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tion/organization and its Cu(II) retention characteristics - the Da in this zone is roughly two orders
of magnitude less than the rest of the profile. These observations have, by the way, quite a number
of similarities with those for Co(II) in-diffusion in the Opalinus clayrock.
CIEMAT carried out a program of ‘filter sandwich’ and block-scale diffusion measurements similar
to that described for the Opalinus clayrock, and obtained generally similar results, i.e. Kd extracted
from fitted Da values are significantly smaller than those observed in comparable batch experiments.
Note finally that FZK/INE carried out similar measurements of Pu(V) migration in COx samples
and obtained similar results as for Opalinus.
Kd values for Cs were determined (CEA) on the same set of ‘variable carbonate content’ COx samples
studied in §3.2. As for De(Cl), the results exhibit a ‘threshold’ effect, with Kd remaining in the
normal range of values for the formation for carbonate fractions below ~70%, then falling off drastically
to roughly 10% of this value. For completeness it can be noted that, when this data is used to
parameterize the distribution of Kd(Cs) values at the formation scale (cf. §3.4), the calculated Cs
flux vs. time curve at the top of the formation is, as expected, identical to that calculated using a
constant Kd for the entire formation.
Boom clay
SCK•CEN carried out in- and through-diffusion experiments with Cs and Sr in Boom clay with the
objective of extracting ‘diffusion-operant Kd’ values for comparison with the Kd values measured in
batch and compacted systems (cf. §4.1). The Da values obtained for both Cs and Sr, respectively
~1.4·10 -13 m²·s -1 and ~9·10 -12 m²·s -1 , match values obtained previously using a variety of experimental
techniques. However, in both cases, it was not possible to extract unambiguous, reliable values
for the ( + Kd) term. Because of this, comparison with measured Kd values was not possible. On
the other hand, it could be shown that batch Kd values tend to overestimate values for the ( + Kd)
term. A coupled sorption/transport simulation implementing the 3 site, cation-exchange model for
Cs+ sorption (cf. §4.1) was used to carry out a sensitivity analysis on the effect of increased pore
diffusion coefficients (related to "surface diffusion" effects) and decreasing available sorption sites.
Results show that good fits to the migration profile required either (i) that the total sorption site
concentration needed to be decreased to a fraction (5%) of that used to model Cs sorption data obtained
on dispersed and compact Boom Clay, or (ii) that the Dp value needed to be raised by a factor
of 16 compared to the Dp of HTO (Dp(HTO)=2.3·10 -10 m²·s -1 ). The extreme nature of both of these
results suggests, irrespective of the underlying hypothesis, that in a compacted system not all sorption
sites seem to be available, a conclusion which tends to go in the same direction as results seen
for the other two clayrocks.
5 Main RTDC3 messages for Clayrock Safety Cases
The overall results of RTDC3 can be summarized by returning to the three questions posed in the
introduction:
Do we have a sound theoretical basis for describing RN speciation in the porosity of highlycompacted
clay materials and clayrocks, in particular the distribution of total RN mass
between dissolved and sorbed species?
A fairly positive response seems justified based on two main results. The first is the observation
that similar equilibrium sorption states (Kd) are observed in dispersed and compacted
materials for moderately sorbing cations (Sr ++ , Cs + , Co ++ ) for all clayrocks. This
implies that the same sorption site populations are accessible under both conditions and
that the corresponding mass action laws are valid. It was not possible to demonstrate this
for highly sorbing RN (actinides…) because of the extremely long times needed to reach
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