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EXAMPLE Pathways of Liquid Effluents in Groundwater and Surface Water (Section 2.4.13 SAR)
FRAMES-2.0 Workshop U.S. Nuclear Regulatory Commission Bethesda, Maryland November 15-16, 2007 Pacific Northwest National Laboratory Richland, Washington
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Purpose Demonstrate Hierarchical Modeling
SSAR assessment: instantaneous mixing, advection, retardation, decay Modeling: mass balance, advection, dispersion, retardation, decay Instantaneous release Long-term release (20-yr leak) Explore Conservative Assumptions Full mixing over the Aquifer Depth Largest Darcy Velocity No Dispersion Register and use an Excel Spreadsheet 2
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Problem Description (direct quote from SAR)
“Reactor Coolant Storage Tank is postulated to rupture, and 80% of its liquid volume (92 m3) is assumed to be released in accordance with Branch Technical Position 11-6…Flow from the rupture is postulated to flood the building and migrate past the building containment structure and sump collection system and enter the subsurface at the top of the building slab…(V)ertical downward flow ensues. A pathway is created that would allow the entire 92 m3 to enter the groundwater system instantaneously.” 3
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Problem Definition No vadose zone, aquifer only
Uses known tank concentrations Instantaneously places 80% (not 100%) of the tank’s mass into only the aquifer’s effective pore space Uses tank’s total liquid volume to estimate the plan view area of contamination NOT an instantaneous release scenario, which requires mass balance checks on water and mass flux rates Mixes contamination over the aquifer depth (conservative?) Maximum Darcy velocity (noted as conservative) Advection, Decay, and Retardation Only (noted as conservative) No Dispersion (noted as conservative) 4
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Fully Mixed With Dispersion Source Upper Aquitard Fully Mixed Plume
River Plume Upper Aquifer Unit Fully Mixed Fully Mixed Upper Aquitard Upper Aquifer Unit Plume Source River With Dispersion 5
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Modules Source, Vadose Zone, and Aquifer Transport
FRAMES Constituent Database Selection Source (User-defined source term) – WFF Vadose Zone Module MEPAS 5.0 Aquifer Module MEPAS 5.0 River Module Exposure/Intake/Risk MEPAS 5.0 Exposure Pathways Module MEPAS 5.0 Receptor Intakes Module MEPAS 5.0 Health Impacts Module 7
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Registering an Excel Worksheet
Model Input Registering an Excel Worksheet (separate presentation) 8
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Constituent Database 12
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“Source-Term” Module 13
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Aquifer Module 14
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River Module 15
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Chronic Exposure Module
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Intake Module Impacts Module 17
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Output Results 18
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Constituent Concentrations in Aquifer at River’s Edge
pCi/mL pCi/mL pCi/mL Years 19
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Comments & Investigations
Modeling provides us with an opportunity to more fully understand the problem. SAR case ignores mass balance of water flux rate. Maximum Darcy Velocity is conservative – Reduce SZ Darcy Velocity one order of magnitude Fully mixed condition is conservative – Increase the aquifer depth 20
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Check to see if a maximum saturated zone Darcy velocity is conservative:
Reduce the saturated zone Darcy velocity by an order of magnitude (i.e., 1/10th) 21
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Reduced by 1/10th 22
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Reduced by 1/10th 23
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Reduced by 1/10th 24
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Reduced by 1/10th 25
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Constituent Concentrations in Aquifer at River’s Edge at 1/10th the Darcy Velocity
pCi/mL pCi/mL pCi/mL Years 26
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Hierarchical Modeling Results
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Check to see if fully mixed conditions in the aquifer is conservative:
Increase the aquifer thickness. This case was not run, but the modeling can provide insight on how aquifer depth can impact the conservative assumption. 28
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Rule-of-Thumb Relationships
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Rule-of-Thumb Relationship Definitions
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Summary Many important factors determine whether scenarios and assumptions are conservative Interdependencies between parameters Duration of release Time of concentration Water mass balance Contaminant mass balance 31
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