Euradwaste '08 - EU Bookshop - Europa

Future Research on Coupled Processes
Most transformations and transport mechanisms in the EBS are driven by processes that show various
degrees of physical (thermo-hydro-mechanical) and chemical coupling. In the past, such transformation
and transport mechanisms in the near-field have been experimentally investigated under
strongly simplified conditions. In future research investigations both “simple” and coupled processes
need further investigations.
Future investigations on the study of uncoupled or weakly coupled processes shall focus on fundamental
mechanisms and the lifetime of individual components of the engineered barriers, for example
the long term corrosion and dissolution processes and rates of spent fuel and glass, or the reaction
of silicon with the near-field materials. For these long term processes methodologies need to
be developed on how to evaluate the long term waste form behaviour for different reference disposal
concepts and scenarios.
Processes with significant THMC coupling, that need further research investigations, are related to
bentonite swelling; bentonite interactions with steel and cement; gas generation and gas transport in
buffer, host rock, shafts and seals; self-sealing of bentonite and clay host rocks; reactive mass transfer
under two phase conditions and formation of reactive barriers; and processes related to drying
and re-saturation of the near-field. Again, experimental and modelling investigations focusing on
more realistic conditions at the detailed and system levels should have high priority, and can be
supported by recent advances in computing. Post-test analysis of large scale validation experiments
is as important as model validation, and complex interacting boundary conditions in the near-field
impacting the waste form behaviour and nuclide transport need to be studied further.
The description of such processes with numerical models is very difficult because of the complexity
of the models and the difficulty to specify the appropriate parameters and boundary conditions. It is
therefore essential to also work towards simplifying these coupled process models in an adequate
way to allow for appropriate sensitivity studies by mapping the possible parameter ranges for the
most important parameters.
Future Research on Heterogeneous Systems
The most important heterogeneities in the EBS, that deserve further investigations, are expected to
occur along the interfaces between the different components, e.g. the canister/buffer interface, and
the buffer/rock interface. Heterogeneities within individual engineered barriers are less important,
mainly because the homogeneity of these barriers can be directly influenced by the fabrication process.
The heterogeneities observed in the bentonite buffer during “short term” experiments relate to
resaturation transients with differential wetting, and are expected not to have any impact on the diffusive
radionuclide transport occurring at later times. The impact of heterogeneities along interfaces
between EBS components on the performance of the whole EBS system could be evaluated
with large scale experiments and numerical models.
Heterogeneities in the near-field host rock can be natural and repository induced. Excavation induced
heterogeneities (EdZ and EDZ) are known to exist for decades and have been studied in the
past intensively in various host rocks, but there are still some open questions on how to address the
uncertainty in large scale behaviour in a reliable manner. Much less is known on the properties of
geological heterogeneities in clay rocks, because these rocks appear as homogeneous at first glance.
However, more recent studies in clay rocks (both indurated and soft) show that they contain many
geological discontinuities which impact the mechanical rock mass behaviour and potentially some
important THM coupled processes in the near-field. Therefore more experimental data is needed to
understand the impact of clay stone discontinuities on the performance of the near-field.
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