There is a strong scientific consensus that deep geological repositories are a safe and effective approach to permanently disposing of spent fuel and high-level radioactive waste. Among the different geological formations considered suitable for hosting geological repositories, crystalline rocks are characterised by their high strength, thereby providing cavity stability, low-heat sensitivity and low permeability.
The NEA established its Expert Group on Geological Repositories in Crystalline Rock Formations – the Crystalline Club (CRC) under the auspices of the NEA Integration Group for the Safety Case (IGSC) in 2017, to examine the technical and scientific studies on crystalline rocks. The group’s key objective is to improve the understanding of crystalline rocks for hosting high-level radioactive waste repositories via the exchange of information and state-of-the-art approaches among member countries.
This month the Crystalline Club held its first technical workshop focusing on “Research Methods and Modern Measuring Equipment Used for Site and Rock Characterisation”, with three sessions held on 11, 18 and 25 March 2021. The workshop featured presentations and case studies on fracture and fault characterisation in crystalline host rocks. The participating experts discussed various geophysical and other methods for the detection of fracture and fault location and dimensions.
Estimation of fracture flow is an integral part of safety case development for deep geological repositories. There are various techniques available for fracture characterisation, including field studies, geological logging and lab testing. Although fractures are common in crystalline rocks, the resulting fracture permeability can be resolved by engineered barriers such as waste containment and bentonite backfill.
The workshop also highlighted new technologies and recent developments in fracture and fault characterisation, such as morphostructural analysis using digital terrain models, borehole televiewer imaging and the Posiva Flow Log method. While international co-operation is key to developing solutions that address common challenges related to fracture and fault characterisation, participants agreed that every country requires a unique approach to crystalline rock characterisation that is tailored to its particular national legislations, geological conditions and repository concept.