Because of their high clay content, Opalinus, "Brauner Dogger", Effingen Member
and Helvetic marl
are in principle suitable types of host rock for an L+ILW repository. This high content of clay leads to rock good self-sealing characteristics, which protect against fissures, and offer a large surface area to bind up radioactive substances; the vicinity of such a deep geological repository would be almost impermeable by water. With the exception of Helvetic marl
, all the host rock formations are located in potential sites in northern Switzerland. In the Alps, large accumulations of Helvetic marl occur in only a few places; the only site with the necessary area for a L+ILW repository is found at Wellenberg.
Opalinus is a clay formation that was formed from marine material deposited approximately 175 million years ago. Its distinctive features are the high clay content and its homogeneity. The opalinus clay layer is more than 100 meters deep, and above and below it there are sediment layers rich in clay and marl with good retention qualities for radioactive substances justifying the selection of this rock as the host a high-level waste (HLW) repository.