implementation of colloid-facilitated transport in … · implementation of colloid-facilitated...

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

TSPA 2002 2

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)

TSPA 2002 3

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

TSPA 2002 4

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

TSPA 2002 5

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.

TSPA 2002 6

Treatment of Irreversible species

Irreversible Pu species:Ic234Ic237Ic238Ic239Ic240

Irreversible Am species:Ic241Ic243

TSPA 2002 7

Treatment of Irreversible species

TSPA 2002 8

Decay of Irreversible species

TSPA 2002 9

TSPA Implementation

TSPA 2002 10

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).

TSPA 2002 11

Colloid Mass Generation (input to Source)

TSPA 2002 12

CDSP Source Inventory

Initial mass of colloidsis set very high (unlimited inventory)

No initial mass of Icspecies given

TSPA 2002 13

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

TSPA 2002 14

TSPA Model - Media in Cell - CDSP Package

Colloids in CDSP Packages

Reversible colloid component

Irreversible colloid component

TSPA 2002 15

TSPA Model - Colloid Transport Parameters

TSPA 2002 16

TSPA Model - Colloid Transport

Mass flux (g/yr) of colloids out of the Waste Form cell

Computes Advective + Diffusive flux

TSPA 2002 17

TSPA Model - Colloid Transport

TSPA 2002 18

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

TSPA 2002 19

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

TSPA 2002 20

Species Solubility

TSPA 2002 21

Calculation of Pu species concentration

Concentration of Pu isotopes irreversiblysorbed to WF colloids-- by multiplying totalPu conc. with mole fraction

TSPA 2002 22

TSPA Model - Colloid Transport

TSPA 2002 23

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.××××××××

TSPA 2002 24

Irreversible Colloid Mass Adjustment

Function to adjust irreversible colloid mass from dissolved mass

TSPA 2002 25

Irreversible Colloid Mass Adjustment

Consequence element to compute discrete changes within the waste form cell

TSPA 2002 26

Irreversible Colloids Logic

TSPA 2002 27

Waste Form Reversible Colloid Material

TSPA 2002 28

Waste Form Reversible Colloid Material

TSPA 2002 29

TSPA Model - Media in Cell - CDSP Package

Colloids in CDSP Packages

Reversible colloid component

Irreversible colloid component