A drop of ink or tea from a tea bag: either substance will spread through water. The force behind this phenomenon is called diffusion. The particles keep moving from the area with high concentration to that with low concentration until they are almost evenly distributed. This becomes visible when ink or tea spreads in a glass of water.
Diffusion also occurs in the Opalinus Clay in which the radioactive waste is to be enclosed. The radioactive particles – known as radionuclides – move through the Opalinus Clay, albeit very slowly. Diffusion experiments are used to investigate how far, how quickly and in what quantity neutral, positively or negatively charged radionuclides move.
Nagra is currently conducting such an experiment together with five partner organisations – including four from abroad. This long-term test, abbreviated to DR-E, is being carried out in the Mont Terri Rock Laboratory. The experiment was launched in January 2023.
First, two vertical holes were drilled into the Opalinus Clay. The geological fault to be investigated was located at a depth of around eight metres, i.e. where the clay rock is fractured and therefore potentially somewhat less tight. A cocktail of radioactive particles consisting of neutral and negatively charged radionuclides has been circulating in the two boreholes since October 2024. The clay binds these less well, which means that it also retains them less well. The cocktail simulates radionuclidesthat can be found in radioactive waste.
“The experiment will last around three years and the data obtained will be analysed in 2028,” says Nagra project manager Raphael Wüst. In contrast to previous diffusion experiments conducted at the Mont Terri Rock Laboratory, this time, a large-scale natural fault zone in the Opalinus Clay was selected. The main objective is to answer this critical question: how would radionuclides behave if the drifts in the future repository were impacted by an extensive geological fault?
Relevant for safety
“In three years, the rock affected by the experiment will be removed, and we will analyse how quickly and in what concentration radioactive substances could move in the damaged Opalinus Clay,” says Raphael Wüst.
The new long-term experiment in Mont Terri is relevant with regard to the safety of the repository. Nagra must demonstrate that a repository constructed into the Opalinus Clay will comply with the legal dose limit over a very long period of time. This means that even in the worst-case scenario, only minute quantities of radioactive particles – within the scope that is still harmless to nature and humans – can be released from the repository into the environment. Previous diffusion experiments have shown that this criterion can be met under controlled conditions. At that time, however, the investigation focused on intact sections of Opalinus Clay. The objective of the ongoing experiment, on the other hand, is to more thoroughly investigate the behaviour of radionuclides in larger fault zones.
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