RD&D-Programme 2004 - SKB

19.2.26 Integrated modelling – radionuclide transport
Conclusions in RD&D 2001 and its review
It was observed in RD&D 2001 regarding model studies that the one-dimensional description
of transport and the dispersion formulation should be evaluated with the aid of more complex
models. It was further observed that Farf31 should be compared with models with a higher
process complexity for retention. The reference was specifically to models with heterogeneity
in diffusion and sorption properties in the matrix. Finally, it was observed that Farf31 should
be updated with respect to how input data are formulated so that information regarding the
transport resistance can be used directly in the model.
In RD&D 2001, it was observed with regard to field studies that further evaluation of True
Block Scale results should be done.
In its evaluation of RD&D 2001, SKI supported the development of transport models for
radionuclides that can take heterogeneity (variable penetration depth) in matrix properties
into account. Further, SKI wanted to see a report that summarized conceptual assumptions,
mathematical formulations and scientific support for Farf31. SKI also observes that if the results
from True indicate that retention can only be described in the context of transport in a threedimensional
network of fractures, SKB should describe the simplification errors that can result
from an application in the one-dimensional Farf31 model.
Newfound knowledge since RD&D 2001
Model studies
In the international Äspö Task Force on Modelling of Groundwater Flow and Transport of
Solutes, Task 4 has been completed and Task 6 is in progress. Task 4 dealt with numerical
modelling of the True-1 experiments; Task 6 deals with upscaling of transport to time and space
scales that are relevant to the needs of the safety assessment.
The modelling in Task 4 has been evaluated, and it turned out that most of the modelling groups
utilized similar conceptualization of the retention processes /19-99/. All groups included matrix
diffusion and equilibrium sorption as the dominant mechanisms in the modelling of the experiment.
However, the predictions exhibit a progressively increasing spread for the more strongly
sorbing tracers (for example cesium). Remaining uncertainties in modelling and evaluation of
results are judged to be mainly due to incomplete knowledge of the flow field in the experiment.
In Task 6, modelling results have been presented for the two initial phases. In these phases,
transport has been modelled in a single fracture (length scale 10 metres) for experimental time
scales and for time scales of relevance to a safety assessment. The purpose of these two exercises
has been to evaluate how different conceptualizations and models behave when the time
scale increases. Final results have not yet been presented. In Task 6C, a fracture network model
with associated retention parameterization has been constructed on a 200-metre scale for use in
the later parts of Task 6 /19-100/. The method presented in this report has also served as a basis
for formulating a strategy for the modelling of transport properties within the site investigation
/19-101/.
An evaluation of how retention mechanisms in connection with radionuclide transport should
be incorporated in modelling in safety assessment has also been done within the EU project
Retrock. Specifically, a number of open questions and recommendations have been compiled.
The final report is expected to be finished in 2004, but it can already be concluded that most of
the questions and recommendations presented are covered by activities in the geosphere part of
this RD&D-programme.
A validity document for SKB’s code for radionuclide transport in the geosphere, Farf31, has
been published /19-102/. The document discusses numerical verification of the code and then
concludes regarding validation that the code is suitable for its stated purposes. These purposes
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