Release of bentonite colloids into freshwater: Present ideas Presentation at Workshop, June 13, 2007
Ivars Neretnieks
Chemical Engineering and Technology
KTH
Release of bentonite colloids into freshwater: Present ideas Presentation at Workshop, June 13, 2007
Ivars Neretnieks
Chemical Engineering and Technology
KTH
Swedish KBS-3 concept
Overview of talk
Background
Bentonite gel/sol properties-Colloids
0:th order model for bentonite loss
Forces acting on colloid particles
Development of swelling and release model
Background and Introduction I
Bentonite buffer around waste canister forms a gel with water and swells
Pore water ionic strength influnces Bentonite swelling pressure
Low ionic strengths,
Background and Introduction II
Passing fresh GW carries away ions from pore water, PW
At Bentonite/GW interface a balance is established between depletion by GW and dissolution of Ca- minerals
Minerals with solubility >CCC will replenish PW concentration to maintain gel
High flowrates of meltwater
Fracture intersecting deposition hole
Physical erosion
Shear force Velocity of gel to Decreasing gel concentration.
Also influenced by cat-ion in clay Shear force by flowing water on gel
“Viscosity” of gel is strongly non-linear at low shear stress: Bingham Fluid
Chemical erosion
At low ionic strength repulsive forces expand the clay and release colloids
Gel turns to sol and particles do not flocculate.
Sol expands due to repulsive forces between particles, which are carried away by the seeping water
Water velocity Fracture aperture Diffusivity Water residence time at
clay/water interface Qeq is used to visualise rate of mass transfer, mol/s of a solute from clay to seeping water
Qeq is the equivalent flowrate, m3/s that carries a solute with the concentration at clay/water interface
Obtained by solving the coupled flow and diffusion equations in a fracture
Potential consequences
When PW concentration in bulk clay can be replenished to >CCC from minerals the solubilisation is determined by rate of depletion of the mineral
Example of clay loss
Qeq=0.01 m3/a
CCCC= 40 g Ca/m3
Cw=0
NCa=0.4 g Ca/a
mCa=0.01 g Ca/g Clay
NClay=40 g Clay/a
400 kg Clay/10 000 a
Is the zero’th order model too pessimistic or may it even be optimistic?
Assumption that clay can disperse into passing water
What can gravity do?
Can clay move sufficiently far into water to be carried away?
Can clay particles be filtered in fracture and clog it?
Can clay particles be filtered in accessory minerals left behind?
Impact of gravity
Clay in a sub-vertical fracture will be pulled downward in addition to repulsion forces due to electrical charges
Could this happen even if PW concentration is above CCC?
Model idea same as for Qeq
Nion=Qeq*Cinterface: Rate of loss of ions
Ions move in chemical potential field = Force to disperse
Balanced by friction. Gives rate of diffusion
Nclay into passing GW
Colloids move in chemical potential field
Gravity field
Inter particle repulsion/attraction forces
Balanced by friction. Gives rate of diffusion
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