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Posiva's Olkiluoto Spalling Experiment

An in-situ experiment to determine rock strength at a depth of 345 m in the ONKALO underground research facility.

Based in large part on precedence from research in crystalline rock at Atomic Energy of Canada Ltd.’s (AECL’s) Underground Research Laboratory (URL) in Canada, and at the Swedish Nuclear Fuel and Waste Management Company’s (SKB’s) Äspö Hard Rock Laboratory (HRL) in Sweden, brittle spalling was postulated as a significant process in the rock mechanics features, events and processes (FEPs) in Posiva’s safety case for Olkiluoto, the location of the ONKALO final disposal facility in Finland (Posiva 2012). Consequently, the Posiva Olkiluoto Spalling Experiment (POSE), an in situ Prediction-Outcome (P-O) experiment, was commissioned to assess this process in ONKALO.

The experiment was planned and executed in three phases, all of which involved predictive thermomechanical simulations using a variety of software along with an extensive back-analysis campaign to identify suitable rock mass parameters using primarily 3DEC, PFC2D and FRACOD.

Originally planned to determine:

  • the in situ spalling strength of Olkiluoto migmatitic gneiss
  • the in situ state of stress at a depth of 345 m
  • act as a Prediction-Outcome (P-O) exercise
Resulted in:
  • better understanding in the local in situ stress state
  • the determination of the dominant rock mass failure mechanism at Olkiluoto
  • in situ observations of the effects of the expected thermal stresses resulting from canister deposition