- 292 - 5.1.2. Reactor operated by the ILL Loss of the electrical power supplies The HFR is supplied from the HTA 20 kV power system managed by Gaz et Electricité de Grenoble. Loss of the 20 kV power system causes loss of the two pumps of the main primary system (reactor coolant pumps) and those of the associated main cooling system; stopping of the reactor coolant pumps causes automatic reactor shutdown. In this situation, two 1800 kVA diesel generator sets situated in building ILL 3 start automatically. These generator sets supply backup power to the CRAB (rod shutdown cooling system) pumps, the associated cooling system pumps (auxiliary backed-up) and the reactor protection system (as well as power for the radiation protection network, the air supply and extraction fans in the reactor building and the safety lighting). The two diesel generator sets each have an autonomy of 40 hours and are fully redundant. If neither of the two diesel generator sets should come into service after loss of the 20 kV system, the three CRAB pumps would be supplied for one hour by three groups of 400 V/50 Hz batteries each delivering 50 kVA. At the end of this period, the residual power is removed by natural convection. The natural convection cooling of the fuel elements in channel 2 and in the handling cask is not affected by loss of the electrical power supplies. Conclusion on the planned measures to protect the facilities against the risk of loss of the electrical power supplies The ILL indicates that by design, core cooling is ensured by natural convection as soon as the reactor is shut down. The shutdown cooling system (CRA) is however supplied by a battery with an autonomy of one hour to ensure cooling by forced convection. It considers that the loss of all the electrical power supplies has no impact on the control of cooling. According to the licensee, the facility could ensure satisfactory cooling of the core and the spent fuel elements (SFE) further to loss of the off-site electrical power supplies and the emergency power sources, without external intervention and with no time limit. The facility is relatively insensitive to electrical power supply losses, on condition that the reactor is shut down and the natural convection valves which ensure passive cooling of the fuel in the reactor are opened. It must be noted however that the ILL does not formally identify these equipment items as "key" SSCs. The ILL indicates in its report that containment isolation and leak-tightness are guaranteed if the emergency power sources are lost. However, in the event of loss of the electrical power supplies and aggravating circumstances, the operability of the gaseous effluents (EG) system in order to maintain the reactor building under negative pressure and thereby guarantee the absences of direct leaks, is not ensured from the emergency control room (PCS). Containment would therefore be degraded due to the loss of annular space pressurisation and loss of the static containment provided by the gaseous effluents system, without any impact in the absence of a nuclear accident, according to the ILL. Measures proposed to reinforce the robustness of the facilities to the loss of the electrical power supplies With regard to the last point concerning the operability of the gaseous effluents system to keep the reactor building under negative pressure, the licensee proposes a modification to the control of the gaseous effluents system which is not entirely taken up by the diesel generator set of the emergency control room (PCS). This modification is scheduled for when the HFR is restarted at the beginning of 2012. The licensee also proposes drafting a specific operating instruction (CPE) for loss of the off-site electrical power supplies, in order optimise the existing procedures and improve the organisational provisions. In its CSA report the licensee also undertook - in accordance with an earlier analysis - to set up an intermediate system that automatically triggers rod drop if the 20 kV supply is lost for more than 2 seconds. In addition, pending creation of the new PCS (PCS3, see the "accident management" section), the ILL plans installing a new emergency diesel generator set (at a higher elevation than 216.2 m NGF see flood risk) to supply the reactor monitoring equipment. Lastly, it has undertaken to consider the emergency shutdown system as a "key" SSC on account of its role in serious accident risk prevention in the event of loss of the electrical power sources. On completion of the review of the ILL's complementary safety assessment, ASN considers that the measures proposed must be implemented. It will issue requests to the licensee in this respect.
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