RD&D-Programme 2004 - SKB

concepts or whose descriptions require only minor modification, and identifying the processes
that are specific for KBS-3H. The results and experience gained from the research, development
and demonstration conducted within the programme will naturally be taken into consideration in
the safety assessment.
14.2 Integrated modelling
The development of many of the modelling tools used in the safety assessment and results
obtained with these tools are described in detail under the relevant sections in the coming
chapters. This includes, for example, modelling of groundwater flow, of the chemical evolution
of the buffer and of the evolution of continental ice sheets. Development of tools for integrated
modelling of a number of processes that span a large part of the system is described below.
14.2.1 System development
The system of coupled processes that controls the long-term evolution of the repository is
complex. In the mathematical modelling of the evolution of the deep repository, the process
system is therefore studied with different process models, each of which illustrates a certain
aspect of the total evolution. Often these models handle thermal, hydraulic, mechanical or
chemical processes. Integration of all aspects of repository evolution therefore consists mainly
of a discussion of the results of the process models within the framework of a safety account.
Recently a system model consisting of several integrated submodels has been developed,
where each of the submodels mimics a process model /14-5/. The system of treated processes
is necessarily simplified. For example, the short-duration saturation phase in the buffer, where
the coupling between the processes is strong, is excluded, as is detailed hydromodelling of the
heterogeneous fractured bedrock.
The first version of the system model does not deal with radionuclide transport. A simplified
description is also provided of these phenomena (see section 14.2.2), and may be incorporated
into future versions of the system model.
A principal reason for development of the system model is to obtain an integrated treatment of
the most important processes in the long-term repository evolution, ensuring that a consistent
input database is used in modelling all aspects of the evolution and that time-dependent output
data from one submodel can be used directly as input data to another. Furthermore, this tool is
controlled directly by the safety analyst, in contrast to the previous situation where a number
of disparate expert groups handled their own process models. Several steps have been taken to
speed up the calculations compared with underlying process models. This permits rapid evaluation
of a number of different sets of input data as well as probabilistic calculations. The model
can be executed probabilistically, but so far mainly deterministic results have been obtained,
partly because the database for probabilistic calculations is incomplete, and partly because it is
often not clear that probabilistic calculations are warranted.
Other reasons for developing the model are to gain insight into the relative importance of
various processes at an overall level and to provide quality assurance in the form of an independent
set of models for important aspects of the evolution of the deep repository. However, the
ambition is not to replace the more detailed process models. They will always be needed to fully
include all details in the mechanisms behind individual processes and as a necessary basis for
the development of simplified models. Furthermore, the expertise behind the development of
the process models will always be needed to describe and justify the common scientific basis
for both types of models.
156 RD&D-Programme 2004