Euradwaste '08 - EU Bookshop - Europa

According to a clearly-defined and demonstrable physical approach to quality, the reliability of the
safety assessment of a long-term repository relies on our capability to describe the different phenomena
occurring over time. In certain cases, those phenomena may be coupled, as in the case of
expectable thermomechanical effects after the emplacement of highly-exothermal packages. As for
processes likely to occur over the very long term (i.e., mostly chemical processes and radionuclide
migration), a sound physical control reduces the propagation of uncertainties relating to an insufficient
description or model. The required parameters to describe that process must also be known
with an appropriate level of precision, and the associated uncertainties must be submitted to essential
sensitivity tests in order to ensure that the repository meets its operational and performance targets.
In order to provide a sound description of the repository’s determining phenomena, I would
like to point out the methodology developed and implemented by Andra, which consists of a phenomenological
analysis of repository situations and a description of the phenomena tracking down
the evolution of the repository at each space and time field. From the previous example, we may
retain the thermal effects relating to the presence of exothermal packages that will need to be taken
into account throughout the heat-release period of the packages. In the case of vitrified waste, heat
releases decrease after a few centuries and cease to determine the behaviour of disposal structures
over time. Inversely, any phenomenon involving radionuclide migration may only be taken into
account once waste containers are corroded. That type of extremely rigorous and systematic analysis
conducted over the entire repository avoids excessive couplings between the thermal, mechanical,
hydraulic and chemical characteristics of the repository, which are not always necessarily described
properly. The reader may refer to Andra’s Dossier 2005 for complementary information on
the selected approach for the phenomenological analysis of repository situations and a sound perspective
view of the phenomena in relation to the space and time dimensions to be taken into account
[4].
Keeping in line with the demonstration capability and, consequently, with the confidence that our
researchers may have in their models, the goal will be to seek the simplest designs possible. The
purpose is not only to describe and model easy geometries, but also to favour certain operating conditions.
With reference once again to the Callovo-Oxfordian argillites mentioned in the Dossier
2005, the temperature is limited to 90°C on the walls of disposal structures, with a view to staying
within a describable operating field without having to go through more complex diphasic behaviours.
By opting for simplicity, not only does modelling become easier, but the demonstration as
well.
However, some inevitable fields of physics remain pending and insufficiently described, as in the
case of gas production and behaviour throughout the lifetime of the repository. All organisations in
charge of 0-waste management have agreed to raise the issue among the priorities for the
7 th Framework Project for Research and Development (FPRD), and the FORGE Project should provide
responses to fulfil the ambitions of the safety demonstration for waste repositories.
Physical modelling is therefore a determining factor. However, we are confronted with space and
time scales that are way above the field of experience. All investigations conducted in many surface
laboratories and all experiments carried out in underground laboratories will only cover several
decades at best, whereas safety demonstrations extend over timescales that are much longer by
three to five orders of magnitude. It is therefore necessary to deepen the analysis over several tens
or hundreds of millennia. Once again, the quality of the analysis will rely on the quality of the
simulation means and of the digital approaches. Qualified methods must be available in order to
change both space and time scales efficiently, to analyse the impact of parameter uncertainties and
to study the nature of the various effects throughout the lifetime of the repository. Generally speaking,
it will be possible to analyse the impact of a leaking package or of a borehole intrusion within
the repository. Today, thanks to the growing efforts in modelling and in digital simulations for
more than 10 years, we may considerer the “black-box era” to be obsolete and welcome rather increasingly
clear and convincing descriptions. The numerous projects supported by the EC will have
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