On 27 December 1999, a severe storm occurred in the vicinity of the “Le Blayais” Nuclear Power Plant located on the banks of the Gironde estuary. The severe storm-driven waves coincident, with high water levels in the Gironde estuary exceeded the worst-case scenario considered at the design of the site protection against flooding, resulting in the scram of three out of four units and severe nuclear island flooding. Many underground rooms sheltering safety-related equipments were flooded, causing the unavailability of all trains of low pressure safety injection and containement spray for two of the units. Moreover, the storm conditions also led to partial temporary loss of external power supplies (loss of the auxiliary 225kV power supplies over the four units, loss of the main grid 400kV at two units).
The flooding has pointed out the possible occurrence of modes of degradation of the safety level affecting simultaneously all the units at a site and has revealed some weaknesses in the site protection against external flooding.
The paper describes the development and management of the flooding event.
It presents the main weaknesses of the existing protection measures in the light of the incident.
It lastely presents the various protection and corrective measures undertaken on the Blayais site in 2000: review of the water level for determining the protections, considering the effect of the heavy wave, raising of the existing dyke, construction of wave breakers, improvement of the warning system, implementation of improved dedicated procedure to bring the plants in a safety state, equipments repair, systematic inspection of the facilities… Le Blayais flooding in December 1999 has led Electricité de France and the regulatory authority to re-examine the flood protection of nuclear sites, as detailed in another dedicated paper.