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eduction in De with increasing proportion of non porous minerals. RTDC3 efforts were therefore
focused (i) on improving the capacity to measure, model and quantify the effects of rock heterogeneity
and bedding on diffusion at the cm scale and (ii) on evaluating the effect of rock mineralogy
on diffusion, in part by comparing diffusion in samples from different clayrock formations.
Detailed analyses of HTO diffusion in dm-scale volumes of Opalinus and Callovo-Oxfordian clayrock
were carried out by CIEMAT using a novel technique consisting of placing a solid source of
radioactive anionic and HTO tracers at the centre of a pluri-dm sized clayrock cylinder. 3D tracer
distribution maps were obtained at the end of the experiment by coring. Numerical modelling by
UDC using a code capable of considering bedding plane relative anisotropy was used (i) to carry
out sensitivity analyses to identify relevant diffusion and retention parameters and (ii) to determine
best estimates for parameter values by solving the inverse problem. Results for the Callovo-
Oxfordian give lowest error values of 4·10 -11 m 2 ·s -1 and 2.23 10 -11 m 2 ·s -1 respectively for
De(bedding parallel) and De(bedding perpendicular), which leads to a diffusion anisotropy ratio of
1.8. This value is of the same order, but greater than, that calculated by the TDD method (§3.2)
which could be due to the fact that the TDD model does not consider possible anisotropy within the
clay domains, i.e. at the scale of the models presented in §3.1. Results for HTO diffusion in the
Opalinus clay give much higher anisotropy ratios, of the order of 10, which might indicate a higher
degree of preferential orientation of clay particles than for the Callovo-Oxfordian.
Effective diffusion coefficients ‘bedding perpendicular’ were measured (CEA) for Cl - and HTO on a
set of Callovo-Oxfordian rock samples having carbonate contents covering the entire observed
range in the formation. Rocks having extreme, and relatively rare, high carbonate contents were of
particular interest since they will necessarily have very low fractions of the clay minerals governing
RN diffusion (and sorption, cf. §4.2). The results for De(Cl) show a ‘threshold’ effect, with De(Cl)
remaining in the normal range of values for the formation (5*10 -12 m 2 .s -1 ) for carbonate fractions
below ~35%, then falling off progressively to roughly 40% of this value as carbonate increases to
70%. This tendency is similar to that predicted by TDD modelling of the effect of increasing the
fraction of non porous grains at the mesoscopic scale (§3.2).
One RTDC3 goal was to develop a comparative database of diffusion properties of different clayrock
formations. The characteristics of 99 TcO4 - , HTO and H 14 CO3 - in clayrock samples originating
from two depths in the Boda claystone formation were determined by II-HAS. While the two depths
show significant differences in mineralogy (e.g. absence or presence of analcime), the De values
measured for the 99 TcO4 - anion and HTO were generally coherent with results on other clayrocks,
i.e. De(TcO4): 4.2·10 -12 m 2 ·s -1 < De(HTO): 1.4·10 -11 m 2 ·s -1 .
3.4 Diffusion at geological and Safety Case (SC) time-space scales
Performance assessment calculations in existing SC (cf. 1,2,3) show that non sorbing RN diffuse
across the entire GBS thickness (roughly 50 meters thick) during typical PA timeframes (10 6 years).
In all of these SC, single values for diffusion-determining parameters (De, Da, ) were used to represent
RN diffusion throughout the entire GBS volume. The parameter values chosen for both base
case (most probable) and sensitivity (pessimistic) calculations are demonstrably valid and robust,
being based on statistical evaluation of measurements made on a representative population of cmdm
scale samples. One of the objectives of RTDC3 was to provide information and methods for
supporting homogeneous representations of GBS diffusion properties. Three complementary approaches
were taken along this line:
A theoretical and statistical analysis of the effects of scaling on RN transport (SCK•CEN),
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