Key points:

• High self-sealing potential of the Opalinus Clay ensures that fractures, when and where formed, do not remain permeable and thus would not jeopardise the barrier integrity over relevant timescales.
• Clay-mineral content is a very good indicator for the self-sealing potential of the host rock and confining units, with values greater than approximately 40 wt.-% (e.g. the Opalinus Clay) exhibiting very high self-sealing and values less than approximately 20 wt.-% exhibiting low or negligible capacity.
• Besides clay-mineral content, sufficient effective stress (or equivalent depth) is required for robust self-sealing. This is achieved in the Opalinus Clay in all the siting regions.
• In view of the long-term development of the repository and related concerns about erosion, a residual overburden of 200 m can be considered as sufficient for robust self-sealing in the Opalinus Clay.