Test and Evaluation of Monitoring Systems (TEM)

The monitoring of the low-pH shotcrete plug at the GTS is currently scheduled to continue until the target saturation of 4.5 MPa in the bentonite buffer has been reached or until plug failure. Continuation of the monitoring demonstrations within the MoDeRn project is under discussion.

A limiting factor for the wireless magneto-inductive (MI) transmission technique is the battery capacity of the data logger and MI transmitter which was originally designed for a 3 year period. The two instruments have been in continuous operation since January 2007. Discussions about the installation of a new wireless sensor network have been initiated.

Numerical simulations demonstrated that temporal changes in the monitored features should produce significant changes in the seismic waveforms. Future steps will address full waveform seismic tomography to investigate whether it can be used to monitor radioactive waste emplacement galleries, given high instrument fidelity and effective noise suppression.

Wireless monitoring

• Within the framework of TEM a wireless magneto-inductive data transmission system was installed. Since January 2007, the data of six sensors (two water content, two pore pressure and two total pressure) has been transmitted continually to the data acquisition unit at distances of ~100 m (in the beginning) and ~30 m (since June 2007)
• Some local magnetic noise sources of unknown origin or low battery power may be responsible for data transmission error messages. Although the error messages appear frequently, the quality of the transmitted data has not been affected. Battery power remained basically constant until the end of 2010.
• The acquired data provides valuable additional information on the heterogeneous evolution of the water saturation and pressures within the ESDRED bentonite buffer.

Seismic tomography

• Numerical simulations demonstrated that realistic changes in the fluid saturation of the bentonite fill can produce significant and detectable changes in seismic data recorded outside of the volume of interest.
• A quantitative methodology to assess data quality requirements for successful monitoring has been developed. The requirements relate to repeatability, reliability and bandwidth. While the available source fulfils these requirements the repeatability of removable hydrophone recordings is limited. Firmly attached geophones show better repeatability at the expense of lower bandwidth.
• The modelling investigations also indicated that crosshole seismic methods are likely to provide useful information for the remote monitoring of high level radioactive waste repositories over repository-scale distances. The monitoring boreholes and tunnels need not intersect the repositories, but the closer they approach the repositories, the higher the resolution and reliability of information provided by the seismic data.

Test and Evaluation of Monitoring Systems (ESDRED / TEM)

Non-intrusive monitoring techniques have the advantage that the technical barrier and surrounding excavation damaged zone are not disturbed by the monitoring system.

Seismic measurements at the ESDRED / TEM project site

Test and Evaluation of Monitoring Systems (ESDRED / TEM)

The full-scale low-pH shotcrete plug was constructed in 2007 in the context of ESDRED and the TEM project has been part of the experiment set-up right from the start. While the overall goals are to demonstrate the construction feasibility and to test the support capacity of the plug, the particular objectives of the TEM project have been:

• To investigate the efficiency of an existing wireless magneto-inductive (MI) transmission technique and
• To evaluate seismic tomography as a non-intrusive monitoring technique.

In addition, the TEM instrumentation provides supplementary data for the hydration process in the bentonite buffer.
Since 2009 these objectives continue to be pursued in association with MoDeRn.

In the current experiment set-up the monitoring concept encompasses three completely separate systems to measure and compile data concerning total pressure, pore pressure and water content in and around the plug and bentonite section:

• A conventional system with sensors being connected to the data acquisition unit via cables
• A wireless system which uses a magneto-inductive technique to transmit data from a data logger “in” the core of the experimental set-up to a data acquisition system in its vicinity
• Non-intrusive seismic tomography which employs 6 boreholes positioned at a certain distance around the low-pH shotcrete plug.

Perspective view of the experimental layout and monitoring installations

Test and Evaluation of Monitoring Systems (ESDRED / TEM)