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An up-scaling methodology linking (i) diffusion parameter value variation as a function of
rock mineralogy measured at the cm scale and (ii) rock mineralogy measured at the GBS
scale, followed by comparative diffusion modelling with a homogeneous model (Andra,
CEA),
Natural tracer based studies (UniBerne, GRS).
A comprehensive evaluation of the potential effects of observed (or induced) spatial variability in
the Boom clay formation properties on RN transport was carried out by SCK•CEN. The study
shows that, from a theoretical standpoint, microscopic flow and transport processes can be upscaled
to the scale of the formation ‘layers’ so long as parameter values are associated with rock
volumes equal to or exceeding that of a representative volume element (RVE) for the Boom clay
(pluri-mm to cm). Consideration of two other types of information, statistical analysis of parameter
values measured on cm scale samples from throughout the formation and in situ measurements integrating
large rock volumes (hydraulic tests, diffusion experiments), allows a strong case to be
made for using parameter values measured on small samples as a basis for determining a representative
value and associated uncertainty applicable to the entire geological formation at the repository
site.
A similar conclusion was reached regarding anion diffusion through the Callovo-Oxfordian formation
using a method developed by Andra and CEA. The approach is based on a logical extension of
the working hypothesis which guided studies at the mesoscopic (§3.2) and macroscopic (§3.3)
scales, i.e. that non sorbing tracer diffusion should be determined largely by effects of rock mineral
composition on porosity organization. It therefore constitutes the last step in up-scaling process understanding
gained at small scales to the parameterization of model for non sorbing RN migration in
the GBS for PA purposes. It consists of three main steps:
Determination of the relationship between diffusion parameter values (De, ) for Cl - and
carbonate mineral content in Callovo-Oxfordian rock samples (cf. §3.3). This relationship
was used to define three rock classes having statistically different De values.
Signal treatment and geo-statistical methods were used (i) to obtain vertical profiles of rock
carbonate content with cm scale resolution from high resolution borehole log data and (ii)
to assign formation intervals (10 cm thick) to one of the three De(Cl) rock classes,
Modelling of diffusion, using the above as a basis for meshing and with an anion source
term located in the centre.
The calculated anion flux vs. time curve at the top of the formation was compared to that calculated
for the same system but with a single De value used for the entire formation, i.e. the configuration
used for PA calculations. The difference is insignificant.
In certain geological contexts information obtained by measuring and modelling spatial distributions
of conservative (non sorbing) natural tracers can provide a powerful argument for supporting
use of Fick’s law representations of non sorbing RN transport at the GBS scale, based on parameter
values measured at laboratory space-time scales. Such an approach requires measurement of natural
tracer concentration profiles within a geological formation (including boundary formations) followed
by model interpretation to extract plausible Fick’s law parameters 4 . UniBerne applied this
approach to studying natural tracers (Cl, Br, He and water isotopes) in the Opalinus Clay and adjacent
formations in the Mont Russelin anticline (Jura, CH). The Cl - distribution shows a regular,
4
The OECD Nuclear Energy Agency ‘CLAYTRAC Project: Natural Tracer Profiles Across Argillaceous Formations’
(2008) treats this aspect in detail.
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