Geologic Scenarios for the Evaluation of Earthquake Impact on the Deep Geological Repository

pdf NAB 24-28 Geologic Scenarios for the Evaluation of Earthquake Impact on the Deep Geological Repository(4.65 MB)

The proposed site for construction of the Swiss deep geological repository (DGR) is located in one of the seismically quietest zones of Switzerland. Nevertheless, seismic activity is not entirely absent and an assessment of the impact of earthquakes on post-closure safety is requested in the Stage 3 requirements of the Deep Geological Repositories Sectoral Plan. 

The Swiss disposal concept addresses the occurrence of future faulting in several ways:

• The repository will be constructed in a ~100 m-thick clay-mineral-rich host rock. The potential disposal zone itself is characterized by the absence of larger-scale faults (i.e. faults mapped by 3D seismic reflection imaging).
• Detailed 3D seismic surveys were conducted to image the subsurface of the siting region and to identify a zone without faults. 
• The potential disposal zone is bounded by regional fault zones to the north and south. These are likely to accommodate potential future deformation due to tectonic processes during the time period for assessment. A safety distance will be maintained to these area-bounding tectonic elements.
• Should faults reactivate or develop in the host rock or in the backfill, self-sealing will seal those features and maintain hydraulic isolation towards bounding formations, including aquifers. The permeabilities of self-sealed faults do not impact radionuclide transport significantly.
• Should smaller-scale safety-relevant faults be penetrated during construction of the waste emplacement tunnels or sealing sections, a suitable safe distance will be maintained to the actual waste containers.
• The frictional properties of the host rock as well as the in-situ stress conditions make it unlikely that a seismic rupture can nucleate. If any deformation would occur, it would be distributed and therefore produce small strains.
• The backfill material will serve as a mechanical buffer against any potential deformation for the emplaced waste packages for high-level waste and spent fuel.

In this report, the occurrence of earthquakes as the part of the assessment basis for the development of a safety case is discussed. As part of this assessment, features, events, and processes (FEPs), which could promote the occurrence of earthquakes, or which could be influenced by direct impacts of earthquakes (shaking and displacement), are screened. Further, the safety case relies on key attributes and safety functions of repository features. Potential impacts on these by earthquakes are reviewed to assess any possible impact. 

At the surface, the damage due to earthquakes is predominantly caused by the shaking movement induced by the seismic waves. It is well established that at depth, the amplitude of seismic waves is reduced. Unlike structures at the surface, backfilled structures in the underground are unable to shear and deform at levels that would be required to cause structural damage. It is concluded that seismic shaking does not pose a threat for post-closure safety. 

The actual rupture, i.e., the displacement discontinuity along the fault activated by an earthquake, could pose a potential threat to post-closure safety as it could create pathways for radionuclides. In three earthquake scenarios the impact of a rupture in different parts of the repository (adjacent to the repository footprint, in the low and intermediate level waste (L/ILW) sealing system, and in the high-level waste (HLW) repository, respectively) are developed and discussed. Features, events, and processes are screened and potential impacts on component-specific safety functions are qualitatively evaluated to facilitate the treatment of these earthquake scenarios in safety assessment.

If during the construction of the repository, further significant faults would be detected, then the impact of such faults on the safety of the particular repository zone would be investigated. Thus, ruptures through safety-relevant structures and barriers may be actively avoided through maintaining a safe distance to such features or other engineering safety measures. Nevertheless, the developed scenarios cover situations where such measures fail. 

In this report we limit the discussion to the detailed development of earthquake scenarios as part of the assessment basis for the safety case. These scenarios will be evaluated in the post-closure safety assessment.