"Salt" Working Group TIMODAZ - THERESA Luxembourg_2009 1 Impact of TMHC-processes on the safety of underground radioactive waste repositories Salt Working

"Salt" Working Group TIMODAZ - THERESA Luxembourg_2009

1

Impact of TMHC-processes on the safetyof underground radioactive waste repositories

Salt Working Group

Chair: Tilmann Rothfuchs, GRSRapporteur: Otto Schulze, BGR


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)


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 ?


4


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)


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)


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


8

Repository System; Release Scenario

D. Buhmann

Access Path to Near Field

Waste ContainerMobilisation

Near Field

Geosphere; GroundwaterBiosphere

= Flow of Contaminated Water

Keep the limits !!


9

Overburden

Host Rock Isolating Rock Zone

Repository

Water bearing strata

Change of philosophy: From release scenario to complete containment

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, 2009 10

= technical barrier


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?


12

Workshop discussion result (1): Need for improvement and refinement of models

• permeability relation(s): what is affecting permeability?

- k = f(Φ, σmin); Φ ≡ εvol (e.g. Stormont, Müller-Lyda, Popp, Heemann, …)

- k = f(σmax/ σeff) (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 ασeff = σmin - α ∙ pfluid

• prerequisite: additional experiments especially tuned to identified requirements


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


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


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, ...

./.

Documents

"Salt" Working Group TIMODAZ - THERESA Luxembourg_2009 1 Impact of TMHC-processes on the safety of underground radioactive waste repositories Salt Working