Safety of the Swiss repository: evidence for clay host formation barrier efficiency from the geological past

In the reference scenario of the safety case for the Swiss repository the peak individual dose for people in the exfiltration area of ca. 0.2E-4 mSv/y is reached 0.8 Mio years after closure of the repository. The largest part of this dose is contributed by I-129. Iodine is non-sorbing on the charged surfaces of the clay minerals in the host rock. In contrast, any cations released from the technical barrier system are effectively retained by sorption within the geological barrier. The calculated dose maximum is 500 times lower than the regulatory limit of 0.1 mS/yr and 2500 times lower than the individual radiation exposure in Switzerland. The repository system therefore offers large safety margins.

The timing and maximum of the I-129 flux from the thick clay rock package forming the natural barrier of the repository into the overlying and underlying aquifers is derived from a 1D diffusion calculation. Confidence into the underlying assumptions and parameters of this calculation is gained by natural in situ analog data. Vertical profiles of Chloride measured in the porewater of the clay rock sequence confirm the assumptions for Iodine migration in the geological barrier. The close similarities between Chloride and Iodine transport in the geosphere allow to directly conclude from geological archives to dose prognosis.

We present data from natural tracers in the pore water of the clay rock sequence indicating that diffusion is the only relevant transport process for radionuclides. The thickness of the diffusion dominated package can be directly derived from the shape of the tracer profiles. Built-up times for the observed vertical distribution of the tracers between the bounding aquifers can be related to the Quaternery landscape evolution. We therefore conclude that the prognosis for the Iodine dominated dose maximum more than 2 orders of magnitude below the regulatory limit is well founded by the observations of Chloride tracer behavior in the geological past.