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Gas migration and two-phase flow through engineered and geological barriers for a deep repository for radioactive waste


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A joint EC/NEA status Report
Nuclear energy and safety



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* *AEN
NE A Communily Researcho
iVor/eariVor/ear ScienceScience andand TechnologyTechnology
Gas Migration and Two-Phase Flow
through Engineered and Geological Barriers
for a Deep Repository for Radioactive Waste
A Joint EC/NEA Status Report
DGDG Research/D.11.3 -- RR && TT programme "Nuclear fissionfission safetysafety 1994-98"1994-98"
Contact: Mr G. A. Cottone
Address: European Commission, rue de la Loi/Wetstraat 200 (M075 5/43), B-W49 Brussels
Tel. (32-2) 29-51589; fax (32-2) 29-54991European Commission Nuclear Energy Agency
« * e
Gas Migration and Two-Phase Flow
throughthrough Engineered andand Geological Barriers
for a Deep Repository for Radioactive Waste
A Joint EC/NEA Status Report
W. R. Rodwell ', A. W. Harris ', S. T. Horseman 2, P. Lalieux 3, W. Muller \
L. Ortiz Amaya 5 and K. Pruess 6
11 AEAAEA Technology, Harwell, UKUK
22 BritishBritish GeologicalGeological Survey,Survey, Keyworth,Keyworth, UKUK
3 OECD Nuclear Energy Agency, Issy-les-Moulineaux, F
4 Institut fur Sicherheitstechnologie (ISTec), Kòln, D
5 Nuclear Reasearch Centre SCK.CEN, Mol, B
66 Lawrence Berkeley NationalNational Laboratory,Laboratory, UniversityUniversity ofof California,California, USAUSA
Science,Science, Research andand Development
Neither the European Commission nor any person acting on behalf of the Commission is responsible for
the use which might be made of the following information.
A great deal of additional information on the European Union is available on the Internet.
It can be accessed through the Europa server (http://europa.eu.int).
Cataloguing datadata cancan bebe foundfound atat thethe endend ofof thisthis publication.publication.
Luxembourg: Office for Official Publications of the European Communities, 1999
ISBN 92-828-8132-6
© European Communities, 1999
Reproduction is authorised provided the source is acknowledged.
Printed inin BelgiumBelgium
Printed on white chlorine-free paperFOREWORD
In underground repositories for radioactive waste, significant quantities of gases may be
generated due to several processes. These gases may migrate through the engineered barrier
system and the natural geological barrier. It is therefore recommended that the potential
impactimpact ofof gasgas accumulation andand migrationmigration onon thethe performance ofof thethe variousvarious barriersbarriers shouldshould
bebe addressedaddressed andand assessedassessed in thethe development of safetysafety casescases for radioactive wastewaste
Significant effort has and continues to be expended in numerous national and international
programmes in attempts to understand and assess this potential impact, notably in the
framework ofof thethe EuropeanEuropean Commission's NuclearNuclear FissionFission Safety programme.programme. TheseThese effortsefforts
have resulted in significantly improved understanding of the relevant processes and produced
an important collection of experimental data.
hi this context, it was considered timely to undertake a review ofthe knowledge gained so far
and establish the current status of the basic understanding ofthe gas issue.
To ensure proper coverage of all the R&D work already carried out within national and
international programmes, and to avoid duplication of efforts, the elaboration of the status
report has been set up under the joint auspices of the European Commission (EC) and the
Nuclear Energy Agency of the Organisation for Economic Co-operation and Development
(NEA/OECD). The work has been commissioned from a group of authoritative experts by the
European Commission, inin thethe frameworkframework ofof itsits R&DR&D programmeprogramme onon NuclearNuclear FissionFission Safely,
and by several national waste management organisations represented within the NEA Co¬
ordinating Group for Site Evaluation and Design of Experiments for Radioactive Waste
Disposal (SEDE).
TheThe presentpresent reportreport providesprovides aa synthesissynthesis ofof availableavailable information,information, identifiesidentifies keykey conceptsconcepts andand
mechanisms andand theirtheir coupling, evaluatesevaluates thethe current approachesapproaches toto modellingmodelling andand assessing
the impact of gas on repository performance, highlights, wherever possible, some of the
unresolved issues, and presents recommendations for further work.
The opinions and conclusions expressed are those of the authors only and do not necessarily
reflect thethe viewsviews ofof thethe funding organisations,organisations, anyany ECEC and/orand/or OECDOECD Member country,country, oror
international organisation.
The status report has been jointly supported by the European Commission and a consortium
ofnational organisations active in the field of radioactive waste management and represented
within the NEA Co-ordinating Group for Site Evaluation and Design of Experiments for
Radioactive Waste Disposal (SEDE):
-- ANDRA, FranceFrance (National(National Radioactive WasteWaste ManagementManagement Agency);Agency);
ENRESA, Spain Agency for Radioactive Waste);
- IPSN, France (Nuclear Protection and Safety Institute);
- ONDRAF/NIRAS, Belgium (National Organisation for Radioactive Waste and
Fissile Materials);
Ontario PowerPower Generation, Canada;
- NAGRA, Switzerland (National Co-operative for the Disposal of Radioactive
- N1REX, United Kingdom;
- SKB, Sweden (Swedish Nuclear Fuel and Waste Management Company);
SKI, Power Inspectorate); and
-- UKEA, UnitedUnited KingdomKingdom (Environment Agency);
- USDOE/YMP, United States (Department ofEnergy, Yucca Mountain Project).
Furthermore, GRS, Germany (Company for Reactor Safety) and SCK/CEN, Belgium (Nuclear
Energy Research Centre) contributed to the status report by providing a thorough review of the
draftdraft versions.versions.
All these organisations are deeply thanked for their support and for their fruitful reviews and
The EC and the NEA wish to express their gratitude to the authors of this report - A.W. Harris,
S.T.S.T. Horseman,Horseman, Ph.Ph. Lalieux,Lalieux, W.W. Muller,Muller, L.L. OrtizOrtiz Amaya, K.K. Pruess andand W.R.W.R. RodwellRodwell -- andand toto
allall theirtheir colleagues,colleagues, tootoo numerous toto bebe quotedquoted here,here, whowho havehave helpedhelped themthem inin theirtheir task.task. InIn
particular, the EC and the NEA wish to commend W.R. Rodwell for his effective management
of the report drafting.
B. Haijtink and Ph. Lalieux were responsible for the co-ordination of this report on behalf of
thethe EC/DGX1IEC/DGX1I andand thethe NEANEA WasteWaste Management Division respectively.respectively.
In recent years it has become apparent that:
a) in many scenarios for the disposal of radioactive waste in geological media, gas
migration and two-phase flows may occur in the engineered barrier systems
associated with a repository and in the geosphere in the vicinity of the repository, and
b) mechanisms may exist by which these flows might potentially have an impact on the
performance of the geosphere or engineered barriers.
Significant effort has been and continues to be expended in numerous national and
international programmes in attempts to understand the potential impact of gas migration,
andand ofof two-phasetwo-phase gas-gas-waterwater processes,processes, onon thethe performance ofof underground radioactive wastewaste
repositories, and to provide modelling tools or approaches that will allow these impacts to be
assessed. In the light of these efforts, it was considered timely and of widespread benefit to
undertake a review (Status Report) to establish the current status of the basic understanding
that exists of gas migration and two-phase gas-water flow processes and their potential
impacts onon thethe performanceperformance ofof engineeredengineered andand geological barriers forfor thethe deepdeep disposaldisposal ofof
radioactive waste.waste. ThisThis reportreport hashas beenbeen preparedprepared toto fulfil thethe needneed for suchsuch anan overview.overview. It
has been prepared as a joint EC (DG XH) and NEA project, with funding providedjointly by
the EC through its R&D programme on Nuclear Fission Safety, and by several national waste
management organisations represented within the NEA Co-ordinating Group for Site
Evaluation and Design of Experiments for Radioactive Waste Disposal (SEDE).
The report is intended to cover work published prior to January 1999, but does also include
some work known to the authors that was being prepared for publication at that time.
The following limitations on the scope and ambitions of the report should be noted:
a)a) TheThe reviewreview isis restricted inin itsits aimsaims toto two-phase flowflow involvinginvolving gasgas andand waterwater only.only.
(The potential presence of non-aqueous phase liquids in a repository environment is
not discussed.)
b) The focus is on long term safety issues, and thus the emphasis is on gas migration and
two-phase flow after repository closure (with account taken of the operational phase
inin asas farfar asas itit determines thethe initialinitial post-closurepost-closure conditions).conditions).
c) Only underground repositories are considered, not surface or near-surface storage
facilities or bunkers.
d) There is no attempt to provide a review of the large topic of single-phase, particularly
groundwater,groundwater, flow.flow.
e) The roles of geomechanics and geochemistry in affecting gas migration are issues
which are highlighted where appropriate, but again the subjects of geomechanics and
geochemistry themselves do not form part of this report.f)f) ThisThis StatusStatus ReportReport isis notnot intendedintended toto provideprovide aa specialistspecialist technicaltechnical résumé thatthat couldcould
substitute for original published material, but to contribute a critical assessment of the
current status at a level of technical detail sufficient to inform the design of technical
programmes and research policy.
Repository HostHost RocksRocks
Many of the issues covered in the review are dependent upon the host-rock type under
consideration. The repository host rock types considered are classified as follows in the
a)a) water-saturatedwater-saturated fractured crystallinecrystalline rock,rock,
b)b) unsaturatedunsaturated fracturedfractured rock,rock,
c) plastic clays,
d) indurated mudrocks, and
e) rock salt.
ThereThere isis somesome arbitrariness inin thesethese classifications, butbut theythey areare consideredconsidered toto provideprovide aa
sufficiently representative range to capture the essential characteristics of those rock types
being used or considered as repository host rocks, even if the above nomenclature does not
provide a truly accurate description of all these host rocks.
Significant differences in the important processes are to be expected between sites in these
various rockrock types,types, andand thesethese differencesdifferences areare highlightedhighlighted inin thethe review.review. However, effortsefforts areare
also made to establish where there are common issues that affect all repository rock types.
In drawing comparisons between gas migration and two-phase flow issues in different
geological settings, it is important to draw a distinction between saturated and unsaturated
environments. Most repository concepts that have been proposed and are reviewed herein
areare forfor aa repository locatedlocated inin tighttight geological formations belowbelow thethe waterwater table,table, inin thethe
saturated zone. For these, the chief pathway through which radionuclides could escape from
the repository is through dissolution and transport in groundwater. Various mechanisms
have been recognised that could cause a gas phase to appear in an otherwise saturated
environment; these include principally gas generation by corrosion, radiolysis, or microbial
degradation, butbut alsoalso degassing fromfrom solutionsolution ofof naturallynaturally present gasgas duedue toto thermalthermal oror
radiation impact oror depressurisationdepressurisation nearnear thethe repositoryrepository excavations.excavations. TheThe presencepresence ofof two-two-
phase (water-gas) conditions could alter groundwater flow and solute transport, and affect
the chemical environment of the waste packages. In addition, gas flow could conceivably
promote the release of volatile radionuclides. A sound understanding of two-phase flow and
gas migration in geologic media, and in engineered barrier materials, is needed for design
andand performanceperformance assessmentassessment ofof aa saturated-zonesaturated-zone repository.
Disposal in an unsaturated site is envisaged in one important case only, namely the Yucca
Mountain site in the USA, where the U.S. civilian nuclear waste programme is unique in its
focus on disposal of high-level wastes in the unsaturated zone, above the water table. There
are fundamental differences in performance assessment issues for this unsaturated site
compared withwith thethe otherother saturated sites,sites, andand thisthis raisedraised thethe issueissue aboutabout whether thethe partsparts ofof
VIthethe reportreport relating toto thethe YuccaYucca Mountain conceptconcept shouldshould bebe presentedpresented separately fromfrom thethe
rest. On balance it was felt that it might be more useful to integrate as far as possible the
discussion of unsaturated site issues with that of saturated site issues so that similarities and
differences could be best highlighted.
Potential Safety IssuesIssues
Gas migration from a repository can lead directly, through the release of radioactive gases at
the surface, to radiological consequences analogous to those that are assessed for the
transport of radionuclides from a repository via other pathways. Gas migration and two-
phase flows can also affect repository performance indirectly through potential impacts on
transport viavia thesethese otherother pathways. TheThe latterlatter effects areare ofof theirtheir naturenature ratherrather complicated,complicated,
coupled processes, and this adds to the difficulty of assessing the consequences of gas
generation in repositories.
The main identified potential safety issues that may arise from gas migration and two-phase
flow effects can be summarised as follows:
a) Overpressurisation and its consequences
If gas cannot escape from a repository as fast as it is generated, the pressure will rise. If the
pressure were to rise to a level at which gas fracturing of the engineered barriers or host rock
could occur, then there would be concern that this would leave pathways along which
radionuclides mightmight bebe transported moremore quicklyquickly thanthan throughthrough thethe originaloriginal structures. AA gas-gas-
pressurised repository could in principle also lead to the risk of "blow out" from intrusive
deep drilling.
Of course, if all the gas generated can be transported from a repository in solution in
groundwater, byby diffusiondiffusion andand advection, thenthen therethere wouldwould bebe nono potentialpotential forfor
b) Release of radioactive and flammable gases at the surface
Some of the gases produced in radioactive waste repositories may be radioactive because
theythey contain,contain, inin particular,particular, 14C,14C, 3H,3H, 222Rn,222Rn, 85Kr85Kr oror 129I.129I. TheseThese maymay bebe transported toto thethe
surfacesurface byby thethe largerlarger volumes of inactive gasesgases produced inin aa repositoryrepository inin water-saturated
rock. The possibility has also been considered that this flux of gas may provide a vehicle for
transporting naturally occurring radon to the surface. Gas-phase transport of volatile
radionuclides, such as 14C or 129I, may occur at Yucca Mountain.
AsAs thethe gasesgases producedproduced inin largestlargest quantities inin repositories inin saturatedsaturated environments areare
usually hydrogen and methane, the possibility that these might produce a flammability
hazard when released to the biosphere needs to be addressed.
VIIc) Effects on movement of contaminated groundwater
Three mechanisms have been envisaged by which gas generated in a repository in a water-
saturated host rock might induce movement of potentially contaminated groundwater. These
i) Forcing of water from a repository by the accumulation of a gas cushion within the
repositoryrepository (or(or withinwithin aa canister).canister).
ii) Entrainment of groundwater in streams of gas bubbles. For this to occur, gas
migration from a repository must involve some contribution in the form of migration
as gas bubbles.
iii)iii) InducedInduced movementmovement ofof groundwatergroundwater asas aa consequence ofof instabilities inin gasgas pathways,pathways,
with pathways continually collapsing and reforming.
d) Transport through attachment at gas-water interfaces
AA varietyvariety ofof colloidal particlesparticles andand chemical speciesspecies areare knownknown toto attachattach toto oror concentrateconcentrate atat
gas-water interfaces, hihi soso farfar asas thethe movement ofof gas-watergas-water interfaces isis involvedinvolved inin gasgas
transport from a repository it may provide a mechanism for transporting radionuclides. This
mechanism is most likely to be significant if bubble transport makes a significant
contribution to gas migration, because of the relatively high interface area that would then be
e) Transport of dissolved contaminants in unsaturated rock
Transport of dissolved radionuclides back to the biosphere from an unsaturated site requires
first their transport from the repository, through unsaturated rock, to the saturated zone below
the repository, by water percolating from the surface (or liberated from the matrix of the host
rock).rock). TheThe extentextent ofofthisthis transport toto thethe saturatedsaturated zonezone willwill depend on:on:
i) The magnitude and spatial variability ofwater flux at the repository horizon.
ii) The dilution and retardation of aqueous solutes in unsaturated flow.
ThisThis transporttransport ofof radionuclidesradionuclides willwill bebe affectedaffected byby coupledcoupled thermal,thermal, chemical andand flowflow
processes in the near field.
It is only in the cases of an unsaturated site for a spent fuel repository (Yucca Mountain) and
of the disposal ofheat generating waste in salt that coupling between heat evolution and two-
phase flow (where the gas phase may be or include steam) has been specifically raised as an
issueissue forfor aa repository concept.concept. (The(The possibility thatthat heatheat evolutionevolution mightmight affectaffect thethe behaviourbehaviour
of bentonite as an engineered barrier in other repository concepts is noted, but this is not a
gas-migration specific issue.)