- 180 - The hydrogen so produced is released in the containment (through the primary system breach, the pressuriser relief tank, or the corium pool) where it is then mixed by the convection movements. In the CSA reports, EDF indicates that Passive Autocatalytic Recombiners (PAR) have been installed on all the reactors in operation in order to reduce the hydrogen concentration in the reactor building (BR) in the event of a severe accident. This installation has been effective since the end of 2007. Associated operating provisions are applicable on the sites. On completion of the CSAs, EDF undertook to study the hydrogen risk in the other peripheral buildings of the reactor containment. The study of the hydrogen risk in the inter-containment space on the 1300 MWe reactors is in progress as part of the periodic safety review associated with their third 10-yearly inspection. In the CSA reports, EDF indicates that the potential cliff-edge effect caused by hydrogen in the containment would be a loss of reactor building (BR) containment in case of ignition of a plume with a high hydrogen concentration in the BR. The recombiners exclude loss of containment through slow deflagration by limiting the quantity of hydrogen in the BR in the event of a severe accident. EDF underlines that the probability of such phenomena occurring is extremely low, especially given the geometrical characteristics of the containment. The containment has a relatively "open" geometry which favours hydrogen mixing and therefore limits the risk of formation of a plume with a high concentration of hydrogen. Installation of the PARs, by reducing the quantity of hydrogen in the containment at a given moment in time, reduces the probability and the consequences of such phenomena. ASN considers that the ongoing R&D studies must be continued to further knowledge of these phenomena. In the CSA report for the Flamanville EPR, EDF describes the planned design measures: hydrogen concentration monitoring is based on two types of device: PARs installed in the reactor building, and rupture and convection flaps and disks, whose opening ensures natural convection within the BR, thereby mixing and homogenising the containment atmosphere. ASN considers these measures satisfactory at this stage of the examination, which is continuing with a view to the commissioning of the Flamanville EPR reactor. 6.3.3 Prevention of reactor containment overpressure The ASN specifications asked EDF to describe the severe accident management measures to prevent reactor containment overpressure. The slow rise in the reactor containment pressure (linked to sump water vaporisation and possibly the formation of non-condensable gases from the decomposition of the basemat concrete by the corium, in the event of corium-concrete interaction (CCI), can lead to exceeding of its design pressure and ultimately loss of its integrity. EDF indicates in the CSA reports that for the reactors in operation, the time before containment is lost due to exceeding of the mechanical characteristics of the reactor, varies from one to several days depending on the assumptions adopted for the studies. EDF considers that this leaves the operator the time to engage action to avoid containment destruction while controlling radioactive release as best possible. The U5 system operating rules were developed in order to avoid containment rupture by overpressure, whatever the circumstances. These rules provide a means of limiting the pressure to a value slightly below the design pressure of the reactor containments by means of the U5 venting and filtration system. Management of such a situation favours a filtered release through a device that can be reclosed if necessary. The reactor building is depressurised by opening two manual valves. In the CSA report, EDF specifies that to exclude any risk of hydrogen combustion in the U5 system that could be induced by condensation of the vapour in the piping, there is a preheating system (venting line conditioning). This conditioning is lost in the event of total loss of the electrical power supplies. Although measures are taken to limit the risk of hydrogen combustion in the U5 venting line (pressure reduction upstream of the line limiting the risk of condensation, recombiners substantially limiting the hydrogen concentration), EDF has undertaken to re-examine the hydrogen risk and its possible impacts on the U5 system. ASN considers that this examination must focus in particular on the impact of the oxygen already present in the U5 pipe and on the risk of hydrogen deflagration and its possible consequences at the U5 system outlet. ASN also considers that for the 900 MWe series, EDF must study the simultaneous use of the U5 system, which is common to two reactors. ASN will require EDF to study the possibilities of improving the U5 venting-filtration
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