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Treatment of Geosphere Retention Phenomena in Safety Assessments – RETROCK Concerted Action

Treatment of Geosphere Retention Phenomena in Safety Assessments – RETROCK Concerted Action. Mikko Nykyri / Safram. Contents. Objectives and scope of RETROCK Working method Results General Process by process Conclusions. Objectives of RETROCK.

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Treatment of Geosphere Retention Phenomena in Safety Assessments – RETROCK Concerted Action

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  1. Treatment of Geosphere Retention Phenomena in Safety Assessments – RETROCK Concerted Action Mikko Nykyri / Safram EURADWASTE’04, Luxemburg

  2. Contents • Objectives and scope of RETROCK • Working method • Results • General • Process by process • Conclusions EURADWASTE’04

  3. 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 EURADWASTE’04

  4. Scope of RETROCKDomain: Deep geological disposal in saturated hard fractured rock Transport and retention processes:

  5. 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 EURADWASTE’04

  6. Project work flow WP1 WP2 WP3 Mapping of treatment of retention in recent PAs In-depth examination of current scientific basis and modelling practices Integration of results. Recommenda-tions for future PAs EURADWASTE’04

  7. Building blocks of retention modelling Basic understanding Supporting process modelling Conceptualisation Data acquisition & upscaling PA modelling EURADWASTE’04

  8. 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 EURADWASTE’04

  9. 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). EURADWASTE’04

  10. 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 EURADWASTE’04

  11. Flow and transport modelling:Model types • Discrete fracture network models (flow and transport) • Flexible: spatial scales, heterogeneity • Expected to dominate in future • Streamtube models (transport) • Stochastic continuum models (flow field) • Fractured rock difficult to be represented by models developed for porous media EURADWASTE’04

  12. Conceptualisation of flow fields “Real” flow field Discrete fracture network Porous medium representation Channel network EURADWASTE’04

  13. 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, b) • Advective travel time • Flow wetted surface over flow rate (FWS/q) • No link to geochemistry EURADWASTE’04

  14. Matrix diffusion • Important phenomenum • Increases time-spreading of releases • Fornon-sorbing radionuclides the only retention mechanism • Well understood process • Modelling tools well-developed ...except the treatment of pore plugging and heterogeneity in pore system EURADWASTE’04

  15. 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 EURADWASTE’04

  16. Sorption (2):SimpleKd 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 EURADWASTE’04

  17. 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 EURADWASTE’04

  18. Colloid-mediated transport processes • Colloids might significantly accelerate transport, if • their concentrationshighand • nuclides attach to colloid particles and • colloidsmove • 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 EURADWASTE’04

  19. 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 EURADWASTE’04

  20. 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 EURADWASTE’04

  21. 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. EURADWASTE’04

  22. 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 EURADWASTE’04

  23. Off-diagonal Onsager processes • Studies clearly suggest that osmosis and hyperfiltration not relevant in repository far-field

  24. Modelling uncertainties vs. PA relevance Gas mediated processes Microbially mediated processes Level of uncertainty Precipitation co-precipit. Colloidal transport Off-diagonal processes Maximum “allowable” level of uncertainty Sorption Matrix diffusion Radioactive decay PA relevance

  25. 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 EURADWASTE’04

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