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of these exchangers, the water is carried to the base of the core structure.
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4. DESIGN OF NUCLEAR REACTORS
The pipe that carries the water to the core has two nozzles, on which natural convection
flappers are fitted. Under normal operating conditions, both natural convection flappers
are maintained in a closed position by the pressure of the water circulating in the primary
coolant pipe. In the event of the pressure in the latter dropping below the threshold which
triggers reactor trip, both flappers automatically open under the effect of gravity, the
water in the pool is let in directly to the base of the core, which is then cooled by natural
convection without changing the direction of the circulation.
Secondary systems
The water in the secondary systems comes from the water system of the Research Centre
at Saclay. After the water passes through the heat exchangers of the primary coolant
systems (core and pool), it is cooled in an atmospheric cooling system comprising four
independent units. The secondary systems have four separate loops, sharing the
atmospheric cooling system. The following two main loops can be identified:
The main system (flowrate 5600 m3/h) which supplies the four primary coolant
system heat exchangers of the core of the OSIRIS reactor and the two heat
exchangers of the primary coolant system of the OSIRIS pool; this system is fitted
with four pumps.
The system used to cool the primary coolant system of the channels and of the core
of the ISIS reactor (flowrate 120 m3/h).
Any leaks in the exchanger pipe of the primary coolant system can be detected by
monitoring the water makeup of this system, the beta and gamma activity of the water in
the secondary system, and the activity of the liquid effluents released into the sewer
network.