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Title Acoustic Emission and Ultrasonic Survey Monitoring at the SKB Prototype Experiment During Heating and Pressurisation

Authors Jonathan R. Haycox, William S. Pettitt, Juan M. Reyes-Montes, R. Paul Young and Lars-Erik Johannesson

Publication Reference Impact of Thermo-Hydro-Mechanical-Chemical (THMC) processes on the safety of underground radioactive waste repositories: An international conference and workshop in the framework of the European Commission TIMODAZ and THERESA projects. Luxembourg, 29th September – 1st October 2009. "Book of Abstracts" pp35.

Abstract The Prototype Repository Experiment at SKB’s Hard Rock Laboratory in Sweden simulates a disposal tunnel in a deep repository for the disposal of high-level radioactive waste under realistic conditions and on a full scale. Acoustic Emission (AE) and ultrasonic monitoring are used as a tool to remotely monitor, and quantitatively measure, the damage and disturbance around the excavated canister deposition boreholes. AE monitoring is a ‘passive’ technique similar to earthquake monitoring but at a smaller distance scale and higher frequencies, able to detect the opening or mobilisation of small fractures with source dimensions of millimetres. Ultrasonic surveys use transmitters to send signals to an array of receivers to ‘actively’ examine the rock. Variations in amplitude and velocity along the raypaths can be interpreted in terms of local changes in the rock properties. This paper describes the results for a 30-month period when the deposition holes were subject to changes in temperature and pressure. Heaters were turned on in March 2003 causing temperatures to increase rapidly. The increase in observed velocity and amplitude values is interpreted as a closure of microcracks and pore spaces in the excavation damaged zone and surrounding stress-disturbed volumes indicating that thermal loads are acting to increase the stresses around the deposition hole. The majority of the AE events locate within 0.2 m of the deposition hole wall in the NE and SW quadrants coinciding with regions of increased compressive stress from stress modelling of the excavations. The stress changes induce small-scale movements on pre-existing microfractures created during excavation, or the opening of new microfractures in weaker volumes of the rock. Temperatures reached a more steady state between February 2005 and August 2005, but there are rapid and significant temporal changes in pressure. This period is characterised by relatively small changes in average velocity and amplitude, and a lower AE rate, suggesting that the rock has stabilised. The primary contribution to this is believed to be the confining pressure applied from the deposition hole, through a combination of thermal and mechanical loading, producing a cessation in AEs as movement on macrofractures and microcracks is inhibited; a response observed in laboratory rock tests and at the Tunnel Sealing Experiment at AECL’s Underground Research Laboratory.

Key Figure

Link http://www.itc-school.org/index.php/Present-Courses/Impact-of-Thermo-Hydro-Mechanical-Chemical-THMC-Processes-on-the-Safety-of-Underground-Repositories.html



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