Nuclear repository should also survive the ice age

A nuclear waste repository must be safe. Even if the glaciers were to advance from the Alps back onto the Central Plateau in the distant future. This is why Nagra had sediments deposited in deep former glacial lakes investigated: The layers are around 600,000 years old, much older than the last ice age around 24,000 years ago. And good news for Nagra: it appears that the underlying rock layers have never been eroded by the ice since then.

A team of researchers from the Swiss Federal Institute of Aquatic Science and Technology (Eawag), the Swiss Federal Institute of Technology (ETH) and the two universities of Basel and Bern examined the sediments drilled by the National Cooperative for the Disposal of Radioactive Waste, Nagra, near the small town of Bülach in the Zurich lowlands. The drill core is 278 meters long and tells almost the entire geological history of the Quaternary period, the most recent 2.6 million years of the earth's history.

The original Griffensee reached as far as Bülach

The researchers were particularly interested in the sediments that had been deposited in an elongated lake that must have reached as far as Bülach - a primordial Griffin Lake, so to speak. This primeval Griffensee trough was carved out by the ice of the Alpine glaciers. The resulting lake was then filled in with sediments. The two questions that arose for the researchers were: How old is the trough? And was it cleared out again during later glacier advances? Because even if all climate signals currently point to warming, a cold period could come again at some point. After all, a deep geological repository for radioactive waste should be safe for over a million years - even from renewed glacier advances.

Pore water as the key

The scientists used a sophisticated method to determine the age of the sediments: they determined the concentrations of helium-4 in minute quantities of water from the pore space of the deposits. This is because the stable helium-4 isotope accumulates as uranium and thorium decay in the pore water. And because the decay takes place at a defined rate, it is possible to deduce when the water was trapped in the sediment.

The results have just been published in the specialist journal "Geology": The dated sediment layers in the ancient Griffin Lake are around 600,000 years old. Although the accuracy of the analysis is mediocre at plus or minus 120,000 years, no one has ever dated such deposits this far back. And above all, the accuracy is sufficient to prove that the sediments are significantly older than the glacial advances of the last glacial period. The trough in the solid rock, which the experts refer to as the Strassberg Trough, was therefore never excavated any further. The Opalinus Clay, which lies over 500 meters deeper and was deposited around 174 million years ago, is undisturbed. In other words, even a cheeky advance of the Rhine-Linth glacier from the Grisons and Glarus regions would not be enough to excavate the nuclear waste that may once have been deposited in the Opalinus Clay, according to the current state of knowledge.

Source: Eawag: Aquatic Research Institute of the ETH Domain