In an accident involving a core meltdown due to the loss of the water inventory, the kinetics are relatively slow: the number of backup systems available ensures that water levels remain sufficient for the core to continue to be properly cooled by natural convection.
The Basic Safety Rules (RFS - Règles Fondamentales de Sûreté) are based on the assumption that certain emergency backup systems fail after 24 hours of use. It is this additional failure that, in this scenario, would result in the exposure of the core and its fusion in air.
The operator and the authorities would therefore have sufficient time to implement their respective emergency response plans (PUI and PPI) before the radiation accident itself (i.e. the core meltdown) occurs.
In the Borax-type accident automatically included in the design basis the kinetics are extremely fast. A small fraction of the fission products is released immediately into the reactor containment; most of the radioactivity, however, is retained in the reactor pool. Although this type of accident comes under the so-called "reflex response phase" of the PPI, the dose rates generated around the installation would be considerably lower than those resulting from a core meltdown in air. Any measures that might need to be taken (evacuation and / or “take cover” order) to protect the populations in the 300 m and 500 m zones could therefore be carried out calmly and carefully, without haste.