- 284 - 5. Loss of the electrical power supplies and cooling systems 5.1 Experimental reactors 5.1.1. CEA reactors (Osiris, JHR, Phénix) Osiris Loss of off-site electrical power supplies In the event of loss of the off-site electrical power grid, two diesel generator sets are automatically brought into service to supply the Osiris reactor with electrical power. Loss of the off-site electrical power supplies and the conventional emergency supplies Loss of a diesel generator set would cause: shutdown of the primary coolant pumps and therefore emergency shutdown of the Osiris reactor. Primary coolant pump shutdown would also lead to passive opening of the natural convection valves after about one minute (making the primary cooling system communicate with the reactor pool) and the establishing of a natural convection flow in the core; loss of the pools and channels cooling system. In this situation it is possible to compensate for evaporation through water make-ups via the pools and channels filling system (flow rate of 30 m3/h) which is connected to the public water supply and supplied by the diesel generator set that is still available. If both diesel generator sets are lost, an ultimate backup diesel-generator set (GUS) dedicated to the facility is put into service manually. However, at present the GUS supplies no pool cooling system or water makeup system. Consequently, the time lapses before the irradiated fuel elements become exposed are as follows: 255 days if the reactor pool communicates with the spent fuel storage channels (nominal situation); more than 230 days for channel 2 if it is isolated; 43 days for the Osiris reactor pool if it is isolated from the spent fuel storage channels by a gate, and assuming that the initial level in the reactor pool is low (exceptional maintenance situation that occurs once a year). The CEA considers this time lapse sufficient to deploy external electrical power supply or water make-up means (the Saclay centre also has two mobile generator sets). In the event of loss of the two diesel-generator sets, power for monitoring the facility is provided by batteries for one hour, a period judged sufficient to start the GUS, which powers the equipment necessary for facility containment management as well as for monitoring the facility. Loss of the off-site electrical power supplies and the conventional emergency supplies and all other on-site electrical power sources In the event of loss of the GUS, the batteries would be available for a limited time. The Centre's mobile generator sets could be brought to the Osiris facility within one to four hours according to the CEA. In the event of loss of the batteries, no electrical power supply is available in the facility. As CEA points out, this means that the radiation protection systems (particularly the permanent irradiation measurement in the reactor building) are no longer powered. To avoid reaching a cliff-edge effect, the CEA indicates that it is necessary to monitor the water level in the tanks and to be able to provide make-ups. The slow kinetics of the phenomenon means that the water levels can be monitored by inspection patrol personnel equipped with mobile radiation detectors.
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