- 135 - If the off-site electrical sources and the abovementioned on-site backup sources should fail, specific equipment is provided to supply certain items that are critical for managing this situation: on each in-service reactor, one ultimate electrical power source provided by a turbine generator (LLS) driven by steam from the steam generators (SG); on the EPR reactor, two batteries dedicated to this situation (called "12-hour" batteries). ASN has asked EDF to study the successive loss of all these electrical power sources in the complementary safety assessments, considering initially that only one reactor is concerned, and in a second phase that all the facilities of a given site are affected simultaneously. The targeted inspections carried out by ASN in 2011 found that the state of the electrical power supplies was generally satisfactory, though a number of shortcomings exist on certain sites. Generally speaking, the consistency of the operating and maintenance documents, the condition of certain items relating to fuel storage, the management of generator set fluids and the periodic inspections of the combustion turbines (TAC) are areas in which many sites could make improvements. 5.1.1 Loss of the off-site electrical power supplies For each reactor, ASN has asked EDF to: describe the facility's design measures that take into account this power loss situation, the backup means provided, and their conditions of use; indicate the length of time the on-site electrical power supplies can function without external backup; specify the measures taken to extend the utilization time of the on-site electrical power supplies (refuelling of the diesel generator sets, etc.); indicate any measure envisaged to increase the robustness of the facility (design change, change in procedures, organisational arrangements, etc.). Loss of the off-site electrical power supplies of a reactor is a situation analysed for the baseline safety standard; it corresponds to loss of the main and auxiliary lines and failure of house load operation. In a situation of off-site electrical power supply loss: the reactor loads are energized by the on-site source, i.e. the backup diesel generator sets; these generator sets start automatically in the event of simultaneous loss of the main and auxiliary systems or a significant voltage drop on the backed-up electrical panels; the control rods drop under gravity, which terminates the nuclear fission reaction and controls the reactivity; the reactor core continues to emit heat (called residual power), which must be removed from the core to prevent its temperature from rising and ultimately damage it; the reactor coolant pumps (RCP) are no longer supplied with electricity, as their power demand is too great for them to be supplied by the generator sets; the flow in the primary system decreases rapidly; after complete stoppage of the RCPs, natural circulation in the primary loops removes the residual power which decreases as a result of the power decay further to the automatic reactor shutdown; on the secondary side, the reactor shutdown trips the turbine and closes the turbine inlet valves; as the steam generator main feedwater pumps (feedwater flow control system - ARE) have stopped due to the initiating event, the feedwater supply terminates until the auxiliary feedwater system (EFWS) starts up; the residual power is removed by the steam generator with opening of the main steam safety relief valves to the atmosphere (GTC-a for the reactors in service, or VDA for the EPR reactor); the spent fuel pool cooling systems are backed up by the reactor emergency generator sets. In the complementary safety assessment (CSA) reports, EDF pointed out that starting the emergency generator sets gives the management team the electrical power sources necessary to bring the reactor to a safe condition if the off-site electrical power supplies are lost.
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