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"Salt" Working Group TIMODAZ - THERESA Luxembourg_2009 1 Impact of TMHC-processes on the safety of underground radioactive waste repositories Salt Working Group Chair: Tilmann Rothfuchs, GRS Rapporteur: Otto Schulze, BGR
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"Salt" Working Group TIMODAZ - THERESA Luxembourg_2009 2 "Salt" Working Group: Main Topics & Results 1. TMHC - Processes Knowledge and understanding of relevant processes → constitutive models ► TM: material behaviour in the far-field ► TMHC: processes in the near-field and evolution of repository system including evolution of barriers (outer natural barrier, host rock and engineered barriers) DRZ evolution (convergence, damage, …) and interfaces (rock - backfill)
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"Salt" Working Group TIMODAZ - THERESA Luxembourg_2009 3 "Salt" Working Group: Main Topics & Results 1. TMHC - Processes Knowledge and understanding of relevant processes → constitutive models ► TM: material behaviour in the far-field ► TMHC: processes in the near-field and evolution of repository system including evolution of barriers (outer natural barrier, host rock and engineered barriers) DRZ evolution (convergence, damage, …) and interfaces (rock - backfill) 2. The requirements for the "Complete Containment" and how to achieve the respective safety proof ► complete containment (FEP - if... then) ► requirements concerning (TS)PA & LTSA (safety case) - tool box complete ?
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"Salt" Working Group TIMODAZ - THERESA Luxembourg_2009 4
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5 1. TMHC - Processes & Models ► relevant processes: TM-far field … TMH(C)-near field ► constitutive modeling ( recent developments: Saltmech_6, 2007 ) TM-far field: ( thermo-mechanical impact - in general: excavation & heat ) transient creep (including recovery of strain hardening), steady-state creep (reliable long-term extrapolation), dilatancy boundary and propagation of damage (EDZ), (◄ Luxembourg 2003) softening and failure, post-failure strength (residual strength), … humidity induced processes (impact on strength and ductility)
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"Salt" Working Group TIMODAZ - THERESA Luxembourg_2009 6 Requirement for modelling the near-field processes ► FEP catalogue - thermal and mechanical impact > stress & stress redistribution - deformation (viscous creep; dilatancy: damage … softening & failure) - impact of fluid pressure on state of stresses (M-H coupling) - impact of moisture on … (M-C coupling)
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"Salt" Working Group TIMODAZ - THERESA Luxembourg_2009 7 Requirement for modelling the near-field processes ► FEP catalogue - thermal and mechanical impact > stress & stress redistribution - deformation (viscous creep; dilatancy: damage … softening & failure) - impact of fluid pressure on state of stresses (M-H coupling) - impact of moisture on … (M-C coupling) Of special importance … - EDZ evolution (micro-fracturing and pathway generation) - EDZ devolution (convergence and re-compaction) (normal and altered evolution scenario) Special emphasis on permeability evolution / devolution … regarding complete containment
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"Salt" Working Group TIMODAZ - THERESA Luxembourg_2009 8 Repository System; Release Scenario D. Buhmann Access Path to Near Field Waste Container Mobilisation Near Field Geosphere; Groundwater Biosphere = Flow of Contaminated Water Keep the limits !!
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"Salt" Working Group TIMODAZ - THERESA Luxembourg_2009 9 Overburden Host Rock Isolating Rock Zone Repository Water bearing strata Change of philosophy: From release scenario to complete containment
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repository Isolating Rock Zone (IRZ) Boundary of IRZ Red line? Black line? What affects the IRZ? drifts crossing IRZ > technical barriers faults or water-bearing layers in the geological environment crossing IRZ IRZ must be chosen before license application Workshop TIMODAZ Luxembourg, D. Buhmann, Sept. 30, 200910 = technical barrier
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"Salt" Working Group TIMODAZ - THERESA Luxembourg_2009 11 Proof of "Complete Containment": Process understanding > Constituve models > Tool box development of constitutive models allowing reliable long-term prediction (THERESA project) model implementation in process level codes for the PA (LTSA - long-term safety analysis) implementation of simplified models for TSPA (total system performance assessment) important challenges: key scenarios: which FEPs may strongly affect the near-field integrity? DRZ evolution with respect to damage and re-compaction? Predictive capabilities of modeling and integration in PA level-models and into tools for the safety case?
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"Salt" Working Group TIMODAZ - THERESA Luxembourg_2009 12 Workshop discussion result (1): Need for improvement and refinement of models permeability relation(s): what is affecting permeability? -k = f(Φ, σ min ); Φ ≡ ε vol (e.g. Stormont, Müller-Lyda, Popp, Heemann, …) -k = f(σ max / σ eff ) (e.g. Peach, Alkan,...) -others: loading geometry, crack orientation, connectivity, aperture, … evolution / devolution of porosity > physical understanding of healing/sealing is variable (dry vs. wet) mechanical / hydraulic coupling: determination of Biot‘s parameter α σ eff = σ min - α ∙ p fluid prerequisite: additional experiments especially tuned to identified requirements
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"Salt" Working Group TIMODAZ - THERESA Luxembourg_2009 13 Workshop discussion result (2): Need for improvement and refinement of models Physical-Chemical Processes After Brine Inflow (worst case scenario) -Gas Generation and Migration (EBS - IRZ) -Gas entry pressure / Infiltration in and out of Salt Barriers (IRZ) -Two-phase flow in rock salt ?? Criterion(s) for Proof of Integrity of the Barrier System (e.g. IRZ) ►Increase of fluid pressure, driven by convergence / gas production - Dilatancy Criterion - Minimum Principal Stress Criterion (hydraulic criterion) backfill material (e.g. crushed salt) in the engineered barrier system
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"Salt" Working Group TIMODAZ - THERESA Luxembourg_2009 14 Crushed salt / granular salt backfill: Reconsolidation of granular salt at ambient temperature is quite well (i.e. quantitatively) understood ► technical demonstration in lab and field tests. Micro-mechanics for ambient re-consolidation as well as for fluid assisted pressure-solution and redeposition are documented. Technical proof for re-consolidation at elevated temperature (HLW ≤ 200 °C) and sound understanding of micro-mechanics is not available. HLW-repository operation in a salt formation requires a sound process understanding with respect to: - the mechanisms of re-consolidation of granular salt backfill - the determination of a permeability-porosity and its evolution
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"Salt" Working Group TIMODAZ - THERESA Luxembourg_2009 15 Summary:... why further improvements - just for fun? Improvement of process understanding and refinement of models: Confidence building and reduction of uncertainties - needed for reliable long term safety assessment (extrapolation on long-term scale; safe predictions) - in general, only short-term experiences available from lab and in situ testing also needed: sensitivity studies to cover uncertainties, e. g. facies, mineral composition,..../.
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"Salt" Working Group TIMODAZ - THERESA Luxembourg_2009 16 Improvements since the EDZ workshop 2003, remaining uncertainties 16 A project on model development for gas permeation showed that gas entry at pressures slightly above minimum principal stress leads to dilatancy-controlled gas flow rather than gas-frac, and at least partial self-sealing was confirmed (Popp & Minkley, 2007). The German cooperation project “Development of constitutive laws” yielded improved material laws that entered in THERESA: A variety of constitutive laws for describing dilatancy, dilatancy-induced permeability increase, and self-sealing were further developed and validated by different modelling teams.
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"Salt" Working Group TIMODAZ - THERESA Luxembourg_2009 17 Host rock (far field) Recent developments in constitutive laws, (THM(C)-near field … (TM-far field)) Criteria of integrity assessment Gas generation and consequences: permeation and migration affecting isolation and transport properties of the host rock EDZ (near field): Improvements since the EDZ workshop 2004, remaining uncertainties Damage evolution … affecting strength and integrity Healing/self-sealing/recompaction Porosity-permeability relation Mechanical hydraulic coupling, models and parameters Backfill (crushed salt) Compaction Porosity-permeability relation Mechanical hydraulic coupling, models and parameters Impact of brine Importance of C-coupling … with respect to waste products and a candidate buffer material Remaining Research Needs of the Salt Option
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The REPOPERM-project 18 Key question from the viewpoint of „complete containment“ What are the hydraulic properties of compacted crushed salt at the end of compaction?” What is the lower limit of backfill porosity/permeability which is critical or uncritical, respectively with regard to a possible water inflow into a HLW repository or a violation of the complete containment Situation at start of the project Porosity measurements in the range of 1-3% and below are severely questionable Project objectives To avoid unnecessary intricate and costly laboratory measurements the relevance of crushed salt properties below 1-3% will be checked first by sensitivity analysis’ based on model calculations
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