- 124 - The Fessenheim NPP is located below the right-bank embankment of the Grand Canal dAlsace (GCA). In the Fessenheim CSA report, EDF recalled that a number of studies had been performed. EDF analyzed four embankment failure modes in these studies, and carried out the following reinforcement work: loss of tightness at the seals: protective embankments built around the site (to divert leaks), reinforcement of the site drainage network (to recover any water that percolated through these protective embankments and discharge it downstream) and monitoring of the body of the embankment (to check that there is no saturation, prevent and detect leaks in a normal situation and after an earthquake) with predetermined alert levels allowing appropriate intervention; failure by internal erosion: injections into the embankment; failure by overtopping26 due to settling caused by an earthquake. ASN considers that the approach adopted by EDF for studying embankment failure is satisfactory. With regard to the state of the embankment and its general understanding, ASN considers that the permanent monitoring and seismic alert measures are appropriate. Similarly, ASN considers that the preventive work completed improves the stability and watertightness of the potentially fragile areas. 3.2.2 Measures envisaged to reinforce the robustness of the facilities to the flooding risk Based on the results presented above, ASN asked EDF: to state whether additional protection measures can be envisaged or implemented (depending on the time between the alert and the flood); to indicate the weak points; to specify any cliff-edge effect27; to identify the buildings and equipment that would be flooded first; to state whether steps could be envisaged to prevent these cliff-edge effects or reinforce the robustness of the facility (design modification, procedural modifications, organisational measures, etc.). In its CSA reports, EDF envisages various solutions according to the cliff-edge effect identified and the maximum scenario which led to this cliff-edge effect. The following table identifies the various EDF proposals: Maximum flood scenario Maximum rainfall scenarios and structural failure scenarios for an earthquake bigger than design-basis When a cliff-edge effect linked to an H3 situation is identified EDF proposes studying a solution to reinforce the protection of the equipment necessary for operation in an H3 situation. EDF proposes studying the plausibility of a risk of the presence of water on the nuclear island platform. Based on the results, EDF will determine whether additional protections are necessary. When a cliff-edge effect linked to an H1 situation is identified For some sites, EDF proposes studying the need for reinforced protection of the pumping station. EDF identified no measures allowing reinforcement of the robustness of the facilities. When a LOOP cliff-edge effect is identified EDF did not propose any measure to reinforce the robustness of the facilities. EDF proposes studying the plausibility of a risk of the presence of water on the transformer platform. Based on the results, EDF will determine whether additional protections are necessary. 26 Overtopping is the river flowing over the top of the embankment. This generally leads to external erosion and rapidly entails breaching of back-filled structures. 27 Cliff-edge effect: major discontinuity in the scenario, leading to a significant and irreversible worsening of the accident
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