"Thud!”. A dull bang reverberates through the fluorescent-lit tunnel. A faint bleep echoes back. Then again: Thud. Bleep. Thud. Bleep. James Irving, a post-doc student at the University of Lausanne, is sitting on a chair in the semi-darkness. Behind him can be seen a brightly lit shotcrete wall with seismometers installed on its surface, sensitive measuring instruments that record seismic waves. After the fourth bang, James pulls the cable 25 centimetres further out of the borehole. The bang is audibly louder. Unimpressed, Magie Maurer sits in front of the monitor screen observing the pattern of sound waves recorded by the computer. Hansruedi Mauer, the project manager from the ETH Zurich Institute for Geophysics (1) occasionally takes up the slack in the cable and coils it.

Measuring with no holes in the concrete seal

The three researchers are in the Nagra Rock Laboratory on the Grimsel Pass at the end of a tunnel 1300 metres long and with 400 metres of mountain above them (2). On behalf of the British company Nirex and in cooperation with Nagra they are currently testing a new measurement method designed to enable the state of storage tunnels in a future deep geological repository for radioactive waste to be checked without damaging the concrete seal several metres thick by drilling holes through it. Maurer explains that “Every hole is a potential weak point.” He says that is why a search is underway for non-destructive measurement methods to check the properties of the repository without needing to have sensors in it.

To test their measurement method, the researchers drilled six holes 25 metres long into the granite around the concrete seal, running past the potential repository tunnel and several metres away from it. Into three of the holes the researchers introduce a sparker that generates seismic waves. The sparker is basically an oversized spark-plug in which a 5000 volt charge builds up, then discharges in a flash. That makes the bang. The researchers fire four shots every 25 centimetres. Thousands of shots are needed to allow enough data to be collected. The sound waves from the shot penetrate through the rock, and behind the concrete seal they hit a bentonite test specimen about one metre thick that completely fills the end of the tunnel. This material scatters the waves back or reflects them. The sound waves, which are really mini-earthquake tremors, will also be reflected differently if the properties of the bentonite change. The researchers want to exploit this to obtain information about changes in the future radioactive waste repository without destroying the concrete seal.

A three-dimensional representation is possible

Each of the 24 highly sensitive seismometers positioned along a cable in the remaining three holes and on the surface of the concrete seal record the sound waves scattered and reflected in the rock and at the bentonite test specimen. The signals are collected and processed in a computer. The boreholes are arranged in such a way that three-dimensional information about the properties of the rock and of the materials introduced into it can be derived from the recorded signals.

IETH Zurich geophysicists led by Hansruedi Maurer (r.) test new measurement methods in the Nagra Rock Laboratory. (Photo: comet photoshopping / Dieter Enz) large

The British client is interested mainly in how materials, for example bentonite or concrete, stored in a deep repository in granite behave under the influence of natural resaturation.

Bentonite makes the seal

Because of its special properties, bentonite is currently the preferred material to encapsulate containers of waste radioactive material, e.g. spent fuel rods from a nuclear power station. Because of its physical properties, the porous clay-like material saturates fully when it comes into contact with water. As a result its volume increases and provides an additional seal for the repository area. The volume of bentonite in the fully saturated state is about three times as large as when dry. For a deep underground repository, for example, this means that groundwater entering the storage tunnel from the rock cannot penetrate through the bentonite layer and cannot leach out radioactive material and carry it away.

Maurer explains that “We are going to start saturating the bentonite test specimen in the rock laboratory at the end of March.” This will also cause its properties to change rather quickly. The geophysicists want to measure these changes again in the third week in April. However, the researchers are interested mainly in how the bentonite changes over a period of years. What happens if the bentonite becomes fully saturated with water? Does its volume stay constant? Will it seal well enough if groundwater enters the tunnel?

ETH Zurich is to contribute to the solution

The experiment is designed to last about three years during which the ETH Zurich geophysicists will have to wait patiently at regular intervals, swallowed up by the mountain in this dark tunnel at a temperature of 13°C. “Thud!”. The bang is now really loud and hurts the ears. The sparker is even closer to the borehole exit. James is wearing ear protection. Three more bangs, two, one, complete. The series of measurements is finished for today. It has gone well. They have finished sooner than planned, and Hansruedi Maurer is visibly satisfied. For him this research is not a question of whether or not one is convinced about nuclear power stations. He says the waste exists now, that is the real situation, like it or not. That’s why ETH Zurich as a national institution is under an obligation to contribute to solving this problem.