The long-term performance of a claystone barrier for a high-level radioactive waste repository in Switzerland: An evaluation using multiple lines of evidence

Andreas Gautschi1
1 Bereich Geologie, Sicherheit, Nagra (Nationale Genossenschaft für die Lagerung radioaktiver Abfälle)

O 4.2 in Endlager und Untertagedeponien

15.04.2016, 10:00-10:15, Audimax A, Geb. 30.95

In Switzerland, the Nuclear Energy Law requires the disposal of all radioactive waste in deep geological repositories. The Swiss program foresees two types of repositories: one for low- and intermediate-level waste (L/ILW) and one for spent fuel (SF), vitrified high-level waste and long-lived ILW. The procedure for selecting the repository sites is defined in the Sectoral Plan for Deep Geological Repositories (SFOE 2008) and the corresponding process is managed by the Swiss Federal Office of Energy. The Sectoral Plan foresees the selection of sites in three stages, the third stage leads to a General License. Stage one of the Sectoral Plan has been concluded with the proposal of four different sedimentary host rocks for the L/ILW repository, for the HLW repository the Opalinus Clay formation in Northern Switzerland has been selected (Nagra 2008). The envisaged repository depth is between 450 and 900 m. The ongoing Stage 2 of the Sectoral Plan aims at narrowing down potential geological siting region (Nagra 2014a,b). During Stage 3 the remaining sites will be investigated in detail (deep boreholes, 3D seismics).

The Opalinus Clay is a 80 – 130 m thick marine claystone formation of Jurassic age (Aalenian). An extensive site and host rock investigation programme has been carried out during the past 30 years comprising extensive 2D seismics surveys, a 3D seismic campaign covering 50 km2 in northeastern Switzerland, a series of deep boreholes within and around the potential siting regions, experiments in the International Mont Terri Underground Rock Laboratory in northwestern Switzerland, and compilations of data from Opalinus Clay in railway and motorway tunnels.

The properties of the geosphere barrier that are relevant from the viewpoint of long-term safety can be summarised as follows:

  • diffusion is the dominant transport mechanism, with advection playing a secondary role, and clay-rich confining units over- and underlying the Opalinus Clay host rock act as a supplementary barrier to migrating radionuclides.
  • faults in the Opalinus Clay do not represent preferential flow-paths, which is attributed to an efficient self-sealing mechanism.
  • stable, reducing geochemical conditions are present and the host rock has favourable sorption properties
  • the self-sealing capacity of the Opalinus Clay minimizes the effects of perturbations caused by the repository (excavation damaged zone)

The main conclusions are supported by multiple lines of evidence demonstrating consistency among hydraulic properties (tests and observations at various scales), pore water geochemistry, laboratory- and in-situ diffusion experiments, as well as distribution of natural tracers across the Opalinus Clay.

 



SFOE (2008): Sectoral Plan for Deep Geological Repositories – Conceptual Part. Department of Environment, Transport, Energy and Communications (DETEC)/SFOE. April 2008, Bern, Switzerland.

Nagra (2008): Vorschlag geologischer Standortgebiete für das SMA- und das HAA-Lager. Geologische Grundlagen.  Nagra Technical Report NTB 08-04. Nagra, Wettingen, Switzerland.

Nagra (2014a): SGT Etappe 2: Vorschlag weiter zu untersuchender geologischer Standortgebiete mit zugehörigen Standortarealen für die Oberflächenanlage : Sicherheitstechnischer Bericht zu SGT Etappe 2 : Sicherheitstechnischer Vergleich und Vorschlag der in Etappe 3 weiter zu untersuchenden geologischen Standortgebiete. Nagra Technischer Bericht NTB 14-01. Nagra, Wettingen, Switzerland.

Nagra (2014b): SGT Etappe 2: Vorschlag weiter zu untersuchender geologischer Standortgebiete mit zugehörigen Standortarealen für die Oberflächenanlage. Geologische Grundlagen. Nagra Technischer Bericht NTB 14-02. Nagra, Wettingen, Switzerland.

 



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