- 138 - for the secondary system, the steam generators are supplied if necessary via the auxiliary feedwater system (EFWS) by two turbine-driven pumps; the residual power is removed to the atmosphere by the main steam safety relief valves GCT-a (turbine bypass system-atmosphere); the spent fuel pool cooling systems are no longer supplied with electricity, which can result in evaporation of the pool water and possibly exposure of the fuel (within a time specified further on), and can ultimately lead to meltdown of the stored fuel. For the EPR reactor, in the event of loss of the off-site electrical power supplies and the conventional backup power supplies: an ultimate backup diesel-generator set (SBO), which is started manually from the control room, supplies the pumps of the EFWS system; the "2-hour" and "12-hour" batteries are charged automatically by the SBO generator when it is in operation; if the reactor is initially under power or in hot shutdown state, the rod cluster control assemblies drop into the core; the residual power is removed by natural circulation; the thermal barrier of the RCPs is cooled automatically by the shutdown sealing system (DEA) supplied by the "2-hour" batteries; for the secondary system, the steam generators are supplied if necessary via motor-driven pumps of the EFWS system which are supplied with electricity by the SBO generators; the residual power is removed by the atmospheric steam dump valves (VDA); if the reactor is shut down and the primary system is just open or fully open, the residual power is removed by evaporation; a low-pressure injection channel of the safety injection system (IRWST - incontainment refuelling water storage tank) supplied by the SBO generator enables primary system makeup to be accomplished and a channel of the ultimate heat removal system in the containment (EVU/SRU) removes the residual power from the containment; a cooling system of the spent fuel pool can be supplied with electricity by an SBO generator. Regarding the capacity and autonomy of the batteries of the reactors in service, EDF has specified in the CSA reports that the storage batteries: ensure automatic power sources switchover; supply power for at least one hour to the instrumentation & control necessary to diagnose the problem and orient the operating team during an electrical power failure. EDF also specified in the CSA reports that operating procedures for lost external and on-site electrical power supply situations provide for operation in "battery saving mode", enabling high-priority functions to be powered for longer by load-shedding lower-priority functions. For the EPR reactor, EDF state in the CSA reports that: four "2-hour" batteries can supply the instrumentation & control, the man-machine interfaces and the containment internal isolation valves for at least two hours; two "12-hour" batteries can supply the instrumentation and control (I&C) dedicated to severe accidents (CCAG), the severe accidents console (CAG), the iodine filtration of the inter-containment space, the containment external isolation valves and the emergency lighting of the control room, of the crisis technical room and of the fallback station, for at least twelve hours. On the EPR reactor, as the "2-hour" batteries are necessary to couple the main generators and ultimate backup generator sets (SBO) to the electrical system, the following cliff-edge effects were identified during the examination prior to the meeting of the advisory committees in November 2011: a common cause failure affecting the four "2-hour" batteries in a situation of off-site electrical power supply loss would lead to the total outage of all the generator sets and a severe accident; the measures necessary for reactor vessel containment and switching over to the severe accidents console must be carried out before these "2-hour" batteries become discharged.
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