TSPA 2002 1
Implementation Of Colloid-Facilitated Transport in the Engineered Barrier System of the TSPA Model
Presented by:Sunil MehtaYucca Mountain Project
Presented at:2002 GoldSim User ConferenceApril 25-26, 2002
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Introduction
• Three types of colloids modeled (two mineralogies)– Waste form colloids (glass-waste derived smectite assumed to
behave as natural smectite colloids)– Corrosion product colloids (“rust”; assumed to be iron
oxyhydroxides)– Groundwater colloids (assumed to be smectite; used as proxy for
all mineral colloids present in natural groundwater)
• Colloid presence based on aqueous conditions– Colloid concentration as f (ionic strength)– Colloid stability as f (ionic strength, pH) - either maximum
possible amount (stable) or ~zero amount (not stable)
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Introduction (continued)
• Colloid CharacterizationWaste Form colloids
– Colloids with Reversibly and Irreversibly attached RNs– Most significant contributor to colloid-facilitated radionuclide
transport– Formed by nucleation of colloids from waste form dissolution
and/or spallation of colloid-sized waste form alteration products
– Only for Co-disposal packages (not for CSNF packages)
Iron (hydr)oxide and Groundwater colloids– Colloids with Reversibly attached RNs only– For both Co-disposal and CSNF packages
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Implementation Requirements
Colloid Model Parameters in TSPA
• Determination of RN mass on colloids - based on ionic strength
• Determination of colloid stability - based on ionic strength and pH
• Determination of Irreversible and Reversible Partitioning of dissolved RN inventory onto colloids at each calculation time step
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Implementation - Assumptions
• Pu, Am (and their daughter products) are irreversibly sorbed oncolloids.
• Pu, Am, Th (Ra, Pb), Pa (Ac) are reversibly sorbed on colloids.
• Radionuclide mass irreversibly sorbed on colloids will be permanently locked within the colloid for transport (ingrowth and decay considered).
• Colloids with irreversibly sorbed RNs can also act as sites for reversible sorption.
• No reversible sorption can occur until the maximum irreversible sorption to colloids is satisfied in a given time step.
• Irreversible attachment of Pu and Am is treated by considering them as separate species -- called Ic species.
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Treatment of Irreversible species
Irreversible Pu species:Ic234Ic237Ic238Ic239Ic240
Irreversible Am species:Ic241Ic243
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Treatment of Irreversible species
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Decay of Irreversible species
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TSPA Implementation
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TSPA Implementation
• In order to adjust the dissolved mass of Pu and Am, the fraction bound irreversibly must be removed from the Waste Form cell
• Thus the mass fraction bound irreversibly must be known
• But this value is not known ahead of time as irreversibly sorbedmass is a function of pH, Ionic strength, and mass in Waste Formpathway.
• Thus a separate calculation has to be performed, where, mass of irreversibly bound Pu and Am is computed first and then removed from the available mass in the Waste Form cell using a feed back loop.
• For this a fictitious species called “Col” was created. It is used to produce mass of colloids in the Waste Form cell based on the stability of colloids (pH and I dependent).
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Colloid Mass Generation (input to Source)
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CDSP Source Inventory
Initial mass of colloidsis set very high (unlimited inventory)
No initial mass of Icspecies given
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Source Term Release to Waste Form Cell
Mass released fromSource to Waste_Formcell is controlled by Glass matrix dissolution rate
The mass of Pu and Amreleased is modified by removing the fraction bound irreversibly as colloids
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TSPA Model - Media in Cell - CDSP Package
Colloids in CDSP Packages
Reversible colloid component
Irreversible colloid component
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TSPA Model - Colloid Transport Parameters
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TSPA Model - Colloid Transport
Mass flux (g/yr) of colloids out of the Waste Form cell
Computes Advective + Diffusive flux
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TSPA Model - Colloid Transport
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Calculation of Pu mass sorbed to WF colloid
Concentration of Waste Form colloids with Irreversibly sorbed Pu
Concentration based on Ionic Strength stability field
Concentration based on pH stability field
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Calculation of Colloid Mass In a Unit Volume
Waste Form colloid mass (g/L) available for Reversible and Irreversible sorption
Conc. (solubility) of Irreversiblysorbed species (mg/L)
Defines the solubility of “Col” species
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Species Solubility
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Calculation of Pu species concentration
Concentration of Pu isotopes irreversiblysorbed to WF colloids-- by multiplying totalPu conc. with mole fraction
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TSPA Model - Colloid Transport
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Irreversible Colloid Mass Computation
Computes mass of irreversible Puspecies: Minimum of
Mass of Pu in pathway
engthTimestep_L ssFlux Colloid_Ma Colloid of Conc.
Pu Irrev. of Conc.××××××××
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Irreversible Colloid Mass Adjustment
Function to adjust irreversible colloid mass from dissolved mass
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Irreversible Colloid Mass Adjustment
Consequence element to compute discrete changes within the waste form cell
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Irreversible Colloids Logic
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Waste Form Reversible Colloid Material
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Waste Form Reversible Colloid Material
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TSPA Model - Media in Cell - CDSP Package
Colloids in CDSP Packages
Reversible colloid component
Irreversible colloid component
