German contribution: CH/HM coupled behaviour of shaft sealing materials

Oliver Czaikowski, Kyra Jantschik, Helge C. Moog, Klaus Wieczorek & Chun-Liang ZhangGRS , Germany

DOPAS: Full-scale demonstration of the

feasibility and performance of plugs

and seals

• Five experiment being designed, implemented and assessed across seven work packages

• Supported by materials development• Three experiments in crystalline

rocks:• DOMPLU (Äspö)• POPLU (ONKALO)• EPSP (Josef)

• One experiment related to clay:• FSS (St Dizier)

• A set of experiments and performance assessment studies related to salt:

• ELSA (generic German concept)

The DOPAS Project

Experiments in crystalline and clay rockDOMPLU (SKB) and POPLU (POSIVA):• confine the backfill in the tunnel,

support backfill saturation and to provide a barrier against water flow / bentonite erosion

• Main components: concrete plug and watertight seal

EPSP (SURAO/CTU):• Composite structure of concrete domes and bentonite

pellets

FSS (ANDRA):• Limit water flow and reduce

groundwater velocity• Swelling clay core and low-

pH concrete containment

German shaft sealing concepts

Reference conceptual design for the German shaft seal. The Gorleben-Bank is a folded anhydrite layer in the rock salt (Müller-Hoeppe et al. 2012a).

German disposal concept• Multiple barrier system consists of technical (disposal

container), geotechnical (sealing elements) and geological (host rock) barriers.

• Barriers shall prohibit intrusion of saline brines to the radioactive waste.

GRS-247

• Investigation of chemical-hydraulic behaviour of cement based sealing materials in rock salt (LAVA)

– Batch experiments with crushed concrete suited to investigation of water-rock interactions. – In-diffusion experiments with concrete and brine in order to determine the rate of alteration of the porous matrix.– Experiments with concrete brine at the contact with the EDZ in order to determine the rate of alteration of the sealing

material owing to advective flow at the boundary to the rock formation.– Accompanied by numerical modelling activities.– Deliverable D3.29

• Investigation of hydro-mechanical behaviour of cement based sealing materials in rock salt (LASA)

– Multistep creep tests on samples of concrete for the determination of creep parameters.– Triaxial compression tests on samples of concrete with axial flow of gas for determination of time-dependent

compaction and damage evolution.– Experimental long-term simulations of the systems rock salt / concrete using large hollow salt cylinders filled with

concrete under varying isostatic load and constant brine pressure.– Accompanied by numerical modelling activities.– Deliverable D3.31

• Hydro-mechanical behaviour of claystone-bentonite-mixture as seal material (THM-Ton)– Investigations on the long-term behaviour of the clay rock (Ucc, TCc, gas flow, swelling properties).– Experiments to characterize the geotechnical properties of compacted claystone-bentonite-mixtures as sealing material

(water retention curves, water re-saturation, water permeability and gas migration, swelling capacities). – Accompanied by numerical modelling activities.– Deliverable D3.32

Issues addressed by GRS within DOPAS

Rock salt from the Excavation

Damaged Zone (EDZ)

Available material for lab tests (in situ / lab)

Sorel concretecrushed salt, magnesium

oxide, MgCl2-brine

Salt concretecrushed salt, blast-

furnace cement, NaCl-brine

Crushed claystone/ bentonite mixtureswith grains d < 10 mm

Drift sealing element Depth 945 m, finished in 1992 Salt concrete

(72% crushed salt, 18% cement, 10% NaCl-brine) 8 m in length, 5.5 m in width, 3.4 m in height

Deformation and damage behaviour of salt concrete

Stress-strain-permeability behaviour of salt concrete samples

Advection experiments with sorel concrete• Investigation of development of permeability of

sorel concrete (MgO-based concrete) in contact with NaCl and MgCl2-based solutions.

Functional principle• Concrete is placed in advection cell, cast in araldite• Inflow: Solution pressure 20 bar.• Outflow: Solution is collected and permeability

calculated.

ResultsSorel concrete / NaCl-based solution• Permeability measureable after 7 to 60 days.• Clear increase of permeability in all samples up to

a level of 10-17 m2/s.• Assumption: no further increase of permeability

because solution passes sample faster than it needs for corrosion if a value around 10-17 m2/s is reached.

Permeability evolution of sorel concrete (corrosion processes)

Sealing in integrated PA: Closer to reality

Sealing material is disturbed→ permeability is increasing Original sealing material

Excavation disturbed Zone (EDZ) around sealing withincreased permeability

Hydraulic sealing capacity of combined samples

Hollow salt cylinder, salt concrete core and salt slurry

Complete combined sample

kgas (concrete) < 1.E-18 m²

kgas(Salt) < 1.E-22 m²

kgas (interface) = f(t)

Phase 1: Compaction of the sample – confining pressure 5 and 10 MPa, sample stays in contact with NaCl-based solution Permeability to NaCl-based solution is around 10-18 m2/s.

kBrine = const.

kBrine = f(time)

Combined system Phase 1: Re-compaction processes (HM)

time [ h ]

Combined systemPhase 2/3: Corrosion processes (CH)

Phase 2: Permeability increases immediately in contact with MgCl2-based solution, probably as result of higher injection pressure. Afterwards, permeability decreases because Brucit (Mg(OH)2) is built and plugs the pores. Additionally, pH decreases to 8-9.Phase 3: The smaller pH-value results in decomposition of portlandite (Ca(OH)2) and CSH-phases (Calcium-silicate-hydrates). Permeability increases.

time [ h ]

Coupled behaviour of sealing materials in contact with surrounding rock• GRS is investigating combined systems of salt concrete seal elements and surrounding rock salt at the

laboratory scale in order to get the temporal evolution of the overall permeability.

Currently, the following results have been obtained:• At dry conditions and at a moderate confining stress up to 5 MPa, reconsolidation is slow. A potentially existing

highly permeable contact seam between the seal element and the rock will not be closed, at least not in the short term.

• With an intact seal element, a confining stress of 5 MPa is, however, sufficient to prevent brine flow along the seal. In the presence of brine, contact seam and EDZ are quickly closed, resulting in overall permeability below 10-20 m2.

• A pre-damaged seal element (e.g., damaged by shrinkage fracturing during construction) will not be reconsolidated at a confining stress of 5 MPa, even if brine is present.

• At a stress level of 10 MPa reconsolidation of the pre-damaged seal element is effective and permeability decreases.

• Chemical alteration processes can be observed when a corrosive brine is present.

The next steps in the experimental investigations will be:• to dismantle the re-consolidated sample and use microscopic methods to investigate pathway reduction and • to perform further experiments to investigate variability of results and derive generally valid material

behaviour.Available physical models of rock salt and salt concrete have been applied already but simulation/ improvement cycles should be performed to advance model qualification.

Conclusions and further R&D work

Acknowledgements

J. DittrichU. Hertes

T. HörbrandJ. Müller

Questions ... The research leading to these results has received funding from the European Atomic Energy Community's (Euratom) Seventh Framework Programme FP7/2007-2013, under Grant Agreement No. 323273 for the DOPAS project andthe Federal Ministry for Economics and Energy (BMWi), represented by the Project Management Agency Karlsruhe (PTKA-WTE), contract no. 02E11122 / 02E11132 / 02E10377.