An interdisciplinary view of the long-term evolution of repository systems across scales: the iCROSS project
Authors: Bosbach Dirk , Geckeis Horst , Heberling Frank , Kolditz Olaf , Kühn Michael , Müller Katharina , Stumpf Thorsten ,   the iCROSS team
Published: November 2021

Abstract. The interdisciplinary project “Integrity of nuclear waste repository
systems – Cross-scale system understanding and analysis (iCROSS)”
combines research competencies of Helmholtz scientists related to the
topics of nuclear, geosciences, biosciences and environmental
simulations in collaborations overarching the research fields energy
and earth and environment. The focus is to understand and analyze
close-to-real long-term evolutionary pathways of radioactive waste
repositories across nanoscales to repository scales. The project is subdivided into work packages dealing with laboratory
studies, field experiments in underground research laboratories
(URLs), advanced modelling studies and the integration and alignment
of data and information using virtual reality methods. In this sense,
the project structure aims at a holistic view on relevant processes
across scales in order to comprehensively simulate potential
repository evolutions. Within the multi-barrier system of a repository for heat-generating
radioactive waste, a number of complex reactions proceed, including
dissolution, redox processes, biochemical reactions, gas evolution and
solid/liquid interface and (co)precipitation reactions. At the same
time, thermal and external mechanical stress has an impact on the
conditions in a deep geological repository. All those processes are
highly coupled, with multiple interdependencies on various scales and
have a strong impact on radionuclide mobility and retention. In recent
years, substantial progress was achieved in describing coupled
thermal-hydro-mechanical-chemical-biological (THM/CB) processes in
numerical simulations. A realistic and concise description of these
coupled processes on different time and spatial scales is, at present,
a largely unresolved scientific and computational challenge. The close
interaction of experimental and simulation teams aims at a more
accurate quantification and assessment of processes and thus, the
reduction of uncertainties and of conservative assumptions and
eventually to a close-to-real perception of the repository evolution. One focus of iCROSS is directed to relevant processes in a clay rock
repository. In this context, the iCROSS team became a full member of
the international Mont Terri consortium and worked in close
collaboration with international and German institutions in URL
projects. Respective experiments specifically deal with coupled
processes at the reactive interfaces in a repository near field
(e.g. the steel/bentonite and bentonite/concrete interfaces). Within
iCROSS, the impact of secondary phase formation on radionuclide
transport is investigated. At Mont Terri, experiments are in
preparation to study radionuclide transport phenomena in clay rock
formations within temperature gradients and in facies exhibiting
significant heterogeneities on different scales (nm to cm). Beside
those studies, high resolution exploration methods for rock
characterization are developed and tested and the effect of
temperature and other boundary conditions on the strength, creep
properties and healing of faults within Opalinus clay are
quantified. Multiphysics models coupled to reactive transport
simulation have been further developed and applied to laboratory and
field experiments. Results are digitally analyzed and illustrated in a
visualization center, in order to enhance the comprehension of coupled
processes in repository systems across scales. The present contribution provides an overview on the project and
reports selected results. The impact of considering complex coupled
processes in repository subsystems for the assessment of the integrity
of a given (generic) repository arrangement is discussed.