- 177 - The measures that can be taken on the reactor fleet to prevent fuel damage aim at restoring a means of injecting water into the reactor vessel in order to - by reflooding the core - cool the fuel and stabilise the situation. The possible measures consist in: if necessary, restoring an electrical panel that can energise the backup systems; deploying an ultimate alignment for injecting water into the vessel of the impacted reactor. On the Flamanville EPR, the various lines of defence (main diesel generators, ultimate backup diesel generator sets (SBO), replenishment of the ASG tank) limit the risk of entry into a severe accident situation. 6.2.2 After the fuel in the reactor vessel has been damaged Beyond this point, a severe accidents management procedure aims at limiting the consequences in the event of core meltdown. If it has been impossible to avoid a severe accident, the operating priorities are turned towards controlling containment and reducing releases. In the CSA report, EDF indicates the existing measures in response to the identified risk in a severe accident situation. They are indicated below and reviewed in detail with the planned or envisaged improvements further to the CSA, in the section relative to "Maintaining containment integrity after fuel damage in the reactor core". Risk due to the production of hydrogen: Since the end of 2007, all the reactors in service are equipped with passive autocatalytic recombiners (PAR). The Flamanville EPR has PARs and devices for monitoring the concentration and distribution of hydrogen in the containment by interconnecting the two parts of the containment and favouring mixing by convection. Risk of slow pressurisation of the containment: On the reactor fleet in service, this risk is dealt with by the existence of the venting-filtration system called "U5" and an associated operating procedure allowing decompression and filtration of the reactor containment in order to maintain its long-term integrity. Filtration is divided between a container internal coarse metallic filter and a sand bed filter (common to two reactors for the 900 MWe series). The opening of this system, which is an ultimate reactor containment protection measure, takes place after 24 hours as from a minimum pressure equal to the containment design pressure (about 5 bars absolute for all the plant series). On the EPR, the EVU system removes heat from the containment and controls its pressure. This safeguard system consists of 2 redundant trains and has a dedicated cooling system which itself has a diversified backup water intake. In the event of loss of the electrical power supplies, while satisfying conditions compatible with operation of the reactor building ultimate heat removal system (EVU), this EVU system can be put into service for 2 days in order to preclude the risk of containment failure. Lastly, the integrity of the containment is maintained for 3 days after the initiating event if the EVU is not put into service. Risk of reactor containment leaktightness fault: On the reactors in service, confirmation of the isolation of the containment penetrations is required as part of the immediate actions on entry into a severe accident situation. The activity is monitored so that restoration measures can be implemented if necessary. The U2 operating procedure (continuous monitoring of containment integrity) which is part of incident/accident operating procedure (CIA) is applicable in a SA situation. Its aim is to monitor the containment integrity under accident conditions and if necessary restore the reactor containment (by isolating the areas concerned, reinjection of highly radioactive effluents, etc.). On the EPR, the containment and the peripheral buildings are designed such that there is no direct leakage path from the reactor containment to the environment. The building ventilation systems are backed up by the main diesel generators and the ultimate backup diesel generator sets (SBO).
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