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Th(IV) sorption on montmorillonite and illite was used to calibrate a predictive sorption
model for Boom clay and Tipping's Humic ion binding model VI was used to account for
NOM interactions. The NOM model has been found to adequately simulate Eu-NOM
interactions and Eu sorption to illite in presence of NOM.
Armines adapted the retention and porosity model developed for bentonite to COx clayrock
by assuming additivity of the various mineralogical contributions, based principally on the
content of interstratified illite/smectite (I/S) minerals and the illite to smectite ratio in the I/S
minerals. The approach led to a reasonable quantitative representation of the CEC and
surface complexation site densities as a function of mineralogical composition. Sorption
isotherms calculated for Cs and Ni were in good agreement with measured Kd data.
4.2 Migration of (strongly) sorbing RN in intact clayrock
The most convincing (from a safety case stand point) approach for quantifying migration of highly
sorbing RN in clayrock is direct measurement of Da values (cf. Fick’s second law, §1) in intact
samples under conditions representative of the GBS. While this approach has been used in SC, it is
much more frequent to estimate Da using based on (i) assumed De values for cationic
RN and (ii) Kd datasets measured on crushed rock samples. The reason for this is evident from
Fick’s second law which tells us that the rate of propagation of a sorbing RN in a clayrock (i.e. the
distance travelled by the migration front away from the source in a given period of time) will be
inversely proportional to its Kd value. For example, for a RN having a Kd = 10 m 3 kg -1 (order of
magnitude for actinides) and De = 10 -11 m 2 s -1 (order of magnitude for HTO), RN mass will have
penetrated roughly 0.5 or 1.5 mm into a typical clayrock after respectively 1 or 10 years of contact.
From a practical standpoint, this means that for a one year experimental time frame, one must be
able to quantify RN mass distribution within a sub-mm thickness of rock in order to determine Da.
Despite such a challenge, RTDC3 considered that it was sufficiently important for safety case confidence
building to take it on.
The work program was guided by two main objectives: (i) determining if Kd values measured in
batch systems under varying conditions could be used to estimate RN sorption behaviour in the intact
rock and (ii) determining if De values for strongly sorbing RN could be estimated based on values
measured for non or weakly sorbing tracers. Studies were carried out on three different clayrocks
(Opalinus clay, Boom clay, Callovo-Oxfordian) using elements representing the range of
sorption behaviour for cationic RN: strongly-complexing (Am, Eu, Pu, U), moderately-sorbing (Cs
Co, Cu), weakly-sorbing (e.g. Sr, Na). The main results concern:
Development and application of new analytical methods for carrying out in-diffusion
experiments and quantifying high Kd tracer migration in intact rock;
Comparison of sorption equilibrium model parameter values determined in batch and
compact rock systems;
Comparison of De + sorption parameter value sets obtained by modelling data from indiffusion
experiments with predictions made using an assumed De value and batch sorption
parameters.
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