- 122 - With regard to the envelope nature of the scenarios utilised, ASN considers that the approach adopted by EDF clearly aims to define maximum augmented hazards covering all the phenomena which could lead to or contribute to flooding, by examining supplementary scenarios for certain sites. The analysis presenting cliff-edge effects induced by the flooding risk, supplied by EDF in the CSA reports, complies with the ASN request. Special case of embankments Following the meeting of the advisory committees in July 2011, the purpose of which was to examine the methodology proposed by the licensees for performance of the CSAs, ASN asked EDF to examine the consequences of a failure of the embankments along the Grand Canal dAlsace close to the Fessenheim site, as well as those of the Donzère canal close to the Tricastin site. Concerning the consequences of a failure of the Donzère-Mondragon canal embankment for Tricastin and the failure of the Grand Canal dAlsace embankments for Fessenheim, EDF provided an answer which should be considered preliminary owing to the lead-times associated with the CSAs. With regard to Tricastin, whether the failure is on the left bank or the right bank of the embankments of the Donzère-Mondragon canal, EDF considers that the existing peripheral protections (sluice-gates, watertight screen) would prevent flooding of the NPP platform. With regard to Fessenheim, the consequences of a failure of the Grand Canal dAlsace embankments would be the presence of a layer of water on the site, liable to lead to a scenario involving total loss of the off-site and onsite power supplies, as well as the potential loss of other nuclear island equipment. Whether for Fessenheim or Tricastin, EDF underlines the absence of any precise study data today available for the height of this layer of water. In the RECS, EDF proposes: Conducting a detailed examination of the ability of the embankments to withstand a level higher than the SSE and to determine a flood flow to be considered beyond the design-basis, In the light of the results, initiating calculation of the corresponding flood fields, If necessary, defining and implementing the appropriate material and organisational countermeasures to prevent the critical situations considered in this kind of analysis, namely significant releases into the environment (for the reactor building case), and fuel uncover (for the fuel storage building case). ASN considers that EDF's undertaking responds in part to its request and that EDF will need to conduct studies giving a precise indication of the water level on the Tricastin site in the event of failure of the Donzère-Mondragon embankments and on the Fessenheim site in the event of failure of the Grand Canal dAlsace embankments and to evaluate the resulting consequences. ASN will issue a requirement on this subject. Strength of the Tricastin embankments The Tricastin Nuclear Power Plant (NPP) is situated alongside the Donzère canal in Mondragon (right bank), to the east of the Rhone river, within the Tricastin nuclear site, which in particular comprises various facilities devoted to the fabrication of nuclear fuel. Cooling of the Tricastin NPP relies on a once-through circuit supplied by the water of the Donzère - Mondragon canal diverted from the Rhone river. EDF has identified two hazards liable to lead to flooding of the site, following failure of the embankments of this canal: earthquake and CMM. In the event of an earthquake, the studies performed by EDF prior to the meeting of the advisory committee in March 2007 concluded that the embankments were stable, subject to effective monitoring and maintenance by their owner, the Compagnie Nationale du Rhône (CNR). Following examination of the dossier, IRSN on the whole confirmed the EDF diagnosis and considered that the two phenomena which could compromise the stability of the embankments are liquefaction and internal erosion at the singularity level of the embarkment body. Concerning the liquefaction risk, piezometry (water height in the embankments) is an essential parameter;
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