- 133 - 5 Loss of electrical power supplies and cooling systems Even after the nuclear chain reaction has stopped, the nuclear fuel in the reactor and the spent fuel pool must be cooled in order to remove the residual power. For this it is necessary to ensure continuity of the electrical power supply to certain key components (for example the cooling system pumps), and the supply of cooling water (from a river or the sea, for example). ASN has therefore asked EDF analyse the induced losses of the following safety systems, in relation to the experience feedback from the Fukushima accident: loss of the electrical power supplies (including the case of total loss of the off-site and on-site electrical supplies); loss of the cooling sources (heat sink); the above two losses combined. ASN considers that EDF's responses on the whole comply with the requested specifications. The analysis of EDF's Complementary Safety Assessment (CSA) reports has shown that some heat sink and electrical power loss scenarios can lead to core meltdown within a few hours in the most unfavourable cases. ASN thus considers it necessary to increase the robustness of the facilities in a number of ways to enable them to cope with long-duration losses of electrical power supplies or cooling means, which could affect all the facilities on a site. ASN will instruct EDF to implement reinforced measures integrated in the "hard core" mentioned in § 8 of this chapter, comprising in particular a diesel generator and a ultimate backup water supply, capable of withstanding large-scale on-site and external hazards exceeding the baseline safety requirments and coping with total loss of electrical power supplies or cooling means, in order to prevent core meltdown in these situations. Pending progressive deployment of these measures, which will take several years, ASN will prescribe the implementation of these provisional measures, such as mobile electricity generating sets, as of 2012. 5.1 Loss of electrical power supplies Each reactor is linked to the electricity transmission system by a line called the "main line". Before delivering the electrical energy produced at the main generator to the electrical power grid, the reactor - via the step-down transformer (TS) - draws the energy it needs to supply the electrical panels that energize the equipment vital for its operation, and the equipment necessary for the safety of the facility. If the main line fails, the reactor can isolate itself from the electricity transmission system and, via the step-down transformer, continue supplying the electrical panels; this procedure is called "house load operation". When the reactor is not producing electricity, or if the main line is out of service, the electrical panels are supplied via a second line called the auxiliary line. In this case the reactor is supplied directly by the electricity transmission system via the auxiliary transformer (TA). To have sufficient on-site electrical power sources, each reactor has redundant conventional backup sources capable of supplying the electrical panels vital for correct operation of the safety equipment. The conventional backup sources for each reactor in service consist of two emergency diesel generator sets, while the EPR reactor has four main generator sets. Each NPP also has an additional on-site emergency power source, whose technology differs according to the plant series involved: for the 900 MWe series, one ultimate backup diesel-generator set (GUS) per site; for the 1300 MWe and N4 series, one combustion turbine (TAC) per site; for the EPR reactor, two ultimate backup diesel-generator sets (SBO Station blackout) per reactor.
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