- 179 - The risks induced by these situations and the severe accident management means for controlling them and mitigating their consequences are presented below, through a description of the existing means and the complementary means envisaged further to the CSAs. 6.3.1 Elimination of the risk of high-pressure fuel damage or core meltdown The ASN specifications required EDF to describe the severe accident management measures to eliminate any possibility of high-pressure damage to the fuel. This is because in a core meltdown accident situation affecting a PWR reactor, and when the primary system is not depressurised (no breach in the primary system and no cooling by the secondary system), meltdown can take place at high pressure; this is called pressure meltdown. In the CSA reports, EDF indicates for the reactors in operation that the prevention of pressure meltdown sequences is based on voluntary opening of the pressuriser SEBIM valve tandems. The opening of the three valve tandems causes rapid depressurisation of the primary system which eliminates the risk of having a highly pressurised reactor vessel when melt-through occurs and the risk of loss of containment through its direct heating. Opening of the valve tandems is required in the majority of situations well before entry into a severe accident on a primary system overheat criterion. In a situation of total loss of the electrical power supplies, valve tandem opening is required in the event of loss of the steam generator supply from the turbine driven auxiliary feedwater pump (TPS ASG). Confirmation of valve opening is required by the severe accident operating documents. EDF indicates that opening the SEBIM valves and keeping them open enables core meltdown to be avoided with the primary system at high pressure, which could lead to substantial pressurisation of the reactor containment atmosphere by fine spraying of the fuel when vessel rupture occurs (phenomenon of direct containment heating (DCH)). EDF specifies in the CSA reports that to fulfil this "primary system depressurisation" function, the current design of the remote control of the pressuriser SEBIM valves requires permanent energising of their solenoids, and therefore the availability of the electrical power source and power cables. A modification to improve SEBIM valve opening reliability, decided before the Fukushima accident and already been applied on certain reactors, is planned for the next 10-yearly inspection of each reactor. The solution chosen by EDF to improve its robustness is to replace the monostable remote control (solenoid) by a bistable control (magnetic latching on control by solenoid). The modification proposed by EDF at the end of the CSAs also aims - in a situation of total loss of electric power sources and exhaustion of the batteries - to control the valve solenoids directly from the relaying rooms from a new stand-alone Mobile Backup Means (MMS). Operation is thus simplified and bypasses all problems of battery autonomy and radiation resistance of the electrical power supply for the valve solenoids. ASN considers that the proposed improvements, which meet the CSA specifications, must be implemented. In the CSA report for the Flamanville 3 EPR, EDF indicates that the EPR is designed with two redundant primary system discharge lines enabling the primary system to be depressurised and avoid the risk of reactor vessel rupture at high pressure, which could lead to loss of containment integrity by DCH. The licensee has one hour after entry into the severe accident situation to open theses lines which are supplied by batteries with 12 hours autonomy. ASN considers the principle of this proposal satisfactory; it will be examined in the framework of the Flamanville 3 EPR reactor commissioning. 6.3.2 Management of the hydrogen risk in the reactor containment The ASN specifications asked EDF to describe the severe accident management measures to prevent any hydrogen deflagration or detonation (container inerting, recombiners or igniters). For the severe accident studies of the PWRs, the hydrogen risk is defined as being the possible loss of reactor containment integrity or of its safety systems further to a hydrogen deflagration. In the CSA reports, EDF indicates that hydrogen can be produced during different phases of an accident: in-vessel, during the phase of core degradation due to the oxidation of the fuel element cladding and other materials present in the reactor vessel; ex-vessel, during the corium/concrete interaction.
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