POSIVA 2000-11
Groundwater salinity at Olkiluoto
and its effects on aspent fuel repository
Timo Vieno
VTT Energy
June 2000
POSIVA OY
T6616nkatu 4, FIN-001 00 HELSINKI, FINLAND
Phone (09) 2280 30 (nat.). (+358-9-) 2280 30 (int.)
Fax (09) 2280 3719 (nat.). (+358-9-) 2280 3719 (int.)
Research organisation and address
Customer
VTT Energy, Nuclear Energy
P.O. Box 1604
FIN-02044 VTT, FINLAND
Posiva Oy
Mikonkatu 15 A
FIN-00100, Helsinki, FINLAND
Contact person
Contact person
I
Jr
JJJ I
7
-
Aimo Hautojarvi /)fAI4drJ r/f:lltb au~·
Timo Vieno
Diary code
Order reference
ENE4-4T-2000
9507/00/AJH, 11.2.2000
Project title and reference code
Report identification & Pages Date
Kaytetyn polttoaineen seka voimalaitos- ja
purkujatteen loppusijoituksen turvallisuustutkimukset I/2000 (45POSIV AOO 1)
ENE4/24/00, 36 p.
12.6.2000
Report title and author(s)
GROUNDWATER SALINITY AT OLKILUOTO
AND ITS EFFECTS ON A SPENT FUEL REPOSITORY
Timo Vieno
Summary
The Olkiluoto island rose from the Baltic Sea 2500 to 3000 years ago. The layered sequence of groundwaters
can be related to climatic and shoreline changes from modern time through former Baltic stages to the
de glaciation phase about 10 000 years ago and even to preglacial times. Fresh ground water is found to the
depth of about 150 metres, brackish between 100 and 400 metres, deeper ground waters are saline. At the
depth of 500 meters, the content of Total Dissolved Solids (TDS) varies between 10 and 25 g/1. The most
saline waters at depths greater than 800 metres have TDS values between 30 and 75 g/1. These deep saline
waters seem to have been undisturbed during the most recent glaciation and even much longer in the past.
Today fresh water infiltrating at the surface gradually displaces brackish and saline groundwater in the
bedrock. Due to the still ongoing postglacial land uplift, Olkiluoto is likely to become an inland site with
brackish or fresh ground water at the depth of 500 metres within the next 10 000 years. During the construction
and operation phases groundwater will be drawn into the repository from the surrounding bedrock. As a
consequence, more saline groundwaters, presently laying 100 to 200 metres below the repository level, may
rise to the disposal level. After the closing of the repository the salinity distribution will gradually return
towards the natural state. During the glacial cycle groundwater salinity may increase, for example, during
freezing of groundwater into permafrost, when dissolved solids concentrate in the remaining water phase, and
in a situation where deep saline ground waters from under the centre of the glacier are pushed to the upper parts
of the bedrock at the periphery of the glacier.
The most significant open issue related to saline ground water is the performance of the tunnel backfill which in
the KBS-3 concept has been planned to consist of a mixture of crushed rock and 10-30% of bentonite. Saline
ground water may significantly decrease the swelling pressure and increase the hydraulic conductivity of such a
backfill. The most promising alternative backfill options are natural mixed-layer clay (Friedland clay) and
crushed rock backfill combined with special sealing structures.
It is recommended that for a repository to be constructed at the depth of about 500 metres at Olkiluoto, all
engineered barriers should be designed to perform properly at groundwater salinities ranging from fresh water
to 35 g/1. Geochemistry and salinity of groundwater will be a key area in the further characterisation of
Olkiluoto, in supporting research, as well as in performance assessment. Posiva will participate in studies and
large-scale experiments on the performance of bentonite-based as well as alternative backfill and buffer
materials in the projects to be launched within the 5th Framework Programme of the European Commission
and in the Prototype Repository in the Hard Rock Laboratory at Aspo.
Principal auth/}'
U/
1/~P.
Timo Vieno
Senior research scientist
R?::d Av:;:.
~~~ftt~~ager, Nuclear Energy
R~~L d2arlv-
Heikki Raiko
Group manager, Nuclear Waste Management
Availability statement
Confidential
The use of the name of the Technical Research Centre of Finland (VTT) in advertising or publication in part of
this report is only permissible by written authorisation from the Technical Research Centre of Finland
mENERGIA
LAHETE
12.6.2000
Dnro ENE4-4T-2000
Posiva Oy
Mikonkatu 15 A
001 00 HELSINKI
Tilauksenne 9507/00/ AJH 11.2.2000
Oheisena Hihetetlliin raportti ENE4/24/00 "Groundwater salinity at Olkiluoto and its effects
on spent fuel repository".
Kopio
VIT ENERGIA
Aimo Hautoj arvi
D
Koivurannantie 1, Jyvaskyla
PL 1603
40101 ]YVASKYLA
Puh. (014) 672 611
Telekopio (014) 672 597
D
Biologinkuja 5, Espoo
PL 1601
02044 VIT
Puh. (09) 4561
Telekopio (09) 460 493
Teleksi 122972 vttha fi
D
Tekniikantie 4 C, Espoo
PL 1604
02044 VIT
Puh. (09) 4561
Telekopio (09) 456 5000
Teleksi 122972 vttha fi
Raportin tunnus - Report code
Posiva-raportti - Posiva Report
POSIV A 2000-11
Posiva Oy
T6616nkatu 4, FIN-001 00 HELSINKI, FINLAND
Puh. (09) 2280 30-lnt. Tel. +358 9 2280 30
Julkaisuaika - Date
June 2000
TekiJci(t)- Author(s)
1o1meks1antaJa(t)
Timo Vieno
VTTEnergy
Posiva Oy
- CommiSSioned by
Nimeke- Title
GROUNDWATER SALINITY AT OLKILUOTO AND ITS EFFECTS ON A SPENT FUEL
REPOSITORY
T11v1stelma- Abstract
The Olkiluoto island rose from the Baltic Sea 2500 to 3000 years ago. The layered sequence of
groundwaters can be related to climatic and shoreline changes from modern time through former
Baltic stages to the deglaciation phase about 10 000 years ago and even to preglacial times. Fresh
ground water is found to the depth of about 150 metres, brackish between 100 and 400 metres,
deeper groundwaters are saline. At the depth of 500 meters, the content of Total Dissolved Solids
(TDS) varies between 10 and 25 g/1. The most saline waters at depths greater than 800 metres have
TDS values between 30 and 75 g/1. These deep saline waters seem to have been undisturbed during
the most recent glaciation and even much longer in the past.
Today fresh water infiltrating at the surface gradually displaces brackish and saline groundwater
in the bedrock. Due to the still ongoing postglacial land uplift, Olkiluoto is likely to become an
inland site with brackish or fresh groundwater at the depth of 500 metres within the next
10 000 years. During the construction and operation phases groundwater will be drawn into the
repository from the surrounding bedrock. As a consequence, more saline groundwaters, presently
laying 100 to 200 metres below the repository level, may rise to the disposal level. After the closing
of the repository the salinity distribution will gradually return towards the natural state. During the
glacial cycle groundwater salinity may increase, for example, during freezing of groundwater into
permafrost, when dissolved solids concentrate in the remaining water phase, and in a situation
where deep saline groundwaters from under the centre of the glacier are pushed to the upper parts of
the bedrock at the periphery of the glacier.
The most significant open issue related to saline groundwater is the performance of the tunnel
backfill which in the KBS-3 concept has been planned to consist of a mixture of crushed rock and
10-30% of bentonite. Saline groundwater may significantly decrease the swelling pressure and
increase the hydraulic conductivity of such a backfill. The most promising alternative backfill
options are natural mixed-layer clay (Friedland clay) and crushed rock backfill combined with
special sealing structures.
It is recommended that for a repository to be constructed at the depth of about 500 metres at
Olkiluoto, all engineered barriers should be designed to perform properly at groundwater salinities
ranging from fresh water to 35 g/1. Geochemistry and salinity of groundwater will be a key area in
the further characterisation of Olkiluoto, in supporting research, as well as in performance
assessment. Posiva will participate in studies and large-scale experiments on the performance of
bentonite-based as well as alternative backfill and buffer materials in the projects to be launched
within the 5th Framework Programme of the European Commission and in the Prototype
Repository in the Hard Rock Laboratory at Aspo.
Avamsanat- Keywords
groundwater, salinity, bentonite, performance, spent fuel, nuclear waste, disposal
ISBN
ISSN
ISBN 951-652-097-9
Sivumaara - Number of pages
ISSN 1239-3096
K1eli - Language
36
English
Raportin tunnus - Report code
Posiva-raportti - Posiva Report
POSIV A 2000-11
Posiva Oy
T6616nkatu 4, FIN-001 00 HELSINKI, FINLAND
Puh. (09) 2280 30 -lnt. Tel. +358 9 2280 30
Julkaisuaika - Date
Kesakuu 2000
TekiJa(t)- Author(s)
T01meks1anta]a(t)- CommiSSioned by
Timo Vieno
VTTEnergy
Posiva Oy
Nimeke- Title
POHJA VEDEN SUOLAISUUS OLKILUODOSSA JA SEN V AIKUTUS
YDINPOLTTOAINEEN LOPPUSIJOITUSTILAAN
KAYTETYN
T11v1stelma- Abstract
Olkiluodon saari kohosi meresta noin 2500 - 3000 vuotta sitten. Sen pohjaveden kerroksellinen
rakenne heijastelee ilmaston ja vedenpinnan korkeuden vaihteluja ltameren jaakauden jalkeisissa
kehitysvaiheissajajopa viimeisinta jaakautta edeltaneita oloja. Makeaa pohjavetta esiintyy noin 150
metrin syvyyteen asti, murtovetta 100 - 400 metrin syvyydella, syvemmalla pohjavesi on suolaista.
500 metrin syvyydella pohjaveden suolaisuus (liuenneiden aineiden kokonaismaara) on 10 - 25 g/1.
800 metrin syvyydessa pohjaveden suolaisuus on 30- 75 g/1. Syvat suolaiset pohjavedet nayttavat
olleen hairiintymattomia viimeisimman jaakauden ajan ja paljon pitempaankin.
Pinnalta suotautuva makea vesi syrjayttaa vahitellen suolaista ja murtovetta kallioperassa. Yha
jatkuvan jaakauden jalkeisen maankohoamisen seurauksena Olkiluodosta on seuraavan 10 000 vuoden kuluessa tulossa sisamaan alue, jossa pohjavesi 500 metrin syvyydella on makeaa tai murtovetta. Loppusijoitustilan rakentamisen ja kayton aikana tilaan virtaa ymparoivasta kalliosta pohjavetta, joka pumpataan maanpinnalle. Taman seurauksena suolaisemmat pohjavedet, jotka nykyaan
ovat 100 - 200 metria loppusijoitustason alapuolella, voivat nousta tilan laheisyyteen. Loppusijoitustilan sulkemisen jalkeen pohjaveden suolaisuusjakauma palautuu vahitellen luonnontilaan.
Jaakausisyklin en vaiheissa pohjaveden suolaisuutta voivat lisata muun muassa ikiroudan
muodostuminen, jolloin pohjavedessa olevat liuenneet aineet konsentroituvat jaljelle jaavaan sulaan
veteen, seka tilanteessa, jossa jaatikon keskiosien alta purkautuvat syvat suolaiset pohjavedet
tyontyvat lahemmaksi maanpintaa jaatikon reunalla.
Merkittavin pohjaveden suolaisuuteen liittyva lisaselvityksia vaativa seikka on suolaisuuden vaikutus loppusijoitustunneleiden tayteaineeseen, joksi on suunniteltu seosta, joka sisaltaa kivimursketta ja 10 -30 % bentoniittisavea. Suolainen pohjavesi voi merkittavasti heikentaa tallaisen seoksen
paisuntapainetta ja kohottaa sen vedenjohtavuutta. Lupaavimmat vaihtoehtoiset tayteaineet ovat
sekakerrosrakenteinen luonnonsavi (esimerkiksi Friedlandin savi Saksassa) seka tunneleiden taytto
pelkalla kivimurskeella ja pohjaveden kulkureittien katkaiseminen sulkurakenteilla.
Noin 500 metrin syvyyteen Olkiluodon kallioperaan rakennettava loppusijoitustila on suunniteltava toimimaan hyvin ainakin pohjavedessa, jonka suolaisuus voi vaihdella alle 1:sta 35:een
grammaan litrassa. Pohjaveden geokemia ja suolaisuus tulevat olemaan keskeisia tutkimuskohteita
Olkiluodon tasmentavissa tutkimuksissa, toimintakykyanalyyseissa seka niita tukevassa perustutkimuksessa. Posiva osallistuu useisiin bentoniittipohjaisia ja vaihtoehtoisia tayteaineita koskeviin
tutkimuksiin ja kokeisiin muun muassa EU:n 5. puiteohjelman projekteissa seka Aspon kalliolaboratorion "Protyyppi loppusijoitustila" -projektissa.
Katsaus pohjautuu useisiin Posivan tutkimusohjelmaan sisaltyneisiin, pohjaveden suolaisuutta ja
sen vaikutuksia selvittaneisiin yksityiskohtaisiin tutkimuksiin.
Avainsanat- Keywords
pohjavesi, suolaisuus, bentoniitti, kaytetty polttoaine, ydinpolttoaine, ydinjate, loppusijoitus
ISBN
ISSN
ISBN 951-652-097-9
ISSN 1239-3096
K1eh - Language
Sivumaara - Number ot pages
36
Englanti
1
TABLE OF CONTENTS
Page
Abstract
Tiivistelma
Preface ....................................................................................................................
2
1
BACKGROUND ..............................................................................................
3
2
INTRODUCTION TO SALINITY ISSUES ........................................................
5
3
GROUNDWATER SALINITY AT OLKILUOTO ............................................... 10
4
FUTURE EVOLUTION .................................................................................... 14
5
EFFECTS ON ENGINEERED BARRIERS ..................................................... 19
5.1 Corrosion of copper .......... ......... ...... .. .. ....... .... ................. .. .. .. .. .. ............ 19
5.2 Buffer and backfill .................................................................................. 22
6
DISCUSSION AND RECOMMENDATIONS ................................................... 30
REFERENCES . .. .... .. ..... ... . ...... .. ...... ... .. .. .. .. .. .. ............. .. ....... .... .. .... ...................... .. .. 32
2
PREFACE
The study has been carried out by VTT Energy on contract for Posiva. On behalf of
Posiva it has been supervised by Aimo Hautojarvi, Jukka-Pekka Salo and Margit Snellman who have provided valuable inputs and comments on the report.
The review is based on several studies recently carried out within Posiva' s programme.
The authors of the original studies - Lasse Ahonen of Geological Survey of Finland,
David Dixon of AECL, Mel Gascoyne of Gascoyne GeoProjects, Ola Karnland of Clay
Technology, Jari Lofman of VTT Energy, Paula Ruotsalainen and eo-workers of
Fintact, Timo Saario and eo-workers of VTT Manufacturing Technology - have been
most helpful by providing figures and other material and commenting drafts of the
report. The author is, of course, solely responsible for the final outcome of the present
report.
3
1
BACKGROUND
Programme context
Based on research and development performed over twenty years and site investigations
carried out at six areas since 1987, Posiva published in May 1999 an Environmental
Impact Assessment (EIA) report (Posiva 1999) and submitted to the Government the
application for the Decision in Principle (DiP) for the disposal facility of spent nuclear
fuel. The EIA considered four alternative sites for the facility: Hastholmen in Loviisa,
Kivetty in Aanekoski, Olkiluoto in Eurajoki, and Romuvaara in Kuhmo. The islands of
Olkiluoto and Hastholmen are the locations of the Finnish nuclear power plants. Kivetty
and Romuvaara are forested areas and are located inland. The DiP application proposed
Olkiluoto as the site of the disposal facility. The EIA report and the DiP application
were supported by the TILA-99 safety assessment (Vieno & Nordman 1999), site
reports (e.g., Anttila et al. 1999a-b ), reports on geochemistry (e.g., Pitkanen et al. 1999),
flow and transport of groundwater (e.g., Lofman 1999, Poteri & Laitinen 1999) and
normal evolution of the repository at the candidate sites (Crawford & Wilmot 1999),
and by several other parallel and background reports dealing with scientific and
engineering aspects, encapsulation and disposal technologies, transportation, operational
safety, and environmental and social impacts of the disposal facility.
The TILA-99 safety assessment concluded that, from the point of view of postclosure
safety, all four candidate sites are suitable to host a repository for spent fuel. The safety
assessment did not give reasons to reject of any of them, neither did release and
transport analyses of radionuclides provide firm grounds to rank one site above the
others. It was, however, noted that there are differences between the sites. The most
obvious ones are related to the coastal location and saline groundwater at Hastholmen
and Olkiluoto, and on the other hand, inland location and non-saline groundwater at
Kivetty and Romuvaara. At Olkiluoto, the salinity of groundwater at the depth of
500 metres is 10- 25 g/1 of TDS (total dissolved solids), and deeper in the bedrock, at
depths greater than 800 metres, highly saline groundwaters with TDS values up to 75 g/1
have been found. One of the recommendations of TILA-99 was that due to the adverse
effects of very saline groundwater on the performance of buffer and backfill, a KBS-3
type repository should not be located at greater depths than about 700 metres at
Olkiluoto and about 800 metres at Hastholmen.
The nuclear energy law requires that a Decision in Principle be made on the need of a
planned nuclear facility. The DiP has to be made by the Government and endorsed by
the Parliament. The main purpose is to judge whether the proposed facility is "in line
with the overall good of the society". The DiP is the first licensing step requiring
political acceptance on the national and local levels. The two other, more technical
licensing steps are the construction permit and the operation license, which according to
the target schedule for nuclear waste management, set forward by the Government in
1983, should follow in 2010 and 2020, respectively.
In the DiP process the consideration of safety issues is left with the regulatory authority,
STUK. STUK submitted its statement and a preliminary safety evaluation (Ruokola
2000) of the proposed disposal facility to the Ministry of Trade and Industry in January
2000. As a basis for the assessment, STUK had obtained a review by a group of
4
independent experts from the international scientific and technical community, and
statements and specific studies from Finnish research organisations. As a conclusion
STUK states that no factors have emerged that would indicate a lack of sufficient
prerequisite for safety of the disposal facility at Olkiluoto. Therefore, from the point of
view of radiation and nuclear safety, the Government has, in STUK' s opinion, sufficient
prerequisite to make a positive Decision in Principle. In the DiP process, a right veto
can be exercised by the municipality of the proposed project. The Eurajoki municipality
council stated its acceptance of the project on January 24, 2000. The Government thus
has the prerequisites to make a positive Decision in Principle. The Government's
decision and finally the approval or rejection by the Parliament are expected later this
year.
Present report
During the past year studies have been continued on the distribution of groundwater
salinity at Olkiluoto, its future evolution and potential effects on the performance of the
engineered barrier system. In November 1999 Posiva organised a Crystalline Group
workshop on implications of high groundwater salinity to safety and technical
implementation of high-level waste disposal, which was attended by representatives of
nuclear waste management organisations in Canada, Japan, Sweden and Switzerland.
Furthermore, Posiva is preparing a research, development and underground
characterisation programme for the period 2001 - 2010. The effects of ground water
salinity at Olkiluoto will be one of the key areas of further research.
The report aims to present an overview of the status of salinity related studies in early
2000 focusing on the occurrence of saline groundwaters at Olkiluoto and elsewhere in
crystalline bedrock and its potential effects on the engineered barriers in a KBS-3 type
repository for spent fuel. The report is organised as follows:
•
The salinity issues are introduced in Chapter 2.
•
The distribution of groundwater salinity at Olkiluoto is presented on the basis of
the recent compilation by Ruotsalainen et al. (2000) and its origin is discussed on
the basis of the geochemical evolution by Pitkanen et al. ( 1999) in Chapter 3.
•
Chapter 4 discusses the future evolution of groundwater flow and salinity on the
basis of the recent (Lofman 2000) and previous ground water flow simulations.
•
The effects of high groundwater salinity on the engineered barriers of the
reference KBS-3 repository design and alternative arrangements of the barrier
system are discussed in Chapter 5.
•
The report is concluded with a discussion of plans and recommendations for
further research and development.
5
2
INTRODUCTION TO SALINITY ISSUES
Classification of ground waters
Ground waters are classified (Davis 1964) in terms of the content of Total Dissolved
Solids (TDS):
•
fresh
TDS < 1 g/1
•
brackish
1 g/1 < TDS < 10 g/1
•
saline
10 g/1 < TDS < 100 g/1
•
brine
TDS > 100 g/1.
In the present report "salinity" refers always to the TDS value. The chloride content is
about 60% of the TDS value. The salinity of ocean water is about 35 g/1, whereas the
salinity of the Baltic Sea water around Olkiluoto is about 6 g/1.
At Olkiluoto, fresh groundwater is found to the depth of about 150 metres, brackish
between 100 and 400 metres, and deeper ground waters are saline. At the depth of
500 metres, the TDS varies between 10 and 25 g/1. The most saline waters at depths
greater than 800 metres have TDS values between 30 and 75 g/1. The distribution of
groundwater salinity at Olkiluoto (Pitkanen et al. 1999, Anttila et al. 1999b,
Ruotsalainen et al. 2000) will be presented in more detail in the next chapter.
Nuclear waste disposal in salt formations
The world's first deep geological repository for long-lived nuclear waste has been
excavated at the depth of about 650 metres in a salt formation in southeastern New
Mexico in the United States. The WIPP repository was taken into operation in 1999 and
will be used for the disposal of transuranic (TRU) waste (materials contaminated with
actinides) from the US nuclear weapon programme. Salt formations have also been
studied as potential host formations for the disposal of long-lived, heat generating highlevel waste (HLW) from reprocessing or spent fuel in several countries, for example,
Germany and the Netherlands.
Deep salt formations are dry and contain no circulating groundwater. Furthermore, due
to the plastic properties of rock salt, the mined excavations will be autonomously and
very effectively sealed due to the creeping of the salt in a relatively short period of time
(within a few hundred years). These properties make salt formations, where available,
as an attractive option for nuclear waste disposal. However, salt formations include
small amounts of brine inclusions. Furthermore, accidental flooding of the repository
before it has been closed and sealed is often considered in the safety assessments.
Therefore, although the disposal concept is quite different from that in crystalline rock,
applicable data can be obtained from the salt programmes, for example, concerning
dissolution of spent fuel (Grambow et al. 1997) and behaviour of radionuclides in
highly saline groundwaters and brine.
Saline groundwaters in crystalline bedrock
Occurrence and origin of saline groundwaters in crystalline bedrock, especially in the
Fennoscandian and Canadian Shields, have been reviewed and discussed by Nurmi et al.
6
(1988), Lampen (1992) and Blomqvist (1999). A highly simplified set of conclusions
from these reviews and studies is:
•
Groundwaters are usually mixtures with different origins and ages.
•
Salinity and age of groundwater are increasing with increasing depth.
•
It is expected that at a sufficient depth, 1000- 2000 metres or more, highly saline
groundwater or brine will eventually be encountered basically everywhere in the
Fennoscandian and Canadian Shields.
•
The deep shield brines are very old(>> 10 000 years) and do not participate in the
local, meteoric circulation of groundwater.
In Finland, brine (TDS > 100 g/1) has been encountered at least in two boreholes: in the
Miihkali serpentinite at Juuka, eastern Finland, a TDS of 170 g/1 has been measured at
the depth of 1020 metres and in the arkosic sandstone at Pori, western coast, a TDS of
120 g/1 has been measured at the depth of 600 metres (Blomqvist 1999).
In Sweden, at the Aspo island the evolution of groundwater seems to have been roughly
similar as at Olkiluoto. The composition of the groundwaters reflects the past phases
and sea levels of the Baltic Sea (Figure 2-1 ). The salinity is, however, somewhat lower
at Aspo than at Olkiluoto. At the depth of about 500 metres, the Aspo groundwater is
interpreted to consist of a mixture of 30o/o meteoric water infiltrated during the past
3000 years, 30o/o glacial meltwater and 30o/o modem and old Baltic Sea water
(Laaksoharju et al. 1998). 10% of the water is of an old (brine) type of groundwater,
which has been isolated from the atmosphere for more than 1.5 million years. The TDS
value of the Aspo reference water used in the SR 97 safety assessment is 11 g/1
(Laaksoharju et al. 1998). The deep saline groundwaters at Aspo (600- 850 m) have a
more dominant brine component and TDS values up to about 20 g/1. In the deep
borehole at Laxemar, near Aspo at the mainland, groundwater is fresh to the depth of
about 900 metres, thereafter the salinity increases steeply and fairly linearly so that the
TDS value is about 80 g/1 (chloride content 47 g/1) in the bottom of the borehole at the
depth of about 1700 metres (Laaksoharju & W allin 1997).
The Finnsjon site is located about 15 kilometres inland from the coastline and the mean
elevation is about 30 metres above sea level (SKB 1999c ). Both fresh and saline
groundwaters have been met at the planned repository level. Saline groundwater is
encountered below a major, gently dipping fracture zone. The saline reference water is
interpreted to consist of about 30% meteoric water, 10% glacial meltwater, 30o/o marine
water and 20% brine, and has a TDS of 10 g/1 (Laaksoharju et al. 1998). Also at
Finnsjon, salinity increases with depth. The third site evaluated in the SR 97 safety
assessment, Gidea, is also about 15 kilometres inland from the coastline. The mean
elevation is about 100 metres above sea level, but also Gidea is situated below the
highest shoreline, which means that the site was covered by water after the most recent
glaciation (Figure 2-1 ). At Gidea ground water is fresh at the depth of 500 metres.
In Canada, AECL has carried out extensive studies on disposal of used CANDU fuel in
titanium and copper containers (Johnson et al. 1994, 1996). The reference groundwater
assumed in the case studies corresponds to the groundwater found at the depth of about
500 metres at the Whiteshell research area and has a TDS content of 11 g/1 (Johnson et
al. 1994 ). At the depth of 1000 metres the salinity of the White shell ground water has
estimated to be about 32 g/1 (Johnson et al. 1994). More recent investigations have
7
>13000 BP
b)
13000 - 10300 BP
Baltic Ice
Lake
Baltic Ice
Lake
Ceberg
~~~--~·~~~--
-~-~~~~--
--------------~~~---·
---------------~~~---·
lOOOkm
500km
500km
10300 - 9500 BP
IOOOkm
9500 - 8000 BP
Ancylus Lake
Yoldia Sea
---- ------ -siliine-- -- ---- ·
IOOOkm
500km
500km
lOOOkm
f)
e)
8000 - 2000 BP
Litorina Sea
Brackish
JV~kJ~
--- ------- -saiTne-------- ·
500km
IOOOkm
---------- -sarrne-------- ·
500km
lOOOkm
Figure 2-1. A conceptual model of groundwater evolution at Aspo (Aberg), Finnsjon
(Beberg) and Gidea (Ceberg) since the most recent glaciation (Laaksoharju 1999).
(Density turnover refers to a situation where the infiltrating saline, dense water
replaces the less saline, lighter groundwater in the bedrock.)
8
found TDS levels as high as 89 g/1 in groundwater seeping into boreholes penetrating
intact rock regions at the depth of 420 metres in the Underground Rock Laboratory
(URL) at Whiteshell (Dixon 2000). Isotope information suggests that fracture-hosted
groundwaters below 500 metres represent stagnant flow system conditions which may
have existed over one million years (Russell et al. 1999). Similarly, pore fluid
compositions within the sparsely fractured rock matrix imply prolonged rock-water
interaction indicating residence times in excess of one million years. The standard
Canadian Shield saline solution (SCSSS) used in many of the laboratory experiments
has a TDS of 55 g/1. At mining areas groundwaters with a TDS value as high as 325 g/1
have been encountered, usually at depths greater than 1000 metres (Frape & Fritz 1987).
Geophysical data from exploration borehole logging have shown that saline
groundwaters at depths between 50 and 250 metres in the sediments of the North Slope
of Alaska appear to be laterally continuous over an area of at least 1 000 km 2 (Gascoyne
2000). Their salinity has been estimated to be as high as 130 g/1. A possible origin for
these saline groundwaters is that they may have been formed as residual fluids resulting
from the rejection of salts during ice formation under conditions of aggrading
permafrost. Saline groundwaters and brines with salinities up to 300 g/1 are commonly
found also in the permafrost terrain of northern Russia (Gascoyne 2000).
Saline groundwaters at coastal areas
Brackish and saline groundwaters of marine origin are commonly found around the
Baltic Sea at areas, which have been covered by the sea at some phase after the most
recent glaciation. For example, Olkiluoto, Hastholmen and Aspo have layered
sequences of groundwaters that can be related to climatic and shoreline changes from
modem time through former Baltic stages and even to preglacial times. The highest hills
of Olkiluoto rose above sea level about 2500 to 3000 years ago (Anttila et al. 1999b),
and the top of Hastholmen about 4000 years ago (Anttila et al. 1999a).
The salinity of the Baltic Sea water has varied at the different phases. The maximum
salinity (in the Baltic Sea south of the islands of Aland) since the most recent glaciation
is estimated to have been 10- 15 g/1 (TDS) at the Litorina Sea phase (Westman et al.
1999), whereas the present salinity around Olkiluoto is about 6 g/1 and the salinity of
ocean water is about 35 g/1. It is evident that the highest groundwater salinities at
Olkiluoto, Hastholmen and Aspo do not originate from seawater infiltrated into the
bedrock after the most recent glaciation. A question is why high salinities at relatively
shallow depths seem to be met more often at coastal areas than inland. An explanation is
that sea and coastal areas are natural discharge areas for deep shield brines that have
travelled over long distances and times through the bedrock. In SITE 94 (SKI 1996) it
was concluded that the deep saline groundwaters at Aspo consist of shield brines which
have originally infiltrated in the southern Swedish highlands and the Caledonian
mountains between Sweden and Norway. Another explanation is that coastal areas have
spent more time under ice sheet, ice lakes and sea, and have thus experienced less of
meteoric groundwater circulation than inland sites at high elevations.
Saline groundwaters are ubiquitous in coastal areas that are currently underlain by
permafrost in Northern America and Russia (Gascoyne 2000). However, it is not clear
9
how much of the salinity has been concentrated by the freezing process and how much
is due to leaching of saline soils and sediments by groundwaters or to the presence of
residual seawater in the sediments.
Salinity and KBS-3
As deep very saline groundwaters seem to be geochemically stable and have long
residence times and are not associated with active meteoric water circulation, it has been
proposed that repositories for high-level nuclear waste could be located in such zones
(e.g., King-Clayton et al. 1997, p. 51; Blomqvist 1999, p. 36). Construction of a
repository at a great depth and high groundwater salinity may, however, involve also
adverse effects on the operational and long-term safety. Problems related to the rock
mechanical stability of the excavations make construction at great depths expensive and
bring about risks for the operational and long-term safety. At Olkiluoto, the construction
conditions at the depth of 500 metres and deeper have been classified as demanding or
very demanding, necessitating systematic rock bolting and fibre-reinforced shotcreting
of the tunnels (Aikas et al. 1999). Other issues, related more directly to the salinity of
groundwater and the KBS-3 disposal concept (based on the engineered barrier system
consisting of copper-iron canister, buffer of compacted bentonite, and tunnel backfill of
a mixture of crushed rock and bentonite), include:
•
Highly saline water is aggressive to construction materials, equipment and sealing
structures.
•
A layered system of different groundwaters is geochemically and geohydrologically complex. Sites at the coast of the Baltic Sea are presently in a transient
phase affected by the still ongoing postglacial land uplift. The effects of land
uplift and varying salinity of groundwater need to be taken into consideration in
evaluating of the geohydrological and geochemical evolution of the sites.
•
At high chloride concentrations, especially when combined with a high
temperature and a low pH, formation of chloride complexes decreases the
immunity of copper and at the same time hinders the formation of a good
passivating oxide (Beverskog & Puigdomenech 1998, Hermansson & Eriksson
1999). However, this process is not considered to be of importance in the
repository conditions where electron acceptors, necessary for the cathodic half of
the corrosion reaction, are not available (Ahonen 1995, 1999; SKB 1999a). The
issue will be discussed in more detail in Section 5 .1.
•
Saline groundwater impairs the swelling capacity and increases the hydraulic
conductivity of the buffer and backfill. The effects are more significant in the
backfill mixture of crushed rock and bentonite than in the buffer consisting of
highly compacted blocks of bentonite (Kamland 1998, Dixon 2000).
•
Salinity affects also groundwater flow, and behaviour and transport of radionuclides. The retardation properties (solubility, sorption, diffusivity) of many
radionuclides depend on the salinity of groundwater. The most significant effect is
the reduced sorption of cations (e.g., Sr, Cs, Ra) in saline water. In the safety
assessments, highest release and dose rates are thus often obtained in scenarios
assuming high flow rates and short transit times of saline groundwater. These
effects have been thoroughly evaluated and discussed in the TILA-99 safety
assessment (Vieno & Nordman 1999) as well as in SR 97 (SKB 1999a), and are
not dealt with in the present report.
10
3
GROUNDWATER SALINITY AT OLKILUOTO
Present distribution of salinity
All salinity measurements, based on water sampling as well as on geophysical methods,
are discussed in detail and the results are compiled by Ruotsalainen et al. (2000).
Figure 3-1 shows a compilation of all salinity measurements in the deep boreholes at
Olkiluoto. Figure 3-2 shows results of the salinity measurements (electrical conductivity
converted into TDS) with the flowmeter, where water is pumped from fractures without
mixing it with the water in the borehole and its electrical conductivity is measured in
situ, and geophysical resistivity logging in the borehole KR11. The latter method
provides a continuous measurement curve of the salinity of the water in the open
borehole. In borehole KR11 salinity seems to increase steeply from about 10 g/1 at the
depth of 750 metres to about 55 g/1 at the depth of 950 metres.
80
• Baltic sw IDS(chem)
X
-- _il.__ _
70
Overburd.IDS(chem)
+ KRl IDS(chem)
<> KRl IDS(in situ)
o KR2 IDS(chem)
0
~
60
o KR2 IDS(in situ)
• KR2 IDS(tube)
• KR..liDS(chem)
so
KR3 IDS(in situ)
4 KR41DS(chem)
~
rJJ 40
A KR41DS(in situ)
•
Q
~
A
+
30
•
20
. ·--•
A KR41DS(tube)
:< KRSIDS(chem)
• •
+ KRSIDS(insitu)
• KR61DS(in situ)
• KR7 IDS(chem)
- KR7 IDS(i n situ)
KR81DS(chem)
KR81DS(in situ)
10
X
KR9 IDS(chem)
X
KR91DS(in situ)
/ KRlO IDS(chem)
I:<
0
0
200
400
600
800
1000
1200
KRlO IDS(in situ)
o KRlO IDS(tube)
+ KRlliDS(insitu)
Depth, m
Figure 3-1. Salinity of groundwater in deep boreholes. "Chem" refers to water pumped
from fractures in packer-isolated bore hole sections and "tube" to water sampled with a
tube sampler from the open borehole. "In situ" refers to water pumped from fractures
in packer-isolated borehole sections with the flowmeter which measures the electrical
conductivity of the water in situ. (Ruotsalainen et al. 2000).
11
60
--
50
-s
40
::::::::
CJ)
c
1-
30
------~--
20 -----
10
''~r---------..r---~
0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
850
900
950
1000
Depth, m
TDS (in situ) g/1
umuuu
TDS (geophysics) g/1
I
Figure 3-2. Salinity of groundwater in borehole KRJJ. Dots present in situ
measurements of water pumped from fractures in packer-isolated borehole sections
with the flowmeter and the continuous line presents resistivity logging in the open
borehole. (Ruotsalainen et al. 2000).
The salinity values in the deep boreholes at the levels of -300 ...-400 metres, -500
metres and -700 ...-800 metres are compiled in Figure 3-3. "Level" refers to the vertical
depth compared to sea level, whereas in Figures 3-1 and 3-2 "depth" refers to borehole
length. At the level of -300 ... -400 metres the TDS values range from 5 to 20 g/1. No
clear signs of dilution with intruding fresh water in areas with high groundwater table
can be seen. At the level of -500 metres the TDS values vary between 10 and 25 g/1.
The lowest value in borehole KR3 may be associated with intrusion of fresh water from
the recharge area towards fracture zones around the borehole (Ruotsalainen et al. 2000).
At the level of -700 ...-800 metres there are measurements from three boreholes. The
measured TDS values range from 30 to 70 g/1. The highest measured TDS is 7 5 g/1 from
a tube sample from the bottom part ( 1000-1050 metres) of borehole KR2 (Ruotsalainen
et al. 2000). The vertical level of this section is about -950 metres.
All salinity measurements discussed in the present report deal with the salinity of water
flowing in fractures. The water within the rock matrix may have a higher salinity than
the water in the fractures (SKB 1999d). The salinity of the fluid in the rock matrix is,
however, not very important from the point of view of the performance of engineered
barriers and near-field transport, although it may have an effect on the transport of
radionuclides in the geosphere.
12
6793000
6792000
>deep boreholes
J:
I
6791000 -l--------f---------+--------i--....::....l..:::;.;;;;=.......J...I..I.....:.......ti!!:.==!IL--____J
1525000 y
1526000
1524000
1527000
1523000
6793000
6792000
deep boreholes
6791ooo L-------+----------+-------~-~~::s~!!.L.~~~:::::_j
1525000 y
1523000
1524000
1526000
1527000
6793000
6792000
deep boreholes
6791000 -L--------+----------+-------~~~~::S~!J:C.2~:=::::..:.:_j
1525000 y
1523000
1524000
1526000
1527000
Figure 3-3. Salinity of groundwater in the deep boreholes at the levels of
-300... -400 m, -500 m, and -700... -800 metres. The thin lines show the equipotentials
of the groundwater table at the surface and the thick line presents the shoreline.
(Ruotsalainen et al. 2000 ).
13
Origin and age of groundwater
Geochemistry and past evolution of Olkiluoto's groundwater have been thoroughly studied
by Pitkanen et al. ( 1999). The main water types and their interpreted origins and formation
ages are presented in Table 3-1. The fresh and brackish groundwaters to the depth of about
500 metres seem to contain a fairly well-developed profile which can be related to the
present, above sea level, conditions which started about 2500 to 3000 years ago, through
former Baltic stages to the de glaciation phase about 10 000 years ago. At the depth of
about 450 metres, the groundwater is interpreted to consist of a mixture of 5% of Litorina
Sea water, 15o/o of glacial meltwater, 50o/o of subglacial water, and 30% of undisturbed
saline water. At the depth of about 550 metres, the groundwater mixture is interpreted to
contain 10% of glacial meltwater, 45% of subglacial water, and 45% of undisturbed saline
water.
The saline groundwaters at depths greater than 500 metres seem to be chemically stable.
Geochemical modelling does not indicate any significant mixing of glacial meltwater in
saline water; the interpreted portion of glacial meltwater is less than 10% in the upper part
of the saline groundwater. Highly saline waters at depths greater than 700 metres seem to
have been undisturbed during the most recent glaciation and even much longer in the past.
Table 3-1. The main water types at Olkiluoto and their interpreted origins and formation
ages (Anttila et al. 1999b, Pitkiinen et al. 1999).
Depth of
occurrence
Water type
Origin of dominant endmembers
Age estimate of
dominant endmember type
(years)
above 150
Fresh-brackish
HC03 -rich water
Meteoric water and
present Baltic Sea water
0-2500
100-300
S04 -rich brackish
Na-Cl water
Litorina Sea water
2 500-7 500
100-500
Brackish N a-Cl
water
Pre-Litorina water
containing fresh glacial
meltwater
7 500-10000
below 500
Saline Ca-N a-Cl
water
Preglacial to Precambrian
saline water
>> 10 000
(metres)
14
4
FUTURE EVOLUTION
Until next glaciation
The transient simulations of groundwater flow and evolution of salinity carried out by
Laitinen & Lofman ( 1996) and Lofman ( 1996, 1999) have been complemented by
Lofman (2000). The most recent study complements the previous ones with several
sensitivity analyses and analyses with alternative conceptual models. The cases include:
•
Base Case: solute transport is modelled with the equivalent-continuum (EC)
model, fracture zones and land uplift are taken into account
•
Case 1: ten-fold flow porosity for solute transport
•
Case 2: solute transport is modelled with the dual-porosity (DP) model
•
Case 3: fracture zones are ignored
•
Case 4: land uplift is ignored
a) with fracture zones
b) without fracture zones
•
Case 5: alternative initial conditions
a) computed with the equivalent-continuum (EC) model
b) computed with the dual-porosity (DP) model
c) salinity is ignored, i.e. fresh water conditions.
Two conceptual models have been used to simulate evolution of salinity: the single
porosity equivalent-continuum (EC) approach and the dual-porosity (DP) approach. In
the DP approach the solute is divided into two interacting parts: the water moving in
the fractures and the stagnant water in the pores of the rock matrix. The dual-porosity
approach is conceptually more realistic than the EC approach, but on the other hand, it
involves more parameters the realistic values of which are difficult to define. The
simulations of Lofman ( 1999), which are indicated as Base Case in Figure 4-1, and
most of those of Lofman (2000) have been performed using the EC approach.
The repository is modelled as a plate having a fairly high, isotropic transmissivity of
5·10- 8 m 2/s. To the depth of 900 metres, the initial present-day salinity of groundwater
is represented by a depth-dependent curve fitted to the measured values. At the depth of
500 metres, the present-day salinity is about 18 g/1 (TDS). At the depth of 900 metres,
the salinity is 72 g/1 and this value is then used to the bottom of the model at the depth
of 1500 metres. Lofman (2000) has used also alternative initial conditions for the
present-day salinity. The alternative present-day salinity distributions were derived
from simulations started at the early Litorina Sea stage, about 7500 years before
present, using for that time point initial conditions based on the geochemical interpretations of Pitkanen et al. ( 1999). The estimated salinity distribution 7500 years ago
differs from the present-day situation in practice only in the upper part of the bedrock,
where groundwater is estimated to have been brackish. The simulations from these
assumed past conditions to the present time result in somewhat lower present-day
salinity as shown in Figure 4-1. The ongoing postglacial land uplift is modelled
according to the model developed by Passe ( 1996). Today the land uplift rate at
Olkiluoto is about 6 mm/year and the remaining uplift during the next 10 000 years is
estimated to be 40 metres. All the modelling approaches and data are presented in detail
in Lofman (1999, 2000).
15
The evolution of groundwater salinity at the repository at the depth of 500 metres is
shown in Figure 4-1. In all cases the salinity is decreasing from the present-day value.
In cases with an initial salinity of 18 g/1, the salinity at 10 000 years ranges from 0 to
7 g/1. In the Case 5 with the lower initial salinity, fresh water conditions (TDS < 1 g/1)
are reached within 4000 years. Also in Case 4, where land uplift is ignored, fresh water
infiltrating at the surface gradually displaces brackish and saline groundwater in the
bedrock, although slower than in the base case with land uplift.
Average salinity concentration at repository
20~--~--------~----------~--------~--------~--------~----~
CASE 2:: DP model
~·
\\\\ .~ . j .
18
CASE 1: ten-fold flow porosity
16
' »"'
14
\
""""""
CASE 4a: without land uplift
\
12
· · CASE 4b: without land uptift and ·
fracture zones
\
"§, 10
\
(J)
0
~
CASE 3: without fracture zones
8
~
.....
.
--~..:.
·\.
' .
\
>~
····~'<-
6
4
2
0
0
2000
4000
6000
8000
10000
t [years]
Figure 4-1. Groundwater salinity at the repository at the depth of 500 metres (Lofman
2000).
Basically Figure 4-1 tells that the zone or bubble of fresh water is increasing in size and
reaches the 500-metre level in a few thousand years. This process started 2500 to 3000
years ago when the highest tops of Olkiluoto rose from the sea. Before that Olkiluoto
was covered by sea and lakes in the Baltic stages after the most recent glaciation as
shown in Figure 2-1. During these stages mostly brackish water infiltrated at the
surface. Before the formation of the bubble of brackish-fresh water the salinity at the
depth of 500 metres may have been higher than today. The past changes in the salinity
at the 500-metre level are, however, not as drastic as one might conclude on the basis of
the simulated future evolution. In Cases Sa and 5b, where the simulations were started
7500 years ago, the salinity has decreased 13 and 5 g/1, respectively, during this period.
16
In an ideal steady state case for a coastal area, where a lens of fresh water is floating
over saline water, and both zones are assumed to be homogenous and a sharp interface
is assumed between the two zones, the depth of the fresh-saline water interface (z) can
be derived from the Ghyben-Herzberg principle (Lofman 2000):
z
=
h
(4-1)
where h is the elevation of the groundwater table above sea level (m), and Po and p are
the densities (kg/m3) of fresh and saline waters, respectively. Lofman (2000) has related
the density and TDS value of groundwater by p = Po + 0.71 X TDS. At Olkiluoto the
maximum elevation of the groundwater table is 10 metres above sea level. For the
average salinity (42 g/1) of the ground water between ground surface and the depth of
1500 metres of the initial salinity model used by Lofman (2000), the Ghyben-Herzberg
principle would locate the fresh-saline water interface at the depth of 335 metres. In
case of a lower salinity, the interface would be located deeper, for example for a TDS of
30 g/1 at the depth of about 560 metres. The highly simplified approach of the GhybenHerzberg principle confirms the results of the numerical simulations: even without any
further land uplift, infiltrating fresh water is able to displace saline water to the depth of
several hundred metres.
Another rough, order-of-magnitude estimate of infiltration of fresh water into the
bedrock may be obtained as follows: If infiltration rate is taken to be 5 mm/year (about
1% of precipitation) and the total porosity of the rock mass is taken to be 1%, then
infiltrating fresh water is displacing groundwater with a rate of 0.5 metre of rock mass
per year, i.e. 500 metres in thousand years.
Upconing during construction and operation
The studies performed for Olkiluoto have not accounted for the effects of construction
and operation of the repository, but the repository has been assumed to be fully
saturated immediately after the sealing with groundwater presently met at the disposal
level. During the construction and operation phases the repository, from which water is
pumped to the ground surface, draws water from all directions. At the surface this
results in a drawdown of the groundwater table - at the Aspo Hard Rock Laboratory the
drawdown is estimated to be about 40 metres (Rhen et al. 1997). Below the repository,
significant upconing of deep, saline groundwaters towards the repository may take place
(Svensson 1997). In the repository, the inflows of less saline waters from upwards and
more saline waters from downwards mix with each other, so that the effects on the
average salinity of inflowing water are less drastic. According to the simulations carried
out by Svensson ( 1997), the average salinity of ground water inflowing into the ramptunnel system at Aspo would be about 6.9 g/1 TDS, with a maximum of 30 g/1 in a
major fracture zone. In the natural state, the salinity at the depth of 450 metres was
estimated to be about 8 g/1. The measured salinity of the inflow into the deep parts of
the tunnel system (in April to November of 1995) ranges from 6 to 15 g/1 (Rhen 1997).
The upconing effects will be studied in detail before and during the construction and
operation of the underground rock characterisation facility at Olkiluoto. However,
already on the basis of the studies carried out at Aspo, it can be estimated that during the
17
50 years of construction and operation of the repository, upconing may result in that
waters which today lie 100 or 200 metres below the disposal depth may be drawn close
to the repository. The effects on the engineered barrier system in the repository are,
however, less significant because also less saline water is drawn from the upper parts of
the bedrock. After the closing of the repository the groundwater table and the salinity
distribution will gradually return towards the natural state.
Glaciation
In the early phase of glaciation, the elevation of Olkiluoto will be further increased
because of lowering of sea level. During the tundra, permafrost and interstadial phases,
recharge into the bedrock is fresh but in average probably lower than today. Decrease of
recharge may lead to diminishing of the fresh water bubble and rise of saline groundwater. Finally in the deglaciation phase, Olkiluoto is likely to experience similar phases
than after the most recent glaciation: intrusion of glacial meltwater, ice lake and sea
covers, land uplift, and above sea level conditions. If the connections to the ocean are
better than after the most recent glaciation, the seawater covering Olkiluoto may be
more saline.
There are glaciation-related processes that may increase the salinity of groundwater
around the repository. During freezing of groundwater into permafrost, dissolved solids
concentrate in the remaining water phase. Deeply penetrating permafrost in the bedrock
would cause relatively pure water to form as ice in fractures and displace residual saline
fluids, under density flow, to greater depths (Gascoyne 2000). However, taking into
account that the initial salinity of the groundwaters above and around a repository at
Olkiluoto at a depth of about 500 metres is not very high and that the increased density
tends to remove the most saline solutions from the freezing zone, it seems unlikely that
this process could result in very high salinity of groundwater around the repository.
During the glaciation a situation may arise where deep groundwaters from under the
centre of the glacier are pushed to the upper parts of the bedrock at the periphery of the
glacier. Svensson (1999) has carried out demonstrative simulations for the Aspo area,
but a similar situation seems at least in theory possible basically at all sites in northern
Europe. The computational domain used in the simulations is three-dimensional with
dimensions of 250 X 10 X 4 km 3 (Figure 4-2). The maximum height of the glacier is
1000 metres. Initial salinity distribution is given as zero salinity down to 1000 metres,
below this level a linear increase with 100 g/1 (TDS) per 1000 metres is prescribed. At
the bottom of the model at 4000 metres, the initial salinity is thus as high as 300 g/1. The
simulation is quasi-steady: All boundary conditions are in a steady state and the ice
margin is assumed to be located at Aspo for the whole period simulated, which is 1000
years.
The results show that under the glacier salinity of groundwater in the deep bedrock is
decreasing as meltwater displaces saline water. The saline water is pushed towards the
ice margin where upconing of salinity can been seen: at the depth of about 500 metres
the salinity at the ice margin could increase to 40 - 60 g/1. As Svensson (1999) points
out, the simulations should be seen only as indicative demonstrations of potential effects
in the postulated situation: There are several uncertainties in the conceptual
assumptions, boundary conditions and data. For example, downwrapping of earth's
18
crust under the glacier, and a forebulge and a possible permafrost zone at the ice margin
have not taken into consideration. The simulations, nevertheless, indicate that there are
processes which potentially may result in a short-term intrusion of deep, more saline
groundwater into the repository.
ICE TUNNELS
4
z
...
10
...
100
...
MELTING RATE
z
Figure 4-2. Computational domain of simulations of Svensson (1999 ). All distances are
in kilometres.
19
5
EFFECTS ON ENGINEERED BARRIERS
5.1
Corrosion of copper
General corrosion
Different conclusions have been presented on whether high groundwater salinity,
especially when combined with high temperature and low pH, could enhance general
corrosion of copper in the reducing repository conditions. Beverskog & Puigdomenech
( 1998) note that presence of chloride increases the corrosion region of copper at the
expense of the immunity and passivity regions in the Pourbaix diagrams. According to
their calculations copper could corrode at 80 - 100 °C at a chloride concentration of
1.5 molal (corresponding to about 90 g/1 TDS when chloride is assumed to constitute
60% of TDS). Even with a chloride concentration of 0.2 molal (10 g/1 TDS) the
calculated equilibrium potentials are close to a corrosion situation at the temperature of
80 - 100 °C. Beverskog & Puigdomenech ( 1998) do, however, not discuss the cathodic
half reaction, i.e. electron acceptors associated with the oxidation of copper, in the
reducing repository conditions.
Based on a literature survey and Pourbaix diagrams calculated with Puighdomenech' s
set of programs and also in part his database, Hermansson & Eriksson ( 1999, p. 84) also
note that the immunity limit for copper on the redox scale descends as a function of
temperature at high chloride concentrations. This means a risk that copper is neither
noble, nor passive at least at higher temperatures in a repository environment containing
large concentrations of chloride. In such a case,. the corrosion rate of the copper canister
would be limited by mass flow rates of reacting species through the bentonite buffer.
Based on a thorough thermodynamic review and evaluation, Ahonen ( 1995) concludes
that salinity of water (high chloride concentration) as such does not cause any risk of
corrosion. Corrosion of copper due to chloride is not possible in any conditions, which
are possible within the disposal system (Ahonen 1995, page 54). A brine containing tens
of grams of chloride per litre can not corrode copper more than about 2·1 o-6 M at pH 4
at 25 °C (Figure 5-1). At pH 6, copper concentration due to corrosion remains below
10-7 M at 25 °C. Increase of temperature from 25 °C to 100 °C has approximately same
effect on copper corrosion as an increase of chloride concentration by one order of
magnitude. Ahonen ( 1999) confirms that no relevant cathodic half reaction (i.e.
oxidiser) has been found, which could act as electron acceptor from metallic copper (i.e.
oxidise copper) in the conditions of deep disposal in saline water environment. There is
a theoretical possibility at pH values below 6 at 100 °C for water (i.e. H+) to accept
electron from copper with subsequent formation of hydrogen gas. Thermodynamic
equilibrium calculations clearly show, however, that this reaction can not leach more
than 1o-6 M copper at a chloride activity of 1 (corresponding to a chloride concentration
of about 50 g/1 and a TDS of about 80 g/1), pH 6 and temperature 100 °C until
equilibrium is attained (Figure 5-1). A similar relationship as in Figure 5-1 has been
obtained by Tax en ( 1990). Tax en's calculations show an equilibrium concentration of
copper less than 1o- 5 M at a chloride concentration of 3 M (corresponding to about
180 g/1 TDS), pH 6 and temperature 100 °C. Hydrogen is formed as a product of this
corrosion reaction, thus, if originally present in water, it will further suppress the
reaction.
20
-1
-2
-3
....... -4
....,
0 -5
...,
..
-
C"'
cu -6
-
:::J
-7
(.)
........
0)
0
_J
-8
-9
-10
-11
-12
0
1
2
3
4
5
6
7
8
9
10
pH
Figure 5-l. Equilibrium concentration of copper (mol!l) in the hydrogen-generating
corrosion reaction where water (i.e. H+) is assumed to be the electron acceptor.
Chloride activity is 1, corresponding to a chloride concentration of about 50 g/l and a
TDS of about 80 g!l. The lines between 25 °C and 100 °C represent temperatures 50 °C
and 75 °C. (Ahonen 1999).
SR 97 (SKB 1999a) also notes that copper forms strong chloride complexes, which
could increase the rate of outwards transport of dissolved copper. It is, however,
estimated that very high chloride concentrations (> 100 g/1, corresponding to about
170 g/1 TDS) in combination with pH values below 3 are required in order for such a
process to be of importance. (Note: There is a misprint on page 205 of SKB 1999a,
there is "< 100 g/1", it should be "> 1OOg/1" as in the original Swedish version of the
SR 97 report).
The key points related to the role of high groundwater salinity in the general corrosion
of copper canisters in the reducing repository conditions may be summarised as follows:
At high chloride concentrations, combined with high temperature and low pH,
formation of chloride complexes decreases the immunity region of copper and at the
same time hinders the formation of a good passivating oxide layer. Corrosion is,
however, possible only if there is an electron acceptor in the system. In the repository
conditions the only relevant electron acceptor is H+. Even in the worst case of high
chloride concentration, high temperature and low pH, the hydrogen-generating
corrosion results in very low concentration of copper (of the order of 1o- 6 M) implying a
negligible extent of corrosion.
21
Because of the somewhat obscure and conflicting statements in the various reports,
there is a need to clarify the conclusions on the above matters and to determine "safe
limits" in terms of chloride concentration, redox capacity, temperature and pH. The
definition of "water stability" corresponding to different partial pressures of hydrogen
(open air vs. the hydrostatic pressure at the disposal depth) should also be explained
more clearly. Efforts leading hopefully to more clear and transparent presentation of the
results and conclusions are underway in association of an updated evaluation of copper
corrosion in a SKB study to which also Posiva' s corrosion experts have contributed.
Localised corrosion
In oxidising conditions corrosion of copper may be significantly enhanced by localised
corrosion in pits. Chloride plays an important role for initiation and propagation of
pitting (Hermansson & Eriksson 1999). Already moderate concentrations of chloride
can contribute to the deepening of the pits by formation of soluble complexes with
copper that continuously diffuse away from the pits. The pitting attack is, however,
expected to slow down as the chloride concentration in the pits increases, which is
observed in field investigations and experiments as long-term effects. Hermansson &
Eriksson (1999, p. 86) explain the long-term slowdown of pitting as follows: In a pit the
chloride concentration increases to a very high level due to inwards migration of cr. In
such an environment CuCh- and CuCb 2- predominate together with naked copper metal,
and there will be no CuCl precipitate. CuCh- and CuCb 2- hydrolyse producing Cu20,
acid (HCl) and chloride. In these hydrolysis reactions, chloride is produced in a surplus
compared with hydrolysis of a supposed CuCl precipitate. At increasing chloride
concentration, the hydrolysis of complexes, and thus also the acid formation, slows
down and virtually comes to an end. As a result the pitting attack also slows down as the
chloride concentration in the pit environment increases.
Literature survey on stress corrosion cracking (SCC) of copper in presence of nitrites,
ammonia, carbonates and acetates (Saario et al. 1999) concludes that nitrites and
carbonates are not expected to cause SCC of copper in the disposal conditions. At
Hastholmen and Olkiluoto ammonium has mainly been found in the brackish
ground water with the Litorina Sea water input at depths between 100 and 200 metres.
The source of ammonium seems to be related to the ancient seawater input. Possible
sources are organic nitrogen compounds in the sediment. The maximum ammonium
concentrations observed at Olkiluoto's and Hastholmen's groundwaters are 1 mg/1 and
3 mg/1, respectively (Anttila et al. 1999a-b). In the deep saline ground waters ammonium
is hardly found, concentrations are below 0.03 - 0.05 mg/1. The maximum
concentrations of ammonium found in the brackish groundwater is of the range of the
concentration found to be critical for SCC of copper in some laboratory tests (Saario et
al. 1999). The susceptibility of copper alloys for SCC caused by ammonia depends on
the concentration of phosphorus in the copper alloy. The planned canister material is
oxygen free high conductivity copper (Cu-OF) with an addition of 40 to 60 ppm
phosphorus (Cu-OFP) to improve the creep strain properties of Cu-OF at high
temperatures (Raiko & Salo 1999). Sato & Nagata (1978) found that the Cu-OFP alloy
is not susceptible to SCC even at very high loads (200 MPa) when the phosphorus
concentration is less than 80 ppm. Furthermore, it has been found out that chloride
inhibits SCC of copper in nitrite solutions (Russell et al. 1999). Most of the test results
reported in the survey by Saario et al. ( 1999) are, however, obtained in test
22
environments (e.g. moist air, gaseous environment, high ammonia content) which do
not correspond to the expected repository conditions. In order to exclude the possibility
of SCC caused by ammonia, Posiva has in cooperation with SKB started an
experimental program where the effects of ammonia on SCC of copper are studied in an
environment simulating the repository conditions.
Saario et al. ( 1999) note that also acetates are known to cause stress corrosion cracking
in pure copper. Acetates are compounds of the organic acetic acid which may be
produced in the bedrock by acetogenic bacteria (Haveman et al. 1998). A wide range of
other bacteria, including methanogenic, sulphate reducing and iron reducing, may use
acetate as an energy source. The bentonite buffer is, however, a hostile environment for
bacteria because of the very low availability of free water in highly compacted bentonite
(Pedersen 1997, Brown & Sherriff 1998). Because of the limited number of
investigations found in the literature, Saario et al. ( 1999) could not determine any
boundary conditions for SCC of copper caused by acetates. Also in this case
experimental verification may be needed. As in the case of SCC caused by ammonia,
also sec caused by acetates is not particularly an issue related to the deep saline
ground waters.
5.2
Buffer and backfill
Desired properties of buffer and backfill
In the KBS-3 concept the buffer consists of blocks of highly compacted bentonite clay.
Bentonite clays have a high swelling capacity in water with a low salinity. The clay
absorbs water and can swell to a volume several times the original dry clay volume. The
volume of the deposition hole in rock is in principle fixed and the bentonite mass may
be balanced to give a desired swelling pressure and thus to provide a homogenous,
impermeable, and yet sufficiently plastic and soft buffer around the canister. The
desired properties of the buffer, several of which are associated with the swelling ability
of the clay, include:
•
chemical and mechanical stability
•
no properties that could have harmful effects on the other barriers
•
suitable swelling pressure ( 1 - 10 MPa): high enough to provide the favourable
properties, but not too high to cause excessively loads on the canister and rock
•
low hydraulic conductivity
•
sufficient thermal conductivity
•
sufficient bearing capacity
•
sufficient plasticity and softness to dampen effects of rock displacements
•
sufficient permeability to corrosion gases that may form in a damaged copper-iron
canister
•
ability to filter colloids
•
ability to filter and limit growth of micro-organisms
•
chemical buffering capacity
•
limitation of transport of corrodents and radionuclides by sorption and due to a
low diffusitivity.
23
The main functions of the backfill used in the top of the deposition hole and in the
tunnels and shafts are:
•
to keep the buffer in place around the canister
•
to contribute towards keeping the tunnels stable
•
to prevent the tunnels and shafts to become a major conductor of groundwater and
transport pathway of radionuclides.
The backfill shall, of course, not have any properties that could cause harmful effects on
the other barriers, and it shall be chemically and mechanically stable. The other desired
properties include sufficient density and low compressibility to keep the buffer in place,
and a low hydraulic conductivity. Some swelling pressure (-100 kPa) would be
beneficial to support the tunnels against rock falls, to establish a tight backfill/rock
contact and to counterbalance the effects of settlement and piping in the backfill.
Sorption capacity for radionuclides is also a favourable property in the backfill material.
In the reference KBS-3 repository design the backfill consists of a mixture of crushed
rock and 10-30o/o of bentonite. In the lower part of the tunnel the backfill can be
compacted mechanically layer by layer. A mixture with a higher proportion of bentonite
is planned to be used in the upper part of the tunnel, where the backfill will be injected
with a spray. Furthermore, it has been recognised that a high proportion of bentonite
(maybe even higher than 30%) and a high density of the backfill mixture is needed in
highly saline water to ensure the desired swelling properties and low hydraulic
con ducti vi ty.
Buffer and backfill materials in the Canadian concept
The buffer and backfill materials in the Canadian reference concepts for spent fuel
disposal (Johnson et al. 1994, 1996; Dixon 2000) differ somewhat from those of the
KBS-3 concept. In the Canadian concept the buffer consists of precompacted blocks of
a 50/50 mixture of silica sand and bentonite clay. It has in principle similar functions
and desired properties as the KBS-3 buffer.
In the Canadian concept, a major part of the disposal rooms and tunnels (80% of their
volume in the case of the in-floor disposal concept) is planned to be backfilled with the
so-called dense (or lower) backfill consisting of a 70/25/5 mixture of crushed granite,
lake clay and bentonite. This material is expected to have only a minimal swelling
capacity. Its primary function is to be a stiff filler with a low hydraulic conductivity.
The dense backfill is planned to be emplaced by direct compaction in layers in all those
regions of the tunnels and rooms where in situ compaction equipment can be used. In
other places large blocks (-1 m 3 ) of precompacted material may be used.
The upper parts of the tunnels and rooms and other regions, where in situ compacted or
precompacted dense backfill cannot be used, will be backfilled with the so-called light
(or upper) backfill consisting of a 50/50 mixture of granite sand and bentonite. The
material will be emplaced using pneumatic and other means of remote placement. The
high content of bentonite ensures that the light backfill, which is used in the most
sensitive regions of the disposal tunnels and rooms, has the required swelling and selfsealing capacity.
24
Special terms: effective clay dry density and void ratio
The swelling properties of bentonite mixtures depend on the content and density of
bentonite. From the point of view of the swelling process, the solid, non-swelling
aggregate in the mixture, like crushed rock, can be considered as a "dead volume".
A quantity often used in characterising the properties of bentonite mixtures is the
effective clay dry density:
.
.
Dry mass of clay
Effective clay dry density (ECDD) = - - - - - - - ' - - - - - - - - ' - - - - Volume occupied by clay + volume of voids
(5-1)
In the following, the calculation of ECDD values is illustrated with two assumed
examples: The specific density of particles of clay, sand or rock is about 2700 kg/m3 .
The ECDD of compacted bentonite with a saturated density of 2000 kg/m3 and a
porosity of 41% is thus 1590 kg/m3 . In a backfill mixture containing 30o/o of bentonite
and having a saturated density of 2100 kg/m3 and a porosity of 35%, the ECDD would
be 970 kg/m3 . The ECDD's of the Canadian reference buffer, light backfill, and dense
backfill are 1250, 950, and 460 kg/m3 , respectively (Dixon 2000).
Another special term used in soil science (and in Figure 5-3 below) is the void ratio:
Void ratio =
Volume of void space
Volume of solid particles
Porosity
1- Porosity
(5-2)
Effects of saline ground water
The swelling capacity of bentonite-based materials results from the mineral structure of
smectite clays. Smectites are composed of very thin platelets, each of which has a high
surface charge on its faces. In the presence of free water, the clay platelets try to move
apart to reduce the repulsion between the platelets. Under certain conditions, the
presence of saline water can limit the potential of the smectite particles to expand. The
decrease in the swelling capacity is relatively greatest in clays with a low (effective clay
dry) density. The extensive experiments on this subject, carried out in soil science and
within the nuclear waste management programmes, and the developed models have
been reviewed by Kamland ( 1998) and Dixon (2000).
Kamland (1998) has improved and applied thermodynamical models to study the effects
of high porewater salinity on the swelling pressure. Maximally pessimistic models
indicate that the swelling pressure decreases with increasing salinity, disappearing
entirely at a N aCl content of about 70 g/1 in bentonite with a saturated density of
2000 kg/m 3 • Calculations based on less pessimistic parameter values correlated with
experimental results show that a swelling pressure can be expected even in a saturated
salt solution with 36 weight-% of NaCl (corresponding to a TDS higher than 400 g/1)
(Figure 5-2).
25
-2150 kg/m 3
- - 2100 kg/m3
-
2050kg/m3
-
2000kg/m3
1950 kg/m 3
1900 kg/m3
1850 kg/m3
NaCI concentration, weight-o/o
Figure 5-2. Swelling pressure as a function of the salinity at different saturated
densities of bentonite (SKB 1999a, based on Figure 2-8 of Karnland (1998)). (NaCl
concentration of 10 weight-% corresponds to a TDS of about 100 g/l. A saturated salt
solution has a NaCl concentration of 36 weight-%. Saturated densities of 2150, 2000
and 1850 kg!m 3 of compacted bentonite correspond, respectively, to effective clay dry
densities of approximately 1830, 1590 and 1350 kg/m 3 .)
Figure 5-3, based on studies carried out by Studds et al. ( 1998) at the University of
Leeds in the United Kingdom, shows the swelling pressure at low to intermediate
densities (ECDD of 200 to 1400 kg/m3 ) for groundwater salinities from 0 to 58 g/1. The
influence of salinity is not discernible in bentonite materials having an ECDD higher
than approximately 1000 kg/m 3 . Such dense bentonites have a swelling pressure higher
than 1 MP a. At lower densities, the swelling pressure decreases with increasing salinity.
At the salinity of 58 g/1, the swelling pressure is negligible (less than 1 kPa) when the
effective clay dry density is less than approximately 700 kg/m 3 . These results are
consistent with those of the Canadian experiments carried out by Dixon and eo-workers
(Dixon 2000).
Figure 5-4 presents the results of a study into the effects of saline (58 g/1) groundwater
percolation on the hydraulic conductivity in bentonite and bentonite-sand mixtures. The
data show that at effective clay dry densities exceeding approximately 1000 kg/m3 the
hydraulic conductivity of bentonite-based materials is not discernibly affected by the
presence of a saline permeant. At the ECDD of about 700 kg/m 3 , the hydraulic
conductivity in the saline solution is about two orders of magnitude higher than in
deionised water, although the conductivity is still fairly low (less than 10-9 m/s) in both
cases.
26
0.18
14
12
0
0.00 molll
X
0.01 molll
<P
0.10 molll
•
1.00 molll
0.21
~
l'f'l
6
10
..........0
8
0.25
....0
~
>
~
'"-"
~
0.30
..........
r.rJ
=
QJ
~
~
~
Q
0.40
6
..
~
Q
~
0.55
4
~
u
QJ
2
•
0
1
10
100
0.92
..........;;,...
1.38
~
~
~
~
1000
Vertical Effective Stress (kPa)
Figure 5-3. Influence of salinity (NaCl) on swelling pressure (vertical effective stress in
the experiment) of bentonite (Dixon 2000, based on Studds et al. 1998). (NaCl
concentration of 1 mol/l equals to a TDS of 58 g/l. In the KBS-3 buffer the effective clay
dry density is about 1.6 Mglm 3 , whereas it is less than 1 Mg/m 3 in the backfill.)
lE-08
58 g/1 Saline Solution
y = 1E-1tx·111.628
,-..._
~
'-"
~
lE-09
R
2
= 0.7998
lE-10
>.
......
....·s:......
lE-11
=
=
u
lE -12
u
"'CS
0
--X
.~
"3~
-=
"'CS
X
lE -13
.....
Deionized Water
-5.4196
y = 1E-12x
2
R = 0.7232
lE-14
X
X
X
,
lE -15
0
0.5
1
1.5
2
Effective Clay Dry Density (Mg/m 3 )
2.5
Figure 5-4. Influence of a saline solution with a TDS of 58 g/l on the hydraulic
conductivity of bentonite materials (Dixon 2000). (In the KBS-3 buffer the effective clay
dry density is about 1.6 Mg/m 3, whereas it is less than 1 Mglm 3 in the backfill.)
27
From the point of view of prevention of colloid formation and the associated erosion of
bentonite into the rock fractures, salinity of groundwater is in principle a positive factor.
Theoretically, the clay could form particles that are small enough to diffuse in the
groundwater. In order for the clay gel to be stable and not be dispersed to a colloid
suspension, the groundwater must contain sufficiently high concentrations of cations.
Highly charged ions are the most effective. For example, if the concentration of Ca2+ is
over 0.1 mM (4 ppm), a stable clay is obtained (SKB 1999b). At Olkiluoto, the calcium
concentration is much higher already in the fresh groundwater in the upper parts of the
bedrock and, of course, increases with the increasing salinity of groundwater (Pitkanen
et al. 1999). Furthermore, very high groundwater velocities, of the order of 300 30 000 m/yr, in the fractures intersecting the deposition hole were needed to initiate
bentonite erosion (Kurosawa et al. 1999, Pusch 1999a).
Implications for KBS-3 buffer and backfill, alternative solutions
The reviews (Kamland 1998, Dixon 2000) confirm that groundwater salinities as high
as 100 g/1 will not significantly affect the swelling pressure and the associated
properties of the highly compacted bentonite buffer with a saturated density of about
2000 kg/m3 . There are, however, uncertainties related to the performance of the KBS-3
backfill mixture of crushed rock and bentonite in highly saline groundwater. Kamland
( 1998) concludes that the positive effects of mixing bentonite into the backfill will be
lost if the system is exposed to brine. Dixon (2000) concludes that in saline groundwaters, where the porewater salinity is between 10 and 75 g/1, an effective clay dry
density of at least 900 kg/m 3 is needed to develop a swelling pressure of at least
100 kPa. It is uncertain whether such a density can be obtained with the planned backfill
mixtures, and emplacement and in-situ compaction methods.
There are several possibilities to improve the performance of the backfill and sealing
systems to obtain the desired functions also in a highly saline environment:
•
Increasing of the bentonite content and density: The bentonite content could be
increased, for example, to the 50% used in the Canadian light backfill. The
density might be increased by using precompacted blocks.
•
A Canadian type of arrangement where two or more backfill materials with
distinctly different properties and functions are used: The major volume of the
tunnels are filled with a dense, uncompressible material having a low hydraulic
conductivity, but only a minimum swelling capacity. The filler could be, for
example, crushed rock with an optimised grain size distribution and a low content
(-5%) of bentonite. This filler is then covered and surrounded with a swelling
backfill, consisting of a mixture with a high (-50%) bentonite content, in the
vicinity of the roof and walls of the tunnel. This swelling mixture or blocks of
highly compacted bentonite could be emplaced also in other sensitive parts: on the
top of the deposition holes and around the sealing structures. Layers of swelling
backfill are needed also in the vertical shafts to support the sealing structures after
eventual settlement of the non-swelling backfill. An issue that needs to be taken
into consideration in planning of the sealing structures is the potentially harmful
effects that hyperalkaline pore fluids from cementitious materials may have on the
performance of bentonite (Kamland 1997).
•
One possibility is to use crushed rock as filler in the tunnels and to block transport
pathways with special sealing structures. In a case of intrusion of highly saline
28
•
•
groundwater, the performance of crushed rock may be even better than that of a
bentonite mixture with a too low effective clay dry density. A low hydraulic
conductivity may be obtained with an optimised grain size distribution of crushed
rock, but it will, nevertheless, be higher than that of the surrounding rock.
Furthermore, due to settlement of the crushed rock backfill and lacking support of
the rock, a flow channel or a conductive excavation damaged rock zone may be
formed at the roof to the tunnel. Plugs play thus an important role in preventing
formation of continuous, highly conductive transport pathways along the tunnels.
Already the reference concept includes massive concrete plugs, with a thickness
up to six metres, at the mouths of the deposition tunnels (Haaramo 1999). One of
their functions is to prevent intrusion of backfill and ground water into the central
tunnel during the operation of the repository. Special sealing structures consisting
of concrete plugs and sealing rings of compacted bentonite blocks inserted in slots
cut in the rock may also be constructed around highly conductive fracture zones
intersecting deposition tunnels. A difficulty with this kind of backfilling and
sealing arrangement is that performance assessment needs to evaluate and model
the long-term evolution of the sealing structures. The TILA-99 safety assessment
(Vieno & Nordman 1999) includes several scenarios where effects of a high
groundwater flow through the backfill on the release of radionuclides from the
repository are evaluated. It should, however, be noted that these analyses deal
only with the outwards transport of radionuclides, and do not address the effects
of high flow and potential intrusion of superficial waters on the other engineered
barriers.
Pusch (1998, 1999b) has proposed smectitic mixed-layer clay, for example
Friedland clay from the Neubrandenburg area in northern Germany, as an
alternative backfill and buffer material. In all key functions, its properties are
evaluated to be equal or superior to those of a 30/70 mixture of Wyoming MX-80
bentonite and crushed rock. Furthermore, the performance of Friedland clay is not
significantly affected by groundwater salinity (Figure 5-5). This is explained by
that the majority of the expandable minerals are of mixed-layer type with almost
the same hydration potential in sodium and calcium form (Pusch 1998). These
favourable properties - together with a long history of utilisation, availability in
large quantities, and competitive costs - make Friedland clay as an interesting
alternative. Pusch (1998), however, notes that it remains to be demonstrated on a
full scale that Friedland clay ground to a suitable grain size distribution can be
sufficiently compacted in the repository.
The isolation of the canister from the tunnel could be improved by deepening of
the deposition hole and by backfilling it to the top with compacted bentonite,
whereas in the reference concept the uppermost metre of the deposition hole is
planned to be filled with the tunnel backfill. Use of compacted bentonite in the
upper part of the deposition hole would, especially, reduce mass transport between
the deposition hole and the excavation damaged rock zone below the tunnel floor.
A potential drawback is that the expanding buffer might cause extra mechanical
disturbances in this zone.
29
3000
C'CS
jt
a..
2500
~
cD
::s 2000
~
U)
U)
Q)
~
a.
I
1500
Cl
c 1000
-
~
Q)
3:
en
500
I
V
-~
-
~
e-0
1700
1800
1900
~
2000
2100
2200
Density at saturation, kg/cubic m .
tn
.......
.....
1,OOE-09
E
~
>
;::;
1,00E-10
-...
0
:::s
-g
-' "'
.........
1,OOE-11
;
~
"'- '
'
~
0
0
0
~
'
1,OOE-12
-
'
~
.....
;
'
..
~"'IJI""
;
lo..
"'C
~
:I:
1,OOE-13
1700
I
1800
1900
2000
2100
2200
Density at saturation, kg/cubic m.
Figure 5-5. Swelling pressure (upper) and hydraulic conductivity (lower) of Friedland
clay as a function of saturated density. The thin curves represent distilled water and the
thick curves a 3.5% CaCl2 solution with a TDS of about 35 g/l. (Pusch 1998).
Taking into account engineering feasibility and long-term performance aspects, as well
as cost aspects in some extent, the most interesting alternatives to improve the KBS-3
backfill concept to endure high ground water salinities are 1) natural mixed-layer clay
(Friedland clay), 2) backfill of crushed rock with special sealing structures, and
3) filling of the deposition hole to the top with highly compacted bentonite. An ultimate
solution to backfill problems would be disposal concepts where there is only a buffer of
highly compacted bentonite in the deposition rooms, for example, horizontal
emplacement of canisters according to the Medium Long Hole (MLH) concept (Autio et
al. 1996).
30
6
DISCUSSION AND RECOMMENDATIONS
Design basis salinity
A design basis TDS value for a repository excavated at the depth of about 500 metres at
Olkiluoto could be, for example, 35 g/1. All the repository systems and engineered
barriers should perform properly at least at groundwater salinities ranging from fresh
water to 35 g/1. Today the salinity at the depth of 500 metres varies from 15 to 25 g/1. A
design basis value of 35 g/1 would allow intrusion of groundwaters presently lying 100
to 200 metres below the 500-metre level. As 35 g/1 is the salinity of ocean water, it
would also take into account the maximum possible salinity of water infiltrating at the
surface.
If the repository were planned to be constructed deeper in the bedrock, the design basis
salinity value needs to be raised. For example, if the repository would be located at a
depth of 700 metres, a possibility of intrusion of highly saline, brine-type groundwaters
(TDS nearing or exceeding 100 g/1) into the repository should be taken into
consideration.
Backfill studies
The most significant open issue related to saline groundwater is the performance of the
tunnel backfill. A salinity of 35 g/1 may significantly impair the performance of the
KBS-3 reference backfill consisting of a mixture of crushed rock and 10-30o/o of
bentonite. It is, therefore, important to continue the studies and large-scale experiments
on the performance of bentonite-based as well as alternative backfill and buffer
materials. Such studies and experiments are underway and planned in projects to be
launched in the 5th Framework Programme of the European Commission and in the
Prototype Repository in the Hard Rock Laboratory at Aspo. The most promising
alternative backfill options are Friedland clay and crushed rock backfill combined with
special sealing structures.
Site characterisation, supporting research, performance assessment
Geochemistry and salinity of groundwater will be a key area in the further
characterisation of Olkiluoto, in supporting research, as well as in performance
assessment. As concerns future evolution, the first priority topics are related to the
effects of upconing during construction and operation of the repository, heat generation
of spent fuel, and land uplift until the next glaciation. Data and basis for calibration and
verification of the simulation models will be obtained from the underground rock
characterisation facility to be constructed at the disposal level. In the long-term
considerations, the effects of climate and sea level changes, permafrost and glaciation
need to be taken into consideration, too. These long-term effects are more speculative in
nature and not particularly specific to the Olkiluoto site. They might be interesting
topics for international cooperation efforts.
The present Olkiluoto conditions are, of course, the starting point for the studies on
barrier performance and data for performance assessment. In the future Posiva' s
programme will put more weight on studies in saline groundwater conditions. However,
31
it should be taken into consideration that within the next 10 000 years Olkiluoto is likely
to become an inland site with brackish or fresh groundwater at the repository level. In
performance assessment, time-dependent modelling of the effects of groundwater
salinity evolution could reduce some of the extra conservatism of the TILA-99 safety
assessment: A high flow of saline groundwater through the repository into the biosphere
can only be a transient situation, not a steady-state case as assumed in some of the most
conservative scenarios of TILA-99.
32
REFERENCES
Ahonen, L. 1995. Chemical stability of copper canisters in deep repository. Helsinki,
Nuclear Waste Commission of Finnish Power Companies, Report YJT -95-19.
Ahonen, L. 1999. Effects of saline water on metallic copper. Helsinki, Posiva, Working
Report 99-5 8.
Aikas, K. (ed.), Hagros, A., Johansson, E., Malmlund, H., Sievanen, U., Tolppanen, P.,
Ahokas, H., Heikkinen, E., Jaaskelainen, P., Ruotsalainen, P. & Saksa, P. 1999.
Engineering rock mass classification of the Olkiluoto investigation site. Helsinki,
Posiva, Working Report 99-55. (In Finnish).
Anttila, P., Ahokas, H., Front, K., Hinkkanen, H., Johansson, E., Paulamaki, S.,
Riekkola, R., Saari, J., Saksa, P., Snellman, M., Wikstrom, L. & Ohberg, A. 1999a.
Final disposal of spent nuclear fuel in Finnish bedrock - Hastholmen site report.
Helsinki, POSIV A 99-08.
Anttila, P., Ahokas, H., Front, K., Hinkkanen, H., Johansson, E., Paulamaki, S.,
Riekkola, R., Saari, J., Saksa, P., Snellman, M., Wikstrom, L. & Ohberg, A. 1999b.
Final disposal of spent nuclear fuel in Finnish bedrock - Olkiluoto site report. Helsinki,
POSIV A 99-10.
Autio, J., Saanio, T., Tolppanen, P., Raiko, H., Vieno, T. & Salo, J.-P. 1996. Assessment
of alternative disposal concepts. Helsinki, POSIV A 96-12.
Beverskog, B. & Puigdomenech, I. 1998. Pourbaix diagrams for the system copperchlorine at 5 - 100 °C. Stockholm, Swedish Nuclear Power Inspectorate, SKI Report
98:19.
Blomqvist, R. 1999. Hydrogeochemistry of deep groundwaters in the central part of the
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engineered barriers of a high-level nuclear waste repository. Stockholm, Swedish Nuclear
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spent U0 2 fuel with saline brines. Luxembourg, European Commission, Report EUR
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the candidate sites in Finland. Helsinki, POSIV A 98-15.
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hydrogeochemistry of bedrock. (To be published in the POSIVA series).
Haaramo, M. 1999. Structural design of the concrete structures used to plug deposition
tunnels. Helsinki, Posiva, Working Report 99-71. (In Finnish).
Haveman, S. A., Pedersen, K. & Ruotsalainen, P. 1998. Geomicrobial investigations of
groundwaters from Olkiluoto, Hastholmen, Kivetty and Romuvaara, Finland. Helsinki,
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Sunder, S., Onofrei, C. & Crosthwaite, J. L. 1996. The disposal of Canada's nuclear fuel
waste: A study of postclosure safety of in-room emplacement of used CANDU fuel in
copper container in permeable plutonic rock- Volume 2: Vault model. Pinawa, Atomic
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Karnland, 0. 1997. Cement I bentonite interaction - Results from 16 month laboratory
tests. Stockholm, Swedish Nuclear Fuel and Waste Management Co (SKB), Technical
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Kamland, 0. 1998. Bentonite swelling pressure in strong NaCl solutions - Correlation
of model calculations to experimentally determined data. Helsinki, POSIVA 98-01.
King-Clayton, L., Chapman, N., Ericsson, L. 0. & Kautsky, F. (eds.) 1997. Glaciation
and hydrogeology - Workshop on the impact of climate change & glaciations on rock
stresses, groundwater flow and hydrochemistry - Past, present and future. Stockholm,
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and dispersion-flocculation behaviour of bentonite particles. W arrendale, Materials
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34
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Aspo Hard Rock Laboratory - Proceeding of the second Aspo International Geochemistry
Workshop, June 6-7, 1995. Stockholm, Swedish Nuclear Fuel and Waste Management Co
(SKB), International Cooperation Report 97-04.
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at Aberg, Beberg and Ceberg. Stockholm, Swedish Nuclear Fuel and Waste Management
Co (SKB), Technical Report 98-03.
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simulations. Helsinki, POSIV A 2000-07.
Nurmi, P. A., Kukkonen, I. T. & Lahermo, P. W. 1998. Geochemistry and origin of
saline groundwaters in the Fennoscandian Shield. Applied Geochemistry 3, 185-303.
Passe, T. 1996. A mathematical model of the shorelevel displacement in Fennoscandia.
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96-24.
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for performance assessment of radioactive waste disposal - Results and conclusions
achieved during the period 1995 to 1997. Stockholm, Swedish Nuclear Fuel and Waste
Management Co (SKB), Technical Report 97-22.
Pitkanen, P., Luukkonen, A., Ruotsalainen, P., Leino-Forsman, H. & Vuorinen, U.
1999. Geochemical modelling of groundwater evolution and residence time at the
Olkiluoto site. Helsinki, POSIV A 98-10.
Posiva 1999. The final disposal facility of spent nuclear fuel - Environmental Impact
Assessment report.
Poteri, A. & Laitinen, M. 1999. Site-to-canister scale flow and transport at the
Hastholmen, Kivetty, Olkiluoto and Romuvaara sites. Helsinki, POSIVA 99-15.
35
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Technical Report TR-98-16.
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LIST OF REPORTS
1 (2)
POSIV A REPORTS 2000, situation 7/2000
POSIV A 2000-01
Interpretation of the Hastholmen in situ state of stress based on core
damage observations
Matti Hakala
Gridpoint Finland Oy
January 2000
ISBN 951-652-087-1
POSIV A 2000-02
Rock mechanics stability at Olkiluoto, Hastholmen, Kivetty and
Romuvaara
Erik Johansson, Jari Rautakorpi
Saanio & Riekkola Oy
February 2000
ISBN 951-652-088-X
POSIV A 2000-03
Sorption and desorption of cesium on rapakivi granite and its minerals
Tuula Huitti, Martti Hakanen
Laboratory of Radiochemistry
Department of Chemistry
University of Helsinki
Antero Lindberg
Geological Survey of Finland
April2000
ISBN 951-652-089-8
POSIVA 2000-04
Porewater salinity and the development of swelling pressure in
bentonite-based buffer and backfill materials
David A. Dixon
Atomic Energy of Canada Limited
June 2000
ISBN 951-652-090-1
POSIV A 2000-05
In-situ failure test in the Research Tunnel at Olkiluoto
forma Autio, Erik Johansson, Timo Kirkkomiiki
Saanio & Riekkola Consulting Engineers
Matti Hakala
Gridpoint Finland Oy
Esa Heikkilii
Helsinki University of Technology
Laboratory of Rock Engineering
May 2000
ISBN 951-652-091-X
POSIV A 2000-06
Regional distribution of microbes in groundwater from Hastholmen,
Kivetty, Olkiluoto and Romuvaara, Finland
Shelley A. Haveman, Emma Larsdotter Nilsson, Karsten Pedersen
Goteborg University, Sweden
June 2000
ISBN 951-652-092-8
LIST OF REPORTS
2 (2)
POSIV A 2000-07
Site scale groundwater flow in Olkiluoto - Complementary simulations
Jari liJfman
VTTEnergy
June 2000
ISBN 951-652-093-6
POSIV A 2000-08
Engin~ering rock mass classification of the Olkiluoto investigation site
Kari Aikiis (editor), Annika Hagros, Erik Johansson,
Hanna Malmlund, Ursula Sieviinen, Pasi Tolppanen
Saanio & Riekkola Consulting Engineers
Henry Ahokas, Eero Heikkinen, Petri Jiiiiskeliiinen,
Paula Ruotsalainen, Pauli Saksa
Fintact Oy
June 2000
ISBN 951-652-094-4
POSIVA 2000-09
A review of published literature on the effects of permafrost on the
hydrogeochemistry of bedrock
M. Gascoyne
Gascoyne GeoProjects Inc., Canada
June 2000
ISBN 951-652-095-2
POSIVA 2000-10
Modelling of the U0 2 dissolution mechanisms in synthetic groundwater
- Experiments carried out under anaerobic and reducing conditions
Esther Cera, Mireia Grive, Jordi Bruno
EnvirosQuantiSci, Spain
Kaija Ollila
VTT Chemical Technology
July 2000
ISBN 951-652-096-0
POSIV A 2000-11
Groundwater salinity at Olkiluoto and its effects on a spent fuel
repository
Timo Vieno
VTIEnergy
June 2000
ISBN 951-652-097-9