Alps laboratory tests methods of storing nuclear waste
intended to stop the radioactive particles: bentonite. This type of claystone swells and seals even the tiniest cracks.
Will this material provide the protective shield as scientists hop? In order to test this, just under twelve years ago researchers walled in two metal cylinders here — exact replicas of nuclear waste containers — using bricks made of bentonite. After the container mockups were entombed, researchers heated them up to 100 degrees Celsius, the typical temperature of spent fuel rods when they are placed in final storage. After five years, the researchers dug out one of the containers and analyzed the encasing stone. Despite the heat, the stone had absorbed sufficient water. “The bentonite swells even under prolonged exposure to heat,” says Blechschmidt
The other container is still heating the mountain to this day. Four strands of cable are used to measure water content and temperature. On the floor of the tunnel lie the rails that were used to roll the containers here. The question remains how to maneuver the 28-ton receptacles actually used by the nuclear industry in such narrow tunnels? “The transport is a risk,” admits Blechschmidt.
The scientists studying the various ways to dispose of nuclear waste are aware of one fact: no matter how deep a storage site is located, at some point in time the smallest radioactive particles, radionuclides, will work their way through the rock and reach the surface. “We just have to ensure that this takes a very long time,” says Paul Bossart, who heads the second Swiss nuclear research site in Mont Terri, “so long in fact that they are no longer radioactive after they have made their way through the containers, the bentonite casing and the host rock.”
At Mont Terri researchers intend to use the radioactive isotopes to test this concept. Since 2006 particles have been directed into the rock. Tritium has proven to be the most rapid element. It has covered 24 centimeters (9.4 inches) in the first year, and an additional six in the second year — an extremely slow movement because the particles are transported through diffusion, not in flowing water. Although claystone contains water, this liquid has not moved for millions of years. The pore water comes from the Jurassic Sea and has not mixed with rainwater over the past six million years, while the mountains in the region have been folded upward. Only the ions dissolved in the water are moving. The laboratory director is relieved to see that this diffusion is extremely slow.
“The region’s geology provides us with security for thousands of years,” Bossart says. Then he reflects on the cars rushing through the nearby freeway tunnel and has another thought: “But what will happen in the distant future if people drill into a deep geological repository because everyone has forgotten where the nuclear waste has been buried?”
