Modelling microbiology, gas
reaction and chemical evolution of
geological disposal facilities
Joe Small
NNL Senior Research Fellow in
Waste and Environmental Geochemistry
Outline
• WG5 expectations
• Specific interests- Biogeochemical Modelling
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Approach for LLW surface disposal, UK
Microbial Gas generation, TVO Finland
Mont Terri Bitumen Nitrate experiment
Cement pH, UK Bigrad Consortium
• Benefit to the safety case
• Proposal ideas
Expectations on the outcome of
WG-5
• An integrated and balanced proposal to address
microbial effects that may be both beneficial or
detrimental to the safety case:
• Beneficial
• Detrimental
• Corrosion (MIC)
• Methanogenesis
• via acetate (LLW/ILW)
• bulk gas
• 14C release
• Mediation of Eh &
radionuclide speciation
• H2 (& CH4) consumption
• oxidation by Fe(III) & SO42• Methanogenesis (H2 only)
• Fate of organic complexants
“The safety case must be able to describe the evolution of the
repository in a way that may be seen as a reasonable
representation of what might happen….”
(Topic 1, IGD-TP Strategic Research Agenda, 2011)
Biogeochemical Modelling
• Concepts and computer model developed in
the UK for LLW surface disposal
• Key component of 2002 (BNFL) and 2011
(LLWR Ltd) safety cases
• Underpins near-field conceptual model
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Eh evolution
pH evolution (neutral & cement wasteform)
Gas generation (CH4, H2)
Radionuclide speciation and solubility (U, Tc)
14
C partitioning to gas, groundwater, carbonate
precipitation
• Generalised Repository Model (GRM)
• Coupled reactive-transport model
• Representation of main microbial metabolic
processes:
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Aerobic
Denitrification
Fe(III) reduction
Sulphate reduction
Fermentation
Methanogenesis
• Rections between:
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H2, organics (cellulose, acetate), reduced minerals
O2, NO3-, Fe(OH)3, SO42-
Gas generation and microbial
processes in repositories
• VLJ Repository, Olkiluoto, Finland
(operated by TVO)
• Large Scale Gas Generation expt.
Examines CH4, CO2, H2 generation from
LLW/ILW
• Initially concrete buffered pH
• High water content
• GRM Biogeochemical modelling
• Constrains rates of gas generation
• Builds confidence in the VLJ safety
case
• Validates microbial modelling approach
for gas generation and chemical
evolution
• GRM and expt. data now used
internationally
• e.g. as a validation test case
• UK, France, Canada, Switzerland
Small, J. Nykyri, M., Helin, M., Hovi, U., Sarlin, T., Itävaara, M. (2008). Experimental and modelling investigations of the biogeochemistry of gas
production from low and intermediate level radioactive waste. Applied Geochemistry 23. 1383-1418
TVO gas generation expt.
100
• Rate of CH4 production modelled
accurately in “blind test”
• Low levels of H2 measured in the
experiment
• H2 from corrosion consumed
locally in waste by SRBs
Model CH4
Volume %
10
Model CO2
1
Model H2
0.1
Expt H2 vol%
Expt CH4 vol%
Expt CO2 vol%
1/1998
1/1999
1/2000
1/2001
1/2002
1/2003
• Heterogeneity in pH allows
microbial processes to develop at
neutral pH in waste drums
• Eventually, concrete buffered
alkaline water is neutralised
1/2004
Average model pH, measured pH
0.01
1/1997
12
Model pH, concrete
11
On-line
measurement
10
Tank water,
drum lid level
9
Tank water,
bottom
Model pH, water
8
Waste drum,
95% biodeg.
7
Waste drum,
6% biodeg.
6
Model pH, waste
5
1997
1998
1999
2000
2001
2002
Date
2003
2004
2005
2006
Effect of nitrate in radioactive
waste disposal
• Nitrate is a strong oxidant (electron acceptor)
• Potential to affect the speciation and transport behaviour of redox
sensitive radionuclides e.g. Se, Tc, U, Np, Pu
• Large amounts of nitrate salts are present in bituminised waste,
produced in France and Belgium
• The Mont Terri Bitumen-Nitrate experiment examines the
biogeochemistry of nitrate interaction with a clay formation.
NO3- &
organics
injected into
borehole in
Opalinus
Clay
1
Measured NO3
0.9
• Migration 2013, Brighton;
Small, J, Abrahamsen L,
Albrecht A, Bleyen N, Valcke E
Modelled NO3 in sampled volume
Nitrate concentration (mM)
• GRM Model aids the
interpretation of expt data
• Has been used to forward
model the effect on redox
potential (Eh) and U and Tc
radionuclide speciation
0.8
Modelled NO3 in filter volume
R eduction by co
injected acetate
0.7
Modelled NO3 adjacent to clay
0.6
Modelled NO3 in first 5mm of clay
0.5
Modelled NO3 in 5-10mm of clay
0.4
0.3
R eduction by F eS 2 and F eC O 3
0.2
0.1
0
0
2
4
6
8
10
12
days after injection
14
16
18
20
Online
monitoring
Microbial Biomass:
1. Modelled Biomass of borehole and EDZ
2. MPN cultivation/ATP/SEM analysis of sampled fluid
5.0E+06
4.5E+06
Total biomass, Cells per ml of fluid
Cell 1: sampled volume
Maximum microbial activity in the
sampled volume related to NO3
reduction by acetate
Cell 2: filter volume
4.0E+06
Cell 3: void adjacent to clay
3.5E+06
Cell 4: 0 to 5mm of clay
Cell 5: 5 to 10mm clay
Nitrate and nitrite reducers utilising
FeCO3 and FeS2
3.0E+06
Cell 6: 10 to 15mm clay
2.5E+06
Continuing growth on the borehole
surface related to stimulation of sulphate
reducing and fermentative processes by
initial denitrification processes.
Effect is uncertain
2.0E+06
Cell 7: 15 to 20mm clay
MPN counts, Moors et al, 2012
1.5E+06
Filtered material (2mls) after 19 days
1.0E+06
5.0E+05
0.0E+00
0
5
10
15
20
25
30
35
40
Time, days
Moors, H., Geissler, A., Boven, P., Selenska-Pobell, S and Leys, N. BN Experiment:
Intermediate results of the microbiological analyses. Mont Terri Project Technical Note 2011-39, 2012.
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Microbial activity under cementitious
conditions (pH 9-13)
• See Jon Lloyd presentation
• GRM models of denitrification, Fe reduction,
and Eh at pH 10
• SO42- reduction and CH4 formation
energetically less favourable at high pH
• Modelled Tc speciation consistent with
observed microbial reduction
• U speciation models (based on NEA
database) predict U(VI) whereas microbial U
reduction observed
Groundwater flow
Host
Rock
Cementitious
Near Field
pH 7
pH 12.5
pH
Eh ?
Biogeochemical
Gradients
Microbial processes?
Tc, U, Np ?
1200
Model Eh
7.E-05
1000
6.E-05
800
Expt. Eh, Anaerobic
600
Eh, mV
5.E-05
Concentration, mol/l
Expt. Eh + lactate
NO2-
4.E-05
Redox couple: NO3/ N2
400
200
3.E-05
0
NO3-
2.E-05
-200
Nitrate (model)
Nitrite (model)
1.E-05
-400
Nitrate Expt
Redox couple: Fe(OH)3/ Fe2+
Nitrite Expt
-600
0.E+00
0
5
10
15
Time days
20
25
30
0
5
10
15
Time, days
20
25
30
The safety assessment perspective: how can
we contribute to a competent safety case?
• Asses the viability of microbial processes under the range of
physical and chemical conditions of ILW, HLW/SF
wasteforms, engineered barriers and geosphere interface
• Realistic consideration of the fate of gases (H2), organics
(CDPs) and electron acceptors (NO3-) released from the
waste and engineered barriers
• Development of microbiological process models (conceptual
and mathematical) for repository evolution and radionuclide
behaviour in the multibarrier system
Remaining issues and a proposal for
future research
• Investigate the viability of specific microbial processes that
may be detrimental to the safety case:
• Sulphate reduction and MIC in bentonite
• Methanogenesis under cementitious conditions
• Develop a microbiological process model that reasonably
describes the fate of repository derived energy sources:
H2 and CH4 gases,
organics, including cellulose degradation products,
nitrates and other electron acceptors present, and
which takes account of physical and chemical conditions heterogeneities
and interfaces which may inhibit or promote microbial activity.
• To test hypotheses for the Eh evolution and speciation of redox sensitive
radionuclides.
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• Utilise and develop current biogeochemical modelling
approaches as an interpretive tool and to represent the
above in the safety case