole seal.
Natural analogues/observations in natureThis has not been done yet. However, knowledge can probably be achieved by studying permafrost areas.
Time perspective

Freezing of borehole seals are relevant during the entire repository lifetime.
Handling in the safety assessment SR-Site

Because of the very restricted water transport capacity, the process is neglected.
Uncertainties

Uncertainties in mechanistic understanding
The freezing of soils is a well known process.
Model simplification uncertainties for the above handling in SR-Site
The reproducibility of the pressure response due to freezing in bentonite is good.
Input data and data uncertainties for the above handling in SR-Site
The key uncertainty is the location and duration of the permafrost front. Other uncertainties are related
to the suction gradient and the assumed hydraulic conductivity. How this affects the conclusions about
the process is discussed in /Birgersson et al. 2010/

9.2

Hydraulic processes

9.2.1

Water uptake and transport under unsaturated conditions

Overview/general description

Water transport in bentonite under unsaturated conditions is a complex process that is dependent on,inter alia, temperature, smectite content, degree of water saturation and water content in the different
parts of the bentonite. The most important driving force for water saturation under deep repository
conditions is a negative capillary pressure in the pores of the bentonite that leads to water uptake
from the surrounding rock. The supply of water from the rock by water transport in fractures and in
the rock matrix is also a decisive factor for the temporal evolution of the process.
For the bentonite part of the investigation holes, the process of water uptake and transport underunsaturated conditions is identical with the process described for the buffer (Section 3.3.1). Therea detailed description of the process is given and it will not be repeated here.
Dependencies between process and borehole seal variables

Table 9-2 summarises how the process influences and is influenced by all variables of the bentonite
seals and how these effects are treated in SR-Site.

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Table 9-2. Direct dependencies between the process “Water uptake and transport under unsaturated
conditions” and the defined borehole seal variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Borehole geometry

Yes

Geometry included in
the models

No

Pore geometryYes. Via void ratio(density) and void
geometry

Variable in the model(Yes) If erosion takes
See 9.4.10 and 9.2.4
place (secondary effect)

Temperature

Yes. Indirect throughwater viscosity andhydrovariables in atemperature gradient

Negligible

Yes Wetting anddrying affects thermal
conductivity

Negligible

Water content

Yes. Via degree ofsaturation

Variable in the model

Yes

Variable in the modelGas content

Yes. Via the gaspressure and degreeof saturation

Variable in the model

Yes

Variable in the model

Hydrovariables(pressure and flows)

Yes. Basic variables.

Variable in the model

Yes

Variable in the modelStress state

Yes

Variable in the model

Yes

Variable in the model

Sealing materials– composition andcontent

Yes. Hydraulicconductivity andretention curve etc.

Sensitivity analyses(Yes). If erosion takes
See 9.4.10 and 9.2.4
place (secondary effect)

Porewater
composition

Yes

Controls some param- Yes
eters in the model

Neglected, of limited
importance

Boundary conditions

Interaction with the rock: The investigation holes are filled with water during emplacement andthe bentonite cylinders are placed in a perforated tube during installation down into the holes. During
installation the slot between the rock surface and the bentonite cylinders are filled with water with ahydraulic head equal to the hydrostatic pressure. This means that there is water enough to expand thebentonite cylinders, make them swell through the perforated holes and come in contact with the rockand only leave a small amount of unsaturated pores (~10%). The remaining water to saturate thebentonite is taken from the rock, either from fractures intersecting the hole or from the rock matrix.
Interaction with the plugs in the holes: Some parts of the holes will be grouted, stabilised andplugged with high performance, low-pH, silica concrete. The upper part will be plugged with coresamples that are cast integral with the rock. These parts will not affect the wetting process.
Model studies/experimental studies

The material model that can be used for modelling the wetting phase is described in the corresponding process description for the buffer (Section 3.3.1) and will not be repeated here.
In SR-Can, no calculations of the wetting of the bentonite in the boreholes were done and are probably not needed for safety reasons, since the small size of the boreholes ensures that full saturation
will be reached within five years. However, some sensitivity analyses will be done for demonstration
purpose /Åkesson et al. 2010a/.
Field tests of bentonite seals in boreholes have been conducted both in Stripa and in Äspö HRL. One
full scale sealing test has also been installed in Olikluoto.

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Experimental studies in the laboratory of the extreme cases with no water available and excesswater available should also be done in order to improve the understanding of the water uptake andhomogenisation of the bentonite cylinders in the holes. Such tests have, so far, only been done withperforated copper tubes around the cylinders.
Natural analogues/observations in nature

Wetting of bentonite in nature has not been studied and would probably not contribute to theknowledge necessary for water uptake assessment in a repository.
Time perspective

The water will penetrate the superficial part of the bentonite cylinders that will swell and come incontact with the rock surface as a gel within a few hours. Then the water will continue to be taken up
by the inner parts of the cylinders and at the same time homogenise. After less than a year, the process
will be dominated by the water supply from the rock. Full saturation and homogenisation is expected
to be reached within five years even in very tight rock but this should be checked with calculations.
Handling in the safety assessment SR-Site

Today’s knowledge of the behaviour is judged to be sufficient for safety reasons but sensitivity
analyses in combination with laboratory tests should be done in order to verify these statements.
Boundary conditions: Sensitivity analyses of the wetting with different rock properties will be done.
Handling of variables influencing this process: Most variables in Table 9-2 are included in the
THM model that will be used for the modelling.
Handling of variables influenced by the process: The THM model is fully coupled so the changes
in variable properties will also be considered.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Modelling of the wetting phase is reported in a general THM modelling report that will undergo adocumented factual- and quality review /Åkesson et al. 2010a/.
Uncertainties

The knowledge of the hydraulic behaviour of unsaturated bentonite material is gained from investigations done on MX-80 while almost no tests have been performed on the other candidate (Milos
bentonite). At the density considered for the buffer material the behaviour should be similar.
Uncertainties in mechanistic understanding
The understanding of the process is deemed sufficient for the needs of the safety assessment. For
further information; see the description of the corresponding buffer process (Section 3.3.1).
Model simplification uncertainties for the above handling in SR-Site
The same models are used and thus the same simplifications are used as for the buffer. They are lessimportant for the seals than for the buffer (see Section 3.3.1)
Input data and data uncertainties for the above handling in SR-Site
The greatest uncertainty/variability concerns the hydraulic conditions in the near-field rock andthe hydraulic interaction between the rock and the bentonite. This will be addressed by analysing a
number of cases with differing assumptions regarding these properties. The data used in the modelling are discussed in the Buffer THM data report /Åkesson et al. 2010b/.

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9.2.2

Water transport under saturated conditions

Overview/general description

Water transport in saturated high density bentonite is a complex interplay between several subprocesses on a microscopic scale. On a macroscopic level, the result is that the permeability of asaturated bentonite is very low, and this is also the essential result for the safety assessment.
Water flow in saturated buffer is a special case of unsaturated flow. The processes involved are the same
as for unsaturated conditions, but can vary widely in importance compared with unsaturated conditions.
The most important mechanism under saturated conditions is transport of water in liquid phase, which
is driven by a water pressure gradient. This transport process can be described by Darcy’s law. The
hydraulic conductivity K is a function of the composition of the buffer, the void ratio, e, the ion concentration in the porewater, ic, and the temperature, T.
The hydraulic conductivity of the reference bentonite (MX-80 and Deponit CA-N) to low-salinity water
after complete saturation is approximately 10–13 m/s at the density 2,000 kg/m3 at water saturation.
The process is identical to the corresponding process in the buffer and is further described in
Section 3.3.2.
Dependencies between process and borehole seal variables

Table 9-3 summarises how the process influences and is influenced by all variables of the bentonite
seals and how these effects are treated in SR-Site.
Although many variables influence the process and vice verse according to Table 9-3 the handlingin the SR-Site will be that the sealed investigation holes are treated as intact rock, which means thatthey are disregarded in the site-scale model. The process is only important for the safety if bentonite
has been lost.
Influence of density, temperature and salinity
The hydraulic conductivity is primarily dependent on the geometry and composition of the bentonite
seal, the density (expressed in Table 9-3 as water content), the ion concentration in the porewater andthe temperature. For further information, see Section 3.3.2.
Boundary conditions

The outer boundary conditions are determined by the hydraulic properties of the rock surrounding theinvestigation hole, in particular the properties of possible fractures intersecting the investigation hole.
Model studies/experimental studies

See Section 3.3.2.
Natural analogues/observations in nature

See Section 3.3.2.
Time perspective

With a hydraulic conductivity of the bentonite of 10-13 m/s, the borehole is at least as impermeable as
the rock matrix without any fractures. This means that even with an impossible axial hydraulic gradient of 1, the time for water to flow from the repository level along the borehole up to the surface
would take in the order of 100 million years. Freezing is treated separately.

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Table 9-3. Direct dependencies between the process “Water transport under saturated conditions”
and the defined borehole seal variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Borehole geometry

Yes, mainly by theflow path.

Given as input data.

Yes, during swellingsince water flow into
the voids makes thebuffer swell if there isspace available.

Included in themechanical model

Pore geometry

Yes, determines thehydraulic conductivity

Included in the valueof the hydraulicconductivity as a function of the density. The
pore geometry can beaffected by a mineraltransformation or achange in porewatercomposition.

Yes, during swellingsince water flow into
the voids makes thebuffer swell if there isspace available.

Included in themechanical model

Temperature

Yes, through waterviscosity and attemperature below 0(see Freezing).

This will be neglected
in SR-Site

Insignificantly.

No

Water content

Influenced via density(pore geometry)

Determines thehydraulic conductivity,
which is included in
the model.

Yes, during swelling
Included in thesince water flow intomechanical model
the voids makes the
bentonite swell if thereis space available.

Gas content

Insignificantly(dissolved gases)

Excluded since itwould reduce hydraulic conductivity

No

Hydrovariables(pressure and flows)

Yes, main variables.

Included in the flow
model

Yes

Included in theflow modelStress state

Yes. If the effective
Included in the
stress on the bentonite mechanical model.
deviates from theswelling pressure thebentonite will change
volume and water willflow into or out from it.

Yes

Included in themechanical model

Sealing materials– composition andcontent

Yes, determines thehydraulic conductivity

Included in the model

No

Porewater
composition

Yes, determines thehydraulic conductivity

Included in the model

Yes

Structural and straymaterials(Insignificant)Replacement ofporewater isdiscussed in 3.5.6

No

Handling in the safety assessment SR-Site

As illustrated by the time perspective the water flow along the holes can be neglected as long as
the bentonite remains its low hydraulic conductivity. This case will thus not be further handled
in SR-Site. If there is a risk that a large part of the bentonite will be lost or transformed to obtainother properties, these cases must be handled. Cases with unsealed boreholes are discussed in the
Geosphere Process Report /SKB 2010i/.
The primary handling of these cases will be to consider how the hydraulic conductivity could vary
for the evolving conditions in the bentonite seal throughout the assessment period.

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339

Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

No references are used.
Uncertainties

So far, very few tests have been performed on Deponit CA-N bentonite, but the results available
indicate that the properties of this material are very similar to the properties of MX-80 at the densities
considered for the buffer material.
See also Section 3.3.2.

9.2.3

Gas transport/dissolution

Overview/General description

Gas processes in the borehole seals are only relevant for the saturation period and only for the
bentonite part of the seal – this is described in Section 3.3.3.
Handling in the safety assessment SR-Site

The process is included in the supporting sensitivity analyses of the saturation phase see Section 9.2.1.

9.2.4

Piping/erosion

Overview/general description

As described for the corresponding buffer process (Section 3.3.4), piping and erosion are primarily
a problem in the unsaturated stage when water flows between different parts of rock cavities. Since
the investigation holes are filled with water from start, erosion will not take place after installation.
There are no high axial hydraulic gradients along the hole so there will be no axial water flow thatcan erode the bentonite. Any large fracture zones that may cause flow along the investigation holeduring installation are presupposed to be grouted and sealed.
Handling in the safety assessment SR-Site

Since piping and erosion of the bentonite cannot occur in stagnant water, the process is ruled out forthe seals in the investigation holes.
However, there is a risk for abrasion of the material during the installation phase. This is discussed in
the Closure production report /SKB 2010f/
The long term erosion by colloid transport is treated in Section 9.4.10

9.3

Mechanical processes

9.3.1 Swelling/mass redistribution
The swelling process has been merged with other stress-strain-related processes that can cause mass
redistribution within the borehole seals namely thermal expansion, creep and a number of interactions.
Overview/general description

Water is absorbed by both unsaturated and saturated bentonite (that is able to physically expand)and causes swelling. If the bentonite is unable to expand freely, a swelling pressure develops, which
locally reaches its peak at full water saturation.
The process of swelling and mass redistribution is described in detail with all influences and dependencies in the buffer Chapter (Section 3.4.1) and will not be repeated here. Instead, a brief descriptionof the mechanical evolution of the bentonite cylinders in the investigation holes is provided.

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During placement of the bentonite cylinders confined in the perforated copper tube into the hole, the slot
between the cylinder and the rock surface is filled with water with a water pressure with a magnitude
that depends on the depth (0-10 MPa water pressure). The high water pressure in combination with thehigh suction in the bentonite means that the surface of the cylinders will be wetted immediately andthere will be a gel expanding from the surface. After less than a few hours, the gel will come in contact
with the rock wall but the water will continue to penetrate into the cylinder and make more bentonite
swell. In this way the gel will consolidate and increase its density at the same time as the bentonite
cylinder slowly decreases its density, at first locally close to the surface and then deeper and deeper.
Due to the perforated copper tube around the bentonite cylinders the swelling will be rather inhomogeneous. The swelling will at first go through the holes and out to the rock surface and then along the rockwall between the tube and the rock wall and fill up the remaining parts. The influence of the copper tube
on the wetting rate is insignificant compared to if there were no tube but the density distribution at end
state will differ significantly due to the friction in the bentonite that will prevent homogenisation. The
density between the tube and the rock wall will be much lower than the average density but still highenough to function as a seal.
There are a combined saturation, swelling and homogenisation of the bentonite that takes place even
if there is no additional water available for the bentonite. This process is complicated and difficult tomodel. Some tools (Code Bright and Abaqus) are available and further developments are ongoing.
The process that may take up to a few years, depending on the availability of water, ends when the
bentonite is saturated and the radial pore pressure distribution equalised. The swelling pressure
against the rock will increase with time and is estimated to end at between 2 and 5 MPa.
Dependencies between process and horehole seal variables

Table 9-4 summarises how the process influences and is influenced by all variables of the bentonite
seals and how these effects are treated in SR-Site.
Table 9-4. Direct dependencies between the process “Swelling/mass redistribution” and thedefined borehole seal variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Process influence on variable
Influence present?
Handling of influence(Yes/No)(How/Why not)Description

Borehole geometry

Yes

Geometry included in
the models

No

Pore geometryYes. Through stressstate and void ratio

Included in themodelsYes

Temperature

Yes. Thermal expansion and change ofretention properties(especially at temperatures below 0).

No, negligible

No

Water content

Yes.

Variable in the model

Yes

Variable in the modelGas content

Yes. Degree ofsaturation and gas
pressure

Included in themodelsYes

Included in themodelsHydrovariables(pressure and flows)

Yes. Basic variables.

Variable in the model

Yes

Variable in the modelStress state

Yes

Variable in the model

Yes

Variable in the model

Sealing materials– composition andcontent

Yes. Determines
many parameters in
the models

Sensitivity analyses

No

Porewater
composition

Yes. Influences many
parameters in themodelsSensitivity analyses

No

Structural and straymaterials

No

TR-10-47

Included in themodels

No341

As shown in Table 9-4, there are many variables that affect the mechanical behaviour and which areincluded in the mechanical models. These dependencies are described for the corresponding buffer
process (Section 3.4.1) and will not be repeated here.
Boundary conditions

The mechanical boundary conditions and interactions are limited since the borehole implies a very
stiff confinement with no complications.
Interaction with the rock
The following mechanical interactions between the bentonite in the investigation holes and the rockhave been identified:
1. Swelling pressure from the buffer is transferred to the rock but is not expected to lead to any
rock movements. The elastic deformation of the rock at the expected swelling pressures is small
due to the rock’s high modulus of elasticity (about 70 GPa).
2. Forces of friction between the bentonite and the rock surface will occur along the borehole in theunperforated parts of the copper tube. Axial friction can also occur if there are concrete seals thatare partly dissolved.
3. Thermal expansion of the buffer may cause stresses on the rock if the bentonite is watersaturated when the temperature increase occurs. The slow temperature increase in combination
with the drainage through the rock implies that this effect is insignificant.
4. Convergence of the holes will occur with time but the effect is insignificant.
5. Intrusion of bentonite into fractures in the rock is very limited due to the small aperture of thefractures and the shear resistance caused by the friction between the bentonite and the fracturesurface. This interaction is treated in Section 3.4.1. It may also occur due the colloid transport out
from the fracture (see Section 9.4.10). Since the mass of bentonite in the hole at the intersecting
fractures is small this may entail local problems with complete loss of bentonite at the fracture.
However, the problem is judged to be local since the friction between the rock and the bentonite
will prevent mass transfer axially. A sensitivity calculation of these effects should be done.
Interaction with concrete plugs and concrete stabilised parts of the hole.
If the concrete degrades and cement is transported away there may be extra space for swelling
bentonite. However, this is judged to be a minor problem since the effect is local. Some sensitivity
calculations should be done.
Model studies/experimental studies
A large number of experimental studies and model studies of the mechanical properties of both
unsaturated and saturated bentonite have been performed. These are treated in the buffer process
Section (3.4.1).
Regarding the behaviour of plugged boreholes, field tests of bentonite seals in boreholes have been
made both in Stripa and in Äspö HRL. One full scale sealing test has also been installed in Olikluoto.
Time perspective

The water saturation phase is expected to take less than a few years, depending on the properties of
the rock. During this time span, the portion of the expansion and homogenisation that is limited by
the consolidation rate (which is controlled by the hydraulic conductivity) also takes place.
Natural analogues/observations in nature

No natural analogues concerning mechanical behaviour have so far been studied. The existence of
relevant natural analogues is probably very limited and difficult to evaluate.

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Handling in the safety assessment SR-Site

Mainly three studies should be done for SR-Site:
1. Analytical modelling of the homogenisation process related to the swelling of bentonite throughthe perforated copper tube
2. Modelling and sensitivity analyses of the saturation processes from installation until equilibrium
is reached.
3. Modelling and sensitivity analyses of the effect of local loss of bentonite.
The effect of an un-sealed borehole will be illustrated in SR-Site.
Adequacy of references supporting the suggested handling in the safetyassessment SR-Site

Modelling of the three cases is reported in a general THM modelling report that will undergoa documented factual- and quality review /Åkesson et al. 2010a/.
Uncertainties

Uncertainties in mechanistic understanding
See the buffer process 3.4.1
Regarding the homogenisation process there are uncertainties in the analytical models and the friction
coefficient but comparisons with measurements have to some extent confirmed the models.
Model simplification uncertainties for the above handling in SR-Can
See the buffer process 3.4.1 and uncertainties in mechanistic understanding.
Input data and data uncertainties for the above handling in SR-Can
See the buffer process 3.4.1 and uncertainties in mechanistic understanding.

9.3.2

Liquefaction

Overview/general description

Liquefaction in the buffer is described in Section 3.4.2 and is not further addressed here.
Handling in the safety assessment SR-Site

The process is ruled out for the bentonite in the investigation holes and would if it could occur nothave any influence on the performance.

9.4

Chemical processes

9.4.1 Advective transport of species
In this context, advection refers to transport of any forms of additional matter, e.g. ions, molecules
or colloids, with porewater flow. The transport direction is thereby principally from volumes of highwater pressure to volumes of lower pressure. The process leads to redistribution of solutes in theporewater and thus affects the porewater composition. There are several possible causes of pressure
gradients in the borehole seals, e.g. external water pressure, affinity for water in the bentonite andtemperature induced volume change of the water. Advection is closely related to water flow in theborehole seals, which is described in Section 9.2.2. Advection may though be different compared topure water flow due to ion equilibrium effects in the case of ions, and due to filtering by nano-sizedpores in the case of colloids.

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Advective transport is of importance in the bentonite part of the borehole seals during the saturation
period. Under saturated conditions, the transport of solutes in the bentonite porewater is expected
to be dominated by diffusion, see Section 9.4.2. Advective flow will probably be significant in the
cement/sand parts of the seals in the long term perspective due to cement degradation.
Dependencies between process and borehole seal variables

Table 9-5 summarises how the process influences and is influenced by all borehole seal variables andhow these effects are treated in SR-Site.
Boundary conditions

The external boundary condition for this process is the hydraulic gradient and the concentration gradients for species across borehole seals. In principle, the difference between two fractures intersected bya plug is of main interest, since the difference within one fracture will be insignificant on the lengthscale of the borehole seal.
Model studies/experimental studies

Field studies of borehole seals are ongoing by SKB and Posiva in Äspö and Olkiluoto.
Table 9-5. Direct dependencies between the process “Advective transport of solutes” and thedefined borehole seal variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence(Yes/No) Description(How/Why not)Process influence on variable
Influence present?
Handling of influence(Yes/No) Description(How/Why not)Borehole geometry

The geometry of the
Included in model
boreholes determines
the cross-section available for advection.

Yes, throughdissolution/
precipitation

Of no concern

Pore geometry

Yes, dominating effectthrough hydraulicconductivity in bothbentonite and concrete

Yes, throughdissolution/
precipitation

Of no concern

Temperature

Insignificant

Insignificant

Water content

Yes, by definition

Assumed to besaturated

Insignificant

Gas content

Yes, by reductionof transport paths,
through dissolution/
precipitation of gas

Assumed to besaturated

Yes, dissolution/
release of gas

Assumed to besaturated

Hydrovariables(pressure and flows)

Yes, by definition

Included in model

Yes, through osmosis

Advection only
considered after lossof bentonite

Stress state

Indirectly through poregeometry

Yes, through osmosisin the bentonite

Advection only
considered after lossof bentonite

Sealing materials– composition andcontent

The content ofmontmorillonite is ofdecisive importancein the bentonite part.
Indirectly throughdissolution/precipitation process in theconcrete part

Advection only
considered after lossof bentonite

Yes, throughprecipitation/
dissolution and ion
exchange processes

Of no concern

Porewater
composition

Yes, by definition

Advection only
considered after lossof bentonite

Yes, by definitions

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Natural analogues/observations in nature

No relevant studies available.
Time perspective

Advective flow transport is important on all timescales. Significant flow will take place in the
cement/sand parts after degradation of the cement.
Handling in the safety assessment SR-Site

Before saturation: No detailed modelling of advection is required. It should be noted that the composition of the porewater will be influenced by the composition of the intruding water during saturation.
The groundwater composition will be considered when defining initial conditions for modelling of thechemical evolution after saturation.
Open boreholes will be considered as a variant in the hydrogeological modelling.
Handling of variables influencing this process:See section 9.2.2
Uncertainties

Uncertainties in mechanistic understanding
See Section 9.2.2 for uncertainties concerning water advective transport.
Model simplification uncertainties SR-Site
Saturated conditions: The process can be modelled with a standard mass-transfer approach.
Input data and data uncertainties in SR-Site
See Section 9.2.2

9.4.2 Diffusive transport of species
An overview of diffusion processes and phenomena as relevant for bentonite and concrete are given in
Sections 3.5.3 and 8.4.2, respectively, these considerations also apply to the concrete- and bentoniteparts of the borehole seals. /Luna et al. 2006/ conclude that the main diffusion parameters (incl.
porosity) will not be significantly affected by an alkaline plume.
Dependencies between process and borehole seal variables

Table 9-6 shows how the process influences and is influenced by all system variables.
Influence on variables of borehole seals: Through transport of dissolved species to and from theconcrete/bentonite system, diffusion will influence the overall composition and rate of alteration.
Influence of variables of borehole seals: Analogous to buffer in case of bentonite; analogous to theconcrete part of bottom plates in case of the concrete part (Sections 3.5.3 and 9.4.2; respectively).
Boundary conditions

A possible transfer of species from the repository system to a borehole seal can be visualised as the
diffusion of dissolved species from one porous medium to another. The transfer of species from theseal to the geosphere can be visualised as the diffusion of dissolved species from the clay or concrete
into flowing water (equivalent flow rate or Qeq).

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Table 9-6. Direct dependencies between the process “Diffusive transport of species” and thedefined borehole seal variables and a short note on the handling in SR-Site.
Variable

Variable influence on process
Influence present?
Handling of influence?
(Yes/No)(How/why not)Description

Process influence on variable
Influence present?
Handling of influence?
(Yes/No)(How/why not)Description

Seal geometry

Yes

Diffusion is neglectedin bore hole seals

No

Pore geometry

Yes, Reflected in themagnitude of diffusion
coefficients

Diffusion is neglectedin bore hole seals

Possibly indirectly
through influence onporewater composition

Temperature

Diffusion is neglected
Yes, influence ofin bore hole seals
temperature on
diffusion in must be
acknowledged, but is
not included (effect is notestablished for concreteand considered as small
for bentonite)

No

Water content

Yes

No

Gas content

Saturated conditions
assumedFor dissolved gases
such as CO2, CH4

Hydrovariables(pressure and flows)

Saturated conditions
assumedNo

Stress state

Indirectly through poregeometry

Possibly indirectly
Neglected
through influence onporewater composition/mineral equilibria/
swelling

Bentonite and concrete composition

Yes, defines magnitudeof diffusivities

Diffusion is neglectedin bore hole seals

Possibly indirectly
through influence onporewater composition/mineral equilibria

Neglected

Porewater
composition

Through effect on
speciation and extent
of EDL in bentonite,
Defines magnitude ofdiffusivities

Diffusion is neglectedin bore hole seals

Through diffusivetransport of main dissolved constituents

Neglected

Diffusion is neglectedin bore hole seals

Neglected

Model studies/experimental studies

Diffusion in the bentonite part of the borehole seals is analogous to diffusion in the buffer and diffusion in the concrete parts of the seals is analogous to diffusion in the concrete part of bottom plates(Sections 3.5.3 and 8.4.2, respectively).
Time perspective

Diffusion could be active in all time perspectives.
Natural analogues/observations in nature

Diffusion in the bentonite part is analogous to diffusion in the buffer, see Section 3.5.3.
Handling in the safety assessment SR-Site, uncertainties

Transport of species with diffusion in the borehole seals is not considered.

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Uncertainties

Not applicable.

9.4.3

Sorption (including exchange of major ions)

Overview/general description

Seals of investigation boreholes consist of bentonite in copper tubes, plus low-pH concrete in the
conductive areas. The sorption processes taking place in these systems are the same as in otherbentonite and concrete parts of the repository system and are described in Sections 3.5.5 and 5.4.3
and 8.4.3, respectively. The effects of concrete-bentonite interaction and the behaviour of organic
concrete admixtures are described in Section 5.4.3.
Handling in the safety assessment SR-Site

Sorption of radionuclides is neglected both in the concrete and the bentonite parts of the borehole
seals. No radionuclide transport is assumed to take place in an intact seal. In the case of a defect seal,
sorption is conservatively neglected.

9.4.4

Alteration of concrete

Overview/general description

This process is in general identical with the corresponding alteration process for concrete in thetunnel plug (Section 5.4.4).
Handling in the safety assessment SR-Site

The pH evolution from the concrete part of the borehole seals will in most respects be identical tothe effect from the plugs (Section 5.4.4). Therefore, the results from modelling of the pH evolution
in and around the concrete plugs will be used to draw conclusions regarding the pH evolution in
and around borehole seals. The evolution of the hydraulic properties of the seals will be studied in asensitivity analysis, see Section 9.2.2.

9.4.5

Aqueous speciation and reactions

Overview/general description

This process is in general identical with the corresponding process in the buffer (Section 3.5.7).
Handling in the safety assessment SR-Site

The process is included in a separate modelling task of the seal (concrete part) degradation, which inprinciple is the same as the plug degradation. This is further described in Section 5.4.4.

9.4.6

Copper corrosion

Overview/general description

Corrosion of copper is described in the Fuel and Canister Process Report /SKB 2010k/ (process
copper corrosion).
Handling in the safety assessment SR-Site

The copper pipes are only important during installation of the borehole seals. No assessment of thecopper in the seals will be done in SR-Site.

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9.4.7

Alteration of impurities in bentonite

Overview/general description

The bentonite material consists not only of montmorillonite, but also of accessory minerals aswell as impurities. In the system under consideration, these can be dissolved and sometimes reprecipitated depending on the prevailing conditions.
The reactions relevant for the borehole seals are basically the same as those in the buffer(Section 3.5.6). However, there are a few differences in the significance of the processes.
•

The temperature is lower and the thermal gradient in the borehole seal is small. Dissolution/
precipitation of minerals due to temperature effects are of minor importance in the seals.

•

The total volume of bentonite in a seal is much less than in a deposition hole. Therefore theeffects of changes in groundwater chemistry will be much faster.

•

The borehole seals are not in direct contact with the deposition holes and changes in the porewaterchemistry will not affect the barriers in the repository

The geochemical evolution of the bentonite part of the borehole seal will be equivalent to that of
the bentonite buffer (see Section 3.5.6 on alteration of impurities of the buffer system part), andtherefore it can be used as analogue.
Handling in the safety assessment SR-Site

As described above, the geochemical evolution in the borehole seals will be of limited importancefor the performance of the repository. The process is therefore neglected in SR-Site. (The resultsfrom the modelling geochemical evolution in the buffer can be used as an analogue).

9.4.8

OsmosisOverview/general description

See buffer Section 3.5.8
Osmosis is of importance only in the bentonite component of the borehole seals.
Handling in the safety assessment SR-Site

The salinity of the groundwater will affect the swelling pressure of the bentonite parts of the seals.
This will be evaluated in the main report of SR-Site in a similar manner to the same process for the
buffer, see buffer Section 3.5.8

9.4.9

Montmorillonite transformation

Overview/general description

Temperature, which is a main factor in montmorillonite transformation in nature, will be relatively low
in the montmorillonite parts of the seals and no significant hydrothermal alteration is consequently
expected. Possible threats are high pH solutions from cement, which may lead to dissolution of themontmorillonite, see buffer Section 3.5.9. However, the cement is supposed to be of low pH type.
Handling in the safety assessment SR-Site

The stability of the montmorillonite itself and the interaction between low-pH cement and bentonite
is handled in the same way as for the buffer, see Section 3.5.9.

9.4.10 Montmorillonite colloid release
Overview/general description

The process is the same as for the buffer (Section 3.5.11).

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Handling in the safety assessment SR-Site

In the case of diluted waters, loss of bentonite from the borehole seal cannot be excluded. Open
boreholes will be treated as a case in the hydrogeological modelling. See also Section 9.2.2.

9.4.11 Microbial processes
Overview/general description

It is not foreseen that microbial processes can interfere with the seals in a way that will influence theperformance of the repository.
Handling in the safety assessment SR-Site

Microbial processes in the borehole seal is not treated in SR-Site. Sulphate reducing bacteria could
potentially affect the copper tubes, but since these have no intended performance after installation,
the process can be neglected.

9.5Radionuclide transport processes

9.5.1

Speciation of radionuclides

Overview/general description

See corresponding process description for the buffer, Section 3.6.1
Handling in the safety assessment SR-Site

This process is indirectly handled through the selection of parameters for radionuclide transport
in the rock. In general, the handling of the process is the same as for the corresponding process in
the geosphere, see process Transport of radionuclides in the water phase in the Geosphere Process
Report /SKB 2010i/.

9.5.2

Transport of radionuclides in the water phase

Overview/general description

If the borehole seals work as intended they do not have to be considered in the radionuclide transport
calculations. If not, they are represented as a pipe with high hydraulic conductivity in the geospheretransport model.
Handling in the safety assessment SR-Site

The borehole seals are considered to be a part of the rock in the radionuclide transport calculations. See
process Transport of radionuclides in the water phase in the Geosphere Process Report /SKB 2010i/.

TR-10-47349

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