RD&D-Programme 2004 - SKB

Newfound knowledge since RD&D 2001
Advection is a process that is generally well-understood /19-53/. Accordingly, no new
knowledge has emerged during the present period which contradicts previous knowledge.
However, some additional support has emerged for existing knowledge. An example is
simulation of advective transport in individual fracture planes /19-54/. Even though the study
mainly deals with diffusion processes, certain conclusions can nonetheless be drawn regarding
how the advective and dispersive processes influence transport. Specifically, it is shown that the
effect of transverse dispersion is small with regard to the transport resistance (F factor).
Simulations of discrete fracture networks have been done to understand the dependence of the
transport resistance (F factor) on various properties of the network /19-55/. The results show
that the assumption of complete mixing, or streamline routing, at fracture intersections does not
significantly affect the results. This is important, since it is difficult to prove which assumption
is most correct. Furthermore, it is shown that the results are sensitive to whether the fracture
intensity, expressed by the so-called P32 value, dramatically decreases. Finally, it is shown that
retention is typically underestimated in the type of channel models /19-56, 19-57/ that are often
used for transport calculation in conjunction with FracMan simulations. In the study in question,
transport has been simulated directly in the two-dimensional fracture planes rather than being
simplified to one-dimensional channels. The results in /19-55/ have also been used in a parallel
study /19-58/. This study shows that the distribution of (bv) –1 , where b is the half-aperture and v
is the velocity of the water, follows a power-law distribution. The transport resistance (F factor)
is in turn dependent on the distribution of (bv) –1 . The results indicate that transport that is
characterized by this type of power-law distribution cannot be described by a classic advectiondispersion
equation, but that more sophisticated methods are needed if analytical approaches
are to be used. The results from /19-55/ have also been used in /19-59/, where it has been
investigated whether the transport resistance can be estimated from quantities measured in the
field. Under certain circumstances the transport resistance can be expressed as a linear function
of the advective travel time and an effective retention aperture. This aperture can be estimated
from transmissivity data or fracture density and porosity data. However, the results indicate that
these simplified estimates tend to overestimate retention, which is why explicit calculation of
the transport resistance in a numerical discrete model is preferable.
Numerical simulations have also been carried out with the code Chan3D for transport in
transient flow fields /19-21/. In this study, transport (advection and matrix diffusion) were
simulated in a flow field that changes with time. The results indicate that transient flow can be
important to incorporate in the analysis of radionuclide transport when shoreline displacement
occurs. This type of study requires a coupled modelling strategy, i.e. that both flow and transport
are simulated in the same model. SKB’s primary modelling strategy is to model flow and
transport separately. To justify such an approach, the limitations must be investigated in detail.
This can be done with the type of modelling tool presented in /19-21/.
Programme
No programme will be initiated to specifically understand advection as a process. However, the
studies of advection and matrix diffusion during transient flow will continue. The objective here
is to understand specifically how transient flow effects that occur due to shoreline displacement
affect radionuclide transport, and how simplified analyses based on separate flow and transport
modelling can be bounded. The purpose is thus to be able to use, in the safety assessment for
example, a coupled model in order to get a feeling for the limitations implied by a simplified
analysis.
A project aimed at studying how the transport resistance (F factor) can be up-scaled from
detailed to block and site scales has been commenced and will continue. Preliminary results
have been presented in /19-60/. The results show how sampling of the transport resistance on
a small scale can be used to calculate the transport resistance on a larger scale by means of
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