- 229 - The scenarios have been devised on the basis of initiating events such as a "reactivity accident" and "loss of cooling", and seeking the sequences that lead to meltdown of fuel elements under water or in air. On this basis the licensee retained the following scenarios as having the potential to produce a cliff-edge effect: explosive core meltdown (BORAX type accident) further to the rupture of the heavy water inlet manifold in the reactor core; in-air meltdown of a spent fuel element in the pile block shortly after reactor shutdown at end of cycle (2.5 h) further to a breach in the primary system or the rupture of a thimble in an experimental channel; in-air meltdown of a spent fuel element having cooled for 24 hours after reactor shutdown, during its unloading from the pile block further to a breach in the primary system or the rupture of a thimble in an experimental channel; in-air meltdown of a fuel element in the handling cask after 50 days of cooling, during the water replacement operation when the fuel element is exposed to air; in-air meltdown of several spent fuel elements stored in canal 2, further to a loss of pool sealing. These scenarios, which have already been studied in the safety frame of reference, lead to a cliff-edge effect - that is to say radiological consequences exceeding those that trigger the PPI - only if they are accumulated with degradation of the "control of releases to the environment" function. This function is degraded in the event of: loss of pressurisation of the annular space between the internal concrete containment and the external metal containment (the pressurisation contributes to static containment); loss of the fans and filtration of the gaseous effluents system (which maintains a negative pressure with respect to the environment in the reactor building); damage to the internal concrete containment, resulting in direct leakage into the environment. This leads the licensee to identify the following key SSCs: for preventing a BORAX type reactivity accident o the heavy water inlet manifold in the core (prevention of the risk of reactivity insertion); o the emergency shutdown system; for preventing the risk of meltdown in air of the fuel element in the pile block: o the primary cooling system (prevention of loss of the water inventory); o the thimbles (prevention of loss of the water inventory); o the thimble safety valves (isolation of the pile block in the event of thimble rupture); o the emergency water system (water make-up in the event of loss of water inventory); for preventing the risk of fuel element meltdown in canal 2: o the canal 2 civil engineering; o the canal 2 liner; for preventing the risk of fuel element meltdown in the handling cask: o the cask water filling system; o the emergency letdown system (system that lowers the fuel element to the bottom of canal 2); for the function controlling releases to the environment: o the gaseous effluent system and containment vessel isolation; o the concrete containment.
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