- 118 - 3.2 Evaluation of safety margins 3.2.1 Estimation of margins in the event of flooding In its specifications, ASN asked EDF to state the flood level the facility could withstand without damage to the fuel (in the reactor vessel or in the pool) and the levels leading to the initiation of accident situation measures. EDF was able to call on the available information (and take account of the studies to confirm the engineer's assessment). In its CSA reports, for the various hazards considered for each site, EDF presented the margins - when available - between the flood level reached and the level of the protections, for the purposes of the current design and reached a conclusion regarding any additional measures to be taken. These tables offer a satisfactory response to the ASN request. EDF also studied a number of situations which it feels are representative when evaluating cliff-edge effects. These cases are summarised below and assume hypotheses going beyond the design-basis, contrary to what was presented hitherto in this part of the report devoted to flooding. In its CSA reports, EDF analysed three types of cliff-edge effects that could be triggered by a flood: 1. Flood causing the loss of site heat-sink (situation H1), initiated by a rise in the water levels leading in turn to loss of the circulating water filtration system (CFI) then submersion of the essential service water system (SEC) pumps. For certain sites, the loss of the SEC pumps occurs before the loss of the filtration system. In its CSA reports, EDF states that: the loss of the filtration system on the sites equipped with rotating drum screens would imply long-term unavailability of certain devices on the filtration system, although without leading to a certain loss of the function, the loss of the chain screen drive motors could lead to long-term unavailability of filtration. In this case, the risk of an H1 situation through clogging cannot be ruled out. For the Fessenheim plant, the pumping station is situated at a higher altitude than the site platform, so the essential service water system can function by gravity in the event of flooding. 2. Flood causing a LOOP (loss of off-site power) situation resulting from a loss of equipment through submersion initiated by at least one of the following events: Loss of all the off-site power substations (HV line outgoing feeders) through equipment submersion. This scenario can directly affect an entire site (except if special corrective measures are taken). Loss of transformers supplying the safety auxiliaries from the off-site grid, these transformers being located inside the site: directly at the output from the generation unit (TP main transformers and TS step-down transformers), TA auxiliary transformers (supply circuit separate from that of the TP and the TS). 3. Flood causing total loss of the electricity sources (H3 situation) associated with the possible loss of the reactor backup systems, this type of effect being initiated by the presence of a layer of water on the nuclear island platform. With regard to flooding caused by an earthquake bigger than design-basis, EDF identified critical cliff-edge effects owing to the positioning of the structure concerned, which are liable to constitute potential sources of flooding following an earthquake of intensity higher than the SSE. Depending on the sites, these cliff-edge effects are the arrival of a layer of water on the nuclear island platform exceeding the building access thresholds, which would lead to an H3 situation, or the arrival of a layer of water causing submersion of the auxiliary transformers, which would lead to a LOOP type situation.
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