139
Honour Lectures /
Conférences honorifiques
13
Nonetheless, prudence dictates consideration of diffusive
transport in terms of long-term performance assessments,
as the results of several simplified transport analyses
suggest that diffusion may be significant under some
scenarios (e.g., Gray and Weber 1984, Shackelford 1989,
Manassero and Shackelford 1994).
4.2.7
Diffusion through Bentonite Buffers for High-
Level Radioactive Waste (HLRW) Disposal
Diffusion of radionuclides through highly compacted
bentonites being considered as buffer barriers in HLRW
disposal scenarios has been an area of substantial research
over the past several decades, and in particular the past
approximate decade. In fact, the number of referenced
publications focused on evaluating diffusion of
radionuclides through bentonite buffer barriers for HLRW
disposal is too voluminous to cite here, but a representative
listing can be found in Shackelford and Moore (2013). The
high number of publications in this area results from the
need for safe and secure, long-term disposal of HLRW
(e.g.,
≥ 10,000 yr)
resulting from the significant past and
present roles of nuclear energy in several countries (e.g.,
Belgium, Canada, France, Japan, Spain, Switzerland,
United Kingdom, and the USA).
In particular, two issues related to radionuclide
diffusion through highly compacted bentonite buffers have
been identified, viz., the influence of surface and/or
interlayer diffusion, and the existence of semipermeable
membrane behavior as a result of ion exclusion
(Shackelford and Moore 2013). Surface or interlayer
diffusion refers to the diffusion of cations, typically metals,
sorbed to clay particles in addition to diffusion of cations
within the mobile pore water between particles, i.e.,
outside the extent of influence of the negative electrical
potentials associated with the individual clay particle
surfaces. This phenomenon is attributed to the excess of
sorbed cations in the diffuse double layers surrounding
negatively charged clay surfaces relative to the
concentration of cations that exists in the mobile pore
water, and is known as interlayer diffusion when referring
to the excess of sorbed cations within the interlayer regions
of smectitic based clays, such as bentonites (Glaus et al.
2007, Appelo et al. 2010). When prevalent, surface and/or
interlayer diffusion can result in enhanced diffusion of
cations, and diminished diffusion of anions, relative to the
diffusion of neutral tracers such as tritium and deuterium
(Appelo et al. 2010). However, Shackelford and Moore
(2013) noted that conflicting results have been reported as
to the significance of surface and/or interlayer diffusion,
and that the phenomenon is likely to be significant only in
high activity clays, such as bentonites, compacted at
relatively high dry densities. Also, the significance of
surface and/or interlayer diffusion will be a function of the
chemical speciation of the diffusing radionuclide.
In terms of semipermeable membrane behavior,
numerous studies have reported significant ion
exclusionary properties of bentonite buffer barriers, but
these properties historically have been taken into account
qualitatively or indirectly by incorporating a correction
(anion exclusion) factor within the form of Fick's first law
(Shackelford and Moore 2013). However, recent advances
in simultaneously testing for both solute diffusion and
semipermeable membrane behavior as previously
documented for GCLs have largely eliminated this
restriction, such that quantification of the effect of
semipermeable membrane behavior of radionuclide
diffusion can now be assessed (e.g., see Fig. 9 and
associated text). As a result of these advancements, and the
continuing need to assess the performance of the
containment structures used to isolate HLRW from the
environment for extensive time frames, diffusion of
radionuclides through bentonite buffer barriers is likely to
remain an important research area for the foreseeable
future.
4.3
Diffusion as an Attenuation Mechanism (Matrix
Diffusion)
The process of matrix diffusion, whereby contaminants
diffuse from interconnected pores or fractures into the
surrounding intact clay or rock matrix, may be an
important attenuation mechanism when the contaminant
transport occurs through structured clay and/or rock
formations (e.g., Foster 1975, Grisak and Pickens 1980,
Neretnieks 1980, Feenstra et al. 1984, Lever et al. 1985,
Rowe and Booker 1990, 1991, Boving and Grathwohl
2001, Polak et al. 2002, Lipson et al. 2005). In this regard,
matrix diffusion has been considered in terms of the
migration of radionuclides resulting from high-level
radioactive waste disposal through fractured crystalline
rocks (Neretnieks 1980, Sato 1999), the migration of
pesticides resulting from agricultural practice through
fractured clayey till (Jorgensen and Fredericia 1992,
Jorgensen and Foged 1994), the migration of leachate
resulting from solid waste landfills through underlying
fractured clayey till (Rowe and Booker 1990, 1991), and
the migration of dense-chlorinated solvents resulting from
industrial spills and disposal practice through fractured
geologic media (Parker and McWhorter 1994, Parker et al.
1994, 1996).
For example, consider the scenario depicted in Fig. 16
after Rowe and Booker (1990, 1991), whereby a clay-lined
(CCL) waste containment facility is underlain by fractured
till that serves as an "attenuation layer" (AL) that could
attenuate the migration of any contaminants emanating
from the containment facility to the underlying confined
aquifer. In this scenario, the greater the ability of the
fractured till to attenuate the migration contaminants, the
more effective the overall or global containment system
(i.e., CCL + AL). In this regard, the fractures may serve as
conduits that facilitate the rate of downward migration of
contaminants, but matrix diffusion of contaminants from
the fractures into the surrounding intact clay matrix and
any subsequent sorption of the contaminants to the
individual clay particles within the matrix pores can
provide for an effective retardation of advancing,
downward contaminant migration.
Matrix diffusion also may be important in attenuating
the migration of contaminants at the local or barrier scale.
For example, Jo et al. (2006) proposed a three-
compartment model that included rate-limited cation
exchange controlled by matrix diffusion to explain
the
extensive tailing of eluted cations that often is observed
during column tests conducted on aggregated soils with
inorganic chemical solutions. As illustrated schematically
in Fig. 17, the pore space in the saturated granular
bentonite was assumed to consist of intergranular,
interparticle, and interlayer (interlaminar) spaces. The
pores between the granules constituted the intergranular
pore space, whereas the interparticle pore spaces existed
between the particles comprising the granules, but outside
the interlayer space between the montmorillonite lamella.
Water in the intergranular pore space was assumed to be
hydraulically mobile. Water in the interparticle and
