RD&D-Programme 2004 - SKB

are, within safety assessment, to calculate radionuclide transport in fractured rock and to obtain
correct results for the type of problems for which the code is intended. A numerical finitevolume
representation of Farf31 has also been developed /19-95/. Comparative calculations
between the standard Farf31 and the numerical Farf31 confirm that the codes give the same
results. This also entails a verification of Farf31. The numerical Farf31 can also be used to
study more complex conditions that those that are possible in the standard Farf31. For example,
the effects of a heterogeneous matrix or other initial and boundary conditions can be studied.
The standard Farf31 has also been reformulated so that the transport resistance (F factor) from
discrete models can be used directly as input data.
The effects of describing one-dimensional advective/dispersive transport in fractures as in
Farf31 have been investigated /19-54/. In this study, a Farf31-like solution is compared with
a complete two-dimensional solution in simulated fracture planes. The study shows that the
effect of the one-dimensional approximation is relatively small with respect to e.g. the resulting
transport resistance.
A project has been conducted to obtain simple measures of the retention capacity of the
geosphere which also includes natural variability and/or uncertainties. This methodology
applied to Äspö is presented in /19-104/. The study is based on groundwater flow modelling
performed in FracMan/PAWorks /19-57/. Such simple measures of the geosphere’s retention
capacity can be used to compare alternative repository volumes within a site or alternative sites
where knowledge (uncertainty) varies between the volumes or the sites.
Field studies
The experiments within the Äspö projects True Block Scale, True Block Scale Continuation
and True-1 Continuation have been under way for the past three years. These experiments
show clearly that retention of tracers can be observed in the results of tracer tests conducted in
the field. Specifically, diffusion into stagnant (immobile) zones has also been demonstrated.
Although the exact distribution of the different types of immobile volumes has not been
established explicitly, a conceptual model has been presented which contains and quantifies
the presumed immobile components /19-67/.
The porosity variation in the altered zone in the fracture wall and further into the intact rock
has been studied under laboratory conditions by means of the PMMA method /19-68, 19-105/.
A porosity gradient from the fracture surface with decreasing porosity into the matrix has been
observed, although it has not been possible to quantify the phenomenon in statistical terms.
Similar measurements have also been done on pieces of breccia (centimetre-scale) and pieces
of breccia fragments (millimetre-scale); also here a gradient can be noted in the larger pieces.
Quantitative porosity determinations of fine-grained fault gouge with elevated clay content have
not been done, but it is hoped that this type of result will emerge in True-1 Continuation, where
a characterization of fracture zones is included as an important component. Both qualitative
information for geometric conceptualization and quantitative data in terms of porosity variation
are expected to emerge.
Sorption properties have been determined (calculated) for intact rock material by utilizing
known mineralogy, CEC (Cation Exchange Capacity) values, selectivity coefficients and
existing hydrogeochemistry. The usefulness of the method is currently being verified by
laboratory experiments where corresponding sorption properties (K d values) are being
measured on altered material and fine-grained fault gouge.
Tools for numerical modelling have been developed and used to show how groundwater flow
and retention processes affect tracer transport in tracer tests performed in the field over length
scales of up to about 100 metres /19-66, 19-67/. These results clearly show that different
modelling tools, from simple one-dimensional codes with an emphasis on the description of
retention to more complex three-dimensional codes with an emphasis on the geological and
hydrogeological description, have been utilized to describe the experiments. The conceptual
RD&D-Programme 2004 271