Host Rock Classification (HRC) system for nuclear waste disposal in crystalline bedrock

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Publications of the Department of Geology D8 Helsinki 2006

Host Rock Classification (HRC) System for Nuclear Waste Disposal in Crystalline Bedrock

Annika Hagros

Academic Dissertation

To be presented with the permission of the Faculty of Science of the University of Helsinki, for public criticism in Auditorium, Arppeanum, Snellmaninkatu 3, University

of Helsinki, on October 3^rd, 2006, at 12 o’clock noon

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Supervised by:

Professor Juha Karhu Department of Geology University of Helsinki Reviewed by:

Professor Emeritus Heikki Niini

Laboratory of Geoenvironmental Technology Helsinki University of Technology

Doctor Johan Andersson JA Stremflow AB Älvsjö

Sweden Opponent:

Professor Ove Stephansson SRC AB

Berlin Germany

Cover photo: A view from Seurasaari, Helsinki, July 2001

ISSN 1795-3499

ISBN 952-10-2607-3 (paperback) ISBN 952-10-2608-1 (PDF) http://ethesis.helsinki.fi/

Helsinki 2006 Yliopistopaino

PhD-thesis No. 191 of the Department of Geology, University of Helsinki

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Annika Hagros: Host Rock Classification (HRC) system for nuclear waste disposal in crystalline bedrock. Academic dissertation, University of Helsinki, 2006. Publications of the Department of Geology D8, ISSN 1795-3499, ISBN 952-10-2607-3 (paperback), ISBN 952-10-2608-1 (pdf-version).

Abstract

A new rock mass classification scheme, the Host Rock Classification system (HRC-system) has been developed for evaluating the suitability of volumes of rock mass for the disposal of high-level nuclear waste in Precambrian crystalline bedrock. To support the development of the system, the requirements of host rock to be used for disposal have been studied in detail and the significance of the various rock mass properties have been examined. The HRC- system considers both the long-term safety of the repository and the constructability in the rock mass. The system is specific to the KBS-3V disposal concept and can be used only at sites that have been evaluated to be suitable at the site scale. By using the HRC-system, it is possible to identify potentially suitable volumes within the site at several different scales (repository, tunnel and canister scales).

The selection of the classification parameters to be included in the HRC-system is based on an extensive study on the rock mass properties and their various influences on the long-term safety, the constructability and the layout and location of the repository. The parameters proposed for the classification at the repository scale include fracture zones, strength/stress ratio, hydraulic conductivity and the Groundwater Chemistry Index. The parameters proposed for the classification at the tunnel scale include hydraulic conductivity, Q´ and fracture zones and the parameters proposed for the classification at the canister scale include hydraulic conductivity, Q´, fracture zones, fracture width (aperture + filling) and fracture trace length.

The parameter values will be used to determine the suitability classes for the volumes of rock to be classified. The HRC-system includes four suitability classes at the repository and tunnel scales and three suitability classes at the canister scale and the classification process is linked to several important decisions regarding the location and acceptability of many components of the repository at all three scales. The HRC-system is, thereby, one possible design tool that aids in locating the different repository components into volumes of host rock that are more suitable than others and that are considered to fulfil the fundamental requirements set for the repository host rock.

The generic HRC-system, which is the main result of this work, is also adjusted to the site-specific properties of the Olkiluoto site in Finland and the classification procedure is demonstrated by a test classification using data from Olkiluoto.

Keywords: host rock, classification, HRC-system, nuclear waste disposal, long-term safety, constructability, KBS-3V, crystalline bedrock, Olkiluoto

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Preface

The classification system (HRC-system) presented in this dissertation is based on the HRC- system developed in the Host Rock Classification project (2001–2005) funded by Posiva Oy. The project’s working group included Ms. Annika Hagros and Mr. Kari Äikäs of Saanio

& Riekkola Consulting Engineers, Dr. Tim McEwen of SAM Ltd and Dr. Pekka Anttila of Fortum Engineering Ltd. The contact person at Posiva Oy was initially Dr. Aimo Hautojärvi and later Ms. Liisa Wikström. This dissertation was compiled by the disputant based on the Phase 2 report (Hagros et al. 2003) and, more specifically, the Phase 3 report (Hagros et al.

2005) produced by the above-mentioned working group. The only chapters of these original reports that were not written by the disputant were Chapters 6, 7, 9 and parts of Chapter 4 of the Phase 2 report and Chapters 4.1.1, 4.1.2 and most of Chapter 2.3 of the Phase 3 report. These sections are not included in this dissertation or have been re-written by the disputant.

In addition to being responsible for most of the writing during the project, the disputant was in charge of editing the reports, of deciding on what kind of text was required from the others and of the overall content of the reports. The language of the original reports was checked by Dr. Tim McEwen. The working group also contributed to the work by presenting ideas, views and facts to support the development of the HRC-system and by giving comments. The system itself was created by the disputant with only minor changes due to the comments of the others.

Particularly in the beginning of the project, Dr. Aimo Hautojärvi also made a significant contribution by initially suggesting the project and by presenting ideas that helped to create a framework for the project. Some versions of the early project plans were written by Mr.

Kari Äikäs. Most of the planning work was, however, done by the disputant, who is also responsible for most of the conclusions presented in the Phase 2 and 3 reports and in this dissertation, except for some parts of Chapter 2.3, which is largely based on the Phase 2 report, where some of the conclusions (presented in the above-mentioned chapters) were made by the other members of the working group.

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Acknowledgements

I am very grateful for the contribution of the HRC working group to this work. I thank Tim McEwen for our intriguing email debates, for his numerous scientific comments and linguistic corrections, which eventually made me a more fluent writer. I thank Kari Äikäs for our fruitful discussions and mental armwrestling, which occasionally was not very far from becoming real armwrestling. Pekka Anttila is thanked for kindly supporting my ideas and for being the balancing force in our project meetings and our numerous enjoyable working meetings.

My employer, Saanio & Riekkola Oy, further contributed to this work by providing a pleasant working environment and the possibility of finishing the work, for which I am deeply grateful. The financial support by Posiva Oy is also kindly acknowledged.

I thank everyone who provided useful comments to the text. The following people have reviewed the original Phase 2 and 3 reports or parts of them: Professor John A. Hudson of Imperial College and Rock Engineering Consultants, Professor Alan Geoffrey Milnes of ETH Zürich and GEA Consulting, Professor Ove Stephansson of Royal Institute of Technology, Dr. Johan Andersson of JA Streamflow AB, Mr. Göran Bäckblom of Conrox, Dr. Raymond Munier and Mr. Björn Magnor of SKB, Dr. Aulis Kärki of Kivitieto Oy, Mr. Kai Front of VTT, Mr. Seppo Paulamäki of Geological Survey of Finland, Mr. Henry Ahokas of JP-Fintact Oy, Dr. Aimo Hautojärvi, Ms. Liisa Wikström and Mr. Kimmo Kemppainen of Posiva Oy and Dr. Timo Vieno, Ms. Margit Snellman, Mr. Erik Johansson and Mr. Antti Öhberg of Saanio

& Riekkola Oy. I also thank Dr. Fredrik Løset of the Norwegian Geotechnical Institute, Dr.

Tomi Seppälä of Helsinki School of Economics, Mr. Antti Poteri and Mr Petteri Pitkänen of VTT, Ms. Tiina Vaittinen and Mr. Jorma Palmén of JP-Fintact Oy, Mr. Jussi Mattila and Ms.

Sanna Riikonen of Posiva Oy and Dr. Nuria Marcos, Mr. Jorma Autio, Ms. Ursula Sievänen, Ms. Paula Keto, Ms. Hanna Malmlund and Mr. Matti Kalliomäki of Saanio & Riekkola Oy for providing information or other kinds of assistance, and Mr. Jukka Sutinen and Mr. Kari Försti of Saanio & Riekkola Oy for the drafting of several figures.

I also thank my supervisor Professor Juha Karhu of the University of Helsinki for all the help and for his unfailing positive attitude, as well as the other members of the “thesis project group”, Dr. Aimo Hautojärvi and Dr. Pekka Anttila, who commented on parts of the manuscript and provided useful data. Professor Matti Eronen of the University of Helsinki is acknowledged for recognising the potential of the work and Professor John A. Hudson is warmly thanked for commenting on the whole manuscript. I am also grateful to my examiners, Professor Emeritus Heikki Niini and Dr. Johan Andersson, for their thorough review and their many valuable comments and suggestions.

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Special thanks are due to all my wonderful workmates, my friends and my family. In particular, I am grateful to my parents for their support and encouragement throughout my life, to my brother and sister, whose constant challenge always made me work harder, and to my dear Tomi for being such an angel during this last, difficult year.

Finally, a very special thank you is sent to the other side, to Dr. Timo Vieno, who unfortunately never got to see this dissertation, the result of his casual suggestion at the lunch table.

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Symbols and acronyms

Greek letters

ε_f flow porosity

s₁ major principal stress s₃ minor principal stress s_h minimum horizontal stress

s_H maximum horizontal stress

s_UCS uniaxial compressive strength

s_V vertical stress

Latin letters and acronyms

2-D two-dimensional

3-D three-dimensional

AECL Atomic Energy of Canada Limited APSE Äspö Pillar Stability Experiment

a_w flow-wetted surface area per volume of flowing water b.s.l. below sea level

CAI Cerchar Abrasion Index

Can1 high suitability class (at the canister scale) Can2 moderate suitability class (at the canister scale) Can3 low suitability class (at the canister scale) CHS_min minimum canister hole separation

CV_K critical value for the hydraulic conductivity CV_S critical value for the strength/stress ratio D&B drill and blast

D_e effective diffusion coefficient DOC dissolved organic carbon

DRI Drilling Rate Index

EDZ excavation damage(d) zone, excavation disturbed zone

Eh redox potential

ESR excavation support ratio

F transport resistance

FW limit value for fracture width

FZI Fracture Zone Index

GCI Groundwater Chemistry Index

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GSI Geological Strength Index

H12 Project to establish a technical basis for HLW disposal in Japan HLW high-level waste

HRC Host Rock Classification

HRL Hard Rock Laboratory

IAEA International Atomic Energy Agency ISRM International Society for Rock Mechanics

J hydraulic gradient

J_a joint alteration number

J_n joint set number

JNC Japan Nuclear Cycle Development Institute

J_r joint roughness number

J_v volumetric joint count

J_w joint water reduction factor

K hydraulic conductivity

K_2m hydraulic conductivity from a 2 m long measurement interval KBS nuclear fuel safety

KBS-3 disposal concept developed in Sweden (see KBS) KBS-3H horizontal disposal concept (see KBS)

KBS-3V vertical disposal concept (see KBS) K_d distribution coefficient

KR borehole

mbsl metres below sea level

MLH Medium Long Hole (disposal concept)

MPa mega pascals

MRMR Modified Rock Mass Rating

N Rock Mass Number

NATM new Austrian tunnelling method

NEA Nuclear Energy Agency

NGI Norwegian Geotechnical Institute

OECD Organisation for Economic Co-operation and Development OL Olkiluoto (refers to surface boreholes at Olkiluoto) ONK ONKALO (refers to boreholes drilled from the ONKALO) ONKALO underground research facility at Olkiluoto

PH pilot hole

q Darcy velocity (flow rate of groundwater per unit area) Q rock mass quality (Q-value); flow rate

Q´ modified Q-value

r correlation

R&D research and development r² coefficient of determination

RCR Rock Condition Rating

RD respect distance

RD_FZ respect distance to fracture zone REL representative elementary length

Rep1 high suitability class (at the repository scale) Rep2 moderate suitability class (at the repository scale) Rep3 low suitability class (at the repository scale)

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Rep4 very low suitability class (at the repository scale) REV representative elementary volume

RG engineering geological; a Finnish rock mass classification system (RG = Rakennus- Geologinen, which is Finnish for Engineering Geological)

Ri symbol for broken rock mass in the RG-system (see RG)

RMi Rock Mass Index

RMR Rock Mass Rating

RMR´ a modification to RMR where the drift orientation is ignored

RQD Rock Quality Designation

RSR Rock Structure Rating

SKB Swedish Nuclear Fuel and Waste Management Co SKBF Swedish Nuclear Fuel Supply Company

SR 97 a Swedish safety assessment SRF stress reduction factor

STUK Finnish Radiation and Nuclear Safety Authority

T transmissivity

T_400m transmissivity at the depth of 400 m TBM tunnel boring machine

TDS total dissolved solids TILA-99 a Finnish safety assessment

TL₁ a (larger) limit value for fracture trace length TL₂ a (smaller) limit value for fracture trace length T_ref transmissivity at a reference depth level TRUE Tracer Retention Understanding Experiments Tun1 high suitability class (at the tunnel scale) Tun2 moderate suitability class (at the tunnel scale) Tun3 low suitability class (at the tunnel scale) Tun4 very low suitability class (at the tunnel scale)

t_w travel time

URL Underground Research Laboratory

VLJ (repository for) low- and medium-level nuclear waste (in Finland) VTT Technical Research Centre of Finland

WL/Q transport resistance (flow wetted surface per flow rate in the channel) YVL nuclear power plant; STUK’s guides for nuclear power plants z vertical depth below ground surface

Z vertical depth below sea level

ZEDEX Zone of Excavation Disturbance Experiment

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1 Introduction

1.1 Background and goals

For several decades, spent nuclear fuel and other high-level nuclear wastes have been produced in many industrialised countries. These wastes remain radioactive for hundreds of thousands of years and the leading concept for managing this waste problem is the deep geological isolation of the high-level wastes. The geological media mainly considered for this include crystalline bedrock, salt formations, clay and sedimentary rocks. In particular, crystalline rocks are being considered in countries that locate in Precambrian shield areas, which are currently very stable in terms of tectonic effects. Due to their stability and also due to their other advantageous properties, Precambrian crystalline rocks are regarded as a favourable host rock for the long-term disposal of radioactive wastes (e.g., International Atomic Energy Agency 1981, Niini et al. 1982). Nevertheless, it is apparent that due to the non-uniform nature of the crystalline rocks, they include sections where the disposal might not be as safe as elsewhere and, therefore, it is necessary to decide on criteria or guidelines regarding the suitability of different parts of the rock mass for disposal. The way in which such criteria or guidelines can be incorporated into a practical system, depends on the disposal concept used at the repository site and the planned course of repository development.

Sweden and Finland are two countries with perhaps the most advanced disposal programmes, based on the so-called KBS-3V disposal concept (SKBF 1983) that was developed by Sweden’s nuclear waste organisation, currently known as the Swedish Nuclear Fuel and Waste Management Co (SKB). In these countries the disposal of low- and intermediate-level nuclear waste has been carried out for years (e.g., in Finland in the two VLJ repositories at Olkiluoto and at Hästholmen, Loviisa). In Finland, the disposal programme for the high-level waste has proceeded to the start of underground constructions at the Olkiluoto site, which has been selected as the potential site for the final disposal of spent nuclear fuel from the Finnish nuclear power plants (Council of State 2000). Olkiluoto, located in Eurajoki, Western Finland, is worldwide the first repository site in crystalline bedrock selected for the disposal of spent fuel from nuclear power plants. The site investigations and research programme and the development of the repository at Olkiluoto are being managed by Posiva Oy.

A need for developing a system for the assessment of the suitability of the host rock for disposal was recognised in Posiva Oy’s programme for research, development and technical design (Posiva 2000), where it was suggested that the criteria for defining potentially suitable volumes of host rock should be incorporated into a rock mass classification system. The development of such a system was also recommended by the Finnish Radiation and Nuclear Safety Authority (STUK), in their Guide YVL 8.4 on the long-term safety of disposal of spent nuclear fuel (STUK 2001a), where it is stated that “the structures of the host rock of importance to groundwater flow, rock movements or other factors relevant to long-term safety, shall be defined and classified”. The external review group assigned by STUK also recommended in their consensus report (Apted et al. 2000) that a Finnish rock mass classification

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scheme should be developed for use during excavation, in order to determine whether the rock mass

“quality” was acceptable for disposal purposes. This recommendation arose from the fact that the existing Finnish engineering geological classification system (RG-system) was not developed for the construction of nuclear repositories.

In addition, it has been suggested that avoidance strategies (i.e. methods to avoid adverse rock conditions or geological structures) and their application during the development of the repository should be clarified (Apted et al. 2000). The development of a system with criteria on the suitability of host rock is also necessitated by the fact that many safety assessments carried out in, for example, Finland and Sweden (Peltonen et al. 1985, SKB 1992, SKB 1999a, Vieno and Nordman 1999, SKB 2004a) have assumed that unfavourable parts of the rock mass – mainly fracture zones – can and will be avoided when constructing the repository. It is, consequently, essential that such avoidance criteria will be clearly defined before the construction of an actual repository begins at any site.

In this work, a Host Rock Classification system (HRC-system) is developed for the evaluation of the suitability of volumes of rock mass for the disposal of high-level nuclear waste in a KBS-3V repository located in Precambrian crystalline bedrock. It was decided to take all Precambrian bedrock into consideration in this work, instead of focusing on only one site, so that the results of this work could be more widely utilised, and also because it was considered more relevant to examine and define the universal aspects of host rock suitability than to discuss them only with reference to a particular site. The work aims at recognising which aspects of host rock suitability are generic and which are site- specific. Since the details of any such system probably need to be site-specific, at least in a quantitative sense, an example of a site-specific HRC-system will also be given by adjusting the basic system to the site-specific properties of Olkiluoto. The existing data from this site will then be used to test the use of the system and to demonstrate the classification process.

The evaluation of host rock suitability can only be based on a definition of what are considered to be potentially suitable host rock conditions. The definition of such conditions, which is to be incorporated into the HRC-system, will be based on the regulatory requirements and the studies of the nuclear waste organisations in countries that have planned the disposal of high-level nuclear waste in Precambrian crystalline bedrock. The examination of these premises for a host rock classification system is one of the main goals of this work. In addition to the long-term safety requirements, any requirements on the construction conditions will also need to be considered. The requirements of the host rock are ultimately based on this definition of suitable host rock, whereas the HRC-system itself does not impose any further requirements. Instead, the purpose of the system is to provide practical criteria or guidelines that can be used to evaluate, in the various stages of repository development, whether the fundamental requirements of the host rock have been met. This is obviously not a straightforward task, and it is acknowledged that the HRC-system proposed in this work is only one possible solution to the problem of creating a system of practical criteria for the host rock. It is also intended that the system will be modified on the basis of site-specific conditions, which need to be studied underground at the proposed repository site before deciding on the final version of the HRC-system to be used at any specific site. Sufficient flexibility must, therefore, be incorporated into the system during its development.

Since it is likely to be impossible to formulate any exact definition of suitable host rock, at least in a quantitative sense, it is also impossible to justify any exact criteria for the host rock suitability.

The main objective of this work is, therefore, to develop principles and methodologies to formulate such criteria, whenever possible, and to propose a system that considers such criteria as far as possible, according to the current concepts of suitable host rock, if such universal (although probably not quantitatively exact) concepts can be identified.

Using a classification system that aims at locating the repository in potentially suitable volumes of rock will increase the likelihood of the repository meeting the safety requirements, although its

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safety will ultimately need to be evaluated using safety assessments. Taking account of the possibility that the available bedrock resources for the repository are rather limited, as well as the advantages of minimising the excavated volume and, thereby, the size of the repository, the main role of the HRC- system is to help distinguish between unsuitable and potentially suitable volumes of rock and, thereby, to contribute to the avoidance of adverse conditions. The benefits of distinguishing between conditions in the rock mass that could be classed as being “favourable” and those that could be considered as being “very favourable” are thought to be negligible.

The developed HRC-system can be used as a basis for developing site-specific classification systems for evaluating the host rock suitability at various repository sites in Precambrian crystalline rock. The final parameter set and the limit values for different parameters need to be decided on site-specifically. Such classification systems have not been developed before, although in Sweden, SKB has previously discussed suitable ranges of parameter values for various rock mass parameters (Andersson et al. 2000). Limit values for a small set of rock mass parameters affecting the suitability of deposition hole locations (with emphasis on the hydrogeological conditions) have also been suggested by Rosén and Gustafson (1995, 1996), who also defined a so-called Positioning Index to estimate the probability of finding suitable canister locations.

Although there is a lack of previous work in developing a comprehensive classification system for evaluating the suitability of volumes of host rock within a repository site, there has, in many countries, been a wide discussion on the suitability of different host rock formations and their properties with reference to the site selection programme that is – in 2006 – still ongoing in all other nuclear waste organisations except for Posiva Oy in Finland. These discussions have clarified the concept of suitable host rock and they will be summarised in Chapter 2 along with a discussion on the actual regulatory requirements of the repository host rock and a more detailed examination of the significance of individual host rock properties. These will be considered when developing the HRC-system, which will be the first rock mass classification system developed specifically to be used at nuclear waste repository sites.

Numerous rock mass classification systems have, however, been developed for the purposes of conventional rock engineering and, although in the case of nuclear waste disposal the aspect of long-term safety can be assumed to be more important than the rock engineering or constructability aspect, these existing classification systems may support the development of the HRC-system and are, therefore, reviewed in Chapter 3. The basic HRC-system will be presented in Chapter 4, and in Chapter 5 the system will be applied to the Olkiluoto site.

1.2 Scope of work

1.2.1 General

It is intended that the work is applicable to:

-

Nuclear waste repositories constructed according to the KBS-3V concept (see Chapter 1.2.2). The HRC-system will not be directly applicable to other disposal concepts, although it is possible that only slight modifications are required if the KBS-3H disposal concept (previously termed MLH, see Autio et al. 1996) is used.

- Repository sites where the host rock is formed of Precambrian crystalline bedrock (see Chapter 1.2.3 for further discussion).

The HRC-system is intended to allow the suitability of the rock mass to be evaluated for the disposal of spent nuclear fuel, and is not intended to serve as a conventional rock mass classification system with a purpose of determining the required level of rock support.

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The classification parameters included in the HRC-system will be selected based on an examination of the various host rock properties and their respective influences. This examination will be carried out from the standpoint of geology, thermal properties, rock mechanics, hydrogeology, chemistry and transport properties, which are considered to cover the main research areas related to the host rock. Due to the multi-disciplinary nature of this work, it has not been possible to go into great detail regarding any single aspect of the host rock, but, instead, the main objective has been to find the right balance between these aspects so that the resulting classification system would be a comprehensive system, where the relative significance of the different parameters is fully considered, whereas the detailed consideration of the individual parameters could later be refined by the specialised experts of the various disciplines.

1.2.2 KBS-3V disposal concept

The HRC-system developed in this work assumes that the spent fuel will be disposed of according to the KBS-3V (previously termed KBS-3) disposal concept (SKBF 1983). “KBS” stands for

“kärnbränslesäkerhet”, which is Swedish for “nuclear fuel safety”, and the letter “V” signifies

“vertical”, i.e. the deposition holes are vertical.

Although no final decision to use the KBS-3V system in Finland has yet been made (another concept that is still being actively studied is the KBS-3H concept mentioned in Chapter 1.2.1), it was recognised already in the 1970s that the most promising disposal concept would be one where the repository is excavated into deep bedrock (Jotuni 1979, Niini and Salmi 1980). The idea of using existing mines for disposal had been rejected based on systematic studies (Niini and Löksy 1980, Niini and Holopainen 1980). The planning of the KBS disposal concept started in the 1970s in Sweden (Nuclear Fuel Safety Project 1977) and the KBS-3 system was described in detail in 1983 by the Swedish Nuclear Fuel Supply Co (SKBF 1983).

In the KBS-3V disposal concept (Figure 1), the spent fuel assemblies will be packaged in copper- iron canisters that can withstand the required mechanical load and are resistant to corrosion. The purpose of the canister is to hinder the release of radionuclides into the surrounding host rock. The canisters are emplaced in vertical deposition holes (also termed “disposal holes”) that are bored into the floors of the deposition tunnels (or “disposal tunnels”). The space between the canister and the rock bordering the deposition hole will be filled with highly compacted bentonite clay (so-called bentonite buffer), which keeps the canister in place, reduces the movement of groundwater around the waste canister and, in case of canister failure, retards the transport of radionuclides into the host rock. The deposition tunnels are linked to central tunnels (also called “main tunnels”). The repository tunnels and access routes, i.e. shafts or access tunnels (ramps), will be backfilled with a suitable backfill material (e.g., a mixture of bentonite and crushed rock) and will be sealed with concrete plugs to prevent them from becoming major conductors for groundwater and also to limit any human intrusion into the repository. The KBS-3V concept has been described in more detail by, for example, SKBF (1983) and Tanskanen and Palmu (2004).

The currently planned range of disposal depths is 400−700 metres in Sweden (SKB 2004b) and approximately 400−500 metres at Olkiluoto, Finland (Tanskanen and Palmu 2004). In other countries the possible range of disposal depths is usually located between 300 m and 1,000 m (e.g., Japan Nuclear Cycle Development Institute 1999, Kim et al. 2001, McKinley et al. 2001, Sykes 2003).

Countries other than Finland and Sweden have not yet decided to use the KBS-3V concept, although in France this concept will be considered if the host rock will be of granitic type (Lebon et al.

2001). In Canada a copper/steel container with a predicted lifetime (at least 100,000 years) similar to that of the KBS-3V canister has been proposed (McCombie 2003, 16) and a repository layout largely similar to that of the KBS-3V system has been used in a safety assessment (Atomic Energy of Canada

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Ltd 1994). However, the Canadian policy on long-term management of high-level wastes has not yet been agreed upon. The disposal concept planned in Korea is also rather similar to KBS-3V, although the canister material will probably be different (Kim et al. 2001).

It has also been assumed in this work that the repository is constructed using the drill and blast (D&B) method, although only minor modifications are probably required if some other method is used.

1.2.3 Geological environments considered

The developed classification system is intended to be applicable to repository sites consisting of a sufficiently large formation of Precambrian (i.e. more than approx. 550 million years old) crystalline bedrock. It is possible that the system can be successfully applied also in younger crystalline rocks, although special attention probably needs to be paid to the tectonic stability of such formations. The countries where the disposal of spent nuclear fuel has been decided to be carried out in Precambrian crystalline bedrock include Sweden and Finland (Milnes 2002, Posiva 2003a). In both countries, several alternative repository sites have been studied in detail (Anttila et al. 1999a, b, c, SKB 2001a, 2005a, b, Posiva 2005). In Canada, the use of a site located somewhere in intrusive igneous rock in the Canadian Precambrian Shield was the leading concept (Atomic Energy of Canada Ltd 1994, 3-4), until the disposal plans were frozen.

Figure 1. An illustration of the basic repository layout according to the KBS-3V disposal concept with an access tunnel (ramp) in addition to three shafts. In reality, the layout will be adapted to the local bedrock conditions, which may result in the repository being divided into several detached repository panels of various sizes. The cross section of a deposition tunnel and a deposition hole is also shown. The length of the deposition holes is some 7−8 m, depending on the canister type (modified from Hagros et al. 2005).

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Other countries where crystalline rock is being considered as, at least, one possible option in the high-level waste management include, for example, Czech Republic, India, China, South Africa, France, Switzerland, Germany, Spain, Hungary, Bulgaria, Argentina, Armenia, Russia, Korea and Japan, although the considered formations are not always of Precambrian age (Thury et al. 1994, Mathur et al. 2001, Wang et al. 2001, Bredell and Raubenheimer 2001, Lebon et al. 2001, Wallner and Bräuer 2001, Astudillo 2001, Ormai et a. 2001, Evstatiev and Kozhukharov 2001, Ninci et al.

2001, Jrbashyan and Ghukasyan 2001, Lebedev et al. 2001, Kim et al. 2001, Masuda and Kawata 2001, Tomas 2003, McCombie 2003). Data from these countries have been used in this study where appropriate, although the main emphasis has been on the data from Finland and Sweden, where the currently assumed disposal concept is KBS-3V. Other disposal concepts may set different requirements on the repository host rock that are difficult to be jointly considered in a single system. In addition to data from the potential repository sites, the study will consider data from generic sites, where detailed investigations have been carried out in underground rock laboratories, most notably at the Precambrian sites of Äspö, Sweden (Bäckblom 1991, Landström and Tullborg 1995) and Pinawa, Canada (Martin 1989, Chandler 2003).

In terms of nuclear waste disposal, the main attribute of Precambrian crystalline bedrock, particularly in the Precambrian shield areas – such as the Fennoscandian Shield – is its current tectonic stability characterised by a low seismicity (e.g., Wahlström and Grünthal 2001) and a lack of volcanic activity. It is known that these shield areas have witnessed tectonically more active times, most recently during late Pleistocene and early Holocene when major earthquakes and significant post-glacial faulting occurred, but their persistence over hundreds of millions of years may indicate a sufficient stability during approximately one million years, when the nuclear wastes pose a threat to the environment.

Crystalline rocks are hard and can fairly well withstand erosion, and they also have a low porosity and, thereby, a low hydraulic conductivity (K), at least outside major fractures and fracture zones.

International Atomic Energy Agency (1981, 20) also lists high rock strength, good chemical stability and moderately good thermal conductivity as favourable properties of crystalline rocks.

Compared with the other two geological media often considered for nuclear waste disposal, namely clay and salt, where the movement of groundwater is minimal, the hydraulic conductivity is likely to be the most problematic property of crystalline rocks, since in these rocks the groundwater flow is highly concentrated in discrete fractures and fracture zones. In stable Precambrian bedrock, the movement of groundwater is basically the only natural process through which the radioactive materials can migrate in the bedrock (Niini et al. 1979). Accordingly, the avoidance of water-conducting fractures and fracture zones when locating the waste canisters is assumed to contribute significantly to the long-term safety of disposal. The properties of Precambrian bedrock that are relevant for the disposal of high-level nuclear wastes have been extensively discussed by Niini et al. (1982).

As the copper-steel canister assumed in the KBS-3V disposal concept (SKBF 1983) has specifically been designed for chemically reducing groundwater conditions typical of the deep bedrock of the Fennoscandian Shield¹ , it will also be assumed here that there will be no significant risk of oxidising conditions in the volumes of host rock considered in the classification process. Otherwise either the disposal site, or the disposal concept should be re-considered, as no classification system can make the problem go away in such conditions.

There is a variety of crystalline (i.e. igneous or metamorphic) rocks that have been proposed to host nuclear waste repositories. They include granites, granodiorites, gneisses, gabbro, even basalt. As all these different rock types have been assumed to be generally suitable for the disposal of nuclear waste, it seems that the overall lithology is not a decisive factor in determining the suitability of the host rock.

Instead, there must be other, more universal properties of crystalline rocks that affect their suitability

1 Reducing groundwater conditions are also expected at the potential disposal depths in the area of the Canadian Shield (Atomic Energy of Canada Ltd 1994, 107).

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and these properties are the ones that the Host Rock Classification system should capture in order to be applicable to all types of Precambrian crystalline rocks.

1.3 Requirements of the classification system

The need to develop a rock mass classification system has been pointed out by the Finnish Radiation and Nuclear Safety Authority (STUK) and its external review group, as discussed in Chapter 1.1.

Subsequently, Posiva has formulated certain objectives for the classification system to be used in Finland (Posiva 2000, 19-20, 65-66):

- The classification system should serve for the design of the repository and the assessment of its suitability.

- The classification system should incorporate criteria for defining potentially suitable volumes of host rock. These criteria will also be used when applying the observational method or “design-as- you-go” methodology (see Bäckblom and Öhberg 2002) during the construction of the repository.

- The classification system should take into account both the long-term safety requirements and the constructability of the repository.

These requirements are fully applicable to a more general classification system and not just to one developed for the Finnish conditions. Regarding the requirement to consider both safety and constructability, it could be mentioned that these are often influenced by the same parameters, implying that it is possible to include both these aspects in one classification scheme. In addition, several other requirements have been recognised during the planning of the HRC-system:

- The classification system must take account of and be applicable to all scales related to the development of a KBS-3V repository system after the selection of a disposal site (i.e. the repository scale, tunnel scale and canister scale need to be considered). If the repository or the deposition tunnels were constructed in unsuitable rock, it would not be possible to find suitable locations for deposition holes, which is why the classification process needs to cover all these scales.

- The classification system must take account of the use that is to be made of the particular part of the repository (access tunnel, central tunnel, deposition tunnel, etc.).

- The classification system should be designed so that classification of host rock can be carried out in all relevant stages of repository development, from the selection of the repository volume within a site until the acceptance of the last deposition hole.

- The classification system must take account of the coupling between the host rock properties.

Because the classification system is designed to consider the overall suitability of the rock mass, the combined effect of rock properties should be regarded as more important than the values of individual parameters, and some favourable properties may even compensate for certain properties that in some other conditions might be considered unfavourable.

- The classification process should be relatively quick to implement, at least at the tunnel scale, so that the tunnelling process is not severely delayed because of the classification system. It is, accordingly, an advantage if the classification can be carried out using the same data that are collected in any case in the repository.

- The classification system must be practical – only properties that can be measured relatively easily are ideal classification parameters, although the significance of the parameters that are difficult to measure needs also to be examined when developing the HRC-system.

- The classification system should be capable of being applied using information from boreholes and from their core and from tunnels, as appropriate, in such a way that the results are compatible.

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- The classification system must be flexible – i.e. it must be possible to modify the system to be applicable at different repository sites and to change it when more accurate site-specific data are obtained, or if it proves to be too demanding or impractical.

- The classification system must take account of the spatial and temporal variations in the rock mass properties, which implies a need to specify when and where the classification parameters will be measured.

- The classification system must be relatively simple and its results unambiguous.

It may prove impossible to meet all these requirements completely, but they all need to be considered as far as possible. The requirement that is most likely to prove problematic is that regarding the required simplicity of the classification system, as it is possible that the HRC-system cannot be simple if all other requirements are met. The intention is to develop a classification system that is as simple as possible, whilst taking into account the other requirements, and an optimised solution needs to be found. This process of optimisation may require some prioritisation of the requirements and their implications.

1.4 Definitions

In this work the term repository scale refers to the scale of the whole repository and the individual repository panels and is mainly concerned with dimensions of up to a few kilometres. The repository panels (i.e. sets of deposition tunnels and one or two central tunnels connecting them) have dimensions of some hundreds of metres at most. The tunnel scale considers aspects that are relevant when constructing single tunnels, the maximum length of which is a few hundred metres and the diameter some 4 m (SKB 1999b, 15; Tanskanen and Palmu 2004, 68-69). The canister scale refers to the scale of individual deposition holes, and mainly relates to a scale of some metres; the separation of adjacent deposition holes is approximately 10 m in the Finnish layout, the length of a deposition hole is likely to be some 7−8 m in both Finnish and Swedish layouts and the diameter of the holes some 1.5−2 m (SKB 1999b, 15; Tanskanen and Palmu 2004, 69).

The term fracture zones refers to faults and other fracture-related discontinuities of the rock mass that have a significant extent (several tens of metres at least), an average thickness of approximately one metre at least and a noticeably higher fracture frequency than in the surrounding rock mass, or some other property that makes the zone significantly weaker than the surrounding rock mass. In different countries such zones have also been termed as structures, discontinuities, deformation zones, weakness zones, crush zones, rupture zones, fissures or faults. Some basic assumptions regarding the definition and modelling of fracture zones will be presented in Chapter 4.2.1 and a classification of fracture zones will be presented in Chapters 4.4.1 and 4.4.2.

The term intact rock refers to the rock matrix with no macroscopically visible fractures. This term has been applied in some of the references used here to denote the rock mass between the fracture zones or any sparsely fractured rock mass, but in this work no specific term will be used for such rock mass.

When quoting some earlier publications, the original terms (intact rock mass or background rock) used in these publications have been applied.

The term EDZ refers to both the excavation damage(d) zone and the excavation disturbed zone. The former is the zone of induced fracturing closest to the opening and the latter, which extends outside the excavation damage(d) zone, may be associated, for example, with aperture changes on natural fractures and elastic deformation of the rock. Such mechanical disturbances are assumed to be recoverable, however, there may be some chemical changes which are irrecoverable (cf. Olsson et al. 1996, Nuclear Energy Agency 2002).

The term fracture width refers to the total width (thickness) of a fracture, i.e. the perpendicular distance separating the adjacent rock walls of any fracture, whether it is open, filled or partially filled

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(cf. Brown 1981, 35). It corresponds, thereby, to the combined thickness of a filling and an aperture.

Where a fracture contains no filling the fracture width is, therefore, equal to the fracture aperture.

When the influence of the host rock on long-term safety is examined, the repository near-field and far-field are usually discussed separately. The near-field is considered to include the repository, the fuel canisters and the engineered barriers, as well as the rock mass immediately adjacent to the repository.

The far-field covers the whole volume of rock mass and soil that can be affected by the repository to any noticeable extent. For practical reasons, it is not possible to define the boundary between the near- and far-fields exactly, but the near-field rock is generally considered to include the excavation damaged zone and the rock mass immediately adjacent to the deposition holes.

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2 Evaluation of host rock suitability

2.1 Introduction

The potential suitability of crystalline bedrock for the disposal of high-level nuclear waste has been a topic for discussions for several decades now. In 1979, Finland hosted an international symposium organised by the International Atomic Energy Agency and Nuclear Energy Agency (1980) that was historically important with reference to the nuclear waste disposal research in Finland and also in many other countries. Precambrian crystalline bedrock (Chapter 1.2.3) was one of the geological formations that were considered as a potential host rock for safe disposal of nuclear wastes. Afterwards, crystalline rocks have been extensively studied in, for example, Finland, Sweden and Canada. In Finland, studies on the potential suitability of this type of host rock were commenced in the 1970s (Niini 1978, Niini and Salmi 1979, Peltonen and Rouhiainen 1980), partly based on even earlier studies on bedrock conditions (e.g., Niini 1968a, Niini et al. 1972).

Niini (1978) and Niini and Salmi (1979) presented a hierarchical grouping of geological factors to be considered when evaluating the suitability of host rock and concluded that fracture zones (faults, crush zones and fractures) and the groundwater flow in the rock mass were of particular significance for the safe disposal of nuclear waste.

In Sweden, the suitable host rock properties were discussed, for example, in the reports that presented the development of the different versions of the KBS disposal concept (e.g., Nuclear Fuel Safety Project 1977, SKBF 1983).

This early work has formed a basis for the more recent research work carried out in Finland and Sweden, which will be discussed below (Chapters 2.2 and 2.3) in the light of the current understanding and views on the role of the host rock, particularly in Finland and Sweden.

2.2 Requirements of the host rock

2.2.1 International and Nordic recommendations

The International Atomic Energy Agency (IAEA) has published recommendations on the properties of a host rock for a nuclear waste repository in their guide on the siting of geological disposal facilities (International Atomic Energy Agency 1994a). Similar guidelines have also been presented jointly by the nuclear safety authorities in the Nordic countries (Denmark, Finland, Iceland, Norway and Sweden) in the so-called “Flagbook” (Nordic 1993).

The guidelines related to the host rock by the International Atomic Energy Agency (1994a) concern the geological setting, future natural changes, hydrogeology, geochemistry and the construction and engineering conditions:

- The geological setting of a repository should be amenable to overall characterisation and have geometrical, physical and chemical characteristics that combine to inhibit the movement of radionuclides from the repository to the environment during the time periods of concern.

- The host rock should not be liable to be affected by future geodynamic phenomena (climatic changes, neotectonics, seismicity, volcanism, diapirism) to such an extent that these could unacceptably impair the isolation capability of the overall disposal system.

- The hydrogeological characteristics and setting of the geological environment should tend to restrict groundwater flow within the repository and should support safe waste isolation for the required times.

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- The physicochemical and geochemical characteristics of the geological and hydrogeological environment should tend to limit the release of radionuclides from the disposal facility to the accessible environment.

- The underground characteristics of the site should permit application of an optimised plan of underground workings and the construction of all excavations in compliance with appropriate mining rules.

More specific recommendations are given for each host rock aspect addressed in these guidelines.

International Atomic Energy Agency emphasises that the guidelines are not intended to be strict preconditions. Instead, it is stated that the system of natural and engineered barriers should be considered as a whole, when assessing the performance of the disposal system (International Atomic Energy Agency 1994a, 11).

Other international publications that discuss the role and properties of a repository host rock include International Atomic Energy Agency and Nuclear Energy Agency (1980), International Atomic Energy Agency (1981, 1983, 1989, 1994b, 2003) and Nuclear Energy Agency (1984).

The recommendation regarding the geology of a disposal site published by the nuclear safety authorities in the Nordic countries (Nordic 1993) is that the site should provide good natural conditions for the containment and isolation of radioactive substances. Thus the site should

- have hydrogeological characteristics that provide low groundwater flow within the repository, a long groundwater transit time from the repository to the biosphere and favourable dispersal characteristics

- have geochemical characteristics that contribute to a low corrosion rate of the canister material, a low dissolution rate of the waste matrix as well as to a low solubility and an effective retardation of the released radioactive substances

- be located in a region of low tectonic and seismic activity

- not be adjacent to any natural resources which are not readily available from other sources - be easy to characterise (Nordic 1993, 38).

The “Flagbook” also suggests that the geological medium becomes increasingly important as a barrier, especially in the long term, after the engineered barriers have become impaired. Even in a shorter term, the host medium needs to be such that it is able to ensure that the engineered barriers operate as intended (Nordic 1993, 38).

Although the international and Nordic guidelines presented above consider the suitability of the host rock mainly from the point of view of the siting process, they can be assumed to include many of the relevant aspects of host rock suitability also at smaller scales. Some of the recommendations mentioned above are only related to the scale of the whole site and are not very useful in the development of the HRC-system except by setting preconditions for the sites where the classification will be used, but other recommendations are relevant at all scales, since they relate to properties that exhibit significant spatial variation within a host rock formation. These include the hydrogeological, geochemical and physical (e.g., mechanical) properties of the rock mass.

2.2.2 National regulations and guidelines Regulations and guidelines in Finland

Regulatory requirements

In Finland, the current regulatory requirements of the host rock have been formulated by the Finnish Radiation and Nuclear Safety Authority (STUK) in Guide YVL 8.4 (STUK 2001a) that specifies the requirements given previously in the Government Decision 478/1999 (Council of State 1999) for

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the safety of disposal of spent nuclear fuel. The Guide addresses the disposal of this spent fuel in a repository in crystalline bedrock at the depth of several hundreds of metres and covers its long-term safety. A summary of the requirements of the disposal site and the host rock based on Guide YVL 8.4 is given below. Many of the requirements refer to site-scale properties and, therefore, provide only a loose framework for this work, but some requirements have direct relevance for the definition of host rock suitability at smaller scales as well.

The basic requirement for the geology of a disposal site, as stated in the Government Decision (478/1999), is that the geological characteristics of the disposal site shall, as a whole, be favourable for the isolation of the disposed radioactive substances from the environment. An area having a feature that is substantially adverse to long-term safety shall not be selected as the disposal site.

The characteristics of the host rock shall be such that it acts as an adequate natural barrier. The natural barrier may be made up of

- the intact rock (mass) around the disposal tunnels, which limits the groundwater flow around the waste canisters

- the host rock where low groundwater flow, reducing and otherwise favourable hydrogeochemical conditions and the retardation of dissolved substances in the rock limit the mobility of radionuclides

- the containment provided by the host rock against natural phenomena and human actions.

In addition, the characteristics of the host rock shall be favourable with respect to the long-term performance of the engineered barriers. Such conditions in the host rock as are of importance to long- term safety shall be stable or predictable for at least several thousand years. Thereafter reasonable estimates of the range of geological changes, due to, for example, large-scale climatic changes, shall be provided and these changes must be considered in the determination of the performance targets for the barriers.

Factors indicating the unsuitability of a disposal site may include - the proximity of exploitable natural resources

- exceptionally high in situ stresses

- predictable anomalously high seismic or tectonic activity

- exceptionally adverse groundwater characteristics, such as the lack of reducing capacity and high concentrations of substances which might substantially impair the performance of the barriers.

The location of the repository shall be favourable with regard to the groundwater flow regime at the site. The disposal depth shall be selected with due regard to long-term safety, taking into account as a minimum

- the geological structures and lithological properties of the host rock

- the trends in in situ stresses, temperature and groundwater flow rate with depth.

The structures of the host rock of importance in terms of groundwater flow, rock movements or other factors relevant to long-term safety, shall be defined and classified. The waste canisters shall be emplaced in the repository so that an adequate distance remains to such major structures of the host rock which might constitute fast transport pathways for the disposed radioactive substances or otherwise impair the performance of the barriers.

Other requirements and guidelines

Before the Guide YVL 8.4 was published, requirements of the repository host rock had been discussed for decades in the process of selecting the repository site. The geological criteria used in the site selection programme of Finland have been presented by McEwen and Äikäs (2000). Suitable properties of the host rock have also been discussed by Posiva (1999, 2000) who presented a list of practical criteria and

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constraints that can be used in the further assessments of suitability of the selected site (Olkiluoto). With reference to long-term safety, it was stated that the host rock should have the following properties:

- The geochemical environment is free of oxygen, represents reducing conditions, has a nearly neutral pH and low sulphide concentrations. The level of groundwater salinity is not too high (dissolved salt content corresponding to TDS < 100 g/l).

- The hydrogeological environment is such that the groundwater fluxes in the near-field are sufficiently low (meaning a small hydraulic gradient and low values of hydraulic conductivity).

- The mechanical strength of rock is sufficient in relation to the prevailing stress situation.

- Large crushed zones with potential to significant rock displacements do not cross the tunnel area (Posiva 1999, App. 7; Posiva 2000, 64).

With reference to constructability and operational safety, the main criterion for host rock was that sufficient volumes of good quality host rock should be identified, the geotechnical environment should allow the use of normal construction methods and the groundwater inflows should not constitute severe problems (Posiva 1999, App. 7; Posiva 2000, 64).

Regulations and guidelines in Sweden

In Sweden, the regulatory requirements of the deep repository are included mainly in the Environmental Code, the Nuclear Activities Act and the Radiation Protection Act. None of these laws include, however, any detailed requirements of the repository host rock, and Andersson et al. (2000, 24) conclude that laws and regulations cannot be used directly to formulate requirements or preferences on the properties of the rock. Such requirements or preferences can only be derived indirectly, based on the impact they may have on the safety of the repository.

Geoscientific factors that affect the suitability of a site to host a repository have been proposed by Rosén and Gustafson (1993). More recently, the properties of the host rock, as well as the requirements and preferences that can be assigned to them, have been discussed in detail by Andersson et al. (2000) with reference to the selection of a disposal site in Sweden. They also discuss how a safety assessment can be used to formulate requirements and preferences regarding the host rock. The following requirements are made on the rock or the placement of the deep repository in the rock:

- The rock in the repository’s deposition zone may not have any ore potential, i.e. it may not contain such valuable minerals that it might justify mining at a depth of hundreds of metres.

- Regional plastic shear zones shall be avoided if it cannot be demonstrated that the properties of the zone do not deviate from those of the rest of the rock. There may, however, be so-called “tectonic lenses” near regional plastic shear zones where the bedrock is homogeneous and relatively unaffected.

- It must be possible to position the repository with respect to the fracture zones on the site. Deposition tunnels and deposition holes for canisters may not pass through or be positioned too close to major regional and major local fracture zones. Deposition holes may not intersect identified local minor fracture zones.

- The rock’s strength, fracture geometry and initial stresses may not be such that large stability problems may arise around tunnels or deposition holes within the deposition area. This is checked by means of a mechanical analysis, where the input values comprise the geometry of the tunnels, the strength and deformation properties of the intact rock, the geometry of the fracture system and the initial rock stresses.

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- The groundwater at repository level may not contain dissolved oxygen. Absence of oxygen is indicated by a negative redox potential (Eh), occurrence of Fe²⁺, or occurrence of sulphide.

- The total salinity (TDS) in the groundwater must be less than 100 g/l at repository level.

In addition to the above requirements, Andersson et al. (2000) list a large number of preferences, i.e.

conditions (usually ranges of parameter values) that are desirable and should be taken into account when positioning the repository in the rock. Some of these will be referred to later in this work where appropriate.

Regulations and guidelines in other countries

In Canada, requirements of the host rock have been discussed in several Regulatory Documents by the Atomic Energy Control Board with reference to the deep repository in crystalline bedrock.

According to the Regulatory Document R-72 (Atomic Energy Control Board 1987), the properties of the natural barriers will be unique to the site chosen. Five geological criteria are, however, formulated and discussed in some detail, although it is emphasised that a single aspect of the geological system may not be critical. The five criteria are:

- The host rock and geological system should have properties such that their combined effect significantly retards the movement or release of radioactive material.

- There should be little likelihood that the host rock will be exploited as a natural resource.

- The repository site should be located in a region that is geologically stable and likely to remain stable.

- Both the host rock and geological system should be capable of withstanding stresses without significant structural deformation, fracturing or breach of the natural barriers.

- The dimensions of the host rock should be such that the repository can be deep underground and well removed from geological discontinuities.

In India, the site selection study was carried out in successive stages and considered a number of criteria, which included aspects of, for example, tectonic stability, size and homogeneity, hydrogeology, as well as thermal, thermomechanical and geochemical properties of the host rock (Mathur et al. 1996).

The general host rock characteristics have been discussed within the framework of several other national site selection programmes. Geological criteria regarding the selection of a suitable disposal site have been formulated, for example, in South Africa (Bredell and Raubenheimer 2001), France (Devillers 1997, Lebon et al. 2001), Switzerland (McCombie et al. 1991, Nagra 1994), Germany (Baltes et al. 2002) and Argentina (Ninci et al. 2001). In addition, a Japanese report series on the H12 Project (Japan Nuclear Cycle Development Institute 1999) discusses the desirable properties of the repository host rock in some detail.

2.2.3 Discussion

The defined requirements of the host rock seem to be very similar in all countries considered here, even though the geological environments or disposal concepts considered are not necessarily the same. It seems that there is a wide agreement on what kind of a host rock is favourable in terms of high-level waste disposal. The regulatory requirements presented in Finland (STUK 2001a) discuss the properties of the host rock in more detail than the current regulations in other countries studied here, and they serve as an important starting point for this work. It has been assumed that the lack of such detailed

Host Rock Classification (HRC) system for nuclear waste disposal in crystalline bedrock