30 March, 2004 EURADWASTE’04, Luxemburg
Treatment of Geosphere Retention Phenomena in Safety Assessments –
RETROCK Concerted Action
Mikko Nykyri / Safram
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Contents
Objectives and scope of RETROCK Working method Results
General Process by process
Conclusions
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Objectives of RETROCK
To examine how the retention and transport of radionuclides are and should be handled in the performance assessment (PA) models Clarify points of consensus and disagreement Assess the importance of open issues and ways
to resolve them Are the simplifications in the modelling justified
and optimal for the purpose? Make recommendations for future work Enhance communication between different players
Scope of RETROCK Domain: Deep geological disposal in saturated hard fractured rock
Transport and retention processes:
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Who’s views are presented?
Project participants Safram (co-ord.) ENRESA with CIEMAT and UPC Nagra with PSI Posiva with VTT SKB with KTH and JA Streamflow SKI with SLU, N. Chapman and J. Geier
External collaborators (questionnaire respondents) NUMO & JNC UK Nirex SwRI / CNWRA
External reviewers: J. Bruno, R. Haggerty, D. Read, P. Robinson
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Project work flow
Mapping of treatment of retention inrecent PAs
In-depth examination
of currentscientific basis and modelling
practices
Integrationof results.
Recommenda-tions for
future PAs
WP1 WP2 WP3
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Building blocks of retention modelling
Data acquisition & upscaling
Basic understanding
Supporting process modelling
PA modelling
Conceptualisation
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General results (1)
Basic understanding and modelling practices rather uniform
PA practitioners confident that the relevant transport and retention processes have been recognized
Input data often satisfy minimum needs of PAs, but are not often sufficient for more realistic modelling. Challenges in up-scaling of experimental data over long spatial and time-scales. Difficulties with field-data and data from natural analogues.
Limited treatment of heterogeneities and time-dependencies
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General results (2)
Possibilities for more realistic treatment of retention and transport in general are seen promising by the experts of different disciplines. Positive vision for future developments
in coupled reactive transport modelling (flow, retention processes, geochemical evolution).
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Flow and transport modelling: Approaches
Flow and transport with separate models Flow modelling with a high degree of
detail Transport modelling with less details
Dual porosity concept Fractures: advection, longitudinal
dispersion Rock matrix: diffusion, sorption
Trend from single 1-D transport pathways towards multiple pathways
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Flow and transport modelling:Model types
1. Discrete fracture network models (flow and transport)
Flexible: spatial scales, heterogeneity Expected to dominate in future
2. Streamtube models (transport) Stochastic continuum models (flow field) Fractured rock difficult to be represented
by models developed for porous media
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Conceptualisation of flow fields
Porous mediumrepresentation
Discrete fracture network
Channel network
“Real” flow field
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Flow and transport modelling:Diverse
Flow distribution has strong coupling to retention. Required resolution of flow field ? Fast pathways difficult to reveal. Existence of long
highly transmissive 'wormholes’? Difficult key parameters needed by models:
Fracture aperture distribution Transport resistance (WL/Q, F factor, ) Advective travel time Flow wetted surface over flow rate (FWS/q)
No link to geochemistry
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Matrix diffusion
Important phenomenumIncreases time-spreading of releasesFor non-sorbing radionuclides the only
retention mechanismWell understood processModelling tools well-developed
...except the treatment of pore plugging and heterogeneity in pore system
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Sorption (1)
In PAs the term sorption captures many
individual mechanisms
Understanding of mechanisms poor
PAs consider sorption in rock pores, but not
on rock fracture surfaces or infills
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Sorption (2):
Simple Kd approach
May satisfy minimum needs of PAs Does not satisfy basic researchers Kd the only way of communication between
sorption researchers and modellers? Easy to model Well-developed sorption databases
Batch experiments with crushed rock non-conservative Kd’s correction factors in use
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Sorption (3):Future?
Mechanistic modelling for more realism
Thermodynamic sorption models coupled with transport models not applicable for PAs in near future (extensive data needs)
Intermediate modelling strategies
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Colloid-mediated transport processes
Colloids might significantly accelerate transport, if their concentrations high and nuclides attach to colloid particles and colloids move
Weak points: Basic understanding insufficient for PA
modelling No tools to predict stability of colloid particles Existing models do not suit for PAs Insufficient field data from relevant systems
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Precipitation and co-precipitation (1):Important?
Generally considered that their benefits dominate and therefore it is conservative to omit them from PAs Sinks for radionuclides Accumulated radionuclides can be dissolved Precipitates may block pores in rock matrix and
affect electrical surface potentials Special cases:
Reactions at redox fronts. Chemical transients caused by glacial meltwaters
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Precipitation and co-precipitation (2):Basic understanding and modelling Precipitation and co-precipitation of
radionuclides Reliable data only for a few nuclides and
mineral phases Modelling tools very limitedly available Data sparse and difficult to acquire
Needed for more realistic modelling: Reactive transport modelling coupled with geochemical evolution modelling
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Microbial effects
Microbial processes contributing directly to retention are not well understood, but it looks conservative to omit them from PAs
Control of redox reactions and consumption of oxygen can affect chemical conditions significantly
Cannot be modelled in PAs before remarkable development in understanding. Rapidly advancing area.
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Gas-mediated transport
Migration of radioactive gases out of scope(e.g. 14C in CH4)
Transport of colloids by gas bubbles To be meaningful, this would require
–abundant gas generation and
–high colloid concentration Modelling capabilities very limited
POTENTIAL GRADIENT X
FLUX J Temperature Hydraulic Chemical Electrical
Heat Thermalconduction
Thermalfiltration
Dufour effect Peltier effect
Fluid Thermalosmosis
Advection Chemicalosmosis
Electricalosmosis
Solute Thermaldiffus. orSoret effect
Hyperfiltration Diffusion Electro-phoresis
Current Seebeck orThompson eff.
Rouss effect Diffusion &Membr. Pot.
Electricalconduction
Off-diagonal Onsager processes
Studies clearly suggest that osmosis and hyperfiltration not relevant in repository far-field
Modelling uncertainties vs. PA relevance
Microbiallymediatedprocesses
Precipitationco-precipit.
Gasmediatedprocesses
Off-diagonalprocesses
Colloidaltransport
Sorption
Matrixdiffusion
Radioactivedecay
Maximum “allowable”
level of uncertainty
Le
vel o
f u
nce
rta
inty
PA relevance
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Conclusions
Satisfied with current situation? PA practitioners generally satisfied with
adequacy of fundamental understanding for their topical PA purposes
Scientific community calls for more explanations for the basis of that satisfaction
Justified level of simplification in modelling is a key issue and often seen differently by process researchers and PA modellers
Strong progress needed in acquisition of data over long spatial and time-scales
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