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International Groundwater Symposium
Valencia, Spain
September 22-24, 2010
Greg Ruskauff1
Nicole DeNovio2
Edward Kwicklis3
1 NNES LLC2 Golder Associates
3 Los Alamos National Laboratory
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BackgroundThe Nevada Test Site was used from
1951 to 1992 for nuclear weapons
testing
Great Basin area of western United
States
Facility area is 3,500 km2
(> Luxembourg)
The tests left behind radioactive residue,
which may be migrating in groundwater
NNSA/NSO established the
Underground Test Area (UGTA)
Subproject to ensure the protection of
the public
Site characterization and modeling used
to negotiate compliance boundaries with
Nevada Department of Environmental
Protection2
Frenchman FlatTen underground shaft tests detonated
between 1965 and 1971
All less than 20 kt
Informally divided into Northern and
Central areas
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What happens near an underground nuclear
test?
Rock and water vaporized immediately around device, forming a cavity
Formation of a chimney as pressure subsides
Alteration of rock properties to some distance away from the test
Cavity size related to yield and depth of burial
The exchange volume, where radionuclides can be found, forms out to ~ 2 cavity radii
Preferential distribution of radionuclides
Higher boiling point radionuclides become incorporated in the nuclear melt glass
Lighter radionuclides condense from vapor and remain in the water and sorbed to cavity/chimney rubble
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Underground Nuclear Tests in Frenchman Flat
Ten tests
Nine conducted in alluvium
One conducted in vitric tuff
The CAMBRIC event (entirely in alluvium)
has been studied extensively by
Lawrence Livermore National Laboratory,
who developed the following conceptual
model:
In alluvium, shock-compressed zones of lower
permeability, porosity limit groundwater flow
through the cavity
Radionuclides assumed to be evenly distributed
within exchange volume (2Rc), and nuclear melt
glass zone.
Supported by several numerical models
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The need for other conceptual models
6
Within 2 cavity radii rocks vary,
especially at the water table
IAEA (1998) reviewed French
underground nuclear tests in basalt
lava in the Pacific
“The Radiological Situation at the
Atolls of Mururoa and Fangataufa”
Conceptualized high permeability
zone out to 2.5 cavity radii
Cavity infilling studies broadly support
spherical zone of enhanced
permeability from test effects
Adopt another conceptual model for
tests with the exchange volume
extending to lava and welded tuff
Consequences of near-test conceptual models
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Alluvium Concept IAEA Concept
Evaluated the effect of different near-field conceptual models on
radionuclide release
Same inventory uncertainty
Different conceptual models received different parameter assignments
Alluvium/vitric tuff – permeability reduction factor from 1.5 to 100
IAEA – permeability enhancement factor of 5 to 100
Different conceptual model results in higher peak concentrations
Basin-Scale Conceptual Model
Deep, complicated geology
Initial model didn’t have data on the depth of the alluvium, thickness of confining units
Cross Section through Center of Model Domain
Alluvial Aquifers
Regional Carbonate
Aquifer
VolcanicAquifers
Aquitards
Top of the Model Domain
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HLCA < HAA
Hydrogeologic Conceptual Model
Easterly component from
leakage across Cane Spring fault
from CP basin
Based on age and head
relationships groundwater flows
from the perimeter of the basin
toward the basin center and out to
the south-south east
For the regional carbonate flow
system, Rock Valley fault system
provides regional drainage that
will focus flow to the southwest
out of Frenchman Flat
Groundwater velocity in
alluvium from 0.1 to 1 m/yr
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1.10.85
0.20
0.43
0.25
Approach to Uncertainty AnalysisExternal project review during 1999 suggested
the need to consider conceptual uncertainty
reflected in basin conceptual model
Revised approach incorporated 5 interpretations
of geology
Tried to focus alternative interpretation in portions
of the model likely to have transport
Several sets of boundary conditions
One different hydrologic conceptual model
Three permeability parameterization
approaches
Several discrete flow model cases of different
geologic models, parameterization approaches
Performed Monte Carlo transport analysis on
each case
Flow model calibration constrained Monte Carlo
analysis
Monte Carlo transport analysis on ensemble
HFM Alternative Description
DETA – detachment
fault alternative
This alternative is a no detachment fault
model.
DISP –
Displacement fault
alternative
This alternative is concerned with the
locations and displacement of basin-
forming faults.
CPBA – CP basin
alternative
The CP basin alternative extends the
UCCU beneath all of CP basin.
BLFA – Basalt Lava-
Flow Aquifer
The BLFA HSU is modeled as a single
continuous flow, rather than three
separate zones
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Pooled Uncertainty Analysis
All discrete cases considered
“Null-space Monte Carlo” of Tonkin and Doherty (2009) (100 realizations)
Calibration constrained uncertainty of flow model parameters
Specified plausible parameter ranges
Heads and flows always calibrate acceptably
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BASE Hydrologic
Conceptual Model
Alternative Hydrologic
Conceptual Model
Which models matter?Try to identify similar behaving
models that can be used as
surrogates for others
Used cavity flow rate as
transport surrogate
Evaluate range to see if relevant
uncertainty is being sampled
Cannot discern that alternative
geologic models have any
importance
Scatter among models using
variable parameterization similar
to alternative geology scatter
Two (NHA and BASE-USGSD)
really are different12
BASE Hydrologic
Conceptual Model
Alternative Hydrologic
Conceptual Model
Alternate geology
interpretations
BASE geology
variable
parameterization
Remaining Geological Uncertainty
Welded tuff under PIN STRIPE modeled as continuous
North-south normal faulting appears to die out, not certain
Is the welded tuff really continuous?
Missed conceptual uncertainty
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Things We LearnedIn one instance, exchange volume altered properties, conceptual model uncertainty was clearly important
We need to be alert to conceptual uncertainty when we are extrapolating outside the information we have
CAMBRIC conceptual model vs. other tests
Refsgaard, J.C., J.P. van der Sluijs, J. Brown, and P. van der Keur. 2006. “A Framework for Dealing with Uncertainty due to Model Structure Error.” In Advances in Water Resources, Vol. 29, 1586-1597.
Can exist at all scales
Having an initial idea of what might be uncertain is useful, but it can’t override the ability to make adjustments
Two of the alternative geologic models were not insightful
It may take some time to understand the base interpretation enough to identify sensitive assumptions (e.g. the geologic uncertainty at PIN STRIPE)
“If the confidence intervals associated with the models predictions are greater than the difference between the predictions, this difference cannot be considered meaningful, at least in a statistical sense.” National Research Council, Ground Water Models Scientific and
Regulatory Applications, 1990.
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International Groundwater Symposium
Valencia, Spain
September 22-24, 2010
Greg Ruskauff1
Nicole DeNovio2
Edward Kwicklis3
1 NNES LLC2 Golder Associates
3 Los Alamos National Laboratory
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