As a measure following on from the reactor accident at Fukushima, Japan, ENSI ruled on 1 April 2011 that the Swiss nuclear power plant operators must furnish proof of their ability to control a very unlikely extreme flood, and must do so by the end of June 2011. In order to provide this proof, the Mühleberg nuclear power plant arranged for model tests to be conducted by the Swiss Federal Institute of Technology (ETH), Zurich. These tests led to the finding that in specified extreme situations, gravel movements between the Wohlensee dam and the Mühleberg nuclear power plant could result in a blockage of the special emergency system water intake. A blockage of this sort could lead to core damage in case of an extreme flood.
The description of the event is only available in German.