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Conceptual Models and Approaches to Understanding Long Term
Performance of
Cementitous Waste FormsPresented by
David S. Kosson, Ph.D., ProfessorVanderbilt University
Consortium for Risk Evaluation with Stakeholder Participation
(CRESP)
To
Advisory Committee on Nuclear Waste
Nuclear Regulatory Commission
July 18, 2006
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Department of Civil and Environmental Engineering
Department of Civil and Environmental Engineering
NRC/ACNW - July 18, 2006 (DRAFT)
Consortium for Risk Evaluation with Stakeholder
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Collaborations
Vanderbilt University
D. Kosson, A. Garrabrants, S. Mahadevan, F. Sanchez, J. Clarke,
S. Lopez
Netherlands Energy Research Centre (ECN)
H. van der Sloot, R.Comans, J.C.L. Meeussen, A. van Zomeren, P.
Seignette
DHI (Denmark)
O. Hjelmar
Savannah River National Lab
C. Langton, G. Flach
Pacific Northwest National Lab
J. Serne, T. Brouns
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Department of Civil and Environmental Engineering
Department of Civil and Environmental Engineering
NRC/ACNW - July 18, 2006 (DRAFT)
Consortium for Risk Evaluation with Stakeholder
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Generic Vault Disposal System
ReinforcingSteel
Waste Form
Clean Grout(high strength)
Muli-layer Cap and Infiltration Control
Drainage Layeror Capillary Break
PerchedWater
Seepage
Infiltration
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Department of Civil and Environmental Engineering
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NRC/ACNW - July 18, 2006 (DRAFT)
Consortium for Risk Evaluation with Stakeholder
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Motivation
Need for realistic (as practical) estimates of long-term
constituent release for near-surface disposal of cementitious and
other non-vitrified waste forms.
ApplicabilityPerformance Assessments and 3116 Determinations
HLW tank closure using groutDisposal of saltstone & similar
wastes at SRNL, INL, ORPPrimary and secondary waste streams from
steam reformingSecondary waste streams from vitrification
Waste Treatment Acceptance CriteriaOperational
ControlsManagement of future wastes from reprocessing (GNEP)
Primary Constituents of ConcernLong lived & Mobile: Tc-99,
Np-237, Se-79, I-129, C-14, U, Mobile: Cs-137, Sr-90, Nitrate,
tritium
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Department of Civil and Environmental Engineering
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NRC/ACNW - July 18, 2006 (DRAFT)
Consortium for Risk Evaluation with Stakeholder
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Broader Questions
What basis should be used to
Define the appropriate type of waste form for specific
wastes?
Estimate long-term waste form and disposal system
performance?
Establish treatment (operational) criteria?
Define monitoring requirements that are pre-emptive to system
failure?
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Department of Civil and Environmental Engineering
Department of Civil and Environmental Engineering
NRC/ACNW - July 18, 2006 (DRAFT)
Consortium for Risk Evaluation with Stakeholder
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Constituent Release by Leaching
Primary Factors System Integrity
Engineered and Institutional Systems
Waste Form PerformancePhysical IntegrityWater ContactMoisture
StatusOxidation Rates and ExtentConstituent Chemistry and Mass
Transport
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Department of Civil and Environmental Engineering
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NRC/ACNW - July 18, 2006 (DRAFT)
Consortium for Risk Evaluation with Stakeholder
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Processes and Impacts
Conceptual ModelMicro-cracks develop, increasing solid-liquid
surface area
Bridging of micro-cracks create macro-cracks
Through-cracks develop over time, leading to convective flow
Ultimate end state may be permeable matrix –equilibrium
release
Physical Integrity & Water Contact Monolithic Matrix
Flow-aroundLow interfacial areaDiffusive release
Stressed MatrixFlow-around/throughHigher interfacial
areaDiffusion-convection
Spalled MatrixHigh permeabilityVery high interfacial
areaEquilibrium-based release
ImpactNeed to account for the sequence of physical states and
rate of changes Influences chemical reactions and constituent
releaseBoth “intact” & “degraded” cases are not realistic
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Department of Civil and Environmental Engineering
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Processes and ImpactsMoisture Transport
Full Saturation
Capillary SaturationContinuous LiquidDiscontinuous Gas
Transition ZoneContinuous Liquid Continuous Gas
Insular SaturationDiscontinuous LiquidContinuous Gas
Completely Dry
Hamb
RH=100%
Hamb
RH=100%
Hamb
RH=100%
Hamb
RH=100%
Hamb
RH
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Department of Civil and Environmental Engineering
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NRC/ACNW - July 18, 2006 (DRAFT)
Consortium for Risk Evaluation with Stakeholder
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Processes and ImpactsOxidation
Rates and Extent
1.4E+012.6E-048.9E-03Conc of O2 [mole/L]
1.1E+040.0000190.21DO2 [cm2/s]
Ratio (A/W)WaterAir
(1) Wilke and Chang, 1955(2) www.swbic.org/education/
env-engr/gastransfer/gastransf.html
oxidation front
O2
occludedpore
Conceptual ModelWaste form pores – two phase system of gas and
liquid; depends on moisture content (saturation)O2 transport via
gaseous diffusion may be important depending on
saturation.Oxidation may lead to change in leaching behavior
Increased Tc-99 release; other constituents
ImpactGas phase transport not considered
Flux of O2 (gas) ~105 > liquid phase flux
Impact to Tc-99 oxidation minimized
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Department of Civil and Environmental Engineering
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Processes and ImpactsCarbonation
CO2
carbonation front
Conceptual ModelCO3-2 + Ca+2 → CaCO3 (s)
Gas phase diffusion of CO2Liquid phase diffusion of HCO3-
Pore water pH decreasedAlters solubility of constituents
(increase or decrease depending on species).
CarbonationExpansive precipitate – internal stress
(cracking)Pore blocking – increases diffusional resistance
(decreases oxidation, release rates).Extent and pore effects depend
on waste form alkalinity and saturation
0.0001
0.001
0.01
0.1
1
10
100
2 4 6 8 10 12 14Leachate pH
As
[mg/
L]
NoncarbonatedCarbonated
ImpactPotential for speciation changes (e.g., As)Pore structure
changesMay have either positive (e.g., pore capping) or detrimental
(i.e., increased solubility) impacts
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Department of Civil and Environmental Engineering
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VaultWall
WasteForm
(high SO4)
Processes and Impacts
Conceptual ModelTransport described by moving dissolution
fronts
Precipitation/reaction processes near external boundaries may
significantly impact release (+ or -)
Dissolution/diffusion of Ca(OH)2 and CSH control pore water
pH
pH gradients alter trace species release
SO4 leaching from waste into vault attacking concrete physical
structure.
Source of SO4 may be waste or external environment
ImpactMass transport estimates do not reflect the dynamic
chemistry and mineralogy of the waste form.Release rates and
extents mechanistically different from simplified assumptions,
limiting predictability.
Leaching of Major Constituents
leac
han
t
Ca moving front
effHD
CCa=CCa,0SCa=Sp,0
SSO4=SSO4,0ε=ε0
Ca(OH)2dissolution
pH
CCa=CCaSCa=0ε>ε0
effCaD
SO4 moving front
effSO4
D
SO4 moving front
effSO4
D
Sulfate species precipitate in cracks and large pores in vault
concrete.
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Department of Civil and Environmental Engineering
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NRC/ACNW - July 18, 2006 (DRAFT)
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Processes and Impacts
Conceptual ModelRelease based on coupled chemistry and mass
transport.Release dependent on:
Moisture conditionspH gradientsRedox chemistryBoundary layer
formation
ImpactPerformance assessments may grossly over- or under-predict
release
Leaching of Trace Constituents
leac
han
t
Ca moving front
effHD
CCa=CCa,0SCa=Sp,0SMe=SMe,0
ε=ε0
Ca(OH)2dissolutionpH
Me moving front
effHDeffMe
D
CCa=CCaSCa=0
CMe=f{pH}
effCa
D
AMD - Selenium
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
0.01 0.1 1 10 100 1000Mean Interval [days]
Mea
n Fl
ux [m
g/m
2s]
AMD-AAMD-BMean
Diffusion Model predicts
flux 102greater than measured
after ~1 year
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Integrated Long-Term Degradation
Chemical degradation and physical stress effects are coupled and
integrated.
Physical stressCyclic loadingFlexural bendingDrying
shrinkageSeismic eventsSettlement
Chemical degradationOxidationLeachingExpansive reactions
CarbonationSulfate attackRebar corrosion
Microcracks• Increase porosity
• Increase interaction pore water/surface
Through-cracks• Preferential flow path• Diffusive and
convective release• Loss of strength
Spalling• Loss of cohesiveness
• Two body problem• Eventual release from
“granular” material
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Department of Civil and Environmental Engineering
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Current Studies on Secondary Waste from ORP
Reducing GroutGround Steel Slag 43 wt%Class F Fly Ash 42 OPC 7
DI Water 7
Synthetic Hanford GroundwaterCaSO4 1.20 mmol/L NaHCO3
1.04Mg(HCO3)2 0.62 CaCl2 0.34 KHCO3 0.19 Ca(HCO3)2 0.18
MotivationTc-99, I-129 in secondary wastes from
vitrification
ObjectiveLeaching assessment of reducing grout for secondary
waste treatment. Comparison with “ANS16.1-type” testing in
synthetic ground water.
mg/kg Added AsAg 243 AgNO3
As(V) 1000 Na2HAsO4▪7H2OBa 500 Ba(NO3)2
Cd 1000 Cd(NO3)2▪4H2OCu 1000 Cu(NO3)2▪2.5H2OCs 1000 CsClI 1214
NaI
Pb 1000 Pb(NO3)2Re 971 KReO4Sb 952 Sb2O3Se 751 KSeO4Zn 1000
Zn(NO3)2
Contaminants in Reducing Grout
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Department of Civil and Environmental Engineering
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Equilibrium – Trace Species
1,000
10,000
100,000
1,000,000
2 4 6 8 10 12 14
Leachant pH
Rhe
nium
[ug/
L]
AMD-AAMD-BAMG-AAMG-B
0.01
0.1
1
10
100
1,000
2 4 6 8 10 12 14
Leachant pH
Ura
nium
[ug/
L]
ML
MDL
AMD-AAMD-BAMG-AAMG-B
0.01
0.1
1
10
100
1,000
1,0000
2 4 6 8 10 12 14
Leachant pH
Iodi
de [u
g/L]
Iodate
AMD-AAMD-BAMG-AAMG-B
Solid-Liquid Partitioning
ComparisonAMD – DI WaterAMG – Synthetic Hanford Ground Water
1
10
100
1,000
10,000
100,000
2 4 6 8 10 12 14
Leachant pH
Stro
ntiu
m [u
g/L]
ML
AMD-AAMD-BAMG-AAMG-B
waste form11.5
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Department of Civil and Environmental Engineering
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Mass Transport TestsAMD - Rhenium
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
0.01 0.1 1 10 100 1000Mean Interval [days]
Mea
n Fl
ux [m
g/m
2s] AMD-A
AMD-BMean
AMD - Selenium
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
0.01 0.1 1 10 100 1000Mean Interval [days]
Mea
n Fl
ux [m
g/m
2s]
AMD-AAMD-BMean
MT001 TestTank Leaching in DI WaterConstituent FluxConstituent
Release
ComparisonAMD – DI WaterFlux @ constant diffusivity (green
dash)AMD - Calcium
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
0.01 0.1 1 10 100 1000Mean Interval [days]
Mea
n Fl
ux [m
g/m
2s]
AMD-AAMD-BMean M
ean
Flux
[mg/
m2
s]AMD - Strontium
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
0.01 0.1 1 10 100 1000Mean Interval [days]
AMD-AAMD-BMean
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Department of Civil and Environmental Engineering
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Synthetic Groundwater
Precipitate on sample AMG-Bafter 6th leaching interval
MT001 Test
ComparisonAMD – DI WaterAMG – Synthetic Hanford Groundwater
Hanford GroundwaterCaSO4 1.20 mmol/L NaHCO3 1.04Mg(HCO3)2 0.62
CaCl2 0.34 KHCO3 0.19 Ca(HCO3)2 0.18
Rhenium
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
0.01 0.1 1 10 100 1000Mean Interval [days]
Mea
n Fl
ux [m
g/m
2s] PNL1-AMD
PNL1-AMG
6th leachateMea
n Fl
ux [m
g/m
2s]
Cesium
Mean Interval [days]
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
0.01 0.1 1 10 100 1000
PNL1-AMDPNL1-AMG
6th leachate
Mea
n Fl
ux [m
g/m
2s]
Selenium
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
0.01 0.1 1 10 100 1000Mean Interval [days]
6th leachate
PNL1-AMDPNL1-AMG
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Department of Civil and Environmental Engineering
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Process- and Mechanism-BasedExperimentation & Modeling
Long-TermPerformance
Estimates
SensitivityAnalysis
UncertaintyAnalysis
Conceptual Model(chemistry & physics)
Mathematical Model & Computer Simulation
Model Verification(comparison to other models & limit
cases)
ModelValidation
ObservationalExperiments
Parametric Experiments(individual processes to obtain parameter
values & constitutive
relations)
Integrative Experiments(multiple processes & field
tests)
Field Scenarios
Independent data
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Department of Civil and Environmental Engineering
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Measure intrinsic leaching characteristics of material
(aqueous-solid partitioning (pH and LS); release kinetics)
Batch extractions & tank leaching (monoliths)Constituent
fraction readily leachedControlling mechanism for release (mineral
dissolution and solubility, solid phase adsorption, aqueous phase
complexation)
Release kinetics for mass transfer parameters
Evaluate release in the context of field scenarioExternal
influencing factors such as carbonation, oxidation
HydrologyMineralogical changes
Use geochemical speciation and mass transfer models to estimate
release for alternative scenarios
Model complexity to match information needsMany scenarios can be
evaluated from single data set
Overarching Framework(Kosson, van der Sloot, Sanchez &
Garrabrants, 2002, Environ. Engr. Sci.,
19, 159-203)
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Department of Civil and Environmental Engineering
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Integrated Use of Testing and Simulation
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LeachXSSoftware-based system for evaluating leaching
Incorporates multiple processes and system configurationsData
management/interpretationGeochemical analysis via ORCHESTRA
(Meeussen, 2003)Database of material leaching information
User Input, Test Results
and ParametersDatabase Access
Calculation Engines
Scenarios(e.g., fill
characteristics,geometry, infiltration,
hydrology)
Materials(Leaching data,
Composition, Physicalcharacteristics)
Regulatory(Regulatory
thresholds andcriteria from different
jurisdictions)
Thermo-dynamic
Databases
ScenarioDatabase
MaterialsLeachingDatabase
RegulatoryDatabase
LeachXS(Materials and
ScenariosEvaluation)
Orchestra(GeochemicalSpeciation and
Reactive TransportSimulator)
Reports(Figures, Tables,
Scenario and MaterialDescriptions)
ExcelSpreadsheets(Data, Figures)
Other Models(Source Term and
Parameters forFate, Transport,and Risk Models)
Output Reporting and
Graphing
User Input, Test Results
and ParametersDatabase Access
Calculation Engines
Scenarios(e.g., fill
characteristics,geometry, infiltration,
hydrology)
Materials(Leaching data,
Composition, Physicalcharacteristics)
Regulatory(Regulatory
thresholds andcriteria from different
jurisdictions)
Thermo-dynamic
Databases
ScenarioDatabase
MaterialsLeachingDatabase
RegulatoryDatabase
LeachXS(Materials and
ScenariosEvaluation)
Orchestra(GeochemicalSpeciation and
Reactive TransportSimulator)
Reports(Figures, Tables,
Scenario and MaterialDescriptions)
ExcelSpreadsheets(Data, Figures)
Other Models(Source Term and
Parameters forFate, Transport,and Risk Models)
Scenarios(e.g., fill
characteristics,geometry, infiltration,
hydrology)
Scenarios(e.g., fill
characteristics,geometry, infiltration,
hydrology)
Materials(Leaching data,
Composition, Physicalcharacteristics)
Materials(Leaching data,
Composition, Physicalcharacteristics)
Regulatory(Regulatory
thresholds andcriteria from different
jurisdictions)
Regulatory(Regulatory
thresholds andcriteria from different
jurisdictions)
Thermo-dynamic
Databases
Thermo-dynamic
Databases
Thermo-dynamic
Databases
ScenarioDatabaseScenarioDatabase
MaterialsLeachingDatabase
MaterialsLeachingDatabase
RegulatoryDatabase
RegulatoryDatabase
RegulatoryDatabase
LeachXS(Materials and
ScenariosEvaluation)
LeachXS(Materials and
ScenariosEvaluation)
LeachXS(Materials and
ScenariosEvaluation)
Orchestra(GeochemicalSpeciation and
Reactive TransportSimulator)
Orchestra(GeochemicalSpeciation and
Reactive TransportSimulator)
Reports(Figures, Tables,
Scenario and MaterialDescriptions)
Reports(Figures, Tables,
Scenario and MaterialDescriptions)
Reports(Figures, Tables,
Scenario and MaterialDescriptions)
ExcelSpreadsheets(Data, Figures)
ExcelSpreadsheets(Data, Figures)
Other Models(Source Term and
Parameters forFate, Transport,and Risk Models)
Other Models(Source Term and
Parameters forFate, Transport,and Risk Models)
Output Reporting and
Graphing LEACHXS
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CHROMIUM SPECIATION IN MORTAR AND WATER
Cr as function of pH
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1 2 3 4 5 6 7 8 9 10 11 12 13 14
pH
Con
cent
ratio
n (m
ol/l)
Cement mortar Model prediction CaCrO4[aq].diss Cr+3.diss
Cr[OH]+2.diss Cr[OH]2+.diss
Partitioning liquid and solid phase, Cr
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1 2 3 4 5 6 7 8 9 10 11 12 13 14
pH
Con
cent
ratio
n (m
ol/l)
Free DOC-boundPOM-bound FeOxideClay Ba[SCr]O4[96%SO4]Cr[OH]3[A]
Cr-Ettringite
Cr fractionation in solution
0%
20%
40%
60%
80%
100%
1 2 3 4 5 6 7 8 9 10 11 12 13 14
pH
Frac
tion
of to
tal
conc
entra
tion
(%)
DOC-bound CaCrO4[aq].diss Cr+3.diss Cr[OH]+2.diss Cr[OH]2+.diss
Cr[OH]3.diss
Cr fractionation in the solid phase
0%
20%
40%
60%
80%
100%
1 2 3 4 5 6 7 8 9 10 11 12 13 14
pH
Frac
tion
of to
tal
conc
entra
tion
(%)
POM-bound FeOxideClay Ba[SCr]O4[96%SO4]Cr[OH]3[A] Cr-Ettringite
LEACHXS
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Leachant Simulation – Boundary Effects
DI Water Hanford GW
DI Water Hanford GW DI Water Hanford GW
DI Water Hanford GW
CO2 equilibrium
CementMaterial
Acidic Soil
Leachant
DI Water Hanford GW LEACHXS
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MODELLING OF 3 LAYER SYSTEM WITH FULL CHEMICAL SPECIATION AND
TRANSPORT
MSW Bottom Ash
Cement
Soil
LEACHXS
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Department of Civil and Environmental Engineering
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NRC/ACNW - July 18, 2006 (DRAFT)
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Suggested Path Forward
Process of continuous improvement, such that assessments
incorporate “state of the art” understanding to extent
practical
Important for current assessments and future nuclear waste
management (legacy and future wastes) Need to define short-term and
long-term needs
Experimental studies coupled with model development and
validation
Formation/effect of boundary layers (e.g., CaCO3, oxidized
layer)Moisture transport and statusOxidation ratesFull geochemical
model (equilibrium & mass transfer) for key systemsPhysical
changes considering key geochemistry and mass transfer
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Department of Civil and Environmental Engineering
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Conclusions
Significant processes are not included in current DOE
performance evaluations that can have major impacts constituent
release.
It is important to have a more robust system understanding and
model for near-term and longer-term DOE waste management
decisions.
CRESP and SRNL, along with others, are currently working
together to provide the needed evaluation system components.