RD&D-Programme 2004 - SKB

work will continue, particularly in international cooperation. Moreover, newfound knowledge
in the subprogrammes presented below (e.g. model development, transport processes, forest
ecosystems, mireland, sediments) will be intercepted.
20.4 Model development
SKB’s modellings of radionuclide transport in the biosphere in the safety assessment have been
carried out with the tools Biopath and Prism. These have been developed by Studsvik with
support from SKB since the 1970s. The tools have been utilized for the KBS studies, SFR,
SKB 91, SR 97 and Safe and have been gradually refined by work by e.g. SKB /20-8 to 20-11/.
The models represent a holistic viewpoint which was pioneering in the environmental field in
the 1970s. At that time they were also regarded as advanced numerical tools. The models were
based on releases around nuclear power plants and were later adapted to a hypothetical deep
repository, but it was still assumed that the release takes place directly in the recipient as an
annual unit release. The model concept has largely been taken over in most models that handle
radionuclide transport in the biosphere in other countries, for example /20-12, 20-13/. The concept
is based to a large extent on the use of generic factors, for example transfer and bioaccumulation
factors, which presumes that the system being modelled is in equilibrium. Furthermore,
the transfer factors are based in many cases on empirical data without a mechanistic explanation
/20-14, 20-15/. The models describe the pathways that affect man and human food, while other
parts of the biosphere are seldom dealt with.
These simplifications may be warranted for safety assessments where doses to man are
overestimated, but they are insufficient for a proper understanding and an explanation of the
simplifications. A thorough validation of the data is moreover difficult when alternative models
and conceptualizations are lacking /20-16/.
It is stated in RD&D 2001 that alternative models are needed to validate the assumptions that
are made. Other models are also needed to be able to make use of site-specific information
about processes and states in the ecosystems. Furthermore, a more realistic description of the
biosphere is needed to satisfy the requirements made by the authorities on an analysis of the
future consequences of the deep repository. To estimate the consequences for surrounding fauna
and flora in accordance with the regulations in /20-13/, models are needed that are based on the
flow of radionuclides in the entire ecosystem and not just for specific pathways that are critical
for man, such as well or cow’s milk.
The use of process-based models is an appropriate way to solve some of these problems.
The transfer between compartments is then based on natural processes such as photosynthesis,
degradation, food ingestion, metabolism, nutrient needs, etc. These processes are coupled and
the flows are driven for the most part by the mass balance between the fixation and degradation
of organic material, which is sustained by other flows of organic and inorganic materials
(e.g. oxygen, carbon dioxide, water, nutrients). Proportional flows of radioactive substances
are associated with these flows. The models are general and can be used for all radionuclides.
Even if data are lacking for transfer factors, good estimates can be made of the concentration
in different compartments and organisms. Another advantage is that the models are scalable to
different site and climatic conditions. Many of the conditions are measurable in the field and
are not nuclide-specific, e.g. geometry of the catchment areas, insolation, water balance, and
composition of the ecosystem.
Conclusions in RD&D 2001 and its review
See section 20.2.
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