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ganic/organic colloids”; WP 4.5 “Process identification and verification by real system analysis”
and, finally, WP 4.6 “Up-scaling of processes”.
In RTDC4, a large experimental work-plan was settled to gather data under well-defined conditions
for assessing the effects of the rock heterogeneities on the distribution of the groundwater flow, matrix
diffusion and sorption. Additional aims of the studies performed were to evaluate the adequacy
of models to describe transport in this complex environment and to develop up-scaling methodologies.
Reactive transport models, at present, do not take into account the complex dynamics inherent to a
heterogeneous reactive transport system increasing the uncertainties on radionuclide migration prediction.
In RTDC 4, efforts were made to quantify effective reactions and transport in heterogeneous
media for a more realistic large scale modelling and to implement concepts and modelling
frameworks in numerical PA tools (WP 4.6) [2]. The dependence of transport parameters on the
scale was analysed and different methodologies were compared. Advantages and limitations of upscaling
methodologies in the context of underground radioactive repositories were evaluated (WP
4.6) [3].
The characterisation of water flow-paths in crystalline rocks is essential for the analysis of the main
migration and/or retention processes: novel and complementary experimental approaches were proposed
to analyse the rock matrix and fractures structure at different scales. The effects of physical
and chemical rock heterogeneity on the main parameters needed for PA calculations (i.e. distribution
coefficients, diffusion coefficients or porosity) were analysed within different work packages.
Even when the processes are well understood, for example in the case of matrix diffusion, it is
sometimes recognised the difficulty to obtain relevant experimental data as input for transport models.
The development of new experimental methodologies may overcome these difficulties and
represents additionally a scientific challenge.
At present, some processes are neglected in PA because they present high degree of uncertainty or
because of data obtained in realistic conditions are too scarce. In crystalline rocks, colloids are neglected
because of their low concentration in the far-field. However, recent studies showed that
bentonite colloids can be generated from the engineered barriers [4] and that they could be particularly
relevant for the migration of high sorbing elements as tri- or tetravalent actinides [5], thus, in
RTDC4, they were thoroughly studied. The complete description of radionuclide transport in the
presence of colloids needs to understand colloidal behaviour in crystalline groundwater (generation,
stability, colloid-RN interactions and rock-colloid interactions). All these aspects were dealt with in
different WPs of RTDC 4.
Lastly, since models developed on the basis of results obtained from laboratory tests might not be
directly applied to field conditions, field studies play an important role. Apart from providing sitespecific
data, they allow investigating the migration processes at a larger spatial scale and under
almost “real” conditions. Successful research programmes have been often based on a continuous
feedback from laboratory to in-situ data.
As an example, in the frame of WP4.5, hydrological and geochemical data from the Swedish Forsmark
and Simpevarp sites are analysed as a support of the safety case for nuclear waste disposal in
fractured crystalline rock in Sweden. In-situ studies were also carried out at the FEBEX site (WP
4.1 and 4.2). The FEBEX experiment reproduces at a real scale a high-level waste repository in
granite and it was installed more than 10 years ago. At moment, it represents the only realistic environment
where the processes affecting RN migration from the bentonite to granite can be studied.
In summary, the main aim of the work carried out within RTDC4 is to obtain (realistic) data, both
from laboratory and in-situ for the validation of existing models, and to improve the knowledge on
less known processes to facilitate their inclusion in PA models (e.g. colloid behaviour). In this paper,
a short description of the main processes studied in RTDC4, including a summary of the main
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