Water flow and contaminant transport in the near field ...

Water flow and contaminant transport in the near field of the near-surface repository in Dessel, Belgium Dirk Mallants Performance Assessments Unit

IAEA Site selection Workshop Buenos Aires-Argentina, 25-29 October 2010

Copyright © 2010 SCK•CEN 1

Outline  Background  Disposal facility description  Approach to assess radionuclide migration from cementitious repository  Concrete durability assessment  Long-term concrete evolution assessment  Integrated model description at repository scale  Examples

 Conclusions

2

Background Final disposal site location for cAt - Dessel municipality

3

Disposal facility - operational phase -

4

Disposal facility Large-scale test facilities

Long-term monitoring of test cover

Demonstration test

5

Disposal facility-operational phase–cross-section From 2016 onwards

6

Facility in its final state Disposal facility cross-section (approx. 2050 ad. onwards)

} Module

Earth cover Monoliths

Filled inspection room and drainage gallery

Embankment 7

Safety concept  Safety functions defined for major System, Structure and Component (SSC)  I1: Reduction of the likelihood of inadvertent human intrusion and of its possible consequences Filled inspection room and drainage gallery

 R1: Limitation of contaminant releases from the waste forms  R2a: Limitation of water flow through the prevention barrier Embankment

 R2b: Limitation of water flow through the retention barrier  R3: Retardation of contaminant migration by chemical retention

8

Approach to assess radionuclide migration from cementitious repository Integrated Safety Assessment [Bq/y] near field

[Bq/m³] geosphere

[Sv/y] biosphere

source

receptor

transport route

9

Multiple engineered barriers

Pores in concrete matrix

ADR model Groundwater 10

ADR model Near field

Source term model

Models for Disposal Facility Simplification: from real 3D to 2D to 1D

Real 3D

Roof

Monolith

1D

Simplified 3D

2D

Floor

11

Chloride migration in monoliths Rebars susceptible to high chloride concentration

R

Cliq t

D

 2 Cliq x

2

v

Cliq x

CDE-model

150 yrs 12

200 yrs

300 yrs

Conceptual model for cover layers-Water infiltration modelling Soil as a porous medium 37.5 m ntent [-]

Vegetation layer Vegetatielaag

Coarsezand sand: Grof

drainage layer

6.25 m 4.4 m

Geomembrane (HDPE, GCL): K~10-11 -10-14 m/s

Loam Leem Clay: Klei infiltration barrier 10000

4

Sand Loam Clay

saturated Sand Loam Clay

2

Log10 (Hydraulic conductivity (cm/d))

1000

100

Matric head (m)

Gravel: Grind intrusion barrier

10

1

0.1

0

-2

-4

dry -6

-8

18 m

-10

0.01 0

0.1

dry

0.2

0.3

Water content [-]

0.4

0.5

-2

0

Log 71 m

saturated

10

2

4

(Matric head [m])

DMa/00/150

13

Experimental field site SCK•CEN Mol

Lysimeter 1

Time Domain Reflectometry

Lysimeter 2

data M5 simulations C5

water content (-)

0.4

0.3

0.2

0.1

0 100

14

150

200

Time (d)

250

300

350

T

Soil layer

b

Transition layer

c

Stony layer Crushed stones

a

Compacted sand

b

Uncompacted sand

c

100

270

300

30 60

360

400

20 30

210

a

b BIOLOGICAL LAYER

Transition layer

c

Stony layer Crushed stones

a

Compacted sand Uncompacted loam

b

Compacted clay

a

Uncompacted clay

b

Compacted clay

c

320 350

Compacted clay

c

Sand layer



b INFILTRATION BARRIER

Geomembrane (HDPE, GCL): K~10-11 -10-14 m/s

445

Sand layer



440

25

25

420

450

465

Long-term monitoring of test cover 30 yrs measurements & modelling

BIO-INTRUSION BARRIER

290

330

350

Soil layer

70

30 40

230

Surface layer

190

200

200

20

30

70

150

60

Depth below surface - cm

130

30

30

100

250

a

170

Surface layer

Alternative 5% slope

90

20

80

50

20

Reference 5% slope 0

Geotextile Geosynthetic clay liner High Density Polyethylene geomembrame

Monitoring of Water Gas Matter fluxes

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Material Science and Radwaste management Challenges in radwaste disposal research

Glenfinnan viaduct, West Highlands of Scotland, 1897

 Disposal facility should protect Man and environment from radioactive waste until levels of radioactivity are acceptably low  Safe disposal based on isolation and containment  Important role for different barrier materials  Natural materials  Soil (sand, gravel/cobbles, clay)  Groundwater sediments

 Engineered materials  Concrete:

– Knowledge on long-term behaviour is key (required service life far greater than for classical civil engineering structures) – Interaction with contaminants over time (immobilisation) 16

Concrete durability – pH & sorption evolution (source: Wang et al. 2009) 13.5

I

II

III

IV

Ca(OH)2

12.5

Na, K

stage I, pH > 12.5

pH

stage II, pH = 12.5

CSH

calcite

stage III, 12.5 > pH > 10

stage IV pH = incoming water

Time

Element Am (+III) C (+IV) Cl (-I) Cs (+I) I (-I) Nb (+V) Ni (+II) Np (+IV)

Sorption on benchmark cement paste Undegraded (II) Degraded (III) Element *** ≈ Pa (+IV) Pu (+IV) *** ≈ Ra (+II) * ≈  Se(+IV) *  Sr (+II) * **** ≈ Tc (+VII) ** ≈ Th (+IV) **** ≈ U (+IV)

(*) Rd < 10 l/kg

* Rd = 10-100 l/kg

** Rd = 102-103 l/kg

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Undegraded (II) **** **** ** ** * (*) **** ****

Degraded (III) ≈ ≈ ≈ ≈ ≈ ≈ ≈ ≈

*** Rd=103 - 104 l/kg **** Rd>104 l/kg

Concrete durability assessment  Long-term safety relies mainly on isolation and chemical containment of cementitious components  Include degradation processes of the cementitious components and their impact on safety functions  Need to define time frames during which the safety functions are active  Physical degradation effects  Chemical degradation effects  Mechanical degradation effects (internal/external)

 Significance of coupled effects  Mechanical-chemical (perturbing effects-sorption)  Chemical-physical (chloride-water flow)

 Significance of fractures and voids on flow and transport

 Main degradation processes  Portlandite and CSH dissolution  Carbonation  ... 18

Concrete durability assessment Understanding

of long-term concrete evolution

Multi-scale modelling

Estimate Mechanical Physical Properties + Chemical evolution Mechanical/physical evolution 19

Concrete durability assessment

Integrated model description Thermal, hydraulic, multi-component reactive chemical and gas transport model

Geochemical model 3D Microstructural model HYMOSTRUC

Multiphysics environment

Auxiliary calculations

Variably saturated flow model slow  velocity

COMSOL

fast velocity

Optimization Data analysis Data manipulation Sequencing ...

s hort pathway

l ong pathway

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Concrete durability assessment At 10°C

Coupled reactive transport modelling  Four distinct states (25°C)  pH > 12.5 due to the presence of high concentrations of alkalis (Na and K)  pH12.5 controlled by the solubility of portlandite (Ca(OH)2);  10.5