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RN migration experiments and model interpretation
A major part of the research carried out in RTDC3 was concentrated on measuring mass transfer of
sorbing RN in compact clayrock-containing systems and interpreting the resulting datasets using
numerical models coupling the effects of diffusion and sorption equilibrium. A wide variety of experimental
techniques were used to study RN migration characteristics in rock volumes ranging
from mm (cf. techniques mentioned above) to dm (laboratory and in-situ experiment) scales. Generally
speaking, two types of information were sought (i) time-dependent evolution of RN tracer
concentration in the source term and, in certain cases, in a ‘sink’ reservoir, and (ii) tracer mass distribution
in the rock volume after a given time(s). Numerical modelling was then used to seek plausible
sets of values for diffusion (e.g. Da, De) and sorption (Kd, complexation model) parameters,
and in certain cases the degree of spatial variability within the rock, leading to the best possible representation
of the experimental dataset. The model results were then compared with those expected
based on initial hypotheses concerning RN migration (e.g. Kd(intact rock) = Kd(batch), De(cations)
= De(HTO), homogeneous porous medium), and conclusions drawn regarding the applicability or
non applicability of the reference model for representing RN migration. The following sections
briefly summarize the principal results of research carried out on each of the three clayrocks:
Opalinus clay, Callovo-Oxfordien and Boom clay.
Opalinus clay
Migration in Opalinus clayrock of a wide range of weakly to strongly sorbing cations (Na + , Sr ++ ,
Co ++ , Cs + , U(VI), Eu(III), Pu(IV)) was studied using a number of differing experimental configurations.
The results of Cs + in-diffusion and through-diffusion experiments carried out by PSI tend to
show that RN migration behaviour is generally coherent with the expected sorption model but is
significantly affected by relatively complex mass transport processes. One hypothesis is that the
porosity is made up of well-connected and ‘dead-end’ pores, which would necessitate considering
at least two Da terms in the diffusion equation, with that for the ‘dead end’ pores representing both
the kinetics of RN mass transport into this volume and RN retention on the sorption sites in contact
with this porosity. In-diffusion studies of Co(II) and Eu(III), with profiles extending to less than 0.5
mm after roughly 400 days for the later, also show evidence of mass transport complexity (dual
path). The general conclusion is that mass transport of sorbing RN in clayrocks is still not well understood,
especially given the difficulties associated with making measurements at such small spatial
scales (modifications of surface layer properties….).
Somewhat different results regarding the transferability of batch Kd to RN migration were obtained
from in-diffusion studies carried out by CIEMAT using three techniques: diffusion from a solid
source term in large-scale (dm) clayrock blocks (Sr), using a ‘filter source sandwich’ configuration
(Sr, Co, Eu, U(VI), Cs) and RBS detection (Eu). Generally speaking, the Kd values estimated by
modelling migration experiment data were found to be significantly smaller (10% or smaller for Co
and Cs) than those measured in batch experiments. Whether this difference is due to real differences
in sorption equilibrium between compact and crushed material, or includes the effects of mass
transport complexities (e.g. kinetic effects related to restricted access to some sorption sites) similar
to those observed by PSI, is not yet clear. On the other hand, the Da range obtained for Cs (3 –
12*10 -14 m 2 .s -1 ) is similar to the value obtained by PSI (~ 6*10 -14 m 2 .s -1 ) and Da values measured
for Co and Eu are intermediate between the Da values for fast and slow diffusion path values fit by
PSI. This suggests that some of the differences between Kd and observed effects on Da might be attributable
to differences in the modelling approach used for taking sorption into account.
Migration of most of these tracers is also being studied by means of the DR in-situ experiment being
carried out by Nagra/PSI at the Mont Terri URL, the first step of which was predictive modelling
(PSI, UDC, GRS) of tracer mass loss from the source solution using a variety of numerical
codes. While preliminary data on tracer loss from the injection interval tend to confirm the general
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