30 march, 2004 euradwaste’04, luxemburg treatment of geosphere retention phenomena in safety...

30 March, 2004 EURADWASTE’04, Luxemburg

Treatment of Geosphere Retention Phenomena in Safety Assessments –

RETROCK Concerted Action

Mikko Nykyri / Safram

30 March, 2004 EURADWASTE’04 2

Contents

Objectives and scope of RETROCK Working method Results

General Process by process

Conclusions


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:


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


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


Building blocks of retention modelling

Data acquisition & upscaling

Basic understanding

Supporting process modelling

PA modelling

Conceptualisation


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


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).


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


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


Conceptualisation of flow fields

Porous mediumrepresentation

Discrete fracture network

Channel network

“Real” flow field


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


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


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


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


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


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


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


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


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.


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


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

30 march, 2004 euradwaste’04, luxemburg treatment of geosphere retention phenomena in safety...

Documents

flow modelling

realistic modelling

relevant transport

transport of radionuclides

streamtube models transport

retention processes

d transport pathways

future pas wp1wp2wp3