Once the resin had been polymerised, the overcoring of the shear zone could begin. The aim of this stage is to allow the shear zone material to be examined in the laboratory for the location of the sorbed radionuclides and the effects of the hyperalkaline solution on the shear zone.

Before any work was carried out, the site was visited by HSK (Hauptabteilung fur die Sicherheit der Kernanlagen, Swiss Federal Nuclear Safety Inspectorate) to ensure the radiation protection measures were of the highest level. Once permission was granted, the task of overcoring could begin.

The overcoring was carried out with 380 mm OD triple-barrel coring equipment. This allows the shear zone to be excavated without damaging the rock fabric or the fault gouge material. To minimise the amounts of potentially radioactive waste produced, the drilling was carried out perpendicular to the shear zone. Around nine metres of granite matrix had to be drilled before each shear zone section could be cored.

Overcoring was carried out by Flotron AG and took over three months of drilling.

Drillers carefully control the triple-barrel drilling equipment during overcoring

Chief driller Ismet Zivcic (Flotron AG) examines the diamond drilling crown

The overcored material was then sub-sampled into smaller sections for a range of different analysis. In the first stage, the cores were checked to ensure the radiation doses were low, then mapped geologically to identify the main shear zone and define the sampling strategy.

Urs Mäder (University of Bern) and Colin Biggin (Nagra) prepare to saw an overcore section (the grey area in the centre of the core is a dummy packer placed down the injection borehole)