- 123 - ASN considers that the current level of embankment monitoring is inadequate and incapable of accurately characterising the piezometry of the canal embankments. Consequently, ASN considers: with regard to the internal erosion risk, EDF will need to identify the local singularities (pipes or buried structures, transition sector between two different types of embankments, etc.) and, as necessary, work will need to be carried out to eliminate the risk of internal erosion in these sectors; pending a study on the vulnerability of the section of the right bank of the embankment, EDF will have to conduct a geotechnical25 survey of its component materials and monitor its piezometry; given the safety issues associated with the resistance of the embankments of the Donzère-Mondragon structure, EDF must check with the CNR that the monitoring and upkeep of these embankments guarantees the long-term effectiveness of their drainage, along with the absence of any disorders. EDF shall in particular ensure that this monitoring is able to confirm the effectiveness of the piezometric device. These actions also aim (in addition to covering the behaviour of the embankment in the event of an earthquake) to ensure the ability of the embankment to withstand a maximum thousand year flood (CMM). In the event of an SSE and the CMM, ASN considers that the Tricastin NPP is not immune to flooding due to failure of the canal embankments In the event of a CMM, the main issue for protection of the Tricastin NPP against the flooding risk concerns the integrity of the Donzère-Mondragon canal structures and maintaining an acceptable water level in the canal, to avoid stressing the embankments beyond their design loadings. The hydraulic facility was designed on the basis of a project flood (9,900 m3/s) corresponding to a flow rate far lower than the flow rate at present used for protection of the Tricastin NPP (flow rate of 13,700 m3/s). Thus, in 2006, EDF and the CNR defined a strategy to protect the Tricastin site, consisting of a combination of several material and operational countermeasures within the Donzère-Mondragon facility. They consist in: Raising the low points and locally consolidating the embankment on the left bank upstream of the guard dams and bund walls in the Donzère reservoir, opposite the town of Donzère; Raising and reinforcing the new navigable channel through the guard dams at the entrance to the canal; Installation of a cofferdam rapid removal system on a reservoir dam sluice-gate; Extension of the operating setpoint beyond the "project flood"; Installation of a canal emergency safety device (DSU). This would consist in creating a lateral weir on the right bank of the canal. ASN considered this strategy to be satisfactory in principle, provided that the work to implement the countermeasures was performed rapidly. However, ASN asked EDF to provide a certain number of complements and justifications in particular regarding the stability of the structures and the embankments. These data have not yet been provided and the countermeasures implementation work has not yet started; however, an agreement between CNR and EDF was signed and the work is scheduled for completion by late 2014. Pending the performance of this work, ASN considers that protection of the Tricastin NPP cannot be guaranteed in the event of a CMM. On 27th May 2011, in its opinion on the continued operation of Tricastin reactor n°1 after thirty years of operation, ASN issued a requirement for performance of this work before 31st December 2014. 25 Soil survey: in-situ survey and laboratory study to define all the physical, chemical and mechanical characteristics of the soils in place.
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