Posiva Oy is responsible for implementing the final disposal program for spent nuclear fuel of its owners, Teollisuuden Voima Oy and Fortum Power & Heat. The concept for geological disposal is based on a multibarrier methodology. Due to the geological concept, the knowledge of the properties of the host rock, such as in situ stress conditions, excavation damaged zone and hydraulic conductivity are highly important for the long term safety analysis. For the repository site characterization, an underground rock facility called ONKALO has been excavated at Olkiluoto Island.
Recent research indicates that the excavation damage zone (EDZ) can be meaningfully separated in to construction induced (EDZCI) and stress induced (EDZSI) Excavation Damage Zones. The EDZ has been thoroughly investigated in ONKALO at the ONK-TKU-3620 niche located at -345 meter depth. In the studies that used various geophysical methods and hydraulic conductivity testing, a horizontal fracture without the characteristics of natural fracture was observed. In ground penetrating radar investigations fairly large reflectors occurred at the depth around 50 – 70 em and Mise-a-Ia-Masse surveys gave indications of electrically highly conductive area. The characteristics of the fracture were confirmed and further observed when the investigation area was cut off using wire sawing. Rock mechanics modelling was used to confirm that the excavation of the niche causes a stress redistribution that can trigger stress induced plasticity or fracture growth beneath the excavated floor.
In the research of Siren et al. (2015) a terminology was established for the Excavation Damage Zone (EDZ) in relation to the birth mechanism for both construction induced (EDZCI) and stress induced (EDZSI) Excavation Damage Zones. Understanding the properties of the EDZ are important for the nuclear waste management industry, which has two rock laboratories in Scandinavia, Äspö Hard Rock Laboratory (HRL) in Sweden and ONKALO in Western Finland. The research by Siren et al. (2015) mostly focused on studying EDZCI solely at Äspö HRL, while also the general EDZ studies in ONKALO were presented. A study area for EDZ in ONKALO is located at ONK-TKU-3620, at which signs of stress induced EDZSI fractures have been detected.
