- 280 - 4. Other extreme natural phenomena 4.1 Design of the facilities The facilities' structures were designed in accordance with the Snow and Wind 65 (NV 65) rules for the oldest ones and the amended versions in effect at the time of their design. At each periodic safety review, the licensees check that the IPS (important for safety) buildings, and the buildings housing IPS systems or equipment (also called EIS - elements important for safety) can withstand the climate conditions defined in the latest rules in effect. 4.2 Experimental reactors 4.2.1. Reactors operated by the CEA (Osiris, JHR, Phénix) Osiris In addition to the flood risk analysis presented earlier, the CEA assessed the risk of localised blockage of the water drainage system induced by debris carried by a very strong wind or hail, associated with a rainfall event. The CEA estimates that the risk of blockage induced by wind is very limited due to the configuration of the site and the drains, and that the risk of flooding due to hail is covered by the flood risks associated with rainfall events exceeding the 100-year storm levels or with the breach of neighbouring infrastructures. Furthermore, the CEA indicates in its CSA report that hail combined with strong winds could cause damage to the cooling tower airfoils, in which case manual reactor shutdown would be necessary when ineffectiveness of the secondary cooling system was observed. Consequently, with regard to natural phenomena, the CEA proposes establishing an instruction requiring the shutdown of the cooling tower and therefore of the reactor in extreme meteorological events such as hail storms that induce a loss of secondary cooling system effectiveness, and at the latest before the safety thresholds of the core inlet temperature probes are reached. Regarding the risk of occurrence of an earthquake exceeding the design-basis earthquake followed by an induced flood, the CEA considers that in the event of an earthquake exceeding 1.5 times the fixed earthquake level applicable at Saclay, localised flooding could occur, particularly with failure of the Saclay site water tower, of the BNI 40 and 101 (the nearest BNI) cooling towers, and rupture of a water pipe coming from the Centre. Given the water volumes involved (3,000 m3) and the flow rate in the water pipe coming from the Centre (150 m3/h), the CEA considered that no impact was expected on the facility. By way of comparison, the volume of water necessary to flood the first EIS's, namely the nuclear ventilation and the electrical cabinets situated at the 4 m level of the facility, is greater than 5,000 m3. The CEA also points out that in this case the backup ventilation system would remain available to maintain negative pressure in the reactor building . In the framework of the complementary safety assessments, the CEA nevertheless proposed implementing the following measures: Using the GUS (ultimate backup diesel generator set) to power the backup ventilation placed at a height of 2 m and the equipment allowing the ventilation to be configured on the Osiris reactor hall in order to filter the ventilation discharges; Using the GUS to back up the annular space lift pumps; Back up the hot layer pump to provide biological protection in the hall; Close the technical gallery proving the link between the purification room at -4 m in the crown gallery, and the cooling towers, to prevent any water ingress resulting from a breach in the ponds; Extend the use of low walls around all the passageways giving access to the - 4 m level from the 0 m level. The CEA's analysis of the impact of extreme meteorological conditions on the Osiris facility raises no particular remarks from ASN.
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